P2PSIP C. Jennings
Internet-Draft Cisco
Intended status: Standards Track B. Lowekamp, Ed.
Expires: September 8, 2009 unaffiliated
E. Rescorla
Network Resonance
S. Baset
H. Schulzrinne
Columbia University
March 07, 2009
REsource LOcation And Discovery (RELOAD) Base Protocol
draft-ietf-p2psip-base-02
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. This document may contain material
from IETF Documents or IETF Contributions published or made publicly
available before November 10, 2008. The person(s) controlling the
copyright in some of this material may not have granted the IETF
Trust the right to allow modifications of such material outside the
IETF Standards Process. Without obtaining an adequate license from
the person(s) controlling the copyright in such materials, this
document may not be modified outside the IETF Standards Process, and
derivative works of it may not be created outside the IETF Standards
Process, except to format it for publication as an RFC or to
translate it into languages other than English.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on September 8, 2009.
Jennings, et al. Expires September 8, 2009 [Page 1]
Internet-Draft RELOAD Base March 2009
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Abstract
In this document the term BCP 78 and BCP 79 refer to RFC 3978 and RFC
3979 respectively. They refer only to those RFCs and not any
documents that update or supersede them.
This document defines REsource LOcation And Discovery (RELOAD), a
peer-to-peer (P2P) signaling protocol for use on the Internet. A P2P
signaling protocol provides its clients with an abstract storage and
messaging service between a set of cooperating peers that form the
overlay network. RELOAD is designed to support a P2P Session
Initiation Protocol (P2PSIP) network, but can be utilized by other
applications with similar requirements by defining new usages that
specify the kinds of data that must be stored for a particular
application. RELOAD defines a security model based on a certificate
enrollment service that provides unique identities. NAT traversal is
a fundamental service of the protocol. RELOAD also allows access
from "client" nodes that do not need to route traffic or store data
for others.
Legal
This documents and the information contained therein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE
IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL
WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY
WARRANTY THAT THE USE OF THE INFORMATION THEREIN WILL NOT INFRINGE
ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS
FOR A PARTICULAR PURPOSE.
Jennings, et al. Expires September 8, 2009 [Page 2]
Internet-Draft RELOAD Base March 2009
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 8
1.1. Basic Setting . . . . . . . . . . . . . . . . . . . . . 9
1.2. Architecture . . . . . . . . . . . . . . . . . . . . . . 10
1.2.1. Usage Layer . . . . . . . . . . . . . . . . . . . . 13
1.2.2. Message Transport . . . . . . . . . . . . . . . . . 14
1.2.3. Storage . . . . . . . . . . . . . . . . . . . . . . 14
1.2.4. Topology Plugin . . . . . . . . . . . . . . . . . . 15
1.2.5. Forwarding and Link Management Layer . . . . . . . . 15
1.3. Security . . . . . . . . . . . . . . . . . . . . . . . . 16
1.4. Structure of This Document . . . . . . . . . . . . . . . 17
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 17
3. Overlay Management Overview . . . . . . . . . . . . . . . . . 19
3.1. Security and Identification . . . . . . . . . . . . . . 19
3.1.1. Shared-Key Security . . . . . . . . . . . . . . . . 20
3.2. Clients . . . . . . . . . . . . . . . . . . . . . . . . 21
3.2.1. Client Routing . . . . . . . . . . . . . . . . . . . 21
3.2.2. Minimum Functionality Requirements for Clients . . . 22
3.3. Routing . . . . . . . . . . . . . . . . . . . . . . . . 22
3.4. Connectivity Management . . . . . . . . . . . . . . . . 25
3.5. Overlay Algorithm Support . . . . . . . . . . . . . . . 26
3.5.1. Support for Pluggable Overlay Algorithms . . . . . . 26
3.5.2. Joining, Leaving, and Maintenance Overview . . . . . 26
3.6. First-Time Setup . . . . . . . . . . . . . . . . . . . . 28
3.6.1. Initial Configuration . . . . . . . . . . . . . . . 28
3.6.2. Enrollment . . . . . . . . . . . . . . . . . . . . . 28
4. Application Support Overview . . . . . . . . . . . . . . . . 28
4.1. Data Storage . . . . . . . . . . . . . . . . . . . . . . 29
4.1.1. Storage Permissions . . . . . . . . . . . . . . . . 30
4.1.2. Usages . . . . . . . . . . . . . . . . . . . . . . . 31
4.1.3. Replication . . . . . . . . . . . . . . . . . . . . 31
4.2. Service Discovery . . . . . . . . . . . . . . . . . . . 32
4.3. Application Connectivity . . . . . . . . . . . . . . . . 32
5. Overlay Management Protocol . . . . . . . . . . . . . . . . . 32
5.1. Message Receipt and Forwarding . . . . . . . . . . . . . 33
5.1.1. Responsible ID . . . . . . . . . . . . . . . . . . . 33
5.1.2. Other ID . . . . . . . . . . . . . . . . . . . . . . 34
5.1.3. Private ID . . . . . . . . . . . . . . . . . . . . . 35
5.2. Symmetric Recursive Routing . . . . . . . . . . . . . . 35
5.2.1. Request Origination . . . . . . . . . . . . . . . . 35
5.2.2. Response Origination . . . . . . . . . . . . . . . . 36
5.3. Message Structure . . . . . . . . . . . . . . . . . . . 36
5.3.1. Presentation Language . . . . . . . . . . . . . . . 37
5.3.1.1. Common Definitions . . . . . . . . . . . . . . . 38
5.3.2. Forwarding Header . . . . . . . . . . . . . . . . . 40
5.3.2.1. Processing Configuration Sequence Numbers . . . . 42
5.3.2.2. Destination and Via Lists . . . . . . . . . . . . 43
Jennings, et al. Expires September 8, 2009 [Page 3]
Internet-Draft RELOAD Base March 2009
5.3.2.3. Route Logging . . . . . . . . . . . . . . . . . . 44
5.3.2.4. Forwarding Options . . . . . . . . . . . . . . . 46
5.3.3. Message Contents Format . . . . . . . . . . . . . . 47
5.3.3.1. Response Codes and Response Errors . . . . . . . 48
5.3.4. Security Block . . . . . . . . . . . . . . . . . . . 50
5.4. Overlay Topology . . . . . . . . . . . . . . . . . . . . 53
5.4.1. Topology Plugin Requirements . . . . . . . . . . . . 53
5.4.2. Methods and types for use by topology plugins . . . 53
5.4.2.1. Join . . . . . . . . . . . . . . . . . . . . . . 53
5.4.2.2. Leave . . . . . . . . . . . . . . . . . . . . . . 54
5.4.2.3. Update . . . . . . . . . . . . . . . . . . . . . 54
5.4.2.4. Route_Query . . . . . . . . . . . . . . . . . . . 55
5.4.2.5. Probe . . . . . . . . . . . . . . . . . . . . . . 56
5.5. Forwarding and Link Management Layer . . . . . . . . . . 58
5.5.1. Attach . . . . . . . . . . . . . . . . . . . . . . . 58
5.5.1.1. Request Definition . . . . . . . . . . . . . . . 59
5.5.1.2. Response Definition . . . . . . . . . . . . . . . 60
5.5.1.3. Using ICE With RELOAD . . . . . . . . . . . . . . 60
5.5.1.4. Collecting STUN Servers . . . . . . . . . . . . . 60
5.5.1.5. Gathering Candidates . . . . . . . . . . . . . . 61
5.5.1.6. Encoding the Attach Message . . . . . . . . . . . 61
5.5.1.7. Verifying ICE Support . . . . . . . . . . . . . . 62
5.5.1.8. Role Determination . . . . . . . . . . . . . . . 62
5.5.1.9. Connectivity Checks . . . . . . . . . . . . . . . 62
5.5.1.10. Concluding ICE . . . . . . . . . . . . . . . . . 62
5.5.1.11. Subsequent Offers and Answers . . . . . . . . . . 63
5.5.1.12. Media Keepalives . . . . . . . . . . . . . . . . 63
5.5.1.13. Sending Media . . . . . . . . . . . . . . . . . . 63
5.5.1.14. Receiving Media . . . . . . . . . . . . . . . . . 63
5.5.2. AttachLite . . . . . . . . . . . . . . . . . . . . . 64
5.5.2.1. Request Definition . . . . . . . . . . . . . . . 64
5.5.2.2. Attach-Lite Connectivity Checks . . . . . . . . . 65
5.5.2.3. Implementation Notes for Attach-Lite . . . . . . 65
5.5.3. Ping . . . . . . . . . . . . . . . . . . . . . . . . 65
5.5.3.1. Request Definition . . . . . . . . . . . . . . . 66
5.5.3.2. Response Definition . . . . . . . . . . . . . . . 66
5.5.4. Config_Update . . . . . . . . . . . . . . . . . . . 66
5.5.4.1. Request Definition . . . . . . . . . . . . . . . 66
5.5.4.2. Response Definition . . . . . . . . . . . . . . . 67
5.6. Overlay Link Layer . . . . . . . . . . . . . . . . . . . 67
5.6.1. Future Support for HIP . . . . . . . . . . . . . . . 68
5.6.2. Reliability for Unreliable Links . . . . . . . . . . 68
5.6.2.1. Framed Message Format . . . . . . . . . . . . . . 68
5.6.2.2. Retransmission and Flow Control . . . . . . . . . 70
5.6.3. Fragmentation and Reassembly . . . . . . . . . . . . 71
6. Data Storage Protocol . . . . . . . . . . . . . . . . . . . . 72
6.1. Data Signature Computation . . . . . . . . . . . . . . . 73
6.2. Data Models . . . . . . . . . . . . . . . . . . . . . . 74
Jennings, et al. Expires September 8, 2009 [Page 4]
Internet-Draft RELOAD Base March 2009
6.2.1. Single Value . . . . . . . . . . . . . . . . . . . . 74
6.2.2. Array . . . . . . . . . . . . . . . . . . . . . . . 75
6.2.3. Dictionary . . . . . . . . . . . . . . . . . . . . . 75
6.3. Access Control Policies . . . . . . . . . . . . . . . . 76
6.3.1. USER-MATCH . . . . . . . . . . . . . . . . . . . . . 76
6.3.2. NODE-MATCH . . . . . . . . . . . . . . . . . . . . . 76
6.3.3. USER-NODE-MATCH . . . . . . . . . . . . . . . . . . 76
6.3.4. NODE-MULTIPLE . . . . . . . . . . . . . . . . . . . 77
6.3.5. USER-MATCH-WITH-ANONYMOUS-CREATE . . . . . . . . . . 77
6.4. Data Storage Methods . . . . . . . . . . . . . . . . . . 77
6.4.1. Store . . . . . . . . . . . . . . . . . . . . . . . 77
6.4.1.1. Request Definition . . . . . . . . . . . . . . . 77
6.4.1.2. Response Definition . . . . . . . . . . . . . . . 81
6.4.1.3. Removing Values . . . . . . . . . . . . . . . . . 82
6.4.2. Fetch . . . . . . . . . . . . . . . . . . . . . . . 83
6.4.2.1. Request Definition . . . . . . . . . . . . . . . 83
6.4.2.2. Response Definition . . . . . . . . . . . . . . . 85
6.4.3. Stat . . . . . . . . . . . . . . . . . . . . . . . . 86
6.4.3.1. Request Definition . . . . . . . . . . . . . . . 86
6.4.3.2. Response Definition . . . . . . . . . . . . . . . 86
6.4.4. Find . . . . . . . . . . . . . . . . . . . . . . . . 88
6.4.4.1. Request Definition . . . . . . . . . . . . . . . 88
6.4.4.2. Response Definition . . . . . . . . . . . . . . . 89
6.4.5. Defining New Kinds . . . . . . . . . . . . . . . . . 90
7. Certificate Store Usage . . . . . . . . . . . . . . . . . . . 90
8. TURN Server Usage . . . . . . . . . . . . . . . . . . . . . . 91
9. Chord Algorithm . . . . . . . . . . . . . . . . . . . . . . . 92
9.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 93
9.2. Reactive vs Periodic Recovery . . . . . . . . . . . . . 93
9.3. Routing . . . . . . . . . . . . . . . . . . . . . . . . 94
9.4. Redundancy . . . . . . . . . . . . . . . . . . . . . . . 94
9.5. Joining . . . . . . . . . . . . . . . . . . . . . . . . 95
9.6. Routing Attaches . . . . . . . . . . . . . . . . . . . . 95
9.7. Updates . . . . . . . . . . . . . . . . . . . . . . . . 96
9.7.1. Sending Updates . . . . . . . . . . . . . . . . . . 97
9.7.2. Receiving Updates . . . . . . . . . . . . . . . . . 98
9.7.3. Stabilization . . . . . . . . . . . . . . . . . . . 99
9.8. Route Query . . . . . . . . . . . . . . . . . . . . . . 100
9.9. Leaving . . . . . . . . . . . . . . . . . . . . . . . . 101
10. Enrollment and Bootstrap . . . . . . . . . . . . . . . . . . 101
10.1. Overlay Configuration . . . . . . . . . . . . . . . . . 101
10.1.1. Relax NG Grammars . . . . . . . . . . . . . . . . . 104
10.2. Discovery Through Enrollment Server . . . . . . . . . . 106
10.3. Credentials . . . . . . . . . . . . . . . . . . . . . . 107
10.3.1. Self-Generated Credentials . . . . . . . . . . . . . 108
10.4. Joining the Overlay Peer . . . . . . . . . . . . . . . . 108
11. Message Flow Example . . . . . . . . . . . . . . . . . . . . 109
12. Security Considerations . . . . . . . . . . . . . . . . . . . 115
Jennings, et al. Expires September 8, 2009 [Page 5]
Internet-Draft RELOAD Base March 2009
12.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 115
12.2. Attacks on P2P Overlays . . . . . . . . . . . . . . . . 116
12.3. Certificate-based Security . . . . . . . . . . . . . . . 116
12.4. Shared-Secret Security . . . . . . . . . . . . . . . . . 117
12.5. Storage Security . . . . . . . . . . . . . . . . . . . . 117
12.5.1. Authorization . . . . . . . . . . . . . . . . . . . 118
12.5.2. Distributed Quota . . . . . . . . . . . . . . . . . 118
12.5.3. Correctness . . . . . . . . . . . . . . . . . . . . 119
12.5.4. Residual Attacks . . . . . . . . . . . . . . . . . . 119
12.6. Routing Security . . . . . . . . . . . . . . . . . . . . 120
12.6.1. Background . . . . . . . . . . . . . . . . . . . . . 120
12.6.2. Admissions Control . . . . . . . . . . . . . . . . . 120
12.6.3. Peer Identification and Authentication . . . . . . . 121
12.6.4. Protecting the Signaling . . . . . . . . . . . . . . 121
12.6.5. Residual Attacks . . . . . . . . . . . . . . . . . . 122
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 122
13.1. Port Registrations . . . . . . . . . . . . . . . . . . . 122
13.2. Overlay Algorithm Types . . . . . . . . . . . . . . . . 123
13.3. Access Control Policies . . . . . . . . . . . . . . . . 123
13.4. Data Kind-ID . . . . . . . . . . . . . . . . . . . . . . 123
13.5. Data Model . . . . . . . . . . . . . . . . . . . . . . . 124
13.6. Message Codes . . . . . . . . . . . . . . . . . . . . . 124
13.7. Error Codes . . . . . . . . . . . . . . . . . . . . . . 125
13.8. Route Log Extension Types . . . . . . . . . . . . . . . 126
13.9. Overlay Link Types . . . . . . . . . . . . . . . . . . . 126
13.10. Forwarding Options . . . . . . . . . . . . . . . . . . . 127
13.11. Probe Information Types . . . . . . . . . . . . . . . . 127
13.12. reload: URI Scheme . . . . . . . . . . . . . . . . . . . 127
13.12.1. URI Registration . . . . . . . . . . . . . . . . . . 128
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 128
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 129
15.1. Normative References . . . . . . . . . . . . . . . . . . 129
15.2. Informative References . . . . . . . . . . . . . . . . . 130
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 133
A.1. Changes since draft-ietf-p2psip-reload-01 . . . . . . . 133
A.2. Changes since draft-ietf-p2psip-reload-00 . . . . . . . 133
A.3. Changes since draft-ietf-p2psip-base-00 . . . . . . . . 133
A.4. Changes since draft-ietf-p2psip-base-01 . . . . . . . . 133
A.5. Changes since draft-ietf-p2psip-base-01a . . . . . . . . 133
Appendix B. Routing Alternatives . . . . . . . . . . . . . . . . 134
B.1. Iterative vs Recursive . . . . . . . . . . . . . . . . . 134
B.2. Symmetric vs Forward response . . . . . . . . . . . . . 134
B.3. Direct Response . . . . . . . . . . . . . . . . . . . . 135
B.4. Relay Peers . . . . . . . . . . . . . . . . . . . . . . 136
B.5. Symmetric Route Stability . . . . . . . . . . . . . . . 136
Appendix C. Why Clients? . . . . . . . . . . . . . . . . . . . . 137
C.1. Why Not Only Peers? . . . . . . . . . . . . . . . . . . 137
C.2. Clients as Application-Level Agents . . . . . . . . . . 138
Jennings, et al. Expires September 8, 2009 [Page 6]
Internet-Draft RELOAD Base March 2009
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 138
Jennings, et al. Expires September 8, 2009 [Page 7]
Internet-Draft RELOAD Base March 2009
1. Introduction
This document defines REsource LOcation And Discovery (RELOAD), a
peer-to-peer (P2P) signaling protocol for use on the Internet. It
provides a generic, self-organizing overlay network service, allowing
nodes to efficiently route messages to other nodes and to efficiently
store and retrieve data in the overlay. RELOAD provides several
features that are critical for a successful P2P protocol for the
Internet:
Security Framework: A P2P network will often be established among a
set of peers that do not trust each other. RELOAD leverages a
central enrollment server to provide credentials for each peer
which can then be used to authenticate each operation. This
greatly reduces the possible attack surface.
Usage Model: RELOAD is designed to support a variety of
applications, including P2P multimedia communications with the
Session Initiation Protocol [I-D.ietf-p2psip-sip]. RELOAD allows
the definition of new application usages, each of which can define
its own data types, along with the rules for their use. This
allows RELOAD to be used with new applications through a simple
documentation process that supplies the details for each
application.
NAT Traversal: RELOAD is designed to function in environments where
many if not most of the nodes are behind NATs or firewalls.
Operations for NAT traversal are part of the base design,
including using ICE to establish new RELOAD or application
protocol connections.
High Performance Routing: The very nature of overlay algorithms
introduces a requirement that peers participating in the P2P
network route requests on behalf of other peers in the network.
This introduces a load on those other peers, in the form of
bandwidth and processing power. RELOAD has been defined with a
simple, lightweight forwarding header, thus minimizing the amount
of effort required by intermediate peers.
Pluggable Overlay Algorithms: RELOAD has been designed with an
abstract interface to the overlay layer to simplify implementing a
variety of structured (DHT) and unstructured overlay algorithms.
This specification also defines how RELOAD is used with Chord,
which is mandatory to implement. Specifying a default "must
implement" overlay algorithm will allow interoperability, while
the extensibility allows selection of overlay algorithms optimized
for a particular application.
Jennings, et al. Expires September 8, 2009 [Page 8]
Internet-Draft RELOAD Base March 2009
These properties were designed specifically to meet the requirements
for a P2P protocol to support SIP. This document defines the base
protocol for the distributed storage and location service, as well as
critical usages for NAT traversal and security. The SIP Usage itself
is described separately in [I-D.ietf-p2psip-sip]. RELOAD is not
limited to usage by SIP and could serve as a tool for supporting
other P2P applications with similar needs. RELOAD is also based on
the concepts introduced in [I-D.ietf-p2psip-concepts].
1.1. Basic Setting
In this section, we provide a brief overview of the operational
setting for RELOAD. See the concepts document for more details. A
RELOAD Overlay Instance consists of a set of nodes arranged in a
partly connected graph. Each node in the overlay is assigned a
numeric Node-ID which, together with the specific overlay algorithm
in use, determines its position in the graph and the set of nodes it
connects to. The figure below shows a trivial example which isn't
drawn from any particular overlay algorithm, but was chosen for
convenience of representation.
+--------+ +--------+ +--------+
| Node 10|--------------| Node 20|--------------| Node 30|
+--------+ +--------+ +--------+
| | |
| | |
+--------+ +--------+ +--------+
| Node 40|--------------| Node 50|--------------| Node 60|
+--------+ +--------+ +--------+
| | |
| | |
+--------+ +--------+ +--------+
| Node 70|--------------| Node 80|--------------| Node 90|
+--------+ +--------+ +--------+
|
|
+--------+
| Node 85|
|(Client)|
+--------+
Because the graph is not fully connected, when a node wants to send a
message to another node, it may need to route it through the network.
For instance, Node 10 can talk directly to nodes 20 and 40, but not
to Node 70. In order to send a message to Node 70, it would first
send it to Node 40 with instructions to pass it along to Node 70.
Different overlay algorithms will have different connectivity graphs,
but the general idea behind all of them is to allow any node in the
Jennings, et al. Expires September 8, 2009 [Page 9]
Internet-Draft RELOAD Base March 2009
graph to efficiently reach every other node within a small number of
hops.
The RELOAD network is not only a messaging network. It is also a
storage network. Records are stored under numeric addresses which
occupy the same space as node identifiers. Nodes are responsible for
storing the data associated with some set of addresses as determined
by their Node-ID. For instance, we might say that every node is
responsible for storing any data value which has an address less than
or equal to its own Node-ID, but greater than the next lowest
Node-ID. Thus, Node-20 would be responsible for storing values
11-20.
RELOAD also supports clients. These are nodes which have Node-IDs
but do not participate in routing or storage. For instance, in the
figure above Node 85 is a client. It can route to the rest of the
RELOAD network via Node 80, but no other node will route through it
and Node 90 is still responsible for all addresses between 81-90. We
refer to non-client nodes as peers.
Other applications (for instance, SIP) can be defined on top of
RELOAD and use these two basic RELOAD services to provide their own
services.
1.2. Architecture
RELOAD is fundamentally an overlay network. Therefore, it can be
divided into components that mimic the layering of the Internet
model[RFC1122].
Jennings, et al. Expires September 8, 2009 [Page 10]
Internet-Draft RELOAD Base March 2009
Application
+-------+ +-------+
| SIP | | XMPP | ...
| Usage | | Usage |
+-------+ +-------+
-------------------------------------- Messaging API
+------------------+ +---------+
| Message |<--->| Storage |
| Transport | +---------+
+------------------+ ^
^ ^ |
| v v
| +-------------------+
| | Topology |
| | Plugin |
| +-------------------+
| ^
v v
+------------------+
| Forwarding & |
| Link Management |
+------------------+
-------------------------------------- Overlay Link API
+-------+ +------+
|TLS | |DTLS | ...
+-------+ +------+
The major components of RELOAD are:
Usage Layer: Each application defines a RELOAD usage; a set of data
kinds and behaviors which describe how to use the services
provided by RELOAD. These usages all talk to RELOAD through a
common Message Transport API.
Message Transport: Handles the end-to-end reliability, manages
request state for the usages, and forwards Store and Fetch
operations to the Storage component. Delivers message responses
to the component initiating the request.
Storage: The Storage component is responsible for processing
messages relating to the storage and retrieval of data. It talks
directly to the Topology Plugin to manage data replication and
migration, and it talks to the Message Transport to send and
receive messages.
Jennings, et al. Expires September 8, 2009 [Page 11]
Internet-Draft RELOAD Base March 2009
Topology Plugin: The Topology Plugin is responsible for implementing
the specific overlay algorithm being used. It uses the Message
Transport component to send and receive overlay management
messages, to the Storage component to manage data replication, and
directly to the Forwarding Layer to control hop-by-hop message
forwarding. This component closely parallels conventional routing
algorithms, but is more tightly coupled to the Forwarding Layer
because there is no single "routing table" equivalent used by all
overlay algorithms.
Forwarding and Link Management Layer: Stores and implements the
routing table by providing packet forwarding services between
nodes. It also handles establishing new links between nodes,
including setting up connections across NATs using ICE.
Overlay Link Layer: TLS [RFC5246] and DTLS [RFC4347] are the "link
layer" protocols used by RELOAD for hop-by-hop communication.
Each such protocol includes the appropriate provisions for per-hop
framing or hop-by-hop ACKs required by unreliable transports.
To further clarify the roles of the various layer, this figure
parallels the architecture with each layer's role from an overlay
perspective and implementation layer in the internet:
Jennings, et al. Expires September 8, 2009 [Page 12]
Internet-Draft RELOAD Base March 2009
| Internet Model |
Real | Equivalent | Reload
Internet | in Overlay | Architecture
--------------+-----------------+------------------------------------
| | +-------+ +-------+
| Application | | SIP | | XMPP | ...
| | | Usage | | Usage |
| | +-------+ +-------+
| | ----------------------------------
| |+------------------+ +---------+
| Transport || Message |<--->| Storage |
| || Transport | +---------+
| |+------------------+ ^
| | ^ ^ |
| | | v v
Application | | | +-------------------+
| (Routing) | | | Topology |
| | | | Plugin |
| | | +-------------------+
| | v ^
| | v
| Network | +------------------+
| | | Forwarding & |
| | | Link Management |
| | +------------------+
| | ----------------------------------
Transport | Link | +-------+ +------+
| | |TLS | |DTLS | ...
| | +-------+ +------+
--------------+-----------------+------------------------------------
Network |
|
Link |
1.2.1. Usage Layer
The top layer, called the Usage Layer, has application usages, such
as the SIP Location Usage, that use the abstract Message Transport
API provided by RELOAD. The goal of this layer is to implement
application-specific usages of the generic overlay services provided
by RELOAD. The usage defines how a specific application maps its
data into something that can be stored in the overlay, where to store
the data, how to secure the data, and finally how applications can
retrieve and use the data.
The architecture diagram shows both a SIP usage and an XMPP usage. A
single application may require multiple usages, for example a SIP
application may also require a voicemail usage. A usage may define
Jennings, et al. Expires September 8, 2009 [Page 13]
Internet-Draft RELOAD Base March 2009
multiple kinds of data that are stored in the overlay and may also
rely on kinds originally defined by other usages.
Because the security and storage policies for each kind are dictated
by the usage defining the kind, the usages may be coupled with the
Storage component to provide security policy enforcement and to
implement appropriate storage strategies according to the needs of
the usage. The exact implementation of such an interface is outside
the scope of this draft.
1.2.2. Message Transport
The Message Transport provides a generic message routing service for
the overlay. The Message Transport layer is responsible for end-to-
end message transactions, including retransmissions. Each peer is
identified by its location in the overlay as determined by its
Node-ID. A component that is a client of the Message Transport can
perform two basic functions:
o Send a message to a given peer specified by Node-ID or to the peer
responsible for a particular Resource-ID.
o Receive messages that other peers sent to a Node-ID or Resource-ID
for which this peer is responsible.
All usages rely on the Message Transport component to send and
receive messages from peers. For instance, when a usage wants to
store data, it does so by sending Store requests. Note that the
Storage component and the Topology Plugin are themselves clients of
the Message Transport, because they need to send and receive messages
from other peers.
The Message Transport API is similar to those described as providing
"Key based routing" (KBR), although as RELOAD supports different
overlay algorithms (including non-DHT overlay algorithms) that
calculate keys in different ways, the actual interface must accept
Resource Names rather than actual keys.
1.2.3. Storage
One of the major functions of RELOAD is to allow nodes to store data
in the overlay and to retrieve data stored by other nodes or by
themselves. The Storage component is responsible for processing data
storage and retrieval messages. For instance, the Storage component
might receive a Store request for a given resource from the Message
Transport. It would then query the appropriate usage before storing
the data value(s) in its local data store and sends a response to the
Message Transport for delivery to the requesting peer. Typically,
these messages will come for other nodes, but depending on the
Jennings, et al. Expires September 8, 2009 [Page 14]
Internet-Draft RELOAD Base March 2009
overlay topology, a node might be responsible for storing data for
itself as well, especially if the overlay is small.
A peer's Node-ID determines the set of resources that it will be
responsible for storing. However, the exact mapping between these is
determined by the overlay algorithm used by the overlay. The Storage
component will only receive a Store request from the Message
Transport if this peer is responsible for that Resource-ID. The
Storage component is notified by the Topology Plugin when the
Resource-IDs for which it is responsible change, and the Storage
component is then responsible for migrating resources to other peers,
as required.
1.2.4. Topology Plugin
RELOAD is explicitly designed to work with a variety of overlay
algorithms. In order to facilitate this, the overlay algorithm
implementation is provided by a Topology Plugin so that each overlay
can select an appropriate overlay algorithm that relies on the common
RELOAD core protocols and code.
The Topology Plugin is responsible for maintaining the overlay
algorithm Routing Table, which is consulted by the Forwarding and
Link Management Layer before routing a message. When connections are
made or broken, the Forwarding and Link Management Layer notifies the
Topology Plugin, which adjusts the routing table as appropriate. The
Topology Plugin will also instruct the Forwarding and Link Management
Layer to form new connections as dictated by the requirements of the
overlay algorithm Topology. The Topology Plugin issues periodic
update requests through Message Transport to maintain and update its
Routing Table.
As peers enter and leave, resources may be stored on different peers,
so the Topology Plugin also keeps track of which peers are
responsible for which resources. As peers join and leave, the
Topology Plugin instructs the Storage component to issue resource
migration requests as appropriate, in order to ensure that other
peers have whatever resources they are now responsible for. The
Topology Plugin is also responsible for providing redundant data
storage to protect against loss of information in the event of a peer
failure and to protect against compromised or subversive peers.
1.2.5. Forwarding and Link Management Layer
The Forwarding and Link Management Layer is responsible for getting a
packet to the next peer, as determined by the Topology Plugin. This
Layer establishes and maintains the network connections as required
by the Topology Plugin. This layer is also responsible for setting
Jennings, et al. Expires September 8, 2009 [Page 15]
Internet-Draft RELOAD Base March 2009
up connections to other peers through NATs and firewalls using ICE,
and it can elect to forward traffic using relays for NAT and firewall
traversal.
This layer provides a fairly generic interface that allows the
topology plugin control the overlay and resource operations and
messages. Since each overlay algorithm is defined and functions
differently, we generically refer to the table of other peers that
the overlay algorithm maintains and uses to route requests
(neighbors) as a Routing Table. The Topology Plugin actually owns
the Routing Table, and forwarding decisions are made by querying the
Topology Plugin for the next hop for a particular Node-ID or
Resource-ID. If this node is the destination of the message, the
message is delivered to the Message Transport.
The Forwarding and Link Management Layer sits on top of the Overlay
Link Layer protocols that carry the actual traffic. This
specification defines how to use DTLS and TLS protocols to carry
RELOAD messages.
1.3. Security
RELOAD's security model is based on each node having one or more
public key certificates. In general, these certificates will be
assigned by a central server which also assigns Node-IDs, although
self-signed certificates can be used in closed networks. These
credentials can be leveraged to provide communications security for
RELOAD messages. RELOAD provides communications security at three
levels:
Connection Level: Connections between peers are secured with TLS
or DTLS.
Message Level: Each RELOAD message must be signed.
Object Level: Stored objects must be signed by the storing peer.
These three levels of security work together to allow peers to verify
the origin and correctness of data they receive from other peers,
even in the face of malicious activity by other peers in the overlay.
RELOAD also provides access control built on top of these
communications security features. Because the peer responsible for
storing a piece of data can validate the signature on the data being
stored, the responsible peer can determine whether a given operation
is permitted or not.
RELOAD also provides a shared secret based admission control feature
using shared secrets and TLS-PSK. In order to form a TLS connection
to any node in the overlay, a new node needs to know the shared
overlay key, thus restricting access to authorized users.
Jennings, et al. Expires September 8, 2009 [Page 16]
Internet-Draft RELOAD Base March 2009
1.4. Structure of This Document
The remainder of this document is structured as follows.
o Section 2 provides definitions of terms used in this document.
o Section 3 provides an overview of the mechanisms used to establish
and maintain the overlay.
o Section 4 provides an overview of the mechanism RELOAD provides to
support other applications.
o Section 5 defines the protocol messages that RELOAD uses to
establish and maintain the overlay.
o Section 6 defines the protocol messages that are used to store and
retrieve data using RELOAD.
o Section 7 defines the Certificate Store Usage that is fundamental
to RELOAD security.
o Section 8 defines the TURN Server Usage needed to locate TURN
servers for NAT traversal.
o Section 9 defines a specific Topology Plugin using Chord.
o Section 10 defines the mechanisms that new RELOAD nodes use to
join the overlay for the first time.
o Section 11 provides an extended example.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
We use the terminology and definitions from the Concepts and
Terminology for Peer to Peer SIP [I-D.ietf-p2psip-concepts] draft
extensively in this document. Other terms used in this document are
defined inline when used and are also defined below for reference.
Terms which are new to this document (and perhaps should be added to
the concepts document) are marked with a (*).
DHT: A distributed hash table. A DHT is an abstract hash table
service realized by storing the contents of the hash table across
a set of peers.
Overlay Algorithm: An overlay algorithm defines the rules for
determining which peers in an overlay store a particular piece of
data and for determining a topology of interconnections amongst
peers in order to find a piece of data.
Jennings, et al. Expires September 8, 2009 [Page 17]
Internet-Draft RELOAD Base March 2009
Overlay Instance: A specific overlay algorithm and the collection of
peers that are collaborating to provide read and write access to
it. There can be any number of overlay instances running in an IP
network at a time, and each operates in isolation of the others.
Peer: A host that is participating in the overlay. Peers are
responsible for holding some portion of the data that has been
stored in the overlay and also route messages on behalf of other
hosts as required by the Overlay Algorithm.
Client: A host that is able to store data in and retrieve data from
the overlay but which is not participating in routing or data
storage for the overlay.
Node: We use the term "Node" to refer to a host that may be either a
Peer or a Client. Because RELOAD uses the same protocol for both
clients and peers, much of the text applies equally to both.
Therefore we use "Node" when the text applies to both Clients and
Peers and the more specific term when the text applies only to
Clients or only to Peers.
Node-ID: A 128-bit value that uniquely identifies a node. Node-IDs
0 and 2^128 - 1 are reserved and are invalid Node-IDs. A value of
zero is not used in the wire protocol but can be used to indicate
an invalid node in implementations and APIs. The Node-ID of
2^128-1 is used on the wire protocol as a wildcard. (*)
Resource: An object or group of objects associated with a string
identifier see "Resource Name" below.
Resource Name: The potentially human readable name by which a
resource is identified. In unstructured P2P networks, the
resource name is sometimes used directly as a Resource-ID. In
structured P2P networks the resource name is typically mapped into
a Resource-ID by using the string as the input to hash function.
A SIP resource, for example, is often identified by its AOR which
is an example of a Resource Name.(*)
Resource-ID: A value that identifies some resources and which is
used as a key for storing and retrieving the resource. Often this
is not human friendly/readable. One way to generate a Resource-ID
is by applying a mapping function to some other unique name (e.g.,
user name or service name) for the resource. The Resource-ID is
used by the distributed database algorithm to determine the peer
or peers that are responsible for storing the data for the
overlay. In structured P2P networks, Resource-IDs are generally
fixed length and are formed by hashing the resource name. In
Jennings, et al. Expires September 8, 2009 [Page 18]
Internet-Draft RELOAD Base March 2009
unstructured networks, resource names may be used directly as
Resource-IDs and may have variable length.
Connection Table: The set of peers to which a node is directly
connected. This includes nodes with which Attach handshakes have
been done but which have not sent any Updates.
Routing Table: The set of peers which a node can use to route
overlay messages. In general, these peers will all be on the
connection table but not vice versa, because some peers will have
Attached but not sent updates. Peers may send messages directly
to peers which are on the connection table but may only route
messages to other peers through peers which are on the routing
table. (*)
Destination List: A list of IDs through which a message is to be
routed. A single Node-ID is a trivial form of destination list.
(*)
Usage: A usage is an application that wishes to use the overlay for
some purpose. Each application wishing to use the overlay defines
a set of data kinds that it wishes to use. The SIP usage defines
the location data kind. (*)
3. Overlay Management Overview
The most basic function of RELOAD is as a generic overlay network.
Nodes need to be able to join the overlay, form connections to other
nodes, and route messages through the overlay to nodes to which they
are not directly connected. This section provides an overview of the
mechanisms that perform these functions.
3.1. Security and Identification
Every node in the RELOAD overlay is identified by a Node-ID. The
Node-ID is used for three major purposes:
o To address the node itself.
o To determine its position in the overlay topology when the overlay
is structured.
o To determine the set of resources for which the node is
responsible.
Each node has a certificate [RFC3280] containing a Node-ID, which is
globally unique.
The certificate serves multiple purposes:
Jennings, et al. Expires September 8, 2009 [Page 19]
Internet-Draft RELOAD Base March 2009
o It entitles the user to store data at specific locations in the
Overlay Instance. Each data kind defines the specific rules for
determining which certificates can access each Resource-ID/Kind-ID
pair. For instance, some kinds might allow anyone to write at a
given location, whereas others might restrict writes to the owner
of a single certificate.
o It entitles the user to operate a node that has a Node-ID found in
the certificate. When the node forms a connection to another
peer, it can use this certificate so that a node connecting to it
knows it is connected to the correct node. In addition, the node
can sign messages, thus providing integrity and authentication for
messages which are sent from the node.
o It entitles the user to use the user name found in the
certificate.
If a user has more than one device, typically they would get one
certificate for each device. This allows each device to act as a
separate peer.
RELOAD supports two certificate issuance models. The first is based
on a central enrollment process which allocates a unique name and
Node-ID to the node a certificate for a public/private key pair for
the user. All peers in a particular Overlay Instance have the
enrollment server as a trust anchor and so can verify any other
peer's certificate.
In some settings, a group of users want to set up an overlay network
but are not concerned about attack by other users in the network.
For instance, users on a LAN might want to set up a short term ad hoc
network without going to the trouble of setting up an enrollment
server. RELOAD supports the use of self-generated and self-signed
certificates. When self-signed certificates are used, the node also
generates its own Node-ID and username. The Node-ID is computed as a
digest of the public key, to prevent Node-ID theft, however this
model is still subject to a number of known attacks (most notably
Sybil attacks [Sybil]) and can only be safely used in closed networks
where users are mutually trusting.
The general principle here is that the security mechanisms (TLS and
message signatures) are always used, even if the certificates are
self-signed. This allows for a single set of code paths in the
systems with the only difference being whether certificate
verification is required to chain to a single root of trust.
3.1.1. Shared-Key Security
RELOAD also provides an admission control system based on shared
keys. In this model, the peers all share a single key which is used
Jennings, et al. Expires September 8, 2009 [Page 20]
Internet-Draft RELOAD Base March 2009
to authenticate the peer-to-peer connections via TLS-PSK/TLS-SRP.
3.2. Clients
RELOAD defines a single protocol that is used both as the peer
protocol and the client protocol for the overlay. This simplifies
implementation, particularly for devices that may act in either role,
and allows clients to inject messages directly into the overlay.
We use the term "peer" to identify a node in the overlay that routes
messages for nodes other than those to which it is directly
connected. Peers typically also have storage responsibilities. We
use the term "client" to refer to nodes that do not have routing or
storage responsibilities. When text applies to both peers and
clients, we will simply refer to such a device as a "node."
RELOAD's client support allows nodes that are not participating in
the overlay as peers to utilize the same implementation and to
benefit from the same security mechanisms as the peers. Clients
possess and use certificates that authorize the user to store data at
its locations in the overlay. The Node-ID in the certificate is used
to identify the particular client as a member of the overlay and to
authenticate its messages.
For more discussion of the motivation for RELOAD's client support,
see Appendix C.
3.2.1. Client Routing
There are two routing options by which a client may be located in an
overlay.
o Establish a connection to the peer responsible for the client's
Node-ID in the overlay. Then requests may be sent from/to the
client using its Node-ID in the same manner as if it were a peer,
because the responsible peer in the overlay will handle the final
step of routing to the client. This will not work in overlays
where NAT or firewall do not allow all clients to form connections
with any other peer.
o Establish a connection with an arbitrary peer in the overlay
(perhaps based on network proximity or an inability to establish a
direct connection with the responsible peer). In this case, the
client will rely on RELOAD's Destination List feature to ensure
reachability. The client can initiate requests, and any node in
the overlay that knows the Destination List to its current
location can reach it, but the client is not directly reachable
directly using only its Node-ID. The Destination List required to
reach it must be learnable via other mechanisms, such as being
Jennings, et al. Expires September 8, 2009 [Page 21]
Internet-Draft RELOAD Base March 2009
stored in the overlay by a usage, if the client is to receive
incoming requests from other members of the overlay.
3.2.2. Minimum Functionality Requirements for Clients
A node may act as a client simply because it does not have the
resources or even an implementation of the topology plugin required
to acts as a peer in the overlay. In order to exchange RELOAD
messages with a peer, a client must meet a minimum level of
functionality. Such a client must:
o Implement RELOAD's connection-management connections that are used
to establish the connection with the peer.
o Implement RELOAD's data retrieval methods (with client
functionality).
o Be able to calculate Resource-IDs used by the overlay.
o Possess security credentials required by the overlay it is
implementing.
A client speaks the same protocol as the peers, knows how to
calculate Resource-IDs, and signs its requests in the same manner as
peers. While a client does not necessarily require a full
implementation of the overlay algorithm, calculating the Resource-ID
requires an implementation of the appropriate algorithm for the
overlay.
RELOAD does not support a separate protocol for clients that do not
meet these functionality requirements. Any such extension would
either entail compromises on the features of RELOAD or require an
entirely new protocol to reimplement the core features of RELOAD.
Furthermore, for SIP and many other applications, a native
application-level protocol already exists that is sufficient for such
a client to interact with a member of the RELOAD overlay.
3.3. Routing
This section will discuss the requirements RELOAD's routing
capabilities must meet, then describe the routing features in the
protocol, and provide a brief overview of how they are used.
Appendix B discusses some alternative designs and the tradeoffs that
would be necessary to support them.
RELOAD's routing capabilities must meet the following requirements:
Jennings, et al. Expires September 8, 2009 [Page 22]
Internet-Draft RELOAD Base March 2009
NAT Traversal: RELOAD must support establishing and using
connections between nodes separated by one or more NATs, including
locating peers behind NATs for those overlays allowing/requiring
it.
Clients: RELOAD must support requests from and to clients that do
not participate in overlay routing.
Client promotion: RELOAD must support clients that become peers at a
later point as determined by the overlay algorithm and deployment.
Low state: RELOAD's routing algorithms must not require
significant state to be stored on intermediate peers.
Return routability in unstable topologies: At some points in
times, different nodes may have inconsistent information about the
connectivity of the routing graph. In all cases, the response to
a request needs to delivered to the node that sent the request and
not to some other node.
To meet these requirements, RELOAD's routing relies on two basic
mechanisms:
Via Lists: The forwarding header used by all RELOAD messages
contains both a Via List (built hop-by-hop as the message is
routed through the overlay) and a Destination List (providing
source-routing capabilities for requests and return-path routing
for responses).
Route_Query: The Route_Query method allows a node to query a peer
for the next hop it will use to route a message. This method is
useful for diagnostics and for iterative routing.
The basic routing mechanism used by RELOAD is Symmetric Recursive.
We will first describe symmetric routing and then discuss its
advantages in terms of the requirements discussed above.
Symmetric recursive routing requires a message follow the path
through the overlay to the destination without returning to the
originating node: each peer forwards the message closer to its
destination. The return path of the response is then the same path
followed in reverse. For example, a message following a route from A
to Z through B and X:
Jennings, et al. Expires September 8, 2009 [Page 23]
Internet-Draft RELOAD Base March 2009
A B X Z
-------------------------------
---------->
Dest=Z
---------->
Via=A
Dest=Z
---------->
Via=A, B
Dest=Z
<----------
Dest=X, B, A
<----------
Dest=B, A
<----------
Dest=A
Note that the preceding Figure does not indicate whether A is a
client or peer, A forwards its request to B and the response is
returned to A in the same manner regardless of A's role in the
overlay.
This figure shows use of full via-lists by intermediate peers B and
X. However, if B and/or X are willing to store state, then they may
elect to truncate the lists, save that information internally (keyed
by the transaction id), and return the response message along the
path from which it was received when the response is received. This
option requires greater state on intermediate peers but saves a small
amount of bandwidth and reduces the need for modifying the message in
route. Selection of this mode of operation is a choice for the
individual peer, the techniques are interoperable even on a single
message. The figure below shows B using full via lists but X
truncating them and saving the state internally.
Jennings, et al. Expires September 8, 2009 [Page 24]
Internet-Draft RELOAD Base March 2009
A B X Z
-------------------------------
---------->
Dest=Z
---------->
Via=A
Dest=Z
---------->
Dest=Z
<----------
Dest=X
<----------
Dest=B, A
<----------
Dest=A
For debugging purposes, a Route Log attribute is available that
stores information about each peer as the message is forwarded.
RELOAD also supports a basic Iterative routing mode (where the
intermediate peers merely return a response indicating the next hop,
but do not actually forward the message to that next hop themselves).
Iterative routing is implemented using the Route_Query method, which
requests this behavior. Note that iterative routing is selected only
by the initiating node. RELOAD does not support an intermediate peer
returning a response that it will not recursively route a normal
request. The willingness to perform that operation is implicit in
its role as a peer in the overlay.
3.4. Connectivity Management
In order to provide efficient routing, a peer needs to maintain a set
of direct connections to other peers in the Overlay Instance. Due to
the presence of NATs, these connections often cannot be formed
directly. Instead, we use the Attach request to establish a
connection. Attach uses ICE [I-D.ietf-mmusic-ice-tcp] to establish
the connection. It is assumed that the reader is familiar with ICE.
Say that peer A wishes to form a direct connection to peer B. It
gathers ICE candidates and packages them up in an Attach request
which it sends to B through usual overlay routing procedures. B does
its own candidate gathering and sends back a response with its
candidates. A and B then do ICE connectivity checks on the candidate
pairs. The result is a connection between A and B. At this point, A
and B can add each other to their routing tables and send messages
directly between themselves without going through other overlay
Jennings, et al. Expires September 8, 2009 [Page 25]
Internet-Draft RELOAD Base March 2009
peers.
There is one special case in which Attach cannot be used: when a
peer is joining the overlay and is not connected to any peers. In
order to support this case, some small number of "bootstrap nodes"
need to be publicly accessible so that new peers can directly connect
to them. Section 10 contains more detail on this.
In general, a peer needs to maintain connections to all of the peers
near it in the Overlay Instance and to enough other peers to have
efficient routing (the details depend on the specific overlay). If a
peer cannot form a connection to some other peer, this isn't
necessarily a disaster; overlays can route correctly even without
fully connected links. However, a peer should try to maintain the
specified link set and if it detects that it has fewer direct
connections, should form more as required. This also implies that
peers need to periodically verify that the connected peers are still
alive and if not try to reform the connection or form an alternate
one.
3.5. Overlay Algorithm Support
The Topology Plugin allows RELOAD to support a variety of overlay
algorithms. This draft defines a DHT based on Chord [Chord], which
is mandatory to implement, but the base RELOAD protocol is designed
to support a variety of overlay algorithms.
3.5.1. Support for Pluggable Overlay Algorithms
RELOAD defines three methods for overlay maintenance: Join, Update,
and Leave. However, the contents of those messages, when they are
sent, and their precise semantics are specified by the actual overlay
algorithm; RELOAD merely provides a framework of commonly-needed
methods that provides uniformity of notation (and ease of debugging)
for a variety of overlay algorithms.
3.5.2. Joining, Leaving, and Maintenance Overview
When a new peer wishes to join the Overlay Instance, it must have a
Node-ID that it is allowed to use. It uses the Node-ID in the
certificate it received from the enrollment server. The details of
the joining procedure are defined by the overlay algorithm, but the
general steps for joining an Overlay Instance are:
o Forming connections to some other peers.
o Acquiring the data values this peer is responsible for storing.
Jennings, et al. Expires September 8, 2009 [Page 26]
Internet-Draft RELOAD Base March 2009
o Informing the other peers which were previously responsible for
that data that this peer has taken over responsibility.
The first thing the peer needs to do is form a connection to some
"bootstrap node". Because this is the first connection the peer
makes, these nodes must have public IP addresses and therefore can be
connected to directly. Once a peer has connected to one or more
bootstrap nodes, it can form connections in the usual way by routing
Attach messages through the overlay to other nodes. Once a peer has
connected to the overlay for the first time, it can cache the set of
nodes it has connected to with public IP addresses for use as future
bootstrap nodes.
Once the peer has connected to a bootstrap node, it then needs to
take up its appropriate place in the overlay. This requires two
major operations:
o Forming connections to other peers in the overlay to populate its
Routing Table.
o Getting a copy of the data it is now responsible for storing and
assuming responsibility for that data.
The second operation is performed by contacting the Admitting Peer
(AP), the node which is currently responsible for that section of the
overlay.
The details of this operation depend mostly on the overlay algorithm
involved, but a typical case would be:
1. JP (Joining Peer) sends a Join request to AP (Admitting Peer)
announcing its intention to join.
2. AP sends a Join response.
3. AP does a sequence of Stores to JP to give it the data it will
need.
4. AP does Updates to JP and to other peers to tell it about its own
routing table. At this point, both JP and AP consider JP
responsible for some section of the Overlay Instance.
5. JP makes its own connections to the appropriate peers in the
Overlay Instance.
After this process is completed, JP is a full member of the Overlay
Instance and can process Store/Fetch requests.
Note that the first node is a special case. When ordinary nodes
cannot form connections to the bootstrap nodes, then they are not
part of the overlay. However, the first node in the overlay can
obviously not connect to others nodes. In order to support this
case, potential first nodes (which must also serve as bootstrap nodes
Jennings, et al. Expires September 8, 2009 [Page 27]
Internet-Draft RELOAD Base March 2009
initially) must somehow be instructed (perhaps by configuration
settings) that they are the entire overlay, rather than not part of
it.
3.6. First-Time Setup
Previous sections addressed how RELOAD works once a node has
connected. This section provides an overview of how users get
connected to the overlay for the first time. RELOAD is designed so
that users can start with the name of the overlay they wish to join
and perhaps a username and password, and leverage that into having a
working peer with minimal user intervention. This helps avoid the
problems that have been experienced with conventional SIP clients
where users are required to manually configure a large number of
settings.
3.6.1. Initial Configuration
In the first phase of the process, the user starts out with the name
of the overlay and uses this to download an initial set of overlay
configuration parameters. The user does a DNS SRV lookup on the
overlay name to get the address of a configuration server. It can
then connect to this server with HTTPS to download a configuration
document which contains the basic overlay configuration parameters as
well as a set of bootstrap nodes which can be used to join the
overlay.
3.6.2. Enrollment
If the overlay is using centralized enrollment, then a user needs to
acquire a certificate before joining the overlay. The certificate
attests both to the user's name within the overlay and to the Node-
IDs which they are permitted to operate. In that case, the
configuration document will contain the address of an enrollment
server which can be used to obtain such a certificate. The
enrollment server may (and probably will) require some sort of
username and password before issuing the certificate. The enrollment
server's ability to restrict attackers' access to certificates in the
overlay is one of the cornerstones of RELOAD's security.
4. Application Support Overview
RELOAD is not intended to be used alone, but rather as a substrate
for other applications. These applications can use RELOAD for a
variety of purposes:
Jennings, et al. Expires September 8, 2009 [Page 28]
Internet-Draft RELOAD Base March 2009
o To store data in the overlay and retrieve data stored by other
nodes.
o As a discovery mechanism for services such as TURN.
o To form direct connections which can be used to transmit
application-level messages.
This section provides an overview of these services.
4.1. Data Storage
RELOAD provides operations to Store and Fetch data. Each location in
the Overlay Instance is referenced by a Resource-ID. However, each
location may contain data elements corresponding to multiple kinds
(e.g., certificate, SIP registration). Similarly, there may be
multiple elements of a given kind, as shown below:
+--------------------------------+
| Resource-ID |
| |
| +------------+ +------------+ |
| | Kind 1 | | Kind 2 | |
| | | | | |
| | +--------+ | | +--------+ | |
| | | Value | | | | Value | | |
| | +--------+ | | +--------+ | |
| | | | | |
| | +--------+ | | +--------+ | |
| | | Value | | | | Value | | |
| | +--------+ | | +--------+ | |
| | | +------------+ |
| | +--------+ | |
| | | Value | | |
| | +--------+ | |
| +------------+ |
+--------------------------------+
Each kind is identified by a Kind-ID, which is a code point assigned
by IANA. As part of the kind definition, protocol designers may
define constraints, such as limits on size, on the values which may
be stored. For many kinds, the set may be restricted to a single
value; some sets may be allowed to contain multiple identical items
while others may only have unique items. Note that a kind may be
employed by multiple usages and new usages are encouraged to use
previously defined kinds where possible. We define the following
data models in this document, though other usages can define their
own structures:
Jennings, et al. Expires September 8, 2009 [Page 29]
Internet-Draft RELOAD Base March 2009
single value: There can be at most one item in the set and any value
overwrites the previous item.
array: Many values can be stored and addressed by a numeric index.
dictionary: The values stored are indexed by a key. Often this key
is one of the values from the certificate of the peer sending the
Store request.
In order to protect stored data from tampering, by other nodes, each
stored value is digitally signed by the node which created it. When
a value is retrieved, the digital signature can be verified to detect
tampering.
4.1.1. Storage Permissions
A major issue in peer-to-peer storage networks is minimizing the
burden of becoming a peer, and in particular minimizing the amount of
data which any peer is required to store for other nodes. RELOAD
addresses this issue by only allowing any given node to store data at
a small number of locations in the overlay, with those locations
being determined by the node's certificate. When a peer uses a Store
request to place data at a location authorized by its certificate, it
signs that data with the private key that corresponds to its
certificate. Then the peer responsible for storing the data is able
to verify that the peer issuing the request is authorized to make
that request. Each data kind defines the exact rules for determining
what certificate is appropriate.
The most natural rule is that a certificate authorizes a user to
store data keyed with their user name X. This rules is used for all
the kinds defined in this specification. Thus, only a user with a
certificate for "alice@example.org" could write to that location in
the overlay. However, other usages can define any rules they choose,
including publicly writable values.
The digital signature over the data serves two purposes. First, it
allows the peer responsible for storing the data to verify that this
Store is authorized. Second, it provides integrity for the data.
The signature is saved along with the data value (or values) so that
any reader can verify the integrity of the data. Of course, the
responsible peer can "lose" the value but it cannot undetectable
modify it.
The size requirements of the data being stored in the overlay are
variable. For instance, a SIP AoR and voicemail differ widely in the
storage size. RELOAD leaves it to the Usage and overlay
Jennings, et al. Expires September 8, 2009 [Page 30]
Internet-Draft RELOAD Base March 2009
configuration to address the size imbalance of various kinds.
4.1.2. Usages
By itself, the distributed storage layer just provides infrastructure
on which applications are built. In order to do anything useful, a
usage must be defined. Each Usage specifies several things:
o Registers Kind-ID code points for any kinds that the Usage
defines.
o Defines the data structure for each of the kinds.
o Defines access control rules for each kinds.
o Defines how the Resource Name is formed that is hashed to form the
Resource-ID where each kind is stored.
o Describes how values will be merged after a network partition.
Unless otherwise specified, the default merging rule is to act as
if all the values that need to be merged were stored and that the
order they were stored in corresponds to the stored time values
associated with (and carried in) their values. Because the stored
time values are those associated with the peer which did the
writing, clock skew is generally not an issue. If two nodes are
on different partitions, clocks, this can create merge conflicts.
However because RELOAD deliberately segregates storage so that
data from different users and peers is stored in different
locations, and a single peer will typically only be in a single
network partition, this case will generally not arise.
The kinds defined by a usage may also be applied to other usages.
However, a need for different parameters, such as different size
limits, would imply the need to create a new kind.
4.1.3. Replication
Replication in P2P overlays can be used to provide:
persistence: if the responsible peer crashes and/or if the storing
peer leaves the overlay
security: to guard against DoS attacks by the responsible peer or
routing attacks to that responsible peer
load balancing: to balance the load of queries for popular
resources.
A variety of schemes are used in P2P overlays to achieve some of
these goals. Common techniques include replicating on neighbors of
the responsible peer, randomly locating replicas around the overlay,
or replicating along the path to the responsible peer.
The core RELOAD specification does not specify a particular
Jennings, et al. Expires September 8, 2009 [Page 31]
Internet-Draft RELOAD Base March 2009
replication strategy. Instead, the first level of replication
strategies are determined by the overlay algorithm, which can base
the replication strategy on the its particular topology. For
example, Chord places replicas on successor peers, which will take
over responsibility should the responsible peer fail [Chord].
If additional replication is needed, for example if data persistence
is particularly important for a particular usage, then that usage may
specify additional replication, such as implementing random
replications by inserting a different well known constant into the
Resource Name used to store each replicated copy of the resource.
Such replication strategies can be added independent of the
underlying algorithm, and their usage can be determined based on the
needs of the particular usage.
4.2. Service Discovery
RELOAD does not currently define a generic service discovery
algorithm as part of the base protocol; although a TURN-specific
discovery mechanism is provided. A variety of service discovery
algorithm can be implemented as extensions to the base protocol, such
as ReDIR [opendht-sigcomm05].
4.3. Application Connectivity
There is no requirement that a RELOAD usage must use RELOAD's
primitives for establishing its own communication if it already
possesses its own means of establishing connections. For example,
one could design a RELOAD-based resource discovery protocol which
used HTTP to retrieve the actual data.
For more common situations, however, the overlay itself is used to
establish a connection rather than an external authority such as DNS,
RELOAD provides connectivity to applications using the same Attach
method as is used for the overlay maintenance. For example, if a
P2PSIP node wishes to establish a SIP dialog with another P2PSIP
node, it will use Attach to establish a direct connection with the
other node. This new connection is separate from the peer protocol
connection, it is a dedicated UDP or TCP flow used only for the SIP
dialog. Each usage specifies which types of connections can be
initiated using Attach.
5. Overlay Management Protocol
This section defines the basic protocols used to create, maintain,
and use the RELOAD overlay network. We start by defining the basic
concept of how message destinations are interpreted when routing
Jennings, et al. Expires September 8, 2009 [Page 32]
Internet-Draft RELOAD Base March 2009
messages. We then describe the symmetric recursive routing model,
which is RELOAD's default routing algorithm. We then define the
message structure and then finally define the messages used to join
and maintain the overlay.
5.1. Message Receipt and Forwarding
When a peer receives a message, it first examines the overlay,
version, and other header fields to determine whether the message is
one it can process. If any of these are incorrect (e.g., the message
is for an overlay in which the peer does not participate) it is an
error. The peer SHOULD generate an appropriate error but local
policy can override this and cause the messages is silently dropped.
Once the peer has determined that the message is correctly formatted,
it examines the first entry on the destination list. There are three
possible cases here:
o The first entry on the destination list is an id for which the
peer is responsible.
o The first entry on the destination list is a an id for which
another peer is responsible.
o The first entry on the destination list is a private id which is
being used for destination list compression.
These cases are handled as discussed below.
5.1.1. Responsible ID
If the first entry on the destination list is a ID for which the node
is responsible, there are several sub-cases.
o If the entry is a Resource-ID, then it MUST be the only entry on
the destination list. If there are other entries, the message
MUST be silently dropped. Otherwise, the message is destined for
this node and it passes it up to the upper layers.
o If the entry is a Node-ID which belongs to this node, then the
message is destined for this node. If this is the only entry on
the destination list, the message is destined for this node and is
passed up to the upper layers. Otherwise the entry is removed
from the destination list and the message is passed it to the
Message Transport. If the message is a response and there is
state for the transaction ID, the state is reinserted into the
destination list first.
o If the entry is a Node-ID which is not equal to this node, then
the node MUST drop the message silently unless the Node-ID
corresponds to a node which is directly connected to this node
(i.e., a client). In that case, it MUST forward the message to
the destination node as described in the next section.
Jennings, et al. Expires September 8, 2009 [Page 33]
Internet-Draft RELOAD Base March 2009
Note that this implies that in order to address a message to "the
peer that controls region X", a sender sends to Resource-ID X, not
Node-ID X.
5.1.2. Other ID
If neither of the other two cases applies, then the peer MUST forward
the message towards the first entry on the destination list. This
means that it MUST select one of the peers to which it is connected
and which is likely to be responsible for the first entry on the
destination list. If the first entry on the destination list is in
the peer's connection table, then it SHOULD forward the message to
that peer directly. Otherwise, it consult the routing table to
forward the message.
Any intermediate peer which forwards a RELOAD message MUST arrange
that if it receives a response to that message the response can be
routed back through the set of nodes through which the request
passed. This may be arranged in one of two ways:
o The peer MAY add an entry to the via list in the forwarding header
that will enable it to determine the correct node.
o The peer MAY keep per-transaction state which will allow it to
determine the correct node.
As an example of the first strategy, if node D receives a message
from node C with via list (A, B), then D would forward to the next
node (E) with via list (A, B, C). Now, if E wants to respond to the
message, it reverses the via list to produce the destination list,
resulting in (D, C, B, A). When D forwards the response to C, the
destination list will contain (C, B, A).
As an example of the second strategy, if node D receives a message
from node C with transaction ID X and via list (A, B), it could store
(X, C) in its state database and forward the message with the via
list unchanged. When D receives the response, it consults its state
database for transaction id X, determines that the request came from
C, and forwards the response to C.
Intermediate peer which modify the via list are not required to
simply add entries. The only requirement is that the peer be able to
reconstruct the correct destination list on the return route. RELOAD
provides explicit support for this functionality in the form of
private IDs, which can replace any number of via list entries. For
instance, in the above example, Node D might send E a via list
containing only the private ID (I). E would then use the destination
list (D, I) to send its return message. When D processes this
destination list, it would detect that I is a private ID, recover the
Jennings, et al. Expires September 8, 2009 [Page 34]
Internet-Draft RELOAD Base March 2009
via list (A, B, C), and reverse that to produce the correct
destination list (C, B, A) before sending it to C. This feature is
called List Compression. I MAY either be a compressed version of the
original via list or an index into a state database containing the
original via list.
Note that if an intermediate peer exits the overlay, then on the
return trip the message cannot be forwarded and will be dropped. The
ordinary timeout and retransmission mechanisms provide stability over
this type of failure.
5.1.3. Private ID
If the first entry on the destination list is a private id (e.g., a
compressed via list), the peer MUST that entry with the original via
list that it replaced indexes and then re-examine the destination
list to determine which case now applies.
5.2. Symmetric Recursive Routing
This Section defines RELOAD's symmetric recursive routing algorithm,
which is the default algorithm used by nodes to route messages
through the overlay. All implementations MUST implement this routing
algorithm. An overlay may be configured to use alternative routing
algorithms, and alternative routing algorithms may be selected on a
per-message basis.
5.2.1. Request Origination
In order to originate a message to a given Node-ID or Resource-ID, a
node constructs an appropriate destination list. The simplest such
destination list is a single entry containing the peer or
Resource-ID. The resulting message will use the normal overlay
routing mechanisms to forward the message to that destination. The
node can also construct a more complicated destination list for
source routing.
Once the message is constructed, the node sends the message to some
adjacent peer. If the first entry on the destination list is
directly connected, then the message MUST be routed down that
connection. Otherwise, the topology plugin MUST be consulted to
determine the appropriate next hop.
Parallel searches for the resource are a common solution to improve
reliability in the face of churn or of subversive peers. Parallel
searches for usage-specified replicas are managed by the usage layer.
However, a single request can also be routed through multiple
adjacent peers, even when known to be sub-optimal, to improve
Jennings, et al. Expires September 8, 2009 [Page 35]
Internet-Draft RELOAD Base March 2009
reliability [vulnerabilities-acsac04]. Such parallel searches MAY BE
specified by the topology plugin.
Because messages may be lost in transit through the overlay, RELOAD
incorporates an end-to-end reliability mechanism. When an
originating node transmits a request it MUST set a 3 second timer.
If a response has not been received when the timer fires, the request
is retransmitted with the same transaction identifier. The request
MAY be retransmitted up to 4 times (for a total of 5 messages).
After the timer for the fifth transmission fires, the message SHALL
be considered to have failed. Note that this retransmission
procedure is not followed by intermediate nodes. They follow the
hop-by-hop reliability procedure described in Section 5.6.2.
The above algorithm can result in multiple requests being delivered
to a node. Receiving nodes MUST generate semantically equivalent
responses to retransmissions of the same request (this can be
determined by transaction id) if the request is received within the
maximum request lifetime (15 seconds). For some requests (e.g.,
FETCH) this can be accomplished merely by processing the request
again. For other requests, (e.g., STORE) it may be necessary to
maintain state for the duration of the request lifetime.
5.2.2. Response Origination
When a peer sends a response to a request, it MUST construct the
destination list by reversing the order of the entries on the via
list. This has the result that the response traverses the same peers
as the request traversed, except in reverse order (symmetric
routing).
5.3. Message Structure
RELOAD is a message-oriented request/response protocol. The messages
are encoded using binary fields. All integers are represented in
network byte order. The general philosophy behind the design was to
use Type, Length, Value fields to allow for extensibility. However,
for the parts of a structure that were required in all messages, we
just define these in a fixed position as adding a type and length for
them is unnecessary and would simply increase bandwidth and
introduces new potential for interoperability issues.
Each message has three parts, concatenated as shown below:
Jennings, et al. Expires September 8, 2009 [Page 36]
Internet-Draft RELOAD Base March 2009
+-------------------------+
| Forwarding Header |
+-------------------------+
| Message Contents |
+-------------------------+
| Security Block |
+-------------------------+
The contents of these parts are as follows:
Forwarding Header: Each message has a generic header which is used
to forward the message between peers and to its final destination.
This header is the only information that an intermediate peer
(i.e., one that is not the target of a message) needs to examine.
Message Contents: The message being delivered between the peers.
From the perspective of the forwarding layer, the contents is
opaque, however, it is interpreted by the higher layers.
Security Block: A security block containing certificates and a
digital signature over the message. Note that this signature can
be computed without parsing the message contents. All messages
MUST be signed by their originator.
The following sections describe the format of each part of the
message.
5.3.1. Presentation Language
The structures defined in this document are defined using a C-like
syntax based on the presentation language used to define TLS.
Advantages of this style include:
o It is easy to write and familiar enough looking that most readers
can grasp it quickly.
o The ability to define nested structures allows a separation
between high-level and low level message structures.
o It has a straightforward wire encoding that allows quick
implementation, but the structures can be comprehended without
knowing the encoding.
o The ability to mechanically (compile) encoders and decoders.
This presentation is to some extent a placeholder. We consider it an
open question what the final protocol definition method and encodings
use. We expect this to be a question for the WG to decide.
Several idiosyncrasies of this language are worth noting.
Jennings, et al. Expires September 8, 2009 [Page 37]
Internet-Draft RELOAD Base March 2009
o All lengths are denoted in bytes, not objects.
o Variable length values are denoted like arrays with angle
brackets.
o "select" is used to indicate variant structures.
For instance, "uint16 array<0..2^8-2>;" represents up to 254 bytes
but only up to 127 values of two bytes (16 bits) each..
5.3.1.1. Common Definitions
The following definitions are used throughout RELOAD and so are
defined here. They also provide a convenient introduction to how to
read the presentation language.
An enum represents an enumerated type. The values associated with
each possibility are represented in parentheses and the maximum value
is represented as a nameless value, for purposes of describing the
width of the containing integral type. For instance, Boolean
represents a true or false:
enum { false (0), true(1), (255)} Boolean;
A boolean value is either a 1 or a 0 and is represented as a single
byte on the wire.
The NodeId, shown below, represents a single Node-ID.
typedef opaque NodeId[16];
A NodeId is a fixed-length 128-bit structure represented as a series
of bytes, most significant byte first. Note: the use of "typedef"
here is an extension to the TLS language, but its meaning should be
relatively obvious.
A ResourceId, shown below, represents a single Resource-ID.
typedef opaque ResourceId<0..2^8-1>;
Like a NodeId, a Resource-ID is an opaque string of bytes, but unlike
Node-IDs, Resource-IDs are variable length, up to 255 bytes (2048
bits) in length. On the wire, each ResourceId is preceded by a
single length byte (allowing lengths up to 255). Thus, the 3-byte
value "Foo" would be encoded as: 03 46 4f 4f.
Jennings, et al. Expires September 8, 2009 [Page 38]
Internet-Draft RELOAD Base March 2009
A more complicated example is IpAddressPort, which represents a
network address and can be used to carry either an IPv6 or IPv4
address:
enum {reserved_addr(0), ipv4_address (1), ipv6_address (2),
(255)} AddressType;
struct {
uint32 addr;
uint16 port;
} IPv4AddrPort;
struct {
uint128 addr;
uint16 port;
} IPv6AddrPort;
struct {
AddressType type;
uint8 length;
select (type) {
case ipv4_address:
IPv4AddrPort v4addr_port;
case ipv6_address:
IPv6AddrPort v6addr_port;
/* This structure can be extended */
} IpAddressPort;
The first two fields in the structure are the same no matter what
kind of address is being represented:
type: the type of address (v4 or v6).
length: the length of the rest of the structure.
By having the type and the length appear at the beginning of the
structure regardless of the kind of address being represented, an
implementation which does not understand new address type X can still
parse the IpAddressPort field and then discard it if it is not
needed.
The rest of the IpAddressPort structure is either an IPv4AddrPort or
Jennings, et al. Expires September 8, 2009 [Page 39]
Internet-Draft RELOAD Base March 2009
an IPv6AddrPort. Both of these simply consist of an address
represented as an integer and a 16-bit port. As an example, here is
the wire representation of the IPv4 address "192.0.2.1" with port
"6100".
01 ; type = IPv4
06 ; length = 6
c0 00 02 01 ; address = 192.0.2.1
17 d4 ; port = 6100
5.3.2. Forwarding Header
The forwarding header is defined as a ForwardingHeader structure, as
shown below.
struct {
uint32 relo_token;
uint32 overlay;
uint16 configuration_sequence;
uint8 ttl;
uint8 reserved;
uint32 fragment;
uint8 version;
uint32 length;
uint64 transaction_id;
uint16 flags;
uint16 via_list_length;
uint16 destination_list_length;
uint16 route_log_length;
uint16 options_length;
Destination via_list[via_list_length];
Destination destination_list
[destination_list_length];
RouteLogEntry route_log[route_log_length];
ForwardingOptions options[options_length];
} ForwardingHeader;
The contents of the structure are:
relo_token: The first four bytes identify this message as a RELOAD
message. The message is easy to demultiplex from STUN messages by
looking at the first bit. This field MUST contain the value
0xc2454c4f (the string 'RELO' with the high bit of the first byte
set.).
Jennings, et al. Expires September 8, 2009 [Page 40]
Internet-Draft RELOAD Base March 2009
overlay: The 32 bit checksum/hash of the overlay being used. The
variable length string representing the overlay name is hashed
with SHA-1 and the low order 32 bits are used. The purpose of
this field is to allow nodes to participate in multiple overlays
and to detect accidental misconfiguration. This is not a security
critical function.
configuration_sequence: The sequence number of the configuration
file.
ttl: An 8 bit field indicating the number of iterations, or hops, a
message can experience before it is discarded. The TTL value MUST
be decremented by one at every hop along the route the message
traverses. If the TTL is 0, the message MUST NOT be propagated
further and MUST be discarded, and a "Error_TTL_Exceeded" error
should be generated. The initial value of the TTL SHOULD be 100
unless defined otherwise by the overlay configuration.
fragment: This field is used to handle fragmentation. The high
order two bits are used to indicate the fragmentation status: If
the high bit (0x80000000) is set, it indicates that the message is
a fragment. If the next bit (0x40000000) is set, it indicates
that this is the last fragment.
The remainder of the field is used to indicate the fragment
offset. [[Open Issue: This is conceptually clear, but the
details are still lacking. Need to define the fragment offset and
total length be encoded in the header. Right now we have 14 bits
reserved with the intention that they be used for fragmenting,
though additional bytes in the header might be needed for
fragmentation.]]
version: The version of the RELOAD protocol being used. This
document describes version 0.1, with a value of 0x01.
length: The count in bytes of the size of the message, including the
header.
transaction_id: A unique 64 bit number that identifies this
transaction and also serves as a salt to randomize the request and
the response. Responses use the same Transaction ID as the
request they correspond to. Transaction IDs are also used for
fragment reassembly.
Jennings, et al. Expires September 8, 2009 [Page 41]
Internet-Draft RELOAD Base March 2009
flags: The flags word contains control flags. Which are ORed
together. There is two currently defined flags: ROUTE-LOG (0x1)
and RESPONSE-ROUTE-LOG (0x2). These flags indicate that the route
log should be included (see Section 5.3.2.3.).
via_list_length: The length of the via list in bytes. Note that in
this field and the following two length fields we depart from the
usual variable-length convention of having the length immediately
precede the value in order to make it easier for hardware decoding
engines to quickly determine the length of the header.
destination_list_length: The length of the destination list in
bytes.
route_log_length: The length of the route log in bytes.
options_length: The length of the header options in bytes.
via_list: The via_list contains the sequence of destinations through
which the message has passed. The via_list starts out empty and
grows as the message traverses each peer.
destination_list: The destination_list contains a sequence of
destinations which the message should pass through. The
destination list is constructed by the message originator. The
first element in the destination list is where the message goes
next. The list shrinks as the message traverses each listed peer.
route_log: Contains a series of route log entries. See
Section 5.3.2.3.
options: Contains a series of ForwardingOptions entries. See
Section 5.3.2.4.
5.3.2.1. Processing Configuration Sequence Numbers
In order to be part of the overlay, a node MUST have a copy of the
overlay configuration document. In order to allow for configuration
document changes, each version of the configuration document has a
sequence number which is monotonically increasing mod 65536. Because
the sequence number may in principle wrap, greater than or less than
are interpreted by modulo arithmetic as in TCP.
When a destination node receives a request, it MUST check that the
configuration_sequence field is equal to its own configuration
sequence number. If they do not match, it MUST generate an error,
either Error_Config_Too_Old or Error_Config_Too_New. In addition, if
the configuration file in the request is too old, it MUST generate a
Jennings, et al. Expires September 8, 2009 [Page 42]
Internet-Draft RELOAD Base March 2009
Config_Update message to update the requesting node. This allows new
configuration documents to propagate quickly throughout the system.
The one exception to this rule is that if the configuration_sequence
field is equal to 0xffff, and the message type is Config_Update, then
the message MUST be accepted regardless of the receiving node's
configuration sequence number.
5.3.2.2. Destination and Via Lists
The destination list and via lists are sequences of Destination
values:
enum {reserved(0), peer(1), resource(2), compressed(3), (255) }
DestinationType;
select (destination_type) {
case peer:
NodeId node_id;
case resource:
ResourceId resource_id;
case compressed:
opaque compressed_id<0..2^8-1>;
/* This structure may be extended with new types */
} DestinationData;
struct {
DestinationType type;
uint8 length;
DestinationData destination_data;
} Destination;
This is a TLV structure with the following contents:
type
The type of the DestinationData PDU. This may be one of "peer",
"resource", or "compressed".
Jennings, et al. Expires September 8, 2009 [Page 43]
Internet-Draft RELOAD Base March 2009
length
The length of the destination_data.
destination_value
The destination value itself, which is an encoded DestinationData
structure, depending on the value of "type".
Note: This structure encodes a type, length, value. The length
field specifies the length of the DestinationData values, which
allows the addition of new DestinationTypes. This allows an
implementation which does not understand a given DestinationType
to skip over it.
A DestinationData can be one of three types:
peer
A Node-ID.
compressed
A compressed list of Node-IDs and/or resources. Because this
value was compressed by one of the peers, it is only meaningful to
that peer and cannot be decoded by other peers. Thus, it is
represented as an opaque string.
resource
The Resource-ID of the resource which is desired. This type MUST
only appear in the final location of a destination list and MUST
NOT appear in a via list. It is meaningless to try to route
through a resource.
5.3.2.3. Route Logging
The route logging feature provides diagnostic information about the
path taken by the message so far and in this manner it is similar in
function to SIP's [RFC3261] Via header field. If the ROUTE-LOG flag
is set in the Flags word, at each hop peers MUST append a route log
entry to the route log element in the header or reject the request.
The order of the route log entry elements in the message is
determined by the order of the peers were traversed along the path.
The first route log entry corresponds to the peer at the first hop
along the path, and each subsequent entry corresponds to the peer at
the next hop along the path. If the ROUTE-LOG flag is set, the route
log entries in the request MUST be copied to the response or the
request rejected. If, and only if, the ROUTE-LOG-RESPONSE flag is
set in a request, the ROUTE-LOG flag MUST be set in the response.
Note that use of the ROUTE-LOG-RESPONSE flag means that the response
will grow on the return path, which may potentially mean that it gets
Jennings, et al. Expires September 8, 2009 [Page 44]
Internet-Draft RELOAD Base March 2009
dropped due to becoming too large for some intermediate hop. Thus,
this option must be used with care.
The route log is defined as follows:
enum { (255) } RouteLogExtensionType;
struct {
RouteLogExtensionType type;
uint16 length;
select (type){
/* Extension values go here */
} extension;
} RouteLogExtension;
enum {
reserved(0),
tcp_tls(1),
udp_dtls(2),
(255)
} OverlayLink;
struct {
opaque version<0..2^8-1>; /* A string */
OverlayLink linkProtocol; /* TCP or UDP */
NodeId id;
uint32 uptime;
IpAddressPort address;
opaque certificate<0..2^16-1>;
RouteLogExtension extensions<0..2^16-1>;
} RouteLogEntry;
struct {
RouteLogEntry entries<0..2^16-1>;
} RouteLog;
The route log consists of an arbitrary number of RouteLogEntry
values, each representing one node through which the message has
passed.
Each RouteLogEntry consists of the following values:
Jennings, et al. Expires September 8, 2009 [Page 45]
Internet-Draft RELOAD Base March 2009
version
A textual representation of the software version
linkProtocol
The Overlay Link Layer protocol, currently either "tcp_tls" or
"udp_dtls".
id
The Node-ID of the peer.
uptime
The uptime of the peer in seconds.
address
The address and port of the peer.
certificate
The peer's certificate. Note that this may be omitted by setting
the length to zero.
extensions
Extensions, if any.
Extensions are defined using a RouteLogExtension structure. New
extensions are defined by defining a new code point for
RouteLogExtensionType and adding a new arm to the RouteLogExtension
structure. The contents of that structure are:
type
The type of the extension.
length
The length of the rest of the structure.
extension
The extension value.
5.3.2.4. Forwarding Options
The Forwarding header can be extended with forwarding header options,
which are a series of ForwardingOptions structures:
Jennings, et al. Expires September 8, 2009 [Page 46]
Internet-Draft RELOAD Base March 2009
enum { (255) } ForwardingOptionsType;
struct {
ForwardingOptionsType type;
uint8 flags;
uint16 length;
select (type) {
/* Option values go here */
} option;
} ForwardingOption;
Each ForwardingOption consists of the following values:
type
The type of the option.
length
The length of the rest of the structure.
flags
Three flags are defined FORWARD_CRITICAL(0x01),
DESTINATION_CRITICAL(0x02), and RESPONSE_COPY(0x04). These flags
MUST NOT be set in a response. If the FORWARD_CRITICAL flag is
set, any node that would forward the message but does not
understand this options MUST reject the request with an 757 error
response. If the DESTINATION_CRITICAL flag is set, any node
generates a response to the message but does not understand the
forwarding option MUST reject the request with an 757 error
response. If the RESPONSE_COPY flag is set, any node generating a
response MUST copy the option from the request to the response and
clear the RESPONSE_COPY, FORWARD_CRITICAL and DESTINATION_CRITICAL
flags.
option
The option value.
5.3.3. Message Contents Format
The second major part of a RELOAD message is the contents part, which
is defined by MessageContents:
struct {
MessageCode message_code;
opaque payload<0..2^24-1>;
} MessageContents;
Jennings, et al. Expires September 8, 2009 [Page 47]
Internet-Draft RELOAD Base March 2009
The contents of this structure are as follows:
message_code
This indicates the message that is being sent. The code space is
broken up as follows.
0 Reserved
1 .. 0x7fff Requests and responses. These code points are always
paired, with requests being odd and the corresponding response
being the request code plus 1. Thus, "probe_request" (the
Probe request) has value 1 and "probe_answer" (the Probe
response) has value 2
0xffff Error
message_body
The message body itself, represented as a variable-length string
of bytes. The bytes themselves are dependent on the code value.
See the sections describing the various RELOAD methods (Join,
Update, Attach, Store, Fetch, etc.) for the definitions of the
payload contents.
5.3.3.1. Response Codes and Response Errors
A peer processing a request returns its status in the message_code
field. If the request was a success, then the message code is the
response code that matches the request (i.e., the next code up). The
response payload is then as defined in the request/response
descriptions.
If the request failed, then the message code is set to 0xffff (error)
and the payload MUST be an error_response PDU, as shown below.
When the message code is 0xffff, the payload MUST be an
ErrorResponse.
public struct {
uint16 error_code;
opaque error_info<0..2^16-1>;
} ErrorResponse;
The contents of this structure are as follows:
Jennings, et al. Expires September 8, 2009 [Page 48]
Internet-Draft RELOAD Base March 2009
error_code
A numeric error code indicating the error that occurred.
error_info
An arbitrary byte string. Unless otherwise specified, this will
be a text string providing further information about what went
wrong.
The following error code values are defined. The numeric values for
these are defined in Section 13.7.
Error_Unauthorized: The requesting peer needs to sign and provide a
certificate. [[TODO: The semantics here don't seem quite
right.]]
Error_Forbidden: The requesting peer does not have permission to
make this request.
Error_Not_Found: The resource or peer cannot be found or does not
exist.
Error_Request_Timeout: A response to the request has not been
received in a suitable amount of time. The requesting peer MAY
resend the request at a later time.
Error_Precondition_Failed: A request can't be completed because some
precondition was incorrect. For instance, the wrong generation
counter was provided
Error_Incompatible_with_Overlay: A peer receiving the request is
using a different overlay, overlay algorithm, or hash algorithm.
Error_Unsupported_Forwarding_Option: A peer receiving the request
with a forwarding options flagged as critical but the peer does
not support this option. See section Section 5.3.2.4.
Error_TTL_Exceeded: A peer receiving the request where the TTL got
decremented to zero. See section Section 5.3.2.
Error_Message_Too_Large: A peer receiving the request that was too
large. See section Section 5.6.
Error_Config_Too_Old: A destination peer received a request with a
configuration sequence that's too old.
Jennings, et al. Expires September 8, 2009 [Page 49]
Internet-Draft RELOAD Base March 2009
Error_Config_Too_New: A destination node received a request with a
configuration sequence that's too new. A node which receives this
error MUST generate a Config_Update message to send a new copy of
the configuration document to the node which generated the error.
5.3.4. Security Block
The third part of a RELOAD message is the security block. The
security block is represented by a SecurityBlock structure:
enum { x509(0), (255) } certificate_type;
struct {
certificate_type type;
opaque certificate<0..2^16-1>;
} GenericCertificate;
struct {
GenericCertificate certificates<0..2^16-1>;
Signature signature;
} SecurityBlock;
The contents of this structure are:
certificates
A bucket of certificates.
signature
A signature over the message contents.
The certificates bucket SHOULD contain all certificates necessary to
verify every signature in both the message and the internal message
objects. This is the only location in the message which contains
certificates, thus allowing for only a single copy of each
certificate. In systems which have some alternate certificate
distribution mechanism, some certificates MAY be omitted. However,
implementors should note that this creates the possibility that
messages may not be immediately verifiable upon receipt of the
certificates must first be retrieved.
Each certificate is represented by a GenericCertificate structure,
which has the following contents:
Jennings, et al. Expires September 8, 2009 [Page 50]
Internet-Draft RELOAD Base March 2009
type
The type of the certificate. Only one type is defined: x509
representing an X.509 certificate
certificate
The encoded version of the certificate. For X.509 certificates,
it is the DER form.
The signature is computed over the payload and parts of forwarding
header. The payload, in case of a Store, may contain an additional
signature computed over a StoreReq structure. All signatures are
formatted using the Signature element. This element is also used in
other contexts where signatures are needed. The input structure to
the signature computation varies depending on the data element being
signed.
enum {reserved(0), cert_hash(1), (255)} SignerIdentityType;
select (identity_type) {
case cert_hash;
HashAlgorithm hash_alg;
opaque certificate_hash<0..2^8-1>;
/* This structure may be extended with new types if necessary*/
} SignerIdentityValue;
struct {
SignerIdentityType identity_type;
uint16 length;
SignerIdentityValue identity[SignerIdentity.length];
} SignerIdentity;
struct {
SignatureAndHashAlgorithm algorithm;
SignerIdentity identity;
opaque signature_value<0..2^16-1>;
} Signature;
The signature construct contains the following values:
algorithm
The signature algorithm in use. The algorithm definitions are
found in the IANA TLS SignatureAlgorithm Registry.
Jennings, et al. Expires September 8, 2009 [Page 51]
Internet-Draft RELOAD Base March 2009
identity
The identity used to form the signature
signature_value
The value of the signature
The only currently permitted identity format is a hash of the
signer's certificate. The hash_alg field is used to indicate the
algorithm used to produce the hash. The certificate_hash contains
the hash of the certificate object as represented in the certificates
structure. The SignerIdentity structure is typed purely to allow for
future (unanticipated) extensibility. [TODO: Should we remove this
extensibility point?]
For signatures over messages the input to the signature is computed
over:
overlay + transaction_id + MessageContents + SignerIdentity
Where overlay and transaction_id come from the forwarding header and
+ indicates concatenation.
[[TODO: Check the inputs to this carefully.]]
The input to signatures over data values is different, and is
described in Section 6.1.
All RELOAD messages MUST be signed. Upon receipt, the receiving node
MUST verify the signature and the authorizing certificate. This
check provides a minimal level of assurance that the sending node is
a valid part of the overlay as well as cryptographic authentication
of the sending node. In addition, responses MUST be checked as
follows:
1. The response to a message sent to a specific Node-Id MUST have
been sent by that Node-Id.
2. The response to a message sent to a Resource-Id MUST have been
sent by a Node-Id which is as close to or closer to the target
Resource-Id than any node in the requesting node's neighbor
table.
The second condition serves as a primitive check for responses from
wildly wrong nodes but is not a complete check. Note that in periods
of churn, it is possible for the requesting node to obtain a closer
neighbor while the request is outstanding. This will cause the
response to be rejected and the request to be retransmitted.
Jennings, et al. Expires September 8, 2009 [Page 52]
Internet-Draft RELOAD Base March 2009
In addition, some methods (especially Store) have additional
authentication requirements, which are described in the sections
covering those methods.
5.4. Overlay Topology
As discussed in previous sections, RELOAD does not itself implement
any overlay topology. Rather, it relies on Topology Plugins, which
allow a variety of overlay algorithms to be used while maintaining
the same RELOAD core. This section describes the requirements for
new topology plugins and the methods that RELOAD provides for overlay
topology maintenance.
5.4.1. Topology Plugin Requirements
When specifying a new overlay algorithm, at least the following need
to be described:
o Joining procedures, including the contents of the Join message.
o Stabilization procedures, including the contents of the Update
message, the frequency of topology probes and keepalives, and the
mechanism used to detect when peers have disconnected.
o Exit procedures, including the contents of the Leave message.
o The length of the Resource-IDs and Node-IDs. For DHTs, the hash
algorithm to compute the hash of an identifier.
o The procedures that peers use to route messages.
o The replication strategy used to ensure data redundancy.
5.4.2. Methods and types for use by topology plugins
This section describes the methods that topology plugins use to join,
leave, and maintain the overlay.
5.4.2.1. Join
A new peer (but which already has credentials) uses the JoinReq
message to join the overlay. The JoinReq is sent to the responsible
peer depending on the routing mechanism described in the topology
plugin. This notifies the responsible peer that the new peer is
taking over some of the overlay and it needs to synchronize its
state.
struct {
NodeId joining_peer_id;
opaque overlay_specific_data<0..2^16-1>;
} JoinReq;
Jennings, et al. Expires September 8, 2009 [Page 53]
Internet-Draft RELOAD Base March 2009
The minimal JoinReq contains only the Node-ID which the sending peer
wishes to assume. Overlay algorithms MAY specify other data to
appear in this request.
If the request succeeds, the responding peer responds with a JoinAns
message, as defined below:
struct {
opaque overlay_specific_data<0..2^16-1>;
} JoinAns;
If the request succeeds, the responding peer MUST follow up by
executing the right sequence of Stores and Updates to transfer the
appropriate section of the overlay space to the joining peer. In
addition, overlay algorithms MAY define data to appear in the
response payload that provides additional info.
In general, nodes which cannot form connections SHOULD report an
error. However, implementations MUST provide some mechanism whereby
nodes can determine they are potentially the first node and take
responsibility for the overlay. This specification does not mandate
any particular mechanism, but a configuration flag or setting seems
appropriate.
5.4.2.2. Leave
The LeaveReq message is used to indicate that a node is exiting the
overlay. A node SHOULD send this message to each peer with which it
is directly connected prior to exiting the overlay.
public struct {
NodeId leaving_peer_id;
opaque overlay_specific_data<0..2^16-1>;
} LeaveReq;
LeaveReq contains only the Node-ID of the leaving peer. Overlay
algorithms MAY specify other data to appear in this request.
Upon receiving a Leave request, a peer MUST update its own routing
table, and send the appropriate Store/Update sequences to re-
stabilize the overlay.
5.4.2.3. Update
Update is the primary overlay-specific maintenance message. It is
used by the sender to notify the recipient of the sender's view of
Jennings, et al. Expires September 8, 2009 [Page 54]
Internet-Draft RELOAD Base March 2009
the current state of the overlay (its routing state) and it is up to
the recipient to take whatever actions are appropriate to deal with
the state change.
The contents of the UpdateReq message are completely overlay-
specific. The UpdateAns response is expected to be either success or
an error.
5.4.2.4. Route_Query
The Route_Query request allows the sender to ask a peer where they
would route a message directed to a given destination. In other
words, a RouteQuery for a destination X requests the Node-ID where
the receiving peer would next route to get to X. A RouteQuery can
also request that the receiving peer initiate an Update request to
transfer his routing table.
One important use of the RouteQuery request is to support iterative
routing. The sender selects one of the peers in its routing table
and sends it a RouteQuery message with the destination_object set to
the Node-ID or Resource-ID it wishes to route to. The receiving peer
responds with information about the peers to which the request would
be routed. The sending peer MAY then Attaches to that peer(s), and
repeats the RouteQuery. Eventually, the sender gets a response from
a peer that is closest to the identifier in the destination_object as
determined by the topology plugin. At that point, the sender can
send messages directly to that peer.
5.4.2.4.1. Request Definition
A RouteQueryReq message indicates the peer or resource that the
requesting peer is interested in. It also contains a "send_update"
option allowing the requesting peer to request a full copy of the
other peer's routing table.
struct {
Boolean send_update;
Destination destination;
opaque overlay_specific_data<0..2^16-1>;
} RouteQueryReq;
The contents of the RouteQueryReq message are as follows:
Jennings, et al. Expires September 8, 2009 [Page 55]
Internet-Draft RELOAD Base March 2009
send_update
A single byte. This may be set to "true" to indicate that the
requester wishes the responder to initiate an Update request
immediately. Otherwise, this value MUST be set to "false".
destination
The destination which the requester is interested in. This may be
any valid destination object, including a Node-ID, compressed ids,
or Resource-ID.
overlay_specific_data
Other data as appropriate for the overlay.
5.4.2.4.2. Response Definition
A response to a successful RouteQueryReq request is a RouteQueryAns
message. This is completely overlay specific.
5.4.2.5. Probe
Probe provides a number of primitive "exploration" services: (1) it
allows node to determine which resources another node is responsible
for (2) it allows some discovery services in multicast settings. A
probe can be addressed to a specific Node-ID, or the peer controlling
a given location (by using a resource ID). In either case, the
target Node-IDs respond with a simple response containing some status
information.
5.4.2.5.1. Request Definition
The ProbeReq message contains a list (potentially empty) of the
pieces of status information that the requester would like the
responder to provide.
enum { responsible_set(1), num_resources(2), (255)}
ProbeInformationType;
struct {
ProbeInformationType requested_info<0..2^8-1>;
} ProbeReq
The two currently defined values for ProbeInformation are:
Jennings, et al. Expires September 8, 2009 [Page 56]
Internet-Draft RELOAD Base March 2009
responsible_set
indicates that the peer should Respond with the fraction of the
overlay for which the responding peer is responsible.
num_resources
indicates that the peer should Respond with the number of
resources currently being stored by the peer.
5.4.2.5.2. Response Definition
A successful ProbeAns response contains the information elements
requested by the peer.
struct {
ProbeInformationType type;
select (type) {
case responsible_set:
uint32 responsible_ppb;
case num_resources:
uint32 num_resources;
/* This type may be extended */
};
} ProbeInformation;
struct {
ProbeInformation probe_info<0..2^16-1>;
} ProbeAns;
A ProbeAns message contains the following elements:
probe_info
A sequence of ProbeInformation structures, as shown below.
Each of the current possible Probe information types is a 32-bit
unsigned integer. For type "responsible_ppb", it is the fraction of
the overlay for which the peer is responsible in parts per billion.
For type "num_resources", it is the number of resources the peer is
storing.
The responding peer SHOULD include any values that the requesting
Jennings, et al. Expires September 8, 2009 [Page 57]
Internet-Draft RELOAD Base March 2009
peer requested and that it recognizes. They SHOULD be returned in
the requested order. Any other values MUST NOT be returned.
5.5. Forwarding and Link Management Layer
Each node maintains connections to a set of other nodes defined by
the topology plugin. This section defines the methods RELOAD uses to
form and maintain connections between nodes in the overlay. Three
methods are defined:
Attach: used to form connections between nodes. When node A wants
to connect to node B, it sends an Attach message to node B through
the overlay. The Attach contains A's ICE parameters. B responds
with its ICE parameters and the two nodes perform ICE to form
connection.
AttachLite: like attach, it is used to form connections between
nodes but instead of using full ICE, it only uses a subset known
as ICE-Lite.
Ping: is a simple request/response which is used to verify
connectivity of the target peer.
5.5.1. Attach
A node sends an Attach request when it wishes to establish a direct
TCP or UDP connection to another node for the purposes of sending
RELOAD messages or application layer protocol messages, such as SIP.
As described in Section 5.1, an Attach may be routed to either a
Node-ID or to a Resource-ID. An Attach routed to a specific Node-ID
will fail if that node is not reached. An Attach routed to a
Resource-ID will establish a connection with the peer currently
responsible for that Resource-ID, which may be useful in establishing
a direct connection to the responsible peer for use with frequent or
large resource updates.
An Attach in and of itself does not result in updating the routing
table of either node. That function is performed by Updates. If
node A has Attached to node B, but not received any Updates from B,
it MAY route messages which are directly addressed to B through that
channel but MUST NOT route messages through B to other peers via that
channel. The process of Attaching is separate from the process of
becoming a peer (using Update) to prevent half-open states where a
node has started to form connections but is not really ready to act
as a peer.
Jennings, et al. Expires September 8, 2009 [Page 58]
Internet-Draft RELOAD Base March 2009
5.5.1.1. Request Definition
An AttachReq message contains the requesting peer's ICE connection
parameters formatted into a binary structure.
typedef opaque IceCandidate<0..2^16-1>;
struct {
opaque ufrag<0..2^8-1>;
opaque password<0..2^8-1>;
uint16 application;
opaque role<0..2^8-1>;
IceCandidate candidates<0..2^16-1>;
} AttachReqAns;
The values contained in AttachReq and AttachAns are:
ufrag
The username fragment (from ICE)
password
The ICE password.
application
A 16-bit port number. This port number represents the IANA
registered port of the protocol that is going to be sent on this
connection. For SIP, this is 5060 or 5061, and for RELOAD is TBD.
By using the IANA registered port, we avoid the need for an
additional registry and allow RELOAD to be used to set up
connections for any existing or future application protocol.
role
An active/passive/actpass attribute from RFC 4145 [RFC4145].
candidates
One or more ICE candidate values in the string representation used
in ordinary ICE. [[OPEN ISSUE: This is convenient for stacks,
but unaesthetic.]] Each candidate has an IP address, IP address
family, port, transport protocol, priority, foundation, component
ID, STUN type and related address. The candidate_list is a list
of string candidate values from ICE.
These values should be generated using the procedures described in
Section 5.5.1.3.
Jennings, et al. Expires September 8, 2009 [Page 59]
Internet-Draft RELOAD Base March 2009
5.5.1.2. Response Definition
If a peer receives an Attach request, it SHOULD follow the process
the request and generate its own response with a AttachReqAns. It
should then begin ICE checks. When a peer receives an Attach
response, it SHOULD parse the response and begin its own ICE checks.
5.5.1.3. Using ICE With RELOAD
This section describes the profile of ICE that is used with RELOAD.
RELOAD implementations MUST implement full ICE. Because RELOAD
always tries to use TCP and then UDP as a fallback, there will be
multiple candidates of the same IP version, which requires full ICE.
In ICE as defined by [I-D.ietf-mmusic-ice], SDP is used to carry the
ICE parameters. In RELOAD, this function is performed by a binary
encoding in the Attach method. This encoding is more restricted than
the SDP encoding because the RELOAD environment is simpler:
o Only a single media stream is supported.
o In this case, the "stream" refers not to RTP or other types of
media, but rather to a connection for RELOAD itself or for SIP
signaling.
o RELOAD only allows for a single offer/answer exchange. Unlike the
usage of ICE within SIP, there is never a need to send a
subsequent offer to update the default candidates to match the
ones selected by ICE.
An agent follows the ICE specification as described in
[I-D.ietf-mmusic-ice] and [I-D.ietf-mmusic-ice-tcp] with the changes
and additional procedures described in the subsections below.
5.5.1.4. Collecting STUN Servers
ICE relies on the node having one or more STUN servers to use. In
conventional ICE, it is assumed that nodes are configured with one or
more STUN servers through some out-of-band mechanism. This is still
possible in RELOAD but RELOAD also learns STUN servers as it connects
to other peers. Because all RELOAD peers implement ICE and use STUN
keepalives, every peer is a STUN server [RFC5389]. Accordingly, any
peer a node knows will be willing to be a STUN server -- though of
course it may be behind a NAT.
A peer on a well-provisioned wide-area overlay will be configured
with one or more bootstrap peers. These peers make an initial list
of STUN servers. However, as the peer forms connections with
additional peers, it builds more peers it can use as STUN servers.
Jennings, et al. Expires September 8, 2009 [Page 60]
Internet-Draft RELOAD Base March 2009
Because complicated NAT topologies are possible, a peer may need more
than one STUN server. Specifically, a peer that is behind a single
NAT will typically observe only two IP addresses in its STUN checks:
its local address and its server reflexive address from a STUN server
outside its NAT. However, if there are more NATs involved, it may
discover that it learns additional server reflexive addresses (which
vary based on where in the topology the STUN server is). To maximize
the chance of achieving a direct connection, a peer SHOULD group
other peers by the peer-reflexive addresses it discovers through
them. It SHOULD then select one peer from each group to use as a
STUN server for future connections.
Only peers to which the peer currently has connections may be used.
If the connection to that host is lost, it MUST be removed from the
list of stun servers and a new server from the same group SHOULD be
selected.
5.5.1.5. Gathering Candidates
When a node wishes to establish a connection for the purposes of
RELOAD signaling or SIP signaling (or any other application protocol
for that matter), it follows the process of gathering candidates as
described in Section 4 of ICE [I-D.ietf-mmusic-ice]. RELOAD utilizes
a single component, as does SIP. Consequently, gathering for these
"streams" requires a single component.
An agent MUST implement ICE-tcp [I-D.ietf-mmusic-ice], and MUST
gather at least one UDP and one TCP host candidate for RELOAD and for
SIP.
The ICE specification assumes that an ICE agent is configured with,
or somehow knows of, TURN and STUN servers. RELOAD provides a way
for an agent to learn these by querying the overlay, as described in
Section 5.5.1.4 and Section 8.
The agent SHOULD prioritize its TCP-based candidates over its UDP-
based candidates in the prioritization described in Section 4.1.2 of
ICE [I-D.ietf-mmusic-ice].
The default candidate selection described in Section 4.1.3 of ICE is
ignored; defaults are not signaled or utilized by RELOAD.
5.5.1.6. Encoding the Attach Message
Section 4.3 of ICE describes procedures for encoding the SDP for
conveying RELOAD or SIP ICE candidates. Instead of actually encoding
an SDP, the candidate information (IP address and port and transport
protocol, priority, foundation, component ID, type and related
Jennings, et al. Expires September 8, 2009 [Page 61]
Internet-Draft RELOAD Base March 2009
address) is carried within the attributes of the Attach request or
its response. Similarly, the username fragment and password are
carried in the Attach message or its response. Section 5.5.1
describes the detailed attribute encoding for Attach. The Attach
request and its response do not contain any default candidates or the
ice-lite attribute, as these features of ICE are not used by RELOAD.
The Attach request and its response also contain a application
attribute, with a value of SIP or RELOAD, which indicates what
protocol is to be run over the connection. The RELOAD Attach request
MUST only be utilized to set up connections for application protocols
that can be multiplexed with STUN.
Since the Attach request contains the candidate information and short
term credentials, it is considered as an offer for a single media
stream that happens to be encoded in a format different than SDP, but
is otherwise considered a valid offer for the purposes of following
the ICE specification. Similarly, the Attach response is considered
a valid answer for the purposes of following the ICE specification.
5.5.1.7. Verifying ICE Support
An agent MUST skip the verification procedures in Section 5.1 and 6.1
of ICE. Since RELOAD requires full ICE from all agents, this check
is not required.
5.5.1.8. Role Determination
The roles of controlling and controlled as described in Section 5.2
of ICE are still utilized with RELOAD. However, the offerer (the
entity sending the Attach request) will always be controlling, and
the answerer (the entity sending the Attach response) will always be
controlled. The connectivity checks MUST still contain the ICE-
CONTROLLED and ICE-CONTROLLING attributes, however, even though the
role reversal capability for which they are defined will never be
needed with RELOAD. This is to allow for a common codebase between
ICE for RELOAD and ICE for SDP.
5.5.1.9. Connectivity Checks
The processes of forming check lists in Section 5.7 of ICE,
scheduling checks in Section 5.8, and checking connectivity checks in
Section 7 are used with RELOAD without change.
5.5.1.10. Concluding ICE
The controlling agent MUST utilize regular nomination. This is to
ensure consistent state on the final selected pairs without the need
for an updated offer, as RELOAD does not generate additional offer/
Jennings, et al. Expires September 8, 2009 [Page 62]
Internet-Draft RELOAD Base March 2009
answer exchanges.
The procedures in Section 8 of ICE are followed to conclude ICE, with
the following exceptions:
o The controlling agent MUST NOT attempt to send an updated offer
once the state of its single media stream reaches Completed.
o Once the state of ICE reaches Completed, the agent can immediately
free all unused candidates. This is because RELOAD does not have
the concept of forking, and thus the three second delay in Section
8.3 of ICE does not apply.
5.5.1.11. Subsequent Offers and Answers
An agent MUST NOT send a subsequent offer or answer. Thus, the
procedures in Section 9 of ICE MUST be ignored.
5.5.1.12. Media Keepalives
STUN MUST be utilized for the keepalives described in Section 10 of
ICE. [[ TODO - this does not define what happens for TCP ]]
5.5.1.13. Sending Media
The procedures of Section 11 apply to RELOAD as well. However, in
this case, the "media" takes the form of application layer protocols
(RELOAD or SIP for example) over TLS or DTLS. Consequently, once ICE
processing completes, the agent will begin TLS or DTLS procedures to
establish a secure connection. The node which sent the Attach
request MUST be the TLS server. The other node MUST be the TLS
client. The nodes MUST verify that the certificate presented in the
handshake matches the identity of the other peer as found in the
Attach message. Once the TLS or DTLS signaling is complete, the
application protocol is free to use the connection.
The concept of a previous selected pair for a component does not
apply to RELOAD, since ICE restarts are not possible with RELOAD.
5.5.1.14. Receiving Media
An agent MUST be prepared to receive packets for the application
protocol (TLS or DTLS carrying RELOAD, SIP or anything else) at any
time. The jitter and RTP considerations in Section 11 of ICE do not
apply to RELOAD or SIP.
Jennings, et al. Expires September 8, 2009 [Page 63]
Internet-Draft RELOAD Base March 2009
5.5.2. AttachLite
An alternative to using the full ICE supported by the Attach request
is to use ICE-Lite with the AttachLite request. This will not work
in all of the scenarios where ICE would work, but in some cases,
particularly those with no NATs or firewalls, it will work.
Configuration for the overlay indicates if this can be used or not.
OPEN ISSUE: We originally envisioned adding support for ICE-Lite
directly to the regular Attach method. However, we found that both
the parameters and processing were completely different, resulting in
almost no overlap between the two methods. Therefore we chose to
separate this out for overlays where the complexities of ICE are not
needed. Note that it is still possible for a node with a public
unfiltered address intending to interoperate to implement Attach
without the candidate gathering phases of ICE and achieve essentially
the same result. If simpler behavior or a better encoding of ICE-
Lite in Attach is developed, such an approach would be preferable.
5.5.2.1. Request Definition
An AttachLiteReq message contains the requesting peer's ICE-Lite
connection parameters formatted into a binary structure. When using
the AttachLite request, both sides act as ICE-Lite hosts.
struct {
IpAddressPort addr_port;
Transport transport;
uint32 priority;
} IceLiteCandidate;
struct {
uint16 application;
IceLiteCandidate candidates<0..2^16-1>;
} AttachLiteReqs;
The values contained in AttachLiteReq are:
application
A 16-bit port number used in the same was as in the Attach
request. This port number represents the IANA registered port of
the protocol that is going to be sent on this connection.
Jennings, et al. Expires September 8, 2009 [Page 64]
Internet-Draft RELOAD Base March 2009
candidates
One or more ICE candidate values. Each one contains an IP address
and family, transport protocol, and port to connect to as well as
a priority.
These values should be generated using the procedures described in
Section 5.5.1.3.
5.5.2.2. Attach-Lite Connectivity Checks
STUN is not used for connectivity checks when doing ICE-Lite, instead
the DTLS or TLS handshake forms the connectivity check. The host
that received the AttachLiteReq MUST initiate TLS or DTLS connections
to candidates provided in the request. When a connection forms, the
node MUST check the certificate is for the node that send
AttachLiteReq and if is not, MUST close the connection.
Since TLS provides the connectivity check, there is no need for the
RFC 4571 [RFC4571] style framing shim for STUN when using TLS and
this is not used for this protocol.
5.5.2.3. Implementation Notes for Attach-Lite
This is a non normative section to help implementors.
At times ICE can seem a bit daunting to gets one head around. For a
simple IPv4 only peer, a simple implementation of Attach-Lite could
be done be doing the following:
o When sending an AttachLiteReq, form one with a candidate with a
priority value of (2^24)*(126)+(2^8)*(65535)+(2^0)*(256-1) that
specifies the UDP port being listened to and another one with the
TCP port.
o When receiving an AttachLiteReq, try to form a connection to each
candidate in the request. Check the certificate receive in the
TLS handshake has the correct Node-ID as the node that send the
AttchLiteReq. If multiple connection succeed, close all but the
one with highest priority.
o Do normal TLS and DTLS with no need for any special framing or
STUN processing.
5.5.3. Ping
Ping is used to test connectivity along a path. A ping can be
addressed to a specific Node-ID, the peer controlling a given
location (by using a resource ID), or to the broadcast Node-ID (all
1s).
Jennings, et al. Expires September 8, 2009 [Page 65]
Internet-Draft RELOAD Base March 2009
5.5.3.1. Request Definition
struct {
} PingReq
5.5.3.2. Response Definition
A successful PingAns response contains the information elements
requested by the peer.
struct {
uint64 response_id;
uint64 time;
} PingAns;
A PingAns message contains the following elements:
response_id
A randomly generated 64-bit response ID. This is used to
distinguish Ping responses in cases where the Ping request is
multicast.
time
The time when the ping responses was created in absolute time,
represented in milliseconds since midnight Jan 1, 1970 which is
the UNIX epoch.
5.5.4. Config_Update
The Config_Update method is used to propagate updated configuration
files across the overlay. Whenever a node detects that another node
has an old configuration file, it MUST generate a Config_Update
request.
5.5.4.1. Request Definition
struct {
opaque config_data<2^24-1>;
} Config_UpdateReq;
The Config_UpdateReq message contains the following elements:
Jennings, et al. Expires September 8, 2009 [Page 66]
Internet-Draft RELOAD Base March 2009
config_data
The contents of the configuration document.
5.5.4.2. Response Definition
struct {
} Config_UpdateReq
The Config_UpdateReq should only be processed if all the following
are true:
o The configuration sequence number in the document is greater than
the current configuration sequence number.
o The configuration document is correctly digitally signed (see
Section 10 for details on signatures.
Otherwise appropriate errors MUST be generated.
If the document is acceptable, then the node MUST reconfigure itself
to match the new document. This may include adding permissions for
new kinds, deleting old kinds, or even, in extreme circumstances,
exiting and reentering the overlay, if, for instance, the DHT
algorithm has changed.
The response for Config_Update is empty.
5.6. Overlay Link Layer
RELOAD can use multiple Overlay Link protocols to send its messages.
Because ICE is used to establish connections (see Section 5.5.1.3),
RELOAD nodes are able to detect which Overlay Link protocols are
offered by other nodes and establish connections between each other.
Any link protocol needs to be able to establish a secure,
authenticated connection, and provide data origin authentication and
message integrity for individual data elements. RELOAD currently
supports two Overlay Link protocols:
o TLS [RFC5246] over TCP
o DTLS [RFC4347] over UDP
Note that although UDP does not properly have "connections", both TLS
and DTLS have a handshake which establishes a stateful association, a
similar stateful construct, and we simply refer to these as
"connections" for the purposes of this document.
If a peer receives a message that is larger than value of max-
message-size defined in the overlay configuration, the peer SHOULD
send an Error_Message_Too_Large error then close the TLS or DTLS
Jennings, et al. Expires September 8, 2009 [Page 67]
Internet-Draft RELOAD Base March 2009
session from which the message was received. Note that this error
can be sent and the session closed before receiving the complete
message. If the forwarding header is larger than the max-message-
size, the receiver SHOULD close the TLS or DTLS session without
sending an error.
5.6.1. Future Support for HIP
The P2PSIP Working Group has expressed interest in supporting a HIP-
based link protocol. Such support would require specifying such
details as:
o How to issue certificates which provided identities meaningful to
the HIP base exchange. We anticipate that this would require a
mapping between ORCHIDs and NodeIds.
o How to carry the HIP I1 and I2 messages. We anticipate that this
would require defining a HIP Tunnel usage.
o How to carry RELOAD messages over HIP.
We leave this work as a topic for another draft.
5.6.2. Reliability for Unreliable Links
When RELOAD is carried over DTLS or another unreliable link protocol,
it needs to be used with a reliability and congestion control
mechanism, which is provided on a hop-by-hop basis, matching the
semantics if TCP were used. The basic principle is that each
message, regardless of if it carries a request or responses, will get
an ACK and be reliably retransmitted. The receiver's job is very
simple, limited to just sending ACKs. All the complexity is at the
sender side. This allows the sending implementation to trade off
performance versus implementation complexity without affecting the
wire protocol.
In order to support unreliable links, each message is wrapped in a
very simple framing layer (FramedMessage) which is only used for each
hop. This layer contains a sequence number which can then be used
for ACKs.
5.6.2.1. Framed Message Format
[[TODO: There had been discussion of always using this, but it's
tied up in the rest of the reliability questions.]]
The definition of FramedMessage is:
Jennings, et al. Expires September 8, 2009 [Page 68]
Internet-Draft RELOAD Base March 2009
enum {data (128), ack (129), (255)} FramedMessageType;
struct {
FramedMessageType type;
select (type) {
case data:
uint32 sequence;
opaque message<0..2^24-1>;
case ack:
uint32 ack_sequence;
uint32 received;
};
} FramedMessage;
The type field of the PDU is set to indicate whether the message is
data or an acknowledgement. Note that these values have been set to
force the first bit to be high, thus allowing easy demultiplexing
with STUN. All FramedMessageType values must be > 128.
If the message is of type "data", then the remainder of the PDU is as
follows:
sequence
the sequence number
message
the message that is being transmitted.
Each connection has it own sequence number space. Initially the
value is zero and it increments by exactly one for each message sent
over that connection.
When the receiver receive a message, it SHOULD immediately send an
ACK message. The receiver MUST keep track of the 32 most recent
sequence numbers received on this association in order to generate
the appropriate ack.
If the PDU is of type "ack", the contents are as follows:
ack_sequence
Jennings, et al. Expires September 8, 2009 [Page 69]
Internet-Draft RELOAD Base March 2009
The sequence number of the message being acknowledged.
received
A bitmask indicating is each of the previous 32 sequence numbers
before this packet had been received as one of the most recently
received 32 packets on this connection. When a packet is received
with a sequence number N, the receiver looks at the sequence
number of the previously 32 packets received on this connection,.
Call the previously received packet number M. And for each of the
previous 32 packets, if the sequence number M is less than N but
greater than N-32, the N-M bit of the received bitmask is set to
one otherwise it is zero.
Note that a bit being set indicates a particular packet was
received but if the bit is set to zero it only means it is unknown
if it was received or not. It might have been received but not in
the 32 most recently received window.
The received field bits in the ACK provide a very high degree of
redundancy for the sender to figure out which packets the receiver
received and can then estimate packet loss rates. If the sender also
keeps track of the time at which recent sequence numbers were sent,
the RTT can be estimated.
5.6.2.2. Retransmission and Flow Control
Because the receiver's role is limited to providing packet
acknowledgements, a wide variety of congestion control algorithms can
be implemented on the sender side while using the same basic wire
protocol. Senders MUST implement a retransmission and congestion
control scheme no more aggressive then TFRC[RFC5348]. One way to do
that is for senders to implement TFRC-SP [RFC4828] and use the
received bitmask to allow the sender to compute packet loss event
rates.
5.6.2.2.1. Trivial Retransmission
An algorithm which will not perform as well as TFRC-SP but is easy to
implement is described in this section and can be used if
implementations don't use a more advanced techniques such as TFRC-SP.
A peer SHOULD retransmit a message if it has not received an ACK for
that messages starting with an interval of RTO ("Retransmission
TimeOut"), doubling after each retransmission. In each
retransmission, the sequence number is incremented. The RTO is an
estimate of the round-trip time (RTT), and is computed as described
in RFC 2988 [RFC2988], with two exceptions. First, the initial value
for RTO SHOULD be configurable (rather than the 3 s recommended in
RFC 2988) and SHOULD be equal to or greater than 500 ms. The
Jennings, et al. Expires September 8, 2009 [Page 70]
Internet-Draft RELOAD Base March 2009
exception cases for this "SHOULD" are when other mechanisms are used
to derive congestion thresholds, or when this is used in non-
Internet environments with known network capacities. In fixed-line
access links, a value of 500 ms is RECOMMENDED. Second, the value of
RTO SHOULD NOT be rounded up to the nearest second. Rather, a 1 ms
accuracy SHOULD be maintained. As with TCP, the usage of Karn's
algorithm is RECOMMENDED [TODO REF KARN87] which means that RTT
estimates SHOULD NOT be computed from transactions that result in the
retransmission of a request. The value for RTO is calculated
separately for each DTLS session.
Retransmissions continue until a response is received, or until a
total of 5 requests have been sent or there has been a hard ICMP
error [RFC1122]. The receiver knows a responses was received by
receiving and ACK with a sequence number that indicates it is a
response to one of the transmissions of this messages. For example,
assuming an RTO of 500 ms, requests would be sent at times 0 ms, 500
ms, 1500 ms, 3500 ms, and 7500 ms. If all retransmissions for a
message fail, the DTLS connection SHOULD be closed.
Once an ACK has been received for a message, the next messages can be
sent but the peer SHOULD ensure that there is at least 10 ms between
sending any two messages.
5.6.3. Fragmentation and Reassembly
In order to allow transmission over datagram protocols such as DTLS,
RELOAD messages may be fragmented. If a message is too large for a
peer to transmit to the next peer it MUST fragment the message. Note
that this implies that intermediate peers may re-fragment messages if
the incoming and outgoing paths have different maximum datagram
sizes. Intermediate peers SHOULD NOT reassemble fragments.
When a message is fragmented, each fragment has a full copy of the
forwarding header but the rest of the messages is split across the
fragments. The fragment offset value is stored in the lower 24 bits
of the fragment field of the forwarding header. The offset is the
number of bytes of the start of data from the end of the forwarding
header so the first fragment has an offset of 0. The first and last
bit indicators MUST be appropriately set. If the message is not
fragmented, then both the first and last fragment are set to 1 and
the offset is 0 resulting in a fragment value of 0xC0000000.
TODO - discuss how to size fragments to leave room for expansion of
forwarding header. Open Issue: Remove route log?
Upon receipt of a fragmented message by the intended peer, the peer
holds the fragments in a holding buffer until the entire message has
Jennings, et al. Expires September 8, 2009 [Page 71]
Internet-Draft RELOAD Base March 2009
been received. The message is then reassembled into a single message
and processed. In order to mitigate denial of service attacks,
receivers SHOULD time out incomplete fragments after 15 seconds.
Note the 15 seconds was derived from looking at the end to end
retransmission time and saving fragments long enough for the full end
to end retransmissions to take place. Ideally the receiver would
have enough buffer space to deal with storing 15 seconds worth of
fragments at whatever rate it receives messages on it89s interfaces,
however, if the receiver runs out of buffer space to reassemble the
messages it SHOULD close the DTLS session.
6. Data Storage Protocol
RELOAD provides a set of generic mechanisms for storing and
retrieving data in the Overlay Instance. These mechanisms can be
used for new applications simply by defining new code points and a
small set of rules. No new protocol mechanisms are required.
The basic unit of stored data is a single StoredData structure:
struct {
uint32 length;
uint64 storage_time;
uint32 lifetime;
StoredDataValue value;
Signature signature;
} StoredData;
The contents of this structure are as follows:
length
The length of the rest of the structure in octets.
storage_time
The time when the data was stored in absolute time, represented in
milliseconds since the Unix epoch of midnight Jan 1, 1970. Any
attempt to store a data value with a storage time before that of a
value already stored at this location MUST generate a
Error_Data_Too_Old error. This prevents rollback attacks. Note
that this does not require synchronized clocks: the receiving
peer uses the storage time in the previous store, not its own
clock.
Jennings, et al. Expires September 8, 2009 [Page 72]
Internet-Draft RELOAD Base March 2009
lifetime
The validity period for the data, in seconds, starting from the
time of store.
value
The data value itself, as described in Section 6.2.
signature
A signature over the data value. Section 6.1 describes the
signature computation. The element is formatted as described in
Section 5.3.4
Each Resource-ID specifies a single location in the Overlay Instance.
However, each location may contain multiple StoredData values
distinguished by Kind-ID. The definition of a kind describes both
the data values which may be stored and the data model of the data.
Some data models allow multiple values to be stored under the same
Kind-ID. Section Section 6.2 describes the available data models.
Thus, for instance, a given Resource-ID might contain a single-value
element stored under Kind-ID X and an array containing multiple
values stored under Kind-ID Y.
6.1. Data Signature Computation
Each StoredData element is individually signed. However, the
signature also must be self-contained and cover the Kind-ID and
Resource-ID even though they are not present in the StoredData
structure. The input to the signature algorithm is:
resource_id + kind + StoredData
Where these values are:
resource
The resource ID where this data is stored.
kind
The Kind-ID for this data.
StoredData
The contents of the stored data value, as described in the
previous sections, with the lifetime set to 0.
[OPEN ISSUE: Should we include the identity in the string that forms
the input to the signature algorithm?.]
Once the signature has been computed, the signature is represented
Jennings, et al. Expires September 8, 2009 [Page 73]
Internet-Draft RELOAD Base March 2009
using a signature element, as described in Section 5.3.4.
6.2. Data Models
The protocol currently defines the following data models:
o single value
o array
o dictionary
These are represented with the StoredDataValue structure:
enum { reserved(0), single_value(1), array(2),
dictionary(3), (255)} DataModel;
struct {
Boolean exists;
opaque value<0..2^32-1>;
} DataValue;
struct {
DataModel model;
select (model) {
case single_value:
DataValue single_value_entry;
case array:
ArrayEntry array_entry;
case dictionary:
DictionaryEntry dictionary_entry;
/* This structure may be extended */
} ;
} StoredDataValue;
We now discuss the properties of each data model in turn:
6.2.1. Single Value
A single-value element is a simple, opaque sequence of bytes. There
may be only one single-value element for each Resource-ID, Kind-ID
pair.
Jennings, et al. Expires September 8, 2009 [Page 74]
Internet-Draft RELOAD Base March 2009
A single value element is represented as a DataValue, which contains
the following two elements:
exists
This value indicates whether the value exists at all. If it is
set to False, it means that no value is present. If it is True,
that means that a value is present. This gives the protocol a
mechanism for indicating nonexistence as opposed to emptiness.
value
The stored data.
6.2.2. Array
An array is a set of opaque values addressed by an integer index.
Arrays are zero based. Note that arrays can be sparse. For
instance, a Store of "X" at index 2 in an empty array produces an
array with the values [ NA, NA, "X"]. Future attempts to fetch
elements at index 0 or 1 will return values with "exists" set to
False.
A array element is represented as an ArrayEntry:
struct {
uint32 index;
DataValue value;
} ArrayEntry;
The contents of this structure are:
index
The index of the data element in the array.
value
The stored data.
6.2.3. Dictionary
A dictionary is a set of opaque values indexed by an opaque key with
one value for each key. A single dictionary entry is represented as
follows
A dictionary element is represented as a DictionaryEntry:
Jennings, et al. Expires September 8, 2009 [Page 75]
Internet-Draft RELOAD Base March 2009
typedef opaque DictionaryKey<0..2^16-1>;
struct {
DictionaryKey key;
DataValue value;
} DictionaryEntry;
The contents of this structure are:
key
The dictionary key for this value.
value
The stored data.
6.3. Access Control Policies
Every kind which is storable in an overlay MUST be associated with an
access control policy. This policy defines whether a request from a
given node to operate on a given value should succeed or fail. It is
anticipated that only a small number of generic access control
policies are required. To that end, this section describes a small
set of such policies and Section 13.3 establishes a registry for new
policies if required. Each policy has a short string identifier
which is used to reference it in the configuration document.
6.3.1. USER-MATCH
In the USER-MATCH policy, a given value MUST be written (or
overwritten) if and only if the request is signed with a key
associated with a certificate whose user name hashes (using the hash
function for the overlay) to the Resource-ID for the resource.
Recall that the certificate may, depending on the overlay
configuration, be self-signed.
6.3.2. NODE-MATCH
In the NODE-MATCH policy, a given value MUST be written (or
overwritten) if and only if the request is signed with a key
associated with a certificate whose Node-ID hashes (using the hash
function for the overlay) to the Resource-ID for the resource.
6.3.3. USER-NODE-MATCH
The USER-NODE-MATCH policy may only be used with dictionary types.
In the USER-NODE-MATCH policy, a given value MUST be written (or
Jennings, et al. Expires September 8, 2009 [Page 76]
Internet-Draft RELOAD Base March 2009
overwritten) if and only if the request is signed with a key
associated with a certificate whose user name hashes (using the hash
function for the overlay) to the Resource-ID for the resource. In
addition, the dictionary key MUST be equal to the Node-ID in the
certificate.
6.3.4. NODE-MULTIPLE
In the NODE-MULTIPLE policy, a given value MUST be written (or
overwritten) if and only if the request is signed with a key
associated with a certificate containing a Node-ID such that
H(Node-ID || i) is equal to the Resource-ID for some small integer
value if i. When this policy is in use, the maximum value of i MUST
be specified, typically in the configuration document.
6.3.5. USER-MATCH-WITH-ANONYMOUS-CREATE
The USER-MATCH-WITH-ANONYMOUS-CREATE policy is like the USER-MATCH
policy except that any user MAY create a new value in a given
location. However, only a user matching the USER-MATCH criteria may
overwrite an existing value. This allows the creation of an
anonymous "drop box" which may be useful for applications like voice
mail.
6.4. Data Storage Methods
RELOAD provides several methods for storing and retrieving data:
o Store values in the overlay
o Fetch values from the overlay
o Find the values stored at an individual peer
These methods are each described in the following sections.
6.4.1. Store
The Store method is used to store data in the overlay. The format of
the Store request depends on the data model which is determined by
the kind.
6.4.1.1. Request Definition
A StoreReq message is a sequence of StoreKindData values, each of
which represents a sequence of stored values for a given kind. The
same Kind-ID MUST NOT be used twice in a given store request. Each
value is then processed in turn. These operations MUST be atomic.
If any operation fails, the state MUST be rolled back to before the
request was received.
Jennings, et al. Expires September 8, 2009 [Page 77]
Internet-Draft RELOAD Base March 2009
The store request is defined by the StoreReq structure:
struct {
KindId kind;
DataModel data_model;
uint64 generation_counter;
StoredData values<0..2^32-1>;
} StoreKindData;
struct {
ResourceId resource;
uint8 replica_number;
StoreKindData kind_data<0..2^32-1>;
} StoreReq;
A single Store request stores data of a number of kinds to a single
resource location. The contents of the structure are:
resource
The resource to store at.
replica_number
The number of this replica. When a storing peer saves replicas to
other peers each peer is assigned a replica number starting from 1
and sent in the Store message. This field is set to 0 when a node
is storing its own data. This allows peers to distinguish replica
writes from original writes.
kind_data
A series of elements, one for each kind of data to be stored.
If the replica number is zero, then the peer MUST check that it is
responsible for the resource and if not reject the request. If the
replica number is nonzero, then the peer MUST check that it expects
to be a replica for the resource and that the request sender is
consistent with being the responsible node (i.e., that the receiving
peer does not know of a better node) and if not reject the request.
Each StoreKindData element represents the data to be stored for a
single Kind-ID. The contents of the element are:
kind
The Kind-ID. Implementations SHOULD reject requests corresponding
to unknown kinds unless specifically configured otherwise.
Jennings, et al. Expires September 8, 2009 [Page 78]
Internet-Draft RELOAD Base March 2009
data_model
The data model of the data. The kind defines what this has to be
so this is redundant in the case where the software interpreting
the messages understands the kind.
generation
The expected current state of the generation counter
(approximately the number of times this object has been written,
see below for details).
values
The value or values to be stored. This may contain one or more
stored_data values depending on the data model associated with
each kind.
The peer MUST perform the following checks:
o The kind_id is known and supported.
o The data_model matches the kind_id.
o The signatures over each individual data element (if any) are
valid.
o Each element is signed by a credential which is authorized to
write this kind at this Resource-ID
o For original (non-replica) stores, the peer MUST check that if the
generation-counter is non-zero, it equals the current value of the
generation-counter for this kind. This feature allows the
generation counter to be used in a way similar to the HTTP Etag
feature.
o The storage time values are greater than that of any value which
would be replaced by this Store.
If all these checks succeed, the peer MUST attempt to store the data
values. For non-replica stores, if the store succeeds and the data
is changed, then the peer must increase the generation counter by at
least one. If there are multiple stored values in a single
StoreKindData, it is permissible for the peer to increase the
generation counter by only 1 for the entire Kind-ID, or by 1 or more
than one for each value. Accordingly, all stored data values must
have a generation counter of 1 or greater. 0 is used in the Store
request to indicate that the generation counter should be ignored for
processing this request, however the responsible peer should increase
the stored generation counter, and should return the correct
generation counter in the response.
For replica Stores, the peer MUST set the generation counter to match
the generation_counter in the message, and MUST NOT check the
generation counter against the current value. Replica Stores MUST
Jennings, et al. Expires September 8, 2009 [Page 79]
Internet-Draft RELOAD Base March 2009
NOT use a generation counter of 0.
When a peer stores data previously stored by another node (e.g., for
replicas or topology shifts) it MUST adjust the lifetime value
downward to reflect the amount of time the value was stored at the
peer.
The properties of stores for each data model are as follows:
Single-value:
A store of a new single-value element creates the element if it
does not exist and overwrites any existing value with the new
value.
Array:
A store of an array entry replaces (or inserts) the given value at
the location specified by the index. Because arrays are sparse, a
store past the end of the array extends it with nonexistent values
(exists=False) as required. A store at index 0xffffffff places
the new value at the end of the array regardless of the length of
the array. The resulting StoredData has the correct index value
when it is subsequently fetched.
Dictionary:
A store of a dictionary entry replaces (or inserts) the given
value at the location specified by the dictionary key.
The following figure shows the relationship between these structures
for an example store which stores the following values at resource
"1234"
o The value "abc" in the single value slot for kind X
o The value "foo" at index 0 in the array for kind Y
o The value "bar" at index 1 in the array for kind Y
Jennings, et al. Expires September 8, 2009 [Page 80]
Internet-Draft RELOAD Base March 2009
Store
resource=1234
/ \
/ \
StoreKindData StoreKindData
kind=X kind=Y
model=Single-Value model=Array
| /\
| / \
StoredData / \
| / \
| StoredData StoredData
StoredDataValue | |
value="abc" | |
| |
StoredDataValue StoredDataValue
index=0 index=1
value="foo" value="bar"
6.4.1.2. Response Definition
In response to a successful Store request the peer MUST return a
StoreAns message containing a series of StoreKindResponse elements
containing the current value of the generation counter for each
Kind-ID, as well as a list of the peers where the data was/will-be
replicated.
struct {
KindId kind;
uint64 generation_counter;
NodeId replicas<0..2^16-1>;
} StoreKindResponse;
struct {
StoreKindResponse kind_responses<0..2^16-1>;
} StoreAns;
The contents of each StoreKindResponse are:
kind
Jennings, et al. Expires September 8, 2009 [Page 81]
Internet-Draft RELOAD Base March 2009
The Kind-ID being represented.
generation
The current value of the generation counter for that Kind-ID.
replicas
The list of other peers at which the data was/will-be replicated.
In overlays and applications where the responsible peer is
intended to store redundant copies, this allows the storing peer
to independently verify that the replicas were in fact stored by
doing its own Fetch.
The response itself is just StoreKindResponse values packed end-to-
end.
If any of the generation counters in the request precede the
corresponding stored generation counter, then the peer MUST fail the
entire request and respond with a Error_Data_Too_Old error. The
error_info in the ErrorResponse MUST be a StoreAns response
containing the correct generation counter for each kind and empty
replicas lists.
If the data being stored is too large for the allowed limit by the
given usage, then the peer MUST fail the request and generate an
Error_Data_Too_Large error.
6.4.1.3. Removing Values
This version of RELOAD (unlike previous versions) does not have an
explicit Remove operation. Rather, values are Removed by storing
"nonexistent" values in their place. Each DataValue contains a
boolean value called "exists" which indicates whether a value is
present at that location. In order to effectively remove a value,
the owner stores a new DataValue with:
exists = false
value = {} (0 length)
Storing nodes MUST treat these nonexistent values the same way they
treat any other stored value, including overwriting the existing
value, replicating them, and aging them out as necessary when
lifetime expires. When a stored nonexistent value's lifetime
expires, it is simply removed from the storing node like any other
stored value expiration. Note that in the case of arrays and
dictionaries, this may create an implicit, unsigned "nonexistent"
value to represent a gap in the data structure. However, this value
isn't persistent nor is it replicated, it's simply synthesized by the
storing node.
Jennings, et al. Expires September 8, 2009 [Page 82]
Internet-Draft RELOAD Base March 2009
6.4.2. Fetch
The Fetch request retrieves one or more data elements stored at a
given Resource-ID. A single Fetch request can retrieve multiple
different kinds.
6.4.2.1. Request Definition
struct {
int32 first;
int32 last;
} ArrayRange;
struct {
KindId kind;
DataModel model;
uint64 generation;
uint16 length;
select (model) {
case single_value: ; /* Empty */
case array:
ArrayRange indices<0..2^16-1>;
case dictionary:
DictionaryKey keys<0..2^16-1>;
/* This structure may be extended */
} model_specifier;
} StoredDataSpecifier;
struct {
ResourceId resource;
StoredDataSpecifier specifiers<0..2^16-1>;
} FetchReq;
The contents of the Fetch requests are as follows:
resource
The resource ID to fetch from.
Jennings, et al. Expires September 8, 2009 [Page 83]
Internet-Draft RELOAD Base March 2009
specifiers
A sequence of StoredDataSpecifier values, each specifying some of
the data values to retrieve.
Each StoredDataSpecifier specifies a single kind of data to retrieve
and (if appropriate) the subset of values that are to be retrieved.
The contents of the StoredDataSpecifier structure are as follows:
kind
The Kind-ID of the data being fetched. Implementations SHOULD
reject requests corresponding to unknown kinds unless specifically
configured otherwise.
model
The data model of the data. This must be checked against the
Kind-ID.
generation
The last generation counter that the requesting peer saw. This
may be used to avoid unnecessary fetches or it may be set to zero.
length
The length of the rest of the structure, thus allowing
extensibility.
model_specifier
A reference to the data value being requested within the data
model specified for the kind. For instance, if the data model is
"array", it might specify some subset of the values.
The model_specifier is as follows:
o If the data is of data model single value, the specifier is empty.
o If the data is of data model array, the specifier contains of a
list of ArrayRange elements, each of which contains two integers.
The first integer is the beginning of the range and the second is
the end of the range. 0 is used to indicate the first element and
0xffffffff is used to indicate the final element. The beginning
of the range MUST be earlier in the array then the end. The
ranges MUST be non-overlapping.
o If the data is of data model dictionary then the specifier
contains a list of the dictionary keys being requested. If no
keys are specified, than this is a wildcard fetch and all key-
value pairs are returned.
The generation-counter is used to indicate the requester's expected
Jennings, et al. Expires September 8, 2009 [Page 84]
Internet-Draft RELOAD Base March 2009
state of the storing peer. If the generation-counter in the request
matches the stored counter, then the storing peer returns a response
with no StoredData values.
Note that because the certificate for a user is typically stored at
the same location as any data stored for that user, a requesting peer
which does not already have the user's certificate should request the
certificate in the Fetch as an optimization.
6.4.2.2. Response Definition
The response to a successful Fetch request is a FetchAns message
containing the data requested by the requester.
struct {
KindId kind;
uint64 generation;
StoredData values<0..2^32-1>;
} FetchKindResponse;
struct {
FetchKindResponse kind_responses<0..2^32-1>;
} FetchAns;
The FetchAns structure contains a series of FetchKindResponse
structures. There MUST be one FetchKindResponse element for each
Kind-ID in the request.
The contents of the FetchKindResponse structure are as follows:
kind
the kind that this structure is for.
generation
the generation counter for this kind.
values
the relevant values. If the generation counter in the request
matches the generation-counter in the stored data, then no
StoredData values are returned. Otherwise, all relevant data
values MUST be returned. A nonexistent value is represented with
"exists" set to False.
There is one subtle point about signature computation on arrays. If
the storing node uses the append feature (where the
index=0xffffffff), then the index in the StoredData that is returned
will not match that used by the storing node, which would break the
Jennings, et al. Expires September 8, 2009 [Page 85]
Internet-Draft RELOAD Base March 2009
signature. In order to avoid this issue, the index value in array is
set to zero before the signature is computed. This implies that
malicious storing nodes can reorder array entries without being
detected. [[OPEN ISSUE: We've considered a number of alternate
designs here that would preserve security against this attack if the
storing node did not use the append feature. However, they are more
complicated for one or both sides. If this attack is considered
serious, we can introduce one of them.]]
6.4.3. Stat
The Stat request is used to get metadata (length, generation counter,
digest, etc.) for a stored element without retrieving the element
itself. The name is from the UNIX stat(2) system call which performs
a similar function for files in a filesystem. It also allows the
requesting node to get a list of matching elements without requesting
the entire element.
6.4.3.1. Request Definition
The Stat request is identical to the Fetch request. It simply
specifies the elements to get metadata about.
struct {
ResourceId resource;
StoredDataSpecifier specifiers<0..2^16-1>;
} StatReq;
6.4.3.2. Response Definition
The Stat response contains the same sort of entries that a Fetch
response would contain, however instead of containing the element
data it contains metadata.
struct {
Boolean exists;
uint32 value_length;
HashAlgorithm hash_algorithm;
opaque hash_value<0..255>;
} MetaData;
struct {
uint32 index;
MetaData value;
Jennings, et al. Expires September 8, 2009 [Page 86]
Internet-Draft RELOAD Base March 2009
} ArrayEntryMeta;
struct {
DictionaryKey key;
MetaData value;
} DictionaryEntryMeta;
struct {
DataModel model;
select (model) {
case single_value:
MetaData single_value_entry;
case array:
ArrayEntryMeta array_entry;
case dictionary:
DictionaryEntryMeta dictionary_entry;
/* This structure may be extended */
} ;
} MetaDataValue;
struct {
uint32 length;
uint64 storage_time;
uint32 lifetime;
MetaDataValue metadata;
} StoredMetaData;
struct {
KindId kind;
uint64 generation;
StoredMetaData values<0..2^32-1>;
} StatKindResponse;
struct {
StatKindResponse kind_responses<0..2^32-1>;
} StatAns;
The structures used in StatAns parallel those used in FetchAns: a
response consists of multiple StatKindResponse values, one for each
kind that was in the request. The contents of the StatKindResponse
are the same as those in the FetchKindResponse, except that the
values list contains StoredMetaData entries instead of StoredData
entries.
Jennings, et al. Expires September 8, 2009 [Page 87]
Internet-Draft RELOAD Base March 2009
The contents of the StoredMetaData structure are the same as the
corresponding fields in StoredData except that there is no signature
field and the value is a MetaDataValue rather than a StoredDataValue.
A MetaDataValue is a variant structure, like a StoredDataValue,
except for the types of each arm, which replace DataValue with
MetaData.
The only really new structure is MetaData, which has the following
contents:
exists
Same as in DataValue
value_length
The length of the stored value.
hash_algorithm
The hash algorithm used to perform the digest of the value.
hash_value
A digest of the value using hash_algorithm.
6.4.4. Find
The Find request can be used to explore the Overlay Instance. A Find
request for a Resource-ID R and a Kind-ID T retrieves the Resource-ID
(if any) of the resource of kind T known to the target peer which is
closes to R. This method can be used to walk the Overlay Instance by
interactively fetching R_n+1=nearest(1 + R_n).
6.4.4.1. Request Definition
The FindReq message contains a series of Resource-IDs and Kind-IDs
identifying the resource the peer is interested in.
struct {
ResourceID resource;
KindId kinds<0..2^8-1>;
} FindReq;
The request contains a list of Kind-IDs which the Find is for, as
indicated below:
Jennings, et al. Expires September 8, 2009 [Page 88]
Internet-Draft RELOAD Base March 2009
resource
The desired Resource-ID
kinds
The desired Kind-IDs. Each value MUST only appear once.
6.4.4.2. Response Definition
A response to a successful Find request is a FindAns message
containing the closest Resource-ID for each kind specified in the
request.
struct {
KindId kind;
ResourceID closest;
} FindKindData;
struct {
FindKindData results<0..2^16-1>;
} FindAns;
If the processing peer is not responsible for the specified
Resource-ID, it SHOULD return a 404 error.
For each Kind-ID in the request the response MUST contain a
FindKindData indicating the closest Resource-ID for that Kind-ID
unless the kind is not allowed to be used with Find in which case a
FindKindData for that Kind-ID MUST NOT be included in the response.
If a Kind-ID is not known, then the corresponding Resource-ID MUST be
0. Note that different Kind-IDs may have different closest Resource-
IDs.
The response is simply a series of FindKindData elements, one per
kind, concatenated end-to-end. The contents of each element are:
kind
The Kind-ID.
closest
The closest resource ID to the specified resource ID. This is 0
if no resource ID is known.
Note that the response does not contain the contents of the data
stored at these Resource-IDs. If the requester wants this, it must
retrieve it using Fetch.
Jennings, et al. Expires September 8, 2009 [Page 89]
Internet-Draft RELOAD Base March 2009
6.4.5. Defining New Kinds
There are two ways to define a new kind. The first is by writing a
document and registering the kind-id with IANA. This is the
preferred method for kinds which may be widely used and reused. The
second method is to simply define the kind and its parameters in the
configuration document using the section of kind-id space set aside
for private use. This method MAY be used to define ad hoc kinds in
new overlays.
However a kind is defined, the definition must include:
o The meaning of the data to be stored (in some textual form).
o The Kind-ID.
o The data model (single value, array, dictionary, etc.)
o The access control model.
In addition, when kinds are registered with IANA, each kind is
assigned a short string name which is used to refer to it in
configuration documents.
While each kind MUST define what data model is used for its data,
that does not mean that it must define new data models. Where
practical, kinds SHOULD use the built-in data models. However, they
MAY define any new required data models. The intention is that the
basic data model set be sufficient for most applications/usages.
7. Certificate Store Usage
The Certificate Store usage allows a peer to store its certificate in
the overlay, thus avoiding the need to send a certificate in each
message - a reference may be sent instead.
A user/peer MUST store its certificate at Resource-IDs derived from
two Resource Names:
o The user name in the certificate.
o The Node-ID in the certificate.
Note that in the second case the certificate is not stored at the
peer's Node-ID but rather at a hash of the peer's Node-ID. The
intention here (as is common throughout RELOAD) is to avoid making a
peer responsible for its own data.
A peer MUST ensure that the user's certificates are stored in the
Overlay Instance. New certificates are stored at the end of the
list. This structure allows users to store and old and new
Jennings, et al. Expires September 8, 2009 [Page 90]
Internet-Draft RELOAD Base March 2009
certificate the both have the same Node-ID which allows for migration
of certificates when they are renewed.
This usage defines the following kind:
Name: CERTIFICATE
Data Model: The data model for CERTIFICATE data is array.
Access Control: NODE-MATCH.
8. TURN Server Usage
The TURN server usage allows a RELOAD peer to advertise that it is
prepared to be a TURN server as defined in [I-D.ietf-behave-turn].
When a node starts up, it joins the overlay network and forms several
connection in the process. If the ICE stage in any of these
connection return a reflexive address that is not the same as the
peers perceived address, then the peers is behind a NAT and not an
candidate for a TURN server. Additionally, if the peers IP address
is in the private address space range, then it is not a candidate for
a TURN server. Otherwise, the peer SHOULD assume it is a potential
TURN server and follow the procedures below.
If the node is a candidate for a TURN server it will insert some
pointers in the overlay so that other peers can find it. The overlay
configuration file specifies a turnDensity parameter that indicates
how many times each TURN server should record itself in the overlay.
Typically this should be set to the reciprocal of the estimate of
what percentage of peers will act as TURN servers. For each value,
called d, between 1 and turnDensity, the peer forms a Resource Name
by concatenating its Peer-ID and the value d. This Resource Name is
hashed to form a Resource-ID. The address of the peer is stored at
that Resource-ID using type TURN-SERVICE and the TurnServer object:
struct {
uint8 iteration;
IpAddressAndPort server_address;
} TurnServer;
The contents of this structure are as follows:
Jennings, et al. Expires September 8, 2009 [Page 91]
Internet-Draft RELOAD Base March 2009
iteration
the d value
server_address
the address at which the TURN server can be contacted.
Note: Correct functioning of this algorithm depends critically on
having turnDensity be an accurate estimate of the true density of
TURN servers. If turnDensity is too high, then the process of
finding TURN servers becomes extremely expensive as multiple
candidate Resource-IDs must be probed.
Peers peers that provide this service need to support the TURN
extensions to STUN for media relay of both UDP and TCP traffic as
defined in [I-D.ietf-behave-turn] and [RFC5382].
[[OPEN ISSUE: This structure only works for TURN servers that have
public addresses. It may be possible to use TURN servers that are
behind well-behaved NATs by first ICE connecting to them. If we
decide we want to enable that, this structure will need to change to
either be a Peer-ID or include that as an option.]]
This usage defines the following kind to indicate that the a peer is
willing to act as a TURN server:
Name TURN-SERVICE
Data Model The TURN-SERVICE kind stores a single value for each
Resource-ID.
Access Control NODE-MULTIPLE, with maximum iteration counter 20.
Peers can find other servers by selecting a random Resource-ID and
then doing a Find request for the appropriate server type with that
Resource-ID. The Find request gets routed to a random peer based on
the Resource-ID. If that peer knows of any servers, they will be
returned. The returned response may be empty if the peer does not
know of any servers, in which case the process gets repeated with
some other random Resource-ID. As long as the ratio of servers
relative to peers is not too low, this approach will result in
finding a server relatively quickly.
9. Chord Algorithm
This algorithm is assigned the name chord-128-2-16+ to indicate it is
based on Chord, uses SHA-1 then truncates that to 128 bit for the
hash function, stores 2 redundant copies of all data, and has finger
tables with at least 16 entries.
Jennings, et al. Expires September 8, 2009 [Page 92]
Internet-Draft RELOAD Base March 2009
9.1. Overview
The algorithm described here is a modified version of the Chord
algorithm. Each peer keeps track of a finger table of 16 entries and
a neighbor table of 6 entries. The neighbor table contains the 3
peers before this peer and the 3 peers after it in the DHT ring. The
first entry in the finger table contains the peer half-way around the
ring from this peer; the second entry contains the peer that is 1/4
of the way around; the third entry contains the peer that is 1/8th of
the way around, and so on. Fundamentally, the chord data structure
can be thought of a doubly-linked list formed by knowing the
successors and predecessor peers in the neighbor table, sorted by the
Node-ID. As long as the successor peers are correct, the DHT will
return the correct result. The pointers to the prior peers are kept
to enable inserting of new peers into the list structure. Keeping
multiple predecessor and successor pointers makes it possible to
maintain the integrity of the data structure even when consecutive
peers simultaneously fail. The finger table forms a skip list, so
that entries in the linked list can be found in O(log(N)) time
instead of the typical O(N) time that a linked list would provide.
A peer, n, is responsible for a particular Resource-ID k if k is less
than or equal to n and k is greater than p, where p is the peer id of
the previous peer in the neighbor table. Care must be taken when
computing to note that all math is modulo 2^128.
9.2. Reactive vs Periodic Recovery
Open Issue: The algorithm currently presented in this section uses
reactive recovery when a neighbor is lost, that information is
immediately propagated. Research in DHT performance by Rhea et al.
indicates that this is not optimal in large-scale networks with churn
[handling-churn-usenix04]. Addressing this issue, however, needs to
take into account the requirements placed on this algorithm. Because
it is the mandatory DHT for RELOAD, the algorithm described here is
designed to meet two primary challenges:
o Scale from small (ten or fewer) overlays on a LAN to global
overlays with millions of nodes
o Simple to implement
One of the challenges these requirements entail is achieving
reasonable performance as the overlay scales without undue
complexity. We have two possibly conflicting concerns:
o A small-scale overlay may not be stable without reactive recovery,
because a single peer represents a large portion of the overlay.
o A large-scale overlay with significant churn may perform poorly,
both in terms of traffic volume and success rates, when using
reactive recovery.
Jennings, et al. Expires September 8, 2009 [Page 93]
Internet-Draft RELOAD Base March 2009
As a result, multiple solutions have been proposed:
o Identify one set of behaviors that achieves adequate functionality
as the overlay scales.
o Add a parameter dictating the type of recovery used by peers in
the overlay, configuring the peers appropriately as they join the
overlay.
o Make the algorithm adaptive, according to the size of the overlay
or the churn rates observed.
At IETF 72, the WG elected to defer a decision on the final choice
until data could be collected on the effectiveness of the strategies.
This section, therefore, retains the reactive recovery model until
evidence supporting a decision is available.
9.3. Routing
If a peer is not responsible for a Resource-ID k, but is directly
connected to a node with Node-ID k, then it routes the message to
that node. Otherwise, it routes the request to the peer in the
routing table that has the largest Node-ID that is in the interval
between the peer and k. The routing table is the union of the
neighbor table and the finger table.
9.4. Redundancy
When a peer receives a Store request for Resource-ID k, and it is
responsible for Resource-ID k, it stores the data and returns a
success response. [[Open Issue: should it delay sending this
success until it has successfully stored the redundant copies?]]. It
then sends a Store request to its successor in the neighbor table and
to that peers successor. Note that these Store requests are
addressed to those specific peers, even though the Resource-ID they
are being asked to store is outside the range that they are
responsible for. The peers receiving these check they came from an
appropriate predecessor in their neighbor table and that they are in
a range that this predecessor is responsible for, and then they store
the data. They do not themselves perform further Stores because they
can determine that they are not responsible for the Resource-ID.
Note that a malicious node can return a success response but not
store the data locally or in the replica set. Requesting peers that
wish to ensure that the replication actually occurred SHOULD [[Open
Issue: SHOULD or MAY?]] contact each peer listed in the replicas
field of the Store response and retrieve a copy of the data.
Jennings, et al. Expires September 8, 2009 [Page 94]
Internet-Draft RELOAD Base March 2009
9.5. Joining
The join process for a joining party (JP) with Node-ID n is as
follows.
1. JP connects to its chosen bootstrap node.
2. JP uses a series of Probes to populate its routing table.
3. JP sends Attach requests to initiate connections to each of the
peers in the connection table as well as to the desired finger
table entries. Note that this does not populate their routing
tables, but only their connection tables, so JP will not get
messages that it is expected to route to other nodes.
4. JP enters all the peers it contacted into its routing table.
5. JP sends a Join to its immediate successor, the admitting peer
(AP) for Node-ID n. The AP sends the response to the Join.
6. AP does a series of Store requests to JP to store the data that
JP will be responsible for.
7. AP sends JP an Update explicitly labeling JP as its predecessor.
At this point, JP is part of the ring and responsible for a
section of the overlay. AP can now forget any data which is
assigned to JP and not AP.
8. AP sends an Update to all of its neighbors with the new values of
its neighbor set (including JP).
9. JP sends Updates to all the peers in its routing table.
In order to populate its routing table, JP sends a Probe via the
bootstrap node directed at Resource-ID n+1 (directly after its own
Resource-ID). This allows it to discover its own successor. Call
that node p0. It then sends a probe to p0+1 to discover its
successor (p1). This process can be repeated to discover as many
successors as desired. The values for the two peers before p will be
found at a later stage when n receives an Update.
In order to set up its neighbor table entry for peer i, JP simply
sends an Attach to peer (n+2^(numBitsInNodeId-i). This will be
routed to a peer in approximately the right location around the ring.
9.6. Routing Attaches
When a peer needs to Attach to a new peer in its neighbor table, it
MUST source-route the Attach request through the peer from which it
learned the new peer's Node-ID. Source-routing these requests allows
the overlay to recover from instability.
All other Attach requests, such as those for new finger table
entries, are routed conventionally through the overlay.
If a peer is unable to successfully Attach with a peer that should be
Jennings, et al. Expires September 8, 2009 [Page 95]
Internet-Draft RELOAD Base March 2009
in its neighborhood, it MUST locate either a TURN server or another
peer in the overlay, but not in its neighborhood, through which it
can exchange messages with its neighbor peer
9.7. Updates
A chord Update is defined as
enum { reserved (0),
peer_ready(1), neighbors(2), full(3), (255) }
ChordUpdateType;
struct {
ChordUpdateType type;
select(type){
case peer_ready: /* Empty */
;
case neighbors:
NodeId predecessors<0..2^16-1>;
NodeId successors<0..2^16-1>;
case full:
NodeId predecessors<0..2^16-1>;
NodeId successors<0..2^16-1>;
NodeId fingers<0..2^16-1>;
};
} ChordUpdate;
The "type" field contains the type of the update, which depends on
the reason the update was sent.
peer_ready: this peer is ready to receive messages. This message
is used to indicate that a node which has Attached is a peer and
can be routed through. It is also used as a connectivity check to
non-neighbor peers.
neighbors: this version is sent to members of the Chord neighbor
table.
full: this version is sent to peers which request an Update with a
RouteQueryReq.
If the message is of type "neighbors", then the contents of the
message will be:
Jennings, et al. Expires September 8, 2009 [Page 96]
Internet-Draft RELOAD Base March 2009
predecessors
The predecessor set of the Updating peer.
successors
The successor set of the Updating peer.
If the message is of type "full", then the contents of the message
will be:
predecessors
The predecessor set of the Updating peer.
successors
The successor set of the Updating peer.
fingers
The finger table if the Updating peer, in numerically ascending
order.
A peer MUST maintain an association (via Attach) to every member of
its neighbor set. A peer MUST attempt to maintain at least three
predecessors and three successors. However, it MUST send its entire
set in any Update message sent to neighbors.
9.7.1. Sending Updates
Every time a connection to a peer in the neighbor table is lost (as
determined by connectivity probes or failure of some request), the
peer should remove the entry from its neighbor table and replace it
with the best match it has from the other peers in its routing table.
It then sends an Update to all its remaining neighbors. The update
will contain all the Node-IDs of the current entries of the table
(after the failed one has been removed). Note that when replacing a
successor the peer SHOULD delay the creation of new replicas for 30
seconds after removing the failed entry from its neighbor table in
order to allow a triggered update to inform it of a better match for
its neighbor table.
If connectivity is lost to all three of the peers that follow this
peer in the ring, then this peer should behave as if it is joining
the network and use Probes to find a peer and send it a Join. If
connectivity is lost to all the peers in the finger table, this peer
should assume that it has been disconnected from the rest of the
network, and it should periodically try to join the DHT.
Jennings, et al. Expires September 8, 2009 [Page 97]
Internet-Draft RELOAD Base March 2009
9.7.2. Receiving Updates
When a peer, N, receives an Update request, it examines the Node-IDs
in the UpdateReq and at its neighbor table and decides if this
UpdateReq would change its neighbor table. This is done by taking
the set of peers currently in the neighbor table and comparing them
to the peers in the update request. There are three major cases:
o The UpdateReq contains peers that would not change the neighbor
set because they match the neighbor table.
o The UpdateReq contains peers closer to N than those in its
neighbor table.
o The UpdateReq defines peers that indicate a neighbor table further
away from N than some of its neighbor table. Note that merely
receiving peers further away does not demonstrate this, since the
update could be from a node far away from N. Rather, the peers
would need to bracket N.
In the first case, no change is needed.
In the second case, N MUST attempt to Attach to the new peers and if
it is successful it MUST adjust its neighbor set accordingly. Note
that it can maintain the now inferior peers as neighbors, but it MUST
remember the closer ones.
The third case implies that a neighbor has disappeared, most likely
because it has simply been disconnected but perhaps because of
overlay instability. N MUST Probe the questionable peers to discover
if they are indeed missing and if so, remove them from its neighbor
table.
After any Probes and Attaches are done, if the neighbor table
changes, the peer sends an Update request to each of its neighbors
that was in either the old table or the new table. These Update
requests are what ends up filling in the predecessor/successor tables
of peers that this peer is a neighbor to. A peer MUST NOT enter
itself in its successor or predecessor table and instead should leave
the entries empty.
If peer N which is responsible for a Resource-ID R discovers that the
replica set for R (the next two nodes in its successor set) has
changed, it MUST send a Store for any data associated with R to any
new node in the replica set. It SHOULD NOT delete data from peers
which have left the replica set.
When a peer N detects that it is no longer in the replica set for a
resource R (i.e., there are three predecessors between N and R), it
SHOULD delete all data associated with R from its local store.
Jennings, et al. Expires September 8, 2009 [Page 98]
Internet-Draft RELOAD Base March 2009
9.7.3. Stabilization
There are four components to stabilization:
1. exchange Updates with all peers in its routing table to exchange
state
2. search for better peers to place in its finger table
3. search to determine if the current finger table size is
sufficiently large
4. search to determine if the overlay has partitioned and needs to
recover
A peer MUST periodically send an Update request to every peer in its
routing table. The purpose of this is to keep the predecessor and
successor lists up to date and to detect connection failures. The
default time is about every ten minutes, but the enrollment server
SHOULD set this in the configuration document using the "chord-128-2-
16+-update-frequency" element (denominated in seconds.) A peer
SHOULD randomly offset these Update requests so they do not occur all
at once. If an Update request fails or times out, the peer MUST mark
that entry in the neighbor table invalid and attempt to reestablish a
connection. If no connection can be established, the peer MUST
attempt to establish a new peer as its neighbor and do whatever
replica set adjustments are required. If a finger table entry is
found to have failed, the peer MUST search for a replacement as
directed below.
A peer MUST periodically select a random entry i from the finger
table and evaluate whether that entry should be replaced. The
default time interval is about every hour, but the enrollment server
SHOULD set this in the configuration document using the "chord-128-2-
16+-probe-frequency" element (denominated in seconds).
To evaluate whether the i'th finger table entry needs to be replaced,
if the Node-ID of the entry is not valid for that finger table entry,
the peer SHOULD search for a better entry. A peer searches for a
better entry using a Probe request. If the Probe returns a different
peer than the one currently in this entry of the finger table, then a
new connection should be formed to replace the old entry in the
finger table.
A peer SHOULD consider the finger table entry valid if it is in the
range [n+2^(numBitsInNodeId-i), n+2^(numBitsInNodeId-(i-1))-
2^(numBitsInNodeId-(i+1))]. When searching for a better entry, the
peer SHOULD send the Probe to a Node-ID selected randomly from that
range. Random selection is preferred over a search for strictly
spaced entries to minimize the effect of churn on overlay routing
[minimizing-churn-sigcomm06]. An implementation or subsequent
specification MAY choose a method for selecting finger table entries
Jennings, et al. Expires September 8, 2009 [Page 99]
Internet-Draft RELOAD Base March 2009
other than choosing randomly within the range. It is RECOMMENDED
that any such alternate methods be employed only on finger table
stabilization and not for the selection of initial finger table
entries unless the alternative method is faster and imposes less
overhead on the overlay.
As an overlay grows, more than 16 entries may be required in the
finger table for efficient routing. To determine if its finger table
is sufficiently large, once an hour the peer should perform a Probe
to determine whether growing its finger table by four entries would
result in it learning at least two peers that it does not already
have in its neighbor table. If so, then the finger table SHOULD be
grown by four entries. Similarly, if the peer observes that its
closest finger table entries are also in its neighbor table, it MAY
shrink its finger table to the minimum size of 16 entries. [[OPEN
ISSUE: there are a variety of algorithms to gauge the population of
the overlay and select an appropriate finger table size. Need to
consider which is the best combination of effectiveness and
simplicity. Also, an example would help here.]]
To detect that a partitioning has occurred and to heal the overlay, a
peer P MUST periodically repeat the discovery process used in the
initial join for the overlay to locate an appropriate bootstrap peer,
B. If an overlay has multiple mechanisms for discovery it should
randomly select a method to locate a bootstrap peer. P should then
send a Probe for its own Node-ID routed through B. If a response is
received from a peer S', which is not P's successor, then the overlay
is partitioned and P should send a Attach to S' routed through B,
followed by an Update sent to S'. (Note that S' may not be in P's
neighbor table once the overlay is healed, but the connection will
allow S' to discover appropriate neighbor entries for itself via its
own stabilization.)
9.8. Route Query
For this topology plugin, the RouteQueryReq contains no additional
information. The RouteQueryAns contains the single node ID of the
next peer to which the responding peer would have routed the request
message in recursive routing:
struct {
NodeId next_id;
} ChordRouteQueryAns;
The contents of this structure are as follows:
Jennings, et al. Expires September 8, 2009 [Page 100]
Internet-Draft RELOAD Base March 2009
next_peer
The peer to which the responding peer would route the message to
in order to deliver it to the destination listed in the request.
If the requester set the send_update flag, the responder SHOULD
initiate an Update immediately after sending the RouteQueryAns.
9.9. Leaving
Peers SHOULD send a Leave request prior to exiting the Overlay
Instance. Any peer which receives a Leave for a peer n in its
neighbor set must remove it from the neighbor set, update its replica
sets as appropriate (including Stores of data to new members of the
replica set) and send Updates containing its new predecessor and
successor tables.
10. Enrollment and Bootstrap
10.1. Overlay Configuration
This specification defines a new content type "application/
p2p-overlay+xml" for an MIME entity that contains overlay
information. An example document is shown below.
Jennings, et al. Expires September 8, 2009 [Page 101]
Internet-Draft RELOAD Base March 2009
false
192.0.0.1:5678
192.0.2.2:6789
30
false
4000
https://example.org
foo
192.0.0.3:5678
300
400
false
asecret
Chord-128-2-16
TODO
single
user-match
1
100
array
user-match
22
4
1
TODO BASE 64 encoded signature block
The file MUST be a well formed XML document and it SHOULD contain an
encoding declaration in the XML declaration. If the charset
parameter of the MIME content type declaration is present and it is
different from the encoding declaration, the charset parameter takes
precedence. Every application conforming to this specification MUST
accept the UTF-8 character encoding to ensure minimal
Jennings, et al. Expires September 8, 2009 [Page 102]
Internet-Draft RELOAD Base March 2009
interoperability. The namespace for the elements defined in this
specification is urn:ietf:params:xml:ns:p2p:config-base and
urn:ietf:params:xml:ns:p2p:config-chord-128-2".
The file can contain multiple "configuration" elements where each one
contains the configuration information for a different overlay. Each
"configuration" has the following attributes:
instance-name: name of the overlay
expiration: time in future at which this overlay configuration is
not longer valid and need to be retrieved again
sequence: a monotonically increasing sequence number between 1 and
65534
Inside each overlay element, the following elements can occur:
topology-plugin This element has an attribute called algorithm-name
that describes the overlay-algorithm being used.
root-cert This element contains a PEM encoded X.509v3 certificate
that is the root trust store used to sign all certificates in this
overlay. There can be more than one of these.
required-kinds This element indicates the kinds that members must
support. It has three attributes:
* kind: either a string representing the kind (the name
registered to IANA) or an integer kind-id allocated out of
private space
* max-count: the maximum number of values which members of the
overlay must support.
* data-model: the data model to be used.
* max-size: the maximum size of individual values.
* access-control: the access control model to be used.
All of these values MUST be provided. If the kind is registered
with IANA, the data-model and access-control attributes MUST match
those in the kind registration. For instance, the example above
indicates that members must support SIP-REGISTRATION with a
maximum of 10 values of up to 1000 bytes each. Multiple required-
kinds elements MAY be present. [TODO: we need some way to
indicate iteration counters for NODE-MULTIPLE. Can some XML
wizard help?]
credential-server This element contains the URL at which the
credential server can be reached in a "url" element. This URL
MUST be of type "https:". More than one credential-server element
may be present.
self-signed-permitted This element indicates whether self-signed
certificates are permitted. If it is set to "true", then self-
signed certificates are allowed, in which case the credential-
server and root-cert elements may be absent. Otherwise, it SHOULD
be absent, but MAY be set "false". This element also contains an
Jennings, et al. Expires September 8, 2009 [Page 103]
Internet-Draft RELOAD Base March 2009
attribute "digest" which indicates the digest to be used to
compute the Node-ID. Valid values for this parameter are "SHA-1"
and "SHA-256".
bootstrap-peer This elements represents the address of one of the
bootstrap peers. It has an attribute called "address" that
represents the IP address (either IPv4 or IPv6, since they can be
distinguished) and an attribute called "port" that represents the
port. More than one bootstrap-peer element may be present.
multicast-bootstrap This element represents the address of a
multicast address and port that may be used for bootstrap and that
peers SHOULD listen on to enable bootstrap. It has an attributed
called "address" that represents the IP address and an attribute
called "port" that represents the port. More than one "multicast-
bootstrap" element may be present.
clients-permitted This element represents whether clients are
permitted or whether all nodes must be peers. If it is set to
"TRUE" or absent, this indicates that clients are permitted. If
it is set to "FALSE" then nodes MUST join as peers.
attach-lite-permitted This element represents whether nodes are
allowed to use the AttachLite request in this overlay. If it is
absent, it is treated as if it was set to "FALSE".
chord-128-2-16+-update-frequency The update frequency for the
Chord-128-2-16+ topology plugin (see Section 9).
chord-128-2-16+-probe-frequency The probe frequency for the Chord-
128-2-16+ topology plugin (see Section 9).
credential-server Base URL for credential server.
shared-secret If shared secret mode is used, this contains the
shared secret.
max-message-size Maximum size in bytes of any message in the
overlay. If this value is not present, the default is 5000.
initial-ttl Initial default TTL (time to live, see section XXX) for
messages. If this value is not present, the default is 100.
The configuration file is a binary file and can not be changed,
including whitepsace changes or the signature will break. The
signature is computed by taking each configuration element and
starting form, and including, the first < at the start of
up to and including the > in and
treating this as a binary blob thats sigend using the standard
SecurityBlock defined in Section 5.3.4. The SecurityBlock is base 64
encoded using base64 alphabet from RFC[RFC4648] and put in the
signature element following the configuration object in the config
file.
10.1.1. Relax NG Grammars
The grammar for the configuration data is:
Jennings, et al. Expires September 8, 2009 [Page 104]
Internet-Draft RELOAD Base March 2009
namespace chord = "urn:ietf:params:xml:ns:p2p:config-chord-128-2"
namespace local = ""
default namespace p2pcf = "urn:ietf:params:xml:ns:p2p:config-base"
namespace rng = "http://relaxng.org/ns/structure/1.0"
anything =
(element * { anything }
| attribute * { text }
| text)*
foreign-elements = element * - (p2pcf:* | local:* | chord:*) { anything }*
hostPort = text
start =
element p2pcf:overlay {
element configuration {
attribute instance-name { text },
attribute expiration { xsd:dateTime },
attribute sequence { xsd:long },
parameter
},
element signature {
attribute algorithm { signature-algorithm-type }?,
xsd:base64Binary
}?
}
signature-algorithm-type |= "rsa-sha1"
parameter &= element topology-plugin { topology-plugin-type }
parameter &= element max-message-size { xsd:int }?
parameter &= element initial-ttl { xsd:int }?
parameter &= element root-cert { text }?
parameter &= element required-kinds { kinds* }
parameter &= element credential-server { xsd:anyURI }?
parameter &=
element self-signed-permitted {
attribute digest { self-signed-digest-type },
xsd:boolean
}?
self-signed-digest-type |= "sha1"
parameter &=
element bootstrap-peer { hostPort
}+
parameter &=
element multicast-bootstrap { hostPort
}*
parameter &= element clients-permitted { xsd:boolean }?
parameter &= element attach-lite-permitted { xsd:boolean }?
parameter &= element shared-secret { xsd:string }?
parameter &= foreign-elements*
kinds =
Jennings, et al. Expires September 8, 2009 [Page 105]
Internet-Draft RELOAD Base March 2009
element kind {
(attribute name { kind-names }
| attribute id { xsd:int }),
kind-paramter
}
kind-paramter &= element max-count { xsd:int }
kind-paramter &= element max-size { xsd:int }
kind-paramter &= element data-model { data-model-type }
data-model-type |= "single"
data-model-type |= "array"
data-model-type |= "dictionary"
kind-paramter &= element access-control { access-control-type }
kind-paramter &= element max-node-muliple { xsd:int }
access-control-type |= "user-match"
access-control-type |= "node-match"
access-control-type |= "user-node-match"
access-control-type |= "node-multiple"
access-control-type |= "user-match-with-anon-create"
kind-paramter &= foreign-elements*
# Chord specific paramters
topology-plugin-type |= "Chord-128-2-16"
kind-names |= "sip-registration"
kind-names |= "turn-service"
parameter &= element chord:update-frequency { xsd:int }?
parameter &= element chord:probe-frequency { xsd:int }?
10.2. Discovery Through Enrollment Server
When a peer first joins a new overlay, it starts with a discovery
process to find an enrollment server. Related work to the approach
used here is described in [I-D.garcia-p2psip-dns-sd-bootstrapping]
and [I-D.matthews-p2psip-bootstrap-mechanisms]. Another scheme for
referencing overlays is described in
[I-D.hardie-p2poverlay-pointers]. The peer first determines the
overlay name. This value is provided by the user or some other out
of band provisioning mechanism. If the name is an IP address, that
is directly used otherwise the peer MUST do a DNS SRV query using a
Service name of "p2p_enroll" and a protocol of tcp to find an
enrollment server.
Once an address for the enrollment servers is determined, the peer
forms an HTTPS connection to that IP address. The certificate MUST
match the overlay name as described in [RFC2818].
Whenever a peer contacts the enrollment server, it MUST fetch a new
copy of the configuration file. To do this, the peer performs a GET
to the URL formed by appending a path of "/p2psip/enroll" to the
overlay name. For example, if the overlay name was example.com, the
Jennings, et al. Expires September 8, 2009 [Page 106]
Internet-Draft RELOAD Base March 2009
URL would be "https://example.com/p2psip/enroll". The result is an
XML configuration file described above, which replaces any previously
learned configuration file for this overlay.
[[OPEN ISSUE: for unsecured overlays or overlays not specified by
domain name, need to specify another way to obtain/validate certs and
to update configuration info]]
10.3. Credentials
If the configuration document contains a credential-server element,
credentials are required to join the Overlay Instance. A peer which
does not yet have credentials MUST contact the credential server to
acquire them.
RELOAD defines its own trivial certificate request protocol. We
would have liked to use an existing protocol, but were concerned
about the implementation burden of even the simplest of those
protocols, such as [RFC5272]) and [RFC5273]. Our objective was to
have a protocol which could be easily implemented in a Web server
which the operator did not control (e.g., in a hosted service) and
was compatible with the existing certificate handling tooling as used
with the Web certificate infrastructure. This means accepting bare
PKCS#10 requests and returning a single bare X.509 certificate.
Although the MIME types for these objects are defined, none of the
existing protocols support exactly this model.
The certificate request protocol is performed over HTTPS. The
request is an HTTP POST with the following properties:
o If authentication is required, there is a URL parameter of
"password" containing the user's password in the clear (hence the
need for HTTPS)
o The body is of content type "application/pkcs10", as defined in
[RFC2311].
o The Accept header contains the type "application/pkix-cert",
indicating the type that is expected in the response.
The credential server MUST authenticate the request using the
provided user name and password. If the authentication succeeds and
the requested user name is acceptable, the server and returns a
certificate. The SubjectAltName field in the certificate contains
the following values:
o One or more Node-IDs which MUST be cryptographically random
[RFC4086]. These MUST be chosen by the credential server in such
a way that they are unpredictable to the requesting user. These
are of type URI and MUST contain RELOAD URIs as described in
Jennings, et al. Expires September 8, 2009 [Page 107]
Internet-Draft RELOAD Base March 2009
Section 13.12 and MUST contain a Destination list with a single
entry of type "node_id".
o The names this user is allowed to use in the overlay, using type
rfc822Name.
The certificate is returned as type "application/pkix-cert", with an
HTTP status code of 200 OK. Certificate processing errors should be
treated as HTTP errors and have appropriate HTTP stats codes. [TODO:
There needs to be some text here about how the interaction with other
HTTP features works. This awaits the example from the apps ADs with
HELD.]
The client MUST check that the certificate returned was signed by one
of the certificates received in the "root-cert" list of the overlay
configuration data. The peer then reads the certificate to find the
Node-IDs it can use.
10.3.1. Self-Generated Credentials
If the "self-signed-permitted" element is present and set to "TRUE",
then a node MUST generate its own self-signed certificate to join the
overlay. The self-signed certificate MAY contain any user name of
the users choice. Users SHOULD make some attempt to make it unique
but this document does not specify any mechanisms for that.
The Node-ID MUST be computed by applying the digest specified in the
self-signed-permitted element to the DER representation of the user's
public key. When accepting a self-signed certificate, nodes MUST
check that the Node-ID and public keys match. This prevents Node-ID
theft.
Once the node has constructed a self-signed certificate, it MAY join
the overlay. Before storing its certificate in the overlay
(Section 7) it SHOULD look to see if the user name is already taken
and if so choose another user name. Note that this only provides
protection against accidental name collisions. Name theft is still
possible. If protection against name theft is desired, then the
enrollment service must be used.
10.4. Joining the Overlay Peer
In order to join the overlay, the peer MUST contact a peer.
Typically this means contacting the bootstrap peers, since they are
guaranteed to have public IP addresses (the system should not
advertise them as bootstrap peers otherwise). If the peer has cached
peers it SHOULD contact them first by sending a Probe request to the
known peer address with the destination Node-ID set to that peer's
Node-ID.
Jennings, et al. Expires September 8, 2009 [Page 108]
Internet-Draft RELOAD Base March 2009
If no cached peers are available, then the peer SHOULD send a Probe
request to the address and port found in the broadcast-peers element
in the configuration document. This MAY be a multicast or anycast
address. The Probe should use the wildcard Node-ID as the
destination Node-ID.
The responder peer that receives the Probe request SHOULD check that
the overlay name is correct and that the requester peer sending the
request has appropriate credentials for the overlay before responding
to the Probe request even if the response is only an error.
When the requester peer finally does receive a response from some
responding peer, it can note the Node-ID in the response and use this
Node-ID to start sending requests to join the Overlay Instance as
described in Section 5.4.
After a peer has successfully joined the overlay network, it SHOULD
periodically look at any peers to which it has managed to form direct
connections. Some of these peers MAY be added to the cached-peers
list and used in future boots. Peers that are not directly connected
MUST NOT be cached. The RECOMMENDED number of peers to cache is 10.
11. Message Flow Example
In the following example, we assume that JP has formed a connection
to one of the bootstrap peers. JP then sends an Attach through that
peer to the admitting peer (AP) to initiate a connection. When AP
responds, JP and AP use ICE to set up a connection and then set up
TLS.
Jennings, et al. Expires September 8, 2009 [Page 109]
Internet-Draft RELOAD Base March 2009
JP PPP PP AP NP NNP BP
| | | | | | |
| | | | | | |
| | | | | | |
|Attach Dest=JP | | | | |
|---------------------------------------------------------->|
| | | | | | |
| | | | | | |
| | |Attach Dest=JP | | |
| | |<--------------------------------------|
| | | | | | |
| | | | | | |
| | |Attach Dest=JP | | |
| | |-------->| | | |
| | | | | | |
| | | | | | |
| | |AttachAns | | |
| | |<--------| | | |
| | | | | | |
| | | | | | |
| | |AttachAns | | |
| | |-------------------------------------->|
| | | | | | |
| | | | | | |
|AttachAns | | | | |
|<----------------------------------------------------------|
| | | | | | |
| | | | | | |
|TLS | | | | | |
|.............................| | | |
| | | | | | |
| | | | | | |
| | | | | | |
| | | | | | |
Once JP has connected to AP, it needs to populate its Routing Table.
In Chord, this means that it needs to populate its neighbor table and
its finger table. To populate its neighbor table, it needs the
successor of AP, NP. It sends an Attach to the Resource-IP AP+1,
which gets routed to NP. When NP responds, JP and NP use ICE and TLS
to set up a connection.
[[TODO: there should be a Probe here before populating]]
Jennings, et al. Expires September 8, 2009 [Page 110]
Internet-Draft RELOAD Base March 2009
JP PPP PP AP NP NNP BP
| | | | | | |
| | | | | | |
| | | | | | |
|Attach AP+1 | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
| | | |Attach AP+1 | |
| | | |-------->| | |
| | | | | | |
| | | | | | |
| | | |AttachAns | |
| | | |<--------| | |
| | | | | | |
| | | | | | |
|AttachAns | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|Attach | | | | | |
|-------------------------------------->| | |
| | | | | | |
| | | | | | |
|TLS | | | | | |
|.......................................| | |
| | | | | | |
| | | | | | |
| | | | | | |
| | | | | | |
JP also needs to populate its finger table (for Chord). It issues a
Attach to a variety of locations around the overlay. The diagram
below shows it sending an Attach halfway around the Chord ring the JP
+ 2^127.
Jennings, et al. Expires September 8, 2009 [Page 111]
Internet-Draft RELOAD Base March 2009
JP NP XX TP
| | | |
| | | |
| | | |
|Attach JP+2<<126 | |
|-------->| | |
| | | |
| | | |
| |Attach JP+2<<126 |
| |-------->| |
| | | |
| | | |
| | |Attach JP+2<<126
| | |-------->|
| | | |
| | | |
| | |AttachAns|
| | |<--------|
| | | |
| | | |
| |AttachAns| |
| |<--------| |
| | | |
| | | |
|AttachAns| | |
|<--------| | |
| | | |
| | | |
|TLS | | |
|.............................|
| | | |
| | | |
| | | |
| | | |
Once JP has a reasonable set of connections he is ready to take his
place in the DHT. He does this by sending a Join to AP. AP does a
series of Store requests to JP to store the data that JP will be
responsible for. AP then sends JP an Update explicitly labeling JP
as its predecessor. At this point, JP is part of the ring and
responsible for a section of the overlay. AP can now forget any data
which is assigned to JP and not AP.
Jennings, et al. Expires September 8, 2009 [Page 112]
Internet-Draft RELOAD Base March 2009
JP PPP PP AP NP NNP BP
| | | | | | |
| | | | | | |
| | | | | | |
|JoinReq | | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
|JoinAns | | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|StoreReq Data A | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|StoreAns | | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
|StoreReq Data B | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|StoreAns | | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
|UpdateReq| | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|UpdateAns| | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
| | | | | | |
| | | | | | |
In Chord, JP's neighbor table needs to contain its own predecessors.
It couldn't connect to them previously because Chord has no way to
route immediately to your predecessors. However, now that it has
received an Update from AP, it has APs predecessors, which are also
its own, so it sends Attaches to them. Below it is shown connecting
to its closest predecessor, PP.
Jennings, et al. Expires September 8, 2009 [Page 113]
Internet-Draft RELOAD Base March 2009
JP PPP PP AP NP NNP BP
| | | | | | |
| | | | | | |
| | | | | | |
|Attach Dest=PP | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
| | |Attach Dest=PP | | |
| | |<--------| | | |
| | | | | | |
| | | | | | |
| | |AttachAns| | | |
| | |-------->| | | |
| | | | | | |
| | | | | | |
|AttachAns| | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|TLS | | | | | |
|...................| | | | |
| | | | | | |
| | | | | | |
|UpdateReq| | | | | |
|------------------>| | | | |
| | | | | | |
| | | | | | |
|UpdateAns| | | | | |
|<------------------| | | | |
| | | | | | |
| | | | | | |
|UpdateReq| | | | | |
|---------------------------->| | | |
| | | | | | |
| | | | | | |
|UpdateAns| | | | | |
|<----------------------------| | | |
| | | | | | |
| | | | | | |
|UpdateReq| | | | | |
|-------------------------------------->| | |
| | | | | | |
| | | | | | |
|UpdateAns| | | | | |
|<--------------------------------------| | |
| | | | | | |
| | | | | | |
Jennings, et al. Expires September 8, 2009 [Page 114]
Internet-Draft RELOAD Base March 2009
Finally, now that JP has a copy of all the data and is ready to route
messages and receive requests, it sends Updates to everyone in its
Routing Table to tell them it is ready to go. Below, it is shown
sending such an update to TP.
JP NP XX TP
| | | |
| | | |
| | | |
|Update | | |
|---------------------------->|
| | | |
| | | |
|UpdateAns| | |
|<----------------------------|
| | | |
| | | |
| | | |
| | | |
12. Security Considerations
12.1. Overview
RELOAD provides a generic storage service, albeit one designed to be
useful for P2PSIP. In this section we discuss security issues that
are likely to be relevant to any usage of RELOAD.
In any Overlay Instance, any given user depends on a number of peers
with which they have no well-defined relationship except that they
are fellow members of the Overlay Instance. In practice, these other
nodes may be friendly, lazy, curious, or outright malicious. No
security system can provide complete protection in an environment
where most nodes are malicious. The goal of security in RELOAD is to
provide strong security guarantees of some properties even in the
face of a large number of malicious nodes and to allow the overlay to
function correctly in the face of a modest number of malicious nodes.
P2PSIP deployments require the ability to authenticate both peers and
resources (users) without the active presence of a trusted entity in
the system. We describe two mechanisms. The first mechanism is
based on public key certificates and is suitable for general
deployments. The second is an admission control mechanism based on
an overlay-wide shared symmetric key.
Jennings, et al. Expires September 8, 2009 [Page 115]
Internet-Draft RELOAD Base March 2009
12.2. Attacks on P2P Overlays
The two basic functions provided by overlay nodes are storage and
routing: some node is responsible for storing a peer's data and for
allowing a peer to fetch other peer's data. Some other set of nodes
are responsible for routing messages to and from the storing nodes.
Each of these issues is covered in the following sections.
P2P overlays are subject to attacks by subversive nodes that may
attempt to disrupt routing, corrupt or remove user registrations, or
eavesdrop on signaling. The certificate-based security algorithms we
describe in this draft are intended to protect overlay routing and
user registration information in RELOAD messages.
To protect the signaling from attackers pretending to be valid peers
(or peers other than themselves), the first requirement is to ensure
that all messages are received from authorized members of the
overlay. For this reason, RELOAD transports all messages over a
secure channel (TLS and DTLS are defined in this document) which
provides message integrity and authentication of the directly
communicating peer. In addition, messages and data are digitally
signed with the sender's private key, providing end-to-end security
for communications.
12.3. Certificate-based Security
This specification stores users' registrations and possibly other
data in an overlay network. This requires a solution to securing
this data as well as securing, as well as possible, the routing in
the overlay. Both types of security are based on requiring that
every entity in the system (whether user or peer) authenticate
cryptographically using an asymmetric key pair tied to a certificate.
When a user enrolls in the Overlay Instance, they request or are
assigned a unique name, such as "alice@dht.example.net". These names
are unique and are meant to be chosen and used by humans much like a
SIP Address of Record (AOR) or an email address. The user is also
assigned one or more Node-IDs by the central enrollment authority.
Both the name and the peer ID are placed in the certificate, along
with the user's public key.
Each certificate enables an entity to act in two sorts of roles:
o As a user, storing data at specific Resource-IDs in the Overlay
Instance corresponding to the user name.
o As a overlay peer with the peer ID(s) listed in the certificate.
Note that since only users of this Overlay Instance need to validate
Jennings, et al. Expires September 8, 2009 [Page 116]
Internet-Draft RELOAD Base March 2009
a certificate, this usage does not require a global PKI. Instead,
certificates are signed by require a central enrollment authority
which acts as the certificate authority for the Overlay Instance.
This authority signs each peer's certificate. Because each peer
possesses the CA's certificate (which they receive on enrollment)
they can verify the certificates of the other entities in the overlay
without further communication. Because the certificates contain the
user/peer's public key, communications from the user/peer can be
verified in turn.
If self-signed certificates are used, then the security provided is
significantly decreased, since attackers can mount Sybil attacks. In
addition, attackers cannot trust the user names in certificates
(though they can trust the Node-IDs because they are
cryptographically verifiable). This scheme is only appropriate for
small deployments, such as a small office or ad hoc overlay set up
among participants in a meeting. Some additional security can be
provided by using the shared secret admission control scheme as well.
Because all stored data is signed by the owner of the data the
storing peer can verify that the storer is authorized to perform a
store at that Resource-ID and also allows any consumer of the data to
verify the provenance and integrity of the data when it retrieves it.
All implementations MUST implement certificate-based security.
12.4. Shared-Secret Security
RELOAD also supports a shared secret admission control scheme that
relies on a single key that is shared among all members of the
overlay. It is appropriate for small groups that wish to form a
private network without complexity. In shared secret mode, all the
peers share a single symmetric key which is used to key TLS-PSK
[RFC4279] or TLS-SRP [RFC5054] mode. A peer which does not know the
key cannot form TLS connections with any other peer and therefore
cannot join the overlay.
One natural approach to a shared-secret scheme is to use a user-
entered password as the key. The difficulty with this is that in
TLS-PSK mode, such keys are very susceptible to dictionary attacks.
If passwords are used as the source of shared-keys, then TLS-SRP is a
superior choice because it is not subject to dictionary attacks.
12.5. Storage Security
When certificate-based security is used in RELOAD, any given
Resource-ID/Kind-ID pair (a slot) is bound to some small set of
certificates. In order to write data in a slot, the writer must
Jennings, et al. Expires September 8, 2009 [Page 117]
Internet-Draft RELOAD Base March 2009
prove possession of the private key for one of those certificates.
Moreover, all data is stored signed by the certificate which
authorized its storage. This set of rules makes questions of
authorization and data integrity - which have historically been
thorny for overlays - relatively simple.
12.5.1. Authorization
When a client wants to store some value in a slot, it first digitally
signs the value with its own private key. It then sends a Store
request that contains both the value and the signature towards the
storing peer (which is defined by the Resource Name construction
algorithm for that particular kind of value).
When the storing peer receives the request, it must determine whether
the storing client is authorized to store in this slot. In order to
do so, it executes the Resource Name construction algorithm for the
specified kind based on the user's certificate information. It then
computes the Resource-ID from the Resource Name and verifies that it
matches the slot which the user is requesting to write to. If it
does, the user is authorized to write to this slot, pending quota
checks as described in the next section.
For example, consider the certificate with the following properties:
User name: alice@dht.example.com
Node-ID: 013456789abcdef
Serial: 1234
If Alice wishes to Store a value of the "SIP Location" kind, the
Resource Name will be the SIP AOR "sip:alice@dht.example.com". The
Resource-ID will be determined by hashing the Resource Name. When a
peer receives a request to store a record at Resource-ID X, it takes
the signing certificate and recomputes the Resource Name, in this
case "alice@dht.example.com". If H("alice@dht.example.com")=X then
the Store is authorized. Otherwise it is not. Note that the
Resource Name construction algorithm may be different for other
kinds.
12.5.2. Distributed Quota
Being a peer in a Overlay Instance carries with it the responsibility
to store data for a given region of the Overlay Instance. However,
if clients were allowed to store unlimited amounts of data, this
would create unacceptable burdens on peers, as well as enabling
trivial denial of service attacks. RELOAD addresses this issue by
requiring configurations to define maximum sizes for each kind of
stored data. Attempts to store values exceeding this size MUST be
Jennings, et al. Expires September 8, 2009 [Page 118]
Internet-Draft RELOAD Base March 2009
rejected (if peers are inconsistent about this, then strange
artifacts will happen when the zone of responsibility shifts and a
different peer becomes responsible for overlarge data). Because each
slot is bound to a small set of certificates, these size restrictions
also create a distributed quota mechanism, with the quotas
administered by the central enrollment server.
Allowing different kinds of data to have different size restrictions
allows new usages the flexibility to define limits that fit their
needs without requiring all usages to have expansive limits.
12.5.3. Correctness
Because each stored value is signed, it is trivial for any retrieving
peer to verify the integrity of the stored value. Some more care
needs to be taken to prevent version rollback attacks. Rollback
attacks on storage are prevented by the use of store times and
lifetime values in each store. A lifetime represents the latest time
at which the data is valid and thus limits (though does not
completely prevent) the ability of the storing node to perform a
rollback attack on retrievers. In order to prevent a rollback attack
at the time of the Store request, we require that storage times be
monotonically increasing. Storing peers MUST reject Store requests
with storage times smaller than or equal to those they are currently
storing. In addition, a fetching node which receives a data value
with a storage time older than the result of the previous fetch knows
a rollback has occurred.
12.5.4. Residual Attacks
The mechanisms described here provide a high degree of security, but
some attacks remain possible. Most simply, it is possible for
storing nodes to refuse to store a value (i.e., reject any request).
In addition, a storing node can deny knowledge of values which it
previously accepted. To some extent these attacks can be ameliorated
by attempting to store to/retrieve from replicas, but a retrieving
client does not know whether it should try this or not, since there
is a cost to doing so.
Although the certificate-based authentication scheme prevents a
single peer from being able to forge data owned by other peers.
Furthermore, although a subversive peer can refuse to return data
resources for which it is responsible it cannot return forged data
because it cannot provide authentication for such registrations.
Therefore parallel searches for redundant registrations can mitigate
most of the affects of a compromised peer. The ultimate reliability
of such an overlay is a statistical question based on the replication
factor and the percentage of compromised peers.
Jennings, et al. Expires September 8, 2009 [Page 119]
Internet-Draft RELOAD Base March 2009
In addition, when a kind is multivalued (e.g., an array data model),
the storing node can return only some subset of the values, thus
biasing its responses. This can be countered by using single values
rather than sets, but that makes coordination between multiple
storing agents much more difficult. This is a trade off that must be
made when designing any usage.
12.6. Routing Security
Because the storage security system guarantees (within limits) the
integrity of the stored data, routing security focuses on stopping
the attacker from performing a DOS attack on the system by misrouting
requests in the overlay. There are a few obvious observations to
make about this. First, it is easy to ensure that an attacker is at
least a valid peer in the Overlay Instance. Second, this is a DOS
attack only. Third, if a large percentage of the peers on the
Overlay Instance are controlled by the attacker, it is probably
impossible to perfectly secure against this.
12.6.1. Background
In general, attacks on DHT routing are mounted by the attacker
arranging to route traffic through or two nodes it controls. In the
Eclipse attack [Eclipse] the attacker tampers with messages to and
from nodes for which it is on-path with respect to a given victim
node. This allows it to pretend to be all the nodes that are
reachable through it. In the Sybil attack [Sybil], the attacker
registers a large number of nodes and is therefore able to capture a
large amount of the traffic through the DHT.
Both the Eclipse and Sybil attacks require the attacker to be able to
exercise control over her peer IDs. The Sybil attack requires the
creation of a large number of peers. The Eclipse attack requires
that the attacker be able to impersonate specific peers. In both
cases, these attacks are limited by the use of centralized,
certificate-based admission control.
12.6.2. Admissions Control
Admission to an RELOAD Overlay Instance is controlled by requiring
that each peer have a certificate containing its peer ID. The
requirement to have a certificate is enforced by using certificate-
based mutual authentication on each connection. Thus, whenever a
peer connects to another peer, each side automatically checks that
the other has a suitable certificate. These peer IDs are randomly
assigned by the central enrollment server. This has two benefits:
Jennings, et al. Expires September 8, 2009 [Page 120]
Internet-Draft RELOAD Base March 2009
o It allows the enrollment server to limit the number of peer IDs
issued to any individual user.
o It prevents the attacker from choosing specific peer IDs.
The first property allows protection against Sybil attacks (provided
the enrollment server uses strict rate limiting policies). The
second property deters but does not completely prevent Eclipse
attacks. Because an Eclipse attacker must impersonate peers on the
other side of the attacker, he must have a certificate for suitable
peer IDs, which requires him to repeatedly query the enrollment
server for new certificates which only will match by chance. From
the attacker's perspective, the difficulty is that if he only has a
small number of certificates the region of the Overlay Instance he is
impersonating appears to be very sparsely populated by comparison to
the victim's local region.
12.6.3. Peer Identification and Authentication
In general, whenever a peer engages in overlay activity that might
affect the routing table it must establish its identity. This
happens in two ways. First, whenever a peer establishes a direct
connection to another peer it authenticates via certificate-based
mutual authentication. All messages between peers are sent over this
protected channel and therefore the peers can verify the data origin
of the last hop peer for requests and responses without further
cryptography.
In some situations, however, it is desirable to be able to establish
the identity of a peer with whom one is not directly connected. The
most natural case is when a peer Updates its state. At this point,
other peers may need to update their view of the overlay structure,
but they need to verify that the Update message came from the actual
peer rather than from an attacker. To prevent this, all overlay
routing messages are signed by the peer that generated them.
[OPEN ISSUE: this allows for replay attacks on requests. There are
two basic defenses here. The first is global clocks and loose anti-
replay. The second is to refuse to take any action unless you verify
the data with the relevant node. This issue is undecided.]
[TODO: I think we are probably going to end up with generic
signatures or at least optional signatures on all overlay messages.]
12.6.4. Protecting the Signaling
The goal here is to stop an attacker from knowing who is signaling
what to whom. An attacker being able to observe the activities of a
specific individual is unlikely given the randomization of IDs and
Jennings, et al. Expires September 8, 2009 [Page 121]
Internet-Draft RELOAD Base March 2009
routing based on the present peers discussed above. Furthermore,
because messages can be routed using only the header information, the
actual body of the RELOAD message can be encrypted during
transmission.
There are two lines of defense here. The first is the use of TLS or
DTLS for each communications link between peers. This provides
protection against attackers who are not members of the overlay. The
second line of defense, if certificate-based security is used, is to
digitally sign each message. This prevents adversarial peers from
modifying messages in flight, even if they are on the routing path.
12.6.5. Residual Attacks
The routing security mechanisms in RELOAD are designed to contain
rather than eliminate attacks on routing. It is still possible for
an attacker to mount a variety of attacks. In particular, if an
attacker is able to take up a position on the overlay routing between
A and B it can make it appear as if B does not exist or is
disconnected. It can also advertise false network metrics in attempt
to reroute traffic. However, these are primarily DoS attacks.
The certificate-based security scheme secures the namespace, but if
an individual peer is compromised or if an attacker obtains a
certificate from the CA, then a number of subversive peers can still
appear in the overlay. While these peers cannot falsify responses to
resource queries, they can respond with error messages, effecting a
DoS attack on the resource registration. They can also subvert
routing to other compromised peers. To defend against such attacks,
a resource search must still consist of parallel searches for
replicated registrations.
13. IANA Considerations
This section contains the new code points registered by this
document. [NOTE TO IANA/RFC-EDITOR: Please replace RFC-AAAA with
the RFC number for this specification in the following list.]
13.1. Port Registrations
IANA has already allocated a port for the main peer to peer protocol.
This port has the name p2p-sip and the port number of 6084. The
names of this port may need to be changed as this draft progresses
and if it does careful instructions will be needed to IANA to ensure
the final RFC and IANA registrations are in sync.
[[TODO - add IANA registration for p2p_enroll SRV and p2p_menroll]]
Jennings, et al. Expires September 8, 2009 [Page 122]
Internet-Draft RELOAD Base March 2009
13.2. Overlay Algorithm Types
IANA SHALL create a "RELOAD Overlay Algorithm Type" Registry.
Entries in this registry are strings denoting the names of overlay
algorithms. The registration policy for this registry is RFC 5226
IETF Review. The initial contents of this registry are:
+-----------------+----------+
| Algorithm Name | RFC |
+-----------------+----------+
| chord-128-2-16+ | RFC-AAAA |
+-----------------+----------+
13.3. Access Control Policies
IANA SHALL create a "RELOAD Access Control Policy" Registry. Entries
in this registry are strings denoting access control policies, as
described in Section 6.3. New entries in this registry SHALL be
registered via RFC 5226 IETF Review. The initial contents of this
registry are:
USER-MATCH
NODE-MATCH
USER-NODE-MATCH
NODE-MULTIPLE
USER-MATCH-WITH-ANONYMOUS-CREATE
13.4. Data Kind-ID
IANA SHALL create a "RELOAD Data Kind-ID" Registry. Entries in this
registry are 32-bit integers denoting data kinds, as described in
Section 4.1.2. Code points in the range 0x00000001 to 0x7fffffff
SHALL be registered via RFC 5226 Standards Action. Code points in
the range 0x8000000 to 0xf0000000 SHALL be registered via RFC 5226
Expert Review. Code points in the range 0xf0000001 to 0xffffffff are
reserved for private use via the kind description mechanism described
in Section 10. The initial contents of this registry are:
Jennings, et al. Expires September 8, 2009 [Page 123]
Internet-Draft RELOAD Base March 2009
+--------------------+------------+----------+
| Kind | Kind-ID | RFC |
+--------------------+------------+----------+
| INVALID | 0 | RFC-AAAA |
| SIP-REGISTRATION | 1 | RFC-AAAA |
| TURN_SERVICE | 2 | RFC-AAAA |
| CERTIFICATE | 3 | RFC-AAAA |
| ROUTING_TABLE_SIZE | 4 | RFC-AAAA |
| SOFTWARE_VERSION | 5 | RFC-AAAA |
| MACHINE_UPTIME | 6 | RFC-AAAA |
| APP_UPTIME | 7 | RFC-AAAA |
| MEMORY_FOOTPRINT | 8 | RFC-AAAA |
| DATASIZE_StoreD | 9 | RFC-AAAA |
| INSTANCES_StoreD | 10 | RFC-AAAA |
| MESSAGES_SENT_RCVD | 11 | RFC-AAAA |
| EWMA_BYTES_SENT | 12 | RFC-AAAA |
| EWMA_BYTES_RCVD | 13 | RFC-AAAA |
| LAST_CONTACT | 14 | RFC-AAAA |
| RTT | 15 | RFC-AAAA |
| Reserved | 0x7fffffff | RFC-AAAA |
| Reserved | 0xffffffff | RFC-AAAA |
+--------------------+------------+----------+
13.5. Data Model
IANA SHALL create a "RELOAD Data Model" Registry. Entries in this
registry are 8-bit integers denoting data models, as described in
Section 6.2. Code points in this registry SHALL be registered via
RFC 5226 IETF Review. The initial contents of this registry are:
+--------------+------+----------+
| Data Model | Code | RFC |
+--------------+------+----------+
| INVALID | 0 | RFC-AAAA |
| SINGLE_VALUE | 1 | RFC-AAAA |
| ARRAY | 2 | RFC-AAAA |
| DICTIONARY | 3 | RFC-AAAA |
| RESERVED | 255 | RFC-AAAA |
+--------------+------+----------+
13.6. Message Codes
IANA SHALL create a "RELOAD Message Code" Registry. Entries in this
registry are 16-bit integers denoting method codes as described in
Section 5.3.3. These codes SHALL be registered via RFC 5226
Standards Action. The initial contents of this registry are:
Jennings, et al. Expires September 8, 2009 [Page 124]
Internet-Draft RELOAD Base March 2009
+-------------------+----------------+----------+
| Message Code Name | Code Value | RFC |
+-------------------+----------------+----------+
| invalid | 0 | RFC-AAAA |
| probe_req | 1 | RFC-AAAA |
| probe_ans | 2 | RFC-AAAA |
| attach_req | 3 | RFC-AAAA |
| attach_ans | 4 | RFC-AAAA |
| unused | 5 | |
| unused | 6 | |
| store_req | 7 | RFC-AAAA |
| store_ans | 8 | RFC-AAAA |
| fetch_req | 9 | RFC-AAAA |
| fetch_ans | 10 | RFC-AAAA |
| remove_req | 11 | RFC-AAAA |
| remove_ans | 12 | RFC-AAAA |
| find_req | 13 | RFC-AAAA |
| find_ans | 14 | RFC-AAAA |
| join_req | 15 | RFC-AAAA |
| join_ans | 16 | RFC-AAAA |
| leave_req | 17 | RFC-AAAA |
| leave_ans | 18 | RFC-AAAA |
| update_req | 19 | RFC-AAAA |
| update_ans | 20 | RFC-AAAA |
| route_query_req | 21 | RFC-AAAA |
| route_query_ans | 22 | RFC-AAAA |
| ping_req | 23 | RFC-AAAA |
| ping_ans | 24 | RFC-AAAA |
| stat_req | 25 | RFC-AAAA |
| stat_ans | 26 | RFC-AAAA |
| attachlite_req | 27 | RFC-AAAA |
| attachlite_ans | 28 | RFC-AAAA |
| reserved | 0x8000..0xfffe | RFC-AAAA |
| error | 0xffff | RFC-AAAA |
+-------------------+----------------+----------+
13.7. Error Codes
IANA SHALL create a "RELOAD Error Code" Registry. Entries in this
registry are 16-bit integers denoting error codes. New entries SHALL
be defined via RFC 5226 Standards Action. The initial contents of
this registry are:
Jennings, et al. Expires September 8, 2009 [Page 125]
Internet-Draft RELOAD Base March 2009
+-------------------------------------+----------------+----------+
| Error Code Name | Code Value | RFC |
+-------------------------------------+----------------+----------+
| invalid | 0 | RFC-AAAA |
| Error_Unauthorized | 1 | RFC-AAAA |
| Error_Forbidden | 2 | RFC-AAAA |
| Error_Not_Found | 3 | RFC-AAAA |
| Error_Request_Timeout | 4 | RFC-AAAA |
| Error_Precondition_Failed | 5 | RFC-AAAA |
| Error_Incompatible_with_Overlay | 6 | RFC-AAAA |
| Error_Unsupported_Forwarding_Option | 7 | RFC-AAAA |
| Error_Data_Too_Large | 8 | RFC-AAAA |
| Error_Data_Too_Old | 9 | RFC-AAAA |
| Error_TTL_Exceeded | 10 | RFC-AAAA |
| Error_Message_Too_Large | 11 | RFC-AAAA |
| reserved | 0x8000..0xfffe | RFC-AAAA |
+-------------------------------------+----------------+----------+
13.8. Route Log Extension Types
IANA SHALL create a "RELOAD Route Log Extension Type Registry." New
entries SHALL be defined via RFC 5226 Specification Required. The
initial contents of this registry are:
+--------------------------+------+---------------+
| Route Log Extension Name | Code | Specification |
+--------------------------+------+---------------+
| invalid | 0 | RFC-AAAA |
| reserved | 255 | RFC-AAAA |
+--------------------------+------+---------------+
13.9. Overlay Link Types
IANA shall create a "RELOAD Overlay Link Type Registry." New entries
SHALL be defined via RFC 5226 Standards Action. This registry SHALL
be initially populated with the following values:
+----------+------+---------------+
| Protocol | Code | Specification |
+----------+------+---------------+
| invalid | 0 | RFC-AAAA |
| tcp_tls | 1 | RFC-AAAA |
| udp_dtls | 2 | RFC-AAAA |
| reserved | 255 | RFC-AAAA |
+----------+------+---------------+
Jennings, et al. Expires September 8, 2009 [Page 126]
Internet-Draft RELOAD Base March 2009
13.10. Forwarding Options
IANA shall create a "Forwarding Option Registry". Entries in this
registry between 1 and 127 SHALL be defined via RFC 5226 Standards
Action. Entries in this registry between 128 and 254 SHALL be
defined via RFC 5226 Specification Required. This registry SHALL be
initially populated with the following values:
+-------------------+------+---------------+
| Forwarding Option | Code | Specification |
+-------------------+------+---------------+
| invalid | 0 | RFC-AAAA |
| reserved | 255 | RFC-AAAA |
+-------------------+------+---------------+
13.11. Probe Information Types
IANA shall create a "RELOAD Probe Information Type Registry".
Entries in this registry SHALL be defined via RFC 5226 Standards
Action. This registry SHALL be initially populated with the
following values:
+-----------------+------+---------------+
| Probe Option | Code | Specification |
+-----------------+------+---------------+
| invalid | 0 | RFC-AAAA |
| responsible_set | 1 | RFC-AAAA |
| requested_info | 2 | RFC-AAAA |
| reserved | 255 | RFC-AAAA |
+-----------------+------+---------------+
13.12. reload: URI Scheme
This section describes the scheme for a reload: URI, which can be
used to refer to either:
o A peer.
o A resource inside a peer.
The reload: URI is defined using a subset of the URI schema
specified in Appendix A of RFC 3986 [REF] and the associated URI
Guidelines [REF: RFC4395] per the following ABNF syntax:
RELOAD-URI = "reload://" destination "@" overlay "/"
[specifier]
destination = 1 * HEXDIG
overlay = reg-name
Jennings, et al. Expires September 8, 2009 [Page 127]
Internet-Draft RELOAD Base March 2009
specifier = 1*HEXDIG
The definitions of these productions are as follows:
destination: a hex-encoded Destination List object.
overlay: the name of the overlay.
specifier : a hex-encoded StoredDataSpecifier indicating the data
element.
If no specifier is present than this URI addresses the peer which can
be reached via the indicated destination list at the indicated
overlay name. If a specifier is present, then the URI addresses the
data value.
13.12.1. URI Registration
The following summarizes the information necessary to register the
reload: URI.
URI Scheme Name: reload
Status: permanent
URI Scheme Syntax: see Section 13.12.
URI Scheme Semantics: The reload: URI is intended to be used as a
reference to a RELOAD peer or resource.
Encoding Considerations: The reload: URI is not intended to be
human-readable text, therefore they are encoded entirely in US-
ASCII.
Applications/protocols that use this URI scheme: The RELOAD
protocol described in RFC-AAAA.
TBD for the rest of this template.
14. Acknowledgments
This draft is a merge of the "REsource LOcation And Discovery
(RELOAD)" draft by David A. Bryan, Marcia Zangrilli and Bruce B.
Lowekamp, the "Address Settlement by Peer to Peer" draft by Cullen
Jennings, Jonathan Rosenberg, and Eric Rescorla, the "Security
Extensions for RELOAD" draft by Bruce B. Lowekamp and James Deverick,
the "A Chord-based DHT for Resource Lookup in P2PSIP" by Marcia
Zangrilli and David A. Bryan, and the Peer-to-Peer Protocol (P2PP)
draft by Salman A. Baset, Henning Schulzrinne, and Marcin
Matuszewski. Thanks to the authors of RFC 5389 for text included
from that.
Jennings, et al. Expires September 8, 2009 [Page 128]
Internet-Draft RELOAD Base March 2009
Thanks to the many people who contributed including: Michael Chen,
TODO - fill in.
15. References
15.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[I-D.ietf-mmusic-ice]
Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols",
draft-ietf-mmusic-ice-19 (work in progress), October 2007.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
[I-D.ietf-behave-turn]
Rosenberg, J., Mahy, R., and P. Matthews, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)",
draft-ietf-behave-turn-12 (work in progress),
November 2008.
[RFC5273] Schaad, J. and M. Myers, "Certificate Management over CMS
(CMC): Transport Protocols", RFC 5273, June 2008.
[RFC5272] Schaad, J. and M. Myers, "Certificate Management over CMS
(CMC)", RFC 5272, June 2008.
[RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
for Transport Layer Security (TLS)", RFC 4279,
December 2005.
[I-D.ietf-mmusic-ice-tcp]
Rosenberg, J., "TCP Candidates with Interactive
Connectivity Establishment (ICE)",
draft-ietf-mmusic-ice-tcp-07 (work in progress),
July 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Jennings, et al. Expires September 8, 2009 [Page 129]
Internet-Draft RELOAD Base March 2009
Security", RFC 4347, April 2006.
[RFC5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification",
RFC 5348, September 2008.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
15.2. Informative References
[RFC4828] Floyd, S. and E. Kohler, "TCP Friendly Rate Control
(TFRC): The Small-Packet (SP) Variant", RFC 4828,
April 2007.
[I-D.ietf-p2psip-concepts]
Bryan, D., Matthews, P., Shim, E., Willis, D., and S.
Dawkins, "Concepts and Terminology for Peer to Peer SIP",
draft-ietf-p2psip-concepts-02 (work in progress),
July 2008.
[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
RFC 5382, October 2008.
[RFC4145] Yon, D. and G. Camarillo, "TCP-Based Media Transport in
the Session Description Protocol (SDP)", RFC 4145,
September 2005.
[RFC4571] Lazzaro, J., "Framing Real-time Transport Protocol (RTP)
and RTP Control Protocol (RTCP) Packets over Connection-
Oriented Transport", RFC 4571, July 2006.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC5054] Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin,
"Using the Secure Remote Password (SRP) Protocol for TLS
Jennings, et al. Expires September 8, 2009 [Page 130]
Internet-Draft RELOAD Base March 2009
Authentication", RFC 5054, November 2007.
[RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[I-D.matthews-p2psip-bootstrap-mechanisms]
Cooper, E., "Bootstrap Mechanisms for P2PSIP",
draft-matthews-p2psip-bootstrap-mechanisms-00 (work in
progress), February 2007.
[I-D.garcia-p2psip-dns-sd-bootstrapping]
Garcia, G., "P2PSIP bootstrapping using DNS-SD",
draft-garcia-p2psip-dns-sd-bootstrapping-00 (work in
progress), October 2007.
[I-D.pascual-p2psip-clients]
Pascual, V., Matuszewski, M., Shim, E., Zhang, H., and S.
Yongchao, "P2PSIP Clients",
draft-pascual-p2psip-clients-01 (work in progress),
February 2008.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
[RFC2311] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L., and
L. Repka, "S/MIME Version 2 Message Specification",
RFC 2311, March 1998.
[I-D.jiang-p2psip-sep]
Jiang, X. and H. Zhang, "Service Extensible P2P Peer
Protocol", draft-jiang-p2psip-sep-01 (work in progress),
February 2008.
[I-D.zheng-p2psip-diagnose]
Yongchao, S. and X. Jiang, "Diagnose P2PSIP Overlay
Network", draft-zheng-p2psip-diagnose-04 (work in
progress), December 2008.
[I-D.hardie-p2poverlay-pointers]
Hardie, T., "Mechanisms for use in pointing to overlay
networks, nodes, or resources",
draft-hardie-p2poverlay-pointers-00 (work in progress),
January 2008.
[I-D.ietf-p2psip-sip]
Jennings, et al. Expires September 8, 2009 [Page 131]
Internet-Draft RELOAD Base March 2009
Jennings, C., Lowekamp, B., Rescorla, E., Baset, S., and
H. Schulzrinne, "A SIP Usage for RELOAD",
draft-ietf-p2psip-sip-00 (work in progress), October 2008.
[Sybil] Douceur, J., "The Sybil Attack", IPTPS 02, March 2002.
[Eclipse] Singh, A., Ngan, T., Druschel, T., and D. Wallach,
"Eclipse Attacks on Overlay Networks: Threats and
Defenses", INFOCOM 2006, April 2006.
[non-transitive-dhts-worlds05]
Freedman, M., Lakshminarayanan, K., Rhea, S., and I.
Stoica, "Non-Transitive Connectivity and DHTs",
WORLDS'05.
[lookups-churn-p2p06]
Wu, D., Tian, Y., and K. Ng, "Analytical Study on
Improving DHT Lookup Performance under Churn", IEEE
P2P'06.
[bryan-design-hotp2p08]
Bryan, D., Lowekamp, B., and M. Zangrilli, "The Design of
a Versatile, Secure P2PSIP Communications Architecture for
the Public Internet", Hot-P2P'08.
[opendht-sigcomm05]
Rhea, S., Godfrey, B., Karp, B., Kubiatowicz, J.,
Ratnasamy, S., Shenker, S., Stoica, I., and H. Yu,
"OpenDHT: A Public DHT and its Uses", SIGCOMM'05.
[Chord] Stoica, I., Morris, R., Liben-Nowell, D., Karger, D.,
Kaashoek, M., Dabek, F., and H. Balakrishnan, "Chord: A
Scalable Peer-to-peer Lookup Protocol for Internet
Applications", IEEE/ACM Transactions on Networking Volume
11, Issue 1, 17-32, Feb 2003.
[vulnerabilities-acsac04]
Srivatsa, M. and L. Liu, "Vulnerabilities and Security
Threats in Structured Peer-to-Peer Systems: A Quantitative
Analysis", ACSAC 2004.
[handling-churn-usenix04]
Rhea, S., Geels, D., Roscoe, T., and J. Kubiatowicz,
"Handling Churn in a DHT", USENIX 2004.
[minimizing-churn-sigcomm06]
Godfrey, P., Shenker, S., and I. Stoica, "Minimizing Churn
in Distributed Systems", SIGCOMM 2006.
Jennings, et al. Expires September 8, 2009 [Page 132]
Internet-Draft RELOAD Base March 2009
Appendix A. Change Log
A.1. Changes since draft-ietf-p2psip-reload-01
o Added the ability to introduce new kinds dynamically.
o Added configuration file updating.
o Major revisions to reliability and flow control algorithms.
o Moved diagnostics out--they no go in a separate draft.
o Removed REMOVE: you now store a "nonexistent" element.
A.2. Changes since draft-ietf-p2psip-reload-00
o Split base protocol from combined draft into new draft.
o Update architecture discussion to address concerns raised about
clarity of roles.
o Moved extensive discussion of routing and client behaviors to
appendix.
o Split Ping into Ping and Probe
o Added AttachLite to provide way to implement ICE-Lite
o added Stat call for retrieving meta-data
o added discussion of periodic vs reactive recovery issue
o changed finger table stabilization to prefer long-lived over best-
match
o updated IANA considerations to be more complete
o changed error codes from http-based
A.3. Changes since draft-ietf-p2psip-base-00
o removed TUNNEL method
o allow implementations more flexibility in picking finger table
entry and revise random range
o decouple overlay configuration from enrollment server
o add error for data too large
o change architecture to overlay perspective from previous revision
and update terminology in document to match
A.4. Changes since draft-ietf-p2psip-base-01
o reordered message routing section to clarify that other routing
algorithms are possible besides symmetric recursive.
o clarified document IPR terms
A.5. Changes since draft-ietf-p2psip-base-01a
o Fragment offset was too small to hold 2^24 bit messages so fixed
this from 16 bits to 32 bits.
Jennings, et al. Expires September 8, 2009 [Page 133]
Internet-Draft RELOAD Base March 2009
o Changed absolute times from seconds to milliseconds
o Added error for messages over max size
o Added error for TTL expired
o Add time in response to PING
o Clarified retransmission and fragmentation algorithm
o Clarified acknowledgement tracking for congestion control
Appendix B. Routing Alternatives
Significant discussion has been focused on the selection of a routing
algorithm for P2PSIP. This section discusses the motivations for
selection of symmetric recursive routing for RELOAD and describes the
extensions that would be required to support additional routing
algorithms.
B.1. Iterative vs Recursive
Iterative routing has a number of advantages. It is easier to debug,
consumes fewer resources on intermediate peers, and allows the
querying peer to identify and route around misbehaving peers
[non-transitive-dhts-worlds05]. However, in the presence of NATs
iterative routing is intolerably expensive because a new connection
must be established for each hop (using ICE) [bryan-design-hotp2p08].
Iterative routing is supported through the Route_Query mechanism and
is primarily intended for debugging. It is also allows the querying
peer to evaluate the routing decisions made by the peers at each hop,
consider alternatives, and perhaps detect at what point the
forwarding path fails.
B.2. Symmetric vs Forward response
An alternative to the symmetric recursive routing method used by
RELOAD is Forward-Only routing, where the response is routed to the
requester as if it is a new message initiating by the responder (in
the previous example, Z sends the response to A as if it were sending
a request). Forward-only routing requires no state in either the
message or intermediate peers.
The drawback of forward-only routing is that it does not work when
the overlay is unstable. For example, if A is in the process of
joining the overlay and is sending a Join request to Z, it is not yet
reachable via forward routing. Even if it is established in the
overlay, if network failures produce temporary instability, A may not
be reachable (and may be trying to stabilize its network connectivity
via Attach messages).
Jennings, et al. Expires September 8, 2009 [Page 134]
Internet-Draft RELOAD Base March 2009
Furthermore, forward-only responses are less likely to reach the
querying peer than symmetric recursive because the forward path is
more likely to have a failed peer than the request path (which was
just tested to route the request) [non-transitive-dhts-worlds05].
An extension to RELOAD that supports forward-only routing but relies
on symmetric responses as a fallback would be possible, but due to
the complexities of determining when to use forward-only and when to
fallback to symmetric, we have chosen not to include it as an option
at this point.
B.3. Direct Response
Another routing option is Direct Response routing, in which the
response is returned directly to the querying node. In the previous
example, if A encodes its IP address in the request, then Z can
simply deliver the response directly to A. In the absence of NATs or
other connectivity issues, this is the optimal routing technique.
The challenge of implementing direct response is the presence of
NATs. There are a number of complexities that must be addressed. In
this discussion, we will continue our assumption that A issued the
request and Z is generating the response.
o The IP address listed by A may be unreachable, either due to NAT
or firewall rules. Therefore, a direct response technique must
fallback to symmetric response [non-transitive-dhts-worlds05].
The hop-by-hop ACKs used by RELOAD allow Z to determine when A has
received the message (and the TLS negotiation will provide earlier
confirmation that A is reachable), but this fallback requires a
timeout that will increase the response latency whenever A is not
reachable from Z.
o Whenever A is behind a NAT it will have multiple candidate IP
addresses, each of which must be advertised to ensure
connectivity, therefore Z will need to attempt multiple
connections to deliver the response.
o One (or all) of A's candidate addresses may route from Z to a
different device on the Internet. In the worst case these nodes
may actually be running RELOAD on the same port. Therefore,
establishing a secure connection to authenticate A before
delivering the response is absolutely necessary. This step
diminishes the efficiency of direct response because multiple
roundtrips are required before the message can be delivered.
o If A is behind a NAT and does not have a connection already
established with Z, there are only two ways the direct response
will work. The first is that A and Z are both behind the same
NAT, in which case the NAT is not involved. In the more common
case, when Z is outside A's NAT, the response will only be
Jennings, et al. Expires September 8, 2009 [Page 135]
Internet-Draft RELOAD Base March 2009
received if A's NAT implements endpoint-independent filtering. As
the choice of filtering mode conflates application transparency
with security [RFC4787], and no clear recommendation is available,
the prevalence of this feature in future devices remains unclear.
An extension to RELOAD that supports direct response routing but
relies on symmetric responses as a fallback would be possible, but
due to the complexities of determining when to use direct response
and when to fallback to symmetric, and the reduced performance for
responses to peers behind restrictive NATs, we have chosen not to
include it as an option at this point.
B.4. Relay Peers
SEP [I-D.jiang-p2psip-sep] has proposed implementing a form of direct
response by having A identify a peer, Q, that will be directly
reachable by any other peer. A uses Attach to establish a connection
with Q and advertises Q's IP address in the request sent to Z. Z
sends the response to Q, which relays it to A. This then reduces the
latency to two hops, plus Z negotiating a secure connection to Q.
This technique relies on the relative population of nodes such as A
that require relay peers and peers such as Q that are capable of
serving as a relay peer. It also requires nodes to be able to
identify which category they are in. This identification problem has
turned out to be hard to solve and is still an open area of
exploration.
An extension to RELOAD that supports relay peers is possible, but due
to the complexities of implementing such an alternative, we have not
added such a feature to RELOAD at this point.
A concept similar to relay peers, essentially choosing a relay peer
at random, has previously been suggested to solve problems of
pairwise non-transitivity [non-transitive-dhts-worlds05], but
deterministic filtering provided by NATs make random relay peers no
more likely to work than the responding peer.
B.5. Symmetric Route Stability
A common concern about symmetric recursive routing has been that one
or more peers along the request path may fail before the response is
received. The significance of this problem essentially depends on
the response latency of the overlay. An overlay that produces slow
responses will be vulnerable to churn, whereas responses that are
delivered very quickly are vulnerable only to failures that occur
over that small interval.
Jennings, et al. Expires September 8, 2009 [Page 136]
Internet-Draft RELOAD Base March 2009
The other aspect of this issue is whether the request itself can be
successfully delivered. Assuming typical connection maintenance
intervals, the time period between the last maintenance and the
request being sent will be orders of magnitude greater than the delay
between the request being forwarded and the response being received.
Therefore, if the path was stable enough to be available to route the
request, it is almost certainly going to remain available to route
the response.
An overlay that is unstable enough to suffer this type of failure
frequently is unlikely to be able to support reliable functionality
regardless of the routing mechanism. However, regardless of the
stability of the return path, studies show that in the event of high
churn, iterative routing is a better solution to ensure request
completion [lookups-churn-p2p06] [non-transitive-dhts-worlds05]
Finally, because RELOAD retries the end-to-end request, that retry
will address the issues of churn that remain.
Appendix C. Why Clients?
There are a wide variety of reasons a node may act as a client rather
than as a peer [I-D.pascual-p2psip-clients]. This section outlines
some of those scenarios and how the client's behavior changes based
on its capabilities.
C.1. Why Not Only Peers?
For a number of reasons, a particular node may be forced to act as a
client even though it is willing to act as a peer. These include:
o The node does not have appropriate network connectivity, typically
because it has a low-bandwidth network connection.
o The node may not have sufficient resources, such as computing
power, storage space, or battery power.
o The overlay algorithm may dictate specific requirements for peer
selection. These may include participation in the overlay to
determine trustworthiness, control the number of peers in the
overlay to reduce overly-long routing paths, or ensure minimum
application uptime before a node can join as a peer.
The ultimate criteria for a node to become a peer are determined by
the overlay algorithm and specific deployment. A node acting as a
client that has a full implementation of RELOAD and the appropriate
overlay algorithm is capable of locating its responsible peer in the
overlay and using CONNECT to establish a direct connection to that
peer. In that way, it may elect to be reachable under either of the
Jennings, et al. Expires September 8, 2009 [Page 137]
Internet-Draft RELOAD Base March 2009
routing approaches listed above. Particularly for overlay algorithms
that elect nodes to serve as peers based on trustworthiness or
population, the overlay algorithm may require such a client to locate
itself at a particular place in the overlay.
C.2. Clients as Application-Level Agents
SIP defines an extensive protocol for registration and security
between a client and its registrar/proxy server(s). Any SIP device
can act as a client of a RELOAD-based P2PSIP overlay if it contacts a
peer that implements the server-side functionality required by the
SIP protocol. In this case, the peer would be acting as if it were
the user's peer, and would need the appropriate credentials for that
user.
Application-level support for clients is defined by a usage. A usage
offering support for application-level clients should specify how the
security of the system is maintained when the data is moved between
the application and RELOAD layers.
Authors' Addresses
Cullen Jennings
Cisco
170 West Tasman Drive
MS: SJC-21/2
San Jose, CA 95134
USA
Phone: +1 408 421-9990
Email: fluffy@cisco.com
Bruce B. Lowekamp (editor)
unaffiliated
2790 Linden Ln
Williamsburg, VA 23185
USA
Email: bbl@lowekamp.net
Jennings, et al. Expires September 8, 2009 [Page 138]
Internet-Draft RELOAD Base March 2009
Eric Rescorla
Network Resonance
2064 Edgewood Drive
Palo Alto, CA 94303
USA
Phone: +1 650 320-8549
Email: ekr@networkresonance.com
Salman A. Baset
Columbia University
1214 Amsterdam Avenue
New York, NY
USA
Email: salman@cs.columbia.edu
Henning Schulzrinne
Columbia University
1214 Amsterdam Avenue
New York, NY
USA
Email: hgs@cs.columbia.edu
Jennings, et al. Expires September 8, 2009 [Page 139]