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P2PSIP                                                       C. Jennings
Internet-Draft                                                     Cisco
Intended status:  Standards Track                       B. Lowekamp, Ed.
Expires:  September 29, 2012                                       Skype
                                                             E. Rescorla
                                                              RTFM, Inc.
                                                                S. Baset
                                                          H. Schulzrinne
                                                     Columbia University
                                                          March 28, 2012


         REsource LOcation And Discovery (RELOAD) Base Protocol
                       draft-ietf-p2psip-base-21

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 must be stored for a particular
   application.  RELOAD defines a security model based on a certificate
   enrollment service that provides unique identities.  NAT traversal is
   a fundamental service of the protocol.  RELOAD also allows access
   from "client" nodes that do not need to route traffic or store data
   for others.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 29, 2012.




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Copyright Notice

   Copyright (c) 2012 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
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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
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   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.

























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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   8
     1.1.   Basic Setting  . . . . . . . . . . . . . . . . . . . . .   9
     1.2.   Architecture . . . . . . . . . . . . . . . . . . . . . .  10
       1.2.1.   Usage Layer  . . . . . . . . . . . . . . . . . . . .  13
       1.2.2.   Message Transport  . . . . . . . . . . . . . . . . .  14
       1.2.3.   Storage  . . . . . . . . . . . . . . . . . . . . . .  15
       1.2.4.   Topology Plugin  . . . . . . . . . . . . . . . . . .  15
       1.2.5.   Forwarding and Link Management Layer . . . . . . . .  16
     1.3.   Security . . . . . . . . . . . . . . . . . . . . . . . .  17
     1.4.   Structure of This Document . . . . . . . . . . . . . . .  18
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .  18
   3.  Overlay Management Overview . . . . . . . . . . . . . . . . .  21
     3.1.   Security and Identification  . . . . . . . . . . . . . .  21
       3.1.1.   Shared-Key Security  . . . . . . . . . . . . . . . .  23
     3.2.   Clients  . . . . . . . . . . . . . . . . . . . . . . . .  23
       3.2.1.   Client Routing . . . . . . . . . . . . . . . . . . .  24
       3.2.2.   Minimum Functionality Requirements for Clients . . .  24
     3.3.   Routing  . . . . . . . . . . . . . . . . . . . . . . . .  25
     3.4.   Connectivity Management  . . . . . . . . . . . . . . . .  28
     3.5.   Overlay Algorithm Support  . . . . . . . . . . . . . . .  28
       3.5.1.   Support for Pluggable Overlay Algorithms . . . . . .  29
       3.5.2.   Joining, Leaving, and Maintenance Overview . . . . .  29
     3.6.   First-Time Setup . . . . . . . . . . . . . . . . . . . .  30
       3.6.1.   Initial Configuration  . . . . . . . . . . . . . . .  31
       3.6.2.   Enrollment . . . . . . . . . . . . . . . . . . . . .  31
       3.6.3.   Diagnostics  . . . . . . . . . . . . . . . . . . . .  31
   4.  RFC 2119 Terminology  . . . . . . . . . . . . . . . . . . . .  31
   5.  Application Support Overview  . . . . . . . . . . . . . . . .  31
     5.1.   Data Storage . . . . . . . . . . . . . . . . . . . . . .  32
       5.1.1.   Storage Permissions  . . . . . . . . . . . . . . . .  33
       5.1.2.   Replication  . . . . . . . . . . . . . . . . . . . .  34
     5.2.   Usages . . . . . . . . . . . . . . . . . . . . . . . . .  34
     5.3.   Service Discovery  . . . . . . . . . . . . . . . . . . .  35
     5.4.   Application Connectivity . . . . . . . . . . . . . . . .  35
   6.  Overlay Management Protocol . . . . . . . . . . . . . . . . .  35
     6.1.   Message Receipt and Forwarding . . . . . . . . . . . . .  36
       6.1.1.   Responsible ID . . . . . . . . . . . . . . . . . . .  36
       6.1.2.   Other ID . . . . . . . . . . . . . . . . . . . . . .  37
       6.1.3.   Opaque ID  . . . . . . . . . . . . . . . . . . . . .  39
     6.2.   Symmetric Recursive Routing  . . . . . . . . . . . . . .  39
       6.2.1.   Request Origination  . . . . . . . . . . . . . . . .  39
       6.2.2.   Response Origination . . . . . . . . . . . . . . . .  40
     6.3.   Message Structure  . . . . . . . . . . . . . . . . . . .  40
       6.3.1.   Presentation Language  . . . . . . . . . . . . . . .  41
         6.3.1.1.  Common Definitions  . . . . . . . . . . . . . . .  42
       6.3.2.   Forwarding Header  . . . . . . . . . . . . . . . . .  44



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         6.3.2.1.  Processing Configuration Sequence Numbers . . . .  46
         6.3.2.2.  Destination and Via Lists . . . . . . . . . . . .  47
         6.3.2.3.  Forwarding Options  . . . . . . . . . . . . . . .  49
       6.3.3.   Message Contents Format  . . . . . . . . . . . . . .  50
         6.3.3.1.  Response Codes and Response Errors  . . . . . . .  51
       6.3.4.   Security Block . . . . . . . . . . . . . . . . . . .  53
     6.4.   Overlay Topology . . . . . . . . . . . . . . . . . . . .  57
       6.4.1.   Topology Plugin Requirements . . . . . . . . . . . .  57
       6.4.2.   Methods and types for use by topology plugins  . . .  58
         6.4.2.1.  Join  . . . . . . . . . . . . . . . . . . . . . .  58
         6.4.2.2.  Leave . . . . . . . . . . . . . . . . . . . . . .  59
         6.4.2.3.  Update  . . . . . . . . . . . . . . . . . . . . .  59
         6.4.2.4.  RouteQuery  . . . . . . . . . . . . . . . . . . .  60
         6.4.2.5.  Probe . . . . . . . . . . . . . . . . . . . . . .  61
     6.5.   Forwarding and Link Management Layer . . . . . . . . . .  63
       6.5.1.   Attach . . . . . . . . . . . . . . . . . . . . . . .  63
         6.5.1.1.  Request Definition  . . . . . . . . . . . . . . .  64
         6.5.1.2.  Response Definition . . . . . . . . . . . . . . .  67
         6.5.1.3.  Using ICE With RELOAD . . . . . . . . . . . . . .  68
         6.5.1.4.  Collecting STUN Servers . . . . . . . . . . . . .  68
         6.5.1.5.  Gathering Candidates  . . . . . . . . . . . . . .  69
         6.5.1.6.  Prioritizing Candidates . . . . . . . . . . . . .  69
         6.5.1.7.  Encoding the Attach Message . . . . . . . . . . .  70
         6.5.1.8.  Verifying ICE Support . . . . . . . . . . . . . .  70
         6.5.1.9.  Role Determination  . . . . . . . . . . . . . . .  71
         6.5.1.10. Full ICE  . . . . . . . . . . . . . . . . . . . .  71
         6.5.1.11. No-ICE  . . . . . . . . . . . . . . . . . . . . .  71
         6.5.1.12. Subsequent Offers and Answers . . . . . . . . . .  72
         6.5.1.13. Sending Media . . . . . . . . . . . . . . . . . .  72
         6.5.1.14. Receiving Media . . . . . . . . . . . . . . . . .  72
       6.5.2.   AppAttach  . . . . . . . . . . . . . . . . . . . . .  72
         6.5.2.1.  Request Definition  . . . . . . . . . . . . . . .  72
         6.5.2.2.  Response Definition . . . . . . . . . . . . . . .  73
       6.5.3.   Ping . . . . . . . . . . . . . . . . . . . . . . . .  74
         6.5.3.1.  Request Definition  . . . . . . . . . . . . . . .  74
         6.5.3.2.  Response Definition . . . . . . . . . . . . . . .  74
       6.5.4.   ConfigUpdate . . . . . . . . . . . . . . . . . . . .  75
         6.5.4.1.  Request Definition  . . . . . . . . . . . . . . .  75
         6.5.4.2.  Response Definition . . . . . . . . . . . . . . .  76
     6.6.   Overlay Link Layer . . . . . . . . . . . . . . . . . . .  76
       6.6.1.   Future Overlay Link Protocols  . . . . . . . . . . .  78
         6.6.1.1.  HIP . . . . . . . . . . . . . . . . . . . . . . .  78
         6.6.1.2.  ICE-TCP . . . . . . . . . . . . . . . . . . . . .  79
         6.6.1.3.  Message-oriented Transports . . . . . . . . . . .  79
         6.6.1.4.  Tunneled Transports . . . . . . . . . . . . . . .  79
       6.6.2.   Framing Header . . . . . . . . . . . . . . . . . . .  79
       6.6.3.   Simple Reliability . . . . . . . . . . . . . . . . .  81
         6.6.3.1.  Stop and Wait Sender Algorithm  . . . . . . . . .  82



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       6.6.4.   DTLS/UDP with SR . . . . . . . . . . . . . . . . . .  83
       6.6.5.   TLS/TCP with FH, No-ICE  . . . . . . . . . . . . . .  83
       6.6.6.   DTLS/UDP with SR, No-ICE . . . . . . . . . . . . . .  83
     6.7.   Fragmentation and Reassembly . . . . . . . . . . . . . .  84
   7.  Data Storage Protocol . . . . . . . . . . . . . . . . . . . .  85
     7.1.   Data Signature Computation . . . . . . . . . . . . . . .  86
     7.2.   Data Models  . . . . . . . . . . . . . . . . . . . . . .  87
       7.2.1.   Single Value . . . . . . . . . . . . . . . . . . . .  88
       7.2.2.   Array  . . . . . . . . . . . . . . . . . . . . . . .  88
       7.2.3.   Dictionary . . . . . . . . . . . . . . . . . . . . .  89
     7.3.   Access Control Policies  . . . . . . . . . . . . . . . .  89
       7.3.1.   USER-MATCH . . . . . . . . . . . . . . . . . . . . .  90
       7.3.2.   NODE-MATCH . . . . . . . . . . . . . . . . . . . . .  90
       7.3.3.   USER-NODE-MATCH  . . . . . . . . . . . . . . . . . .  90
       7.3.4.   NODE-MULTIPLE  . . . . . . . . . . . . . . . . . . .  90
     7.4.   Data Storage Methods . . . . . . . . . . . . . . . . . .  91
       7.4.1.   Store  . . . . . . . . . . . . . . . . . . . . . . .  91
         7.4.1.1.  Request Definition  . . . . . . . . . . . . . . .  91
         7.4.1.2.  Response Definition . . . . . . . . . . . . . . .  95
         7.4.1.3.  Removing Values . . . . . . . . . . . . . . . . .  97
       7.4.2.   Fetch  . . . . . . . . . . . . . . . . . . . . . . .  97
         7.4.2.1.  Request Definition  . . . . . . . . . . . . . . .  98
         7.4.2.2.  Response Definition . . . . . . . . . . . . . . . 100
       7.4.3.   Stat . . . . . . . . . . . . . . . . . . . . . . . . 101
         7.4.3.1.  Request Definition  . . . . . . . . . . . . . . . 101
         7.4.3.2.  Response Definition . . . . . . . . . . . . . . . 101
       7.4.4.   Find . . . . . . . . . . . . . . . . . . . . . . . . 103
         7.4.4.1.  Request Definition  . . . . . . . . . . . . . . . 103
         7.4.4.2.  Response Definition . . . . . . . . . . . . . . . 104
       7.4.5.   Defining New Kinds . . . . . . . . . . . . . . . . . 105
   8.  Certificate Store Usage . . . . . . . . . . . . . . . . . . . 105
   9.  TURN Server Usage . . . . . . . . . . . . . . . . . . . . . . 106
   10. Chord Algorithm . . . . . . . . . . . . . . . . . . . . . . . 108
     10.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . 109
     10.2.  Hash Function  . . . . . . . . . . . . . . . . . . . . . 109
     10.3.  Routing  . . . . . . . . . . . . . . . . . . . . . . . . 109
     10.4.  Redundancy . . . . . . . . . . . . . . . . . . . . . . . 110
     10.5.  Joining  . . . . . . . . . . . . . . . . . . . . . . . . 110
     10.6.  Routing Attaches . . . . . . . . . . . . . . . . . . . . 111
     10.7.  Updates  . . . . . . . . . . . . . . . . . . . . . . . . 111
       10.7.1.  Handling Neighbor Failures . . . . . . . . . . . . . 113
       10.7.2.  Handling Finger Table Entry Failure  . . . . . . . . 114
       10.7.3.  Receiving Updates  . . . . . . . . . . . . . . . . . 114
       10.7.4.  Stabilization  . . . . . . . . . . . . . . . . . . . 115
         10.7.4.1. Updating neighbor table . . . . . . . . . . . . . 115
         10.7.4.2. Refreshing finger table . . . . . . . . . . . . . 115
         10.7.4.3. Adjusting finger table size . . . . . . . . . . . 116
         10.7.4.4. Detecting partitioning  . . . . . . . . . . . . . 117



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     10.8.  Route query  . . . . . . . . . . . . . . . . . . . . . . 117
     10.9.  Leaving  . . . . . . . . . . . . . . . . . . . . . . . . 118
   11. Enrollment and Bootstrap  . . . . . . . . . . . . . . . . . . 119
     11.1.  Overlay Configuration  . . . . . . . . . . . . . . . . . 119
       11.1.1.  Relax NG Grammar . . . . . . . . . . . . . . . . . . 125
     11.2.  Discovery Through Configuration Server . . . . . . . . . 128
     11.3.  Credentials  . . . . . . . . . . . . . . . . . . . . . . 128
       11.3.1.  Self-Generated Credentials . . . . . . . . . . . . . 130
     11.4.  Searching for a Bootstrap Node . . . . . . . . . . . . . 130
     11.5.  Contacting a Bootstrap Node  . . . . . . . . . . . . . . 130
   12. Message Flow Example  . . . . . . . . . . . . . . . . . . . . 131
   13. Security Considerations . . . . . . . . . . . . . . . . . . . 137
     13.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . 137
     13.2.  Attacks on P2P Overlays  . . . . . . . . . . . . . . . . 138
     13.3.  Certificate-based Security . . . . . . . . . . . . . . . 138
     13.4.  Shared-Secret Security . . . . . . . . . . . . . . . . . 139
     13.5.  Storage Security . . . . . . . . . . . . . . . . . . . . 140
       13.5.1.  Authorization  . . . . . . . . . . . . . . . . . . . 140
       13.5.2.  Distributed Quota  . . . . . . . . . . . . . . . . . 141
       13.5.3.  Correctness  . . . . . . . . . . . . . . . . . . . . 141
       13.5.4.  Residual Attacks . . . . . . . . . . . . . . . . . . 141
     13.6.  Routing Security . . . . . . . . . . . . . . . . . . . . 142
       13.6.1.  Background . . . . . . . . . . . . . . . . . . . . . 142
       13.6.2.  Admissions Control . . . . . . . . . . . . . . . . . 143
       13.6.3.  Peer Identification and Authentication . . . . . . . 143
       13.6.4.  Protecting the Signaling . . . . . . . . . . . . . . 144
       13.6.5.  Routing Loops and Dos Attacks  . . . . . . . . . . . 144
       13.6.6.  Residual Attacks . . . . . . . . . . . . . . . . . . 144
   14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 145
     14.1.  Well-Known URI Registration  . . . . . . . . . . . . . . 145
     14.2.  Port Registrations . . . . . . . . . . . . . . . . . . . 145
     14.3.  Overlay Algorithm Types  . . . . . . . . . . . . . . . . 146
     14.4.  Access Control Policies  . . . . . . . . . . . . . . . . 146
     14.5.  Application-ID . . . . . . . . . . . . . . . . . . . . . 147
     14.6.  Data Kind-ID . . . . . . . . . . . . . . . . . . . . . . 147
     14.7.  Data Model . . . . . . . . . . . . . . . . . . . . . . . 148
     14.8.  Message Codes  . . . . . . . . . . . . . . . . . . . . . 148
     14.9.  Error Codes  . . . . . . . . . . . . . . . . . . . . . . 150
     14.10. Overlay Link Types . . . . . . . . . . . . . . . . . . . 150
     14.11. Overlay Link Protocols . . . . . . . . . . . . . . . . . 151
     14.12. Forwarding Options . . . . . . . . . . . . . . . . . . . 151
     14.13. Probe Information Types  . . . . . . . . . . . . . . . . 152
     14.14. Message Extensions . . . . . . . . . . . . . . . . . . . 152
     14.15. reload URI Scheme  . . . . . . . . . . . . . . . . . . . 152
       14.15.1. URI Registration . . . . . . . . . . . . . . . . . . 153
     14.16. Media Type Registration  . . . . . . . . . . . . . . . . 154
     14.17. XML Name Space Registration  . . . . . . . . . . . . . . 155
       14.17.1. Config URL . . . . . . . . . . . . . . . . . . . . . 155



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       14.17.2. Config Chord URL . . . . . . . . . . . . . . . . . . 155
   15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 155
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . . 156
     16.1.  Normative References . . . . . . . . . . . . . . . . . . 156
     16.2.  Informative References . . . . . . . . . . . . . . . . . 158
   Appendix A.  Routing Alternatives . . . . . . . . . . . . . . . . 161
     A.1.   Iterative vs Recursive . . . . . . . . . . . . . . . . . 161
     A.2.   Symmetric vs Forward response  . . . . . . . . . . . . . 162
     A.3.   Direct Response  . . . . . . . . . . . . . . . . . . . . 162
     A.4.   Relay Peers  . . . . . . . . . . . . . . . . . . . . . . 163
     A.5.   Symmetric Route Stability  . . . . . . . . . . . . . . . 164
   Appendix B.  Why Clients? . . . . . . . . . . . . . . . . . . . . 164
     B.1.   Why Not Only Peers?  . . . . . . . . . . . . . . . . . . 164
     B.2.   Clients as Application-Level Agents  . . . . . . . . . . 165
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 165




































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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.  It
   provides a generic, self-organizing overlay network service, allowing
   nodes to efficiently route messages to other nodes and to efficiently
   store and retrieve data in the overlay.  RELOAD provides several
   features that are critical for a successful P2P protocol for the
   Internet:


   Security Framework:  A P2P network will often be established among a
      set of peers that do not trust each other.  RELOAD leverages a
      central enrollment server to provide credentials for each peer
      which can then be used to authenticate each operation.  This
      greatly reduces the possible attack surface.

   Usage Model:  RELOAD is designed to support a variety of
      applications, including P2P multimedia communications with the
      Session Initiation Protocol [I-D.ietf-p2psip-sip].  RELOAD allows
      the definition of new application usages, each of which can define
      its own data types, along with the rules for their use.  This
      allows RELOAD to be used with new applications through a simple
      documentation process that supplies the details for each
      application.

   NAT Traversal:  RELOAD is designed to function in environments where
      many if not most of the nodes are behind NATs or firewalls.
      Operations for NAT traversal are part of the base design,
      including using ICE to establish new RELOAD or application
      protocol connections.

   High Performance Routing:  The very nature of overlay algorithms
      introduces a requirement that peers participating in the P2P
      network route requests on behalf of other peers in the network.
      This introduces a load on those other peers, in the form of
      bandwidth and processing power.  RELOAD has been defined with a
      simple, lightweight forwarding header, thus minimizing the amount
      of effort required by intermediate peers.

   Pluggable Overlay Algorithms:  RELOAD has been designed with an
      abstract interface to the overlay layer to simplify implementing a
      variety of structured (e.g., distributed hash tables) and
      unstructured overlay algorithms.  The idea here is that RELOAD
      provides a generic structure that should 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



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      messages efficiently within it.  This specification also defines
      how RELOAD is used with the Chord based DHT algorithm, which is
      mandatory to implement.  Specifying a default "must implement"
      overlay algorithm promotes interoperability, while extensibility
      allows selection of overlay algorithms optimized for a particular
      application.

   These properties were designed specifically to meet the requirements
   for a P2P protocol to support SIP.  This document defines the base
   protocol for the distributed storage and location service, as well as
   critical usages for NAT traversal and security.  The SIP Usage itself
   is described separately in [I-D.ietf-p2psip-sip].  RELOAD is not
   limited to usage by SIP and could serve as a tool for supporting
   other P2P applications with similar needs.

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 which, together with the specific
   overlay algorithm in use, determines its position in the graph and
   the set of nodes it connects to.  The figure below shows a trivial
   example which isn't drawn from any particular overlay algorithm, but
   was chosen for convenience of representation.

            +--------+              +--------+              +--------+
            | Node 10|--------------| Node 20|--------------| Node 30|
            +--------+              +--------+              +--------+
                |                       |                       |
                |                       |                       |
            +--------+              +--------+              +--------+
            | Node 40|--------------| Node 50|--------------| Node 60|
            +--------+              +--------+              +--------+
                |                       |                       |
                |                       |                       |
            +--------+              +--------+              +--------+
            | Node 70|--------------| Node 80|--------------| Node 90|
            +--------+              +--------+              +--------+
                                        |
                                        |
                                    +--------+
                                    | Node 85|
                                    |(Client)|
                                    +--------+

   Because the graph is not fully connected, when a node wants to send a
   message to another node, it may need to route it through the network.



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   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 storage rather
   than for bulk storage of large objects.  Records are stored under
   numeric addresses 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 all addresses between 81-90.  We
   refer to non-client nodes as peers.

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

1.2.  Architecture

   RELOAD is fundamentally an overlay network.  The following figure
   shows the layered RELOAD architecture.

















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            Application

        +-------+  +-------+
        | SIP   |  | XMPP  |  ...
        | Usage |  | Usage |
        +-------+  +-------+
    ------------------------------------ Messaging Service Boundary
    +------------------+     +---------+
    |     Message      |<--->| Storage |
    |    Transport     |     +---------+
    +------------------+           ^
           ^       ^               |
           |       v               v
           |     +-------------------+
           |     |    Topology       |
           |     |     Plugin        |
           |     +-------------------+
           |         ^
           v         v
        +------------------+
        |  Forwarding &    |
        | Link Management  |
        +------------------+
    ------------------------------------ Overlay Link Service Boundary
         +-------+  +------+
         |TLS    |  |DTLS  |  ...
         +-------+  +------+

   The major components of RELOAD are:


   Usage Layer:  Each application defines a RELOAD usage; a set of data
      Kinds and behaviors which describe how to use the services
      provided by RELOAD.  These usages all talk to RELOAD through a
      common Message Transport Service.

   Message Transport:  Handles end-to-end reliability, manages request
      state for the usages, and forwards Store and Fetch operations to
      the Storage component.  Delivers message responses to the
      component initiating the request.

   Storage:  The Storage component is responsible for processing
      messages relating to the storage and retrieval of data.  It talks
      directly to the Topology Plugin to manage data replication and
      migration, and it talks to the Message Transport component to send
      and receive messages.





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   Topology Plugin:  The Topology Plugin is responsible for implementing
      the specific overlay algorithm being used.  It uses the Message
      Transport component to send and receive overlay management
      messages, to the Storage component to manage data replication, and
      directly to the Forwarding Layer to control hop-by-hop message
      forwarding.  This component closely parallels conventional routing
      algorithms, but is more tightly coupled to the Forwarding Layer
      because there is no single "routing table" equivalent used by all
      overlay algorithms.

   Forwarding and Link Management Layer:  Stores and implements the
      routing table by providing packet forwarding services between
      nodes.  It also handles establishing new links between nodes,
      including setting up connections across NATs using ICE.

   Overlay Link Layer:  Responsible for actually transporting traffic
      directly between nodes.  Each such protocol includes the
      appropriate provisions for per-hop framing or hop-by-hop ACKs
      required by unreliable transports.  TLS [RFC5246] and DTLS
      [RFC4347] are the currently defined "link layer" protocols used by
      RELOAD for hop-by-hop communication.  New protocols can be
      defined, as described in Section 6.6.1 and Section 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.

   To further clarify the roles of the various layers, this figure
   parallels the architecture with each layer's role from an overlay
   perspective and implementation layer in the internet:





















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                | Internet Model  |
    Real        |   Equivalent    |          Reload
   Internet     |   in Overlay    |       Architecture
   -------------+-----------------+------------------------------------
                |                 |    +-------+  +-------+
                |  Application    |    | SIP   |  | XMPP  |  ...
                |                 |    | Usage |  | Usage |
                |                 |    +-------+  +-------+
                |                 |  ----------------------------------
                |                 |+------------------+     +---------+
                |   Transport     ||     Message      |<--->| Storage |
                |                 ||    Transport     |     +---------+
                |                 |+------------------+           ^
                |                 |       ^       ^               |
                |                 |       |       v               v
   Application  |                 |       |     +-------------------+
                |   (Routing)     |       |     |    Topology       |
                |                 |       |     |     Plugin        |
                |                 |       |     +-------------------+
                |                 |       |         ^
                |                 |       v         v
                |    Network      |    +------------------+
                |                 |    |  Forwarding &    |
                |                 |    | Link Management  |
                |                 |    +------------------+
                |                 |  ----------------------------------
   Transport    |      Link       |     +-------+  +------+
                |                 |     |TLS    |  |DTLS  |  ...
                |                 |     +-------+  +------+
   -------------+-----------------+------------------------------------
     Network    |
                |
       Link     |

1.2.1.  Usage Layer

   The top layer, called the Usage Layer, has application usages, such
   as the SIP Registration Usage [I-D.ietf-p2psip-sip], 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
   softphone application may also require a voicemail usage.  A usage



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   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 Plugin are themselves clients of
   the Message Transport, because they need to send and receive messages
   from other peers.

   The Message Transport Service is responsible for end-to-end
   reliability, 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), although as RELOAD supports
   different overlay algorithms (including non-DHT overlay algorithms)
   that calculate keys in different ways, the actual interface must
   accept Resource Names rather than actual keys.




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   Stability of the underlying network supporting the overlay (the
   Internet) and congestion control between overlay neighbors, which
   exchange routing updates and data replicas in addition to forwarding
   end-to-end messages, is handled by the Forwarding and Link Management
   layer described below.

   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, more appropriate means of
   improving network performance, such as the Topology Plugin removing
   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 to allow nodes to store data
   in the overlay and to retrieve data stored by other nodes or by
   themselves.  The Storage component is responsible for processing data
   storage and retrieval messages.  For instance, the Storage component
   might receive a Store request for a given resource from the Message
   Transport.  It would then query the appropriate usage before storing
   the data value(s) in its local data store and sending a response to
   the Message Transport for delivery to the requesting node.
   Typically, these messages will come from other 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 Plugin when the Resource-IDs for which it is
   responsible change, and the Storage component is then responsible for
   migrating resources to other peers, as required.

1.2.4.  Topology Plugin

   RELOAD is explicitly designed to work with a variety of overlay
   algorithms.  In order to facilitate this, the overlay algorithm
   implementation is provided by a Topology Plugin so that each overlay
   can select an appropriate overlay algorithm that relies on the common
   RELOAD core protocols and code.



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   The Topology Plugin is responsible for maintaining the overlay
   algorithm Routing Table, which is consulted by the Forwarding and
   Link Management Layer before routing a message.  When connections are
   made or broken, the Forwarding and Link Management Layer notifies the
   Topology Plugin, which adjusts the routing table as appropriate.  The
   Topology Plugin will also instruct the Forwarding and Link Management
   Layer to form new connections as dictated by the requirements of the
   overlay algorithm Topology.  The Topology Plugin issues periodic
   update requests through Message Transport to maintain and update its
   Routing Table.

   As peers enter and leave, resources may be stored on different peers,
   so the Topology Plugin also keeps track of which peers are
   responsible for which resources.  As peers join and leave, the
   Topology Plugin instructs the Storage component to issue resource
   migration requests as appropriate, in order to ensure that other
   peers have whatever resources they are now responsible for.  The
   Topology Plugin is also responsible for providing 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.

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 Plugin.  This
   Layer establishes and maintains the network connections as required
   by the Topology Plugin.  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 relationship between this layer and the Message
   Transport Layer is similar to the relationship between link-level
   congestion control and retransmission in modern wireless networks is
   to Internet transport protocols.

   This layer provides a generic interface that allows the topology
   plugin to control the overlay and resource operations and messages.
   Since each overlay algorithm is defined and functions differently, we
   generically refer to the table of other peers that the overlay
   algorithm maintains and uses to route requests (neighbors) as a
   Routing Table.  The Topology Plugin actually owns the Routing Table,
   and forwarding decisions are made by querying the Topology Plugin for
   the next hop for a particular Node-ID or Resource-ID.  If this node
   is the destination of the message, the message is delivered to the



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   Message Transport.

   This layer also utilizes a framing header to encapsulate messages as
   they are forwarding 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:

   Connection Level:    Connections between peers are secured with TLS,
      DTLS, or potentially some to be defined future protocol.
   Message Level:    Each RELOAD message is signed.
   Object Level:    Stored objects is signed by the creating peer.

   These three levels of security work together to allow peers to verify
   the origin and correctness of data they receive from other peers,
   even in the face of malicious activity by other peers in the overlay.
   RELOAD also provides access control built on top of these
   communications security features.  Because the peer responsible for
   storing a piece of data can validate the signature on the data being
   stored, the responsible peer can determine whether a given operation
   is permitted or not.

   RELOAD also provides an optional shared secret based admission
   control feature using shared secrets and TLS-PSK.  In order to form a
   TLS connection to any node in the overlay, a new node needs to know
   the shared overlay key, thus restricting access to authorized users
   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.







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1.4.  Structure of This Document

   The remainder of this document is structured as follows.

   o  Section 2 provides definitions of terms used in this document.
   o  Section 3 provides an overview of the mechanisms used to establish
      and maintain the overlay.
   o  Section 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 Usage that is fundamental
      to RELOAD security.
   o  Section 9 defines the TURN Server Usage needed to locate TURN
      servers for NAT traversal.
   o  Section 10 defines a specific Topology Plugin using 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.  Terminology

   Terms used in this document are defined inline when used and are also
   defined below for reference.


   DHT:  A distributed hash table.  A DHT is an abstract hash table
      service realized by storing the contents of the hash table across
      a set of peers.

   Overlay Algorithm:  An overlay algorithm defines the rules for
      determining which peers in an overlay store a particular piece of
      data and for determining a topology of interconnections amongst
      peers in order to find a piece of data.

   Overlay Instance:  A specific overlay algorithm and the collection of
      peers that are collaborating to provide read and write access to
      it.  There can be any number of overlay instances running in an IP
      network at a time, and each operates in isolation of the others.








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   Peer:  A host that is participating in the overlay.  Peers are
      responsible for holding some portion of the data that has been
      stored in the overlay and also route messages on behalf of other
      hosts as required by the Overlay Algorithm.

   Client:  A host that is able to store data in and retrieve data from
      the overlay but which is not participating in routing or data
      storage for the overlay.

   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.

   Node:  We use the term "Node" to refer to a host that may be either a
      Peer or a Client.  Because RELOAD uses the same protocol for both
      clients and peers, much of the text applies equally to both.
      Therefore we use "Node" when the text applies to both Clients and
      Peers and the more specific term (i.e. client or peer) when the
      text applies only to Clients or only to Peers.

   Node-ID:  A fixed-length value that uniquely identifies a node.
      Node-IDs of all 0s and all 1s are reserved and are invalid Node-
      IDs.  A value of zero is not used in the wire protocol but can be
      used to indicate an invalid node in implementations and APIs.  The
      Node-ID of all 1s is used on the wire protocol as a wildcard.

   Joining Peer:  A node that is attempting to become a Peer in a
      particular Overlay.

   Admitting Peer:  A Peer in the Overlay which helps the Joining Peer
      join the Overlay.

   Bootstrap Node:  A network node used by Joining Peers to help locate
      the Admitting Peer.

   Peer Admission:  The act of admitting a peer (the "Joining Peer" )
      into an Overlay.  After the admission process is over, the joining
      peer is a fully-functional peer of the overlay.  During the
      admission process, the joining peer may need to present
      credentials to prove that it has sufficient authority to join the
      overlay.

   Resource:  An object or group of objects associated with a string
      identifier.  See "Resource Name" below.






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   Resource Name:  The potentially human readable name by which a
      resource is identified.  In unstructured P2P networks, the
      resource name is sometimes used directly as a Resource-ID.  In
      structured P2P networks the resource name is typically mapped into
      a Resource-ID by using the string as the input to hash function.
      Structured and unstructured P2P networks are described in
      [RFC5694].  A SIP resource, for example, is often identified by
      its AOR which is an example of a Resource Name.

   Resource-ID:  A value that identifies some resources and which is
      used as a key for storing and retrieving the resource.  Often this
      is not human friendly/readable.  One way to generate a Resource-ID
      is by applying a mapping function to some other unique name (e.g.,
      user name or service name) for the resource.  The Resource-ID is
      used by the distributed database algorithm to determine the peer
      or peers that are responsible for storing the data for the
      overlay.  In structured P2P networks, Resource-IDs are generally
      fixed length and are formed by hashing the resource name.  In
      unstructured networks, resource names may be used directly as
      Resource-IDs and may be variable lengths.

   Connection Table:  The set of nodes to which a node is directly
      connected.  This includes nodes with which Attach handshakes have
      been done but which have not sent any Updates.

   Routing Table:  The set of peers which a node can use to route
      overlay messages.  In general, these peers will all be on the
      connection table but not vice versa, because some peers will have
      Attached but not sent updates.  Peers may send messages directly
      to peers that are in the connection table but may only route
      messages to other peers through peers that are in the routing
      table.

   Destination List:  A list of IDs through which a message is to be
      routed, in strict order.  A single Node-ID or a Resource-ID is a
      trivial form of destination list.  When multiple Node-IDs are
      specified (no more than one Resource-ID is permitted, and it MUST
      be the last entry) a Destination List is a loose source route.

   Usage:  A usage is an application that wishes to use the overlay for
      some purpose.  Each application wishing to use the overlay defines
      a set of data Kinds that it wishes to use.  The SIP usage defines
      the location data Kind.







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   Transaction ID:  A randomly chosen identifier selected by the
      originator of a request and used to correlate requests and
      responses.

   The term "maximum request lifetime" is the maximum time a request
   will wait for a response; it defaults to 15 seconds.  The term
   "successor replacement hold-down time" is the amount of time to wait
   before starting replication when a new successor is found; it
   defaults to 30 seconds.


3.  Overlay Management Overview

   The most basic function of RELOAD is as a generic overlay network.
   Nodes need to be able to join the overlay, form connections to other
   nodes, and route messages through the overlay to nodes to which they
   are not directly connected.  This section provides an overview of the
   mechanisms that perform these functions.

3.1.  Security and Identification

   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
   must be retrieved securely.  The initial configuration document is
   either initially fetched over HTTPS or manually provisioned;
   subsequent configuration document updates are received either by
   periodically refreshing from 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 tracing back to the initial
   configuration document.

   Every node in the RELOAD overlay is identified by a Node-ID.  The
   Node-ID is used for three major purposes:

   o  To address the node itself.
   o  To determine its position in the overlay topology when the overlay
      is structured.
   o  To determine the set of resources for which the node is
      responsible.

   Each node has a certificate [RFC5280] containing a Node-ID, which is
   unique within an overlay instance.

   The certificate serves multiple purposes:




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   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 (D)TLS
      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.

   In some settings, a group of users want to set up an overlay network
   but are not concerned about attack by other users in the network.
   For instance, users on a LAN might want to set up a short term ad hoc
   network without going to the trouble of setting up an enrollment
   server.  RELOAD supports the use of self-generated, self-signed
   certificates.  When self-signed certificates are used, the node also
   generates its own Node-ID and username.  The Node-ID is computed as a
   digest of the public key, to prevent Node-ID theft.  Note that the
   relevant cryptographic property for the digest is preimage
   resistance.  Collision-resistance is not required since an attacker
   who can create two nodes with the same Node-ID but different public
   key obtains no advantage.  This model is still subject to a number of
   known attacks (most notably Sybil attacks [Sybil]) and can only be
   safely used in closed networks where users are mutually trusting.
   Another drawback of this approach is that user's data is then tied to
   their keys, so if a key is changed any data stored under their
   Node-ID must then be re-stored.  This is not an issue for centrally-
   issued Node-IDs provided that the CA re-issues the same Node-ID when
   a new certificate is generated.

   The general principle here is that the security mechanisms (TLS and
   message signatures) are always used, even if the certificates are



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   self-signed.  This allows for a single set of code paths in the
   systems with the only difference being whether certificate
   verification is required to chain to a single root of trust.

3.1.1.  Shared-Key Security

   RELOAD also provides an admission control system based on shared
   keys.  In this model, the peers all share a single key which is used
   to authenticate the peer-to-peer connections via TLS-PSK/TLS-SRP.

3.2.  Clients

   RELOAD defines a single protocol that is used both as the peer
   protocol and as the client protocol for the overlay.  This simplifies
   implementation, particularly for devices that may act in either role,
   and allows clients to inject messages directly into the overlay.

   We use the term "peer" to identify a node in the overlay that routes
   messages for nodes other than those to which it is directly
   connected.  Peers 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."

   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 a first-class
   entity.  From the perspective of a peer, a client is simply a node
   which has not yet sent any Updates or Joins.  It might never do so
   (if it's a client) or it might eventually do so (if it's just a node
   that's 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 must know that it intends to be a
   client, but this localizes complexity only to that node.

   For more discussion of the motivation for RELOAD's client support,
   see Appendix B.







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3.2.1.  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 relay in
      cases where NATs or firewalls prevent a client from forming a
      direct connections with its responsible peer.  Note that clients
      that choose this option need to process Update messages from the
      peer.  Those updates can indicate that the peer no longer is
      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 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 receive incoming
      requests from other members of the overlay, the Destination List
      required to reach it must 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 MAY
      proceed to use the connection without performing an Attach.  A
      client wishing to connect using this mechanism with a certificate
      with multiple Node-IDs can use a Ping to probe the Node-ID of the
      node to which it is connected before doing the Attach.

3.2.2.  Minimum Functionality Requirements for Clients

   A node may act as a client simply because it does not have the
   resources or even an implementation of the topology plugin required
   to act as a peer in the overlay.  In order to exchange RELOAD
   messages with a peer, a client MUST meet a minimum level of
   functionality.  Such a client MUST:

   o  Implement RELOAD's connection-management 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.





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   o  Possess security credentials required by the overlay it is
      implementing.

   A client speaks the same protocol as the peers, knows how to
   calculate Resource-IDs, and signs its requests in the same manner as
   peers.  While a client does not necessarily require a full
   implementation of the overlay algorithm, calculating the Resource-ID
   requires an implementation of the appropriate algorithm for the
   overlay.

3.3.  Routing

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

   RELOAD's routing provides the following capabilities:

   Resource-based routing:    RELOAD supports routing messages based
      soley 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 Plugins.
   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, located via either of the
      mechanisms described above.
   Bridging overlays:    Similar to how a Destination List is used to
      reach a client attached via an arbitrary peer, RELOAD can route
      messages between two different overlays by building a destination
      list that includes a peer (or client) with connectivity to both
      networks.
   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 re-route 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.



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   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 must flow 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.
   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.

   The basic routing mechanism used by RELOAD 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
   is then the same path followed in reverse.  For example, a message
   following a route from A to Z through B and X:

   A         B         X         Z
   -------------------------------

   ---------->
   Dest=Z
            ---------->
            Via=A
            Dest=Z
                      ---------->
                      Via=A,B
                      Dest=Z


                      <----------
                       Dest=X,B,A
            <----------
               Dest=B,A
   <----------
        Dest=A

   Note that the preceding Figure does not indicate whether A is a



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   client or peer:  A forwards its request to B and the response is
   returned to A in the same manner regardless of A's role in the
   overlay.

   This figure shows use of full via-lists by intermediate peers B and
   X. However, if B and/or X are willing to store state, then they may
   elect to truncate the lists, save that information internally (keyed
   by the transaction id), and return the response message along the
   path from which it was received when the response is received.  This
   option requires greater state 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
   -------------------------------

   ---------->
   Dest=Z
            ---------->
            Via=A
            Dest=Z
                      ---------->
                      Via=X1
                      Dest=Z

                      <----------
                        Dest=X,X1
              <----------
                 Dest=B,A
   <----------
        Dest=A

   As before, when B receives the message, he creates 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 forwards to Z with only X1 as the via list.
   When the response arrives at X, it maps X1 back to [A, B] and then
   inverts it to produce the new destination list [B, A] and routes it
   to B.

   RELOAD also supports a basic Iterative routing mode (where the
   intermediate peers merely return a response indicating the next hop,
   but do not actually forward the message to that next hop themselves).
   Iterative routing is implemented using the RouteQuery method, which
   requests this behavior.  Note that iterative routing is selected only



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   by the initiating node.

3.4.  Connectivity Management

   In order to provide efficient routing, a peer needs to maintain a set
   of direct connections to other peers in the Overlay Instance.  Due to
   the presence of NATs, these connections often cannot be formed
   directly.  Instead, we use the Attach request to establish a
   connection.  Attach uses ICE [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. It
   gathers ICE candidates and packages them up in an Attach request
   which it sends to B through usual overlay routing procedures.  B does
   its own candidate gathering and sends back a response with its
   candidates.  A and B then do ICE connectivity checks on the candidate
   pairs.  The result is a connection between A and B. At this point, A
   and B can add each other to their routing tables and send messages
   directly between themselves without going through other overlay
   peers.

   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, some 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 node connects to a node at an
   arbitrary IP address, rather than to its responsible peer, as
   described in the second bullet point of Section 3.2.1.

   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 depend on the specific overlay).  If a
   peer cannot form a connection to some other peer, this isn't
   necessarily a disaster; overlays can route correctly even without
   fully connected links.  However, a peer should try to maintain the
   specified link set and if it detects that it has fewer direct
   connections, should form more as required.  This also implies that
   peers need to periodically verify that the connected peers are still
   alive and if not try to reform the connection or form an alternate
   one.

3.5.  Overlay Algorithm Support

   The Topology Plugin allows RELOAD to support a variety of overlay
   algorithms.  This 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



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   to implement this DHT is fully contained in this specification but it
   is easier to understand if you are familiar with Chord [Chord] based
   DHTs.  A nice tutorial can be found at [wikiChord].

3.5.1.  Support for Pluggable Overlay Algorithms

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

3.5.2.  Joining, Leaving, and Maintenance Overview

   When a new peer wishes to join the Overlay Instance, it MUST have a
   Node-ID that it is allowed to use and a set of credentials which
   match that Node-ID.  When an enrollment server is used that Node-ID
   will be in the certificate the node received from the enrollment
   server.  The details of the joining procedure are defined by the
   overlay algorithm, but the general steps for joining an Overlay
   Instance are:

   o  Forming connections to some other peers.
   o  Acquiring the data values this peer is responsible for storing.
   o  Informing the other peers which were previously responsible for
      that data that this peer has taken over responsibility.

   The first thing the peer needs to do is to form a connection to some
   "bootstrap node".  Because this is the first connection the peer
   makes, these nodes MUST have 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.  Once a peer has
   connected to the overlay for the first time, it can cache the set of
   past adjacencies which have public IP address and attempt to use them
   as future bootstrap nodes.  Note that this requires some notion of
   which addresses are likely to be public as discussed in Section 9.

   Once 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  Forming connections to other peers in the overlay to populate its
      Routing Table.




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   o  Getting a copy of the data it is now responsible for storing and
      assuming responsibility for that data.

   The second operation is performed by contacting the Admitting Peer
   (AP), the node which is currently responsible for that section of the
   overlay.

   The details of this operation depend mostly on the overlay algorithm
   involved, but a typical case would be:

   1.  JP (Joining Peer) sends a Join request to AP (Admitting Peer)
       announcing its intention to join.
   2.  AP sends a Join response.
   3.  AP does a sequence of Stores to JP to give it the data it will
       need.
   4.  AP does Updates to JP and to other peers to tell it about its own
       routing table.  At this point, both JP and AP consider JP
       responsible for some section of the Overlay Instance.
   5.  JP makes its own connections to the appropriate peers in the
       Overlay Instance.

   After this process is completed, JP is a full member of the Overlay
   Instance and can process Store/Fetch requests.

   Note that the first node is a special case.  When ordinary nodes
   cannot form connections to the bootstrap nodes, then they are not
   part of the overlay.  However, the first node in the overlay can
   obviously not connect to other nodes.  In order to support this case,
   potential first nodes (which must also serve as bootstrap nodes
   initially) must somehow be instructed (perhaps by configuration
   settings) that they are the entire overlay, rather than not part of
   it.

   Note that clients do not perform either of these operations.

3.6.  First-Time Setup

   Previous sections addressed how RELOAD works once a node has
   connected.  This section provides an overview of how users get
   connected to the overlay for the first time.  RELOAD is designed so
   that users can start with the name of the overlay they wish to join
   and perhaps a username and password, and leverage that into having a
   working peer with minimal user intervention.  This helps avoid the
   problems that have been experienced with conventional SIP clients
   where users are required to manually configure a large number of
   settings.





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3.6.1.  Initial Configuration

   In the first phase of the process, the user starts out with the name
   of the overlay and uses this to download an initial set of overlay
   configuration parameters.  The node does a DNS SRV lookup on the
   overlay name to get the address of a configuration server.  It can
   then connect to this server with HTTPS [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 expected domain name for HTTPS is the name of
   the overlay.

   If a node already has the valid configuration document that it
   received by some out of band method, this step can be skipped.  Note
   that that out of band method MUST provide authentication and
   integrity, because the configuration document contains the trust
   anchors for the system.

3.6.2.  Enrollment

   If the overlay is using centralized enrollment, then a user needs to
   acquire a certificate before joining the overlay.  The certificate
   attests both to the user's name within the overlay and to the Node-
   IDs which they are permitted to operate.  In that case, the
   configuration document will contain the address of an enrollment
   server which can be used to obtain such a certificate.  The
   enrollment server may (and probably will) require some sort of
   username and password before issuing the certificate.  The enrollment
   server's ability to restrict attackers' access to certificates in the
   overlay is one of the cornerstones of RELOAD's security.

3.6.3.  Diagnostics

   Significant advice around managing a RELAOD overlay and extensions
   for diagnostics are described in [I-D.ietf-p2psip-diagnostics].


4.  RFC 2119 Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].


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



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   variety of purposes:

   o  To store data in the overlay and retrieve data stored by other
      nodes.
   o  As a discovery mechanism for services such as TURN.
   o  To form direct connections which can be used to transmit
      application-level messages 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, SIP registration).  Similarly, there may be
   multiple elements of a given Kind, as shown below:

                      +--------------------------------+
                      |            Resource-ID         |
                      |                                |
                      | +------------+  +------------+ |
                      | |   Kind 1   |  |   Kind 2   | |
                      | |            |  |            | |
                      | | +--------+ |  | +--------+ | |
                      | | | Value  | |  | | Value  | | |
                      | | +--------+ |  | +--------+ | |
                      | |            |  |            | |
                      | | +--------+ |  | +--------+ | |
                      | | | Value  | |  | | Value  | | |
                      | | +--------+ |  | +--------+ | |
                      | |            |  +------------+ |
                      | | +--------+ |                 |
                      | | | Value  | |                 |
                      | | +--------+ |                 |
                      | +------------+                 |
                      +--------------------------------+

   Each Kind is identified by a Kind-ID, which is a code point 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; some sets may be allowed
   to contain multiple identical items while others may only have unique
   items.  Note that a Kind may be employed by multiple usages and new
   usages are encouraged to use previously defined Kinds where possible.
   We define the following data models in this document, though other
   usages can define their own structures:



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   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.

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 is required to store for other nodes.  RELOAD
   addresses this issue by only allowing any given node to store data at
   a small number of locations in the overlay, with those locations
   being determined by the node's certificate.  When a peer uses a Store
   request to place data at a location authorized by its certificate, it
   signs that data with the private key that corresponds to its
   certificate.  Then the peer responsible for storing the data is able
   to verify that the peer issuing the request is authorized to make
   that request.  Each data Kind defines the exact rules for determining
   what certificate is appropriate.

   The most natural rule is that a certificate authorizes a user to
   store data keyed with their user name X. This rule is used for all
   the Kinds defined in this specification.  Thus, only a user with a
   certificate for "alice@example.org" could write to that location in
   the overlay.  However, other usages can define any rules they choose,
   including publicly writable values.

   The digital signature over the data serves two purposes.  First, it
   allows the peer responsible for storing the data to verify that this
   Store is authorized.  Second, it provides integrity for the data.
   The signature is saved along with the data value (or values) so that
   any reader can verify the integrity of the data.  Of course, the
   responsible peer can "lose" the value but it cannot 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



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   configuration to limit size imbalance 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,
   or 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 are 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 should the responsible peer fail [Chord].

   If additional replication is needed, for example if data persistence
   is particularly important for a particular usage, then that usage may
   specify additional replication, such as implementing random
   replications by inserting a different well known constant into the
   Resource Name used to store each replicated copy of the resource.
   Such replication strategies can be added independent of the
   underlying algorithm, and their usage can be determined based on the
   needs of the particular usage.

5.2.  Usages

   By itself, the distributed storage layer just provides infrastructure
   on which applications are built.  In order to do anything useful, a
   usage must be defined.  Each Usage needs to specify several things:

   o  Registers Kind-ID code points for any Kinds that the Usage
      defines.
   o  Defines the data structure for each of the Kinds.
   o  Defines access control rules for each of the Kinds.
   o  Defines how the Resource Name is formed that is hashed to form the
      Resource-ID where each Kind is stored.





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   o  Describes how values will be merged after a network partition.
      Unless otherwise specified, the default merging rule is to act as
      if all the values that need to be merged were stored and 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 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 Kinds defined by a usage may also be applied to other usages.
   However, a need for different parameters, such as different size
   limits, would imply the need to create a new Kind.

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] or [I-D.ietf-p2psip-service-discovery].

5.4.  Application Connectivity

   There is no requirement that a RELOAD usage must use RELOAD's
   primitives for establishing its own communication if it already
   possesses its own means of establishing connections.  For example,
   one could design a RELOAD-based resource discovery protocol which
   used HTTP to retrieve the actual data.

   For more common situations, however, 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
   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 UDP or TCP flow used only for the SIP dialog.


6.  Overlay Management Protocol

   This section defines the basic protocols used to create, maintain,



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   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.  We then define the
   message structure and then finally define 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 (e.g., the message
   is for an overlay in which the peer does not participate) 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
   messages 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.

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) sub-cases
   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 and it verify the signature and pass it up to the upper
      layers.



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   o  If the entry is a Node-ID which equals this node's Node-ID, then
      the message is destined for this node.  If this is the only entry
      on the destination list, the message is destined for this node and
      so the node passes it up 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, the message is destined for
      this node and it passes it up to the upper layers.
   o  If the entry is a Node-ID which 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 later case, it MUST forward the message
      to the destination node as described in the next section.

   Note that this implies that in order to address a message to "the
   peer that controls region X", a sender sends to Resource-ID X, not
   Node-ID X.

6.1.2.  Other ID

   If neither of the other three cases applies, then the peer MUST
   forward the message towards the first entry on the destination list.
   This means that it MUST select one of the peers to which it is
   connected and which is likely to be responsible for the first entry
   on the destination list.  If the first entry on the destination list
   is in the peer's connection table, then it SHOULD forward the message
   to that peer directly.  Otherwise, 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.  There are two major ways of accomplishing this:

   o  The peer can add an entry to the via list in the forwarding header
      that will enable it to determine the correct node.
   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



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   contain (C, B, A).

   As an example of the second strategy, if node D receives a message
   from node C with transaction ID X and via list (A, B), it could store
   (X, C) in its state database and forward the message with the via
   list unchanged.  When D receives the response, it consults its state
   database for transaction id X, determines that the request came from
   C, and forwards the response to C.

   Intermediate 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 a 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 a opaque id include a
   compressed version of the original via list or 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 or 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 extension, such as
   [I-D.jiang-p2psip-relay], may define mechanisms for determining when
   this state does not need to be retained.

   None of the above mechanisms are required for responses, since there
   is no need to ensure that subsequent requests follow the same path.

   To be precise on the responsibility of the intermediate node, suppose
   that an intermediate node, A, receives a message from node B with via
   list X-Y-Z.  Node A MUST implement an algorithm that ensures that A



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   returns a response to this request to node B with the destination
   list B-Z-Y-X, provided that the node to which A forwards the request
   follows the same contract.  Node A normally learns the Node-ID B is
   using via an Attach, but a node using a certificate with a single
   Node-ID MAY elect to not send an Attach (see Section 3.2.1 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 in the destination list is an opaque id (e.g., a
   compressed via list), the peer MUST replace that 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.

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.  I.e., a node in an overlay which supports SRR and
   routing algorithm XXX might use SRR some of the time and XXX some of
   the time.

6.2.1.  Request Origination

   In order to originate a message to a given Node-ID or Resource-ID, a
   node constructs an appropriate destination list.  The simplest such
   destination list is a single entry containing the Node-ID or
   Resource-ID.  The resulting message uses the normal overlay routing
   mechanisms to forward the message to that destination.  The node can
   also construct a more complicated destination list for source
   routing.

   Once the message is constructed, the node sends the message to some
   adjacent peer.  If the first entry on the destination list is
   directly connected, then the message MUST be routed down that
   connection.  Otherwise, the topology plugin MUST be consulted to
   determine the appropriate next hop.

   Parallel requests for a resource are a common solution to improve



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   reliability in the face of churn or of 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 known to be sub-optimal, to improve
   reliability [vulnerabilities-acsac04].  Such parallel searches MAY be
   specified by the topology plugin, in which case it would return
   multiple next hops and the request would be routed to all of them.

   Because messages may be lost in transit through the overlay, RELOAD
   incorporates an end-to-end reliability mechanism.  When an
   originating node transmits a request it MUST set a timer to the
   current overlay-reliability-timer.  If a response has not been
   received when the timer fires, the request is retransmitted with the
   same transaction identifier.  The request MAY be retransmitted up to
   4 times (for a total of 5 messages).  After the timer for the fifth
   transmission fires, the message SHALL be considered to have failed.
   Note that this retransmission procedure is not followed by
   intermediate nodes.  They follow the hop-by-hop reliability procedure
   described in Section 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 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).

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 fields to allow for extensibility.  However,
   for the parts of a structure that were required in all messages, we
   just define these in a fixed position, as adding a type and length
   for them is unnecessary and would simply increase bandwidth and



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   introduces new potential for interoperability issues.

   Each message has three parts, concatenated as shown below:

     +-------------------------+
     |    Forwarding Header    |
     +-------------------------+
     |    Message Contents     |
     +-------------------------+
     |     Security Block      |
     +-------------------------+


   The contents of these parts are as follows:

   Forwarding Header:  Each message has a generic header which is used
      to forward the message between peers and to its final destination.
      This header is the only information that an intermediate peer
      (i.e., one that is not the target of a message) needs to examine.

   Message Contents:  The message being delivered between the peers.
      From the perspective of the forwarding layer, the contents are
      opaque, however, they are interpreted by the higher layers.

   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.

   The following sections describe the format of each part of the
   message.

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 familiar enough looking that most readers can grasp it quickly.
   o  The ability to define nested structures allows a separation
      between high-level and low-level message structures.
   o  It has a straightforward wire encoding that allows quick
      implementation, but the structures can be comprehended without
      knowing the encoding.
   o  The ability to mechanically compile encoders and decoders.

   Several idiosyncrasies of this language are worth noting.




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   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.

6.3.1.1.  Common Definitions

   The following definitions are used throughout RELOAD and so are
   defined here.  They also provide a convenient introduction to how to
   read the presentation language.

   An enum represents an enumerated type.  The values associated with
   each possibility are represented in parentheses and the maximum value
   is represented as a nameless value, for purposes of describing the
   width of the containing integral type.  For instance, Boolean
   represents a true or false:

         enum { false (0), true(1), (255)} Boolean;


   A boolean value is either a 1 or a 0.  The max value of 255 indicates
   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:  the use of
   "typedef" here is an extension to the TLS language, but its meaning
   should be relatively obvious.  Note 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



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   NodeIds, ResourceIds are variable length, up to 254 bytes (2040 bits)
   in length.  On the wire, each ResourceId is preceded by a single
   length byte (allowing lengths up to 255).  Thus, the 3-byte value
   "FOO" would be encoded as:  03 46 4f 4f.  Note the < range > syntax
   defines a variable length element that does include 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 {reservedAddr(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:





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   type:  the type of address (v4 or v6).
   length:  the length of the rest of the structure.

   By having the type and the length appear at the beginning of the
   structure regardless of the kind of address being represented, an
   implementation which does not understand new address type X can still
   parse the IpAddressPort field and then discard it if it is not
   needed.

   The rest of the IpAddressPort structure is either an IPv4AddrPort or
   an IPv6AddrPort.  Both of these simply consist of an address
   represented as an integer and a 16-bit port.  As an example, here is
   the wire representation of the IPv4 address "192.0.2.1" with port
   "6100".

             01           ; type    = IPv4
             06           ; length  = 6
             c0 00 02 01  ; address = 192.0.2.1
             17 d4        ; port    = 6100

   Unless a given structure that uses a select explicitly allows for
   unknown types in the select, any unknown type SHOULD be treated as an
   parsing error and the whole message 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];
          ForwardingOptions  options[options_length];
        } ForwardingHeader;



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   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 charters what would be
      allowable as a DNS name.

   configuration_sequence:  The sequence number of the configuration
      file.

   version:  The version of the RELOAD protocol being used.  This is a
      fixed point integer between 0.1 and 25.4.  This document describes
      version 0.1, with a value of 0x01. [[ Note to RFC Editor:  Please
      update this to version 1.0 with value of 0x0a and remove this
      note. ]]

   ttl:  An 8 bit field indicating the number of iterations, or hops, a
      message can experience before it is discarded.  The TTL value MUST
      be decremented by one at every hop along the route the message
      traverses 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 and a
      "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 message with a TTL greater than the current value of
      initial-ttl (or the 100 default) 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 to 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








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   length:  The count in bytes of the size of the message, including the
      header.

   transaction_id:  A unique 64 bit number that identifies this
      transaction and also 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
      they correspond to.  Transaction IDs are also used for fragment
      reassembly.

   max_response_length:  The maximum size in bytes of a response.  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.

   destination_list:  The destination_list contains a sequence of
      destinations which the message should pass through.  The
      destination list is constructed by the message originator.  The
      first element in the destination list is where the message goes
      next.  The list shrinks as the message traverses each listed peer.

   options:  Contains a series of ForwardingOptions entries.  See
      Section 6.3.2.3.

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 has a
   sequence number which is monotonically increasing mod 65536.  Because



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   the sequence number may in principle wrap, greater than or less than
   are interpreted by modulo arithmetic as in TCP.

   When a destination node receives a request, it MUST check that the
   configuration_sequence field is equal to its own configuration
   sequence number.  If they do not match, it MUST generate an error,
   either Error_Config_Too_Old or Error_Config_Too_New. In addition, if
   the configuration file in the request is too old, it MUST generate a
   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 0xffff, 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 out a new configuration.

6.3.2.2.  Destination and Via Lists

   The destination list and via lists are sequences of Destination
   values:

     enum {reserved(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 */



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     } Destination;


   If a destination structure has its first bit set to 1, then it is a
   16 bit integer.  If the first bit is not set, then it is a structure
   starting with DestinationType.  If it is a 16 bit integer, it is
   treated as if it were a full structure with a DestinationType of
   opaque_id_type and a opaque_id that was 2 bytes long with the value
   of the 16 bit integer.  When the destination structure is not a 16
   bit integer, it is the TLV structure with the following contents:

   type
      The type of the DestinationData Payload Data Unit (PDU).  This may
      be one of "node", "resource", or "opaque_id_type".

   length
      The length of the destination_data.

   destination_data
      The destination value itself, which is an encoded DestinationData
      structure, depending on the value of "type".

   Note:  This structure encodes a type, length, value.  The length
      field specifies the length of the DestinationData values, which
      allows the addition of new DestinationTypes.  This allows an
      implementation which does not understand a given DestinationType
      to skip over it.

   A DestinationData can be one of three types:

   node
      A Node-ID.

   opaque
      A compressed list of Node-IDs and/or resources.  Because this
      value was compressed by one of the peers, it is only meaningful to
      that peer and cannot be decoded by other peers.  Thus, it is
      represented as an opaque string.

   resource
      The Resource-ID of the resource which is desired.  This type MUST
      only appear in the final location of a destination list and MUST
      NOT appear in a via list.  It is meaningless to try to route
      through a resource.

   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



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   connection table entry has changed, the identifier SHOULD be split
   between an index and a generation counter for that index.  At
   startup, the generation counters should be initialized to random
   values.  An implementation could 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 node, 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 node
   confirms that the generation counter matches the current generation
   counter of that index before forwarding the message.  If it does not
   match, the message is silently dropped.

6.3.2.3.  Forwarding Options

   The Forwarding header can be extended with forwarding header options,
   which are a series of ForwardingOptions structures:

    enum { reservedForwarding(0), (255) }
      ForwardingOptionsType;

    struct {
      ForwardingOptionsType     type;
      uint8                     flags;
      uint16                    length;
      select (type) {
            /* This type may be extended */
      } option;
    } ForwardingOption;


   Each ForwardingOption consists of the following values:


   type
      The type of the option.  This structure allows for unknown options
      types.

   length
      The length of the rest of the structure.








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   flags
      Three flags are defined FORWARD_CRITICAL(0x01),
      DESTINATION_CRITICAL(0x02), and RESPONSE_COPY(0x04).  These flags
      MUST NOT be set in a response.  If the FORWARD_CRITICAL flag is
      set, any node that would forward the message but does not
      understand this options MUST reject the request with an
      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.

   option
      The option value.

6.3.3.  Message Contents Format

   The second major part of a RELOAD message is the contents part, which
   is defined by MessageContents:

   enum { reservedMessagesExtension(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>;
     MessageExtensions      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.







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      0  Reserved

      1 .. 0x7fff  Requests and responses.  These code points are always
         paired, with requests being odd and the corresponding response
         being the request code plus 1.  Thus, "probe_request" (the
         Probe request) has value 1 and "probe_answer" (the Probe
         response) has value 2

      0xffff  Error
      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.
   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 must 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 (extension-dependent).

6.3.3.1.  Response Codes and Response Errors

   A peer processing a request returns its status in the message_code
   field.  If the request was a success, then the message code is the
   response code that matches the request (i.e., the next code up).  The
   response payload is then as defined in the request/response
   descriptions.

   If the request has failed, then the message code is set to 0xffff
   (error) and the payload MUST be an error_response PDU, as shown



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   below.

   When the message code is 0xffff, the payload MUST be an
   ErrorResponse.

         public struct {
           uint16             error_code;
           opaque             error_info<0..2^16-1>;
         } ErrorResponse;


   The contents of this structure are as follows:


   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 providing further information
      about what went wrong.  Developers are encouraged to put enough
      diagnostic information to be useful in error_info.

   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 peer cannot be found or does not
      exist.

   Error_Request_Timeout:  A response to the request has not been
      received in a suitable amount of time.  The requesting 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.






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   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 peer receiving the request
      with a forwarding options flagged as critical but the peer does
      not support this option.  See section Section 6.3.2.3.

   Error_TTL_Exceeded:  A peer receiving the request where the TTL got
      decremented to zero.  See section Section 6.3.2.

   Error_Message_Too_Large:  A peer receiving the request that was too
      large.  See section Section 6.6.

   Error_Response_Too_Large:  A peer would have generated a response
      that is too large per the max_response_length field.

   Error_Config_Too_Old:  A destination peer received a request with a
      configuration sequence that's too old.  See Section 6.3.2.1.

   Error_Config_Too_New:  A destination node received a request with a
      configuration sequence that's too new.  See Section 6.3.2.1.

   Error_Unknown_Kind:  A destination node received a request with an
      unknown Kind-ID.  See Section 7.4.1.2.

   Error_In_Progress:  An Attach is already in progress to this peer.
      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 doesn't fit the other error codes.  When this message is sent,
      implementations SHOULD provide some meaningful description in
      error_info to aid in debugging.

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:








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   struct {
      CertificateType     type;
      opaque              certificate<0..2^16-1>;
   } GenericCertificate;

   struct {
      GenericCertificate certificates<0..2^16-1>;
      Signature          signature;
   } SecurityBlock;

   The contents of this structure are:


   certificates
      A bucket of certificates.

   signature
      A signature over the message contents.

   The certificates bucket SHOULD contain all 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 for 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 certifcates, such as shared secret security (Section 13.4)
   is used, it is strongly RECOMMENDED that implementors include all
   referenced certificates, otherwise there is the possibility that
   messages may not be immediately verifiable because certificates must
   first be retrieved.

   NOTE TO IMPLEMENTERS:  This requirement implies that a peer storing
   data is obligated to retain certificates for the data it holds
   regardless of whether it is responsible for or actually holding the
   certificates for the Certificate Store usage.

   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 draft.






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   certificate
      The encoded version of the certificate.  For X.509 certificates,
      it is the DER form.

   The signature is computed over the payload and parts of the
   forwarding header.  The payload, in case of a Store, may contain an
   additional signature computed over a StoreReq structure.  All
   signatures are formatted using the Signature element.  This element
   is also used in other contexts where signatures are needed.  The
   input structure to the signature computation varies depending on the
   data element being signed.

     enum { reservedSignerIdentity(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;

     struct  {
        SignatureAndHashAlgorithm     algorithm;   // From TLS
        SignerIdentity                identity;
        opaque                        signature_value<0..2^16-1>;
     } Signature;

   The signature construct contains the following values:



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   algorithm
      The signature algorithm in use.  The algorithm definitions are
      found in the IANA TLS SignatureAlgorithm Registry and
      HashAlgorithm registries.  All implementations MUST support
      RSASSA-PKCS1-v1_5 [RFC3447] signatures with SHA-256 hashes.

   identity
      The identity used to form the signature.

   signature_value
      The value of the signature.

   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 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.

   The input to signatures over data values is different, and is
   described in Section 7.1.

   All RELOAD messages MUST be signed.  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 as well as cryptographic
   authentication of the sending node.  In addition, responses MUST be
   checked as follows by the requesting node:





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   1.  The response to a message sent to a specific Node-ID MUST have
       been sent by that Node-ID.
   2.  The response to a message sent to a Resource-Id MUST have been
       sent by a Node-ID which is as close to or closer to the target
       Resource-Id than any node in the requesting node's neighbor
       table.

   The second condition serves as a primitive check for responses from
   wildly wrong nodes but is not a complete check.  Note that in periods
   of churn, it is possible for the requesting node to obtain a closer
   neighbor while the request is outstanding.  This will cause the
   response to be rejected and the request to be retransmitted.

   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 does not itself implement
   any overlay topology.  Rather, it relies on Topology Plugins, which
   allow a variety of overlay algorithms to be used while maintaining
   the same RELOAD core.  This section describes the requirements for
   new topology plugins and the methods that RELOAD provides for overlay
   topology maintenance.

6.4.1.  Topology Plugin Requirements

   When specifying a new overlay algorithm, at least the following need
   to be described:

   o  Joining procedures, including the contents of the Join message.
   o  Stabilization procedures, including the contents of the Update
      message, the frequency of topology probes and keepalives, and the
      mechanism used to detect when peers have disconnected.
   o  Exit procedures, including the contents of the Leave message.
   o  The length of the Resource-IDs.  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.



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6.4.2.  Methods and types for use by topology plugins

   This section describes the methods that topology plugins use to join,
   leave, and maintain the overlay.

6.4.2.1.  Join

   A new peer (but one that already has credentials) uses the JoinReq
   message to join the overlay.  The JoinReq is sent to the responsible
   peer depending on the routing mechanism described in the topology
   plugin.  This notifies the responsible peer that the new peer is
   taking over some of the overlay and it needs to synchronize its
   state.

         struct {
            NodeId                joining_peer_id;
            opaque                overlay_specific_data<0..2^16-1>;
         } JoinReq;


   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 MUST reject the message with an Error_Forbidden
   error.

   Because joins may only be executed 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 peer.)  This also prevents replay attacks
   provided that DTLS anti-replay is used.

   If the request succeeds, the responding peer responds with a JoinAns
   message, as defined below:

         struct {
            opaque                overlay_specific_data<0..2^16-1>;
         } JoinAns;

   If the request succeeds, the responding peer MUST follow up by
   executing the right sequence of Stores and Updates to transfer the
   appropriate section of the overlay space to the joining peer.  In
   addition, overlay algorithms MAY define data to appear in the
   response payload that provides additional info.

   Joining nodes MUST verify that the signature on the JoinAns message
   matches the expected target (i.e., the adjacency over which they are



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   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 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 MUST reject
   the message with an Error_Forbidden error.

   Because leaves may only be executed 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.

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 (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.



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   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 the completion of the
   corresponding ICE check and TLS connection.  Note that the sender of
   a such 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.

   One important use of the RouteQuery request is to support iterative
   routing.  The sender selects one of the peers in its routing table
   and sends it a RouteQuery message with the destination_object set to
   the Node-ID or Resource-ID it wishes to route to.  The receiving peer
   responds with information about the peers to which the request would
   be routed.  The sending peer MAY then 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_object as determined by the topology plugin.  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 allowing 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>;



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         } 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 ids, or
      Resource-ID.

   overlay_specific_data
      Other data as appropriate for the overlay.

6.4.2.4.2.  Response Definition

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

6.4.2.5.  Probe

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

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 { reservedProbeInformation(0), responsible_set(1),
               num_resources(2), uptime(3),  (255)}
             ProbeInformationType;

        struct {
          ProbeInformationType     requested_info<0..2^8-1>;
        } ProbeReq




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   The currently defined values for ProbeInformation 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.

   uptime
      indicates that the peer should Respond with how long the peer has
      been up in seconds.

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;






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   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.

6.5.  Forwarding and Link Management Layer

   Each node maintains connections to a set of other nodes defined by
   the topology plugin.  This section defines the methods RELOAD uses to
   form and maintain connections between nodes in the overlay.  Three
   methods are defined:

   Attach:    used to form 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:    is 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
   TCP or UDP 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 3.2.1

   As described in Section 6.1, an Attach may be routed to either a
   Node-ID or to a Resource-ID.  An Attach routed to a specific Node-ID
   will fail if that node is not reached.  An Attach routed to a
   Resource-ID will establish a connection with the peer currently
   responsible for that Resource-ID, which may be useful in establishing
   a direct connection to the responsible peer for use with frequent or



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   large resource updates.

   An Attach in and of itself does not result in updating the routing
   table of either node.  That function is performed by Updates.  If
   node A has Attached to node B, but not received any Updates from B,
   it MAY route messages which are directly addressed to B through that
   channel but MUST NOT route messages through B to other peers via that
   channel.  The process of Attaching is separate from the process of
   becoming a peer (using 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.

6.5.1.1.  Request Definition

   An Attach request message contains the requesting node ICE connection
   parameters formatted into a binary structure.


































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        enum { reservedOverlayLink(0), DTLS-UDP-SR(1),
                DTLS-UDP-SR-NO-ICE(3), TLS-TCP-FH-NO-ICE(4),
                (255) } OverlayLinkType;

        enum { reservedCand(0), host(1), srflx(2), prflx(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:
              ;          /* Nothing */
            case srflx:
            case prflx:
            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).

   password







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      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 PDU.
   The remaining values are specified as follows:

   addr_port
      corresponds to the connection-address and port productions.

   overlay_link
      corresponds to the OverlayLinkType production, Overlay Link
      protocols used with No-ICE MUST specify "No-ICE" in their
      description.  Future overlay link values can be added be defining
      new OverlayLinkType values in the IANA registry in Section 14.10.
      Future extensions to the encapsulation or framing that provide for
      backward compatibility with that specified by a previously defined
      OverlayLinkType values MUST use that previous value.
      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 foundation production.

   priority
      corresponds to the priority production.

   type
      corresponds to the cand-type production.








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   rel_addr_port
      corresponds to the rel-addr and rel-port productions.  Only
      present for type "relay".

   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 it 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 it 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 is in progress, then it
      SHOULD generate an Error_In_Progress error instead of an
      AttachReqAns.
   o  If it 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 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-endien
         unsigned integers, it MUST generate an Error_In_Progress error
         to this Attach request, 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.

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







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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], 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 only allows for a single offer/answer exchange.  Unlike the
      usage of ICE within SIP, there is never a need to send a
      subsequent offer to update the default candidates to match the
      ones selected by ICE.

   An agent follows the ICE specification as described in [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 STUN servers to use.  In
   conventional ICE, it is assumed that nodes are configured with one or
   more STUN servers through some out of band mechanism.  This is still
   possible in RELOAD but RELOAD also learns STUN servers as it connects
   to other peers.  Because all RELOAD peers implement ICE and use STUN
   keepalives, every peer is a capable of responding to STUN Binding
   requests [RFC5389].  Accordingly, any peer that a node knows about
   can be used like a STUN server -- though of course it may be behind a
   NAT.

   A peer on a well-provisioned wide-area overlay will be configured
   with one or more bootstrap nodes.  These nodes make an initial list
   of STUN servers.  However, as the peer forms connections with
   additional peers, it builds more peers 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 there are more NATs involved, it may
   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



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   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
   Section 6.5.1.4 and Section 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 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 of 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

   However, standardization of additional protocols for use with ICE is
   expected, including TCP[I-D.ietf-mmusic-ice-tcp] and protocols such
   as SCTP and DCCP.  UDP encapsulations for SCTP and DCCP would expand
   the available Overlay Link protocols available for RELOAD.  When
   additional protocols are available, the following prioritization is
   RECOMMENDED:





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   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, or
      those protocols encapsulated using UDP.
   o  Second highest priority is assigned to protocols that offer well-
      understood congestion and flow control but 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 SR overlay link protocol is an
      example of such a protocol.

   Head of line blocking is undesireable 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 an Overlay Link protocol is not optimal.

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
   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 Section 5.1 and 6.1
   of ICE.  Since RELOAD requires full ICE from all agents, this check
   is not required.




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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.

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 (D)TLS handshake MUST match the node
   that sent the AttachReqAns and if it does not, the connection MUST be



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   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 which 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, SIP or anything else) 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,
   except for the purpose of the connection:  it is used to transport
   non-RELOAD "media".  A separate request is used to avoid implementor
   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.




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        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 connection set up with this request is
   responsible for providing sufficiently frequent keep traffic for NAT
   and Firewall keep alive and for deciding when to close the
   connection.

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>;



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        } 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 broadcast Node-ID
   (2^128-1).

6.5.3.1.  Request Definition

        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;



   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.






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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

        enum { reservedConfigUpdate(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;

   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.







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   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

        struct {
        } ConfigUpdateAns


   If the ConfigUpdateReq is of type "config" it MUST only be processed
   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 only be processed if
   it is correctly digitally signed by an acceptable Kind signer (i.e.,
   one listed in the current configuration file).  Details on kind-
   signer field in the configuration file is 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 "Error_Forbidden".  This should not happen in correctly
   functioning overlays.

   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 reentering the overlay, if, for instance, the DHT
   algorithm has changed.

   If an implementation receives repeated ConfigUpdates which it cannot
   verify with sequence numbers substantially in advance of its own
   configuration document, it SHOULD contact the configuration server to
   get the latest configuration file in order to avoid permanent
   breakage.  The details of this are left up to the implementation.

   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



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   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 [RFC4347] 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 [RFC4347] 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 which establishes a similar, stateful
   association, and we simply refer to these as "connections" for the
   purposes of this document.

   If a peer receives a message that is larger than value of max-
   message-size defined in the overlay configuration, the peer SHOULD
   send an Error_Message_Too_Large error 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 receiving the complete
   message.  If the forwarding header is larger than the max-message-
   size, the receiver SHOULD close the TLS or DTLS session without
   sending an error.

   The Framing Header (FH) is used to frame messages and provide timing
   when used on a reliable stream-based transport protocol.  Simple
   Reliability (SR) makes use of the FH to provide congestion control
   and semi-reliability when using unreliable message-oriented transport
   protocols.  We will first define each of these algorithms, then
   define overlay link protocols that use them.

   Note:  We expect future Overlay Link protocols to define replacements
   for all components of these protocols, including the framing header.
   These protocols have been chosen for simplicity of implementation and
   reasonable performance.

   Note to implementers:  There are inherent tradeoffs in utilizing
   short timeouts to determine when a link has failed.  To balance the
   tradeoffs, an implementation SHOULD quickly act to remove entries
   from the routing table when there is reason to suspect the link has
   failed.  For example, in a Chord derived overlay algorithm, a closer
   finger table entry could be substituted for an entry in the finger
   table that has experienced a timeout.  That entry can be restored if
   it proves to resume functioning, or replaced at some point in the
   future if necessary.  End-to-end retransmissions will handle any lost
   messages, but only if the failing entries do not remain in the finger



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   table for subsequent retransmissions.

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 not involved
      in the association to determine the contents of that traffic.

   Any new overlay protocol MUST be defined via RFC 5226 Standards
   Action; 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 provided identities meaningful to
      the HIP base exchange.  We anticipate that this would require a
      mapping between ORCHIDs and NodeIds.
   o  How to carry the HIP I1 and I2 messages.
   o  How to carry RELOAD messages over HIP.

   [I-D.ietf-hip-reload-instance] documents work in progress on using
   RELOAD with the HIP BONE.







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6.6.1.2.  ICE-TCP

   The ICE-TCP draft [I-D.ietf-mmusic-ice-tcp] 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, good
   congestion control, and 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, motivated by
   the situation that UDP is well-supported by modern NAT hardware, and
   similar performance can be achieved to native implementation.
   Currently SCTP, DCCP, and a generic tunneling extension are being
   proposed for message-oriented protocols.  Once ICE traversal has been
   specified for these tunneled 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 only used 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:














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        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 1 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.

   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.







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   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 previously 32 packets received on this connection.  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 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 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, 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.

   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].






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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
   round-trip time (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
   chosen larger, and this is RECOMMENDED if it is known in advance
   (such as on high latency access links) that the round-trip time 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.

   Retransmissions continue until a response is received, or until a
   total of 5 requests have been sent or there has been a hard ICMP
   error [RFC1122] or a TLS alert.  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



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   messages.  For example, assuming an RTO of 500 ms, requests would be
   sent at times 0 ms, 500 ms, 1500 ms, 3500 ms, and 7500 ms.  If all
   retransmissions for a message fail, then the sending node SHOULD
   close the connection routing the message.

   To determine when a link may 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 10ms 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 Simple Reliability 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 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 30s 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.

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



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   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 in
   appropriate 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.  When a
   message must be fragmented, it SHOULD be split into equal-sized
   fragments that are no larger than the 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 is only assembled at the final destination.  This option
   has been chosen as it is far easier to implement than 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 attacks,
   receivers SHOULD time out incomplete fragments after maximum request
   lifetime (15 seconds).  Note 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
   the receiver runs out of buffer space to reassemble the messages 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



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   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" and 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 octets 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 time 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 a 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 during unpartitioning) 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 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 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.

7.1.  Data Signature Computation

   Each StoredData element is individually signed.  However, the
   signature also must be self-contained and cover the Kind-ID and



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   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.

   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 necessarily 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.  However, it is
   permitted to have a value signed which expires after a certificate's
   validity period (though this will likely cause verification failure
   at some future time.)

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
   dataModel is known from the Kind being stored.




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        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;


   We now discuss the properties of each data model in turn:

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,
      that means that a value is present.  This gives the protocol a
      mechanism for indicating nonexistence as opposed to emptiness.

   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.



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   A array element is represented as an ArrayEntry:


         struct {
           uint32                  index;
           DataValue               value;
         } ArrayEntry;



   The contents of this structure are:

   index
      The index of the data element in the array.

   value
      The stored data.

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
   follows:

   A dictionary element 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.

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



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   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.  I.e., 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 only be used 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.

7.3.4.  NODE-MULTIPLE

   In the NODE-MULTIPLE policy, a given value MUST be written (or
   overwritten) if and only if 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



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   maximum value of i MUST be specified in the Kind definition.

   Note that as i is not carried on the wire, the verifier MUST iterate
   through potential i values up to the maximum value in order 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 each 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 before the
   request was received.

   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



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   resource location.  The contents of the structure are:

   resource
      The resource to store at.

   replica_number
      The number of this replica.  When a storing peer saves replicas to
      other peers each peer is assigned a replica number starting from 1
      and sent in the Store message.  This field is set to 0 when a node
      is storing its own data.  This allows peers to distinguish replica
      writes from original writes.

   kind_data
      A series of elements, one for each Kind of data to be stored.

   If the replica number is zero, then the peer MUST check that it is
   responsible for the resource and, if not, reject the request.  If the
   replica number is nonzero, then the peer MUST check that it expects
   to be a replica for the resource and that the request sender is
   consistent with being the responsible node (i.e., that the receiving
   peer does not know of a better node) and, if not, reject the request.

   Each StoreKindData element represents the data to be stored for a
   single Kind-ID.  The contents of the element are:

   kind
      The Kind-ID.  Implementations MUST reject requests corresponding
      to unknown Kinds.

   generation_counter
      The expected current state of the generation counter
      (approximately the number of times this object has been written;
      see below for details).

   values
      The value or values to be stored.  This may contain one or more
      stored_data values depending on the data model associated with
      each Kind.

   The peer MUST perform the following checks:

   o  The Kind-ID is known and supported.
   o  The 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.



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   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 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 non-zero, it equals the current value of the
      generation counter for this Kind.  This feature allows the
      generation counter to be used in a way similar to the HTTP Etag
      feature.
   o  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.
   o  The storage time values are greater than that of any value which
      would be replaced by this Store.
   o  The size and number of the stored values is 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

   If all these checks succeed, the peer MUST attempt to store the data
   values.  For non-replica stores, if the store succeeds and the data
   is changed, then the peer MUST increase the generation counter by at
   least one.  If there are multiple stored values in a single
   StoreKindData, it is permissible for the peer to increase the
   generation counter by only 1 for the entire Kind-ID, or by 1 or more
   than one for each value.  Accordingly, all stored data values MUST
   have a generation counter of 1 or greater. 0 is used in the Store
   request to indicate that the generation counter should be ignored for
   processing this request; however the responsible peer should increase
   the stored generation counter and should return the correct
   generation counter in the response.

   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



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   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.

   The properties of stores for each data model are as follows:

   Single-value:
      A store of a new single-value element creates the element if it
      does not exist and overwrites any existing value with the new
      value.

   Array:
      A store of an array entry replaces (or inserts) the given value at
      the location specified by the index.  Because arrays are sparse, a
      store past the end of the array extends it with nonexistent values
      (exists=False) as required.  A store at index 0xffffffff places
      the new value at the end of the array regardless of the length of
      the array.  The resulting StoredData has the correct index value
      when it is subsequently fetched.

   Dictionary:
      A store of a dictionary entry replaces (or inserts) the given
      value at the location specified by the dictionary key.

   The following figure shows the relationship between these structures
   for an example store which stores the following values at resource
   "1234"

   o  The value "abc" in the single value location for Kind X
   o  The value "foo" at index 0 in the array for Kind Y
   o  The value "bar" at index 1 in the array for Kind Y











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                                     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"


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
   containing 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;




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   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 peer
      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 peer 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.

   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 node does
   not know about, an Error_Unknown_Kind MUST be generated.  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 that in fact a
   Kind was used which was defined in the configuration document).





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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 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 there is still a window of vulnerability for replay attack 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
   lifetime expires.  When a stored nonexistent value's lifetime
   expires, it is simply removed from the storing node like any other
   stored value expiration.

   Note that in the case of arrays and dictionaries, 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.
   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.












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7.4.2.1.  Request Definition

        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.

   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:






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   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 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,
      than this is a wildcard fetch and all key-value pairs are
      returned.

   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.

   Note that because the certificate for a user is typically stored at
   the same location as any data stored for that user, a requesting node
   that does not already have the user's certificate should request the
   certificate in the Fetch as an optimization.




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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 of Section 7.4.1.3 and have not yet expired also
      have exists = false but have valid signatures from the node which
      did the store.

   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



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   implies that implementations which wish to store data for long
   periods of time must have certificates with appropriate expiry dates
   or re-store periodically.  Implementations MAY return the subset of
   values with valid signatures, but in that case 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;



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.










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        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 (model) {
            case single_value:
              MetaData              single_value_entry;

            case array:
              ArrayEntryMeta        array_entry;

            case dictionary:
              DictionaryEntryMeta   dictionary_entry;

            /* This structure may be extended */
          };
        } MetaDataValue;

        struct {
          uint32                  value_length;
          uint64                  storage_time;
          uint32                  lifetime;
          MetaDataValue           metadata;
        } StoredMetaData;

        struct {
          KindId                 kind;
          uint64                 generation;
          StoredMetaData         values<0..2^32-1>;
        } StatKindResponse;

        struct {
          StatKindResponse      kind_responses<0..2^32-1>;
        } StatAns;



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   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 really new structure is MetaData, which has the following
   contents:

   exists
      Same as in DataValue

   value_length
      The length of the stored value.

   hash_algorithm
      The hash algorithm used to perform the digest of the value.

   hash_value
      A digest of the value using hash_algorithm.

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).

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;




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   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 only appear once, and if
      not 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.

   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.








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   closest
      The closest resource ID to the specified resource ID.  This is 0
      if no resource ID is known.

   Note that the response does not contain the contents of the data
   stored at these Resource-IDs.  If the requester wants this, it must
   retrieve it using Fetch.

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,
   that 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.


8.  Certificate Store Usage

   The Certificate Store usage allows a peer to store its certificate in
   the overlay, thus avoiding the need to send a certificate in each
   message.

   A user/peer MUST store its certificate at Resource-IDs derived from
   two Resource Names:

   o  The user name in the certificate.





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   o  The Node-ID in the certificate.

   Note that in the second case the certificate is not stored at the
   peer's Node-ID but rather at a hash of the peer's Node-ID.  The
   intention here (as is common throughout RELOAD) is to avoid making a
   peer responsible for its own data.

   A peer MUST ensure that the user's certificates are stored in the
   Overlay Instance.  New certificates are stored at the end of the
   list.  This structure allows users to store 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
   SHOULD NOT be a candidate for a TURN server.  Otherwise, the peer
   SHOULD assume it is a potential TURN server and follow the procedures
   below.

   If the node is a candidate for a TURN server it will insert some
   pointers in the overlay so that other peers can find it.  The overlay
   configuration file specifies a turn-density parameter that indicates
   how many times each TURN server SHOULD record itself in the overlay.



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   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;
          IpAddressAndPort        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 an 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,
      then 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.
   Access Control  NODE-MULTIPLE, with maximum iteration 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



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   relative to peers is not too low, this approach will result in
   finding a server relatively quickly.

   NOTE TO IMPLEMENTERS:  As the access control for this usage is not
   CERTIFICATE_BY_NODE or CERTIFICATE_BY_USER, 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 it is an
   adaptation of the basic Chord based DHT algorithm.

   This algorithm differs from the originally presented Chord algorithm
   [Chord].  It has been updated based on more recent research results
   and implementation experiences, and to adapt it to the RELOAD
   protocol.  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 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
      reduce load, nor does it search for optimal matches of the finger
      table entries.
   o  RELOAD uses 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 (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  This 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,



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      but is believed to be unstable in large (>1000) overlays 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 is a modified version of the Chord
   algorithm.  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
   half-way around the ring from this peer; the second entry contains
   the peer that is 1/4 of the way around; the third entry contains the
   peer that is 1/8th of the way around, and so on.  Fundamentally, the
   chord data structure can be thought of a doubly-linked list formed by
   knowing the successors and predecessor peers in the neighbor table,
   sorted by the Node-ID.  As long as the successor peers are correct,
   the DHT will return the correct result.  The pointers to the prior
   peers are kept to enable 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, so that entries in the linked list can be found in O(log(N))
   time instead of the typical O(N) time that a linked list would
   provide.

   A peer, n, is responsible for a particular Resource-ID k if k is less
   than or equal to n and k is greater than p, where p is the 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 plugin, the size of the Resource-ID is
   128 bits.  The hash of a Resource-ID MUST be computed using SHA-1
   [RFC3174]then truncating the SHA-1 result to the most significant 128
   bits.

10.3.  Routing

   The routing table is 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



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   routing table that has the largest Node-ID that is in the interval
   between the peer and k.  If no such node is found, it 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 returns a
   success response.  It MUST then sends a Store request to its
   successor in the neighbor table and to that peer's 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 they came from an appropriate predecessor in their
   neighbor table and that they are in a range that this predecessor is
   responsible for, and then they MUST store the data.  They do not
   themselves perform further Stores because they can determine that
   they are not responsible for the Resource-ID.

   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 party (JP) with Node-ID n is as
   follows.

   1.  JP MUST connect to its chosen bootstrap node.
   2.  JP SHOULD send an Attach request to the admitting peer (AP) for
       Node-ID n.  The "send_update" flag should be used to acquire the
       routing table for AP.
   3.  JP SHOULD send Attach requests to initiate connections to each of
       the peers in the neighbor table as well as to the desired finger
       table entries.  Note that this does not populate their routing
       tables, but only their connection tables, so JP will not get
       messages that it is expected to route to other nodes.
   4.  JP MUST enter all the peers it has contacted into its routing
       table.
   5.  JP MUST send a Join to AP.  The AP sends the response to the
       Join.





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   6.  AP MUST do a series of Store requests to JP to store the data
       that JP will be responsible for.
   7.  AP MUST send JP an Update explicitly labeling JP as its
       predecessor.  At this point, JP is part of the ring and
       responsible for a section of the overlay.  AP can now forget any
       data which is assigned to JP and not AP.
   8.  The AP MUST send an Update to all of its neighbors with the new
       values of its neighbor set (including JP).
   9.  The JP MUST send Updates to all the peers in its neighbor table.

   If JP sends an Attach to AP with send_update, it immediately knows
   most of its expected neighbors from AP's routing table update and can
   directly connect to them.  This is the RECOMMENDED procedure.

   If for some reason JP does not get AP's routing table, it can still
   populate its neighbor table incrementally.  It sends a Ping directed
   at Resource-ID n+1 (directly after its own Resource-ID).  This allows
   it to discover its own successor.  Call that node p0.  It then sends
   a ping to p0+1 to discover its successor (p1).  This process can be
   repeated to discover as many successors as desired.  The values for
   the two peers before p will be found at a later stage when n receives
   an Update.  An alternate procedure is to send Attaches to those nodes
   rather than pings, which forms the connections immediately but may be
   slower if the nodes need to collect ICE candidates, thus reducing
   parallelism.

   In order to set up its finger table entry for peer i, JP simply sends
   an Attach to peer (n+2^(128-i).  This will be routed to a peer in
   approximately the right location around the ring.

   The joining peer 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.

10.7.  Updates

   An Update for this DHT is defined as




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        enum { reserved (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.

   If the message is of type "neighbors", then the contents of the
   message will be:








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   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.

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, it then sends 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 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 effects the peer's range of responsible IDs,
   then the Update MUST be sent to all nodes in its Connection Table.

   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



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   procedures are beyond the scope of this document.  In no event does
   an attempt to reestablish connectivity with a lost neighbor allow the
   peer to remain in 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 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, all references to
   the failed peer are 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 initiates a search for a new
   finger table entry as described below.

10.7.3.  Receiving Updates

   When a peer, N, receives an Update request, it examines the Node-IDs
   in the UpdateReq and at its neighbor table and decides if this
   UpdateReq would change its neighbor table.  This is done by taking
   the set of peers currently in the neighbor table and comparing them
   to the peers in the update request.  There are two major cases:

   o  The UpdateReq contains peers that match N's neighbor table, so no
      change is needed to the neighbor set.
   o  The UpdateReq contains peers N does not know about that should be
      in N'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, N MUST attempt to Attach to the new peers and if
   it is successful it MUST adjust its neighbor set accordingly.  Note
   that it can maintain the now inferior peers as neighbors, but it MUST
   remember the closer ones.

   After any Pings and Attaches are done, if the neighbor table changes
   and the peer is using reactive recovery, the peer sends 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



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   the entries empty.

   If peer N is responsible for a Resource-ID R, and N discovers that
   the replica set for R (the next two nodes in its successor set) has
   changed, it MUST send a Store for any data associated with R to any
   new node in the replica set.  It SHOULD NOT delete data from peers
   which have left the replica set.

   When a peer N detects that it is no longer in the replica set for a
   resource R (i.e., there are three predecessors between N and R), it
   SHOULD delete all data associated with R from its local store.

   When a peer discovers that its range of responsible IDs have 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 neighbor table

   A peer MUST periodically send an Update request to every peer in its
   Connection 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.

10.7.4.2.  Refreshing finger table

   A peer MUST periodically search for new peers to replace invalid
   entries in the finger table.  A finger table entry i is valid if it
   is in the range [ n+2^( 128-i ) , n+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.

   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).




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   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=0 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 in 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 MAY choose to keep connections to multiple peers that can act
   for a given finger table entry.

10.7.4.3.  Adjusting finger table size

   If the finger table has less 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.

   Note to implementers:  Although log(N) entries are all that are
   required for optimal performance, careful implementation of
   stabilization will result in no additional traffic being generated



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   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 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 a 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.

10.8.  Route query

   For this topology plugin, the RouteQueryReq contains no additional
   information.  The RouteQueryAns contains the single node ID of the
   next peer to which the responding peer would have routed the request
   message in recursive routing:


      struct {
         NodeId                  next_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.





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10.9.  Leaving

   To support extensions, such as [I-D.ietf-p2psip-self-tuning], Peers
   SHOULD send a Leave request to all members of their neighbor table
   prior to exiting the Overlay Instance.  The overlay_specific_data
   field MUST contain the ChordLeaveData structure defined below:

              enum { reserved (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;


   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
   follows procedures as if it had detected a peer failure as described
   in Section 10.7.1.




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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 an 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
   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>
          <multicast-bootstrap address="192.0.0.1" />



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          <multicast-bootstrap address="233.252.0.1" port="6084" />
          <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">
                  <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==



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              </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 namespace for the elements defined in
   this specification is urn:ietf:params:xml:ns:p2p:config-base and
   urn:ietf:params:xml:ns:p2p:config-chord".

   The file can 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
   provides a signature over the preceding configuration element.  Each
   configuration element has the following attributes:

   instance-name:  name of the overlay
   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 be in UTC.
   sequence:  a monotonically increasing sequence number between 0 and
      2^16-2

   Inside each overlay element, the following elements can occur:

   topology-plugin  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.




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   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.
   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 IP address is in
      typical hexadecimal form using standard period and colon
      separators as specified in [RFC5952].  More than one bootstrap-
      peer 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 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.
   multicast-bootstrap   This element represents the address of a
      multicast, broadcast, or anycast address and port that may be used
      for bootstrap.  Nodes SHOULD listen on the address.  It has an
      attributed called "address" that represents the IP address and an
      optional attribute called "port" that represents the port and
      defaults to 6084.  More than one "multicast-bootstrap" element may
      be present.
   clients-permitted   This element represents whether clients are
      permitted or whether all nodes must be peers.  If it is set to
      "true" or absent, this indicates that clients are permitted.  If
      it is set to "false" then nodes are not allowed to remain clients
      after the initial join.  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".





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   chord-update-interval   The update frequency for the Chord-reload
      topology plugin (see Section 10).
   chord-ping-interval   The ping frequency for the Chord-reload
      topology plugin (see Section 10).
   chord-reactive   Whether reactive recovery should be used for this
      overlay.  Set to "true" or "false".  Default if missing is "true".
      (see Section 10).
   shared-secret  If shared secret mode is used, this 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 (time to live, see Section 6.3.2)
      for messages.  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.
   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.
   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 only be
      present for the lifetime of the certificate.  Technically
      speaking, bad node-ids may be reused once their certificates have
      expired, the requirement for node-ids to be pseudo randomly
      generated gives this event a vanishing probability.






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   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 [w3c-xml-namespaces]
      Section 2.3.  More than one mandatory-extension element may be
      present.

   Inside each overlay element, the required-kinds elements can 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 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 contains 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.
   max-node-multiple:  This is optional and only used when the access
      control is NODE-MULTIPLE.  This indicates the maximum value for
      the i counter.  This is 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 required-kinds elements MAY be
   present.

   The kind-block element also MUST contain a "kind-signature" element.
   This signature is computed across the kind from the beginning of the
   first < of the kind to the end of the last > of the kind in the same
   way as the signature element described later in this section.

   The configuration file needs to be treated as a binary blob that
   cannot be changed - including any whitespace changes - or the
   signature will break.  The signature is computed by taking each
   configuration element and starting from, and including, the first <
   at the start of <configuration> up to and including the > in



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   </configuration> and treating this as a binary blob that is signed
   using the standard SecurityBlock defined in Section 6.3.4.  The
   SecurityBlock is base 64 encoded using the base64 alphabet from
   RFC[RFC4648] and put in the signature element following the
   configuration object in the configuration file.  Any configuration
   file through the overlay (as opposed to directly from the
   configuration server) MUST be signed by one of the configure-signers
   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 re-join.  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 re-joining the overlay may result in serious performance
   problems, including total collapse of the network if configuration
   parameters are not properly considered.  Such an event may be
   necessary in case of a compromised CA or similar problem, but for
   large overlays 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 {



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         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 }*
   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 }?



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                }*

   parameter &= element multicast-bootstrap {
                   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

   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 }?



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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 is
   provided by the user or some other out of band provisioning
   mechanism.  The out of band mechanisms 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 "p2psip-enroll" and a protocol of TCP to find a
   configuration server and form the URL by appending a path of "/.well-
   known/p2psip-enroll" 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/p2psip-enroll".

   Once an address and URL for the configuration server is determined,
   the peer MUST form an HTTPS connection to that IP address.  The
   certificate MUST match the overlay name as described in [RFC2818].
   Then 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, which MUST replace any
   previously learned configuration file for this overlay.

   For overlays that do not use a configuration server, nodes need to
   obtain the configuration information needed to join the overlay
   through some out of band approach such an XML configuration file sent
   over email.

11.3.  Credentials

   If the configuration document contains a 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
   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.



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   The certificate request protocol is performed over HTTPS.  The
   request is an HTTP POST with the parameter encodes as described in
   [RFC2388] and the following properties:

   o  If authentication is required, there is an form parameter of
      "password" and "username" containing the user's name and password
      in the clear (hence the need for HTTPS)
   o  If more than one Node-ID is required, there is an 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].
   o  The Accept header MUST contain the type "application/pkix-cert",
      indicating the type that is expected in the response.

   The enrollment server MUST authenticate the request using the
   provided user 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 is acceptable, the server generates and returns a
   certificate for the certificate signing request in the "csr"
   parameter of the request.  The SubjectAltName field in the
   certificate contains 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 they are unpredictable to the requesting user.  E.g., the
      user MUST NOT be informed of potential (random) Node-IDs prior to
      authenticating.  Each is placed in the subjectAltName using the
      uniformResourceIdentifier type and MUST contain RELOAD URIs as
      described in Section 14.15 and 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) return
      the same Node-ID, thus avoiding the need for implementations to
      re-store all their data when their certificates expire.
   o  A single name this user is allowed to use in the overlay, using
      type rfc822Name.  Enrollment servers SHOULD take care to only
      allow legal characters in the name (e.g., no embedded NULs),
      rather than simply accepting any name provided by the user.

   The certificate is returned as type "application/pkix-cert" as
   defined in [RFC2585], with an HTTP status code of 200 OK.
   Certificate processing errors should be treated as HTTP errors and
   have appropriate HTTP status codes.  In particular, password errors
   SHOULD be returned as 401 Unauthorized. [[ OPEN ISSUE:  We know this
   isn't right and have a question out to the apps AD. ]]




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   The client MUST check that the certificate returned 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-IDs it can use.

11.3.1.  Self-Generated Credentials

   If the "self-signed-permitted" element is present in the
   configuration and set to "true", then a node MUST generate its own
   self-signed certificate to join the overlay.  The self-signed
   certificate MAY contain any user name of the users choice.

   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.  When accepting a self-signed certificate, nodes
   MUST check that the Node-ID and public keys match.  This prevents
   Node-ID theft.

   Once the node has constructed a self-signed certificate, it MAY join
   the overlay.  Before storing its certificate in the overlay
   (Section 8) it SHOULD look to see if the user name is already taken
   and if so choose another user name.  Note that this only provides
   protection against accidental name collisions.  Name theft is still
   possible.  If protection against name theft is desired, then the
   enrollment service must be used.

11.4.  Searching for a Bootstrap Node

   If no cached bootstrap nodes are available and the configuration file
   has an multicast-bootstrap element, then the node SHOULD send a Ping
   request over UDP to the address and port found to each multicast-
   bootstrap element found in the configuration document.  This MAY be a
   multicast, broadcast, or anycast address.  The Ping should use the
   wildcard Node-ID as the destination Node-ID.

   The responder node that receives the Ping request SHOULD check that
   the overlay name is correct and that the requester peer sending the
   request has appropriate credentials for the overlay before responding
   to the Ping request even if the response is only an error.

11.5.  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 it has
   previously been connected with in this overlay, as an optimization it



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   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 sends an
   Attach request over this connection with the destination Node-ID set
   to the joining node's Node-ID.

   When the requester node finally does receive a response from some
   responding node, it can note the Node-ID in the response and use this
   Node-ID to start sending requests to join the Overlay Instance as
   described in Section 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:
   JP = joining peer, 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 peer.

   In the following example, we assume that JP has formed a connection
   to one of the bootstrap nodes.  JP then sends an Attach through that
   peer to a resource ID of itself (JP).  It gets routed to the
   admitting peer (AP) because JP is not yet part of the overlay.  When
   AP responds, JP and AP use ICE to set up a connection and then set up
   TLS.  Once AP has connected to JP, AP sends to JP an Update to
   populate its Routing Table.  The following example shows the Update
   happening after the TLS connection is formed but it could also happen
   before in which case the Update would often be routed through other
   nodes.












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       JP        PPP       PP        AP        NP        NNP       BP
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |Attach Dest=JP     |         |         |         |         |
        |---------------------------------------------------------->|
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |Attach Dest=JP     |         |         |
        |         |         |<--------------------------------------|
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |Attach Dest=JP     |         |         |
        |         |         |-------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |AttachAns          |         |         |
        |         |         |<--------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |AttachAns          |         |         |
        |         |         |-------------------------------------->|
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |AttachAns          |         |         |         |         |
        |<----------------------------------------------------------|
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |TLS      |         |         |         |         |         |
        |.............................|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |Update   |         |         |         |         |         |
        |<----------------------------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateAns|         |         |         |         |         |
        |---------------------------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |


   The JP then forms connections to the appropriate neighbors, such as
   NP, by sending an Attach which gets routed via other nodes.  When NP
   responds, JP and NP use ICE and TLS to set up a connection.




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       JP        PPP       PP        AP        NP        NNP       BP
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |Attach NP          |         |         |         |         |
        |---------------------------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |Attach NP|         |         |
        |         |         |         |-------->|         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |AttachAns|         |         |
        |         |         |         |<--------|         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |AttachAns          |         |         |         |         |
        |<----------------------------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |Attach   |         |         |         |         |         |
        |-------------------------------------->|         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |TLS      |         |         |         |         |         |
        |.......................................|         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |


   JP 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 it sending an Attach halfway around the Chord
   ring to the JP + 2^127.















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       JP        NP        XX        TP
        |         |         |         |
        |         |         |         |
        |         |         |         |
        |Attach JP+2<<126   |         |
        |-------->|         |         |
        |         |         |         |
        |         |         |         |
        |         |Attach JP+2<<126   |
        |         |-------->|         |
        |         |         |         |
        |         |         |         |
        |         |         |Attach JP+2<<126
        |         |         |-------->|
        |         |         |         |
        |         |         |         |
        |         |         |AttachAns|
        |         |         |<--------|
        |         |         |         |
        |         |         |         |
        |         |AttachAns|         |
        |         |<--------|         |
        |         |         |         |
        |         |         |         |
        |AttachAns|         |         |
        |<--------|         |         |
        |         |         |         |
        |         |         |         |
        |TLS      |         |         |
        |.............................|
        |         |         |         |
        |         |         |         |
        |         |         |         |
        |         |         |         |

   Once JP has a reasonable set of connections, it is ready to take its
   place in the DHT.  It does this by sending a Join to AP.  AP does a
   series of Store requests to JP to store the data that JP will be
   responsible for.  AP then sends JP an Update explicitly labeling JP
   as its predecessor.  At this point, JP is part of the ring and
   responsible for a section of the overlay.  AP can now forget any data
   which is assigned to JP and not AP.









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       JP        PPP       PP        AP        NP        NNP       BP
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |JoinReq  |         |         |         |         |         |
        |---------------------------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |JoinAns  |         |         |         |         |         |
        |<----------------------------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |StoreReq Data A    |         |         |         |         |
        |<----------------------------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |StoreAns |         |         |         |         |         |
        |---------------------------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |StoreReq Data B    |         |         |         |         |
        |<----------------------------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |StoreAns |         |         |         |         |         |
        |---------------------------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateReq|         |         |         |         |         |
        |<----------------------------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateAns|         |         |         |         |         |
        |---------------------------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |

   In Chord, JP's neighbor table needs to contain its own predecessors.
   It couldn't connect to them previously because it did not yet know
   their addresses.  However, now that it has received an Update from
   AP, it has AP's predecessors, which are also its own, so it sends
   Attaches to them.  Below it is shown connecting to AP's closest
   predecessor, PP.






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       JP        PPP       PP        AP        NP        NNP       BP
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |Attach Dest=PP     |         |         |         |         |
        |---------------------------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |Attach Dest=PP     |         |         |
        |         |         |<--------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |AttachAns|         |         |         |
        |         |         |-------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |AttachAns|         |         |         |         |         |
        |<----------------------------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |TLS      |         |         |         |         |         |
        |...................|         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateReq|         |         |         |         |         |
        |------------------>|         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateAns|         |         |         |         |         |
        |<------------------|         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateReq|         |         |         |         |         |
        |---------------------------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateAns|         |         |         |         |         |
        |<----------------------------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateReq|         |         |         |         |         |
        |-------------------------------------->|         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateAns|         |         |         |         |         |
        |<--------------------------------------|         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |



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   Finally, now that JP has a copy of all the data and is ready to route
   messages and receive requests, it sends Updates to everyone in its
   Routing Table to tell them it is ready to go.  Below, it is shown
   sending such an update to TP.


           JP        NP        XX        TP
            |         |         |         |
            |         |         |         |
            |         |         |         |
            |Update   |         |         |
            |---------------------------->|
            |         |         |         |
            |         |         |         |
            |UpdateAns|         |         |
            |<----------------------------|
            |         |         |         |
            |         |         |         |
            |         |         |         |
            |         |         |         |


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
   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.




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13.2.  Attacks on P2P Overlays

   The two basic functions provided by overlay nodes are storage and
   routing:  some node is responsible for storing a peer's data and for
   allowing a third peer to fetch this stored data.  Other nodes are
   responsible for routing messages to and from the storing nodes.  Each
   of these issues is covered in the following sections.

   P2P overlays are subject to attacks by subversive nodes that may
   attempt to disrupt routing, corrupt or remove user registrations, or
   eavesdrop on signaling.  The certificate-based security algorithms we
   describe in this specification are intended to protect overlay
   routing and user registration information in RELOAD messages.

   To protect the signaling from attackers pretending to be valid peers
   (or peers other than themselves), the first requirement is to ensure
   that all messages are received from authorized members of the
   overlay.  For this reason, RELOAD transports all messages over a
   secure channel (TLS and DTLS are defined in this document) which
   provides message integrity and authentication of the directly
   communicating peer.  In addition, messages and data are digitally
   signed with the sender's private key, providing end-to-end security
   for communications.

13.3.  Certificate-based Security

   This specification stores users' registrations and possibly other
   data in an overlay network.  This requires a solution to securing
   this data as well as securing, as well as possible, the routing in
   the overlay.  Both types of security are based on requiring that
   every entity in the system (whether user or peer) authenticate
   cryptographically using an asymmetric key pair tied to a certificate.

   When a user enrolls in the Overlay Instance, they request or are
   assigned a unique name, such as "alice@dht.example.net".  These names
   are unique and are meant to be chosen and used by humans much like a
   SIP Address of Record (AOR) or an email address.  The user is also
   assigned one or more Node-IDs by the central enrollment authority.
   Both the name and the Node-ID are placed in the certificate, along
   with the user's public key.

   Each certificate enables an entity to act in two sorts of roles:

   o  As a user, storing data at specific Resource-IDs in the Overlay
      Instance corresponding to the user name.
   o  As a overlay peer with the Node-ID(s) listed in the certificate.

   Note that since only users of this Overlay Instance need to validate



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   a certificate, this usage does not require a global PKI.  Instead,
   certificates are signed by a central enrollment authority which acts
   as the certificate authority for the Overlay Instance.  This
   authority signs each peer's certificate.  Because each peer possesses
   the CA's certificate (which they receive on enrollment) they can
   verify the certificates of the other entities in the overlay without
   further communication.  Because the certificates contain the user/
   peer's public key, communications from the user/peer can be verified
   in turn.

   If self-signed certificates are used, then the security provided is
   significantly decreased, since attackers can mount Sybil attacks.  In
   addition, attackers cannot trust the user names in certificates
   (though they can trust the Node-IDs because they are
   cryptographically verifiable).  This scheme 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 peer can verify that the storer is authorized to perform a
   store at that Resource-ID and also 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 (though certificates may of course include 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 peers, this form of compromise is not fatal.

   All implementations MUST implement certificate-based security.

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 share a single symmetric key which is used to key TLS-PSK
   [RFC4279] or TLS-SRP [RFC5054] mode.  A peer which does not know the
   key cannot form TLS connections with any other peer and therefore
   cannot join the overlay.

   One natural approach to a shared-secret scheme is to use a user-
   entered password as the key.  The difficulty with this is that in
   TLS-PSK mode, such keys are very susceptible to dictionary attacks.



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   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 client wants to store some value, it first digitally signs the
   value with its own private key.  It then sends a Store request that
   contains both the value and the signature towards the storing peer
   (which is defined by the Resource Name construction algorithm for
   that particular Kind of value).

   When the storing peer receives the request, it must determine whether
   the storing client is authorized to store 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 the certificate with the following properties:

          User name: alice@dht.example.com
          Node-ID:   013456789abcdef
          Serial:    1234

   If Alice wishes to Store a value of the "SIP Location" Kind, the
   Resource Name will be the SIP AOR "sip:alice@dht.example.com".  The
   Resource-ID will be determined by hashing the Resource Name.  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.






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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 clients to store unlimited amounts of
   data would create unacceptable burdens on peers and would also enable
   trivial denial of service attacks.  RELOAD addresses this issue by
   requiring configurations to define maximum sizes for each Kind of
   stored data.  Attempts to store values exceeding this size MUST be
   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
   peer to verify the integrity of the stored value.  Some more care
   needs to be taken to prevent version rollback attacks.  Rollback
   attacks on storage are prevented by the use of store times and
   lifetime values in each store.  A lifetime represents the latest time
   at which the data is valid and thus limits (though does not
   completely prevent) the ability of the storing node to perform a
   rollback attack on retrievers.  In order to prevent a rollback attack
   at the time of the Store request, we require that storage times be
   monotonically increasing.  Storing peers MUST reject Store requests
   with storage times smaller than or equal to those they are currently
   storing.  In addition, a fetching node which receives a data value
   with a storage time older than the result of the previous fetch knows
   a rollback has occurred.

13.5.4.  Residual Attacks

   The mechanisms described here provides a high degree of security, but
   some attacks remain possible.  Most simply, it is possible for
   storing nodes to refuse to store a value (i.e., reject any request).
   In addition, a storing node can deny knowledge of values which it has
   previously accepted.  To some extent these attacks can be ameliorated
   by attempting to store to/retrieve from replicas, but a retrieving
   client does not know whether it should try this or not, since there
   is a cost to doing so.




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   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 node can return only some subset of the values, thus
   biasing its responses.  This can be countered by using single values
   rather than sets, but that makes coordination between multiple
   storing agents much more difficult.  This is a trade off that must be
   made when designing any usage.

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 peer in the Overlay Instance.  Second, this is a DOS attack
   only.  Third, if a large percentage of the peers on the Overlay
   Instance are controlled by the attacker, it is probably impossible to
   perfectly secure against this.

13.6.1.  Background

   In general, attacks on DHT routing are mounted by the attacker
   arranging to route traffic through one or two nodes it controls.  In
   the Eclipse attack [Eclipse] the attacker tampers with messages to
   and from nodes for which it is on-path with respect to a given victim
   node.  This allows it to pretend to be all the nodes that are
   reachable through it.  In the Sybil attack [Sybil], the attacker
   registers a large number of nodes and is therefore able to capture a
   large amount of the traffic through the DHT.

   Both the Eclipse and Sybil attacks require the attacker to be able to
   exercise control over her 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, these attacks are limited by the use of centralized,
   certificate-based admission control.





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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 only applies 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 are
   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
   the enrollment server uses strict rate limiting policies).  The
   second property deters but does not completely prevent Eclipse
   attacks.  Because an Eclipse attacker must impersonate peers on the
   other side of the attacker, he must have a certificate for suitable
   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 he only has a
   small number of certificates, the region of the Overlay Instance he
   is impersonating appears to be very sparsely populated by comparison
   to the victim's local region.

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 this, all overlay
   routing messages are signed by the peer that generated them.

   Replay is typically prevented for messages that impact the topology



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   of the overlay by having the information come directly, or be
   verified by, the nodes that claimed to have generated the update.
   Data storage replay detection is done by signing time of the node
   that generated the signature on the store request thus providing a
   time based replay protection but the time synchronization is only
   needed 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.

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 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



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   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.


14.  IANA Considerations

   This section contains the new code points registered by this
   document.  [NOTE TO IANA/RFC-EDITOR:  Please replace RFC-AAAA with
   the RFC number for this specification in the following list.]

14.1.  Well-Known URI Registration

   IANA SHALL make the following "Well Known URI" registration as
   described in [RFC5785]:

   [[Note to RFC Editor - this paragraph can be removed before
   publication. ]] A review request was sent to
   wellknown-uri-review@ietf.org on October 12, 2010.

           +----------------------------+----------------------+
           | URI suffix:                | p2psip-enroll        |
           | Change controller:         | IETF <iesg@ietf.org> |
           | Specification document(s): | [RFC-AAAA]           |
           | Related information:       | None                 |
           +----------------------------+----------------------+

14.2.  Port Registrations

   [[Note to RFC Editor - this paragraph can be removed before
   publication. ]] IANA has already allocated a TCP port for the main
   peer to peer protocol.  This port has the name p2p-sip and the port
   number of 6084.  IANA needs to update this registration to be defined
   for UDP as well as TCP.




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   IANA SHALL make the following port registration:

   +------------------------------+------------------------------------+
   | Registration Technical       | Cullen Jennings <fluffy@cisco.com> |
   | Contact                      |                                    |
   | Registration Owner           | IETF <iesg@ietf.org>               |
   | Transport Protocol           | TCP & UDP                          |
   | Port Number                  | 6084                               |
   | Service Name                 | p2psip-enroll                      |
   | Description                  | Peer to Peer Infrastructure        |
   |                              | Enrollment                         |
   | Reference                    | [RFC-AAAA]                         |
   +------------------------------+------------------------------------+

14.3.  Overlay Algorithm Types

   IANA SHALL create a "RELOAD Overlay Algorithm Type" Registry.
   Entries in this registry are strings denoting the names of overlay
   algorithms.  The registration policy for this registry is RFC 5226
   IETF Review.  The initial contents of this registry are:

                       +----------------+----------+
                       | Algorithm Name |      RFC |
                       +----------------+----------+
                       | CHORD-RELOAD   | RFC-AAAA |
                       | EXP-OVERLAY    | RFC-AAAA |
                       +----------------+----------+

   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 SHALL create a "RELOAD Access Control Policy" Registry.  Entries
   in this registry are strings denoting access control policies, as
   described in Section 7.3.  New entries in this registry SHALL be
   registered via RFC 5226 Standards Action.  The initial contents of
   this registry are:

                      +-----------------+----------+
                      | Access Policy   |      RFC |
                      +-----------------+----------+
                      | USER-MATCH      | RFC-AAAA |
                      | NODE-MATCH      | RFC-AAAA |
                      | USER-NODE-MATCH | RFC-AAAA |
                      | NODE-MULTIPLE   | RFC-AAAA |




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                      | EXP-MATCH       | RFC-AAAA |
                      +-----------------+----------+

   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.

14.5.  Application-ID

   IANA SHALL create a "RELOAD Application-ID" Registry.  Entries in
   this registry are 16-bit integers denoting application Kinds.  Code
   points in the range 0x0001 to 0x7fff SHALL be registered via RFC 5226
   Standards Action.  Code points in the range 0x8000 to 0xf000 SHALL be
   registered via RFC 5226 Expert Review.  Code points in the range
   0xf001 to 0xfffe are reserved for private use.  The initial contents
   of this registry are:

     +-------------+----------------+-------------------------------+
     | Application | Application-ID |                 Specification |
     +-------------+----------------+-------------------------------+
     | INVALID     |              0 |                      RFC-AAAA |
     | SIP         |           5060 | Reserved for use by SIP Usage |
     | SIP         |           5061 | Reserved for use by SIP Usage |
     | Reserved    |         0xffff |                      RFC-AAAA |
     +-------------+----------------+-------------------------------+

14.6.  Data Kind-ID

   IANA SHALL create a "RELOAD Data Kind-ID" Registry.  Entries in this
   registry are 32-bit integers denoting data Kinds, as described in
   Section 5.2.  Code points in the range 0x00000001 to 0x7fffffff SHALL
   be registered via RFC 5226 Standards Action.  Code points in the
   range 0x8000000 to 0xf0000000 SHALL be registered via RFC 5226 Expert
   Review.  Code points in the range 0xf0000001 to 0xfffffffe are
   reserved for private use via the Kind description mechanism described
   in Section 11.  The initial contents of this registry are:

              +---------------------+------------+----------+
              | Kind                |    Kind-ID |      RFC |
              +---------------------+------------+----------+
              | INVALID             |          0 | RFC-AAAA |
              | TURN-SERVICE        |          2 | RFC-AAAA |
              | CERTIFICATE_BY_NODE |          3 | RFC-AAAA |
              | CERTIFICATE_BY_USER |         16 | RFC-AAAA |
              | Reserved            | 0x7fffffff | RFC-AAAA |
              | Reserved            | 0xfffffffe | RFC-AAAA |
              +---------------------+------------+----------+




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14.7.  Data Model

   IANA SHALL create a "RELOAD Data Model" Registry.  Entries in this
   registry denoting data models, as described in Section 7.2.  Code
   points in this registry SHALL be registered via RFC 5226 Standards
   Action.  The initial contents of this registry are:

                         +------------+----------+
                         | Data Model |      RFC |
                         +------------+----------+
                         | INVALID    | RFC-AAAA |
                         | SINGLE     | RFC-AAAA |
                         | ARRAY      | RFC-AAAA |
                         | DICTIONARY | RFC-AAAA |
                         | EXP-DATA   | RFC-AAAA |
                         | RESERVED   | RFC-AAAA |
                         +------------+----------+

   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 SHALL create a "RELOAD Message Code" Registry.  Entries in this
   registry are 16-bit integers denoting method codes as described in
   Section 6.3.3.  These codes SHALL be registered via RFC 5226
   Standards Action.  The initial contents of this registry are:























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      +---------------------------------+----------------+----------+
      | Message Code Name               |     Code Value |      RFC |
      +---------------------------------+----------------+----------+
      | invalid                         |              0 | RFC-AAAA |
      | probe_req                       |              1 | RFC-AAAA |
      | probe_ans                       |              2 | RFC-AAAA |
      | attach_req                      |              3 | RFC-AAAA |
      | attach_ans                      |              4 | RFC-AAAA |
      | unused                          |              5 |          |
      | unused                          |              6 |          |
      | store_req                       |              7 | RFC-AAAA |
      | store_ans                       |              8 | RFC-AAAA |
      | fetch_req                       |              9 | RFC-AAAA |
      | fetch_ans                       |             10 | RFC-AAAA |
      | unused (was remove_req)         |             11 | RFC-AAAA |
      | unused (was remove_ans)         |             12 | RFC-AAAA |
      | find_req                        |             13 | RFC-AAAA |
      | find_ans                        |             14 | RFC-AAAA |
      | join_req                        |             15 | RFC-AAAA |
      | join_ans                        |             16 | RFC-AAAA |
      | leave_req                       |             17 | RFC-AAAA |
      | leave_ans                       |             18 | RFC-AAAA |
      | update_req                      |             19 | RFC-AAAA |
      | update_ans                      |             20 | RFC-AAAA |
      | route_query_req                 |             21 | RFC-AAAA |
      | route_query_ans                 |             22 | RFC-AAAA |
      | ping_req                        |             23 | RFC-AAAA |
      | ping_ans                        |             24 | RFC-AAAA |
      | stat_req                        |             25 | RFC-AAAA |
      | stat_ans                        |             26 | RFC-AAAA |
      | unused (was attachlite_req)     |             27 | RFC-AAAA |
      | unused (was attachlite_ans)     |             28 | RFC-AAAA |
      | app_attach_req                  |             29 | RFC-AAAA |
      | app_attach_ans                  |             30 | RFC-AAAA |
      | unused (was app_attachlite_req) |             31 | RFC-AAAA |
      | unused (was app_attachlite_ans) |             32 | RFC-AAAA |
      | config_update_req               |             33 | RFC-AAAA |
      | config_update_ans               |             34 | RFC-AAAA |
      | exp_a_req                       |             35 | RFC-AAAA |
      | exp_a_ans                       |             36 | RFC-AAAA |
      | exp_b_req                       |             37 | RFC-AAAA |
      | exp_b_ans                       |             38 | RFC-AAAA |
      | reserved                        | 0x8000..0xfffe | RFC-AAAA |
      | error                           |         0xffff | RFC-AAAA |
      +---------------------------------+----------------+----------+

   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



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   not meant for vendor specific use of any sort and MUST NOT be used
   for operational deployments.

14.9.  Error Codes

   IANA SHALL create a "RELOAD Error Code" Registry.  Entries in this
   registry are 16-bit integers denoting error codes.  New entries SHALL
   be defined via RFC 5226 Standards Action.  The initial contents of
   this registry are:

    +-------------------------------------+----------------+----------+
    | Error Code Name                     |     Code Value |      RFC |
    +-------------------------------------+----------------+----------+
    | invalid                             |              0 | RFC-AAAA |
    | Unused                              |              1 | RFC-AAAA |
    | Error_Forbidden                     |              2 | RFC-AAAA |
    | Error_Not_Found                     |              3 | RFC-AAAA |
    | Error_Request_Timeout               |              4 | RFC-AAAA |
    | Error_Generation_Counter_Too_Low    |              5 | RFC-AAAA |
    | Error_Incompatible_with_Overlay     |              6 | RFC-AAAA |
    | Error_Unsupported_Forwarding_Option |              7 | RFC-AAAA |
    | Error_Data_Too_Large                |              8 | RFC-AAAA |
    | Error_Data_Too_Old                  |              9 | RFC-AAAA |
    | Error_TTL_Exceeded                  |             10 | RFC-AAAA |
    | Error_Message_Too_Large             |             11 | RFC-AAAA |
    | Error_Unknown_Kind                  |             12 | RFC-AAAA |
    | Error_Unknown_Extension             |             13 | RFC-AAAA |
    | Error_Response_Too_Large            |             14 | RFC-AAAA |
    | Error_Config_Too_Old                |             15 | RFC-AAAA |
    | Error_Config_Too_New                |             16 | RFC-AAAA |
    | Error_In_Progress                   |             17 | RFC-AAAA |
    | Error_Exp_A                         |             18 | RFC-AAAA |
    | Error_Exp_B                         |             19 | RFC-AAAA |
    | Error_Invalid_Message               |             20 | RFC-AAAA |
    | reserved                            | 0x8000..0xfffe | RFC-AAAA |
    +-------------------------------------+----------------+----------+

   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 MUST NOT be used for operational
   deployments.

14.10.  Overlay Link Types

   IANA SHALL create a "RELOAD Overlay Link Registry".  For more
   information on the link types defeind here, see Section 6.6.  New
   entries SHALL be defined via RFC 5226 Standards Action.  This
   registry SHALL be initially populated with the following values:



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               +--------------------+------+---------------+
               | Protocol           | Code | Specification |
               +--------------------+------+---------------+
               | reserved           |    0 |      RFC-AAAA |
               | DTLS-UDP-SR        |    1 |      RFC-AAAA |
               | DTLS-UDP-SR-NO-ICE |    3 |      RFC-AAAA |
               | TLS-TCP-FH-NO-ICE  |    4 |      RFC-AAAA |
               | EXP-LINK           |    5 |      RFC-AAAA |
               | reserved           |  255 |      RFC-AAAA |
               +--------------------+------+---------------+

   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 SHALL create an "Overlay Link Protocol Registry".  Entries in
   this registry SHALL be defined via RFC 5226 Standards Action.  This
   registry SHALL be initially populated with the following valuse:

                     +---------------+---------------+
                     | Link Protocol | Specification |
                     +---------------+---------------+
                     | TLS           |      RFC-AAAA |
                     | EXP-PROTOCOL  |      RFC-AAAA |
                     +---------------+---------------+

   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.

14.12.  Forwarding Options

   IANA SHALL create a "Forwarding Option Registry".  Entries in this
   registry between 1 and 127 SHALL be defined via RFC 5226 Standards
   Action.  Entries in this registry between 128 and 254 SHALL be
   defined via RFC 5226 Specification Required.  This registry SHALL be
   initially populated with the following values:

               +-------------------+------+---------------+
               | Forwarding Option | Code | Specification |
               +-------------------+------+---------------+
               | invalid           |    0 |      RFC-AAAA |
               | exp-forward       |    1 |      RFC-AAAA |
               | reserved          |  255 |      RFC-AAAA |
               +-------------------+------+---------------+




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   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 SHALL create a "RELOAD Probe Information Type Registry".
   Entries in this registry SHALL be defined via RFC 5226 Standards
   Action.  This registry SHALL be initially populated with the
   following values:

                +-----------------+------+---------------+
                | Probe Option    | Code | Specification |
                +-----------------+------+---------------+
                | invalid         |    0 |      RFC-AAAA |
                | responsible_set |    1 |      RFC-AAAA |
                | num_resources   |    2 |      RFC-AAAA |
                | uptime          |    3 |      RFC-AAAA |
                | exp-probe       |    4 |      RFC-AAAA |
                | reserved        |  255 |      RFC-AAAA |
                +-----------------+------+---------------+

   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.

14.14.  Message Extensions

   IANA SHALL create a "RELOAD Extensions Registry".  Entries in this
   registry SHALL be defined via RFC 5226 Specification Required.  This
   registry SHALL be initially populated with the following values:

               +-----------------+--------+---------------+
               | Extensions Name |   Code | Specification |
               +-----------------+--------+---------------+
               | invalid         |      0 |      RFC-AAAA |
               | exp-ext         |      1 |      RFC-AAAA |
               | reserved        | 0xFFFF |      RFC-AAAA |
               +-----------------+--------+---------------+

   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:



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   o  A peer.
   o  A resource inside a peer.

   The reload URI is defined using a subset of the URI schema specified
   in Appendix A of RFC 3986 [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 hex-encoded Destination List object (i.e., multiple
      concatenated Destination objects with no length prefix prior to
      the object as a whole.)

   overlay:    the name of the overlay.

   specifier :  a hex-encoded StoredDataSpecifier indicating the data
      element.

   If no specifier is present then this URI addresses the peer which can
   be reached via the indicated destination list at the indicated
   overlay name.  If a specifier is present, then the URI addresses the
   data value.

14.15.1.  URI Registration

   [[ Note to RFC Editor - please remove this paragraph before
   publication. ]] A review request was sent to uri-review@ietf.org on
   Oct 7, 2010.

   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-AAAA
   URI Scheme Semantics:  The reload URI is intended to be used as a
      reference to a RELOAD peer or resource.






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   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-AAAA.
   Interoperability considerations:  See RFC-AAAA.
   Security considerations:  See RFC-AAAA
   Contact:  Cullen Jennings <fluffy@cisco.com>
   Author/Change controller:  IESG
   References:  RFC-AAAA

14.16.  Media Type Registration

   [[ Note to RFC Editor - please remove this paragraph before
   publication. ]] A review request was sent to ietf-types@iana.org on
   May 27, 2011.

   Type name:  application

   Subtype name:  p2p-overlay+xml

   Required parameters:  none

   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-AAAA.  In the case when the
   media includes a "shared-secret" element, then the contents of the
   file MUST be kept confidential or else anyone that can see the
   shared-secret and effect the RELOAD overlay network.

   Interoperability considerations:  No known interoperability
   consideration beyond those identified for application/xml in
   [RFC3023].

   Published specification:  RFC-AAAA

   Applications that use this media type:  The type is used to configure
   the peer to peer overlay networks defined in RFC-AAAA.

   Additional information:  The syntax for this media type is specified
   in Section 11.1 of RFC-AAAA.  The contents MUST be valid XML
   compliant with the relax NG grammar specified in RFC-AAAA and use the
   UTF-8[RFC3629] character encoding.



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   Magic number(s):  none

   File extension(s):  relo

   Macintosh file type code(s):  none

   Person & email address to contact for further information:  Cullen
   Jennings <c.jennings@ieee.org>

   Intended usage:  COMMON

   Restrictions on usage:  None

   Author:  Cullen Jennings <c.jennings@ieee.org>

   Change controller:  IESG

14.17.  XML Name Space Registration

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

14.17.1.  Config URL

   URI:  urn:ietf:params:xml:ns:p2p:config-base

   Registrant Contact:  The 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)" draft by David A. Bryan, Marcia Zangrilli and Bruce B.
   Lowekamp, the "Address Settlement by Peer to Peer" draft by Cullen
   Jennings, Jonathan Rosenberg, and Eric Rescorla, the "Security
   Extensions for RELOAD" draft by Bruce B. Lowekamp and James Deverick,
   the "A Chord-based DHT for Resource Lookup in P2PSIP" by Marcia
   Zangrilli and David A. Bryan, and the Peer-to-Peer Protocol (P2PP)



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   draft by Salman A. Baset, Henning Schulzrinne, and Marcin
   Matuszewski.  Thanks to the authors of RFC 5389 for text included
   from that.  Vidya Narayanan provided many comments and improvements.

   The ideas and text for the Chord specific extension data to the Leave
   mechanisms was 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, and David Bryan.  Special thanks to Marc Petit-
   Huguenin who provided an amazing amount of detailed review.


16.  References

16.1.  Normative References

   [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.

   [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.

   [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



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              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.

   [RFC4279]  Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
              for Transport Layer Security (TLS)", RFC 4279,
              December 2005.

   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, April 2006.

   [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.

   [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.



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   [RFC6091]  Mavrogiannopoulos, N. and D. Gillmor, "Using OpenPGP Keys
              for Transport Layer Security (TLS) Authentication",
              RFC 6091, February 2011.

   [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.

   [w3c-xml-namespaces]
              Bray, T., Hollander, D., Layman, A., Tobin, R., and Henry
              S. , "Namespaces in XML 1.0 (Third Edition)".

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.

   [Eclipse]  Singh, A., Ngan, T., Druschel, T., and D. Wallach,
              "Eclipse Attacks on Overlay Networks: Threats and
              Defenses", INFOCOM 2006, April 2006.

   [I-D.ietf-hip-reload-instance]
              Keranen, A., Camarillo, G., and J. Maenpaa, "Host Identity
              Protocol-Based Overlay Networking Environment (HIP BONE)
              Instance Specification for REsource LOcation And Discovery
              (RELOAD)", draft-ietf-hip-reload-instance-04 (work in
              progress), October 2011.

   [I-D.ietf-mmusic-ice-tcp]
              Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach,
              "TCP Candidates with Interactive Connectivity
              Establishment (ICE)", draft-ietf-mmusic-ice-tcp-16 (work
              in progress), November 2011.

   [I-D.ietf-p2psip-diagnostics]
              Bryan, D., Jiang, X., Even, R., and H. Song, "P2PSIP
              Overlay Diagnostics", draft-ietf-p2psip-diagnostics-08
              (work in progress), December 2011.

   [I-D.ietf-p2psip-self-tuning]
              Maenpaa, J., Camarillo, G., and J. Hautakorpi, "A Self-
              tuning Distributed Hash Table (DHT) for REsource LOcation



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              And Discovery (RELOAD)", draft-ietf-p2psip-self-tuning-05
              (work in progress), January 2012.

   [I-D.ietf-p2psip-service-discovery]
              Maenpaa, J. and G. Camarillo, "Service Discovery Usage for
              REsource LOcation And Discovery (RELOAD)",
              draft-ietf-p2psip-service-discovery-04 (work in progress),
              January 2012.

   [I-D.ietf-p2psip-sip]
              Jennings, C., Lowekamp, B., Rescorla, E., Baset, S., and
              H. Schulzrinne, "A SIP Usage for RELOAD",
              draft-ietf-p2psip-sip-07 (work in progress), January 2012.

   [I-D.jiang-p2psip-relay]
              Jiang, X., Zong, N., Even, R., and Y. Zhang, "An extension
              to RELOAD to support Direct Response and Relay Peer
              routing", draft-jiang-p2psip-relay-05 (work in progress),
              March 2011.

   [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.

   [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.

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 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.



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   [RFC5201]  Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
              "Host Identity Protocol", RFC 5201, April 2008.

   [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. 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.

   [Sybil]    Douceur, J., "The Sybil Attack", IPTPS 02, March 2002.

   [UnixTime]
              Wikipedia, "Unix Time", <http:/wikipedia.org/wiki/
              Unix_time>.

   [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.

   [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.

   [lookups-churn-p2p06]
              Wu, D., Tian, Y., and K. Ng, "Analytical Study on
              Improving DHT Lookup Performance under Churn",  IEEE
              P2P'06.

   [minimizing-churn-sigcomm06]
              Godfrey, P., Shenker, S., and I. Stoica, "Minimizing Churn



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              in Distributed Systems",  SIGCOMM 2006.

   [non-transitive-dhts-worlds05]
              Freedman, M., Lakshminarayanan, K., Rhea, S., and I.
              Stoica, "Non-Transitive Connectivity and DHTs",
               WORLDS'05.

   [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.

   [vulnerabilities-acsac04]
              Srivatsa, M. and L. Liu, "Vulnerabilities and Security
              Threats in Structured Peer-to-Peer Systems: A Quantitative
              Analysis",  ACSAC 2004.

   [wikiChord]
              Wikipedia, "Chord (peer-to-peer)",
              <http://en.wikipedia.org/wiki/Chord_(peer-to-peer)>.


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.







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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 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].

   An extension to RELOAD that supports forward-only routing but relies
   on symmetric responses as a fallback would be possible, but due to
   the complexities of determining when to use forward-only and when to
   fallback to symmetric, we have chosen not to include it as an option
   at this point.

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 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



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      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 because multiple
      roundtrips are required before the message can be delivered.
   o  If A is behind a NAT and does not have a connection already
      established with Z, there are only two ways the direct response
      will work.  The first is that A and Z 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 only be received if
      A's NAT implements endpoint-independent filtering.  As the choice
      of filtering mode conflates application transparency with security
      [RFC4787], and no clear recommendation is available, the
      prevalence of this feature in future devices remains unclear.

   An extension to RELOAD that supports direct response routing but
   relies on symmetric responses as a fallback would be possible, but
   due to the complexities of determining when to use direct response
   and when to fallback to symmetric, and the reduced performance for
   responses to peers behind restrictive NATs, we have chosen not to
   include it as an option at this point.

A.4.  Relay Peers

   [I-D.jiang-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, plus Z negotiating a secure connection to Q.

   This technique relies on the relative population of nodes such as A
   that require relay peers and peers such as Q that are capable of
   serving as a relay peer.  It also requires nodes to be able to
   identify which category they are in.  This identification problem has
   turned out to be hard to solve and is still an open area of
   exploration.

   An extension to RELOAD that supports relay peers is possible, but due
   to the complexities of implementing such an alternative, we have not



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   added such a feature to RELOAD at this point.

   A concept similar to relay peers, essentially choosing a relay peer
   at random, has previously been suggested to solve problems of
   pairwise non-transitivity [non-transitive-dhts-worlds05], but
   deterministic filtering provided by NATs 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.

   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:



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   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; or 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
   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.
















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Authors' Addresses

   Cullen Jennings
   Cisco
   170 West Tasman Drive
   MS: SJC-21/2
   San Jose, CA  95134
   USA

   Phone:  +1 408 421-9990
   Email:  fluffy@cisco.com


   Bruce B. Lowekamp (editor)
   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




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