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rfc:rfc6940

Internet Engineering Task Force (IETF) C. Jennings Request for Comments: 6940 Cisco Category: Standards Track B. Lowekamp, Ed. ISSN: 2070-1721 Skype

                                                           E. Rescorla
                                                            RTFM, Inc.
                                                              S. Baset
                                                        H. Schulzrinne
                                                   Columbia University
                                                          January 2014
       REsource LOcation And Discovery (RELOAD) Base Protocol

Abstract

 This specification 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 need to 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.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6940.

Jennings, et al. Standards Track [Page 1] RFC 6940 RELOAD Base January 2014

Copyright Notice

 Copyright (c) 2014 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
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
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 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 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.

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   7
   1.1.  Basic Setting . . . . . . . . . . . . . . . . . . . . . .   8
   1.2.  Architecture  . . . . . . . . . . . . . . . . . . . . . .  10
     1.2.1.  Usage Layer . . . . . . . . . . . . . . . . . . . . .  13
     1.2.2.  Message Transport . . . . . . . . . . . . . . . . . .  13
     1.2.3.  Storage . . . . . . . . . . . . . . . . . . . . . . .  14
     1.2.4.  Topology Plug-in  . . . . . . . . . . . . . . . . . .  15
     1.2.5.  Forwarding and Link Management Layer  . . . . . . . .  16
   1.3.  Security  . . . . . . . . . . . . . . . . . . . . . . . .  16
   1.4.  Structure of This Document  . . . . . . . . . . . . . . .  17
 2.  Requirements Language . . . . . . . . . . . . . . . . . . . .  18
 3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .  18
 4.  Overlay Management Overview . . . . . . . . . . . . . . . . .  21
   4.1.  Security and Identification . . . . . . . . . . . . . . .  21
     4.1.1.  Shared-Key Security . . . . . . . . . . . . . . . . .  23
   4.2.  Clients . . . . . . . . . . . . . . . . . . . . . . . . .  23
     4.2.1.  Client Routing  . . . . . . . . . . . . . . . . . . .  24
     4.2.2.  Minimum Functionality Requirements for Clients  . . .  25
   4.3.  Routing . . . . . . . . . . . . . . . . . . . . . . . . .  25

Jennings, et al. Standards Track [Page 2] RFC 6940 RELOAD Base January 2014

   4.4.  Connectivity Management . . . . . . . . . . . . . . . . .  29
   4.5.  Overlay Algorithm Support . . . . . . . . . . . . . . . .  30
     4.5.1.  Support for Pluggable Overlay Algorithms  . . . . . .  30
     4.5.2.  Joining, Leaving, and Maintenance Overview  . . . . .  30
   4.6.  First-Time Setup  . . . . . . . . . . . . . . . . . . . .  32
     4.6.1.  Initial Configuration . . . . . . . . . . . . . . . .  32
     4.6.2.  Enrollment  . . . . . . . . . . . . . . . . . . . . .  32
     4.6.3.  Diagnostics . . . . . . . . . . . . . . . . . . . . .  33
 5.  Application Support Overview  . . . . . . . . . . . . . . . .  33
   5.1.  Data Storage  . . . . . . . . . . . . . . . . . . . . . .  33
     5.1.1.  Storage Permissions . . . . . . . . . . . . . . . . .  34
     5.1.2.  Replication . . . . . . . . . . . . . . . . . . . . .  35
   5.2.  Usages  . . . . . . . . . . . . . . . . . . . . . . . . .  36
   5.3.  Service Discovery . . . . . . . . . . . . . . . . . . . .  36
   5.4.  Application Connectivity  . . . . . . . . . . . . . . . .  36
 6.  Overlay Management Protocol . . . . . . . . . . . . . . . . .  37
   6.1.  Message Receipt and Forwarding  . . . . . . . . . . . . .  37
     6.1.1.  Responsible ID  . . . . . . . . . . . . . . . . . . .  38
     6.1.2.  Other ID  . . . . . . . . . . . . . . . . . . . . . .  38
     6.1.3.  Opaque ID . . . . . . . . . . . . . . . . . . . . . .  40
   6.2.  Symmetric Recursive Routing . . . . . . . . . . . . . . .  41
     6.2.1.  Request Origination . . . . . . . . . . . . . . . . .  41
     6.2.2.  Response Origination  . . . . . . . . . . . . . . . .  42
   6.3.  Message Structure . . . . . . . . . . . . . . . . . . . .  42
     6.3.1.  Presentation Language . . . . . . . . . . . . . . . .  43
       6.3.1.1.  Common Definitions  . . . . . . . . . . . . . . .  44
     6.3.2.  Forwarding Header . . . . . . . . . . . . . . . . . .  46
       6.3.2.1.  Processing Configuration Sequence Numbers . . . .  49
       6.3.2.2.  Destination and Via Lists . . . . . . . . . . . .  50
       6.3.2.3.  Forwarding Option . . . . . . . . . . . . . . . .  52
     6.3.3.  Message Contents Format . . . . . . . . . . . . . . .  53
       6.3.3.1.  Response Codes and Response Errors  . . . . . . .  54
     6.3.4.  Security Block  . . . . . . . . . . . . . . . . . . .  57
   6.4.  Overlay Topology  . . . . . . . . . . . . . . . . . . . .  60
     6.4.1.  Topology Plug-in Requirements . . . . . . . . . . . .  60
     6.4.2.  Methods and Types for Use by Topology Plug-ins  . . .  61
       6.4.2.1.  Join  . . . . . . . . . . . . . . . . . . . . . .  61
       6.4.2.2.  Leave . . . . . . . . . . . . . . . . . . . . . .  62
       6.4.2.3.  Update  . . . . . . . . . . . . . . . . . . . . .  63
       6.4.2.4.  RouteQuery  . . . . . . . . . . . . . . . . . . .  63
       6.4.2.5.  Probe . . . . . . . . . . . . . . . . . . . . . .  65
   6.5.  Forwarding and Link Management Layer  . . . . . . . . . .  67
     6.5.1.  Attach  . . . . . . . . . . . . . . . . . . . . . . .  67
       6.5.1.1.  Request Definition  . . . . . . . . . . . . . . .  68
       6.5.1.2.  Response Definition . . . . . . . . . . . . . . .  70
       6.5.1.3.  Using ICE with RELOAD . . . . . . . . . . . . . .  71
       6.5.1.4.  Collecting STUN Servers . . . . . . . . . . . . .  71
       6.5.1.5.  Gathering Candidates  . . . . . . . . . . . . . .  72

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       6.5.1.6.  Prioritizing Candidates . . . . . . . . . . . . .  72
       6.5.1.7.  Encoding the Attach Message . . . . . . . . . . .  73
       6.5.1.8.  Verifying ICE Support . . . . . . . . . . . . . .  74
       6.5.1.9.  Role Determination  . . . . . . . . . . . . . . .  74
       6.5.1.10. Full ICE  . . . . . . . . . . . . . . . . . . . .  74
       6.5.1.11. No-ICE  . . . . . . . . . . . . . . . . . . . . .  75
       6.5.1.12. Subsequent Offers and Answers . . . . . . . . . .  75
       6.5.1.13. Sending Media . . . . . . . . . . . . . . . . . .  75
       6.5.1.14. Receiving Media . . . . . . . . . . . . . . . . .  75
     6.5.2.  AppAttach . . . . . . . . . . . . . . . . . . . . . .  75
       6.5.2.1.  Request Definition  . . . . . . . . . . . . . . .  76
       6.5.2.2.  Response Definition . . . . . . . . . . . . . . .  77
     6.5.3.  Ping  . . . . . . . . . . . . . . . . . . . . . . . .  77
       6.5.3.1.  Request Definition  . . . . . . . . . . . . . . .  77
       6.5.3.2.  Response Definition . . . . . . . . . . . . . . .  77
     6.5.4.  ConfigUpdate  . . . . . . . . . . . . . . . . . . . .  78
       6.5.4.1.  Request Definition  . . . . . . . . . . . . . . .  78
       6.5.4.2.  Response Definition . . . . . . . . . . . . . . .  79
   6.6.  Overlay Link Layer  . . . . . . . . . . . . . . . . . . .  80
     6.6.1.  Future Overlay Link Protocols . . . . . . . . . . . .  81
       6.6.1.1.  HIP . . . . . . . . . . . . . . . . . . . . . . .  82
       6.6.1.2.  ICE-TCP . . . . . . . . . . . . . . . . . . . . .  82
       6.6.1.3.  Message-Oriented Transports . . . . . . . . . . .  82
       6.6.1.4.  Tunneled Transports . . . . . . . . . . . . . . .  82
     6.6.2.  Framing Header  . . . . . . . . . . . . . . . . . . .  83
     6.6.3.  Simple Reliability  . . . . . . . . . . . . . . . . .  84
       6.6.3.1.  Stop and Wait Sender Algorithm  . . . . . . . . .  85
     6.6.4.  DTLS/UDP with SR  . . . . . . . . . . . . . . . . . .  86
     6.6.5.  TLS/TCP with FH, No-ICE . . . . . . . . . . . . . . .  86
     6.6.6.  DTLS/UDP with SR, No-ICE  . . . . . . . . . . . . . .  87
   6.7.  Fragmentation and Reassembly  . . . . . . . . . . . . . .  87
 7.  Data Storage Protocol . . . . . . . . . . . . . . . . . . . .  88
   7.1.  Data Signature Computation  . . . . . . . . . . . . . . .  90
   7.2.  Data Models . . . . . . . . . . . . . . . . . . . . . . .  91
     7.2.1.  Single Value  . . . . . . . . . . . . . . . . . . . .  91
     7.2.2.  Array . . . . . . . . . . . . . . . . . . . . . . . .  92
     7.2.3.  Dictionary  . . . . . . . . . . . . . . . . . . . . .  92
   7.3.  Access Control Policies . . . . . . . . . . . . . . . . .  93
     7.3.1.  USER-MATCH  . . . . . . . . . . . . . . . . . . . . .  93
     7.3.2.  NODE-MATCH  . . . . . . . . . . . . . . . . . . . . .  93
     7.3.3.  USER-NODE-MATCH . . . . . . . . . . . . . . . . . . .  93
     7.3.4.  NODE-MULTIPLE . . . . . . . . . . . . . . . . . . . .  94
   7.4.  Data Storage Methods  . . . . . . . . . . . . . . . . . .  94
     7.4.1.  Store . . . . . . . . . . . . . . . . . . . . . . . .  94
       7.4.1.1.  Request Definition  . . . . . . . . . . . . . . .  94
       7.4.1.2.  Response Definition . . . . . . . . . . . . . . . 100
       7.4.1.3.  Removing Values . . . . . . . . . . . . . . . . . 101

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     7.4.2.  Fetch . . . . . . . . . . . . . . . . . . . . . . . . 102
       7.4.2.1.  Request Definition  . . . . . . . . . . . . . . . 102
       7.4.2.2.  Response Definition . . . . . . . . . . . . . . . 104
     7.4.3.  Stat  . . . . . . . . . . . . . . . . . . . . . . . . 105
       7.4.3.1.  Request Definition  . . . . . . . . . . . . . . . 105
       7.4.3.2.  Response Definition . . . . . . . . . . . . . . . 106
     7.4.4.  Find  . . . . . . . . . . . . . . . . . . . . . . . . 107
       7.4.4.1.  Request Definition  . . . . . . . . . . . . . . . 108
       7.4.4.2.  Response Definition . . . . . . . . . . . . . . . 108
     7.4.5.  Defining New Kinds  . . . . . . . . . . . . . . . . . 109
 8.  Certificate Store Usage . . . . . . . . . . . . . . . . . . . 110
 9.  TURN Server Usage . . . . . . . . . . . . . . . . . . . . . . 110
 10. Chord Algorithm . . . . . . . . . . . . . . . . . . . . . . . 112
   10.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . 113
   10.2.  Hash Function  . . . . . . . . . . . . . . . . . . . . . 114
   10.3.  Routing  . . . . . . . . . . . . . . . . . . . . . . . . 114
   10.4.  Redundancy . . . . . . . . . . . . . . . . . . . . . . . 114
   10.5.  Joining  . . . . . . . . . . . . . . . . . . . . . . . . 115
   10.6.  Routing Attaches . . . . . . . . . . . . . . . . . . . . 116
   10.7.  Updates  . . . . . . . . . . . . . . . . . . . . . . . . 117
     10.7.1.  Handling Neighbor Failures . . . . . . . . . . . . . 118
     10.7.2.  Handling Finger Table Entry Failure  . . . . . . . . 119
     10.7.3.  Receiving Updates  . . . . . . . . . . . . . . . . . 119
     10.7.4.  Stabilization  . . . . . . . . . . . . . . . . . . . 120
       10.7.4.1.  Updating the Neighbor Table  . . . . . . . . . . 120
       10.7.4.2.  Refreshing the Finger Table  . . . . . . . . . . 121
       10.7.4.3.  Adjusting Finger Table Size  . . . . . . . . . . 122
       10.7.4.4.  Detecting Partitioning . . . . . . . . . . . . . 122
   10.8.  Route Query  . . . . . . . . . . . . . . . . . . . . . . 123
   10.9.  Leaving  . . . . . . . . . . . . . . . . . . . . . . . . 123
 11. Enrollment and Bootstrap  . . . . . . . . . . . . . . . . . . 124
   11.1.  Overlay Configuration  . . . . . . . . . . . . . . . . . 124
     11.1.1.  RELAX NG Grammar . . . . . . . . . . . . . . . . . . 132
   11.2.  Discovery through Configuration Server . . . . . . . . . 134
   11.3.  Credentials  . . . . . . . . . . . . . . . . . . . . . . 135
     11.3.1.  Self-Generated Credentials . . . . . . . . . . . . . 137
   11.4.  Contacting a Bootstrap Node  . . . . . . . . . . . . . . 138
 12. Message Flow Example  . . . . . . . . . . . . . . . . . . . . 138
 13. Security Considerations . . . . . . . . . . . . . . . . . . . 144
   13.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . 144
   13.2.  Attacks on P2P Overlays  . . . . . . . . . . . . . . . . 145
   13.3.  Certificate-Based Security . . . . . . . . . . . . . . . 145
   13.4.  Shared-Secret Security . . . . . . . . . . . . . . . . . 147
   13.5.  Storage Security . . . . . . . . . . . . . . . . . . . . 147
     13.5.1.  Authorization  . . . . . . . . . . . . . . . . . . . 147
     13.5.2.  Distributed Quota  . . . . . . . . . . . . . . . . . 148
     13.5.3.  Correctness  . . . . . . . . . . . . . . . . . . . . 148
     13.5.4.  Residual Attacks . . . . . . . . . . . . . . . . . . 149

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   13.6.  Routing Security . . . . . . . . . . . . . . . . . . . . 149
     13.6.1.  Background . . . . . . . . . . . . . . . . . . . . . 150
     13.6.2.  Admissions Control . . . . . . . . . . . . . . . . . 150
     13.6.3.  Peer Identification and Authentication . . . . . . . 151
     13.6.4.  Protecting the Signaling . . . . . . . . . . . . . . 151
     13.6.5.  Routing Loops and DoS Attacks  . . . . . . . . . . . 152
     13.6.6.  Residual Attacks . . . . . . . . . . . . . . . . . . 152
 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 153
   14.1.  Well-Known URI Registration  . . . . . . . . . . . . . . 153
   14.2.  Port Registrations . . . . . . . . . . . . . . . . . . . 153
   14.3.  Overlay Algorithm Types  . . . . . . . . . . . . . . . . 154
   14.4.  Access Control Policies  . . . . . . . . . . . . . . . . 154
   14.5.  Application-ID . . . . . . . . . . . . . . . . . . . . . 155
   14.6.  Data Kind-ID . . . . . . . . . . . . . . . . . . . . . . 155
   14.7.  Data Model . . . . . . . . . . . . . . . . . . . . . . . 156
   14.8.  Message Codes  . . . . . . . . . . . . . . . . . . . . . 156
   14.9.  Error Codes  . . . . . . . . . . . . . . . . . . . . . . 158
   14.10. Overlay Link Types . . . . . . . . . . . . . . . . . . . 159
   14.11. Overlay Link Protocols . . . . . . . . . . . . . . . . . 159
   14.12. Forwarding Options . . . . . . . . . . . . . . . . . . . 160
   14.13. Probe Information Types  . . . . . . . . . . . . . . . . 160
   14.14. Message Extensions . . . . . . . . . . . . . . . . . . . 161
   14.15. Reload URI Scheme  . . . . . . . . . . . . . . . . . . . 161
     14.15.1.  URI Registration  . . . . . . . . . . . . . . . . . 162
   14.16. Media Type Registration  . . . . . . . . . . . . . . . . 162
   14.17. XML Namespace Registration . . . . . . . . . . . . . . . 163
     14.17.1.  Config URL  . . . . . . . . . . . . . . . . . . . . 164
     14.17.2.  Config Chord URL  . . . . . . . . . . . . . . . . . 164
 15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 164
 16. References  . . . . . . . . . . . . . . . . . . . . . . . . . 165
   16.1.  Normative References . . . . . . . . . . . . . . . . . . 165
   16.2.  Informative References . . . . . . . . . . . . . . . . . 167
 Appendix A.  Routing Alternatives . . . . . . . . . . . . . . . . 171
   A.1.  Iterative vs. Recursive . . . . . . . . . . . . . . . . . 171
   A.2.  Symmetric vs. Forward Response  . . . . . . . . . . . . . 171
   A.3.  Direct Response . . . . . . . . . . . . . . . . . . . . . 172
   A.4.  Relay Peers . . . . . . . . . . . . . . . . . . . . . . . 173
   A.5.  Symmetric Route Stability . . . . . . . . . . . . . . . . 173
 Appendix B.  Why Clients? . . . . . . . . . . . . . . . . . . . . 174
   B.1.  Why Not Only Peers? . . . . . . . . . . . . . . . . . . . 174
   B.2.  Clients as Application-Level Agents . . . . . . . . . . . 175

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1. Introduction

 This document defines REsource LOcation And Discovery (RELOAD), a
 peer-to-peer (P2P) signaling protocol for use on the Internet.
 RELOAD provides a generic, self-organizing overlay network service,
 allowing nodes to route messages to other nodes and to 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 (SIP) [SIP-RELOAD].  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 Interactive Connectivity Establishment (ICE)
    [RFC5245] to establish new RELOAD or application protocol
    connections.
 Optimized 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 for intermediate peers.
 Pluggable Overlay Algorithms:  RELOAD has been designed with an
    abstract interface to the overlay layer to simplify implementing a
    variety of structured (e.g., distributed hash tables (DHTs)) and
    unstructured overlay algorithms.  The idea here is that RELOAD
    provides a generic structure that can fit most types of overlay
    topologies (ring, hyperspace, etc.).  To instantiate an actual
    network, you combine RELOAD with a specific overlay algorithm,
    which defines how to construct the overlay topology and route
    messages efficiently within it.  This specification also defines

Jennings, et al. Standards Track [Page 7] RFC 6940 RELOAD Base January 2014

    how RELOAD is used with the Chord-based [Chord] DHT algorithm,
    which is mandatory to implement.  Specifying a default "mandatory-
    to-implement" overlay algorithm promotes interoperability, while
    extensibility allows selection of overlay algorithms optimized for
    a particular application.
 Support for Clients:  RELOAD clients differ from RELOAD peers
    primarily in that they do not store information on behalf of other
    nodes in the overlay.  Rather, they use the overlay only to locate
    users and resources, as well as to store information and to
    contact other nodes.
 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 usage for NAT traversal.  The SIP Usage itself is described
 separately in [SIP-RELOAD].  RELOAD is not limited to usage by SIP
 and could serve as a tool for supporting other P2P applications with
 similar needs.

1.1. Basic Setting

 In this section, we provide a brief overview of the operational
 setting for RELOAD.  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 for the lifetime of the node, which,
 together with the specific overlay algorithm in use, determines its
 position in the graph and the set of nodes it connects to.  The
 Node-ID is also tightly coupled to the certificate (see
 Section 13.3).  The figure below shows a trivial example which isn't
 drawn from any particular overlay algorithm, but was chosen for
 convenience of representation.

Jennings, et al. Standards Track [Page 8] RFC 6940 RELOAD Base January 2014

    +--------+              +--------+              +--------+
    | 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
 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, albeit one designed for small-scale transient
 storage rather than for bulk storage of large objects.  Records are
 stored under numeric addresses, called Resource-IDs, which occupy the
 same space as node identifiers.  Peers 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 peer 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 addresses in the range [81..90].
 We refer to non-client nodes as peers.

Jennings, et al. Standards Track [Page 9] RFC 6940 RELOAD Base January 2014

 Other applications (for instance, SIP) can be defined on top of
 RELOAD and can use these two basic RELOAD services to provide their
 own services.

1.2. Architecture

 RELOAD is fundamentally an overlay network.  The following figure
 shows the layered RELOAD architecture.
          Application
      +-------+  +-------+
      | SIP   |  | XMPP  |  ...
      | Usage |  | Usage |
      +-------+  +-------+
  ------------------------------------ Messaging Service Boundary
  +------------------+     +---------+
  |     Message      |<--->| Storage |
  |    Transport     |     +---------+
  +------------------+           ^
         ^       ^               |
         |       v               v
         |     +-------------------+
         |     |    Topology       |
         |     |    Plug-in        |
         |     +-------------------+
         |         ^
         v         v
      +------------------+
      |  Forwarding &    |
      | Link Management  |
      +------------------+
  ------------------------------------ Overlay Link Service Boundary
       +-------+  +-------+
       |TLS    |  |DTLS   |  ...
       |Overlay|  |Overlay|
       |Link   |  |Link   |
       +-------+  +-------+
 The major components of RELOAD are:
 Usage Layer:  Each application defines a RELOAD Usage, which is 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 Service.

Jennings, et al. Standards Track [Page 10] RFC 6940 RELOAD Base January 2014

 Message Transport:  Handles end-to-end reliability, manages request
    state for the usages, and forwards Store and Fetch operations to
    the Storage component.  It 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 Plug-in to manage data replication and
    migration, and it talks to the Message Transport component to send
    and receive messages.
 Topology Plug-in:  The Topology Plug-in is responsible for
    implementing the specific overlay algorithm being used.  It uses
    the Message Transport component to send and receive overlay
    management messages, the Storage component to manage data
    replication, and the Forwarding Layer to control hop-by-hop
    message forwarding.  This component superficially 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.  The Topology Plug-in
    has two functions: constructing the local forwarding instructions
    and selecting the operational topology (i.e., creating links by
    sending overlay management messages).
 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 for overlay links across NATs
    using ICE.
 Overlay Link Layer:  Responsible for actually transporting traffic
    directly between nodes.  Transport Layer Security (TLS) [RFC5246]
    and Datagram Transport Layer Security (DTLS) [RFC6347] are the
    currently defined "overlay link layer" protocols used by RELOAD
    for hop-by-hop communication.  Each such protocol includes the
    appropriate provisions for per-hop framing and hop-by-hop ACKs
    needed by unreliable underlying transports.  New protocols can be
    defined, as described in Sections 6.6.1 and 11.1.  As this
    document defines only TLS and DTLS, we use those terms throughout
    the remainder of the document with the understanding that some
    future specification may add new overlay link layers.

Jennings, et al. Standards Track [Page 11] RFC 6940 RELOAD Base January 2014

 To further clarify the roles of the various layers, the following
 figure parallels the architecture with each layer's role from an
 overlay perspective and implementation layer in the Internet:
  Internet    | Internet Model  |
  Model       |   Equivalent    |          Reload
              |   in Overlay    |       Architecture
 -------------+-----------------+------------------------------------
              |                 |    +-------+  +-------+
              |  Application    |    | SIP   |  | XMPP  |  ...
              |                 |    | Usage |  | Usage |
              |                 |    +-------+  +-------+
              |                 |  ----------------------------------
              |                 |+------------------+     +---------+
              |   Transport     ||     Message      |<--->| Storage |
              |                 ||    Transport     |     +---------+
              |                 |+------------------+           ^
              |                 |       ^       ^               |
              |                 |       |       v               v
 Application  |                 |       |     +-------------------+
              |   (Routing)     |       |     |     Topology      |
              |                 |       |     |     Plug-in       |
              |                 |       |     +-------------------+
              |                 |       |         ^
              |                 |       v         v
              |    Network      |    +------------------+
              |                 |    |  Forwarding &    |
              |                 |    | Link Management  |
              |                 |    +------------------+
              |                 |  ----------------------------------
 Transport    |      Link       |     +-------+  +------+
              |                 |     |TLS    |  |DTLS  |  ...
              |                 |     +-------+  +------+
 -------------+-----------------+------------------------------------
   Network    |
              |
     Link     |
 In addition to the above components, nodes may communicate with a
 central provisioning infrastructure (not shown) to get configuration
 information, authentication credentials, and the initial set of nodes
 to communicate with to join the overlay.

Jennings, et al. Standards Track [Page 12] RFC 6940 RELOAD Base January 2014

1.2.1. Usage Layer

 The top layer, called the Usage Layer, has application usages, such
 as the SIP Registration Usage [SIP-RELOAD], that use the abstract
 Message Transport Service 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
 voicemail feature in a softphone application that stores links to the
 messages in the overlay would require a different usage than the type
 of rendezvous service of XMPP or SIP.  A usage may define 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 specification.

1.2.2. Message Transport

 The Message Transport component provides a generic message routing
 service for the overlay.  The Message Transport layer is responsible
 for end-to-end message transactions.  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 the receiving 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 Plug-in are themselves clients of
 the Message Transport, because they need to send and receive messages
 from other peers.

Jennings, et al. Standards Track [Page 13] RFC 6940 RELOAD Base January 2014

 The Message Transport Service is responsible for end-to-end
 reliability, which is accomplished by timer-based retransmissions.
 Unlike the Internet transport layer, however, this layer does not
 provide congestion control.  RELOAD is a request-response protocol,
 with no more than two pairs of request-response messages used in
 typical transactions between pairs of nodes; therefore, there are no
 opportunities to observe and react to end-to-end congestion.  As with
 all Internet applications, implementers are strongly discouraged from
 writing applications that react to loss by immediately retrying the
 transaction.
 The Message Transport Service is similar to those described as
 providing "key-based routing" (KBR) [wikiKBR], although as RELOAD
 supports different overlay algorithms (including non-DHT overlay
 algorithms) that calculate keys (storage indices, not encryption
 keys) in different ways, the actual interface needs to accept
 Resource Names rather than actual keys.
 The Forwarding and Link Management layers are responsible for
 maintaining the overlay in the face of changes in the available nodes
 and underlying network supporting the overlay (the Internet).  They
 also handle congestion control between overlay neighbors, and
 exchange routing updates and data replicas in addition to forwarding
 end-to-end messages.
 Real-world experience has shown that a fixed timeout for the end-to-
 end retransmission timer is sufficient for practical overlay
 networks.  This timer is adjustable via the overlay configuration.
 As the overlay configuration can be rapidly updated, this value could
 be dynamically adjusted at coarse time scales, although algorithms
 for determining how to accomplish this are beyond the scope of this
 specification.  In many cases, however, other means of improving
 network performance, such as having the Topology Plug-in remove lossy
 links from use in overlay routing or reducing the overall hop count
 of end-to-end paths, will be more effective than simply increasing
 the retransmission timer.

1.2.3. Storage

 One of the major functions of RELOAD is storage of data, that is,
 allowing 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 sending a response to the Message Transport for delivery to
 the requesting node.  Typically, these messages will come from other

Jennings, et al. Standards Track [Page 14] RFC 6940 RELOAD Base January 2014

 nodes, but depending on the 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 in use.  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 Plug-in when the Resource-IDs for which it
 is responsible change, and the Storage component is then responsible
 for migrating resources to other peers.

1.2.4. Topology Plug-in

 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 Plug-in so that each overlay
 can select an appropriate overlay algorithm that relies on the common
 RELOAD core protocols and code.
 The Topology Plug-in 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 Plug-in, which adjusts the Routing Table as appropriate.
 The Topology Plug-in 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 Plug-in
 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 Plug-in also keeps track of which peers are
 responsible for which resources.  As peers join and leave, the
 Topology Plug-in 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 Plug-in is also responsible for providing for redundant data
 storage to protect against loss of information in the event of a peer
 failure and to protect against compromised or subversive peers.

Jennings, et al. Standards Track [Page 15] RFC 6940 RELOAD Base January 2014

1.2.5. Forwarding and Link Management Layer

 The Forwarding and Link Management Layer is responsible for getting a
 message to the next peer, as determined by the Topology Plug-in.
 This layer establishes and maintains the network connections as
 needed by the Topology Plug-in.  This layer is also responsible for
 setting 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.
 Congestion control is implemented at this layer to protect the
 Internet paths used to form the link in the overlay.  Additionally,
 retransmission is performed to improve the reliability of end-to-end
 transactions.  The relation of this layer to the Message Transport
 Layer can be likened to the relation of the link-level congestion
 control and retransmission in modern wireless networks ` to Internet
 transport protocols.
 This layer provides a generic interface that allows the Topology
 Plug-in to control the overlay and resource operations and messages.
 Because 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 as a Routing Table.
 The Topology Plug-in actually owns the Routing Table, and forwarding
 decisions are made by querying the Topology Plug-in 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.
 This layer also utilizes a framing header to encapsulate messages as
 they are forwarded along each hop.  This header aids reliability
 congestion control, flow control, etc.  It has meaning only in the
 context of that individual link.
 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:

Jennings, et al. Standards Track [Page 16] RFC 6940 RELOAD Base January 2014

 Connection level:  Connections between nodes are secured with TLS,
    DTLS, or potentially some to-be-defined future protocol.
 Message level:  Each RELOAD message is signed.
 Object Level:  Stored objects are signed by the creating node.
 These three levels of security work together to allow nodes to verify
 the origin and correctness of data they receive from other nodes,
 even in the face of malicious activity by other nodes 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, it can determine whether or not a given operation is
 permitted.
 RELOAD also provides an optional shared-secret-based admission
 control feature using shared secrets and TLS pre-shared keys (PSK) or
 TLS Secure Remote Password (SRP).  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 only.  This
 feature is used together with certificate-based access control, not
 as a replacement for it.  It is typically used when self-signed
 certificates are being used but would generally not be used when the
 certificates were all signed by an enrollment server.

1.4. Structure of This Document

 The remainder of this document is structured as follows.
 o  Section 3 provides definitions of terms used in this document.
 o  Section 4 provides an overview of the mechanisms used to establish
    and maintain the overlay.
 o  Section 5 provides an overview of the mechanism RELOAD provides to
    support other applications.
 o  Section 6 defines the protocol messages that RELOAD uses to
    establish and maintain the overlay.
 o  Section 7 defines the protocol messages that are used to store and
    retrieve data using RELOAD.
 o  Section 8 defines the Certificate Store Usages.
 o  Section 9 defines the TURN Server Usage needed to locate TURN
    (Traversal Using Relays around NAT) servers for NAT traversal.

Jennings, et al. Standards Track [Page 17] RFC 6940 RELOAD Base January 2014

 o  Section 10 defines a specific Topology Plug-in using a Chord-based
    algorithm.
 o  Section 11 defines the mechanisms that new RELOAD nodes use to
    join the overlay for the first time.
 o  Section 12 provides an extended example.

2. Requirements Language

 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].

3. Terminology

 Terms in this document are defined in-line when used and are also
 defined below for reference.  The definitions in this section use
 terminology and concepts that are not explained until later in the
 specification.
 Admitting Peer (AP):  A peer in the overlay which helps the Joining
    Node join the Overlay.
 Bootstrap Node:  A network node used by Joining Nodes to help locate
    the Admitting Peer.
 Client:  A host that is able to store data in and retrieve data from
    the overlay, but does not participate in routing or data storage
    for the overlay.
 Configuration Document:  An XML document containing all the Overlay
    Parameters for one overlay instance.
 Connection Table:  Contains connection information for the set of
    nodes to which a node is directly connected, which include nodes
    that are not yet available for routing.
 Destination List:  A list of Node-IDs, Resource-IDs, and Opaque IDs
    through which a message is to be routed, in strict order.  A
    single Node-ID, Resource-ID, or Opaque ID is a trivial form of
    Destination List.  When multiple Node-IDs are specified, a
    Destination List is a loose source route.  The list is reduced hop
    by hop, and does not include the source but does include the
    destination.

Jennings, et al. Standards Track [Page 18] RFC 6940 RELOAD Base January 2014

 DHT:  A distributed hash table.  A DHT is an abstract storage service
    realized by storing the contents of the hash table across a set of
    peers.
 ID:  A generic term for any kind of identifiers in an Overlay.  This
    document specifies an ID as being an Application-ID, a Kind-ID, a
    Node-ID, a transaction ID, a component ID, a response ID, a
    Resource-ID, or an Opaque ID.
 Joining Node (JN):  A node that is attempting to become a peer in a
    particular Overlay.
 Kind:  A Kind defines a particular type of data that can be stored in
    the overlay.  Applications define new Kinds to store the data they
    use.  Each Kind is identified with a unique integer called a
    Kind-ID.
 Kind-ID:  A unique 32-bit value identifying a Kind.  Kind-IDs are
    either private or allocated by IANA (see Section 14.6).
 Maximum Request Lifetime:  The maximum time a request will wait for a
    response.  This value is equal to the value of the overlay
    reliability value (defined in Section 11.1) multiplied by the
    number of transmissions (defined in Section 6.2.1), and so
    defaults to 15 seconds.
 Node:  The term "node" refers 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
    we use the more specific term (i.e., "client" or "peer") when the
    text applies only to clients or only to peers.
 Node-ID:  A value of fixed but configurable length that uniquely
    identifies a node.  Node-IDs of all 0s and all 1s are reserved.  A
    value of 0 is not used in the wire protocol, but can be used to
    indicate an invalid node in implementations and APIs.  The Node-ID
    of all 1s is used on the wire protocol as a wildcard.
 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.
 Overlay Instance:  A specific overlay algorithm and the collection of
    peers that are collaborating to provide read and write access to
    it.  Any number of overlay instances can be running in an IP
    network at a time, and each operates in isolation of the others.

Jennings, et al. Standards Track [Page 19] RFC 6940 RELOAD Base January 2014

 Overlay Parameters:  A set of values that are shared among all nodes
    in an overlay.  The overlay parameters are distributed in an XML
    document called the Configuration Document.
 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 they are responsible for routing
    messages on behalf of other hosts as needed by the Overlay
    Algorithm.
 Peer Admission:  The act of admitting a node (the Joining Node) into
    an Overlay.  After the admission process is over, the Joining Node
    is a fully functional peer of the overlay.  During the admission
    process, the Joining Node may need to present credentials to prove
    that it has sufficient authority to join the overlay.
 Resource:  An object or group of objects stored in a P2P network.
 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
    unstructured networks, Resource Names may be used directly as
    Resource-IDs and may be of variable length.
 Resource Name:  The 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.  Structured and unstructured
    P2P networks are described in [RFC5694].  A SIP resource, for
    example, is often identified by its AOR (address-of-record), which
    is an example of a Resource Name.
 Responsible Peer:  The peer that is responsible for a specific
    resource, as defined by the Topology Plug-in algorithm.
 Routing Table:  The set of directly connected peers which a node can
    use to forward overlay messages.  In normal operation, these peers
    will all be in the Connection Table, but not vice versa, because
    some peers may not yet be available for routing.  Peers may send

Jennings, et al. Standards Track [Page 20] RFC 6940 RELOAD Base January 2014

    messages directly to peers that are in their Connection Tables,
    but may forward messages to peers that are not in their Connection
    Table only through peers that are in the Routing Table.
 Successor Replacement Hold-Down Time:  The amount of time to wait
    before starting replication when a new successor is found; it
    defaults to 30 seconds.
 Transaction ID:  A randomly chosen identifier selected by the
    originator of a request that is used to correlate requests and
    responses.
 Usage:  The definition of a set of data structures (data Kinds) that
    an application wants to store in the overlay.  A usage may also
    define a set of network protocols (Application IDs) that can be
    tunneled over TLS or DTLS direct connections between nodes.  For
    example, the SIP Usage defines a SIP registration data Kind, which
    contains information on how to reach a SIP endpoint, and two
    Application IDs corresponding to the SIP and SIPS protocols.
 User:  A physical person identified by the certificates assigned to
    them.
 User Name:  A name identifying a user of the overlay, typically used
    as a Resource Name or as a label on a resource that identifies the
    user owning the resource.

4. 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.

4.1. Security and Identification

 The overlay parameters are specified in a Configuration Document.
 Because the parameters include security-critical information, such as
 the certificate signing trust anchors, the Configuration Document
 needs to be retrieved securely.  The initial Configuration Document
 is either initially fetched over HTTPS or manually provisioned.
 Subsequent Configuration Document updates are received either as a
 result of being refreshed periodically by the configuration server,
 or, more commonly, by being flood-filled through the overlay, which
 allows for fast propagation once an update is pushed.  In the latter
 case, updates are via digital signatures that trace back to the
 initial Configuration Document.

Jennings, et al. Standards Track [Page 21] RFC 6940 RELOAD Base January 2014

 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 the node's position in the overlay topology (if the
    overlay is structured; overlays do not need to be structured).
 o  To determine the set of resources for which the node is
    responsible.
 Each node has a certificate [RFC5280] containing its Node-ID in a
 subjectAltName extension, which is unique within an overlay instance.
 The certificate serves multiple purposes:
 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 uses this certificate so that a node connecting to it
    knows it is connected to the correct node.  (Technically, a TLS or
    DTLS association with client authentication is formed.)  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 multiple certificate issuance models.  The first is
 based on a central enrollment process, which allocates a unique name
 and Node-ID and puts them in a certificate 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.
 The second model is useful in settings, when 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

Jennings, et al. Standards Track [Page 22] RFC 6940 RELOAD Base January 2014

 setting up an enrollment server.  RELOAD supports the use of self-
 generated, self-signed certificates.  When self-signed certificates
 are used, the node also generates its own Node-ID and user name.  The
 Node-ID is computed as a digest of the public key, to prevent Node-ID
 theft.  Note that the relevant cryptographic property for the digest
 is partial preimage resistance.  Collision resistance is not needed,
 because an attacker who can create two nodes with the same Node-ID
 but a different public key obtains no advantage.  This model is still
 subject to a number of known attacks (most notably, Sybil attacks
 [Sybil]) and can be safely used only in closed networks where users
 are mutually trusting.  Another drawback of this approach is that the
 user's data is then tied to their key, so if a key is changed, any
 data stored under their Node-ID needs to be re-stored.  This is not
 an issue for centrally issued Node-IDs provided that the
 Certification Authority (CA) reissues the same Node-ID when a new
 certificate is generated.
 The general principle here is that the security mechanisms (TLS or
 DTLS at the data link layer and message signatures at the message
 transport layer) 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
 used to chain to a single root of trust.

4.1.1. 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
 to authenticate the peer-to-peer connections via TLS-PSK [RFC4279] or
 TLS-SRP [RFC5054].

4.2. Clients

 RELOAD defines a single protocol that is used both as the peer
 protocol and as the client protocol for the overlay.  Having a single
 protocol 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 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 devices as "nodes".

Jennings, et al. Standards Track [Page 23] RFC 6940 RELOAD Base January 2014

 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
 certain 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.
 In RELOAD, unlike some other designs, clients are not first-class
 entities.  From the perspective of a peer, a client is a node that
 has connected to the overlay, but that has not yet taken steps to
 insert itself into the overlay topology.  It might never do so (if
 it's a client), or it might eventually do so (if it's just a node
 that is taking a long time to join).  The routing and storage rules
 for RELOAD provide for correct behavior by peers regardless of
 whether other nodes attached to them are clients or peers.  Of
 course, a client implementation needs to know that it intends to be a
 client, but this localizes complexity only to that node.
 For more discussion about the motivation for RELOAD's client support,
 see Appendix B.

4.2.1. Client Routing

 Clients may insert themselves in the overlay in two ways:
 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 may require a TURN [RFC5766]
    relay in cases where NATs or firewalls prevent a client from
    forming a direct connection with its responsible peer.  Note that
    clients that choose this option need to process Update messages
    from the peer (Section 6.4.2.3).  These updates can indicate that
    the peer is no longer responsible for the client's Node-ID.  The
    client would then need to form a connection to the appropriate
    peer.  Failure to do so will result in the client no longer
    receiving messages.
 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
    (Section 6.3.2.2) 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 using only its Node-ID.  If the client is to

Jennings, et al. Standards Track [Page 24] RFC 6940 RELOAD Base January 2014

    receive incoming requests from other members of the overlay, the
    Destination List needed to reach the client needs to be learnable
    via other mechanisms, such as being stored in the overlay by a
    usage.  A client connected this way using a certificate with only
    a single Node-ID can proceed to use the connection without
    performing an Attach (Section 6.5.1).  A client wishing to connect
    using this mechanism with a certificate with multiple Node-IDs can
    use a Ping (Section 6.5.3) to probe the Node-ID of the node to
    which it is connected before performing the Attach.

4.2.2. Minimum Functionality Requirements for Clients

 A node may act as a client simply because it does not have the
 capacity or need to act as a peer in the overlay, or because it does
 not even have an implementation of the Topology Plug-in defined in
 Section 6.4.1, needed to act as a peer in the overlay.  In order to
 exchange RELOAD messages with a peer, a client needs to meet a
 minimum level of functionality.  Such a client will:
 o  Implement RELOAD's connection-management operations 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 needed by the overlay that 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 an appropriate algorithm for the
 overlay.

4.3. Routing

 This section discusses the capabilities of RELOAD's routing layer and
 the protocol features used to implement the capabilities, and
 provides a brief overview of how they are used.  Appendix A discusses
 some alternative designs and the trade-offs that would be necessary
 to support them.

Jennings, et al. Standards Track [Page 25] RFC 6940 RELOAD Base January 2014

 RELOAD's routing provides the following capabilities:
 Resource-based Routing:   RELOAD supports routing messages based
    solely on the name of the resource.  Such messages are delivered
    to a node that is responsible for that resource.  Both structured
    and unstructured overlays are supported, so the route may not be
    deterministic for all Topology Plug-ins.
 Node-based Routing:   RELOAD supports routing messages to a specific
    node in the overlay.
 Clients:   RELOAD supports requests from and to clients that do not
    participate in overlay routing.  The clients are located via
    either of the mechanisms described above.
 NAT Traversal:   RELOAD supports establishing and using connections
    between nodes separated by one or more NATs, including locating
    peers behind NATs for those overlays allowing/requiring it.
 Low State:   RELOAD's routing algorithms do not require significant
    state (i.e., state linear or greater in the number of outstanding
    messages that have passed through it) to be stored on intermediate
    peers.
 Routability in Unstable Topologies:   Overlay topology changes
    constantly in an overlay of moderate size due to the failure of
    individual nodes and links in the system.  RELOAD's routing allows
    peers to reroute messages when a failure is detected, and replies
    can be returned to the requesting node as long as the peers that
    originally forwarded the successful request do not fail before the
    response is returned.
 RELOAD's routing utilizes three basic mechanisms:
 Destination Lists:   While, in principle, it is possible to just
    inject a message into the overlay with a single Node-ID as the
    destination, RELOAD provides a source-routing capability in the
    form of "Destination Lists".  A Destination List provides a list
    of the nodes through which a message flows in order (i.e., it is
    loose source routed).  The minimal Destination List contains just
    a single value.
 Via Lists:   In order to allow responses to follow the same path as
    requests, each message also contains a "Via List", which is
    appended to by each node a message traverses.  This Via List can
    then be inverted and used as a Destination List for the response.

Jennings, et al. Standards Track [Page 26] RFC 6940 RELOAD Base January 2014

 RouteQuery:   The RouteQuery 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 (see
    Section 6.4.2.4).
 The basic routing mechanism that RELOAD uses is symmetric recursive.
 We will first describe symmetric recursive routing and then discuss
 its advantages in terms of the requirements discussed above.
 Symmetric recursive routing requires that a request message follow a
 path through the overlay to the destination: each peer forwards the
 message closer to its destination.  The return path of the response
 goes through the same nodes as the request (though it may also go
 through some new intermediate nodes due to topology changes).  Note
 that a failure on the reverse path caused by a topology change after
 the request was sent will be handled by the end-to-end retransmission
 of the response as described in Section 6.2.1.  For example, the
 following figure shows a message following a route from A to Z
 through B and X:
 A         B         X         Z
 -------------------------------
  1. ———>

Dest=Z

  1. ———>

Via=A

          Dest=Z
                    ---------->
                    Via=A,B
                    Dest=Z
                    <----------
                     Dest=X,B,A
          <----------
             Dest=B,A
 <----------
      Dest=A
 Note that this 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 and save the truncated information
 internally using the transaction ID as a key to allow it to be
 retrieved later.  Later, when the response message arrives, the

Jennings, et al. Standards Track [Page 27] RFC 6940 RELOAD Base January 2014

 transaction ID would be used to recover the truncated information and
 return the response message along the path from which the request
 arrived.  This option requires a greater amount of state to be stored
 on intermediate peers, but saves a small amount of bandwidth and
 reduces the need for modifying the message en 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 to X1 and
 saving the state internally.
 A         B         X         Z
 -------------------------------
  1. ———>

Dest=Z

  1. ———>

Via=A

          Dest=Z
                    ---------->
                    Via=X1
                    Dest=Z
                    <----------
                      Dest=X,X1
            <----------
               Dest=B,A
 <----------
      Dest=A
 As before, when B receives the message, B creates a Via List
 consisting of [A].  However, instead of sending [A, B], X creates an
 opaque ID X1 which maps internally to [A, B] (perhaps by being an
 encryption of [A, B]) and then forwards to Z with only X1 as the Via
 List.  When the response arrives at X, it maps X1 back to [A, B],
 then inverts it to produce the new Destination List [B, A], and
 finally routes it to B.
 RELOAD also supports a basic iterative "routing" mode, in which 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 RouteQuery method (see
 Section 6.4.2.4), which requests this behavior.  Note that iterative
 routing is selected only by the initiating node.

Jennings, et al. Standards Track [Page 28] RFC 6940 RELOAD Base January 2014

4.4. 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 Interactive Connectivity Establishment (ICE)
 [RFC5245] 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, either
 to join the overlay or to add more connections in its Routing Table.
 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 MAY send messages directly between themselves without going
 through other overlay peers.  In other words, A and B are in each
 other's Connection Tables.  They MAY then execute an Update process,
 resulting in additions to each other's Routing Tables, and may then
 become able to route messages through each other to other overlay
 nodes.
 There are two cases where Attach is not used.  The first is when a
 peer is joining the overlay and is not connected to any peers.  In
 order to support this case, a small number of bootstrap nodes
 typically need to be publicly accessible so that new peers can
 directly connect to them.  Section 11 contains more detail on this.
 The second case is when a client connects to a peer at an arbitrary
 IP address, rather than to its responsible peer, as described in the
 second bullet point of Section 4.2.1.
 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 on what "enough" and "near" mean
 depend on the specific overlay).  If a peer cannot form a connection
 to some other peer, this is not necessarily a disaster; overlays can
 route correctly even without fully connected links.  However, a peer
 needs to try to maintain the specified Routing Table defined by the
 Topology Plug-in algorithm and needs to form new connections if it
 detects that it has fewer direct connections than specified by the
 algorithm.  This also implies that peers, in accordance with the
 Topology Plug-in algorithm, need to periodically verify that the
 connected peers are still alive and, if not, need to try to re-form
 the connections or form alternate ones.  See Section 10.7.4.3 for an
 example on how a specific overlay algorithm implements these
 constraints.

Jennings, et al. Standards Track [Page 29] RFC 6940 RELOAD Base January 2014

4.5. Overlay Algorithm Support

 The Topology Plug-in allows RELOAD to support a variety of overlay
 algorithms.  This specification defines a DHT based on Chord, which
 is mandatory to implement, but the base RELOAD protocol is designed
 to support a variety of overlay algorithms.  The information needed
 to implement this DHT is fully contained in this specification, but
 it is easier to understand if you are familiar with Chord-based
 [Chord] DHTs.  A nice tutorial can be found at [wikiChord].

4.5.1. Support for Pluggable Overlay Algorithms

 RELOAD defines three methods for overlay maintenance: Join, Update,
 and Leave.  However, the contents of these messages, when they are
 sent, and their precise semantics are specified by the actual overlay
 algorithm, which is specified by configuration for all nodes in the
 overlay and thus is known to nodes before they attempt to join the
 overlay.  RELOAD merely provides a framework of commonly needed
 methods that provide uniformity of notation (and ease of debugging)
 for a variety of overlay algorithms.

4.5.2. Joining, Leaving, and Maintenance Overview

 When a new peer wishes to join the Overlay Instance, it will need a
 Node-ID that it is allowed to use and a set of credentials which
 match that Node-ID.  When an enrollment server is used, the Node-ID
 used is the one found in the certificate 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  Form connections to some other peers.
 o  Acquire the data values this peer is responsible for storing.
 o  Inform 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 to form a connection to some
 bootstrap node.  Because this is the first connection the peer makes,
 these nodes will need public IP addresses so that they 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.  After a peer has
 connected to the overlay for the first time, it can cache the set of
 past adjacencies which have public IP addresses and can attempt to
 use them as future bootstrap nodes.  Note that this requires some

Jennings, et al. Standards Track [Page 30] RFC 6940 RELOAD Base January 2014

 notion of which addresses are likely to be public as discussed in
 Section 9.
 After a 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  Form connections to other peers in the overlay to populate its
    Routing Table.
 o  Get a copy of the data it is now responsible for storing, and
    assume responsibility for that data.
 The second operation is performed by contacting the Admitting Peer
 (AP), the node which is currently responsible for the relevant
 section of the overlay.
 The details of this operation depend mostly on the overlay algorithm
 involved, but a typical case would be:
 1.  JN sends a Join request to AP announcing its intention to join.
 2.  AP sends a Join response.
 3.  AP does a sequence of Stores to JN to give it the data it will
     need.
 4.  AP does Updates to JN and to other peers to tell them about its
     own Routing Table.  At this point, both JN and AP consider JN
     responsible for some section of the Overlay Instance.
 5.  JN makes its own connections to the appropriate peers in the
     Overlay Instance.
 After this process completes, JN 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 other nodes.  In order to support this case,
 potential first nodes (which can also initially serve as bootstrap
 nodes) need to somehow be instructed that they are the entire
 overlay, rather than part of an existing overlay (e.g., by comparing
 their IP address to the bootstrap IP addresses in the configuration
 file).
 Note that clients do not perform either of these operations.

Jennings, et al. Standards Track [Page 31] RFC 6940 RELOAD Base January 2014

4.6. First-Time Setup

 Previous sections addressed how RELOAD works after 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 an account name and password, and can leverage these into
 having a working peer with minimal user intervention.  This helps
 avoid the problems that have been experienced with conventional SIP
 clients in which users need to manually configure a large number of
 settings.

4.6.1. Initial Configuration

 In the first phase of the setup process, the user starts with the
 name of the overlay and uses it to download an initial set of overlay
 configuration parameters.  The node does a DNS SRV [RFC2782] lookup
 on the overlay name to get the address of a configuration server.  It
 can then connect to this server with HTTPS [RFC2818] 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.  The details of the relationships between names in
 the HTTPS certificates and the overlay names are described in
 Section 11.2.
 If a node already has the valid Configuration Document that it
 received by an out-of-band method, this step can be skipped.  Note
 that this out-of-band method needs to provide authentication and
 integrity, because the Configuration Document contains the trust
 anchors used by the overlay.

4.6.2. 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 this case, the
 Configuration Document will contain the address of an enrollment
 server which can be used to obtain such a certificate and will also
 contain the trust anchor, so this document must be retrieved securely
 (see Section 11.2).  The enrollment server may (and probably will)
 require some sort of account name for the user and a password before
 issuing the certificate.  The enrollment server's ability to ensure
 attackers cannot get a large number of certificates for the overlay
 is one of the cornerstones of RELOAD's security.

Jennings, et al. Standards Track [Page 32] RFC 6940 RELOAD Base January 2014

4.6.3. Diagnostics

 Significant advice around managing a RELOAD overlay and extensions
 for diagnostics are described in [P2P-DIAGNOSTICS].

5. 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:
 o  To store data in the overlay and to 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 without using the overlay.
 This section provides an overview of these services.

5.1. 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 and 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  | |                 |
                    | | +--------+ |                 |
                    | +------------+                 |
                    +--------------------------------+

Jennings, et al. Standards Track [Page 33] RFC 6940 RELOAD Base January 2014

 Each Kind is identified by a Kind-ID, which is a code point either
 assigned by IANA or allocated out of a private range.  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, while some sets may be
 allowed to contain multiple identical items, and others may have only
 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,
 although other usages can define their own structures:
 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 individually digitally signed by the node which
 created it.  When a value is retrieved, the digital signature can be
 verified to detect tampering.  If the certificate used to verify the
 stored value signature expires, the value can no longer be retrieved
 (although it may not be immediately garbage collected by the storing
 node), and the creating node will need to store the value again if it
 desires that the stored value continue to be available.

5.1.1. 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 needs to store for other nodes.  RELOAD
 addresses this issue by allowing any given node to store data only 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.  Thus, only a user with a
 certificate for "alice@example.org" could write to that location in

Jennings, et al. Standards Track [Page 34] RFC 6940 RELOAD Base January 2014

 the overlay (see Section 11.3).  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 undetectably
 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
 configuration to limit size imbalances of various Kinds.

5.1.2. 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,
 and replicating along the path to the responsible peer.
 The core RELOAD specification does not specify a particular
 replication strategy.  Instead, the first level of replication
 strategies is determined by the overlay algorithm, which can base the
 replication strategy on its particular topology.  For example, Chord
 places replicas on successor peers, which will take over
 responsibility if the responsible peer fails [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 independently of the
 underlying algorithm, and their usage can be determined based on the
 needs of the particular usage.

Jennings, et al. Standards Track [Page 35] RFC 6940 RELOAD Base January 2014

5.2. Usages

 By itself, the distributed storage layer provides only the
 infrastructure on which applications are built.  In order to do
 anything useful, a usage needs to be defined.  Each usage needs to
 specify several things:
 o  Register Kind-ID code points for any Kinds that the usage defines
    (Section 14.6).
 o  Define the data structure for each of the Kinds (the value member
    in Section 7.2).  If the data structure contains character
    strings, conversion rules between characters and the binary
    storage need to be specified.
 o  Define access control rules for each of the Kinds (Section 7.3).
 o  Define how the Resource Name is used to form the Resource-ID where
    each Kind is stored.
 o  Describe how values will be merged when a network partition is
    being healed.
 The Kinds defined by a usage may also be applied to other usages.
 However, a need for different parameters, such as a different access
 control model, would imply the need to create a new Kind.

5.3. Service Discovery

 RELOAD does not currently define a generic service discovery
 algorithm as part of the base protocol, although a simplistic TURN-
 specific discovery mechanism is provided.  A variety of service
 discovery algorithms can be implemented as extensions to the base
 protocol, such as the service discovery algorithm ReDIR
 [opendht-sigcomm05] and [REDIR-RELOAD].

5.4. Application Connectivity

 There is no requirement that a RELOAD Usage needs to 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, it is the overlay itself --
 rather than an external authority such as DNS -- which is used to
 establish a connection.  RELOAD provides connectivity to applications
 using the AppAttach method.  For example, if a P2PSIP node wishes to

Jennings, et al. Standards Track [Page 36] RFC 6940 RELOAD Base January 2014

 establish a SIP dialog with another P2PSIP node, it will use
 AppAttach to establish a direct connection with the other node.  This
 new connection is separate from the peer protocol connection.  It is
 a dedicated DTLS or TLS flow used only for the SIP dialog.

6. 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
 messages.  We then describe the symmetric recursive routing model,
 which is RELOAD's default routing algorithm.  Finally, we define the
 message structure and the messages used to join and maintain the
 overlay.

6.1. Message Receipt and Forwarding

 When a node 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, as defined in
 Section 6.3.2, it is an error and the message MUST be discarded.  The
 peer SHOULD generate an appropriate error, but local policy can
 override this and cause the message to be silently dropped.
 Once the peer has determined that the message is correctly formatted
 (note that this does not include signature-checking on intermediate
 nodes as the message may be fragmented), 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.  A peer is always responsible for the
    wildcard Node-ID.  Handling of this case is described in
    Section 6.1.1.
 o  The first entry on the Destination List is an ID for which another
    peer is responsible.  Handling of this case is described in
    Section 6.1.2.
 o  The first entry on the Destination List is an opaque ID that is
    being used for Destination List compression.  Handling of this
    case is described in Section 6.1.3.  Note that opaque IDs can be
    distinguished from Node-IDs and Resource-IDs on the wire as
    described in Section 6.3.2.2.
 These cases are handled as discussed below.

Jennings, et al. Standards Track [Page 37] RFC 6940 RELOAD Base January 2014

6.1.1. Responsible ID

 If the first entry on the Destination List is an ID for which the
 peer is responsible, there are several (mutually exclusive) subcases
 to consider.
 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, so the node MUST verify the signature as described in
    Section 7.1 and MUST pass it to the upper layers.  "Upper layers"
    is used here to mean the components above the "Overlay Link
    Service Boundary" line in the figure in Section 1.2.
 o  If the entry is a Node-ID which equals this node's Node-ID, then
    the message is destined for this node.  If it is the only entry on
    the Destination List, the message is destined for this node and so
    the node passes it to the upper layers.  Otherwise, the node
    removes the entry from the Destination List and repeats the
    routing process with the next entry on the Destination List.  If
    the message is a response and list compression was used, then the
    node first modifies the Destination List to reinsert the saved
    state, e.g., by unpacking any opaque IDs.
 o  If the entry is the wildcard Node-ID (all "1"s), the message is
    destined for this node, and the node passes the message to the
    upper layers.  A message with a wildcard Node-ID as its first
    entry is never forwarded; it is consumed locally.
 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 the latter case, the node MUST attempt to
    forward the message to the destination node as described in the
    next section (though this may fail for connectivity reasons,
    because the TTL has expired, or because of some other error.)
 Note that this process 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.

6.1.2. Other ID

 If the first entry on the Destination List is neither an opaque ID
 nor an ID the peer is responsible for, then the peer MUST forward the
 message towards that entry.  This means that it MUST select one of
 the peers to which it is connected and which is most likely to be
 responsible (according to the Topology Plug-in) for the first entry

Jennings, et al. Standards Track [Page 38] RFC 6940 RELOAD Base January 2014

 on the Destination List.  For the CHORD-RELOAD topology, the routing
 to the most likely responsible node is explained in Section 10.3.  If
 the first entry on the Destination List is in the peer's Connection
 Table, the peer MUST forward the message to that peer directly.
 Otherwise, the peer consults the Routing Table to forward the
 message.
 Any intermediate peer which forwards a RELOAD request MUST ensure
 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.  The peer selects one of these approaches:
 o  The peer can add an entry to the Via List in the forwarding header
    that will enable it to determine the correct node.  This is done
    by appending to the Via List the Node-ID of the node from which
    the request was received.
 o  The peer can keep per-transaction state which will allow it to
    determine the correct node.
 As an example of the first strategy, consider an example with nodes
 A, B, C, D, and E.  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 (as assigned by A) 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 peers which modify the Via List are not required to
 simply add entries.  The only requirement is that the peer MUST be
 able to reconstruct the correct Destination List on the return route.
 RELOAD provides explicit support for this functionality in the form
 of opaque 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 opaque 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 an opaque ID, recover the
 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".  Possibilities for an opaque ID include a

Jennings, et al. Standards Track [Page 39] RFC 6940 RELOAD Base January 2014

 compressed and/or encrypted version of the original Via List and an
 index into a state database containing the original Via List, but the
 details are a local matter.
 No matter what mechanism for storing Via List state is used, 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.
 Note that if an intermediate peer retains per-transaction state
 instead of modifying the Via List, it needs some mechanism for timing
 out that state; otherwise, its state database will grow without
 bound.  Whatever algorithm is used, unless a FORWARD_CRITICAL
 forwarding option (Section 6.3.2.3) or an overlay configuration
 option explicitly indicates this state is not needed, the state MUST
 be maintained for at least the value of the overlay-reliability-timer
 configuration parameter and MAY be kept longer.  Future extensions,
 such as [P2PSIP-RELAY], may define mechanisms for determining when
 this state does not need to be retained.
 There is no requirement to ensure that a request issued after the
 receipt of a response follows the same path as the response.  As a
 consequence, there is no requirement to use either of the mechanisms
 described above (Via List or state retention) when processing a
 response message.
 A node receiving a request from another node MUST ensure that any
 response to that request exits that node with a Destination List
 equal to the concatenation of the Node-ID of the node from which the
 request was received with the Via List in the original request.  The
 intermediate node normally learns the Node-ID that the other node is
 using via an Attach, but a node using a certificate with a single
 Node-ID MAY elect not to send an Attach (see Section 4.2.1, bullet
 2).  If a node with a certificate with multiple Node-IDs attempts to
 route a message other than a Ping or Attach through a node without
 performing an Attach, the receiving node MUST reject the request with
 an Error_Forbidden error.  The node MUST implement support for
 returning responses to a Ping or Attach request made by a Joining
 Node Attaching to its responsible peer.

6.1.3. Opaque ID

 If the first entry on the Destination List is an opaque ID (e.g., a
 compressed Via List), the peer MUST replace the entry with the
 original Via List that it replaced and then re-examine the
 Destination List to determine which of the three cases in Section 6.1
 now applies.

Jennings, et al. Standards Track [Page 40] RFC 6940 RELOAD Base January 2014

6.2. 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.  That is, a node in an overlay which supports
 Symmetric Recursive Routing and some other routing algorithm called
 XXX might use Symmetric Recursive Routing some of the time and XXX at
 other times.

6.2.1. Request Origination

 In order to originate a message to a given Node-ID or Resource-ID, a
 node MUST construct an appropriate Destination List.  The simplest
 such Destination List is a single entry containing the Node-ID or
 Resource-ID.  The resulting message MUST be forwarded to its
 destination via the normal overlay routing mechanisms.  The node MAY
 also construct a more complicated Destination List for source
 routing.
 Once the message is constructed, the node sends the message to an
 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 Plug-in MUST be consulted to
 determine the appropriate next hop.
 Parallel requests for a resource are a common solution to improve
 reliability in the face of churn or subversive peers.  Parallel
 searches for usage-specified replicas are managed by the usage layer,
 for instance, by having the usage store data at multiple
 Resource-IDs, with the requesting node sending requests to each of
 those Resource-IDs.  However, a single request MAY also be routed
 through multiple adjacent peers, even when they are known to be
 suboptimal, to improve reliability [vulnerabilities-acsac04].  Such
 parallel searches MAY be specified by the Topology Plug-in, in which
 case it would return multiple next hops and the request would be
 routed to all of them.
 Because messages can 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 timer to the
 current overlay-reliability-timer.  If a response has not been
 received when the timer fires, the request MUST be 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 MUST be considered to have failed.

Jennings, et al. Standards Track [Page 41] RFC 6940 RELOAD Base January 2014

 Although the originating node will be doing both end-to-end and hop-
 by-hop retransmissions, the end-by-end retransmission procedure is
 not followed by intermediate nodes.  They follow the hop-by-hop
 reliability procedure described in Section 6.6.3.
 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 the 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.

6.2.2. Response Origination

 When a peer sends a response to a request using this routing
 algorithm, 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) and possibly with extra nodes
 (loose routing).

6.3. 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 (TLV) 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 only increase bandwidth
 and introduce new potential interoperability issues.
 Each message has three parts, which are concatenated, as shown below:
   +-------------------------+
   |    Forwarding Header    |
   +-------------------------+
   |    Message Contents     |
   +-------------------------+
   |     Security Block      |
   +-------------------------+

Jennings, et al. Standards Track [Page 42] RFC 6940 RELOAD Base January 2014

 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.
    Section 6.3.2 describes the format of this part.
 Message Contents:  The message being delivered between the peers.
    From the perspective of the forwarding layer, the contents are
    opaque; however, they are interpreted by the higher layers.
    Section 6.3.3 describes the format of this part.
 Security Block:  A security block containing certificates and a
    digital signature over the "Message Contents" section.  Note that
    this signature can be computed without parsing the message
    contents.  All messages MUST be signed by their originator.
    Section 6.3.4 describes the format of this part.

6.3.1. Presentation Language

 The structures defined in this document are defined using a C-like
 syntax based on the presentation language used to define TLS
 [RFC5246].  Advantages of this style include:
 o  It is familiar enough 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  It is possible to mechanically compile encoders and decoders.
 Several idiosyncrasies of this language are worth noting:
 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,
 which corresponds to up to 127 values of two bytes (16 bits) each.

Jennings, et al. Standards Track [Page 43] RFC 6940 RELOAD Base January 2014

 A repetitive structure member shares a common notation with a member
 containing a variable-length block of data.  The latter always starts
 with "opaque", whereas the former does not.  For instance, the
 following denotes a variable block of data:
                   opaque data<0..2^32-1>;
 whereas the following denotes a list of 0, 1, or more instances of
 the Name element:
                   Name names<0..2^32-1>;

6.3.1.1. Common Definitions

 This section provides an introduction to the presentation language
 used throughout RELOAD.
 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.  The max value of 255 indicates
 that this is represented as a single byte on the wire.
 The NodeId, shown below, represents a single Node-ID.
           typedef opaque       NodeId[NodeIdLength];
 A NodeId is a fixed-length structure represented as a series of
 bytes, with the most significant byte first.  The length is set on a
 per-overlay basis within the range of 16-20 bytes (128 to 160 bits).
 (See Section 11.1 for how NodeIdLength is set.)  Note that the use of
 "typedef" here is an extension to the TLS language, but its meaning
 should be relatively obvious.  Also note that the [ size ] syntax
 defines a fixed-length element that does not include the length of
 the element in the on-the-wire encoding.
 A ResourceId, shown below, represents a single Resource-ID.
           typedef opaque       ResourceId<0..2^8-1>;
 Like a NodeId, a ResourceId is an opaque string of bytes, but unlike
 NodeIds, ResourceIds are variable length, up to 254 bytes (2040 bits)
 in length.  On the wire, each ResourceId is preceded by a single

Jennings, et al. Standards Track [Page 44] RFC 6940 RELOAD Base January 2014

 length byte (allowing lengths up to 255 bytes).  Thus, the 3-byte
 value "FOO" would be encoded as: 03 46 4f 4f.  Note the < range >
 syntax defines a variable length element that includes the length of
 the element in the on-the-wire encoding.  The number of bytes to
 encode the length on the wire is derived by range; i.e., it is the
 minimum number of bytes which can encode the largest range value.
 A more complicated example is IpAddressPort, which represents a
 network address and can be used to carry either an IPv6 or IPv4
 address:
      enum { invalidAddressType(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 (IPv4 or IPv6).
 length:  The length of the rest of the structure.

Jennings, et al. Standards Track [Page 45] RFC 6940 RELOAD Base January 2014

 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
 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
 "6084".
           01           ; type    = IPv4
           06           ; length  = 6
           c0 00 02 01  ; address = 192.0.2.1
           17 c4        ; port    = 6084
 Unless a given structure that uses a select explicitly allows for
 unknown types in the select, any unknown type SHOULD be treated as a
 parsing error, and the whole message SHOULD be discarded with no
 response.

6.3.2. Forwarding Header

 The forwarding header is defined as a ForwardingHeader structure, as
 shown below.
      struct {
        uint32             relo_token;
        uint32             overlay;
        uint16             configuration_sequence;
        uint8              version;
        uint8              ttl;
        uint32             fragment;
        uint32             length;
        uint64             transaction_id;
        uint32             max_response_length;
        uint16             via_list_length;
        uint16             destination_list_length;
        uint16             options_length;
        Destination        via_list[via_list_length];
        Destination        destination_list
                             [destination_list_length];
        ForwardingOption   options[options_length];
      } ForwardingHeader;

Jennings, et al. Standards Track [Page 46] RFC 6940 RELOAD Base January 2014

 The contents of the structure are:
 relo_token:  The first four bytes identify this message as a RELOAD
    message.  This field MUST contain the value 0xd2454c4f (the string
    "RELO" with the high bit of the first byte set).
 overlay:  The 32-bit checksum/hash of the overlay being used.  This
    MUST be formed by taking the lower 32 bits of the SHA-1 [RFC3174]
    hash of the overlay name.  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.  The
    overlay name MUST consist of a sequence of characters that would
    be allowable as a DNS name.  Specifically, as it is used in a DNS
    lookup, it will need to be compliant with the grammar for the
    domain as specified in Section 2.3.1 of [RFC1035].
 configuration_sequence:  The sequence number of the configuration
    file.  See Section 6.3.2.1 for details.
 version:  The version of the RELOAD protocol being used times 10.
    RELOAD version numbers are fixed-point decimal numbers between
    fixed-point integer between 0.1 and 25.4.  This document describes
    version 1.0, with a value of 0x0a.  (Note that versions used prior
    to the publication of this RFC used version number 0.1.)  Nodes
    MUST reject messages with other versions.
 ttl:  An 8-bit field indicating the number of iterations, or hops, a
    message can experience before it is discarded.  The TTL (time-to-
    live) value MUST be decremented by one at every hop along the
    route the message traverses just before transmission.  If a
    received message has a TTL of 0 and the message is not destined
    for the receiving node, then the message MUST NOT be propagated
    further, and an Error_TTL_Exceeded error should be generated.  The
    initial value of the TTL SHOULD be 100 and MUST NOT exceed 100
    unless defined otherwise by the overlay configuration.
    Implementations which receive messages with a TTL greater than the
    current value of initial-ttl (or the default of 100) MUST discard
    the message and send an Error_TTL_Exceeded error.
 fragment:  This field is used to handle fragmentation.  The high bit
    (0x80000000) MUST be set for historical reasons.  If the next bit
    (0x40000000) is set to 1, it indicates that this is the last (or
    only) fragment.  The next six bits (0x20000000 through 0x01000000)
    are reserved and SHOULD be set to zero.  The remainder of the
    field is used to indicate the fragment offset; see Section 6.7 for
    details.

Jennings, et al. Standards Track [Page 47] RFC 6940 RELOAD Base January 2014

 length:  The count in bytes of the size of the message, including the
    header, after the eventual fragmentation.
 transaction_id:  A unique 64-bit number that identifies this
    transaction and also allows receivers to disambiguate transactions
    which are otherwise identical.  In order to provide a high
    probability that transaction IDs are unique, they MUST be randomly
    generated.  Responses use the same transaction ID as the request
    to which they correspond.  Transaction IDs are also used for
    fragment reassembly.  See Section 6.7 for details.
 max_response_length:  The maximum size in bytes of a response.  This
    is used by requesting nodes to avoid receiving (unexpected) very
    large responses.  If this value is non-zero, responding peers MUST
    check that any response would not exceed it and if so generate an
    Error_Incompatible_with_Overlay value.  This value SHOULD be set
    to zero for responses.
 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.
 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.  In stateless cases, the
    previous hop that the message is from is appended to the Via List
    as specified in Section 6.1.2.
 destination_list:  The destination_list contains a sequence of
    destinations through which the message should pass.  The
    Destination List is constructed by the message originator.  The
    first element on the Destination List is where the message goes
    next.  Generally, the list shrinks as the message traverses each
    listed peer, though if list compression is used, this may not be
    true.
 options:  Contains a series of ForwardingOption entries.  See
    Section 6.3.2.3.

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6.3.2.1. 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 MUST
 contain a sequence number which MUST be monotonically increasing mod
 65535.  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, the node 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, the
 node MUST generate a ConfigUpdate 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 65535 and the message type
 is ConfigUpdate, then the message MUST be accepted regardless of the
 receiving node's configuration sequence number.  Since 65535 is a
 special value, peers sending a new configuration when the
 configuration sequence is currently 65534 MUST set the configuration
 sequence number to 0 when they send a new configuration.

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6.3.2.2. Destination and Via Lists

 The Destination List and Via List are sequences of Destination
 values:
   enum { invalidDestinationType(0), node(1), resource(2),
          opaque_id_type(3), /* 128-255 not allowed */ (255) }
        DestinationType;
   select (destination_type) {
    case node:
           NodeId               node_id;
    case resource:
           ResourceId           resource_id;
    case opaque_id_type:
           opaque               opaque_id<0..2^8-1>;
        /* This structure may be extended with new types */
   } DestinationData;
   struct {
      DestinationType         type;
      uint8                   length;
      DestinationData         destination_data;
   } Destination;
   struct {
      uint16               opaque_id; /* Top bit MUST be 1 */
   } Destination;
 If the destination structure is a 16-bit integer, then the first bit
 MUST be set to 1, and it MUST be treated as if it were a full
 structure with a DestinationType of opaque_id_type and an opaque_id
 that was 2 bytes long with the value of the 16-bit integer.  If the
 destination structure starts with DestinationType, then the first bit
 MUST be set to 0, and the destination structure must use a TLV
 structure with the following contents:
 type
    The type of the DestinationData Payload Data Unit (PDU).  It may
    be one of "node", "resource", or "opaque_id_type".
 length
    The length of the destination_data.

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 destination_data
    The destination value itself, which is an encoded DestinationData
    structure that depends on the value of "type".
 Note that the destination structure encodes a Type, Length, Value.
 The Length field specifies the length of the DestinationData values,
 which allows the addition of new DestinationTypes.  It also allows an
 implementation which does not understand a given DestinationType to
 skip over it.
 A DestinationData can be one of three types:
 node
    A Node-ID.
 opaque
    A compressed list of Node-IDs and an eventual Resource-ID.
    Because this value has been compressed by one of the peers, it is
    meaningful only 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
    appear only 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.
 One possible encoding of the 16-bit integer version as an opaque
 identifier is to encode an index into a Connection Table.  To avoid
 misrouting responses in the event a response is delayed and the
 Connection Table entry has changed, the identifier SHOULD be split
 between an index and a generation counter for that index.  When a
 Node first joins the overlay, the generation counters SHOULD be
 initialized to random values.  An implementation MAY use 12 bits for
 the Connection Table index and 3 bits for the generation counter.
 (Note that this does not suggest a 4096-entry Connection Table for
 every peer, only the ability to encode for a larger Connection
 Table.)  When a Connection Table slot is used for a new connection,
 the generation counter is incremented (with wrapping).  Connection
 Table slots are used on a rotating basis to maximize the time
 interval between uses of the same slot for different connections.
 When routing a message to an entry in the Destination List encoding a
 Connection Table entry, the peer MUST confirm that the generation
 counter matches the current generation counter of that index before
 forwarding the message.  If it does not match, the message MUST be
 silently dropped.

Jennings, et al. Standards Track [Page 51] RFC 6940 RELOAD Base January 2014

6.3.2.3. Forwarding Option

 The Forwarding header can be extended with forwarding header options,
 which are a series of ForwardingOption structures:
  enum { invalidForwardingOptionType(0), (255) }
    ForwardingOptionType;
  struct {
    ForwardingOptionType      type;
    uint8                     flags;
    uint16                    length;
    select (type) {
          /* This type may be extended */
    };
  } ForwardingOption;
 Each ForwardingOption consists of the following values:
 type
    The type of the option.  This structure allows for unknown options
    types.
 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 peer that would forward the message but does not
    understand this option MUST reject the request with an
    Error_Unsupported_Forwarding_Option error response.  If the
    DESTINATION_CRITICAL flag is set, any node that generates a
    response to the message but does not understand the forwarding
    option MUST reject the request with an
    Error_Unsupported_Forwarding_Option error response.  If the
    RESPONSE_COPY flag is set, any node generating a response MUST
    copy the option from the request to the response except that the
    RESPONSE_COPY, FORWARD_CRITICAL, and DESTINATION_CRITICAL flags
    MUST be cleared.
 length
    The length of the rest of the structure.  Note that a 0 length may
    be reasonable if the mere presence of the option is meaningful and
    no value is required.
 option
    The option value.

Jennings, et al. Standards Track [Page 52] RFC 6940 RELOAD Base January 2014

6.3.3. Message Contents Format

 The second major part of a RELOAD message is the contents part, which
 is defined by MessageContents:
 enum { invalidMessageExtensionType(0),
        (2^16-1) } MessageExtensionType;
 struct {
   MessageExtensionType  type;
   Boolean               critical;
   opaque                extension_contents<0..2^32-1>;
 } MessageExtension;
 struct {
   uint16                 message_code;
   opaque                 message_body<0..2^32-1>;
   MessageExtension       extensions<0..2^32-1>;
 } MessageContents;
 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:
    0x0  Invalid Message Code.  This code will never be assigned.
    0x1 .. 0x7FFF  Requests and responses.  These code points are
       always paired, with requests being an odd value and the
       corresponding response being the request code plus 1.  Thus,
       "probe_request" (the Probe request) has the value 1 and
       "probe_answer" (the Probe response) has the value 2
    0x8000 .. 0xFFFE  Reserved
    0xFFFF  Error
    The message codes are defined in Section 14.8.
 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.

Jennings, et al. Standards Track [Page 53] RFC 6940 RELOAD Base January 2014

 extensions
    Extensions to the message.  Currently no extensions are defined,
    but new extensions can be defined by the process described in
    Section 14.14.
 All extensions have the following form:
 type
    The extension type.
 critical
    Whether this extension needs to be understood in order to process
    the message.  If critical = True and the recipient does not
    understand the message, it MUST generate an
    Error_Unknown_Extension error.  If critical = False, the recipient
    MAY choose to process the message even if it does not understand
    the extension.
 extension_contents
    The contents of the extension (which are extension dependent).
 The subsections 6.4.2, 6.5, and 7 describe structures that are
 inserted inside the message_body member, depending on the value of
 the message_code value.  For example, a message_code value of
 join_req means that the structure named JoinReq is inserted inside
 message_body.  This document does not contain a mapping between
 message_code values and structure names, as the conversion between
 the two is obvious.
 Similarly, this document uses the name of the structure without the
 "Req" or "Ans" suffix to mean the execution of a transaction
 consisting of the matching request and answer.  For example, when the
 text says "perform an Attach", it must be understood as performing a
 transaction composed of an AttachReq and an AttachAns.

6.3.3.1. Response Codes and Response Errors

 A node processing a request MUST return its status in the
 message_code field.  If the request was a success, then the message
 code MUST be set to 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 has failed, then the message code MUST be set to
 0xffff (error) and the payload MUST be an error_response message, as
 shown below.

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 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:
 error_code
    A numeric error code indicating the error that occurred.
 error_info
    An optional arbitrary byte string.  Unless otherwise specified,
    this will be a UTF-8 text string that provides further information
    about what went wrong.  Developers are encouraged to include
    enough diagnostic information to be useful in error_info.  The
    specific text to be used and any relevant language or encoding
    thereof is left to the implementation.
 The following error code values are defined.  The numeric values for
 these are defined in Section 14.9.
 Error_Forbidden
    The requesting node does not have permission to make this request.
 Error_Not_Found
    The resource or node 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 node MAY resend the request at a
    later time.
 Error_Data_Too_Old
    A store cannot be completed because the storage_time precedes the
    existing value.
 Error_Data_Too_Large
    A store cannot be completed because the requested object exceeds
    the size limits for that Kind.
 Error_Generation_Counter_Too_Low
    A store cannot be completed because the generation counter
    precedes the existing value.

Jennings, et al. Standards Track [Page 55] RFC 6940 RELOAD Base January 2014

 Error_Incompatible_with_Overlay
    A peer receiving the request is using a different overlay, overlay
    algorithm, or hash algorithm, or some other parameter that is
    inconsistent with the overlay configuration.
 Error_Unsupported_Forwarding_Option
    A node received the request with a forwarding options flagged as
    critical, but the node does not support this option.  See
    Section 6.3.2.3.
 Error_TTL_Exceeded
    A peer received the request in which the TTL was decremented to
    zero.  See Section 6.3.2.
 Error_Message_Too_Large
    A peer received a request that was too large.  See Section 6.6.
 Error_Response_Too_Large
    A node would have generated a response that is too large per the
    max_response_length field.
 Error_Config_Too_Old
    A destination node received a request with a configuration
    sequence that is too old.  See Section 6.3.2.1.
 Error_Config_Too_New
    A destination node received a request with a configuration
    sequence that is too new.  See Section 6.3.2.1.
 Error_Unknown_Kind
    A destination peer received a request with an unknown Kind-ID.
    See Section 7.4.1.2.
 Error_In_Progress
    An Attach to this peer is already in progress.  See
    Section 6.5.1.2.
 Error_Unknown_Extension
    A destination node received a request with an unknown extension.
 Error_Invalid_Message
    Something about this message is invalid, but it does not fit the
    other error codes.  When this message is sent, implementations
    SHOULD provide some meaningful description in error_info to aid in
    debugging.

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 Error_Exp_A
    For the purposes of experimentation.  It is not meant for vendor-
    specific use of any sort and MUST NOT be used for operational
    deployments.
 Error_Exp_B
    For the purposes of experimentation.  It is not meant for vendor-
    specific use of any sort and MUST NOT be used for operational
    deployments.

6.3.4. Security Block

 The third part of a RELOAD message is the security block.  The
 security block is represented by a SecurityBlock structure:
 struct {
    CertificateType     type;   // From RFC 6091
    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.
 The certificates bucket SHOULD contain all the certificates necessary
 to verify every signature in both the message and the internal
 message objects, except for those certificates in a root-cert element
 of the current configuration file.  This is the only location in the
 message which contains certificates, thus allowing only a single copy
 of each certificate to be sent.  In systems that have an alternative
 certificate distribution mechanism, some certificates MAY be omitted.
 However, unless an alternative mechanism for immediately generating
 certificates, such as shared secret security (Section 13.4) is used,
 implementers MUST include all referenced certificates.
 NOTE TO IMPLEMENTERS: This requirement implies that a peer storing
 data is obligated to retain certificates for the data that it holds.

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 Each certificate is represented by a GenericCertificate structure,
 which has the following contents:
 type
    The type of the certificate, as defined in [RFC6091].  Only the
    use of X.509 certificates is defined in this document.
 certificate
    The encoded version of the certificate.  For X.509 certificates,
    it is the Distinguished Encoding Rules (DER) form.
 The signature is computed over the payload and parts of the
 forwarding header.  In case of a Store, the payload MUST contain an
 additional signature computed as described in Section 7.1.  All
 signatures MUST be formatted using the Signature element.  This
 element is also used in other contexts where signatures are needed.
 The input structure to the signature computation MAY vary depending
 on the data element being signed.
   enum { invalidSignerIdentityType(0),
          cert_hash(1), cert_hash_node_id(2),
          none(3)
          (255) } SignerIdentityType;
   struct {
     select (identity_type) {
       case cert_hash;
         HashAlgorithm      hash_alg;              // From TLS
         opaque             certificate_hash<0..2^8-1>;
       case cert_hash_node_id:
         HashAlgorithm      hash_alg;              // From TLS
         opaque             certificate_node_id_hash<0..2^8-1>;
       case none:
         /* empty */
       /* This structure may be extended with new types if necessary*/
     };
   } SignerIdentityValue;
   struct {
     SignerIdentityType     identity_type;
     uint16                 length;
     SignerIdentityValue    identity[SignerIdentity.length];
   } SignerIdentity;

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   struct {
      SignatureAndHashAlgorithm     algorithm;   // From TLS
      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 and HashAlgorithm
    registries.  All implementations MUST support RSASSA-PKCS1-v1_5
    [RFC3447] signatures with SHA-256 hashes [RFC6234].
 identity
    The identity, as defined in the two paragraphs following this
    list, used to form the signature.
 signature_value
    The value of the signature.
    Note that storage operations allow for special values of algorithm
    and identity.  See the Store Request definition (Section 7.4.1.1)
    and the Fetch Response definition (Section 7.4.2.2).
 There are two permitted identity formats, one for a certificate with
 only one Node-ID and one for a certificate with multiple Node-IDs.
 In the first case, the cert_hash type MUST be used.  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
 (i.e., the DER-encoded certificate).
 In the second case, the cert_hash_node_id type MUST be used.  The
 hash_alg is as in cert_hash, but the cert_hash_node_id is computed
 over the NodeId used to sign concatenated with the certificate; i.e.,
 H(NodeId || certificate).  The NodeId is represented without any
 framing or length fields, as simple raw bytes.  This is safe because
 NodeIds are a fixed length for a given overlay.
 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.

Jennings, et al. Standards Track [Page 59] RFC 6940 RELOAD Base January 2014

 The input to signatures over data values is different and is
 described in Section 7.1.
 All RELOAD messages MUST be signed.  Intermediate nodes do not verify
 signatures.  Upon receipt (and fragment reassembly, if needed), the
 destination node MUST verify the signature and the authorizing
 certificate.  If the signature fails, the implementation SHOULD
 simply drop the message and MUST NOT process it.  This check provides
 a minimal level of assurance that the sending node is a valid part of
 the overlay, and it provides cryptographic authentication of the
 sending node.  In addition, responses MUST be checked as follows by
 the requesting node:
 1.  The response to a message sent to a Node-ID MUST have been sent
     by that Node-ID unless the response has been sent to the wildcard
     Node-ID.
 2.  The response to a message sent to a Resource-ID MUST have been
     sent by a Node-ID which is at least as close to the target
     Resource-ID as 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.
 In addition, some methods (especially Store) have additional
 authentication requirements, which are described in the sections
 covering those methods.

6.4. Overlay Topology

 As discussed in previous sections, RELOAD defines a default overlay
 topology (CHORD-RELOAD) but allows for other topologies through the
 use of Topology Plug-ins.  This section describes the requirements
 for new Topology Plug-ins and the methods that RELOAD provides for
 overlay topology maintenance.

6.4.1. Topology Plug-in Requirements

 When specifying a new overlay algorithm, at least the following MUST
 be described:
 o  Joining procedures, including the contents of the Join message.

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 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 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.
 All overlay algorithms MUST specify maintenance procedures that send
 Updates to clients and peers that have established connections to the
 peer responsible for a particular ID when the responsibility for that
 ID changes.  Because tracking this information is difficult, overlay
 algorithms MAY simply specify that an Update is sent to all members
 of the Connection Table whenever the range of IDs for which the peer
 is responsible changes.

6.4.2. Methods and Types for Use by Topology Plug-ins

 This section describes the methods that Topology Plug-ins use to
 join, leave, and maintain the overlay.

6.4.2.1. Join

 A new peer (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 Plug-in.
 This message notifies the responsible peer that the new peer is
 taking over some of the overlay and that it needs to synchronize its
 state.
       struct {
          NodeId                joining_peer_id;
          opaque                overlay_specific_data<0..2^16-1>;
       } JoinReq;
 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.  Receivers of the JoinReq MUST verify that
 the joining_peer_id field matches the Node-ID used to sign the
 message and, if not, the message MUST be rejected with an
 Error_Forbidden error.

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 Because joins may be executed only between nodes which are directly
 adjacent, receiving peers MUST verify that any JoinReq they receive
 arrives from a transport channel that is bound to the Node-ID to be
 assumed by the Joining Node.  Implementations MUST use DTLS
 anti-replay mechanisms, thus preventing replay attacks.
 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 Node.  In
 addition, overlay algorithms MAY define data to appear in the
 response payload that provides additional information.
 Joining Nodes MUST verify that the signature on the JoinAns message
 matches the expected target (i.e., the adjacency over which they are
 joining).  If not, they MUST discard the message.
 In general, nodes which cannot form connections SHOULD report an
 error to the user.  However, implementations MUST provide some
 mechanism whereby nodes can determine that they are potentially the
 first node and can take responsibility for the overlay.  (The idea is
 to avoid having ordinary nodes try to become responsible for the
 entire overlay during a partition.)  This specification does not
 mandate any particular mechanism, but a configuration flag or setting
 seems appropriate.

6.4.2.2. 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.
       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.
 Receivers of the LeaveReq MUST verify that the leaving_peer_id field
 matches the Node-ID used to sign the message and, if not, the message
 MUST be rejected with an Error_Forbidden error.

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 Because leaves may be executed only between nodes which are directly
 adjacent, receiving peers MUST verify that any LeaveReq they receive
 arrives from a transport channel that is bound to the Node-ID to be
 assumed by the leaving peer.  This also prevents replay attacks,
 provided that DTLS anti-replay is used.
 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.
 LeaveAns is an empty message.

6.4.2.3. 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
 the current state of the overlay (that is, its routing state), and it
 is up to the recipient to take whatever actions are appropriate to
 deal with the state change.  In general, peers send Update messages
 to all their adjacencies whenever they detect a topology shift.
 When a peer receives an Attach request with the send_update flag set
 to True (Section 6.4.2.4.1), it MUST send an Update message back to
 the sender of the Attach request after completion of the
 corresponding ICE check and TLS connection.  Note that the sender of
 such an Attach request may not have joined the overlay yet.
 When a peer detects through an Update that it is no longer
 responsible for any data value it is storing, it MUST attempt to
 Store a copy to the correct node unless it knows the newly
 responsible node already has a copy of the data.  This prevents data
 loss during large-scale topology shifts, such as the merging of
 partitioned overlays.
 The contents of the UpdateReq message are completely overlay
 specific.  The UpdateAns response is expected to be either success or
 an error.

6.4.2.4. RouteQuery

 The RouteQuery 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 for the
 node that the receiving peer would next route to in order to get to
 X.  A RouteQuery can also request that the receiving peer initiate an
 Update request to transfer the receiving peer's Routing Table.

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 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 field
 set to the Node-ID or Resource-ID to which it wishes to route.  The
 receiving peer responds with information about the peers to which the
 request would be routed.  The sending peer MAY then use the Attach
 method to attach to that peer(s) and repeat the RouteQuery.
 Eventually, the sender gets a response from a peer that is closest to
 the identifier in the destination field as determined by the Topology
 Plug-in.  At that point, the sender can send messages directly to
 that peer.

6.4.2.4.1. Request Definition

 A RouteQueryReq message indicates the peer or resource that the
 requesting node is interested in.  It also contains a "send_update"
 option that allows the requesting node 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:
 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, opaque ID, or
    Resource-ID.
    Note: If implementations are using opaque IDs for privacy
    purposes, answering RouteQueryReqs for opaque IDs will allow the
    requester to translate an opaque ID.  Implementations MAY wish to
    consider limiting the use of RouteQuery for opaque IDs in such
    cases.
 overlay_specific_data
    Other data as appropriate for the overlay.

Jennings, et al. Standards Track [Page 64] RFC 6940 RELOAD Base January 2014

6.4.2.4.2. Response Definition

 A response to a successful RouteQueryReq request is a RouteQueryAns
 message.  This message is completely overlay specific.

6.4.2.5. Probe

 Probe provides primitive "exploration" services: it allows a node to
 determine which resources another node is responsible for.  A probe
 can be addressed to a specific Node-ID or to the peer controlling a
 given location (by using a Resource-ID).  In either case, the target
 node responds with a simple response containing some status
 information.

6.4.2.5.1. 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 { invalidProbeInformationType(0), responsible_set(1),
             num_resources(2), uptime(3), (255) }
           ProbeInformationType;
      struct {
        ProbeInformationType     requested_info<0..2^8-1>;
      } ProbeReq;
 The currently defined values for ProbeInformationType are:
 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.  Note that multiple
    values under the same Resource-ID are counted only once.
 uptime
    Indicates that the peer should Respond with how long the peer has
    been up, in seconds.

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6.4.2.5.2. Response Definition

 A successful ProbeAns response contains the information elements
 requested by the peer.
       struct {
         select (type) {
           case responsible_set:
             uint32             responsible_ppb;
           case num_resources:
             uint32             num_resources;
           case uptime:
             uint32             uptime;
           /* This type may be extended */
         };
       } ProbeInformationData;
       struct {
         ProbeInformationType    type;
         uint8                   length;
         ProbeInformationData    value;
       } ProbeInformation;
       struct {
         ProbeInformation        probe_info<0..2^16-1>;
       } ProbeAns;
 A ProbeAns message contains a sequence of ProbeInformation
 structures.  Each has a "length" indicating the length of the
 following value field.  This structure allows for unknown option
 types.
 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.  For the type "uptime", it is the number of seconds the peer
 has been up.
 The responding peer SHOULD include any values that the requesting
 node requested and that it recognizes.  They SHOULD be returned in
 the requested order.  Any other values MUST NOT be returned.

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6.5. Forwarding and Link Management Layer

 Each node maintains connections to a set of other nodes defined by
 the Topology Plug-in.  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 RELOAD connections between nodes using ICE for NAT
    traversal.  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.  Attach also allows two
    nodes to connect via No-ICE instead of full ICE.
 AppAttach
    Used to form application-layer connections between nodes.
 Ping
    A simple request/response which is used to verify connectivity of
    the target peer.

6.5.1. Attach

 A node sends an Attach request when it wishes to establish a direct
 Overlay Link connection to another node for the purpose of sending
 RELOAD messages.  A client that can establish a connection directly
 need not send an Attach, as described in the second bullet of
 Section 4.2.1.
 As described in Section 6.1, an Attach may be routed to either a
 Node-ID or 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 has not received any Updates from
 B, it MAY route messages which are directly addressed to B through
 that channel, but it 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 Join and Update), to prevent half-
 open states where a node has started to form connections but is not
 really ready to act as a peer.  Thus, clients (unlike peers) can
 simply Attach without sending Join or Update.

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6.5.1.1. Request Definition

 An Attach request message contains the requesting node ICE connection
 parameters formatted into a binary structure.
      enum { invalidOverlayLinkType(0), DTLS-UDP-SR(1),
             DTLS-UDP-SR-NO-ICE(3), TLS-TCP-FH-NO-ICE(4),
             (255) } OverlayLinkType;
      enum { invalidCandType(0),
             host(1), srflx(2), /* RESERVED(3), */ relay(4),
             (255) } CandType;
      struct {
        opaque                name<0..2^16-1>;
        opaque                value<0..2^16-1>;
      } IceExtension;
      struct {
        IpAddressPort         addr_port;
        OverlayLinkType       overlay_link;
        opaque                foundation<0..255>;
        uint32                priority;
        CandType              type;
        select (type) {
          case host:
            ;          /* Empty */
          case srflx:
          case relay:
            IpAddressPort     rel_addr_port;
        };
        IceExtension          extensions<0..2^16-1>;
      } IceCandidate;
      struct {
        opaque                ufrag<0..2^8-1>;
        opaque                password<0..2^8-1>;
        opaque                role<0..2^8-1>;
        IceCandidate          candidates<0..2^16-1>;
        Boolean               send_update;
      } AttachReqAns;
 The values contained in AttachReqAns are:
 ufrag
    The username fragment (from ICE).

Jennings, et al. Standards Track [Page 68] RFC 6940 RELOAD Base January 2014

 password
    The ICE password.
 role
    An active/passive/actpass attribute from RFC 4145 [RFC4145].  This
    value MUST be "passive" for the offerer (the peer sending the
    Attach request) and "active" for the answerer (the peer sending
    the Attach response).
 candidates
    One or more ICE candidate values, as described below.
 send_update
    Has the same meaning as the send_update field in RouteQueryReq.
 Each ICE candidate is represented as an IceCandidate structure, which
 is a direct translation of the information from the ICE string
 structures, with the exception of the component ID.  Since there is
 only one component, it is always 1, and thus left out of the
 structure.  The remaining values are specified as follows:
 addr_port
    Corresponds to the ICE connection-address and port productions.
 overlay_link
    Corresponds to the ICE transport production.  Overlay Link
    protocols used with No-ICE MUST specify "No-ICE" in their
    description.  Future overlay link values can be added by defining
    new OverlayLinkType values in the IANA registry as described in
    Section 14.10.  Future extensions to the encapsulation or framing
    that provide for backward compatibility with the previously
    specified encapsulation or framing values MUST use the same
    OverlayLinkType value that was previously defined.
    OverlayLinkType protocols are defined in Section 6.6
    A single AttachReqAns MUST NOT include both candidates whose
    OverlayLinkType protocols use ICE (the default) and candidates
    that specify "No-ICE".
 foundation
    Corresponds to the ICE foundation production.
 priority
    Corresponds to the ICE priority production.
 type
    Corresponds to the ICE cand-type production.

Jennings, et al. Standards Track [Page 69] RFC 6940 RELOAD Base January 2014

 rel_addr_port
    Corresponds to the ICE rel-addr and rel-port productions.  It is
    present only for types "relay", "prfix", and "srflx".
 extensions
    ICE extensions.  The name and value fields correspond to binary
    translations of the equivalent fields in the ICE extensions.
 These values should be generated using the procedures described in
 Section 6.5.1.3.

6.5.1.2. Response Definition

 If a peer receives an Attach request, it MUST determine how to
 process the request as follows:
 o  If the peer has not initiated an Attach request to the originating
    peer of this Attach request, it MUST process this request and
    SHOULD generate its own response with an AttachReqAns.  It should
    then begin ICE checks.
 o  If the peer has already sent an Attach request to and received the
    response from the originating peer of this Attach request and, as
    a result, an ICE check and TLS connection are in progress, then it
    SHOULD generate an Error_In_Progress error instead of an
    AttachReqAns.
 o  If the peer has already sent an Attach request to but not yet
    received the response from the originating peer of this Attach
    request, it SHOULD apply the following tie-breaker heuristic to
    determine how to handle this Attach request and the incomplete
    Attach request it has sent out:
  • If the peer's own Node-ID is smaller when compared as big-

endian unsigned integers, it MUST cancel retransmission of its

       own incomplete Attach request.  It MUST then process this
       Attach request, generate an AttachReqAns response, and proceed
       with the corresponding ICE check.
  • If the peer's own Node-ID is larger when compared as big-endian

unsigned integers, it MUST generate an Error_In_Progress error

       to this Attach request, and then proceed to wait for and
       complete the Attach and the corresponding ICE check it has
       originated.
 o  If the peer is overloaded or detects some other kind of error, it
    MAY generate an error instead of an AttachReqAns.

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 When a peer receives an Attach response, it SHOULD parse the response
 and begin its own ICE checks.

6.5.1.3. Using ICE with RELOAD

 This section describes the profile of ICE that is used with RELOAD.
 RELOAD implementations MUST implement full ICE.
 In ICE, as defined by [RFC5245], the Session Description Protocol
 (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 other
    application-layer protocols, such as SIP.
 o  RELOAD allows only 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 [RFC5245] with
 the changes and additional procedures described in the subsections
 below.

6.5.1.4. Collecting STUN Servers

 ICE relies on the node having one or more Session Traversal Utilities
 for NAT (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.
 A peer on a well-provisioned wide-area overlay will be configured
 with one or more bootstrap nodes.  These nodes make an initial list
 of STUN servers.  However, as the peer forms connections with
 additional peers, it builds more peers that it can use like STUN
 servers.
 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 more NATs are involved, a peer may

Jennings, et al. Standards Track [Page 71] RFC 6940 RELOAD Base January 2014

 learn 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 MUST be
 selected unless there are no others servers in the group, in which
 case some other peer MAY be used.

6.5.1.5. Gathering Candidates

 When a node wishes to establish a connection for the purposes of
 RELOAD signaling or application signaling, it follows the process of
 gathering candidates as described in Section 4 of ICE [RFC5245].
 RELOAD utilizes a single component.  Consequently, gathering for
 these "streams" requires a single component.  In the case where a
 node has not yet found a TURN server, the agent would not include a
 relayed candidate.
 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
 Sections 6.5.1.4 and 9.
 The default candidate selection described in Section 4.1.4 of ICE is
 ignored; defaults are not signaled or utilized by RELOAD.
 An alternative to using the full ICE supported by the Attach request
 is to use the No-ICE mechanism by providing candidates with "No-ICE"
 Overlay Link protocols.  Configuration for the overlay indicates
 whether or not these Overlay Link protocols can be used.  An overlay
 MUST be either all ICE or all No-ICE.
 No-ICE will not work in all the scenarios where ICE would work, but
 in some cases, particularly those with no NATs or firewalls, it will
 work.

6.5.1.6. Prioritizing Candidates

 Standardization of additional protocols for use with ICE is expected,
 including TCP [RFC6544] and protocols such as the Stream Control
 Transmission Protocol (SCTP) [RFC4960] and Datagram Congestion
 Control Protocol (DCCP) [RFC4340].  UDP encapsulations for SCTP and
 DCCP would expand the Overlay Link protocols available for RELOAD.

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 When additional protocols are available, the following prioritization
 is RECOMMENDED:
 o  Highest priority is assigned to protocols that offer well-
    understood congestion and flow control without head-of-line
    blocking, for example, SCTP without message ordering, DCCP, and
    those protocols encapsulated using UDP.
 o  Second highest priority is assigned to protocols that offer well-
    understood congestion and flow control, but that have head-of-line
    blocking, such as TCP.
 o  Lowest priority is assigned to protocols encapsulated over UDP
    that do not implement well-established congestion control
    algorithms.  The DTLS/UDP with Simple Reliability (SR) overlay
    link protocol is an example of such a protocol.
 Head-of-line blocking is undesirable in an Overlay Link protocol,
 because the messages carried on a RELOAD link are independent, rather
 than stream-oriented.  Therefore, if message N on a link is lost,
 delaying message N+1 on that same link until N is successfully
 retransmitted does nothing other than increase the latency for the
 transaction of message N+1, as they are unrelated to each other.
 Therefore, while the high quality, performance, and availability of
 modern TCP implementations makes them very attractive, their
 performance as Overlay Link protocols is not optimal.
 Note that none of the protocols defined in this document meets these
 conditions, but it is expected that new Overlay Link protocols
 defined in the future will fill this gap.

6.5.1.7. Encoding the Attach Message

 Section 4.3 of ICE describes procedures for encoding the SDP for
 conveying RELOAD candidates.  Instead of actually encoding an SDP
 message, the candidate information (IP address and port and transport
 protocol, priority, foundation, type, and related 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 6.5.1 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.
 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

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 the ICE specification.  Similarly, the Attach response is considered
 a valid answer for the purposes of following the ICE specification.

6.5.1.8. Verifying ICE Support

 An agent MUST skip the verification procedures in Sections 5.1 and
 6.1 of ICE.  Since RELOAD requires full ICE from all agents, this
 check is not required.

6.5.1.9. 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.

6.5.1.10. Full ICE

 When the overlay uses ICE, connectivity checks and nominations are
 used as in regular ICE.

6.5.1.10.1. 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.

6.5.1.10.2. Concluding ICE

 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.

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6.5.1.10.3. Media Keepalives

 STUN MUST be utilized for the keepalives described in Section 10 of
 ICE.

6.5.1.11. No-ICE

 No-ICE is selected when either side has provided "no ICE" Overlay
 Link candidates.  STUN is not used for connectivity checks when doing
 No-ICE; instead, the DTLS or TLS handshake (or similar security layer
 of future overlay link protocols) forms the connectivity check.  The
 certificate exchanged during the TLS or DTLS handshake MUST match the
 node which sent the AttachReqAns, and if it does not, the connection
 MUST be closed.

6.5.1.12. 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.

6.5.1.13. Sending Media

 The procedures of Section 11 of ICE apply to RELOAD as well.
 However, in this case, the "media" takes the form of application-
 layer protocols (e.g., RELOAD) over TLS or DTLS.  Consequently, once
 ICE processing completes, the agent will begin TLS or DTLS procedures
 to establish a secure connection.  The node that sent the Attach
 request MUST be the TLS server.  The other node MUST be the TLS
 client.  The server MUST request TLS client authentication.  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.

6.5.1.14. Receiving Media

 An agent MUST be prepared to receive packets for the application
 protocol (TLS or DTLS carrying RELOAD) at any time.  The jitter and
 RTP considerations in Section 11 of ICE do not apply to RELOAD.

6.5.2. AppAttach

 A node sends an AppAttach request when it wishes to establish a
 direct connection to another node for the purposes of sending
 application-layer messages.  AppAttach is nearly identical to Attach,

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 except for the purpose of the connection: it is used to transport
 non-RELOAD "media".  A separate request is used to avoid implementer
 confusion between the two methods (this was found to be a real
 problem with initial implementations).  The AppAttach request and its
 response contain an application attribute, which indicates what
 protocol is to be run over the connection.

6.5.2.1. Request Definition

 An AppAttachReq message contains the requesting node's ICE connection
 parameters formatted into a binary structure.
      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>;
      } AppAttachReq;
 The values contained in AppAttachReq and AppAttachAns are:
 ufrag
    The username fragment (from ICE).
 password
    The ICE password.
 application
    A 16-bit Application-ID, as defined in the Section 14.5.  This
    number represents the IANA-registered application that is going to
    send data on this connection.
 role
    An active/passive/actpass attribute from RFC 4145 [RFC4145].
 candidates
    One or more ICE candidate values.
 The application using the connection that is set up with this request
 is responsible for providing traffic of sufficient frequency to keep
 the NAT and Firewall binding alive.  Applications will often send
 traffic every 25 seconds to ensure this.

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6.5.2.2. Response Definition

 If a peer receives an AppAttach request, it SHOULD process the
 request and generate its own response with a AppAttachAns.  It should
 then begin ICE checks.  When a peer receives an AppAttach response,
 it SHOULD parse the response and begin its own ICE checks.  If the
 Application ID is not supported, the peer MUST reply with an
 Error_Not_Found error.
      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>;
      } AppAttachAns;
 The meaning of the fields is the same as in the AppAttachReq.

6.5.3. Ping

 Ping is used to test connectivity along a path.  A ping can be
 addressed to a specific Node-ID, to the peer controlling a given
 location (by using a Resource-ID), or to the wildcard Node-ID.

6.5.3.1. Request Definition

 The PingReq structure is used to make a Ping request.
      struct {
        opaque<0..2^16-1> padding;
      } PingReq;
 The Ping request is empty of meaningful contents.  However, it may
 contain up to 65535 bytes of padding to facilitate the discovery of
 overlay maximum packet sizes.

6.5.3.2. Response Definition

 A successful PingAns response contains the information elements
 requested by the peer.
       struct {
         uint64                 response_id;
         uint64                 time;
       } PingAns;

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 A PingAns message contains the following elements:
 response_id
    A randomly generated 64-bit response ID.  This is used to
    distinguish Ping responses.
 time
    The time when the Ping response was created, represented in the
    same way as storage_time, defined in Section 7.

6.5.4. ConfigUpdate

 The ConfigUpdate method is used to push updated configuration data
 across the overlay.  Whenever a node detects that another node has
 old configuration data, it MUST generate a ConfigUpdate request.  The
 ConfigUpdate request allows updating of two kinds of data: the
 configuration data (Section 6.3.2.1) and the Kind information
 (Section 7.4.1.1).

6.5.4.1. Request Definition

 The ConfigUpdateReq structure is used to provide updated
 configuration information.
      enum { invalidConfigUpdateType(0), config(1), kind(2), (255) }
           ConfigUpdateType;
      typedef uint32           KindId;
      typedef opaque           KindDescription<0..2^16-1>;
      struct {
        ConfigUpdateType       type;
        uint32                 length;
        select (type) {
          case config:
                      opaque             config_data<0..2^24-1>;
          case kind:
                      KindDescription    kinds<0..2^24-1>;
          /* This structure may be extended with new types */
        };
      } ConfigUpdateReq;

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 The ConfigUpdateReq message contains the following elements:
 type
    The type of the contents of the message.  This structure allows
    for unknown content types.
 length
    The length of the remainder of the message.  This is included to
    preserve backward compatibility and is 32 bits instead of 24 to
    facilitate easy conversion between network and host byte order.
 config_data (type==config)
    The contents of the Configuration Document.
 kinds (type==kind)
    One or more XML kind-block productions (see Section 11.1).  These
    MUST be encoded with UTF-8 and assume a default namespace of
    "urn:ietf:params:xml:ns:p2p:config-base".

6.5.4.2. Response Definition

 The ConfigUpdateAns structure is used to respond to a ConfigUpdateReq
 request.
      struct {
      } ConfigUpdateAns;
 If the ConfigUpdateReq is of type "config", it MUST be processed only
 if all the following are true:
 o  The sequence number in the document is greater than the current
    configuration sequence number.
 o  The Configuration Document is correctly digitally signed (see
    Section 11 for details on signatures).
 Otherwise, appropriate errors MUST be generated.
 If the ConfigUpdateReq is of type "kind", it MUST be processed only
 if it is correctly digitally signed by an acceptable Kind signer
 (i.e., one listed in the current configuration file).  Details on the
 kind-signer field in the configuration file are described in
 Section 11.1.  In addition, if the Kind update conflicts with an
 existing known Kind (i.e., it is signed by a different signer), then
 it should be rejected with an Error_Forbidden error.  This should not
 happen in correctly functioning overlays.

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 If the update is acceptable, then the node MUST reconfigure itself to
 match the new information.  This may include adding permissions for
 new Kinds, deleting old Kinds, or even, in extreme circumstances,
 exiting and re-entering the overlay, if, for instance, the DHT
 algorithm has changed.
 If an implementation misses enough ConfigUpdates that include key
 changes, it is possible that it will no longer be able to verify new
 valid ConfigUpdates.  In this case, the only available recovery
 mechanism is to attempt to retrieve a new Configuration Document,
 typically by the mechanisms used for initial bootstrapping.  It is up
 to implementers whether or how to decide to employ this sort of
 recovery mechanism.
 The response for ConfigUpdate is empty.

6.6. Overlay Link Layer

 RELOAD can use multiple Overlay Link protocols to send its messages.
 Because ICE is used to establish connections (see Section 6.5.1.3),
 RELOAD nodes are able to detect which Overlay Link protocols are
 offered by other nodes and establish connections between them.  Any
 link protocol needs to be able to establish a secure, authenticated
 connection and to provide data origin authentication and message
 integrity for individual data elements.  RELOAD currently supports
 three Overlay Link protocols:
 o  DTLS [RFC6347] over UDP with Simple Reliability (SR)
    (OverlayLinkType=DTLS-UDP-SR)
 o  TLS [RFC5246] over TCP with Framing Header, No-ICE
    (OverlayLinkType=TLS-TCP-FH-NO-ICE)
 o  DTLS [RFC6347] over UDP with SR, No-ICE
    (OverlayLinkType=DTLS-UDP-SR-NO-ICE)
 Note that although UDP does not properly have "connections", both TLS
 and DTLS have a handshake that establishes a similar, stateful
 association.  We refer to these as "connections" for the purposes of
 this document.
 If a peer receives a message that is larger than the value of max-
 message-size defined in the overlay configuration, the peer SHOULD
 send an Error_Message_Too_Large error and then close the TLS or DTLS
 session from which the message was received.  Note that this error
 can be sent and the session closed before the peer receives the
 complete message.  If the forwarding header is larger than the max-

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 message-size, the receiver SHOULD close the TLS or DTLS session
 without sending an error.
 The RELOAD mechanism requires that failed links be quickly removed
 from the Routing Table so end-to-end retransmission can handle lost
 messages.  Overlay Link protocols MUST be designed with a mechanism
 that quickly signals a likely failure, and implementations SHOULD
 quickly act to remove a failed link from the Routing Table when
 receiving this signal.  The entry can be restored if it proves to
 resume functioning, or it can be replaced at some point in the future
 if necessary.  Section 10.7.2 contains more details specific to the
 CHORD-RELOAD Topology Plug-in.
 The Framing Header (FH) is used to frame messages and provide timing
 when used on a reliable stream-based transport protocol.  Simple
 Reliability (SR) uses the FH to provide congestion control and
 partial reliability when using unreliable message-oriented transport
 protocols.  We will first define each of these algorithms in Sections
 6.6.2 and 6.6.3, and then define Overlay Link protocols that use them
 in Sections 6.6.4, 6.6.5, and 6.6.6.
 Note: We expect future Overlay Link protocols to define replacements
 for all components of these protocols, including the Framing Header.
 The three protocols that we will discuss have been chosen for
 simplicity of implementation and reasonable performance.

6.6.1. Future Overlay Link Protocols

 It is possible to define new link-layer protocols and apply them to a
 new overlay using the "overlay-link-protocol" configuration directive
 (see Section 11.1.).  However, any new protocols MUST meet the
 following requirements:
 Endpoint authentication:  When a node forms an association with
    another endpoint, it MUST be possible to cryptographically verify
    that the endpoint has a given Node-ID.
 Traffic origin authentication and integrity:  When a node receives
    traffic from another endpoint, it MUST be possible to
    cryptographically verify that the traffic came from a given
    association and that it has not been modified in transit from the
    other endpoint in the association.  The overlay link protocol MUST
    also provide replay prevention/detection.
 Traffic confidentiality:  When a node sends traffic to another
    endpoint, it MUST NOT be possible for a third party that is not
    involved in the association to determine the contents of that
    traffic.

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 Any new overlay protocol MUST be defined via Standards Action
 [RFC5226].  See Section 14.11.

6.6.1.1. HIP

 In a Host Identity Protocol Based Overlay Networking Environment (HIP
 BONE) [RFC6079], HIP [RFC5201] provides connection management (e.g.,
 NAT traversal and mobility) and security for the overlay network.
 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 provide identities meaningful to
    the HIP base exchange.  We anticipate that this would require a
    mapping between Overlay Routable Cryptographic Hash Identifiers
    (ORCHIDs) and NodeIds.
 o  How to carry the HIP I1 and I2 messages.
 o  How to carry RELOAD messages over HIP.
 [HIP-RELOAD] documents work in progress on using RELOAD with the HIP
 BONE.

6.6.1.2. ICE-TCP

 The ICE-TCP RFC [RFC6544] allows TCP to be supported as an Overlay
 Link protocol that can be added using ICE.

6.6.1.3. Message-Oriented Transports

 Modern message-oriented transports offer high performance and good
 congestion control, and they avoid head-of-line blocking in case of
 lost data.  These characteristics make them preferable as underlying
 transport protocols for RELOAD links.  SCTP without message ordering
 and DCCP are two examples of such protocols.  However, currently they
 are not well-supported by commonly available NATs, and specifications
 for ICE session establishment are not available.

6.6.1.4. Tunneled Transports

 As of the time of this writing, there is significant interest in the
 IETF community in tunneling other transports over UDP, which is
 motivated by the situation that UDP is well-supported by modern NAT
 hardware and by the fact that performance similar to a native
 implementation can be achieved.  Currently, SCTP, DCCP, and a generic
 tunneling extension are being proposed for message-oriented
 protocols.  Once ICE traversal has been specified for these tunneled

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 protocols, they should be straightforward to support as overlay link
 protocols.

6.6.2. Framing Header

 In order to support unreliable links and to allow for quick detection
 of link failures when using reliable end-to-end transports, each
 message is wrapped in a very simple framing layer (FramedMessage),
 which is used only for each hop.  This layer contains a sequence
 number which can then be used for ACKs.  The same header is used for
 both reliable and unreliable transports for simplicity of
 implementation.
 The definition of FramedMessage is:
      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.
 If the message is of type "data", then the remainder of the PDU is as
 follows:
 sequence
    The sequence number.  This increments by one for each framed
    message sent over this transport session.
 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.

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 When the receiver receives 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
    The sequence number of the message being acknowledged.
 received
    A bitmask indicating if each of the previous 32 sequence numbers
    before this packet has been among the 32 packets most recently
    received on this connection.  When a packet is received with a
    sequence number N, the receiver looks at the sequence number of
    the 32 previously received packets on this connection.  We call
    the previously received packet number M.  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 set to zero.  Note that a bit being set to one
    indicates positively that a particular packet was received, but a
    bit being set to zero means only that it is unknown whether or not
    the packet has been received, because it might have been received
    before the 32 most recently received packets.
 The received field bits in the ACK provide a high degree of
 redundancy so that the sender can figure out which packets the
 receiver has received and can then estimate packet loss rates.  If
 the sender also keeps track of the time at which recent sequence
 numbers have been sent, the RTT (round-trip time) can be estimated.
 Note that because retransmissions receive new sequence numbers,
 multiple ACKs may be received for the same message.  This approach
 provides more information than traditional TCP sequence numbers, but
 care must be taken when applying algorithms designed based on TCP's
 stream-oriented sequence number.

6.6.3. Simple Reliability

 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.  The basic
 principle is that each message, regardless of whether or not it
 carries a request or response, will get an ACK and be reliably
 retransmitted.  The receiver's job is very simple, and is 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.

Jennings, et al. Standards Track [Page 84] RFC 6940 RELOAD Base January 2014

 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.  The sender algorithm used MUST meet the requirements of
 [RFC5405].

6.6.3.1. Stop and Wait Sender Algorithm

 This section describes one possible implementation of a sender
 algorithm for Simple Reliability.  It is adequate for overlays
 running on underlying networks with low latency and loss (LANs) or
 low-traffic overlays on the Internet.
 A node MUST NOT have more than one unacknowledged message on the DTLS
 connection at a time.  Note that because retransmissions of the same
 message are given new sequence numbers, there may be multiple
 unacknowledged sequence numbers in use.
 The RTO (Retransmission TimeOut) is based on an estimate of the RTT.
 The value for RTO is calculated separately for each DTLS session.
 Implementations can use a static value for RTO or a dynamic estimate,
 which will result in better performance.  For implementations that
 use a static value, the default value for RTO is 500 ms.  Nodes MAY
 use smaller values of RTO if it is known that all nodes are within
 the local network.  The default RTO MAY be set to a larger value,
 which is RECOMMENDED if it is known in advance (such as on high-
 latency access links) that the RTT is larger.
 Implementations that use a dynamic estimate to compute the RTO MUST
 use the algorithm described in RFC 6298 [RFC6298], with the exception
 that the value of RTO SHOULD NOT be rounded up to the nearest second,
 but instead rounded up to the nearest millisecond.  The RTT of a
 successful STUN transaction from the ICE stage is used as the initial
 measurement for formula 2.2 of RFC 6298.  The sender keeps track of
 the time each message was sent for all recently sent messages.  Any
 time an ACK is received, the sender can compute the RTT for that
 message by looking at the time the ACK was received and the time when
 the message was sent.  This is used as a subsequent RTT measurement
 for formula 2.3 of RFC 6298 to update the RTO estimate.  (Note that
 because retransmissions receive new sequence numbers, all received
 ACKs are used.)
 An initiating node SHOULD retransmit a message if it has not received
 an ACK after an interval of RTO (transit nodes do not retransmit at
 this layer).  The node MUST double the time to wait after each
 retransmission.  For each retransmission, the sequence number MUST be
 incremented.

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 Retransmissions continue until a response is received, until a total
 of 5 requests have been sent, until there has been a hard ICMP error
 [RFC1122], or until a TLS alert indicating the end of the connection
 has been sent or received.  The sender knows a response was received
 when it receives an ACK with a sequence number that indicates it is a
 response to one of the transmissions of this message.  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, then the sending node SHOULD close the connection
 routing the message.
 To determine when a link might be failing without waiting for the
 final timeout, observe when no ACKs have been received for an entire
 RTO interval, and then wait for three retransmissions to occur beyond
 that point.  If no ACKs have been received by the time the third
 retransmission occurs, it is RECOMMENDED that the link be removed
 from the Routing Table.  The link MAY be restored to the Routing
 Table if ACKs resume before the connection is closed, as described
 above.
 A sender MUST wait 10 ms between receipt of an ACK and transmission
 of the next message.

6.6.4. DTLS/UDP with SR

 This overlay link protocol consists of DTLS over UDP while
 implementing the SR protocol.  STUN connectivity checks and
 keepalives are used.  Any compliant sender algorithm may be used.

6.6.5. TLS/TCP with FH, No-ICE

 This overlay link protocol consists of TLS over TCP with the framing
 header.  Because ICE is not used, STUN connectivity checks are not
 used upon establishing the TCP connection, nor are they used for
 keepalives.
 Because the TCP layer's application-level timeout is too slow to be
 useful for overlay routing, the Overlay Link implementation MUST use
 the framing header to measure the RTT of the connection and calculate
 an RTO as specified in Section 2 of [RFC6298].  The resulting RTO is
 not used for retransmissions, but rather as a timeout to indicate
 when the link SHOULD be removed from the Routing Table.  It is
 RECOMMENDED that such a connection be retained for 30 seconds to
 determine if the failure was transient before concluding the link has
 failed permanently.
 When sending candidates for TLS/TCP with FH, No-ICE, a passive
 candidate MUST be provided.

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6.6.6. DTLS/UDP with SR, No-ICE

 This overlay link protocol consists of DTLS over UDP while
 implementing the Simple Reliability protocol.  Because ICE is not
 used, no STUN connectivity checks or keepalives are used.

6.7. Fragmentation and Reassembly

 In order to allow transmission over datagram protocols such as DTLS,
 RELOAD messages may be fragmented.
 Any node along the path can fragment the message, but only the final
 destination reassembles the fragments.  When a node takes a packet
 and fragments it, each fragment has a full copy of the forwarding
 header, but the data after the forwarding header is broken up into
 appropriately sized chunks.  The size of the payload chunks needs to
 take into account space to allow the Via and Destination Lists to
 grow.  Each fragment MUST contain a full copy of the Via List,
 Destination List, and ForwardingOptions and MUST contain at least 256
 bytes of the message body.  If these elements cannot fit within the
 MTU of the underlying datagram protocol, RELOAD fragmentation is not
 performed, and IP-layer fragmentation is allowed to occur.  The
 length field MUST contain the size of the message after
 fragmentation.  When a message MUST be fragmented, it SHOULD be split
 into equal-sized fragments that are no larger than the Path MTU
 (PMTU) of the next overlay link minus 32 bytes.  This is to allow the
 Via List to grow before further fragmentation is required.
 Note that this fragmentation is not optimal for the end-to-end
 path -- a message may be refragmented multiple times as it traverses
 the overlay, but it is assembled only at the final destination.  This
 option has been chosen as it is far easier to implement than end-to-
 end (e2e) PMTU discovery across an ever-changing overlay and it
 effectively addresses the reliability issues of relying on IP-layer
 fragmentation.  However, Ping can be used to allow e2e PMTU discovery
 to be implemented if desired.
 Upon receipt of a fragmented message by the intended peer, the peer
 holds the fragments in a holding buffer until the entire message has
 been received.  The message is then reassembled into a single message
 and processed.  In order to mitigate denial-of-service (DoS) attacks,
 receivers SHOULD time out incomplete fragments after the maximum
 request lifetime (15 seconds).  This time 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 as many
 fragments as can arrive in the maximum request lifetime.  However, if

Jennings, et al. Standards Track [Page 87] RFC 6940 RELOAD Base January 2014

 the receiver runs out of buffer space to reassemble a message, it
 MUST drop the message.
 The fragment field of the forwarding header is used to encode
 fragmentation information.  The offset is the number of bytes between
 the end of the forwarding header and the start of the data.  The
 first fragment therefore has an offset of 0.  The last fragment
 indicator MUST be appropriately set.  If the message is not
 fragmented, it is simply treated as if it is the only fragment: the
 last fragment bit is set and the offset is 0, resulting in a fragment
 value of 0xC0000000.
 Note: The reason for this definition of the fragment field is that
 originally, the high bit was defined in part of the specification as
 "is fragmented", so there was some specification ambiguity about how
 to encode messages with only one fragment.  This ambiguity was
 resolved in favor of always encoding as the "last" fragment with
 offset 0, thus simplifying the receiver code path, but resulting in
 the high bit being redundant.  Because messages MUST be set with the
 high bit set to 1, implementations SHOULD discard any message with it
 set to 0.  Implementations (presumably legacy ones) which choose to
 accept such messages MUST either ignore the remaining bits or ensure
 that they are 0.  They MUST NOT try to interpret as fragmented
 messages with the high bit set low.

7. 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 size of the StoredData structure, in bytes, excluding the size
    of length itself.

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 storage_time
    The time when the data was stored, represented as the number of
    milliseconds elapsed since midnight Jan 1, 1970 UTC, not counting
    leap seconds.  This will have the same values for seconds as
    standard UNIX or POSIX time.  More information can be found at
    [UnixTime].  Any attempt to store a data value with a storage time
    before that of a value already stored at this location MUST
    generate an Error_Data_Too_Old error.  This prevents rollback
    attacks.  The node SHOULD make a best-effort attempt to use a
    correct clock to determine this number.  However, the protocol
    does not require synchronized clocks: the receiving peer uses the
    storage time in the previous store, not its own clock.  Clock
    values are used so that when clocks are generally synchronized,
    data may be stored in a single transaction, rather than querying
    for the value of a counter before the actual store.
    If a node attempting to store new data in response to a user
    request (rather than as an overlay maintenance operation such as
    occurs when healing the overlay from a partition) is rejected with
    an Error_Data_Too_Old error, the node MAY elect to perform its
    store using a storage_time that increments the value used with the
    previous store (this may be obtained by doing a Fetch).  This
    situation may occur when the clocks of nodes storing to this
    location are not properly synchronized.
 lifetime
    The validity period for the data, in seconds, starting from the
    time the peer receives the StoreReq.
 value
    The data value itself, as described in Section 7.2.
 signature
    A signature, as defined in Section 7.1.
 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 7.2 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.

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7.1. Data Signature Computation

 Each StoredData element is individually signed.  However, the
 signature also must be self-contained and must 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 || storage_time || StoredDataValue ||
    SignerIdentity
 where || indicates concatenation and where these values are:
 resource_id
    The Resource-ID where this data is stored.
 kind
    The Kind-ID for this data.
 storage_time
    The contents of the storage_time data value.
 StoredDataValue
    The contents of the stored data value, as described in the
    previous sections.
 SignerIdentity
    The signer identity, as defined in Section 6.3.4.
 Once the signature has been computed, the signature is represented
 using a signature element, as described in Section 6.3.4.
 Note that there is no necessary relationship between the validity
 window of a certificate and the expiry of the data it is
 authenticating.  When signatures are verified, the current time MUST
 be compared to the certificate validity period.  Stored data MAY be
 set to expire after the signing certificate's validity period.  Such
 signatures are not considered valid after the signing certificate
 expires.  Implementations may "garbage collect" such data at their
 convenience, either by purging it automatically (perhaps by setting
 the upper bound on data storage to the lifetime of the signing
 certificate) or by simply leaving it in place until it expires
 naturally and relying on users of that data to notice the expired
 signing certificate.

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7.2. Data Models

 The protocol currently defines the following data models:
 o  single value
 o  array
 o  dictionary
 These are represented with the StoredDataValue structure.  The actual
 data model is known from the Kind being stored.
      struct {
        Boolean                exists;
        opaque                 value<0..2^32-1>;
      } DataValue;
      struct {
        select (DataModel) {
          case single_value:
            DataValue             single_value_entry;
          case array:
            ArrayEntry            array_entry;
          case dictionary:
            DictionaryEntry       dictionary_entry;
          /* This structure may be extended */
        };
      } StoredDataValue;
 The following sections discuss the properties of each data model.

7.2.1. Single Value

 A single-value element is a simple sequence of bytes.  There may be
 only one single-value element for each Resource-ID, Kind-ID pair.
 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,
    this means that a value is present.  This gives the protocol a
    mechanism for indicating nonexistence as opposed to emptiness.

Jennings, et al. Standards Track [Page 91] RFC 6940 RELOAD Base January 2014

 value
    The stored data.

7.2.2. 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.
 An 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.

7.2.3. 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
 a DictionaryEntry:
       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.

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7.3. 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 14.4 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.
 In the following policies, the term "signer" refers to the signer of
 the StoredValue object and, in the case of non-replica stores, to the
 signer of the StoreReq message.  That is, in a non-replica store,
 both the signer of the StoredValue and the signer of the StoreReq
 MUST conform to the policy.  In the case of a replica store, the
 signer of the StoredValue MUST conform to the policy, and the
 StoreReq itself MUST be checked as described in Section 7.4.1.1.

7.3.1. USER-MATCH

 In the USER-MATCH policy, a given value MUST be written (or
 overwritten) if and only if the signer's certificate has a user name
 which 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.

7.3.2. NODE-MATCH

 In the NODE-MATCH policy, a given value MUST be written (or
 overwritten) if and only if the signer's certificate has a specified
 Node-ID which hashes (using the hash function for the overlay) to the
 Resource-ID for the resource and that Node-ID is the one indicated in
 the SignerIdentity value cert_hash.

7.3.3. USER-NODE-MATCH

 The USER-NODE-MATCH policy may be used only with dictionary types.
 In the USER-NODE-MATCH policy, a given value MUST be written (or
 overwritten) if and only if the signer's certificate has a user name
 which 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, and that Node-ID MUST be
 the one indicated in the SignerIdentity value cert_hash.

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7.3.4. NODE-MULTIPLE

 In the NODE-MULTIPLE policy, a given value MUST be written (or
 overwritten) if and only if the signer's certificate contains a
 Node-ID such that H(Node-ID || i) is equal to the Resource-ID for
 some small integer value of i and that Node-ID is the one indicated
 in the SignerIdentity value cert_hash.  When this policy is in use,
 the maximum value of i MUST be specified in the Kind definition.
 Note that because i is not carried on the wire, the verifier MUST
 iterate through potential i values, up to the maximum value, to
 determine whether a store is acceptable.

7.4. 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  Stat: Get metadata about values in the overlay.
 o  Find the values stored at an individual peer.
 These methods are described in the following sections.

7.4.1. 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.

7.4.1.1. 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 what it was
 before the request was received.

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 The store request is defined by the StoreReq structure:
     struct {
         KindId                 kind;
         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 at which to store.
 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, that is 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.  Different topologies may
    choose to allocate or interpret the replica number differently
    (see Section 10.4).
 kind_data
    A series of elements, one for each Kind of data to be stored.
 The peer MUST check that it is responsible for the resource if the
 replica number is zero; if it is not, the peer must reject the
 request.  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) if the replica number is nonzero; if the request sender
 is not consistent, it should 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 MUST reject requests corresponding
    to unknown Kinds.

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 generation_counter
    The expected current state of the generation counter
    (approximately the number of times that 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 signatures over each individual data element, if any, are
    valid.  If this check fails, the request MUST be rejected with an
    Error_Forbidden error.
 o  Each element is signed by a credential which is authorized to
    write this Kind at this Resource-ID.  If this check fails, the
    request MUST be rejected with an Error_Forbidden error.
 o  For original (non-replica) stores, the StoreReq is signed by a
    credential which is authorized to write this Kind at this
    Resource-ID.  If this check fails, the request MUST be rejected
    with an Error_Forbidden error.
 o  For replica stores, the StoreReq is signed by a Node-ID which is a
    plausible node to either have originally stored the value or have
    been in the replica set.  What this means is overlay specific, but
    in the case of the Chord-based DHT defined in this specification,
    replica StoreReqs MUST come from nodes which are either in the
    known replica set for a given resource or which are closer than
    some node in the replica set.  If this check fails, the request
    MUST be rejected with an Error_Forbidden error.
 o  For original (non-replica) stores, the peer MUST check that if the
    generation counter is nonzero, 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  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
    NOT use a generation counter of 0.

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 o  The storage time values are greater than that of any values which
    would be replaced by this Store.
 o  The size and number of the stored values are consistent with the
    limits specified in the overlay configuration.
 o  If the data is signed with identity_type set to "none" and/or
    SignatureAndHashAlgorithm values set to {0, 0} ("anonymous" and
    "none"), the StoreReq MUST be rejected with an Error_forbidden
    error.  Only synthesized data returned by the storage can use
    these values (see Section 7.4.2.2)
 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 1.  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 1 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.
 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 adjustment SHOULD be implemented by an algorithm
 equivalent to the following: at the time the peer initially receives
 the StoreReq, it notes the local time T.  When it then attempts to do
 a StoreReq to another node, it should decrement the lifetime value by
 the difference between the current local time and T.
 Unless otherwise specified by the usage, if a peer attempts to store
 data previously stored by another node (e.g., for replicas or
 topology shifts) and that store fails with either an
 Error_Generation_Counter_Too_Low or an Error_Data_Too_Old error, the
 peer MUST fetch the newer data from the peer generating the error and
 use that to replace its own copy.  This rule allows resynchronization
 after partitions heal.
 When a network partition is being healed and unless otherwise
 specified, the default merging rule is to act as if all the values
 that need to be merged were stored and as if 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

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 generally not an issue.  If two nodes are on different partitions,
 write to the same location, and have clock skew, 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 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.

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 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" is in the single-value location for Kind X.
 o  The value "foo" at index 0 is in the array for Kind Y.
 o  The value "bar" at index 1 is in the array for Kind Y.
                                   Store
                              resource=1234
                            replica_number = 0
                                 /      \
                                /        \
                    StoreKindData        StoreKindData
                kind=X (Single-Value)    kind=Y (Array)
              generation_counter = 99    generation_counter = 107
                         |                    /\
                         |                   /  \
                     StoredData             /    \
           storage_time = xxxxxxx          /      \
                 lifetime = 86400         /        \
                 signature = XXXX        /          \
                         |               |           |
                         |        StoredData       StoredData
                         |    storage_time =       storage_time =
                         |          yyyyyyyy       zzzzzzz
                         |  lifetime = 86400       lifetime = 33200
                         |  signature = YYYY       signature = ZZZZ
                         |               |           |
                  StoredDataValue        |           |
                   value="abc"           |           |
                                         |           |
                                StoredDataValue  StoredDataValue
                                      index=0      index=1
                                   value="foo"    value="bar"

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7.4.1.2. Response Definition

 In response to a successful Store request, the peer MUST return a
 StoreAns message containing a series of StoreKindResponse elements,
 which contains the current value of the generation counter for each
 Kind-ID, as well as a list of the peers where the data will be
 replicated by the node processing the request.
      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
    The Kind-ID being represented.
 generation_counter
    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 node
    to independently verify that the replicas have in fact been
    stored.  It does this verification by using the Stat method (see
    Section 7.4.3).  Note that the storing node is not required to
    perform this verification.
 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 an Error_Generation_Counter_Too_Low
 error.  The error_info in the ErrorResponse MUST be a StoreAns
 response containing the correct generation counter for each Kind and
 the replica list, which will be empty.  For original (non-replica)
 stores, a node which receives such an error SHOULD attempt to fetch
 the data and, if the storage_time value is newer, replace its own
 data with that newer data.  This rule improves data consistency in
 the case of partitions and merges.

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 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.
 If any type of request tries to access a data Kind that the peer does
 not know about, the peer MUST fail the request and generate an
 Error_Unknown_Kind error.  The error_info in the Error_Response is:
            KindId        unknown_kinds<0..2^8-1>;
 which lists all the Kinds that were unrecognized.  A node which
 receives this error MUST generate a ConfigUpdate message which
 contains the appropriate Kind definition (assuming which, in fact, a
 Kind which was defined in the configuration document was used).

7.4.1.3. Removing Values

 RELOAD 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" set to
 False:
    exists = False
    value = {} (0 length)
 The owner SHOULD use a lifetime for the nonexistent value that is at
 least as long as the remainder of the lifetime of the value it is
 replacing.  Otherwise, it is possible for the original value to be
 accidentally or maliciously re-stored after the storing node has
 expired it.  Note that a window of vulnerability for replay attack
 still exists after the original lifetime has expired (as with any
 store).  This attack can be mitigated by doing a nonexistent store
 with a very long lifetime.
 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 the
 lifetime expires.  When a stored nonexistent value's lifetime
 expires, it is simply removed from the storing node, as happens when
 any other stored value expires.
 Note that in the case of arrays and dictionaries, expiration may
 create an implicit, unsigned "nonexistent" value to represent a gap
 in the data structure, as might happen when any value is aged out.

Jennings, et al. Standards Track [Page 101] RFC 6940 RELOAD Base January 2014

 However, this value isn't persistent, nor is it replicated.  It is
 simply synthesized by the storing node.

7.4.2. 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.

7.4.2.1. Request Definition

 Fetch requests are defined by the FetchReq structure:
      struct {
        int32            first;
        int32            last;
      } ArrayRange;
      struct {
        KindId                  kind;
        uint64                  generation;
        uint16                  length;
        select (DataModel) {
          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. Standards Track [Page 102] RFC 6940 RELOAD Base January 2014

 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.
 DataModel
    The data model of the data.  This is not transmitted on the wire,
    but comes from the definition of the Kind.
 generation
    The last generation counter that the requesting node 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 model is single value, the specifier is empty.
 o  If the data model is array, the specifier contains 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 first
    integer MUST be less than or equal to the second.  While multiple
    ranges MAY be specified, they MUST NOT overlap.
 o  If the data model is dictionary, then the specifier contains a
    list of the dictionary keys being requested.  If no keys are
    specified, then this is a wildcard fetch and all key-value pairs
    are returned.

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 The generation counter is used to indicate the requester's expected
 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.

7.4.2.2. 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 (i.e., one which the
    node has no knowledge of) is represented by a synthetic value with
    "exists" set to False and has an empty signature.  Specifically,
    the identity_type is set to "none", the SignatureAndHashAlgorithm
    values are set to {0, 0} ("anonymous" and "none", respectively),
    and the signature value is of zero length.  This removes the need
    for the responding node to do signatures for values which do not
    exist.  These signatures are unnecessary, as the entire response
    is signed by that node.  Note that entries which have been removed
    by the procedure given in Section 7.4.1.3 and which have not yet
    expired also have exists = False, but have valid signatures from
    the node which did the store.

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 Upon receipt of a FetchAns message, nodes MUST verify the signatures
 on all the received values.  Any values with invalid signatures
 (including expired certificates) MUST be discarded.  Note that this
 implies that implementations which wish to store data for long
 periods of time must have certificates with appropriate expiration
 dates or must re-store periodically.  Implementations MAY return the
 subset of values with valid signatures, but in that case, they SHOULD
 somehow signal to the application that a partial response was
 received.
 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
 signature.  In order to avoid this issue, the index value in the
 array is set to zero before the signature is computed.  This implies
 that malicious storing nodes can reorder array entries without being
 detected.

7.4.3. 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 file system.  It also
 allows the requesting node to get a list of matching elements without
 requesting the entire element.

7.4.3.1. 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;

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7.4.3.2. 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;
      } ArrayEntryMeta;
      struct {
        DictionaryKey          key;
        MetaData               value;
      } DictionaryEntryMeta;
      struct {
        select (DataModel) {
          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                  value_length;
        uint64                  storage_time;
        uint32                  lifetime;
        MetaDataValue           metadata;
      } StoredMetaData;

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      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.
 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 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 using hash_algorithm on the value field of the DataValue,
    including its 4 leading length bytes.

7.4.4. 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 closest to R.  This method can be used to walk the
 Overlay Instance by iteratively fetching R_n+1=nearest(1 + R_n).

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7.4.4.1. Request Definition

 The FindReq message contains a Resource-ID and a series of 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:
 resource
    The desired Resource-ID.
 kinds
    The desired Kind-IDs.  Each value MUST appear only once.
    Otherwise, the request MUST be rejected with an error.

7.4.4.2. Response Definition

 A response to a successful Find request is a FindAns message
 containing the closest Resource-ID on the peer 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 an Error_Not_Found error code.
 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.

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 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.  It 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.

7.4.5. 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 needs to define what data model is used for its data,
 this does not mean that it must define new data models.  Where
 practical, Kinds should use the existing data models.  The intention
 is that the basic data model set be sufficient for most applications/
 usages.

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8. Certificate Store Usage

 The Certificate Store Usage allows a node to store its certificate in
 the overlay.
 A user/node 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 for a peer is not
 stored at its Node-ID but rather at a hash of its Node-ID.  The
 intention here (as is common throughout RELOAD) is to avoid making a
 peer responsible for its own data.
 New certificates are stored at the end of the list.  This structure
 allows users to store an old and a new certificate that both have the
 same Node-ID, which allows for migration of certificates when they
 are renewed.
 This usage defines the following Kinds:
 Name:  CERTIFICATE_BY_NODE
 Data Model:  The data model for CERTIFICATE_BY_NODE data is array.
 Access Control:  NODE-MATCH
 Name:  CERTIFICATE_BY_USER
 Data Model:  The data model for CERTIFICATE_BY_USER data is array.
 Access Control:  USER-MATCH

9. 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 [RFC5766].  When a node
 starts up, it joins the overlay network and forms several connections
 in the process.  If the ICE stage in any of these connections returns
 a reflexive address that is not the same as the peer's perceived
 address, then the peer is behind a NAT and SHOULD NOT be a candidate
 for a TURN server.  Additionally, if the peer's IP address is in the
 private address space range as defined by [RFC1918], then it is also

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 SHOULD NOT be a candidate for a TURN server.  Otherwise, the peer
 SHOULD assume that 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 turn-density 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.  If the turn-
 density is not set to zero, for each value, called d, between 1 and
 turn-density, the peer forms a Resource Name by concatenating its
 Node-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;
        IpAddressPort           server_address;
      } TurnServer;
 The contents of this structure are as follows:
 iteration
    The d value.
 server_address
    The address at which the TURN server can be contacted.
 Note:  Correct functioning of this algorithm depends on having turn-
    density be a reasonable estimate of the reciprocal of the
    proportion of nodes in the overlay that can act as TURN servers.
    If the turn-density value in the configuration file is too low,
    the process of finding TURN servers becomes more expensive, as
    multiple candidate Resource-IDs must be probed to find a TURN
    server.
 Peers that provide this service need to support the TURN extensions
 to STUN for media relay, as defined in [RFC5766].
 This usage defines the following Kind to indicate that 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.

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 Access Control:  NODE-MULTIPLE, with a maximum iteration of counter
    20.
 Peers MAY find other servers by selecting a random Resource-ID and
 then doing a Find request for the appropriate Kind-ID 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.
 Note to implementers: The certificates used by TurnServer entries
 need to be retained, as described in Section 6.3.4.

10. Chord Algorithm

 This algorithm is assigned the name CHORD-RELOAD to indicate that it
 is an adaptation of the basic Chord-based DHT algorithm.
 This algorithm differs from the Chord algorithm that was originally
 presented in [Chord].  It has been updated based on more recent
 research results and implementation experiences, and to adapt it to
 the RELOAD protocol.  Here is a short list of differences:
 o  The original Chord algorithm specified that a single predecessor
    and a successor list be stored.  The CHORD-RELOAD algorithm
    attempts to have more than one predecessor and successor.  The
    predecessor sets help other neighbors learn their successor list.
 o  The original Chord specification and analysis called for iterative
    routing.  RELOAD specifies recursive routing.  In addition to the
    performance implications, the cost of NAT traversal dictates
    recursive routing.
 o  Finger Table entries are indexed in the opposite order.  Original
    Chord specifies finger[0] as the immediate successor of the peer.
    CHORD-RELOAD specifies finger[0] as the peer 180 degrees around
    the ring from the peer.  This change was made to simplify
    discussion and implementation of variable-sized Finger Tables.
    However, with either approach, no more than O(log N) entries
    should typically be stored in a Finger Table.
 o  The stabilize() and fix_fingers() algorithms in the original Chord
    algorithm are merged into a single periodic process.
    Stabilization is implemented slightly differently because of the
    larger neighborhood, and fix_fingers is not as aggressive to

Jennings, et al. Standards Track [Page 112] RFC 6940 RELOAD Base January 2014

    reduce load, nor does it search for optimal matches of the Finger
    Table entries.
 o  RELOAD allows for a 128-bit hash instead of a 160-bit hash, as
    RELOAD is not designed to be used in networks with close to or
    more than 2^128 nodes or objects (and it is hard to see how one
    would assemble such a network).
 o  RELOAD uses randomized finger entries, as described in
    Section 10.7.4.2.
 o  The CHORD-RELOAD algorithm allows the use of either reactive or
    periodic recovery.  The original Chord paper used periodic
    recovery.  Reactive recovery provides better performance in small
    overlays, but is believed to be unstable in large overlays
    (greater than 1000) with high levels of churn
    [handling-churn-usenix04].  The overlay configuration file
    specifies a "chord-reactive" element that indicates whether
    reactive recovery should be used.

10.1. Overview

 The algorithm described here, CHORD-RELOAD, is a modified version of
 the Chord algorithm.  In Chord (and in the algorithm described here),
 nodes are arranged in a ring, with node n being adjacent to nodes n-1
 and n+1 and with all arithmetic being done modulo 2^{k}, where k is
 the length of the Node-ID in bits, so that node 2^{k} - 1 is directly
 before node 0.
 Each peer keeps track of a Finger Table and a Neighbor Table.  The
 Neighbor Table contains at least the three peers before and after
 this peer in the DHT ring.  There may not be three entries in all
 cases, such as small rings or while the ring topology is changing.
 The first entry in the Finger Table contains the peer halfway around
 the ring from this peer, the second entry contains the peer that is
 1/4th of the way around, the third entry contains the peer that is
 1/8th of the way around, and so on.  Fundamentally, the Chord DHT can
 be thought of as 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 the insertion 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
 [wikiSkiplist] 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, where N represents the number of nodes in the DHT.

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 The Neighbor Table and Finger Table entries contain logical Node-IDs
 as values, but the actual mapping of an IP level addressing
 information to reach that Node-ID is kept in the Connection Table.
 A peer, x, is responsible for a particular Resource-ID, k, if k is
 less than or equal to x and k is greater than p, where p is the
 Node-ID of the previous peer in the Neighbor Table.  Care must be
 taken when computing to note that all math is modulo 2^128.

10.2. Hash Function

 For this Chord-based Topology Plug-in, the size of the Resource-ID is
 128 bits.  The hash of a Resource-ID MUST be computed using SHA-1
 [RFC3174], and then the SHA-1 result MUST be truncated to the most
 significant 128 bits.

10.3. Routing

 The Routing Table is conceptually the union of the Neighbor Table and
 the Finger Table.
 If a peer is not responsible for a Resource-ID k, but is directly
 connected to a node with Node-ID k, then it MUST route the message to
 that node.  Otherwise, it MUST route 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. If no such node is found, the peer finds the
 smallest Node-ID that is greater than k and MUST route the message to
 that node.

10.4. Redundancy

 When a peer receives a Store request for Resource-ID k and it is
 responsible for Resource-ID k, it MUST store the data and return a
 success response.  It MUST then send a Store request to its successor
 in the Neighbor Table and to that peer's successor, incrementing the
 replica number for each 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 SHOULD check that they
 came from an appropriate predecessor in their Neighbor Table and that
 they are in a range that this predecessor is responsible for.  Then,
 they MUST 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 this Topology Plug-in does not use the replica number for
 purposes other than knowing the difference between a replica and a
 non-replica.

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 Managing replicas as the overlay changes is described in
 Section 10.7.3.
 The sequential replicas used in this overlay algorithm protect
 against peer failure but not against malicious peers.  Additional
 replication from the Usage is required to protect resources from such
 attacks, as discussed in Section 13.5.4.

10.5. Joining

 The join process for a Joining Node (JN) with Node-ID n is as
 follows:
 1.  JN MUST connect to its chosen bootstrap node, as specified in
     Section 11.4.
 2.  JN SHOULD send an Attach request to the Admitting Peer (AP) for
     Resource-ID n+1.  The "send_update" flag can be used to acquire
     the Routing Table of AP.
 3.  JN SHOULD send Attach requests to initiate connections to each of
     the peers in the Neighbor Table as well as to the desired peers
     in the Finger Table.  Note that this does not populate their
     Routing Tables, but only their Connection Tables, so JN will not
     get messages that it is expected to route to other nodes.
 4.  JN MUST enter into its Routing Table all the peers that it has
     successfully contacted.
 5.  JN MUST send a Join to AP.  The AP MUST send the response to the
     Join.
 6.  AP MUST do a series of Store requests to JN to store the data
     that JN will be responsible for.
 7.  AP MUST send JN an Update explicitly labeling JN as its
     predecessor.  At this point, JN is part of the ring and is
     responsible for a section of the overlay.  AP MAY now forget any
     data which is assigned to JN and not AP.  AP SHOULD NOT forget
     any data where AP is the replica set for the data.
 8.  The AP MUST send an Update to all of its neighbors (including JN)
     with the new values of its neighbor set (including JN).
 9.  JN MUST send Updates to all of the peers in its Neighbor Table.

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 If JN sends an Attach to AP with send_update, it immediately knows
 most of its expected neighbors from AP's Routing Table update and MAY
 directly connect to them.  This is the RECOMMENDED procedure.
 If for some reason JN does not get AP's Routing Table, it MAY still
 populate its Neighbor Table incrementally.  It SHOULD send a Ping
 directed at Resource-ID n+1 (directly after its own Resource-ID).
 This allows JN to discover its own successor.  Call that node p0.  JN
 then SHOULD send a Ping to p0+1 to discover its successor (p1).  This
 process MAY 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.  An alternate procedure is to send
 Attaches to those nodes rather than Pings, which form the connections
 immediately, but may be slower if the nodes need to collect ICE
 candidates.
 In order to set up its i'th Finger Table entry, JN MUST send an
 Attach to peer n+2^(128-i).  This will be routed to a peer in
 approximately the right location around the ring.  (Note that the
 first entry in the Finger Table has i=1 and not i=0 in this
 formulation.)
 The Joining Node MUST NOT send any Update message placing itself in
 the overlay until it has successfully completed an Attach with each
 peer that should be in its Neighbor Table.

10.6. 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.

Jennings, et al. Standards Track [Page 116] RFC 6940 RELOAD Base January 2014

10.7. Updates

 An Update for this DHT is defined as:
      enum { invalidChordUpdateType(0),
             peer_ready(1), neighbors(2), full(3), (255) }
           ChordUpdateType;
      struct {
         uint32                 uptime;
         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 "uptime" field contains the time this peer has been up in
 seconds.
 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.

Jennings, et al. Standards Track [Page 117] RFC 6940 RELOAD Base January 2014

 If the message is of type "neighbors", then the contents of the
 message will be:
 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 of 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, even though this will not be
 possible if the ring is very small.  It is RECOMMENDED that O(log(N))
 predecessors and successors be maintained in the neighbor set.  There
 are many ways to estimate N, some of which are discussed in
 [DHT-RELOAD].

10.7.1. Handling Neighbor Failures

 Every time a connection to a peer in the Neighbor Table is lost (as
 determined by connectivity pings or the failure of some request), the
 peer MUST remove the entry from its Neighbor Table and replace it
 with the best match it has from the other peers in its Routing Table.
 If using reactive recovery, the peer MUST send an immediate Update to
 all nodes in its Neighbor Table.  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 the successor
 replacement hold-down time (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 the neighbor failure affects the peer's range of responsible IDs,
 then the Update MUST be sent to all nodes in its Connection Table.

Jennings, et al. Standards Track [Page 118] RFC 6940 RELOAD Base January 2014

 A peer MAY attempt to reestablish connectivity with a lost neighbor
 either by waiting additional time to see if connectivity returns or
 by actively routing a new Attach to the lost peer.  Details for these
 procedures are beyond the scope of this document.  In the case of an
 attempt to reestablish connectivity with a lost neighbor, the peer
 MUST be removed from the Neighbor Table.  Such a peer is returned to
 the Neighbor Table once connectivity is reestablished.
 If connectivity is lost to all successor peers in the Neighbor Table,
 then this peer SHOULD behave as if it is joining the network and MUST
 use Pings 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.

10.7.2. Handling Finger Table Entry Failure

 If a Finger Table entry is found to have failed (as determined by
 connectivity pings or the failure of some request), all references to
 the failed peer MUST be removed from the Finger Table and replaced
 with the closest preceding peer from the Finger Table or Neighbor
 Table.
 If using reactive recovery, the peer MUST initiate a search for a new
 Finger Table entry, as described below.

10.7.3. Receiving Updates

 When a peer x 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 two major cases:
 o  The UpdateReq contains peers that match x's Neighbor Table, so no
    change is needed to the neighbor set.
 o  The UpdateReq contains peers that x does not know about that
    should be in x's Neighbor Table; i.e., they are closer than
    entries in the Neighbor Table.
 In the first case, no change is needed.
 In the second case, x MUST attempt to Attach to the new peers, and if
 it is successful, it MUST adjust its neighbor set accordingly.  Note
 that x can maintain the now inferior peers as neighbors, but it MUST
 remember the closer ones.

Jennings, et al. Standards Track [Page 119] RFC 6940 RELOAD Base January 2014

 After any Pings and Attaches are done, if the Neighbor Table changes
 and the peer is using reactive recovery, the peer MUST send an Update
 request to each member of its Connection Table.  These Update
 requests are what end 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 x is responsible for a Resource-ID R and x 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 peer x detects that it is no longer in the replica set for a
 resource R (i.e., there are three predecessors between x and R), it
 SHOULD delete all data associated with R from its local store.
 When a peer discovers that its range of responsible IDs has changed,
 it MUST send an Update to all entries in its Connection Table.

10.7.4. Stabilization

 There are four components to stabilization:
 1.  Exchange Updates with all peers in its Neighbor 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.

10.7.4.1. Updating the Neighbor Table

 A peer MUST periodically send an Update request to every peer in its
 Neighbor Table.  The purpose of this is to keep the predecessor and
 successor lists up to date and to detect failed peers.  The default
 time is about every ten minutes, but the configuration server SHOULD
 set this in the Configuration Document using the "chord-update-
 interval" element (denominated in seconds).  A peer SHOULD randomly
 offset these Update requests so they do not occur all at once.

Jennings, et al. Standards Track [Page 120] RFC 6940 RELOAD Base January 2014

10.7.4.2. Refreshing the Finger Table

 A peer MUST periodically search for new peers to replace invalid
 entries in the Finger Table.  For peer x, the i'th Finger Table entry
 is valid if it is in the range [ x+2^( 128-i ),
 x+2^( 128-(i-1) )-1 ].  Invalid entries occur in the Finger
 Table when a previous Finger Table entry has failed or when no peer
 has been found in that range.
 Two possible methods for searching for new peers for the Finger
 Table entries are presented:
 Alternative 1: A peer selects one entry in the Finger Table from
 among the invalid entries.  It pings for a new peer for that Finger
 Table entry.  The selection SHOULD be exponentially weighted to
 attempt to replace earlier (lower i) entries in the Finger Table.  A
 simple way to implement this selection is to search through the
 Finger Table entries from i=1, and each time an invalid entry is
 encountered, send a Ping to replace that entry with probability 0.5.
 Alternative 2: A peer monitors the Update messages received from its
 connections to observe when an Update indicates a peer that would be
 used to replace an invalid Finger Table entry, i, and flags that
 entry in the Finger Table.  Every "chord-ping-interval" seconds, the
 peer selects from among those flagged candidates using an
 exponentially weighted probability, as above.
 When searching for a better entry, the peer SHOULD send the Ping 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 other than choosing randomly within
 the range.  Any such alternate methods SHOULD 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.
 A peer SHOULD NOT send Ping requests looking for new finger table
 entries more often than the configuration element "chord-ping-
 interval", which defaults to 3600 seconds (one per hour).
 A peer MAY choose to keep connections to multiple peers that can act
 for a given Finger Table entry.

Jennings, et al. Standards Track [Page 121] RFC 6940 RELOAD Base January 2014

10.7.4.3. Adjusting Finger Table Size

 If the Finger Table has fewer than 16 entries, the node SHOULD
 attempt to discover more fingers to grow the size of the table to 16.
 The value 16 was chosen to ensure high odds of a node maintaining
 connectivity to the overlay even with strange network partitions.
 For many overlays, 16 Finger Table entries will be enough, but as an
 overlay grows very large, more than 16 entries may be required in the
 Finger Table for efficient routing.  An implementation SHOULD be
 capable of increasing the number of entries in the Finger Table to
 128 entries.
 Although log(N) entries are all that are required for optimal
 performance, careful implementation of stabilization will result in
 no additional traffic being generated when maintaining a Finger
 Table larger than log(N) entries.  Implementers are encouraged to
 make use of RouteQuery and algorithms for determining where new
 Finger Table entries may be found.  Complete details of possible
 implementations are outside the scope of this specification.
 A simple approach to sizing the Finger Table is to ensure that the
 Finger Table is large enough to contain at least the final successor
 in the peer's Neighbor Table.

10.7.4.4. Detecting Partitioning

 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 node,
 B.  P SHOULD then send a Ping for its own Node-ID routed through B.
 If a response is received from peer S', which is not P's successor,
 then the overlay is partitioned and P SHOULD send an 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.)
 Future specifications may describe alternative mechanisms for
 determining when to repeat the discovery process.

Jennings, et al. Standards Track [Page 122] RFC 6940 RELOAD Base January 2014

10.8. Route Query f.in 3

     For CHORD-RELOAD, 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_peer;
    } ChordRouteQueryAns;
 The contents of this structure are as follows:
 next_peer
    The peer to which the responding peer would route the message in
    order to deliver it to the destination listed in the request.
 If the requester has set the send_update flag, the responder SHOULD
 initiate an Update immediately after sending the RouteQueryAns.

10.9. Leaving

 To support extensions, such as [DHT-RELOAD], peers SHOULD send a
 Leave request to all members of their Neighbor Table before exiting
 the Overlay Instance.  The overlay_specific_data field MUST contain
 the ChordLeaveData structure, defined below:
            enum { invalidChordLeaveType(0),
                    from_succ(1), from_pred(2), (255) }
                  ChordLeaveType;
             struct {
               ChordLeaveType         type;
                select (type) {
                  case from_succ:
                    NodeId            successors<0..2^16-1>;
                  case from_pred:
                    NodeId           predecessors<0..2^16-1>;
                };
             } ChordLeaveData;

Jennings, et al. Standards Track [Page 123] RFC 6940 RELOAD Base January 2014

 The "type" field indicates whether the Leave request was sent by a
 predecessor or a successor of the recipient:
 from_succ
    The Leave request was sent by a successor.
 from_pred
    The Leave request was sent by a predecessor.
 If the type of the request is "from_succ", the contents will be:
 successors
    The sender's successor list.
 If the type of the request is "from_pred", the contents will be:
 predecessors
    The sender's predecessor list.
 Any peer which receives a Leave for a peer n in its neighbor set MUST
 follow procedures as if it had detected a peer failure as described
 in Section 10.7.1.

11. Enrollment and Bootstrap

 The section defines the format of the configuration data as well the
 process to join a new overlay.

11.1. Overlay Configuration

 This specification defines a new content type
 "application/p2p-overlay+xml" for a MIME entity that contains overlay
 information.  An example document is shown below:
 <?xml version="1.0" encoding="UTF-8"?>
 <overlay xmlns="urn:ietf:params:xml:ns:p2p:config-base"
    xmlns:ext="urn:ietf:params:xml:ns:p2p:config-ext1"
    xmlns:chord="urn:ietf:params:xml:ns:p2p:config-chord">
    <configuration instance-name="overlay.example.org" sequence="22"
        expiration="2002-10-10T07:00:00Z" ext:ext-example="stuff" >
        <topology-plugin> CHORD-RELOAD </topology-plugin>
        <node-id-length>16</node-id-length>
        <root-cert>
 MIIDJDCCAo2gAwIBAgIBADANBgkqhkiG9w0BAQUFADBwMQswCQYDVQQGEwJVUzET
 MBEGA1UECBMKQ2FsaWZvcm5pYTERMA8GA1UEBxMIU2FuIEpvc2UxDjAMBgNVBAoT
 BXNpcGl0MSkwJwYDVQQLEyBTaXBpdCBUZXN0IENlcnRpZmljYXRlIEF1dGhvcml0
 eTAeFw0wMzA3MTgxMjIxNTJaFw0xMzA3MTUxMjIxNTJaMHAxCzAJBgNVBAYTAlVT
 MRMwEQYDVQQIEwpDYWxpZm9ybmlhMREwDwYDVQQHEwhTYW4gSm9zZTEOMAwGA1UE

Jennings, et al. Standards Track [Page 124] RFC 6940 RELOAD Base January 2014

 ChMFc2lwaXQxKTAnBgNVBAsTIFNpcGl0IFRlc3QgQ2VydGlmaWNhdGUgQXV0aG9y
 aXR5MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDDIh6DkcUDLDyK9BEUxkud
 +nJ4xrCVGKfgjHm6XaSuHiEtnfELHM+9WymzkBNzZpJu30yzsxwfKoIKugdNUrD4
 N3viCicwcN35LgP/KnbN34cavXHr4ZlqxH+OdKB3hQTpQa38A7YXdaoz6goW2ft5
 Mi74z03GNKP/G9BoKOGd5QIDAQABo4HNMIHKMB0GA1UdDgQWBBRrRhcU6pR2JYBU
 bhNU2qHjVBShtjCBmgYDVR0jBIGSMIGPgBRrRhcU6pR2JYBUbhNU2qHjVBShtqF0
 pHIwcDELMAkGA1UEBhMCVVMxEzARBgNVBAgTCkNhbGlmb3JuaWExETAPBgNVBAcT
 CFNhbiBKb3NlMQ4wDAYDVQQKEwVzaXBpdDEpMCcGA1UECxMgU2lwaXQgVGVzdCBD
 ZXJ0aWZpY2F0ZSBBdXRob3JpdHmCAQAwDAYDVR0TBAUwAwEB/zANBgkqhkiG9w0B
 AQUFAAOBgQCWbRvv1ZGTRXxbH8/EqkdSCzSoUPrs+rQqR0xdQac9wNY/nlZbkR3O
 qAezG6Sfmklvf+DOg5RxQq/+Y6I03LRepc7KeVDpaplMFGnpfKsibETMipwzayNQ
 QgUf4cKBiF+65Ue7hZuDJa2EMv8qW4twEhGDYclpFU9YozyS1OhvUg==
        </root-cert>
        <root-cert> YmFkIGNlcnQK </root-cert>
        <enrollment-server>https://example.org</enrollment-server>
        <enrollment-server>https://example.net</enrollment-server>
        <self-signed-permitted
                  digest="sha1">false</self-signed-permitted>
        <bootstrap-node address="192.0.0.1" port="6084" />
        <bootstrap-node address="192.0.2.2" port="6084" />
        <bootstrap-node address="2001:DB8::1" port="6084" />
        <turn-density> 20 </turn-density>
        <clients-permitted> false </clients-permitted>
        <no-ice> false </no-ice>
        <chord:chord-update-interval>
            400</chord:chord-update-interval>
        <chord:chord-ping-interval>30</chord:chord-ping-interval>
        <chord:chord-reactive> true </chord:chord-reactive>
        <shared-secret> password </shared-secret>
        <max-message-size>4000</max-message-size>
        <initial-ttl> 30 </initial-ttl>
        <overlay-reliability-timer> 3000 </overlay-reliability-timer>
        <overlay-link-protocol>TLS</overlay-link-protocol>
        <configuration-signer>47112162e84c69ba</configuration-signer>
        <kind-signer> 47112162e84c69ba </kind-signer>
        <kind-signer> 6eba45d31a900c06 </kind-signer>
        <bad-node> 6ebc45d31a900c06 </bad-node>
        <bad-node> 6ebc45d31a900ca6 </bad-node>
        <ext:example-extension> foo </ext:example-extension>
        <mandatory-extension>
            urn:ietf:params:xml:ns:p2p:config-ext1
        </mandatory-extension>
        <required-kinds>
          <kind-block>
            <kind name="SIP-REGISTRATION">

Jennings, et al. Standards Track [Page 125] RFC 6940 RELOAD Base January 2014

                <data-model>SINGLE</data-model>
                <access-control>USER-MATCH</access-control>
                <max-count>1</max-count>
                <max-size>100</max-size>
            </kind>
            <kind-signature>
                 VGhpcyBpcyBub3QgcmlnaHQhCg==
            </kind-signature>
          </kind-block>
          <kind-block>
            <kind id="2000">
                <data-model>ARRAY</data-model>
                <access-control>NODE-MULTIPLE</access-control>
                <max-node-multiple>3</max-node-multiple>
                <max-count>22</max-count>
                <max-size>4</max-size>
                <ext:example-kind-extension> 1
                        </ext:example-kind-extension>
            </kind>
            <kind-signature>
               VGhpcyBpcyBub3QgcmlnaHQhCg==
            </kind-signature>
          </kind-block>
        </required-kinds>
    </configuration>
    <signature> VGhpcyBpcyBub3QgcmlnaHQhCg== </signature>
    <configuration instance-name="other.example.net">
    </configuration>
    <signature> VGhpcyBpcyBub3QgcmlnaHQhCg== </signature>
  </overlay>
 The file MUST be a well-formed XML document, and it SHOULD contain an
 encoding declaration in the XML declaration.  The file MUST use the
 UTF-8 character encoding.  The namespaces for the elements defined in
 this specification are urn:ietf:params:xml:ns:p2p:config-base and
 urn:ietf:params:xml:ns:p2p:config-chord.
 Note that elements or attributes that are defined as type xsd:boolean
 in the RELAX NG schema (Section 11.1.1) have two lexical
 representations, "1" or "true" for the concept true, and "0" or
 "false" for the concept false.  Whitespace and case processing
 follows the rules of [OASIS.relax_ng] and XML Schema Datatypes
 [W3C.REC-xmlschema-2-20041028].

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 The file MAY contain multiple "configuration" elements, where each
 one contains the configuration information for a different overlay.
 Each configuration element MAY be followed by signature elements that
 provide a signature over the preceding configuration element.  Each
 configuration element has the following attributes:
 instance-name
    The name of the overlay (referred to as "overlay name" in this
    specification)
 expiration
    Time in the future at which this overlay configuration is no
    longer valid.  The node SHOULD retrieve a new copy of the
    configuration at a randomly selected time that is before the
    expiration time.  Note that if the certificates expire before a
    new configuration is retried, the node will not be able to
    validate the configuration file.  All times MUST conform to the
    Internet date/time format defined in [RFC3339] and be specified
    using UTC.
 sequence
    A monotonically increasing sequence number between 0 and 2^16-2.
 Inside each overlay element, the following elements can occur:
 topology-plug-in
    This element defines the overlay algorithm being used.  If
    missing, the default is "CHORD-RELOAD".
 node-id-length
    This element contains the length of a NodeId (NodeIdLength), in
    bytes.  This value MUST be between 16 (128 bits) and 20 (160
    bits).  If this element is not present, the default of 16 is used.
 root-cert
    This element contains a base-64-encoded X.509v3 certificate that
    is a root trust anchor used to sign all certificates in this
    overlay.  There can be more than one root-cert element.
 enrollment-server
    This element contains the URL at which the enrollment server can
    be reached in a "url" element.  This URL MUST be of type "https:".
    More than one enrollment-server element MAY be present.  Note that
    there is no necessary relationship between the overlay name/
    configuration server name and the enrollment server name.

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 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 enrollment-server and root-cert
    elements MAY be absent.  Otherwise, it SHOULD be absent, but MAY
    be set to "false".  This element also contains an attribute
    "digest", which indicates the digest to be used to compute the
    Node-ID.  Valid values for this parameter are "sha1" and "sha256",
    representing SHA-1 [RFC3174] and SHA-256 [RFC6234], respectively.
    Implementations MUST support both of these algorithms.
 bootstrap-node
    This element represents the address of one of the bootstrap nodes.
    It has an attribute called "address" that represents the IP
    address (either IPv4 or IPv6, since they can be distinguished) and
    an optional attribute called "port" that represents the port and
    defaults to 6084.  The IPv6 address is in typical hexadecimal form
    using standard period and colon separators as specified in
    [RFC5952].  More than one bootstrap-node element MAY be present.
 turn-density
    This element is a positive integer that represents the approximate
    reciprocal of density of nodes that can act as TURN servers.  For
    example, if 5% of the nodes can act as TURN servers, this element
    would be set to 20.  If it is not present, the default value is 1.
    If there are no TURN servers in the overlay, it is set to zero.
 clients-permitted
    This element represents whether clients are permitted or whether
    all nodes must be peers.  If clients are permitted, the element
    MUST be set to "true" or be absent.  If the nodes are not allowed
    to remain clients after the initial join, the element MUST be set
    to "false".  There is currently no way for the overlay to enforce
    this.
 no-ice
    This element represents whether nodes are REQUIRED to use the
    "No-ICE" Overlay Link protocols in this overlay.  If it is absent,
    it is treated as if it were set to "false".
 chord-update-interval
    The update frequency for the CHORD-RELOAD Topology Plug-in (see
    Section 10).
 chord-ping-interval
    The Ping frequency for the CHORD-RELOAD Topology Plug-in (see
    Section 10).

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 chord-reactive
    Whether reactive recovery SHOULD be used for this overlay.  It is
    set to "true" or "false".  If missing, the default is "true" (see
    Section 10).
 shared-secret
    If shared secret mode is used, this element contains the shared
    secret.  The security guarantee here is that any agent which is
    able to access the Configuration Document (presumably protected by
    some sort of HTTP access control or network topology) is able to
    recover the shared secret and hence join the overlay.
 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 for messages (see Section 6.3.2).  If this
    value is not present, the default is 100.
 overlay-reliability-timer
    Default value for the end-to-end retransmission timer for
    messages, in milliseconds.  If not present, the default value is
    3000.  The value MUST be at least 200 milliseconds, which means
    the minimum time delay before dropping a link is 1000
    milliseconds.
 overlay-link-protocol
    Indicates a permissible overlay link protocol (see Section 6.6.1
    for requirements for such protocols).  An arbitrary number of
    these elements may appear.  If none appear, then this implies the
    default value, "TLS", which refers to the use of TLS and DTLS.  If
    one or more elements appear, then no default value applies.
 kind-signer
    This contains a single Node-ID in hexadecimal and indicates that
    the certificate with this Node-ID is allowed to sign Kinds.
    Identifying kind-signer by Node-ID instead of certificate allows
    the use of short-lived certificates without constantly having to
    provide an updated configuration file.
 configuration-signer
    This contains a single Node-ID in hexadecimal and indicates that
    the certificate with this Node-ID is allowed to sign
    configurations for this instance-name.  Identifying the signer by
    Node-ID instead of certificate allows the use of short-lived
    certificates without constantly having to provide an updated
    configuration file.

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 bad-node
    This contains a single Node-ID in hexadecimal and indicates that
    the certificate with this Node-ID MUST NOT be considered valid.
    This allows certificate revocation.  An arbitrary number of these
    elements can be provided.  Note that because certificates may
    expire, bad-node entries need be present only for the lifetime of
    the certificate.  Technically speaking, bad Node-IDs may be reused
    after their certificates have expired.  The requirement for
    Node-IDs to be pseudorandomly generated gives this event a
    vanishing probability.
 mandatory-extension
    This element contains the name of an XML namespace that a node
    joining the overlay MUST support.  The presence of a mandatory-
    extension element does not require the extension to be used in the
    current configuration file, but can indicate that it may be used
    in the future.  Note that the namespace is case-sensitive, as
    specified in Section 2.3 of [w3c-xml-namespaces].  More than one
    mandatory-extension element MAY be present.
 Inside each configuration element, the required-kinds element MAY
 also occur.  This element indicates the Kinds that members MUST
 support and contains multiple kind-block elements that each define a
 single Kind that MUST be supported by nodes in the overlay.  Each
 kind-block consists of a single kind element and a kind-signature.
 The kind element defines the Kind.  The kind-signature is the
 signature computed over the kind element.
 Each kind element has either an id attribute or a name attribute.
 The name attribute is a string representing the Kind (the name
 registered to IANA), while the id is an integer Kind-ID allocated out
 of private space.
 In addition, the kind element MUST contain the following elements:
 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.

Jennings, et al. Standards Track [Page 130] RFC 6940 RELOAD Base January 2014

 The kind element MAY also contain the following element:
 max-node-multiple
    If the access control is NODE-MULTIPLE, this element MUST be
    included.  This indicates the maximum value for the i counter.  It
    MUST be an integer greater than 0.
 All of the non-optional values MUST be provided.  If the Kind is
 registered with IANA, the data-model and access-control elements MUST
 match those in the Kind registration, and clients MUST ignore them in
 favor of the IANA versions.  Multiple kind-block elements MAY be
 present.
 The kind-block element also MUST contain a "kind-signature" element.
 This signature is computed across the kind element from the beginning
 of the first < of the kind element to the end of the last > of the
 kind element in the same way as the signature element described later
 in this section. kind-block elements MUST be signed by a node listed
 in the kind-signers block of the current configuration.  Receivers
 MUST verify the signature prior to accepting a kind-block.
 The configuration element MUST be treated as a binary blob that
 cannot be changed -- including any whitespace changes -- or the
 signature will break.  The signature MUST be computed by taking each
 configuration element and starting from, and including, the first <
 at the start of <configuration> up to and including the > in </
 configuration> and treating this as a binary blob that MUST be signed
 using the standard SecurityBlock defined in Section 6.3.4.  The
 SecurityBlock MUST be base-64 encoded using the base64 alphabet from
 [RFC4648] and MUST be put in the signature element following the
 configuration object in the configuration file.  Any configuration
 file MUST be signed by one of the configuration-signer elements from
 the previous extant configuration.  Recipients MUST verify the
 signature prior to accepting the configuration file.
 When a node receives a new configuration file, it MUST change its
 configuration to meet the new requirements.  This may require the
 node to exit the DHT and rejoin.  If a node is not capable of
 supporting the new requirements, it MUST exit the overlay.  If some
 information about a particular Kind changes from what the node
 previously knew about the Kind (for example, the max size), the new
 information in the configuration files overrides any previously
 learned information.  If any Kind data was signed by a node that is
 no longer allowed to sign Kinds, that Kind MUST be discarded along
 with any stored information of that Kind.  Note that forcing an
 avalanche restart of the overlay with a configuration change that
 requires rejoining the overlay may result in serious performance
 problems, including total collapse of the network if configuration

Jennings, et al. Standards Track [Page 131] RFC 6940 RELOAD Base January 2014

 parameters are not properly considered.  Such an event may be
 necessary in case of a compromised CA or similar problem, but for
 large overlays, it should be avoided in almost all circumstances.

11.1.1. RELAX NG Grammar

 The grammar for the configuration data is:
 namespace chord = "urn:ietf:params:xml:ns:p2p:config-chord"
 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 }*
 foreign-attributes = attribute * - (p2pcf:*|local:*|chord:*)
                      { text }*
 foreign-nodes = (foreign-attributes | foreign-elements)*
 start =  element p2pcf:overlay {
       overlay-element
 }
 overlay-element &=  element configuration {
             attribute instance-name { xsd:string },
             attribute expiration { xsd:dateTime }?,
             attribute sequence { xsd:long }?,
             foreign-attributes*,
             parameter
         }+
 overlay-element &= element signature {
             attribute algorithm { signature-algorithm-type }?,
             xsd:base64Binary
         }*
 signature-algorithm-type |= "rsa-sha1"
 signature-algorithm-type |=  xsd:string # signature alg extensions
 parameter &= element topology-plugin { topology-plugin-type }?
 topology-plugin-type |= xsd:string # topo plugin extensions
 parameter &= element max-message-size { xsd:unsignedInt }?
 parameter &= element initial-ttl { xsd:int }?
 parameter &= element root-cert { xsd:base64Binary }*

Jennings, et al. Standards Track [Page 132] RFC 6940 RELOAD Base January 2014

 parameter &= element required-kinds { kind-block* }?
 parameter &= element enrollment-server { xsd:anyURI }*
 parameter &= element kind-signer {  xsd:string }*
 parameter &= element configuration-signer {  xsd:string }*
 parameter &= element bad-node {  xsd:string }*
 parameter &= element no-ice { xsd:boolean }?
 parameter &= element shared-secret { xsd:string }?
 parameter &= element overlay-link-protocol { xsd:string }*
 parameter &= element clients-permitted { xsd:boolean }?
 parameter &= element turn-density { xsd:unsignedByte }?
 parameter &= element node-id-length { xsd:int }?
 parameter &= element mandatory-extension { xsd:string }*
 parameter &= foreign-elements*
 parameter &=
     element self-signed-permitted {
         attribute digest { self-signed-digest-type },
         xsd:boolean
     }?
 self-signed-digest-type |= "sha1"
 self-signed-digest-type |=  xsd:string # signature digest extensions
 parameter &= element bootstrap-node {
                 attribute address { xsd:string },
                 attribute port { xsd:int }?
              }*
 kind-block = element kind-block {
     element kind {
         (  attribute name { kind-names }
            | attribute id { xsd:unsignedInt } ),
         kind-parameter
     } &
     element kind-signature  {
         attribute algorithm { signature-algorithm-type }?,
         xsd:base64Binary
     }?
 }
 kind-parameter &= element max-count { xsd:int }
 kind-parameter &= element max-size { xsd:int }
 kind-parameter &= element max-node-multiple { xsd:int }?
 kind-parameter &= element data-model { data-model-type }
 data-model-type |= "SINGLE"
 data-model-type |= "ARRAY"
 data-model-type |= "DICTIONARY"
 data-model-type |=  xsd:string # data model extensions

Jennings, et al. Standards Track [Page 133] RFC 6940 RELOAD Base January 2014

 kind-parameter &= element access-control { access-control-type }
 access-control-type |= "USER-MATCH"
 access-control-type |= "NODE-MATCH"
 access-control-type |= "USER-NODE-MATCH"
 access-control-type |= "NODE-MULTIPLE"
 access-control-type |= xsd:string # access control extensions
 kind-parameter &= foreign-elements*
 kind-names |= "TURN-SERVICE"
 kind-names |= "CERTIFICATE_BY_NODE"
 kind-names |= "CERTIFICATE_BY_USER"
 kind-names |= xsd:string # kind extensions
 # Chord specific parameters
 topology-plugin-type |= "CHORD-RELOAD"
 parameter &= element chord:chord-ping-interval { xsd:int }?
 parameter &= element chord:chord-update-interval { xsd:int }?
 parameter &= element chord:chord-reactive { xsd:boolean }?

11.2. Discovery through Configuration Server

 When a node first enrolls in a new overlay, it starts with a
 discovery process to find a configuration server.
 The node MAY start by determining the overlay name.  This value MUST
 be provided by the user or some other out-of-band provisioning
 mechanism.  The out-of-band mechanism MAY also provide an optional
 URL for the configuration server.  If a URL for the configuration
 server is not provided, the node MUST do a DNS SRV query using a
 Service name of "reload-config" and a protocol of TCP to find a
 configuration server and form the URL by appending a path of
 "/.well-known/reload-config" to the overlay name.  This uses the
 "well-known URI" framework defined in [RFC5785].  For example, if the
 overlay name was example.com, the URL would be
 "https://example.com/.well-known/reload-config".
 Once an address and URL for the configuration server are determined,
 the peer MUST form an HTTPS connection to that IP address.  If an
 optional URL for the configuration server was provided, the
 certificate MUST match the domain name from the URL as described in
 [RFC2818]; otherwise, the certificate MUST match the overlay name as
 described in [RFC2818].  If the HTTPS certificates pass the name
 matching, the node MUST fetch a new copy of the configuration file.
 To do this, the peer performs a GET to the URL.  The result of the
 HTTP GET is an XML configuration file described above.  If the XML is
 not valid or the instance-name attribute of the overlay-element in
 the XML does not match the overlay name, this configurations file

Jennings, et al. Standards Track [Page 134] RFC 6940 RELOAD Base January 2014

 SHOULD be discarded.  Otherwise, the new configuration MUST replace
 any previously learned configuration file for this overlay.
 For overlays that do not use a configuration server, nodes MUST
 obtain the configuration information needed to join the overlay
 through some out-of-band approach, such as an XML configuration file
 sent over email.

11.3. Credentials

 If the Configuration Document contains an enrollment-server element,
 credentials are REQUIRED to join the Overlay Instance.  A peer which
 does not yet have credentials MUST contact the enrollment server to
 acquire them.
 RELOAD defines its own trivial certificate request protocol.  We
 would have liked to have used an existing protocol, but were
 concerned about the implementation burden of even the simplest of
 those protocols, such as [RFC5272] and [RFC5273].  The 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
 which 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 MUST be performed over HTTPS.  The
 server certificate MUST match the overlay name as described in
 [RFC2818].  The request MUST be an HTTP POST with the parameters
 encoded as described in [RFC2388] and with the following properties:
 o  If authentication is required, there MUST be form parameters of
    "password" and "username" containing the user's account name and
    password in the clear (hence the need for HTTPS).  The username
    and password strings MUST be UTF-8 strings compared as binary
    objects.  Applications using RELOAD SHOULD define any needed
    string preparation as per [RFC4013] or its successor documents.
 o  If more than one Node-ID is required, there MUST be a form
    parameter of "nodeids" containing the number of Node-IDs required.
 o  There MUST be a form parameter of "csr" with a content type of
    "application/pkcs10", as defined in [RFC2311], that contains the
    certificate signing request (CSR).
 o  The Accept header MUST contain the type "application/pkix-cert",
    indicating the type that is expected in the response.

Jennings, et al. Standards Track [Page 135] RFC 6940 RELOAD Base January 2014

 The enrollment server MUST authenticate the request using the
 provided account name and password.  The reason for using the RFC
 2388 "multipart/form-data" encoding is so that the password parameter
 will not be encoded in the URL, to reduce the chance of accidental
 leakage of the password.  If the authentication succeeds and the
 requested user name in the CSR is acceptable, the server MUST
 generate and return a certificate for the CSR in the "csr" parameter
 of the request.  The SubjectAltName field in the certificate MUST
 contain the following values:
 o  One or more Node-IDs which MUST be cryptographically random
    [RFC4086].  Each MUST be chosen by the enrollment server in such a
    way that it is unpredictable to the requesting user.  For example,
    the user MUST NOT be informed of potential (random) Node-IDs prior
    to authenticating.  Each is placed in the subjectAltName using the
    uniformResourceIdentifier type, each MUST contain RELOAD URI, as
    described in Section 14.15, and each MUST contain a Destination
    List with a single entry of type "node_id".  The enrollment server
    SHOULD maintain a mapping of users to Node-IDs and if the same
    user returns (e.g., to have their certificate re-issued), the
    enrollment server should return the same Node-IDs, thus avoiding
    the need for implementations to re-store all their data when their
    certificates expire.
 o  A single name (the "user name") that this user is allowed to use
    in the overlay, using type rfc822Name.  Enrollment servers SHOULD
    take care to allow only legal characters in the name (e.g., no
    embedded NULs), rather than simply accepting any name provided by
    the user.  In some usages, the right side of the user name will
    match the overlay name, but there is no requirement for this match
    in this specification.  Applications using this specification MAY
    define such a requirement or MAY otherwise limit the allowed range
    of allowed user names.
 The SubjectAltName field in the certificate MUST NOT contain any
 identities other than those listed above.  The subject distinguished
 name in the certificate MUST be empty.
 The certificate MUST be returned as type "application/pkix-cert", as
 defined in [RFC2585], with an HTTP status code of 200 OK.

Jennings, et al. Standards Track [Page 136] RFC 6940 RELOAD Base January 2014

 Certificate processing errors SHOULD result in an HTTP return code of
 403 Forbidden, along with a body of type "text/plain" and body that
 consists of one of the tokens defined in the following list:
 failed_authentication
    The account name and password combination used in the HTTPS
    request was not valid.
 username_not_available
    The requested user name in the CSR was not acceptable.
 Node-IDs_not_available
    The number of Node-IDs requested was not acceptable.
 bad_CSR
    There was some other problem with the CSR.
 If the client receives an unknown token in the body, it SHOULD treat
 it as a failure for an unknown reason.
 The client MUST check that the returned certificate chains back to
 one of the certificates received in the "root-cert" list of the
 overlay configuration data (including PKIX BasicConstraints checks).
 The node then reads the certificate to find the Node-ID it can use.

11.3.1. Self-Generated Credentials

 If the "self-signed-permitted" element is present in the
 configuration and is 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 user's choice.
 For self-signed certificates containing only one Node-ID, 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
 (more specifically, the subjectPublicKeyInfo) and taking the high-
 order bits.  For self-signed certificates containing multiple
 Node-IDs, the index of the Node-ID (from 1 to the number of Node-IDs
 needed) must be prepended as a 4-byte big-endian integer to the DER
 representation of the user's public key and taking the high-order
 bits.  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.  It MUST store its certificate in the overlay
 (Section 8), but SHOULD look to see if the user name is already taken
 and, if so, choose another user name.  Note that this provides
 protection only against accidental name collisions.  Name theft is

Jennings, et al. Standards Track [Page 137] RFC 6940 RELOAD Base January 2014

 still possible.  If protection against name theft is desired, then
 the enrollment service MUST be used.

11.4. Contacting a Bootstrap Node

 In order to join the overlay, the Joining Node MUST contact a node in
 the overlay.  Typically this means contacting the bootstrap nodes,
 since they are reachable by the local peer or have public IP
 addresses.  If the Joining Node has cached a list of peers that it
 has previously been connected with in this overlay, as an
 optimization it MAY attempt to use one or more of them as bootstrap
 nodes before falling back to the bootstrap nodes listed in the
 configuration file.
 When contacting a bootstrap node, the Joining Node MUST first form
 the DTLS or TLS connection to the bootstrap node and then send an
 Attach request over this connection with the destination Resource-ID
 set to the Joining Node's Node-ID plus 1.
 When the requester node finally does receive a response from some
 responding node, it MUST use the Node-ID in the response to start
 sending requests to join the Overlay Instance as described in
 Section 6.4.
 After a node has successfully joined the overlay network, it will
 have direct connections to several peers.  Some MAY be added to the
 cached bootstrap nodes list and used in future boots.  Peers that are
 not directly connected MUST NOT be cached.  The suggested number of
 peers to cache is 10.  Algorithms for determining which peers to
 cache are beyond the scope of this specification.

12. Message Flow Example

 The following abbreviations are used in the message flow diagrams:
 JN = Joining Node, AP = Admitting Peer, NP = next peer after the AP,
 NNP = next next peer which is the peer after NP, PP = previous peer
 before the AP, PPP = previous previous peer which is the peer before
 the PP, BP = bootstrap node.
 In the following example, we assume that JN has formed a connection
 to one of the bootstrap nodes.  JN then sends an Attach through that
 peer to a Resource-ID of itself plus 1 (JN+1).  It gets routed to the
 AP, because JN is not yet part of the overlay.  When AP responds, JN
 and the AP use ICE to set up a connection and then set up DTLS.  Once
 AP has connected to JN, AP sends to JN an Update to populate its
 Routing Table.  The following example shows the Update happening
 after the DTLS connection is formed, but it could also happen before,
 in which case the Update would often be routed through other nodes.

Jennings, et al. Standards Track [Page 138] RFC 6940 RELOAD Base January 2014

     JN        PPP       PP        AP        NP        NNP       BP
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |AttachReq Dest=JN+1|         |         |         |         |
      |---------------------------------------------------------->|
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |AttachReq Dest=JN+1|         |
      |         |         |         |<----------------------------|
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |AttachAns          |         |
      |         |         |         |---------------------------->|
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |AttachAns          |         |         |         |         |
      |<----------------------------------------------------------|
      |         |         |         |         |         |         |
      |ICE      |         |         |         |         |         |
      |<===========================>|         |         |         |
      |         |         |         |         |         |         |
      |TLS      |         |         |         |         |         |
      |<...........................>|         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |UpdateReq|         |         |         |         |         |
      |<----------------------------|         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |UpdateAns|         |         |         |         |         |
      |---------------------------->|         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
                               Figure 1

Jennings, et al. Standards Track [Page 139] RFC 6940 RELOAD Base January 2014

 The JN then forms connections to the appropriate neighbors, such as
 NP, by sending an Attach which gets routed via other nodes.  When NP
 responds, JN and NP use ICE and DTLS to set up a connection.
     JN        PPP       PP        AP        NP        NNP       BP
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |AttachReq NP       |         |         |         |         |
      |---------------------------->|         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |AttachReq NP       |         |
      |         |         |         |-------->|         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |AttachAns|         |         |
      |         |         |         |<--------|         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |AttachAns|         |         |         |         |         |
      |<----------------------------|         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |ICE      |         |         |         |         |         |
      |<=====================================>|         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |TLS      |         |         |         |         |         |
      |<.....................................>|         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
                               Figure 2

Jennings, et al. Standards Track [Page 140] RFC 6940 RELOAD Base January 2014

 The JN also needs to populate its Finger Table (for the Chord-based
 DHT).  It issues an Attach to a variety of locations around the
 overlay.  The diagram below shows JN sending an Attach halfway around
 the Chord ring to the JN + 2^127.
     JN        NP        XX        TP
      |         |         |         |
      |         |         |         |
      |         |         |         |
      |AttachReq JN+2<<126|         |
      |-------->|         |         |
      |         |         |         |
      |         |         |         |
      |         |AttachReq JN+2<<126|
      |         |-------->|         |
      |         |         |         |
      |         |         |         |
      |         |         |AttachReq JN+2<<126
      |         |         |-------->|
      |         |         |         |
      |         |         |         |
      |         |         |AttachAns|
      |         |         |<--------|
      |         |         |         |
      |         |         |         |
      |         |AttachAns|         |
      |         |<--------|         |
      |         |         |         |
      |         |         |         |
      |AttachAns|         |         |
      |<--------|         |         |
      |         |         |         |
      |ICE      |         |         |
      |<===========================>|
      |         |         |         |
      |TLS      |         |         |
      |<...........................>|
      |         |         |         |
      |         |         |         |
                               Figure 3

Jennings, et al. Standards Track [Page 141] RFC 6940 RELOAD Base January 2014

 Once JN has a reasonable set of connections, it is ready to take its
 place in the DHT.  It does this by sending a Join to AP.  AP sends a
 series of Store requests to JN to store the data that JN will be
 responsible for.  AP then sends JN an Update that explicitly labels
 JN as its predecessor.  At this point, JN is part of the ring and is
 responsible for a section of the overlay.  AP can now forget any data
 which is assigned to JN and not to AP.
     JN        PPP       PP        AP        NP        NNP       BP
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |JoinReq  |         |         |         |         |         |
      |---------------------------->|         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |JoinAns  |         |         |         |         |         |
      |<----------------------------|         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |StoreReq Data A    |         |         |         |         |
      |<----------------------------|         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |StoreAns |         |         |         |         |         |
      |---------------------------->|         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |StoreReq Data B    |         |         |         |         |
      |<----------------------------|         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |StoreAns |         |         |         |         |         |
      |---------------------------->|         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |UpdateReq|         |         |         |         |         |
      |<----------------------------|         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |UpdateAns|         |         |         |         |         |
      |---------------------------->|         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
                               Figure 4

Jennings, et al. Standards Track [Page 142] RFC 6940 RELOAD Base January 2014

 In Chord, JN's Neighbor Table needs to contain its own predecessors.
 It couldn't connect to them previously, because it did not yet know
 their addresses.  However, now that it has received an Update from
 AP, as in the previous diagram, it has AP's predecessors, which are
 also its own, so it sends Attaches to them.  Below, it is shown
 connecting only to AP's closest predecessor, PP.
     JN        PPP       PP        AP        NP        NNP       BP
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |AttachReq Dest=PP  |         |         |         |         |
      |---------------------------->|         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |         |         |AttachReq Dest=PP  |         |         |
      |         |         |<--------|         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |         |         |AttachAns|         |         |         |
      |         |         |-------->|         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |AttachAns|         |         |         |         |         |
      |<----------------------------|         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |TLS      |         |         |         |         |         |
      |...................|         |         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |UpdateReq|         |         |         |         |         |
      |------------------>|         |         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |UpdateAns|         |         |         |         |         |
      |<------------------|         |         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |UpdateReq|         |         |         |         |         |
      |---------------------------->|         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |UpdateAns|         |         |         |         |         |
      |<----------------------------|         |         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |UpdateReq|         |         |         |         |         |

Jennings, et al. Standards Track [Page 143] RFC 6940 RELOAD Base January 2014

      |-------------------------------------->|         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
      |UpdateAns|         |         |         |         |         |
      |<--------------------------------------|         |         |
      |         |         |         |         |         |         |
      |         |         |         |         |         |         |
                               Figure 5
 Finally, now that JN 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.
         JN        NP        XX        TP
          |         |         |         |
          |         |         |         |
          |         |         |         |
          |UpdateReq|         |         |
          |---------------------------->|
          |         |         |         |
          |         |         |         |
          |UpdateAns|         |         |
          |<----------------------------|
          |         |         |         |
          |         |         |         |
          |         |         |         |
          |         |         |         |
                               Figure 6

13. Security Considerations

13.1. 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.  More background
 information can be found in [RFC5765].
 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

Jennings, et al. Standards Track [Page 144] RFC 6940 RELOAD Base January 2014

 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.

13.2. Attacks on P2P Overlays

 The two basic functions provided by overlay nodes are storage and
 routing: some peer is responsible for storing a node's data and for
 allowing a third node to fetch this stored data, while other peers
 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 specification are intended to protect overlay
 routing and user registration information in RELOAD messages.
 To protect the signaling from attackers pretending to be valid nodes
 (or nodes other than themselves), the first requirement is to ensure
 that all messages are received from authorized members of the
 overlay.  For this reason, RELOAD MUST transport 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 MUST be digitally
 signed with the sender's private key, providing end-to-end security
 for communications.

13.3. Certificate-Based Security

 This specification stores users' registrations and possibly other
 data in an overlay network.  This requires a solution both to
 securing this data and to 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
 MUST be 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

Jennings, et al. Standards Track [Page 145] RFC 6940 RELOAD Base January 2014

 MUST also be assigned one or more Node-IDs by the central enrollment
 authority.  Both the name and the Node-IDs 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 Node-IDs listed in the certificate.
 Note that since only users of this Overlay Instance need to validate
 a certificate, this usage does not require a global Public Key
 Infrastructure (PKI).  Instead, certificates MUST be signed by a
 central enrollment authority which acts as the certificate authority
 for the Overlay Instance.  This authority signs each node's
 certificate.  Because each node possesses the CA's certificate (which
 they receive upon enrollment), they can verify the certificates of
 the other entities in the overlay without further communication.
 Because the certificates contain the user's/node's public key,
 communications from the user/node can, in turn, be verified.
 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
 (although they can trust the Node-IDs, because they are
 cryptographically verifiable).  This scheme may be appropriate for
 some small deployments, such as a small office or an ad hoc overlay
 set up among participants in a meeting where all hosts on the network
 are trusted.  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 node can verify that the storer is authorized to perform a
 store at that Resource-ID and also can allow any consumer of the data
 to verify the provenance and integrity of the data when it retrieves
 it.
 Note that RELOAD does not itself provide a revocation/status
 mechanism (although certificates may, of course, include Online
 Certificate Status Protocol [OCSP] responder information).  Thus,
 certificate lifetimes SHOULD be chosen to balance the compromise
 window versus the cost of certificate renewal.  Because RELOAD is
 already designed to operate in the face of some fraction of malicious
 nodes, this form of compromise is not fatal.
 All implementations MUST implement certificate-based security.

Jennings, et al. Standards Track [Page 146] RFC 6940 RELOAD Base January 2014

13.4. 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 MUST share a single symmetric key which is used to key TLS-PSK
 or TLS-SRP 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.

13.5. Storage Security

 When certificate-based security is used in RELOAD, any given
 Resource-ID/Kind-ID pair is bound to some small set of certificates.
 In order to write data, the writer must prove possession of the
 private key for one of those certificates.  Moreover, all data is
 stored, signed with the same private key that was used to authorize
 the storage.  This set of rules makes questions of authorization and
 data integrity, which have historically been thorny for overlays,
 relatively simple.

13.5.1. Authorization

 When a node wants to store some value, it MUST first digitally sign
 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 node is authorized to store at this Resource-ID/Kind-ID
 pair.  Determining this requires comparing the user's identity to the
 requirements of the access control model (see Section 7.3).  If it
 satisfies those requirements, the user is authorized to write,
 pending quota checks, as described in the next section.
 For example, consider a certificate with the following properties:
        User name: alice@dht.example.com
        Node-ID:   013456789abcdef
        Serial:    1234

Jennings, et al. Standards Track [Page 147] RFC 6940 RELOAD Base January 2014

 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.  Because
 SIP Location uses the USER-NODE-MATCH policy, it first verifies that
 the user name in the certificate hashes to the requested Resource-ID.
 It then verifies that the Node-ID in the certificate matches the
 dictionary key being used for the store.  If both of these checks
 succeed, the Store is authorized.  Note that because the access
 control model is different for different Kinds, the exact set of
 checks will vary.

13.5.2. Distributed Quota

 Being a peer in an Overlay Instance carries with it the
 responsibility to store data for a given region of the Overlay
 Instance.  However, allowing nodes to store unlimited amounts of data
 would create unacceptable burdens on peers and would also enable
 trivial denial-of-service (DoS) 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
 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
 Resource-ID/Kind-ID pair is bound to a small set of certificates,
 these size restrictions also create a distributed quota mechanism,
 with the quotas administered by the central configuration 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.

13.5.3. Correctness

 Because each stored value is signed, it is trivial for any retrieving
 node to verify the integrity of the stored value.  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 (although 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, it is REQUIRED that storage times be monotonically
 increasing.  Storing peers MUST reject Store requests with storage
 times smaller than or equal to those that 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 that a
 rollback has occurred.

Jennings, et al. Standards Track [Page 148] RFC 6940 RELOAD Base January 2014

13.5.4. Residual Attacks

 The mechanisms described here provide a high degree of security, but
 some attacks remain possible.  Most simply, it is possible for
 storing peers to refuse to store a value (i.e., they reject any
 request).  In addition, a storing peer can deny knowledge of values
 which it has previously accepted.  To some extent, these attacks can
 be ameliorated by attempting to store to and retrieve from replicas,
 but a retrieving node does not know whether or not it should try
 this, as there is a cost to doing so.
 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 effects 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.
 In addition, when a Kind is multivalued (e.g., an array data model),
 the storing peer 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.

13.6. 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 that misroutes 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 node in the Overlay Instance.  Second, this is a DoS attack
 only.  Third, if a large percentage of the nodes on the Overlay
 Instance are controlled by the attacker, it is probably impossible to
 perfectly secure against this.

Jennings, et al. Standards Track [Page 149] RFC 6940 RELOAD Base January 2014

13.6.1. Background

 In general, attacks on DHT routing are mounted by the attacker
 arranging to route traffic through one or two nodes that 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 Node-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, RELOAD attempts to mitigate these attacks by the use of
 centralized, certificate-based admission control.

13.6.2. Admissions Control

 Admission to a RELOAD Overlay Instance is controlled by requiring
 that each peer have a certificate containing its Node-ID.  The
 requirement to have a certificate is enforced by using certificate-
 based mutual authentication on each connection.  (Note: the following
 applies only when self-signed certificates are not used.)  Whenever a
 peer connects to another peer, each side automatically checks that
 the other has a suitable certificate.  These Node-IDs MUST be
 randomly assigned by the central enrollment server.  This has two
 benefits:
 o  It allows the enrollment server to limit the number of Node-IDs
    issued to any individual user.
 o  It prevents the attacker from choosing specific Node-IDs.
 The first property allows protection against Sybil attacks (provided
 that 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, the attacker must have a certificate for
 suitable Node-IDs, which requires him to repeatedly query the
 enrollment server for new certificates, which will match only by
 chance.  From the attacker's perspective, the difficulty is that if
 the attacker has only 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.

Jennings, et al. Standards Track [Page 150] RFC 6940 RELOAD Base January 2014

13.6.3. 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 having a peer accept
 Update messages from an attacker, all overlay routing messages are
 signed by the peer that generated them.
 For messages that impact the topology of the overlay, replay is
 typically prevented by having the information come directly from, or
 be verified by, the nodes that claimed to have generated the update.
 Data storage replay detection is done by signing the time of the node
 that generated the signature on the Store request, thus providing a
 time-based replay protection, but the time synchronization is needed
 only between peers that can write to the same location.

13.6.4. Protecting the Signaling

 The goal here is to stop an attacker from knowing who is signaling
 what to whom.  An attacker is unlikely to be able to observe the
 activities of a specific individual, given the randomization of IDs
 and 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 is to digitally sign each message.  This
 prevents adversarial peers from modifying messages in flight, even if
 they are on the routing path.

Jennings, et al. Standards Track [Page 151] RFC 6940 RELOAD Base January 2014

13.6.5. Routing Loops and DoS Attacks

 Source-routing mechanisms are known to create the possibility for DoS
 amplification, especially by the induction of routing loops
 [RFC5095].  In order to limit amplification, the initial-ttl value in
 the configuration file SHOULD be set to a value slightly larger than
 the longest expected path through the network.  For Chord, experience
 has shown that log(2) of the number of nodes in the network + 5 is a
 safe bound.  Because nodes are required to enforce the initial-ttl as
 the maximum value, an attacker cannot achieve an amplification factor
 greater than initial-ttl, thus limiting the additional capabilities
 provided by source routing.
 In order to prevent the use of loops for targeted implementation
 attacks, implementations SHOULD check the Destination List for
 duplicate entries and discard such records with an
 "Error_Invalid_Message" error.  This does not completely prevent
 loops, but it does require that at least one attacker node be part of
 the loop.

13.6.6. 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 an
 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.

Jennings, et al. Standards Track [Page 152] RFC 6940 RELOAD Base January 2014

14. IANA Considerations

 This section contains the new code points registered by this
 document.

14.1. Well-Known URI Registration

 IANA has registered a "well-known URI" as described in [RFC5785]:
         +----------------------------+----------------------+
         | URI suffix:                | reload-config        |
         | Change controller:         | IETF <iesg@ietf.org> |
         | Specification document(s): | RFC 6940             |
         | Related information:       | None                 |
         +----------------------------+----------------------+

14.2. Port Registrations

 IANA has already allocated a TCP port for the main peer-to-peer
 protocol.  This port had the name p2psip-enroll and the port number
 of 6084.  Per this document, IANA has updated this registration to
 change the service name to reload-config.
 IANA has made the following port registration:
 +-----------------------------+-------------------------------------+
 | Registration Technical      | IETF Chair <chair@ietf.org>         |
 | Contact                     |                                     |
 | Registration Owner          | IETF <iesg@ietf.org>                |
 | Transport Protocol          | TCP                                 |
 | Port Number                 | 6084                                |
 | Service Name                | reload-config                       |
 | Description                 | Peer-to-Peer Infrastructure         |
 |                             | Configuration                       |
 +-----------------------------+-------------------------------------+

Jennings, et al. Standards Track [Page 153] RFC 6940 RELOAD Base January 2014

14.3. Overlay Algorithm Types

 IANA has created a "RELOAD Overlay Algorithm Types" Registry.
 Entries in this registry are strings denoting the names of overlay
 algorithms, as described in Section 11.1 of [RFC6940].  The
 registration policy for this registry is "IETF Review" [RFC522].  The
 initial contents of this registry are:
                    +----------------+-----------+
                    | Algorithm Name | Reference |
                    +----------------+-----------+
                    | CHORD-RELOAD   |  RFC 6940 |
                    | EXP-OVERLAY    |  RFC 6940 |
                    +----------------+-----------+
 The value EXP-OVERLAY has been made available for the purposes of
 experimentation.  This value is not meant for vendor-specific use of
 any sort, and it MUST NOT be used for operational deployments.

14.4. Access Control Policies

 IANA has created a "RELOAD Access Control Policies" Registry.
 Entries in this registry are strings denoting access control
 policies, as described in Section 7.3 of [RFC6940].  New entries in
 this registry SHALL be registered via Standards Action [RFC5226].
 The initial contents of this registry are:
                    +-----------------+-----------+
                    | Access Policy   | Reference |
                    +-----------------+-----------+
                    | USER-MATCH      |  RFC 6940 |
                    | NODE-MATCH      |  RFC 6940 |
                    | USER-NODE-MATCH |  RFC 6940 |
                    | NODE-MULTIPLE   |  RFC 6940 |
                    | EXP-MATCH       |  RFC 6940 |
                    +-----------------+-----------+
 The value EXP-MATCH has been made available for the purposes of
 experimentation.  This value is not meant for vendor-specific use of
 any sort, and it MUST NOT be used for operational deployments.

Jennings, et al. Standards Track [Page 154] RFC 6940 RELOAD Base January 2014

14.5. Application-ID

 IANA has created a "RELOAD Application-ID" Registry.  Entries in this
 registry are 16-bit integers denoting Application-IDs, as described
 in Section 6.5.2 of [RFC6940].  Code points in the range 1 to 32767
 SHALL be registered via Standards Action [RFC5226].  Code points in
 the range 32768 to 61440 SHALL be registered via Expert Review
 [RFC5226].  Code points in the range 61441 to 65534 are reserved for
 private use.  The initial contents of this registry are:
   +-------------+----------------+-------------------------------+
   | Application | Application-ID |                 Specification |
   +-------------+----------------+-------------------------------+
   | INVALID     |              0 |                      RFC 6940 |
   | SIP         |           5060 | Reserved for use by SIP Usage |
   | SIP         |           5061 | Reserved for use by SIP Usage |
   | Reserved    |          65535 |                      RFC 6940 |
   +-------------+----------------+-------------------------------+

14.6. Data Kind-ID

 IANA has created a "RELOAD Data Kind-ID" registry.  Entries in this
 registry are 32-bit integers denoting data Kinds, as described in
 Section 5.2 of [RFC6940].  Code points in the range 0x00000001 to
 0x7FFFFFFF SHALL be registered via Standards Action [RFC5226].  Code
 points in the range 0x8000000 to 0xF0000000 SHALL be registered via
 Expert Review [RFC5226].  Code points in the range 0xF0000001 to
 0xFFFFFFFE are reserved for private use via the Kind description
 mechanism described in Section 11 of [RFC6940].  The initial contents
 of this registry are:
           +---------------------+------------+-----------+
           | Kind                |    Kind-ID | Reference |
           +---------------------+------------+-----------+
           | INVALID             |        0x0 |  RFC 6940 |
           | TURN-SERVICE        |        0x2 |  RFC 6940 |
           | CERTIFICATE_BY_NODE |        0x3 |  RFC 6940 |
           | CERTIFICATE_BY_USER |       0x10 |  RFC 6940 |
           | Reserved            | 0x7fffffff |  RFC 6940 |
           | Reserved            | 0xfffffffe |  RFC 6940 |
           +---------------------+------------+-----------+

Jennings, et al. Standards Track [Page 155] RFC 6940 RELOAD Base January 2014

14.7. Data Model

 IANA has created a "RELOAD Data Model" registry.  Entries in this
 registry are strings denoting data models, as described in
 Section 7.2 of [RFC6940].  New entries in this registry SHALL be
 registered via Standards Action [RFC5226].  The initial contents of
 this registry are:
                      +------------+-----------+
                      | Data Model | Reference |
                      +------------+-----------+
                      | INVALID    |  RFC 6940 |
                      | SINGLE     |  RFC 6940 |
                      | ARRAY      |  RFC 6940 |
                      | DICTIONARY |  RFC 6940 |
                      | EXP-DATA   |  RFC 6940 |
                      | RESERVED   |  RFC 6940 |
                      +------------+-----------+
 The value EXP-DATA has been made available for the purposes of
 experimentation.  This value is not meant for vendor-specific use of
 any sort, and it MUST NOT be used for operational deployments.

14.8. Message Codes

 IANA has created a "RELOAD Message Codes" registry.  Entries in this
 registry are 16-bit integers denoting method codes, as described in
 Section 6.3.3 of [RFC6940].  These codes SHALL be registered via
 Standards Action [RFC5226].  The initial contents of this registry
 are:

Jennings, et al. Standards Track [Page 156] RFC 6940 RELOAD Base January 2014

 +-------------------------------------+----------------+-----------+
 | Message Code Name                   |     Code Value | Reference |
 +-------------------------------------+----------------+-----------+
 | invalidMessageCode                  |            0x0 |  RFC 6940 |
 | probe_req                           |            0x1 |  RFC 6940 |
 | probe_ans                           |            0x2 |  RFC 6940 |
 | attach_req                          |            0x3 |  RFC 6940 |
 | attach_ans                          |            0x4 |  RFC 6940 |
 | Unassigned                          |            0x5 |           |
 | Unassigned                          |            0x6 |           |
 | store_req                           |            0x7 |  RFC 6940 |
 | store_ans                           |            0x8 |  RFC 6940 |
 | fetch_req                           |            0x9 |  RFC 6940 |
 | fetch_ans                           |            0xA |  RFC 6940 |
 | Unassigned (was remove_req)         |            0xB |  RFC 6940 |
 | Unassigned (was remove_ans)         |            0xC |  RFC 6940 |
 | find_req                            |            0xD |  RFC 6940 |
 | find_ans                            |            0xE |  RFC 6940 |
 | join_req                            |            0xF |  RFC 6940 |
 | join_ans                            |           0x10 |  RFC 6940 |
 | leave_req                           |           0x11 |  RFC 6940 |
 | leave_ans                           |           0x12 |  RFC 6940 |
 | update_req                          |           0x13 |  RFC 6940 |
 | update_ans                          |           0x14 |  RFC 6940 |
 | route_query_req                     |           0x15 |  RFC 6940 |
 | route_query_ans                     |           0x16 |  RFC 6940 |
 | ping_req                            |           0x17 |  RFC 6940 |
 | ping_ans                            |           0x18 |  RFC 6940 |
 | stat_req                            |           0x19 |  RFC 6940 |
 | stat_ans                            |           0x1A |  RFC 6940 |
 | Unassigned (was attachlite_req)     |           0x1B |  RFC 6940 |
 | Unassigned (was attachlite_ans)     |           0x1C |  RFC 6940 |
 | app_attach_req                      |           0x1D |  RFC 6940 |
 | app_attach_ans                      |           0x1E |  RFC 6940 |
 | Unassigned (was app_attachlite_req) |           0x1F |  RFC 6940 |
 | Unassigned (was app_attachlite_ans) |           0x20 |  RFC 6940 |
 | config_update_req                   |           0x21 |  RFC 6940 |
 | config_update_ans                   |           0x22 |  RFC 6940 |
 | exp_a_req                           |           0x23 |  RFC 6940 |
 | exp_a_ans                           |           0x24 |  RFC 6940 |
 | exp_b_req                           |           0x25 |  RFC 6940 |
 | exp_b_ans                           |           0x26 |  RFC 6940 |
 | Reserved                            | 0x8000..0xFFFE |  RFC 6940 |
 | error                               |         0xFFFF |  RFC 6940 |
 +-------------------------------------+----------------+-----------+

Jennings, et al. Standards Track [Page 157] RFC 6940 RELOAD Base January 2014

 The values exp_a_req, exp_a_ans, exp_b_req, and exp_b_ans have been
 made available for the purposes of experimentation.  These values are
 not meant for vendor-specific use of any sort, and they MUST NOT be
 used for operational deployments.

14.9. Error Codes

 IANA has created a "RELOAD Error Code" registry.  Entries in this
 registry are 16-bit integers denoting error codes, as described in
 Section 6.3.3.1 of [RFC6940].  New entries SHALL be defined via
 Standards Action [RFC5226].  The initial contents of this registry
 are:
 +-------------------------------------+----------------+-----------+
 | Error Code Name                     |     Code Value | Reference |
 +-------------------------------------+----------------+-----------+
 | invalidErrorCode                    |            0x0 |  RFC 6940 |
 | Unassigned                          |            0x1 |           |
 | Error_Forbidden                     |            0x2 |  RFC 6940 |
 | Error_Not_Found                     |            0x3 |  RFC 6940 |
 | Error_Request_Timeout               |            0x4 |  RFC 6940 |
 | Error_Generation_Counter_Too_Low    |            0x5 |  RFC 6940 |
 | Error_Incompatible_with_Overlay     |            0x6 |  RFC 6940 |
 | Error_Unsupported_Forwarding_Option |            0x7 |  RFC 6940 |
 | Error_Data_Too_Large                |            0x8 |  RFC 6940 |
 | Error_Data_Too_Old                  |            0x9 |  RFC 6940 |
 | Error_TTL_Exceeded                  |            0xA |  RFC 6940 |
 | Error_Message_Too_Large             |            0xB |  RFC 6940 |
 | Error_Unknown_Kind                  |            0xC |  RFC 6940 |
 | Error_Unknown_Extension             |            0xD |  RFC 6940 |
 | Error_Response_Too_Large            |            0xE |  RFC 6940 |
 | Error_Config_Too_Old                |            0xF |  RFC 6940 |
 | Error_Config_Too_New                |           0x10 |  RFC 6940 |
 | Error_In_Progress                   |           0x11 |  RFC 6940 |
 | Error_Exp_A                         |           0x12 |  RFC 6940 |
 | Error_Exp_B                         |           0x13 |  RFC 6940 |
 | Error_Invalid_Message               |           0x14 |  RFC 6940 |
 | Reserved                            | 0x8000..0xFFFE |  RFC 6940 |
 +-------------------------------------+----------------+-----------+
 The values Error_Exp_A and Error_Exp_B have been made available for
 the purposes of experimentation.  These values are not meant for
 vendor-specific use of any sort, and they MUST NOT be used for
 operational deployments.

Jennings, et al. Standards Track [Page 158] RFC 6940 RELOAD Base January 2014

14.10. Overlay Link Types

 IANA has created a "RELOAD Overlay Link Registry".  Entries in this
 registry are 8-bit integers, as described in Section 6.5.1.1 of
 [RFC6940].  For more information on the link types defined here, see
 Section 6.6 of [RFC6940].  New entries SHALL be defined via Standards
 Action [RFC5226].  This registry has been initially populated with
 the following values:
               +--------------------+------+-----------+
               | Protocol           | Code | Reference |
               +--------------------+------+-----------+
               | INVALID-PROTOCOL   |    0 |  RFC 6940 |
               | DTLS-UDP-SR        |    1 |  RFC 6940 |
               | DTLS-UDP-SR-NO-ICE |    3 |  RFC 6940 |
               | TLS-TCP-FH-NO-ICE  |    4 |  RFC 6940 |
               | EXP-LINK           |    5 |  RFC 6940 |
               | Reserved           |  255 |  RFC 6940 |
               +--------------------+------+-----------+
 The value EXP-LINK has been made available for the purposes of
 experimentation.  This value is not meant for vendor-specific use of
 any sort, and it MUST NOT be used for operational deployments.

14.11. Overlay Link Protocols

 IANA has created a "RELOAD Overlay Link Protocol Registry".  Entries
 in this registry are strings denoting protocols as described in
 Section 11.1 of this document and SHALL be defined via Standards
 Action [RFC5226].  This registry has been initially populated with
 the following values:
                     +---------------+-----------+
                     | Link Protocol | Reference |
                     +---------------+-----------+
                     | TLS           |  RFC 6940 |
                     | EXP-PROTOCOL  |  RFC 6940 |
                     +---------------+-----------+
 The value EXP-PROTOCOL has been made available for the purposes of
 experimentation.  This value is not meant for vendor-specific use of
 any sort, and it MUST NOT be used for operational deployments.

Jennings, et al. Standards Track [Page 159] RFC 6940 RELOAD Base January 2014

14.12. Forwarding Options

 IANA has created a "RELOAD Forwarding Option Registry".  Entries in
 this registry are 8-bit integers denoting options, as described in
 Section 6.3.2.3 of [RFC6940].  Values between 1 and 127 SHALL be
 defined via Standards Action [RFC5226].  Entries in this registry
 between 128 and 254 SHALL be defined via Specification Required
 [RFC5226].  This registry has been initially populated with the
 following values:
            +-------------------------+------+-----------+
            | Forwarding Option       | Code | Reference |
            +-------------------------+------+-----------+
            | invalidForwardingOption |    0 |  RFC 6940 |
            | exp-forward             |    1 |  RFC 6940 |
            | Reserved                |  255 |  RFC 6940 |
            +-------------------------+------+-----------+
 The value exp-forward has been made available for the purposes of
 experimentation.  This value is not meant for vendor-specific use of
 any sort, and it MUST NOT be used for operational deployments.

14.13. Probe Information Types

 IANA has created a "RELOAD Probe Information Type Registry".  Entries
 are 8-bit integers denoting types as described in Section 6.4.2.5.1
 of [RFC6940] and SHALL be defined via Standards Action [RFC5226].
 This registry has been initially populated with the following values:
               +--------------------+------+-----------+
               | Probe Option       | Code | Reference |
               +--------------------+------+-----------+
               | invalidProbeOption |    0 |  RFC 6940 |
               | responsible_set    |    1 |  RFC 6940 |
               | num_resources      |    2 |  RFC 6940 |
               | uptime             |    3 |  RFC 6940 |
               | exp-probe          |    4 |  RFC 6940 |
               | Reserved           |  255 |  RFC 6940 |
               +--------------------+------+-----------+
 The value exp-probe has been made available for the purposes of
 experimentation.  This value is not meant for vendor-specific use of
 any sort, and it MUST NOT be used for operational deployments.

Jennings, et al. Standards Track [Page 160] RFC 6940 RELOAD Base January 2014

14.14. Message Extensions

 IANA has created a "RELOAD Extensions Registry".  Entries in this
 registry are 8-bit integers denoting extensions as described in
 Section 6.3.3 of [RFC6940] and SHALL be defined via Specification
 Required [RFC5226].  This registry has been initially populated with
 the following values:
         +-----------------------------+--------+-----------+
         | Extensions Name             |   Code | Reference |
         +-----------------------------+--------+-----------+
         | invalidMessageExtensionType |    0x0 |  RFC 6940 |
         | exp-ext                     |    0x1 |  RFC 6940 |
         | Reserved                    | 0xFFFF |  RFC 6940 |
         +-----------------------------+--------+-----------+
 The value exp-ext has been made available for the purposes of
 experimentation.  This value is not meant for vendor-specific use of
 any sort, and it MUST NOT be used for operational deployments.

14.15. Reload URI Scheme

 This section describes the scheme for a reload URI, which can be used
 to refer to either:
 o  A peer, e.g., as used in a certificate (see Section 11.3 of
    [RFC6940]).
 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 [RFC3986] and the associated URI Guidelines
 [RFC4395] per the following ABNF syntax:
    RELOAD-URI = "reload://" destination "@" overlay "/"
             [specifier]
    destination = 1*HEXDIG
    overlay = reg-name
    specifier = 1*HEXDIG
 The definitions of these productions are as follows:
 destination
    A hexadecimal-encoded Destination List object (i.e., multiple
    concatenated Destination objects with no length prefix prior to
    the object as a whole).

Jennings, et al. Standards Track [Page 161] RFC 6940 RELOAD Base January 2014

 overlay
    The name of the overlay.
 specifier
    A hexadecimal-encoded StoredDataSpecifier indicating the data
    element.
 If no specifier is present, this URI addresses the peer which can be
 reached via the indicated Destination List at the indicated overlay
 name.  If a specifier is present, the URI addresses the data value.

14.15.1. URI Registration

 The following summarizes the information necessary to register the
 reload URI.
 URI Scheme Name:  reload
 Status:   permanent
 URI Scheme Syntax:  see Section 14.15 of RFC 6940
 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, so it is encoded entirely in US-ASCII.
 Applications/protocols that Use this URI Scheme:  The RELOAD protocol
    described in RFC 6940.
 Interoperability Considerations:  See RFC 6940.
 Security Considerations:  See RFC 6940
 Contact:  Cullen Jennings <fluffy@cisco.com>
 Author/Change Controller:  IESG
 References:  RFC 6940

14.16. Media Type Registration

 Type Name: application
 Subtype Name: p2p-overlay+xml
 Required Parameters: none

Jennings, et al. Standards Track [Page 162] RFC 6940 RELOAD Base January 2014

 Optional Parameters: none
 Encoding Considerations: Must be binary encoded.
 Security Considerations: This media type is typically not used to
 transport information that needs to be kept confidential.  However,
 there are cases where it is integrity of the information is
 important.  For these cases, using a digital signature is
 RECOMMENDED.  One way of doing this is specified in RFC 6940.  In the
 case when the media includes a shared-secret element, the contents of
 the file MUST be kept confidential or else anyone who can see the
 shared secret can affect the RELOAD overlay network.
 Interoperability Considerations: No known interoperability
 consideration beyond those identified for application/xml in
 [RFC3023].
 Published Specification: RFC 6940
 Applications that Use this Media Type: The type is used to configure
 the peer-to-peer overlay networks defined in RFC 6940.
 Additional Information: The syntax for this media type is specified
 in Section 11.1 of [RFC6940].  The contents MUST be valid XML that is
 compliant with the RELAX NG grammar specified in RFC 6940 and that
 use the UTF-8[RFC3629] character encoding.
    Magic Number(s): none
    File Extension(s): relo
    Macintosh File Type Code(s): none
 Person & Email Address to Contact for Further Information: Cullen
 Jennings <fluffy@cisco.com>
 Intended Usage: COMMON
 Restrictions on Usage: None
 Author: Cullen Jennings <fluffy@cisco.com>
 Change Controller: IESG

14.17. XML Namespace Registration

 This document registers two URIs for the config and config-chord XML
 namespaces in the IETF XML registry defined in [RFC3688].

Jennings, et al. Standards Track [Page 163] RFC 6940 RELOAD Base January 2014

14.17.1. Config URL

 URI: urn:ietf:params:xml:ns:p2p:config-base
 Registrant Contact: IESG.
 XML: N/A, the requested URIs are XML namespaces

14.17.2. Config Chord URL

 URI: urn:ietf:params:xml:ns:p2p:config-chord
 Registrant Contact: The IESG.
 XML: N/A, the requested URIs are XML namespaces

15. Acknowledgments

 This specification is a merge of the "REsource LOcation And Discovery
 (RELOAD)" document by David A. Bryan, Marcia Zangrilli, and Bruce B.
 Lowekamp; the "Address Settlement by Peer to Peer" document by Cullen
 Jennings, Jonathan Rosenberg, and Eric Rescorla; the "Security
 Extensions for RELOAD" document 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) document by Salman A. Baset, Henning Schulzrinne, and Marcin
 Matuszewski.  Thanks to the authors of [RFC5389] for text included
 from that document.  Vidya Narayanan provided many comments and
 improvements.
 The ideas and text for the Chord-specific extension data to the Leave
 mechanisms were provided by Jouni Maenpaa, Gonzalo Camarillo, and
 Jani Hautakorpi.
 Thanks to the many people who contributed, including Ted Hardie,
 Michael Chen, Dan York, Das Saumitra, Lyndsay Campbell, Brian Rosen,
 David Bryan, Dave Craig, and Julian Cain.  Extensive last call
 comments were provided by Jouni Maenpaa, Roni Even, Gonzalo
 Camarillo, Ari Keranen, John Buford, Michael Chen, Frederic-Philippe
 Met, Mary Barnes, Roland Bless, David Bryan, and Polina Goltsman.
 Special thanks to Marc Petit-Huguenin, who provided an amazing amount
 of detailed review.
 Dean Willis and Marc Petit-Huguenin helped resolve and provided text
 to fix many comments received during the IESG review.

Jennings, et al. Standards Track [Page 164] RFC 6940 RELOAD Base January 2014

16. References

16.1. Normative References

 [OASIS.relax_ng]
            Bray, T. and M. Murata, "RELAX NG Specification", December
            2001.
 [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
            E. Lear, "Address Allocation for Private Internets", BCP
            5, RFC 1918, February 1996.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2388]  Masinter, L., "Returning Values from Forms: multipart/
            form-data", RFC 2388, August 1998.
 [RFC2585]  Housley, R. and P. Hoffman, "Internet X.509 Public Key
            Infrastructure Operational Protocols: FTP and HTTP", RFC
            2585, May 1999.
 [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
            specifying the location of services (DNS SRV)", RFC 2782,
            February 2000.
 [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
 [RFC3023]  Murata, M., St. Laurent, S., and D. Kohn, "XML Media
            Types", RFC 3023, January 2001.
 [RFC3174]  Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1
            (SHA1)", RFC 3174, September 2001.
 [RFC3339]  Klyne, G., Ed. and C. Newman, "Date and Time on the
            Internet: Timestamps", RFC 3339, July 2002.
 [RFC3447]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography
            Standards (PKCS) #1: RSA Cryptography Specifications
            Version 2.1", RFC 3447, February 2003.
 [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
            10646", STD 63, RFC 3629, November 2003.
 [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifier (URI): Generic Syntax", STD 66, RFC
            3986, January 2005.

Jennings, et al. Standards Track [Page 165] RFC 6940 RELOAD Base January 2014

 [RFC4279]  Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
            for Transport Layer Security (TLS)", RFC 4279, December
            2005.
 [RFC4395]  Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
            Registration Procedures for New URI Schemes", BCP 35, RFC
            4395, February 2006.
 [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
            Encodings", RFC 4648, October 2006.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            May 2008.
 [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
            (ICE): A Protocol for Network Address Translator (NAT)
            Traversal for Offer/Answer Protocols", RFC 5245, April
            2010.
 [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.2", RFC 5246, August 2008.
 [RFC5272]  Schaad, J. and M. Myers, "Certificate Management over CMS
            (CMC)", RFC 5272, June 2008.
 [RFC5273]  Schaad, J. and M. Myers, "Certificate Management over CMS
            (CMC): Transport Protocols", RFC 5273, June 2008.
 [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
            "Session Traversal Utilities for NAT (STUN)", RFC 5389,
            October 2008.
 [RFC5405]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
            for Application Designers", BCP 145, RFC 5405, November
            2008.
 [RFC5766]  Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
            Relays around NAT (TURN): Relay Extensions to Session
            Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.
 [RFC5952]  Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
            Address Text Representation", RFC 5952, August 2010.
 [RFC6091]  Mavrogiannopoulos, N. and D. Gillmor, "Using OpenPGP Keys
            for Transport Layer Security (TLS) Authentication", RFC
            6091, February 2011.

Jennings, et al. Standards Track [Page 166] RFC 6940 RELOAD Base January 2014

 [RFC6234]  Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
            (SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011.
 [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
            "Computing TCP's Retransmission Timer", RFC 6298, June
            2011.
 [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
            Security Version 1.2", RFC 6347, January 2012.
 [W3C.REC-xmlschema-2-20041028]
            Malhotra, A. and P. Biron, "XML Schema Part 2: Datatypes
            Second Edition", World Wide Web Consortium Recommendation
            REC-xmlschema-2-20041028, October 2004,
            <http://www.w3.org/TR/2004/REC-xmlschema-2-20041028>.
 [w3c-xml-namespaces]
            Bray, T., Hollander, D., Layman, A., Tobin, R., and
            University of Edinburgh and W3C, "Namespaces in XML 1.0
            (Third Edition)", December 2008.

16.2. Informative References

 [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, 2001.
 [DHT-RELOAD]
            Maenpaa, J. and G. Camarillo, "A Self-tuning Distributed
            Hash Table (DHT) for REsource LOcation And Discovery
            (RELOAD)", Work in Progress, August 2013.
 [Eclipse]  Singh, A., Ngan, T., Druschel, T., and D. Wallach,
            "Eclipse Attacks on Overlay Networks: Threats and
            Defenses", INFOCOM 2006, April 2006.
 [P2P-DIAGNOSTICS]
            Song, H., Jiang, X., Even, R., and D. Bryan, "P2P Overlay
            Diagnostics", Work in Progress, August 2013.
 [P2PSIP-RELAY]
            Zong, N., Jiang, X., Even, R., and Y. Zhang, "An extension
            to RELOAD to support Relay Peer Routing", Work in
            Progress, October 2013.

Jennings, et al. Standards Track [Page 167] RFC 6940 RELOAD Base January 2014

 [REDIR-RELOAD]
            Maenpaa, J. and G. Camarillo, "Service Discovery Usage for
            REsource LOcation And Discovery (RELOAD)", Work in
            Progress, August 2013.
 [RFC1035]  Mockapetris, P., "Domain names - implementation and
            specification", STD 13, RFC 1035, November 1987.
 [RFC1122]  Braden, R., "Requirements for Internet Hosts -
            Communication Layers", STD 3, RFC 1122, October 1989.
 [RFC2311]  Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L., and
            L. Repka, "S/MIME Version 2 Message Specification", RFC
            2311, March 1998.
 [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
            January 2004.
 [RFC4013]  Zeilenga, K., "SASLprep: Stringprep Profile for User Names
            and Passwords", RFC 4013, February 2005.
 [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
            Requirements for Security", BCP 106, RFC 4086, June 2005.
 [RFC4145]  Yon, D. and G. Camarillo, "TCP-Based Media Transport in
            the Session Description Protocol (SDP)", RFC 4145,
            September 2005.
 [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
            Congestion Control Protocol (DCCP)", RFC 4340, March 2006.
 [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
            (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
            RFC 4787, January 2007.
 [RFC4960]  Stewart, R., "Stream Control Transmission Protocol", RFC
            4960, September 2007.
 [RFC5054]  Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin,
            "Using the Secure Remote Password (SRP) Protocol for TLS
            Authentication", RFC 5054, November 2007.
 [RFC5095]  Abley, J., Savola, P., and G. Neville-Neil, "Deprecation
            of Type 0 Routing Headers in IPv6", RFC 5095, December
            2007.
 [RFC5201]  Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
            "Host Identity Protocol", RFC 5201, April 2008.

Jennings, et al. Standards Track [Page 168] RFC 6940 RELOAD Base January 2014

 [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
            Housley, R., and W. Polk, "Internet X.509 Public Key
            Infrastructure Certificate and Certificate Revocation List
            (CRL) Profile", RFC 5280, May 2008.
 [RFC5694]  Camarillo, G., Ed., and IAB, "Peer-to-Peer (P2P)
            Architecture: Definition, Taxonomies, Examples, and
            Applicability", RFC 5694, November 2009.
 [RFC5765]  Schulzrinne, H., Marocco, E., and E. Ivov, "Security
            Issues and Solutions in Peer-to-Peer Systems for Realtime
            Communications", RFC 5765, February 2010.
 [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
            Uniform Resource Identifiers (URIs)", RFC 5785, April
            2010.
 [RFC6079]  Camarillo, G., Nikander, P., Hautakorpi, J., Keranen, A.,
            and A. Johnston, "HIP BONE: Host Identity Protocol (HIP)
            Based Overlay Networking Environment (BONE)", RFC 6079,
            January 2011.
 [RFC6544]  Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach,
            "TCP Candidates with Interactive Connectivity
            Establishment (ICE)", RFC 6544, March 2012.
 [RFC7086]  Keranen, A., Camarillo, G., and J. Maenpaa, "Host Identity
            Protocol-Based Overlay Networking Environment (HIP BONE)
            Instance Specification for REsource LOcation And Discovery
            (RELOAD)", RFC 7086, January 2014.
 [SIP-RELOAD]
            Jennings, C., Lowekamp, B., Rescorla, E., Baset, S.,
            Schulzrinne, H., and T. Schmidt, "A SIP Usage for RELOAD",
            Work in Progress, July 2013.
 [Sybil]    Douceur, J., "The Sybil Attack", IPTPS 02, March 2002.
 [UnixTime] Wikipedia, "Unix Time", 2013, <http://en.wikipedia.org/w/
            index.php?title=Unix_time&oldid=551527446>.
 [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, 2008.

Jennings, et al. Standards Track [Page 169] RFC 6940 RELOAD Base January 2014

 [handling-churn-usenix04]
            Rhea, S., Geels, D., Roscoe, T., and J. Kubiatowicz,
            "Handling Churn in a DHT", In Proc. of the USENIX Annual
            Technical Conference June 2004 USENIX 2004, 2004.
 [lookups-churn-p2p06]
            Wu, D., Tian, Y., and K. Ng, "Analytical Study on
            Improving DHT Lookup Performance under Churn", IEEE
            P2P'06, 2006.
 [minimizing-churn-sigcomm06]
            Godfrey, P., Shenker, S., and I. Stoica, "Minimizing Churn
            in Distributed Systems", SIGCOMM 2006, 2006.
 [non-transitive-dhts-worlds05]
            Freedman, M., Lakshminarayanan, K., Rhea, S., and I.
            Stoica, "Non-Transitive Connectivity and DHTs", WORLDS'05,
            2005.
 [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, 2005.
 [vulnerabilities-acsac04]
            Srivatsa, M. and L. Liu, "Vulnerabilities and Security
            Threats in Structured Peer-to-Peer Systems: A Quantitative
            Analysis", ACSAC 2004, 2004.
 [wikiChord]
            Wikipedia, "Chord (peer-to-peer)", 2013,
            <http://en.wikipedia.org/w/
            index.php?title=Chord_%28peer-to-peer%29&oldid=549516287>.
 [wikiKBR]  Wikipedia, "Key-based routing", 2013, <en.wikipedia.org/w/
            index.php?title=Key-based_routing&oldid=543850833>.
 [wikiSkiplist]
            Wikipedia, "Skip list", 2013, <http://en.wikipedia.org/w/
            index.php?title=Skip_list&oldid=551304213>.

Jennings, et al. Standards Track [Page 170] RFC 6940 RELOAD Base January 2014

Appendix A. Routing Alternatives

 Significant discussion has been focused on the selection of a routing
 algorithm for P2PSIP.  This section discusses the motivations for
 selecting symmetric recursive routing for RELOAD and describes the
 extensions that would be required to support additional routing
 algorithms.

A.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 RouteQuery mechanism and
 is primarily intended for debugging.  It 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.

A.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 were a new message initiated 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-only 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).
 Furthermore, forward-only responses are less likely to reach the
 querying peer than symmetric recursive ones are, because the forward
 path is more likely to have a failed peer than is the request path
 (which was just tested to route the request)
 [non-transitive-dhts-worlds05].

Jennings, et al. Standards Track [Page 171] RFC 6940 RELOAD Base January 2014

 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 routing and
 when to fallback to symmetric routing, we have chosen not to include
 it as an option at this point.

A.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 routing 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, it is
    absolutely necessary to establish a secure connection to
    authenticate A before delivering the response.  This step
    diminishes the efficiency of direct response routing, because
    multiple round-trips 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 must both be 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 be received
    only if A's NAT implements endpoint-independent filtering.  As the

Jennings, et al. Standards Track [Page 172] RFC 6940 RELOAD Base January 2014

    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
 routing and when to fallback to symmetric routing, and the reduced
 performance for responses to peers behind restrictive NATs, we have
 chosen not to include it as an option at this point.

A.4. Relay Peers

 [P2PSIP-RELAY] 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, and Z is 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 pair-
 wise non-transitivity [non-transitive-dhts-worlds05], but
 deterministic filtering provided by NATs makes random relay peers no
 more likely to work than the responding peer.

A.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. Standards Track [Page 173] RFC 6940 RELOAD Base January 2014

 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 B. Why Clients?

 There are a wide variety of reasons a node may act as a client rather
 than as a peer.  This section outlines some of those scenarios and
 how the client's behavior changes based on its capabilities.

B.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 participating in the overlay to
    determine trustworthiness, controlling the number of peers in the
    overlay to reduce overly long routing paths, and ensuring 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 Attach to establish a direct connection to that
 peer.  In that way, it may elect to be reachable under either of the

Jennings, et al. Standards Track [Page 174] RFC 6940 RELOAD Base January 2014

 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.

B.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.

Jennings, et al. Standards Track [Page 175] RFC 6940 RELOAD Base January 2014

Authors' Addresses

 Cullen Jennings
 Cisco
 400 3rd Avenue SW, Suite 350
 Calgary
 Canada
 EMail: fluffy@cisco.com
 Bruce B. Lowekamp (editor)
 Skype
 Palo Alto, CA
 USA
 EMail: bbl@lowekamp.net
 Eric Rescorla
 RTFM, Inc.
 2064 Edgewood Drive
 Palo Alto, CA  94303
 USA
 Phone: +1 650 678 2350
 EMail: ekr@rtfm.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. Standards Track [Page 176]

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