P2PSIP Working Group                                            D. Bryan
Internet-Draft                                    SIPeerior Technologies                                         Cogent Force, LLC
Intended status: Informational                               P. Matthews
Expires: January 8, 2009                                    Unaffiliated April 28, 2011                                   Alcatel-Lucent
                                                                 E. Shim
                                                Locus Telecommunications
                                                             Avaya, Inc.
                                                               D. Willis
                                                       Softarmor Systems
                                                              S. Dawkins
                                                            Huawei (USA)
                                                            July 7, 2008
                                                        October 25, 2010

             Concepts and Terminology for Peer to Peer SIP
                     draft-ietf-p2psip-concepts-02

Status
                     draft-ietf-p2psip-concepts-03

Abstract

   This document defines concepts and terminology for the use of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent the
   Session Initiation Protocol in a peer-to-peer environment where the
   traditional proxy-registrar and message routing functions are
   replaced by a distributed mechanism.  These mechansims may be
   implemented using a distributed hash table or other IPR claims distributed data
   mechanism with similar external properties.  This document includes a
   high-level view of which he or she is aware
   have been or will be disclosed, the functional relationships between the network
   elements defined herein, a conceptual model of operations, and any an
   outline of which he or she becomes
   aware will be disclosed, the related problems addressed by the P2PSIP working group
   and the RELOAD protocol ([I-D.ietf-p2psip-base],
   [I-D.ietf-p2psip-sip]) defined by the working group.

Status of this Memo

   This Internet-Draft is submitted in accordance full conformance with Section 6 the
   provisions of BCP 78 and BCP 79.

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

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   The list of current Internet-Drafts can be accessed at
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   This Internet-Draft will expire on January 8, 2009.

Abstract April 28, 2011.

Copyright Notice
   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document defines concepts is subject to BCP 78 and terminology for use of the Session
   Initiation Protocol IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in a peer-to-peer environment where effect on the
   traditional proxy-registrar date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and message routing functions are
   replaced by a distributed mechanism implemented using a distributed
   hash table or other distributed data mechanism restrictions with similar external
   properties.  This respect
   to this document.  Code Components extracted from this document includes a high-level view must
   include Simplified BSD License text as described in Section 4.e of
   the
   functional relationships between the network elements defined herein,
   a conceptual model of operations, Trust Legal Provisions and an outline are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the related open
   problems being addressed by IETF Trust the P2PSIP working group.  As right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document matures, may not be modified
   outside the IETF Standards Process, and derivative works of it is expected to define may
   not be created outside the general framework IETF Standards Process, except to format
   it for
   P2PSIP. publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Author's  Editor's Notes and Changes To This Version . . . . . . . . . .  4
     1.1.  Author's Notes . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Changes from Previous Version  . . . . . . . . . . . . . .  4
   2.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  High Level Description . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Services . . . . . . . . . . . . . . . . . . . . . . . . .  6  5
     3.2.  Clients  . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.3.  Protocol . . . . . . . . . . . . . . .  Relationship Between P2PSIP and RELOAD . . . . . . . . . .  6
     3.4.  Relationship of Peer and Client Protocols  . . . . . . . .  7
     3.5.  Relationship Between P2PSIP and SIP  . . . . . . . . . . .  7
     3.6.  6
     3.5.  Relationship Between P2PSIP and Other AoR
           Dereferencing Approaches . . . . . . . . . . . . . . . . .  7
     3.7.
     3.6.  NAT Issues . . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Reference Model  . . . . . . . . . . . . . . . . . . . . . . .  8  7
   5.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . . 10  9
   6.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 14 13
     6.1.  The Distributed Database Function  . . . . . . . . . . . . 14 13
     6.2.  Using the Distributed Database Function  . . . . . . . . . 16 14
     6.3.  NAT Traversal  . . . . . . . . . . . . . . . . . . . . . . 19 15
     6.4.  Locating and Joining an Overlay  . . . . . . . . . . . . . 21 15
     6.5.  Possible Client Behavior . . . . . . . . . . .  Clients and Connecting Unmodified SIP Devices  . . . . . . 22 16
     6.6.  Interacting with non-P2PSIP entities . . . . . . . . . . . 22
     6.7.  Architecture . . . . . . . . . . . . . . . . . . . . . . . 23 17
   7.  Additional Questions .  Open Issues  . . . . . . . . . . . . . . . . . . . . 24
     7.1.  Selecting between Multiple Peers offering the Same
           Service . . . . . 17
   8.  Informative References . . . . . . . . . . . . . . . . . . . . 24
     7.2.  Visibility of Messages to Intermediate Peers 17
   Authors' Addresses . . . . . . . 25
     7.3.  Using C/S SIP and P2PSIP Simultaneously in a Single UA . . 25
     7.4.  Clients, Peers, and Services . . . . . . . . . . . . . . . 25
     7.5.  Relationships 19

1.  Editor's Notes and Changes To This Version

   This version of Domains to Overlays . . . . . . . . . . . 25

   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 26
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 26

   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26

   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 26
     11.2. Informative References . . . . . . . . . . . . . . . . . . 27

   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28
   Intellectual Property and Copyright Statements . . . . . . . . . . 30

1.  Author's Notes and Changes To This Version

1.1.  Author's Notes

   The editors are currently considering the draft represents a rather substantial revision
   to from the
   previous version.  Until -02, this document work was tracking open questions
   and being used to better reflect help reach consensus on a draft.  With the evolving direction
   eselection of RELOAD as the
   working group.  This version incorporates only minor revisions from protocol for this WG, the -01 version focus of the document.

   In particular, the authors intend
   group turned to make completing the following more
   substantial changes, RELOAD drafts, and solicit the opinion of the WG on these
   changes, as well as to solicit suggestions for text for the new
   sections:

   o  Document directed the current view of
   editors to update the working group that document to reflect the protocols
      being developed decisions made in P2PSIP should be more broadly applicable than
      just for peer-to-peer networks of SIP endpoints.

   o  The authors plan to add a section that documents the history of
      various design decisions, and at the same time remove this
      discussion from other parts of the text.  The authors feel that
      this historical information is important, but also feel that a
      reader needs to be able to quickly
   RELOAD upon completion.

   Please see what the current state of
      the P2PSIP work is today.  An exception would be an early
      explanation of the fact that P2PSIP doesn't use SIP Section 7 for the peer
      protocol, a frequent source of confusion to many people new to the
      WG.

   o  The definition text is somewhat out of date, and should be revised
      (with some terms added and others eliminated, as appropriate)

   o  Incorporate the descriptions of the applications scenarios
      currently described in draft-bryan-p2psip-app-scenarios-00 into
      this document.

1.2.  Changes from Previous Version

   Changes to this version include removal of the prefix "P2PSIP" before
   each definition, and clarification on the issue of clients,
   reflecting the consensus list of the WG. major open issues.

2.  Background

   One of the fundamental problems in multimedia communication between
   Internet nodes is that of discovering the host at which a given user can be
   reached.  In the Session Initiation Protocol (SIP) [RFC3261] this
   problem is expressed as the problem of mapping an Address of Record
   (AoR) for a user into one or more Contact URIs [RFC3986].  The AoR is
   a name for the user that is independent of the host or hosts where
   the user can be contacted, while a Contact URI indicates the host
   where the user can be contacted.

   In the common SIP-using architectures that we refer to as
   "Conventional SIP" or "Client/Server SIP", there is a relatively
   fixed hierarchy of SIP routing proxies and SIP user agents.  To
   deliver a SIP INVITE to the host or hosts at which the user can be
   contacted, a SIP UA follows the procedures specified in [RFC3263] to
   determine the IP address of a SIP proxy, and then sends the INVITE to
   that proxy.  The proxy will then, in turn, deliver the SIP INVITE to
   the hosts where the user can be contacted.

   This document gives a high-level description of an alternative
   solution to this problem.  In this alternative solution, the
   relatively fixed hierarchy of Client/Server SIP is replaced by a
   peer-to-peer overlay network.  In this peer-to-peer overlay network,
   the various AoR to Contact URI mappings are not centralized at proxy/
   registrar nodes but are instead distributed amongst the peers in the
   overlay.

   The details of this alternative solution are currently being worked
   out in specified by the P2PSIP working group. RELOAD
   protocol.  The RELOAD base draft [I-D.ietf-p2psip-base] defines a
   mechanism to distribute using a Distributed Hash Table (DHT) and
   specifies the wire protocol, security, and authentication mechanisms
   needed to convey this information.  This document describes DHT protocol was designed
   specifically with the basic
   concepts purpose of such enabling a peer-to-peer overlay, and lists distributed SIP registrar
   in mind.  While designing the open questions protocol other applications were
   considered, and when possible design decisions were made that still need allow
   RELOAD to be resolved.  As used in other instances where a DHT is desirable, but
   only when making such decisions did not add undue complexity to the work proceeds, it
   RELOAD protocol.  The RELOAD sip draft [I-D.ietf-p2psip-sip]
   specifies how RELOAD is expected
   that this document will develop into a high-level architecture
   document for used with the SIP protocol to enable a
   distributed, server-less SIP solution.

3.  High Level Description

   A P2PSIP Overlay is a collection of nodes organized in a peer-to-peer
   fashion for the purpose of enabling real-time communication using the
   Session Initiation Protocol (SIP).  Collectively, the nodes in the
   overlay provide a distributed mechanism for mapping names to overlay
   locations.  This provides for the mapping of Addresses of Record
   (AoRs) to Contact URIs, thereby providing the "location server"
   function of [RFC3261].  An Overlay also provides a transport function
   by which SIP messages can be transported between any two nodes in the
   overlay.

   A P2PSIP Overlay consists of one or more nodes called Peers.  The
   peers in the overlay collectively run a distributed database
   algorithm.  This distributed database algorithm allows data to be
   stored on peers and retrieved in an efficient manner.  It may also
   ensure that a copy of a data item is stored on more than one peer, so
   that the loss of a peer does not result in the loss of the data item
   to the overlay.

   One use of this distributed database is to store the information
   required to provide the mapping between AoRs and Contact URIs for the
   distributed location function.  This provides a location function
   within each overlay that is an alternative to the location functions
   described in [RFC3263].  However, the model of [RFC3263] is used
   between overlays.

3.1.  Services

   The nature of peer-to-peer computing is that each peer offers
   services to other peers to allow the overlay to collectively provide
   larger functions.  In P2PSIP, peers offer storage and transport
   services to allow the distributed database function and distributed
   transport function to be implemented.  Additionally, the RELOAD
   protocol offers a simplistic discovery mechanism specific to the TURN
   [RFC5766] protocol used for NAT traversal.  It is expected that
   individual peers may also offer other services.  Some of these additional services as an enhancement to
   P2PSIP functionality (for example, a STUN server service
   [I-D.ietf-behave-rfc3489bis]) may be required example to allow the overlay to
   form and operate, while others (for example, a voicemail service) may
   be enhancements support voicemail) or to the basic P2PSIP functionality. support
   other applications beyond SIP.  To allow peers to offer support these additional services, the distributed
   database may need to store information about services.  For example,
   it
   peers may need to store additional information about which peers offer which
   services, and perhaps what sort of capacity each peer has in the overlay.

   [I-D.ietf-p2psip-service-discovery] describes the mechanism used in
   P2PSIP for
   delivering each listed service. resource discovery.

3.2.  Clients

   An overlay may or may not also include one or more nodes called
   clients.  The role of a client  Clients are supported in the P2PSIP model is still under
   discussion, with a number of suggestions for roles being put forth.
   The group has reached consensus RELOAD protocol as peers that clients will be able to store
   have not joined the overlay, and retrieve information from therefore do not route messages or
   store information.  Clients access the overlay.  Section 6.5 discusses services of the
   possible roles RELOAD
   protocol by connecting to a peer which performs operations on the
   behalf of the client.  Note that in RELOAD there is no distinct
   client protocol.  Instead, a client in connects using the same protocol,
   but never joins the overlay as a peer.  For more detail.

3.3.  Protocol

   Peers information, see
   [I-D.ietf-p2psip-base].

   Note that in an overlay need to speak some protocol between themselves to
   maintain the overlay and context of P2PSIP, there is an additional entity
   that is sometimes referred to store and retrieve data.  Until as a better
   name is found, this protocol has been dubbed client.  A special peer may be a
   member of the in the P2PSIP Peer
   Protocol.  While overlay and may present the use functionality
   of one or all of a SIP for registrar, proxy or redirect server to
   conventional SIP devices (SIP clients).  In this protocol was proposed as way, existing, non-
   modified SIP clients may connect to the
   working group was forming, network.  These unmodified
   SIP devices do not speak the group RELOAD protocol, and this is currently working toward a
   new protocol.

3.4.  Relationship distinct
   concept from the notion of Peer client discussed in the previous
   paragraph.

3.3.  Relationship Between P2PSIP and Client Protocols

   To allow clients to communicate with peers, another protocol is
   required.  Until a better name is found, this RELOAD

   The RELOAD protocol has been
   dubbed defined by the P2PSIP Client Protocol.  The details of this protocol are
   also very much under debate. working group implements a
   DHT primarily for use by server-less, peer-to-peer SIP deployments.
   However, if the client RELOAD protocol exists,
   then it could be used for other applications as
   well.  As such, a "P2PSIP" deployment is agreed that it should generally assumed to be a logical subset of the peer
   protocol.  In other words, the syntax
   use of the peer and client
   protocols may be completely different, RELOAD to implement distributed SIP, but any operation supported by
   client protocol it is also supported by the peer protocol.  This implies
   that clients cannot do anything possible that peers cannot also do.

3.5.
   RELOAD is used as a mechanism to distribute other applications,
   completely unrelated to SIP.

3.4.  Relationship Between P2PSIP and SIP

   Since P2PSIP is about peer-to-peer networks for real-time
   communication, it is expected that most (if not all) peers and clients will be
   coupled with SIP entities. entities (although RELOAD may be used for other
   applications than P2PSIP).  For example, one peer might be coupled
   with a SIP UA, another might be coupled with a SIP proxy, while a
   third might be coupled with a SIP-to-PSTN gateway.  For such nodes, we think of
   the peer or client portion of the node as
   being is logically distinct from the
   SIP entity portion.  However, there is no hard requirement that every
   P2PSIP node (peer or client) be coupled to a SIP entity, and some proposed architectures include peer nodes
   that entity.  As an
   example, additional peers could be placed in the overlay to provide
   additional storage or redundancy for the RELOAD overlay, but might
   not have no any direct SIP function whatsoever.

3.6. capabilities.

3.5.  Relationship Between P2PSIP and Other AoR Dereferencing Approaches

   OPEN ISSUE: Many of the "decisions" made have been moved out of the
   main document.  This one, however, seems to point out a difference.
   Should this section be moved or removed?

   As noted above, the fundamental task of P2PSIP is turning an AoR into
   a Contact.  This task might be approached using zeroconf techniques
   such as multicast DNS and DNS Service Discovery (as in Apple's
   Bonjour protocol), link-local multicast name resolution [RFC4795],
   and dynamic DNS [RFC2136].

   These alternatives were discussed in the P2PSIP Working Group, and
   not pursued as a general solution for a number of reasons related to
   scalability, the ability to work in a disconnected state, partition
   recovery, and so on.  However, there does seem to be some continuing
   interest in the possibility of using DNS-SD and mDNS for
   bootstrapping of P2PSIP overlays.

3.7.

3.6.  NAT Issues

   Network Address Translators (NATs) are impediments to establishing
   and maintaining peer-to-peer networks, since NATs hinder direct
   communication between peers.  Some peer-to-peer network architectures
   avoid this problem by insisting that all peers exist in the same
   address space.  However, in the P2PSIP model, it has been agreed RELOAD provides capabilities that allow
   peers can live to be located in multiple address spaces interconnected by NATs.

   This implies that Peer Protocol connections must be able
   NATs, to allow RELOAD messages to traverse NATs, and to assist in
   transmitting application-level messages (for example SIP messages)
   across NATs.  It also means that the peers must collectively provide a
   distributed transport function that allows

4.  Reference Model

   The following diagram shows a peer to send P2PSIP Overlay consisting of a SIP
   message to any other peer in the overlay - without this function two
   peers in different IP address spaces might not be able to exchange
   SIP messages.

4.  Reference Model

   The following diagram shows a P2PSIP Overlay consisting of a number
   of Peers, one Client, and an ordinary number
   of Peers, one Client, and an ordinary SIP UA.  It illustrates a
   typical P2PSIP overlay but does not limit other compositions or
   variations; for example, Proxy Peer P might also talk to a ordinary
   SIP proxy as well.  The figure is not intended to cover all possible
   architecture variations in this document. variations, but simply to show a deployment with many
   common P2PSIP elements.

                                                  --->PSTN
     +------+    N     +------+     +---------+  /
     |      |    A     |      |     | Gateway |-/
     |  UA  |####T#####|  UA  |#####|   Peer  |########
     | Peer |    N     | Peer |     |    G    |       #   P2PSIP   RELOAD
     |  E   |    A     |  F   |     +---------+       #   Client   P2PSIP
     |      |    T     |      |                       #   Protocol
     +------+    N     +------+                       #    |
        #        A                                    #    |
      NATNATNATNAT                                    #    |
        #                                             #    |   \__/
      NATNATNATNAT                              +-------+  v   /  \
        #        N                              |       |=====/       |#####/ UA \
     +------+    A       P2PSIP Overlay         | Peer  |    /Client\
     |      |    T                              |   Q   |    |___C__|
     |  UA  |    N                              |       |
     | Peer |    A                              +-------+
     |  D   |    T                                    #
     |      |    N                                    #
     +------+    A                                    # P2PSIP RELOAD
        #        T                                    # Peer P2PSIP
        #        N    +-------+        +-------+      # Protocol
        #        A    |       |        |       |      #
        #########T####| Proxy |########| Redir |#######
                 N    | Peer  |        | Peer  |
                 A    |   P   |        |   R   |
                 T    +-------+        +-------+
                        |                 /
                        | SIP            /
                  \__/  /               /
                   /\  / ______________/ SIP
                  /  \/ /
                 / UA \/
                /______\
                SIP UA A

   Figure: P2PSIP Overlay Reference Model

   Here, the large perimeter depicted by "#" represents a stylized view
   of the Overlay (the actual connections could be a mesh, a ring, or
   some other structure).  Around the periphery of the Overlay
   rectangle, we have a number of Peers.  Each peer is labeled with its
   coupled SIP entity -- for example, "Proxy Peer P" means that peer P
   which is coupled with a SIP proxy.  In some cases, a peer or client
   might be coupled with two or more SIP entities.  In this diagram we
   have a PSTN gateway coupled with peer "G", three peers ("D", "E" and
   "F") which are each coupled with a UA, a peer "P" which is coupled
   with a SIP proxy, an ordinary peer "Q", "Q" with no SIP capabilities, and
   one peer "R" which is coupled with a SIP Redirector.  Note that
   because these are all Peers, each is responsible for storing Resource
   Records and transporting messages around the Overlay.

   To the left, two of the peers ("D" and "E") are behind network
   address translators (NATs).  These peers are included in the P2PSIP
   overlay and thus participate in storing resource records and routing
   messages, despite being behind the NATs.

   On the right side, we have a client "C", which uses the RELOAD
   Protocol to communicate with Proxy Peer "Q".  The Client "C" uses
   RELOAD to obtain information from the overlay, but has not inserted
   itself into the overlay, and therefore does not participate in
   routing messages or storing information.

   Below the Overlay, we have a conventional SIP UA "A" which is not
   part of the Overlay, either directly as a peer or indirectly as a
   client.  It speaks neither does not speak the RELOAD P2PSIP protocol, and is not
   participating in the overlay as either a Peer nor Client protocols. Client.  Instead,
   it uses SIP to interact with the Overlay.

   On the right side, we have a client "C", Overlay via an adapter peer or peers
   which uses the Client
   Protocol depicted by "=" to communicate with Proxy Peer "Q".  The
   Client "C" could communicate with a different peer, for example peer
   "F", if it establishes a connection to "F" instead of or in addition
   to "Q".  The exact role that this client plays in the network is
   still under discussion (see Section 6.5). overlay using RELOAD.

   Both the SIP proxy coupled with peer "P" and the SIP redirector
   coupled with peer "R" can serve as adapters between ordinary SIP
   devices and the Overlay.  Each accepts standard SIP requests and
   resolves the next-hop by using the P2PSIP overlay Peer Protocol protocol to interact with
   the routing knowledge of the Overlay, then processes the SIP requests
   as appropriate (proxying or redirecting towards the next-hop).  Note
   that proxy operation is bidirectional - the proxy may be forwarding a
   request from an ordinary SIP device to the Overlay, or from the
   P2PSIP overlay to an ordinary SIP device.

   The PSTN Gateway at peer "G" provides a similar sort of adaptation to
   and from the public switched telephone network (PSTN).

5.  Definitions

   This section defines a number of concepts that are key to
   understanding the P2PSIP work.

   Overlay Network:  An overlay network is a computer network which is
      built on top of another network.  Nodes in the overlay can be
      thought of as being connected by virtual or logical links, each of
      which corresponds to a path, perhaps through many physical links,
      in the underlying network.  For example, many peer-to-peer
      networks are overlay networks because they run on top of the
      Internet.  Dial-up Internet is an overlay upon the telephone
      network. <http://en.wikipedia.org/wiki/P2P_overlay>

   P2P Network:  A peer-to-peer (or P2P) computer network is a network
      that relies primarily on the computing power and bandwidth of the
      participants in the network rather than concentrating it in a
      relatively low number of servers.  P2P networks are typically used
      for connecting nodes via largely ad hoc connections.  Such
      networks are useful for many purposes.  Sharing content files (see
      <http://en.wikipedia.org/wiki/File_sharing>) containing audio,
      video, data or anything in digital format is very common, and
      realtime
      real-time data, such as telephony traffic, is also exchanged using
      P2P technology. <http://en.wikipedia.org/wiki/Peer-to-peer>.  A
      P2P Network may also be called a "P2P Overlay" or "P2P Overlay
      Network" or "P2P Network Overlay", since its organization is not
      at the physical layer, but is instead "on top of" an existing
      Internet Protocol network.

   P2PSIP:  A suite of communications protocols related to the Session
      Initiation Protocol (SIP) [RFC3261] that enable SIP to use peer-
      to-peer techniques for resolving the targets of SIP requests,
      providing SIP message transport, and providing other SIP-related
      functions.  The exact contents of this protocol suite are still
      under discussion, but is likely to  At present, these protocols include the P2PSIP Peer
      Protocol
      [I-D.ietf-p2psip-base], [I-D.ietf-p2psip-sip],
      [I-D.ietf-p2psip-diagnostics], [I-D.ietf-p2psip-service-discovery]
      and may include a P2PSIP Client Protocol (see definitions
      below). [I-D.ietf-p2psip-self-tuning].

   User:  A human that interacts with the overlay through SIP UAs
      located on peers and clients (and perhaps other ways).

      The following terms are defined here only within the scope of
      P2PSIP.  These terms may have conflicting definitions in other
      bodies of literature.  Some earlier versions of this document
      prefixed each term with "P2PSIP" to clarify the term's scope.
      This prefixing has been eliminated from the text; however the
      scoping still applies.

   Overlay Name:  A human-friendly name that identifies a specific
      P2PSIP Overlay.  This is in the format of (a portion of) a URI,
      but may or may not have a related record in the DNS.

   Peer:  A node participating in a P2PSIP Overlay that provides storage
      and transport services to other nodes in that P2PSIP Overlay.
      Each Peer has a unique identifier, known as a Peer-ID, within the
      Overlay.  Each Peer may be coupled to one or more SIP entities.
      Within the Overlay, the peer is capable of performing several
      different operations, including: joining and leaving the overlay,
      transporting SIP messages within the overlay, storing information
      on behalf of the overlay, putting information into the overlay,
      and getting information from the overlay.

   Peer-ID:  Information that uniquely identifies each Peer within a
      given Overlay.  This value is not human-friendly -- in a DHT
      approach, this is a numeric value in the hash space.  These Peer-
      IDs are completely independent of the identifier of any user of a
      user agent associated with a peer.  (Note: This is often called a
      "Node-ID" in the P2P literature).

   Client:  A node participating in a P2PSIP Overlay that is less
      capable than a Peer in some way.  The role of a Client is still
      under debate, with a number of competing proposals (see the
      discussion on this later in the document).  It has been agreed
      that they do have the ability to add, modify, inspect, and delete
      information in the overlay.  Note that the term client does not
      imply that this node is a SIP UAC.  Some have suggested that the
      word 'client' be changed to something else to avoid both this
      confusion and the implication of a client-server relationship.

   User Name:  A human-friendly name for a user.  This name must be
      unique within the overlay, but may be unique in a wider scope.
      User Names are formatted so that they can be used within a URI
      (likely a SIP URI), perhaps in combination with the Overlay Name.

   Service:  A capability contributed by a peer to an overlay or to the
      members of an overlay.  It is expected that not all peers and
      clients will offer the same set of services, so a means of finding
      peers (and perhaps clients) that offer a particular service is
      required.  Services might include routing of requests, storing of
      routing data, storing of other data, STUN discovery, STUN relay,
      and many other things.  This model posits a requirement for a
      service locator function, possibly including supporting
      information such as the capacity of a peer to provide a specific
      service or descriptions of the policies under which a peer will
      provide that service.  We currently expect that we will need to be
      able to search for available service providers within each
      overlay.  We think we might need to be able to make searches based
      on network locality or path minimalization.

   Service Name:  A unique, human-friendly, name for a service.

   Resource:  Anything about which information can be stored in the
      overlay.  Both Users and Services are examples of Resources.

   Resource-ID:  A non-human-friendly value that uniquely identifies a
      resource and which is used as a key for storing and retrieving
      data about the resource.  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.

   Resource Record:  A block of data, stored using distributed database
      mechanism of the Overlay, that includes information relevant to a
      specific resource.  We presume that there may be multiple types of
      resource records.  Some may hold data about Users, and others may
      hold data about Services, and the working group may define other
      types.  The types, usages, and formats of the records are a
      question for future study.

   Responsible Peer  The Peer that is responsible for storing the
      Resource Record for a Resource.  In the literature, the term "Root
      Peer" is also used for this concept.

   Peer Protocol:  The protocol spoken between P2PSIP Overlay peers to
      share information and organize the P2PSIP Overlay Network.

   Client Protocol:  The protocol spoken between Clients and Peers.  It
      is used to store and retrieve information from the P2P Overlay.
      The nature of this protocol, and even its existence, is under
      discussion.  However, if it exists, it has been agreed that the
      Client Protocol is a functional subset of the P2P Peer Protocol,
      but may differ in syntax and protocol implementation (i.e., may
      not be syntactically related).

   Peer Protocol Connection / P2PSIP Client Protocol Connection:  The
      TCP, UDP or other transport layer protocol connection over which
      the Peer Protocol (or respectively the Client protocol) is
      transported.

   Neighbors:  The set of P2PSIP Peers that either a Peer or Client know
      of directly and can reach without further lookups.

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

   Bootstrap Peer:  A Peer in the Overlay that is the first point of
      contact for a Joining Peer.  It selects the peer that will serve
      as the Admitting Peer and helps the joining peer contact the
      admitting peer.

   Admitting Peer:  A Peer in the Overlay which helps the Joining Peer
      join the Overlay.  The choice of the admitting peer may depend on
      the joining peer (e.g., depend on the joining peer's Peer-ID).
      For example, the admitting peer might be chosen as the peer which
      is "closest" in the logical structure of the overlay to the future
      position of the joining peer.  The selection of the admitting peer
      is typically done by the bootstrap peer.  It is allowable for the
      bootstrap peer to select itself as the admitting peer.

   Bootstrap Server:  A network node used by Joining Peers to locate a
      Bootstrap Peer.  A Bootstrap Server may act as a proxy for
      messages between the Joining Peer and the Bootstrap Peer.  The
      Bootstrap Server itself is typically a stable host with a DNS name
      that is somehow communicated (for example, through configuration)
      to peers that want to join the overlay.  A Bootstrap Server is NOT
      required to be a peer or client, though it may be if desired.

   Peer Admission:  The act of admitting a node (the "Joining Peer")
      into an Overlay as a Peer.  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 Record Insertion:  The act of inserting a P2PSIP Resource
      Record into the distributed database.  Following insertion, the
      data will be stored at one or more peers.  The data can be
      retrieved or updated using the Resource-ID as a key.

6.  Discussion

6.1.  The Distributed Database Function

   A P2PSIP Overlay functions as a distributed database.  The database
   serves as a way to store information about things called Resources.
   A piece of information, called a Resource Record, can be stored by
   and retrieved from the database using a key associated with the
   Resource Record called its Resource-ID.  Each Resource must have a
   unique Resource-ID.  In addition to uniquely identifying the
   Resource, the Resource-ID is also used by the distributed database
   algorithm to determine the peer or peers that store the Resource
   Record in the overlay.

   It is expected that the P2PSIP working group will standardize the
   way(s) certain types of resources are represented in the distributed
   database.

   One type of resource representation that the working group is
   expected to standardize is information about users.  Users are humans
   that can use the overlay to do things like making and receiving
   calls.  Information stored in the resource record associated with a
   user might include things like the full name of the user and the
   location of the UAs that the user is using.

   Before information about a user can be stored in the overlay, a user
   needs a User Name.  The User Name is a human-friendly identifier that
   uniquely identifies the user within the overlay.  The User Name is
   not a Resource-ID, rather the Resource-ID is derived from the User
   Name using some mapping function (often a cryptographic hash
   function) defined by the distributed database algorithm used by the
   overlay.

   The overlay may also require that the user have a set of credentials.
   Credentials may be required to authenticate the user and/or to show
   that the user is authorized to use the overlay.

   Another type of resource representation that the working group is
   expected to standardize is information about services.  Services
   represent actions that a peer (and perhaps a client) can do to
   benefit other peers and clients in the overlay.  Information that
   might be stored in the resource record associated with a service
   might include the peers (and perhaps clients) offering the service.

   Each service has a human-friendly Service Name that uniquely
   identifies the service.  Like User Names, the Service Name is not a
   resource-id, rather the resource-id is derived from the service name
   using some function defined by the distributed database algorithm
   used by the overlay.

   It is expected that the working group will standardize at least one
   service.  For each standardized service, the working group will
   likely specify the service name, the nature and format of the
   information stored in the resource record associated with the
   service, and the protocol used to access the service.

   The overlay may require that the peer (or client) have a set of
   credentials for a service.  For example, credentials might be
   required to show that the peer (or client) is authorized to offer the
   service, or to show that the peer (or client) is a providing a
   trustworthy implementation of the service.

   It is expected that the P2PSIP WG will not standardize how a User
   Name is obtained, nor how the credentials associated with a User Name
   or a Service Name are obtained, but merely standardize at least one
   acceptable format for each.  To ensure interoperability, it is
   expected that at least one of these formats will be specified as
   "mandatory-to-implement".

   A class of algorithms known as Distributed Hash Tables
   <http://en.wikipedia.org/wiki/P2P_overlay> are one way to implement
   the Distributed Database.  In particular, both the Chord and Bamboo
   algorithms have been suggested as good choices for the distributed
   database algorithm.  However, no decision has been taken so far.

6.2.  Using the Distributed Database Function

   There are a number of ways the distributed database described in the
   previous section might be used to establish multimedia sessions using
   SIP.  In this section, we give four possibilities as examples.  It
   seems likely that the working group will standardize at least one way
   (not necessarily one of the four listed here), but no decisions have
   been taken yet.

   The first option is to store the contact information for a user in
   the resource record for the user.  A peer Y that is a contact point
   for this user adds contact information to this resource record.  The
   resource record itself is stored with peer Z in the network, where
   peer Z is chosen by the distributed database algorithm.

   When the SIP entity coupled with peer X has an INVITE message
   addressed to this user, it retrieves the resource record from peer Z.
   It then extracts the contact information for the various peers that
   are a contact point for the user, including peer Y, and forwards the
   INVITE onward.

   This exchange is illustrated in the following figure.  The notation
   "Put(U@Y)" is used to show the distributed database operation of
   updating the resource record for user U with the contract Y, and
   "Get(U)" illustrates the distributed database operation of retrieving
   the resource record for user U. Note that the messages between the
   peers X, Y and Z may actually travel via intermediate peers (not
   shown) as part of the distributed lookup process or so as to traverse
   intervening NATs.

   Peer X           Peer Z           Peer Y
    |                 |                |
    |                 |       Put(U@Y) |
    |                 |<---------------|
    |                 | Put-Resp(OK)   |
    |                 |--------------->|
    |                 |                |
    | Get(U)          |                |
    |---------------->|                |
    |    Get-Resp(U@Y)|                |
    |<----------------|                |
    | INVITE(To:U)    |                |
    |--------------------------------->|
    |                 |                |

   The second option also involves storing the contact information for a
   user in the resource record of the user.  However, SIP entity at peer
   X, rather than retrieving the resource record from peer Z, instead
   forwards the INVITE message to the proxy at peer Z. The proxy at peer
   Z then uses the information in the resource record and forwards the
   INVITE onwards to the SIP entity at peer Y and the other contacts.

   Peer X           Peer Z           Peer Y
    |                 |                |
    |                 |       Put(U@Y) |
    |                 |<---------------|
    |                 | Put-Resp(OK)   |
    |                 |--------------->|
    |                 |                |
    | INVITE(To:U)    |                |
    |-----------------| INVITE(To:U)   |
    |                 |--------------->|
    |                 |                |

   The third option is for a single peer W to place its contact
   information into the resource record for the user (stored with peer
   Z).  A peer Y that is a contact point for the user retrieves the
   resource record from peer Z, extracts the contact information for
   peer W, and then uses the standard SIP registration mechanism
   [RFC3261] to register with peer W. When the SIP entity at peer X has
   to forward an INVITE request, it retrieves the resource record and
   extracts the contact information for W. It then forwards the INVITE
   to the proxy at peer W, which proxies it onward to peer Y and the
   other contacts.

   Peer X           Peer Z           Peer Y           Peer W
    |                 |                |                 |
    |                 |       Put(U@W) |                 |
    |                 |<---------------------------------|
    |                 | Put-Resp(OK)   |                 |
    |                 |--------------------------------->|
    |                 |                |                 |
    |                 |                |                 |
    |                 |                | REGISTER(To:U)  |
    |                 |                |---------------->|
    |                 |                |             200 |
    |                 |                |<----------------|
    |                 |                |                 |
    |                 |                |                 |
    | Get(U)          |                |                 |
    |---------------->|                |                 |
    |    Get-Resp(U@W)|                |                 |
    |<----------------|                |                 |
    | INVITE(To:U)    |                |                 |
    |--------------------------------------------------->|
    |                 |                |    INVITE(To:U) |
    |                 |                |<----------------|
    |                 |                |                 |

   The fourth option works as in option 3, with the exception that,
   rather than X retrieving the resource record from Z, peer X forwards
   the INVITE to a SIP proxy at Z, which proxies it onward to W and
   hence to Y.

   Peer X           Peer Z           Peer Y           Peer W
    |                 |                |                 |
    |                 |       Put(U@W) |                 |
    |                 |<---------------------------------|
    |                 | Put-Resp(OK)   |                 |
    |                 |--------------------------------->|
    |                 |                |                 |
    |                 |                |                 |
    |                 |                | REGISTER(To:U)  |
    |                 |                |---------------->|
    |                 |                |             200 |
    |                 |                |<----------------|
    |                 |                |                 |
    |                 |                |                 |
    | INVITE(To:U)    |                |                 |
    |---------------->| INVITE(To:U)   |                 |
    |                 |--------------------------------->|
    |                 |                |    INVITE(To:U) |
    |                 |                |<----------------|
    |                 |                |                 |

   The pros and cons of option 1 and 3 are briefly discussed in
   [Using-an-External-DHT].

6.3.  NAT Traversal

   Two approaches to NAT Traversal for P2PSIP Peer Protocol have been
   suggested.  The working group has not made any decision yet on the
   approach that will be selected.

   The first, the traditional approach adopted by most peer-to-peer
   networks today, divides up the peers in the network into two groups:
   those with public IP addresses and those without.  The networks then
   select a subset of the former group and elevate them to "super peer"
   status, leaving the remaining peers as "ordinary peers".  Since super
   peers all have public IP addresses, there are no NAT problems when
   communicating between them.  The network then associates each
   ordinary peer with (usually just one) super peer in a client-server
   relationship.  Once this is done, an ordinary peer X can communicate
   with another ordinary peer Y by sending the message to X's super
   peer, which forwards it to Y's super peer, which forwards it to Y.
   The connection between an ordinary peer and its super peer is
   initiated by the ordinary peer, which makes it easy to traverse any
   intervening NATs.  In this approach, the number of hops between two
   peers is at most 3.

   The second approach treats all peers as equal and establishes a
   partial mesh clients (and perhaps other ways).

      The following terms are defined here only within the scope of connections between them.  Messages from one peer
      P2PSIP.  These terms may have conflicting definitions in other
      bodies of literature.  Some earlier versions of this document
      prefixed each term with "P2PSIP" to
   another are then routed along clarify the edges term's scope.
      This prefixing has been eliminated from the text; however the
      scoping still applies.

   Overlay Name:  A human-friendly name that identifies a specific
      P2PSIP Overlay.  This is in the mesh format of connections
   until they reach their destination.  To make (a portion of) a URI,
      but may or may not have a related record in the routing efficient DNS.

   Peer:  A node participating in a P2PSIP Overlay that provides storage
      and transport services to avoid the use of standard Internet routing protocols, the
   partial mesh is organized other nodes in that P2PSIP Overlay.
      Each Peer has a structured manner.  If unique identifier, known as a Peer-ID, within the structure
   is based on any
      Overlay.  Each Peer may be coupled to one of a number of common DHT algorithms, then or more SIP entities.
      Within the
   maximum number of hops between any two peers is log N, where N Overlay, the peer is capable of performing several
      different operations, including: joining and leaving the
   number overlay,
      transporting SIP messages within the overlay, storing information
      on behalf of peers in the overlay, putting information into the overlay,
      and getting information from the overlay.

   The first approach

   Peer-ID:  Information that uniquely identifies each Peer within a
      given Overlay.  This value is not human-friendly -- in a DHT
      approach, this is significantly more efficient than the second a numeric value in
   overlays with large numbers of peers.  However, the first approach
   assumes there hash space.  These Peer-
      IDs are a sufficient number of peers with public IP
   addresses to serve as super peers.  In some usage scenarios
   envisioned for P2PSIP, this assumption does not hold.  For example,
   this approach fails completely in independent of the case where every peer is behind identifier of any user of a distinct NAT.

   The second approach, while less efficient in overlays
      user agent associated with larger
   numbers of peers, a peer.  (Note: This is efficient often called a
      "Node-ID" in smaller overlays and the P2P literature).

   Client:  A node participating in a P2PSIP Overlay but that does not
      store information or forward messages.  A client can also be made to
   work in many use cases where the first approach fails.

   Both
      thought of these approaches assume as a method of setting up Peer Protocol
   connections between peers.  Many such methods exist; peer that has not joined the now expired
   [I-D.iab-nat-traversal-considerations] is an attempt to give overlay

   User Name:  A human-friendly name for a fairly
   comprehensive list along with user.  This name must be
      unique within the overlay, but may be unique in a discussion of their pros and cons.
   After wider scope.
      User Names are formatted so that they can be used within a consideration of the various techniques, URI
      (likely a SIP URI), perhaps in combination with the P2PSIP working
   group has decided Overlay Name.

   Service:  A capability contributed by a peer to an overlay or to select the Unilateral Self-Address Fixing method
   [RFC3424]
      members of NAT Traversal, an overlay.  Not all peers and in particular clients will offer the ICE
   [I-D.ietf-mmusic-ice] implementation
      same set of this approach.

   The above discussion covers NAT traversal services, and P2PSIP provides service discovery
      mechanisms to locate services.

   Service Name:  A unique, human-friendly, name for Peer Protocol
   connections.  For Client Protocol connections, the approach depends
   on a service.

   Resource:  Anything about which information can be stored in the role adopted for clients
      overlay.  Both Users and we defer the discussion on Services are examples of Resources.

   Resource-ID:  A non-human-friendly value that
   point until the role becomes clearer.

   In addition to Peer Protocol and Client Protocol messages, uniquely identifies a P2PSIP
   Overlay must also provide
      resource and which is used as a solution to the NAT Traversal problem key for
   SIP messages.  If it does not, there is no reliable storing and retrieving
      data about the resource.  One way for a peer
   behind one NAT to send generate a SIP INVITE to Resource-ID is by
      applying a peer behind another NAT.
   One way mapping function to solve this problem some other unique name (e.g., User
      Name or Service Name) for the resource.  The Resource-ID is to transport SIP messages along Peer
   and Client Protocol connections: this could be done either used
      by
   encapsulating the SIP messages inside Peer and Client Protocol
   messages or by multiplexing SIP with distributed database algorithm to determine the Peer (resp.Client) Protocol
   on a Peer (resp. Client) Protocol connection.

   Finally, it should be noted peer or
      peers that are responsible for storing the NAT traversal problem data for media
   connections signaled using SIP is outside the scope overlay.

   Resource Record:  A block of data, stored using distributed database
      mechanism of the P2PSIP
   working group.  As discussed in [I-D.ietf-sipping-nat-scenarios], the
   current recommendation is to use ICE.

6.4.  Locating and Joining an Overlay

   Before a peer can attempt Overlay, that includes information relevant to join a P2PSIP overlay, it must first
   obtain a Peer-ID
      specific resource.  We presume that there may be multiple types of
      resource records.  Some may hold data about Users, and optionally a set others may
      hold data about Services, and the working group may define other
      types.  The types, usages, and formats of credentials. the records are a
      question for future study.

   Responsible Peer  The Peer-ID is
   an identifier Peer that will uniquely identify the peer within is responsible for storing the
   overlay, while
      Resource Record for a Resource.  In the credentials show that literature, the peer term "Root
      Peer" is allowed also used for this concept.

   Peer Protocol:  The protocol spoken between P2PSIP Overlay peers to join
      share information and organize the overlay.

   The P2PSIP WG will not standardize how Overlay Network.  In
      P2PSIP, this is implemented using the peer-ID RELOAD
      [I-D.ietf-p2psip-base] protocol.

   Client Protocol:  The protocol spoken between Clients and Peers.  In
      P2PSIP and RELOAD, this is the
   credentials are obtained, but merely standardize at least one
   acceptable format for each.  To ensure interoperability, it same protocol syntactically as the
      Peer Protocol.  The only difference is
   expected that at least one of these formats will be specified as
   "mandatory-to-implement".

   Once a peer (the "joining peer") has a peer-ID Clients are not
      routing messages or routing information, and optionally a set
   of credentials, it have not (or can attempt to join not)
      insert themselves into the overlay.  To do this, it
   needs to locate a bootstrap peer for the Overlay.

   A bootstrap peer is a peer that serves as the first point of contact
   for

   Peer Protocol Connection / P2PSIP Client Protocol Connection:  The
      TLS, DTLS, TCP, UDP or other transport layer protocol connection
      over which the joining peer. RELOAD Peer Protocol messages are transported.

   Neighbors:  The joining peer uses a bootstrap mechanism to
   locate a bootstrap peer.  Locating a bootstrap peer might be done in
   any one of a number set of different ways:

   o  By remembering peers P2PSIP Peers that were part of the overlay the last time
      the peer was part of the overlay;

   o  Through a multicast discovery mechanism;

   o  Through manual configuration; or

   o  By contacting a P2PSIP Bootstrap Server, and using its help Peer or Client know of
      directly and can reach without further lookups.

   Joining Peer:  A node that is attempting to
      locate become a bootstrap peer.

   The joining peer might reasonably try each of the methods (and
   perhaps others) in some order or in parallel until it succeeds Peer in
   finding a bootstrap peer.

   The job of
      particular Overlay.

   Bootstrap Peer:  A Peer in the bootstrap peer Overlay that is simple: refer the joining peer to first point of
      contact for a
   peer (called Joining Peer.  It selects the "admitting peer") peer that will help serve
      as the Admitting Peer and helps the joining peer contact the
      admitting peer.

   Admitting Peer:  A Peer in the Overlay which helps the Joining Peer
      join the network. Overlay.  The choice of the admitting peer will often may depend on
      the joining node - for peer (e.g., depend on the joining peer's Peer-ID).
      For example, the admitting peer may might be a chosen as the peer that
   will become a neighbor which
      is "closest" in the logical structure of the overlay to the future
      position of the joining peer.  The selection of the admitting peer in
      is typically done by the overlay. bootstrap peer.  It is
   possible that allowable for the
      bootstrap peer might also serve to select itself as the admitting peer.

   The admitting peer will help

   Bootstrap Server:  A network node used by Joining Peers to locate a
      Bootstrap Peer.  A Bootstrap Server may act as a proxy for
      messages between the joining peer learn about other Joining Peer and the Bootstrap Peer.  The
      Bootstrap Server itself is typically a stable host with a DNS name
      that is somehow communicated (for example, through configuration,
      specification on a web page, or using DHCP) to peers
   in that want to
      join the overlay and establish connections overlay.  A Bootstrap Server is NOT required to them as appropriate. be a peer
      or client, though it may be if desired.

   Peer Admission:  The act of admitting peer and/or the other peers in a node (the "Joining Peer")
      into an Overlay as a Peer.  After the overlay will also do
   whatever else admission process is required to help over,
      the joining peer become is a fully-
   functional peer.  The details fully-functional peer of how this is done will depend on the
   distributed database algorithm used in the overlay.

   At various stages in this
      During the admission process, the joining peer may be asked need to present its
      credentials to show prove that it is authorized has sufficient authority to join the
      overlay.  Similarly,

   Resource Record Insertion:  The act of inserting a P2PSIP Resource
      Record into the various peers contacted may distributed database.  Following insertion, the
      data will be asked stored at one or more peers.  The data can be
      retrieved or updated using the Resource-ID as a key.

6.  Discussion

6.1.  The Distributed Database Function

   A P2PSIP Overlay functions as a distributed database.  The database
   serves as a way to
   present their credentials so store information about Resources.  A piece of
   information, called a Resource Record, can be stored by and retrieved
   from the database using a key associated with the Resource Record
   called its Resource-ID.  Each Resource must have a unique
   Resource-ID.  In addition to uniquely identifying the Resource, the
   Resource-ID is also used by the distributed database algorithm to
   determine the joining peer can verify or peers that it is
   really joining store the Resource Record in the
   overlay.

   Users are humans that can use the overlay it wants to.

6.5.  Possible Client Behavior

   As mentioned above, a number of people have proposed to do things like making
   and receiving calls.  Information stored in the resource record
   associated with a second type user can include things like the full name of
   P2PSIP entity, known as a "P2PSIP client".  The consensus the
   user and the location of the
   group is UAs that the need for entities to store and retrieve user is using (the users
   SIP AoR).  Full details of how this is implemented using RELOAD are
   provided in [I-D.ietf-p2psip-sip]

   Before information
   from about a user can be stored in the Overlay without participating overlay, a user
   needs a User Name.  The User Name is recognized, but a human-friendly identifier that for
   now, little time will spent.  This section presents some of
   uniquely identifies the
   alternatives that have been suggested for user within the possible role of a
   client. overlay.  In one approach, RELOAD, users
   are issued certificates, which in the case of centrally signed
   certificates, identify the User Name as well as a client interacts with certain number of
   Resource-IDs where the user may store their information.  For more
   information, see [I-D.ietf-p2psip-base].

   The P2PSIP overlay through
   an associated suite of protocols also standardizes information about how
   to locate services.  Services represent actions that a peer (or (and
   perhaps several such peers) using the Client
   Protocol.  The client does not run the distributed database
   algorithm, does not store resource records, and is not involved in
   routing messages a client) can do to benefit other peers or clients.  Through interactions
   with its associated peer, a client can insert, modify, examine, and
   remove resource records.  A client may also send SIP messages to its
   associated peer for routing through clients in the
   overlay.  In this approach,  Information that might be stored in the resource record
   associated with a
   client service might include the peers (and perhaps
   clients) offering the service.  Service discovery for P2PSIP is
   defined in [I-D.ietf-p2psip-service-discovery].

   Each service has a node human-friendly Service Name that wants to take advantage of uniquely
   identifies the overlay, but service.  Like User Names, the Service Name is not a
   resource-id, rather the resource-id is
   unable or unwilling to contribute resources back to derived from the overlay.
   This may be achieved service name
   using a subset of some function defined by the Peer Protocol.  Such a
   device need not speak SIP.

   For SIP devices, another distributed database algorithm
   used by the overlay.

   A class of algorithms known as Distributed Hash Tables
   <http://en.wikipedia.org/wiki/P2P_overlay> are one way to realize this functionality implement
   the Distributed Database.  The RELOAD protocol is for a
   Peer to behave as a [RFC3261] proxy/registrar.  SIP devices then use
   standard SIP mechanisms to add, update, and remove registrations and
   to send SIP messages to peers extensible and other clients.  The authors here
   refer
   allows many different DHTs to these devices simply as a "SIP UA", not be implemented, but specifies a "P2PSIP Client",
   mandatory to distinguish it from implement DHT in the concept described above.

6.6.  Interacting with non-P2PSIP entities

   It is possible for network nodes that are not peers or clients to
   interact with form of a P2PSIP overlay.  Such nodes would do this through
   mechanisms not defined by modified Chord DHT.  For
   more information, see [Chord]

6.2.  Using the P2PSIP working group provided they can
   find Distributed Database Function

   While there are a peer or client that supports that mechanism and which will do
   any related P2PSIP operations necessary.  In this section, we briefly
   describe two number of ways this might the distributed database described
   in the previous section can be done.  (Note that these are just
   examples and used to establish multimedia sessions
   using SIP, the descriptions here are not recommendations).

   One example basic mechanism defined in the RELOAD base draft and
   SIP usage is summarized below.  This is a peer that also acts as very simplistic overview.
   For more detailed information, please see the RELOAD base draft.

   Contact information for a standard SIP proxy and
   registrar.  SIP UAs can interact with it using mechanisms defined user is stored in
   [RFC3261].  The peer inserts registrations the resource record for users learned from
   these UAs into
   that user.  Assume that a user is using a device, here called peer A,
   which serves as the distributed database, and retrieves contact point for this user.  The user adds
   contact information when proxying INVITE messages.

   Another example to this resource record, as authorized by the
   RELOAD certificate mechanism.  The resource record itself is a stored
   with peer that has a fully-qualified domain name
   (FQDN) that matches Z in the name of network, where peer Z is chosen by the overlay and acts as a SIP proxy
   for calls coming into particular
   distributed database algorithm in use by the overlay.  A

   When the SIP entity coupled with peer B has an INVITE message
   addressed to
   "user@overlay-name" arrives at this user, it retrieves the resource record from peer (using Z.
   It then extracts the mechanisms in
   [RFC3263]) contact information for the various peers that
   are a contact point for the user, including peer A, and this uses the
   overlay to establish a connection to peer then looks up A, including any
   appropriate NAT traversal (the details of which are not shown).

   Note that RELOAD is used only to establish the user connection.  Once the
   connection is established, messages between the peers are sent using
   ordinary SIP.

   This exchange is illustrated in the following figure.  The notation
   "Store(U@A)" is used to show the distributed database operation of
   updating the resource record for user U with the contract A, and proxies
   "Fetch(U)" illustrates the call onto it.

6.7.  Architecture

   There has been much debate in distributed database operation of
   retrieving the group over what an appropriate
   architecture resource record for P2PSIP should be.  Currently, the group is
   investigating architectures that involve a P2P layer user U. Note that is distinct
   from the applications that run on messages
   between the overlay.
         __________________________ peers A, B and Z may actually travel via intermediate
   peers (not shown) as part of the distributed lookup process or so as
   to traverse intervening NATs.

         Peer B           Peer Z           Peer A
         |                    |                   |    SIP, other apps...
         |                    |       ___________________|         Store(U@Y)|
         |                    |<------------------|
         |   P2P Layer                    |Store-Resp(OK)     |
        |______|___________________|
         |     Transport Layer                    |------------------>|
         |
        |__________________________|

   The P2P layer implements the Peer Protocol (and the Client Protocol,
   if such a protocol exists).  Applications access this P2P layer for
   various overlay-related services.  Applications are also free to
   bypass this layer                    |                   |
         |Fetch(U)            |                   |
         |------------------->|                   |
         |     Fetch-Resp(U@Y)|                   |
         |<-------------------|                   |
         |                    |                   |
          (RELOAD IS USED TO ESTABLISH CONNECTION)
         |                    |                   |
         | SIP INVITE(To:U)   |                   |
         |--------------------------------------->|
         |                    |                   |

6.3.  NAT Traversal

   NAT Traversal in P2PSIP using RELOAD treats all peers as equal and access the existing transport layer protocols
   (e.g., TCP, UDP, etc.) directly.

   A notable feature
   establishes a partial mesh of this architecture is that it envisions connections between them.  Messages
   from one peer to another are routed along the use
   of protocols other than SIP edges in the overlay.  Though the working group
   is primarily focused on the use mesh of SIP in peer-to-peer overlays, this
   architecture envisions a future in which other protocols can play a
   role.

   The group initially considered another architecture.  In this
   alternative architecture, the Peer Protocol was defined as an
   extension to SIP.  That is, that
   connections until they reach their destination.  To make the necessary operations for forming routing
   efficient and maintaining to avoid the overlay and for storing and retrieving resource
   records in use of standard Internet routing
   protocols, the distributed database were defined as extensions to
   SIP.  Each peer partial mesh is organized in the overlay was viewed as a SIP proxy that would
   forward structured manner.  If
   the overlay maintenance and distributed database query
   messages (expressed in SIP) structure is based on behalf any one of other peers.

   This architecture was eventually rejected by a number of common DHT
   algorithms, then the working group for maximum number of hops between any two peers is
   log N, where N is the following reasons:

   o  The architecture was totally focused on SIP, and made it difficult
      to use other protocols number of peers in the overlay.

   o  In SIP, proxies are assumed to be trusted parties.  Relying on  Existing
   connections, along with the
      peers ICE NAT traversal techniques [RFC5245],
   are used to route the message as proxies exposes the SIP messages establish new connections between peers, and also to
      attacks from untrusted proxies that SIP's design does not
      anticipate.  A design that does not
   allow the applications running on peers to modify the
      SIP message and ideally prevents them from reading it is
      preferable.

   o  SIP was seen as a "heavy-weight" protocol for this task.  SIP uses establish a text-based encoding which is very flexible, but leads connection to both
      large messages
   communicate with one another.

6.4.  Locating and slow processing times at proxies.  This was
      seen Joining an Overlay

   Before a peer can attempt to be join a poor match for P2PSIP, where P2PSIP overlay, it must first
   obtain a distributed database
      lookup operation requires O(log N) peers to receive, process and
      forward the message.

   More discussion on this alternate approach Peer-ID, configuration information, and why it was rejected
   can be found on the P2PSIP mailing list in optionally a thread that started on
   20 March 2007.

7.  Additional Questions

   This section lists some additional questions set of
   credentials.  The Peer-ID is an identifier that will uniquely
   identify the proposed P2PSIP
   Working Group may need to consider in the process of defining peer within the
   Peer and Client protocols.

7.1.  Selecting between Multiple Peers offering overlay, while the Same Service

   If a P2PSIP network contains two or more peers credentials show that offer
   the same
   service, then how does a peer or client that wishes is allowed to use that
   service select join the peer to use?  This question comes up in a number
   of contexts:

   o  When two or more peers are willing to serve as overlay.

   The P2PSIP WG does not impose a STUN Relay, particular mechanism for how
      do we select a peer that is close in the netpath sense and is
      otherwise appropriate
   peer-ID and the credentials are obtained, but the RELOAD base draft
   does specify the format for the call?

   o  When two or more peers are willing to serve as PSTN gateways, configuration information, and
   specifies how
      do we select an appropriate gateway for this information may be obtained, along with
   credentials and a call that Peer-ID, from an offline enrollment server.

   Once the configuration information is both
      netpath efficient and provides good quality or inexpensive PSTN
      routing?

   It has been suggested that, at least initially, obtained, the working group
   should restrict itself to defining RELOAD base draft
   specifies a mechanism that can return whereby a list
   of peers offering peer may obtain a service multicast-bootstrap
   address in the configuration file, and not define can broadcast to this address
   to attempt to locate a bootstrap peer.  Additionally, the mechanism peer may
   store previous peers it has seen and attempt to use these as
   bootstrap peers, or may obtain an address for
   selecting a bootstrap peer from that list.

7.2.  Visibility of Messages to Intermediate Peers

   When transporting SIP messages through the overlay, are by
   some other mechanism.  For more information, see the headers
   and/or bodies RELOAD base
   draft.

   The job of the SIP messages visible bootstrap peer is simple: refer the joining peer to a
   peer (called the peers "admitting peer") that will help the
   messages happen to pass through?  If they are, what types of security
   risks does this pose in joining peer
   join the presence of peers that have been
   compromised in some way?

7.3.  Using C/S SIP and P2PSIP Simultaneously in a Single UA

   If a given UA is capable network.  The choice of operating in both P2PSIP and conventional
   SIP modalities (especially simultaneously), is it possible admitting peer will often depend on
   the joining node - for it to
   use and respond to example, the same AOR using both conventional and P2PSIP?
   An example of such a topology might admitting peer may be a UA that registers an AOR
   (say, "sip:alice@example.com") conventionally with a registrar and
   then inserts a resource record for that resource into a P2PSIP
   topology, such peer that both conventional SIP users and P2PSIP users
   (within the overlay or
   will become a federation thereof) would be able to contact
   the user without necessarily traversing some sort neighbor of gateway.  Is
   this something the joining peer in the overlay.  It is
   possible that we want to make work?

7.4.  Clients, Peers, and Services

   1.  Do all the bootstrap peer might also serve as the admitting
   peer.

   The admitting peer will help the joining peer learn about other peers providing routing, storage,
   in the overlay and all establish connections to them as appropriate.  The
   admitting peer and/or the other services,
       or do only some peers provide certain services?

   2.  What services, if any, must all peers provide?

   3.  How we can we describe in the capacity of a overlay will also do
   whatever else is required to help the joining peer for delivering become a
       given service?

7.5.  Relationships fully-
   functional peer.  The details of Domains to Overlays

   1.  Can there be names from more than one domain how this is done will depend on the
   distributed database algorithm used by the overlay.

   At various stages in a single overlay?

   2.  Can there this process, the joining peer may be names from one domain in more than a single overlay?
       If so, how do we route Client/Server SIP requests asked to
   present its credentials to show that it is authorized to join the right
       overlay?

   3.  Can
   overlay.  Similarly, the domain of an AoR various peers contacted may be in more than one overlay?

   4.  Should we have a "default overlay" asked to search for peers
   present their credentials so the joining peer can verify that it is
   really joining the overlay it wants to.

6.5.  Clients and Connecting Unmodified SIP Devices

   As mentioned above, in many
       domains?

8.  Security Considerations

   Building a P2PSIP system has many security considerations, many RELOAD, from the perspective of the protocol,
   clients are simply peers that do not store information, do not route
   messages, and which we have only begun to consider.  We anticipate not inserted themselves into the overlay.
   The same protocol is used for the actual message exchanged.  Note
   that while the protocol documents describing is the actual protocols will deal more
   thoroughly with security topics.

   One critical security issue that same, the client need not implement
   all the capabilities of a peer.  If, for example, it never routes
   messages, it will not need to be addressed capable of processing such messages,
   or understanding a DHT.

   For SIP devices, another way to realize this functionality is
   providing for the privacy a
   Peer to behave as a [RFC3261] proxy/registrar.  SIP devices then use
   standard SIP mechanisms to add, update, and integrity of remove registrations and
   to send SIP messages being routed
   by peer nodes, when those peer nodes might well be hostile.  This is to peers and other clients.  The authors here
   refer to these devices simply as a "SIP UA", not a departure "P2PSIP Client",
   to distinguish it from Client/Server SIP, where the proxies are generally
   operated concept described above.

6.6.  Architecture

   The architecture adopted by enterprises or service providers with whom the users of RELOAD to implement P2PSIP is shown
   below.  An application, for example SIP UAs have (or another application using
   RELOAD) uses RELOAD to locate other peers and (optionally) to
   establish connections to those peers, potentially across NATs.
   Messages may still be exchanged directly between the peers.  The
   overall block diagram for the architecture is as follows:

        __________________________
       |                          |
       |    SIP, other apps...    |
       |       ___________________|
       |      |   RELOAD Layer    |
       |______|___________________|
       |     Transport Layer      |
       |__________________________|

7.  Open Issues

   OPEN ISSUE: Should we include a trust relationship.

9.  IANA Considerations

   This document presently raises no IANA considerations.

10.  Acknowledgements

   This document draws heavily from section that documents previous
   decisions made, to preserve the contributions of many
   participants historical debate and prevent past
   issues from being raised in the P2PSIP Mailing List.  Particular thanks to
   Henning Schulzrinne and Cullen Jennings who spent time future, or simply rely on phone calls
   related the mailing
   list to this text.

11.  References

11.1.  Normative References

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, address these concerns?

   OPEN ISSUE: Should we include the use cases from
   draft-bryan-p2psip-app-scenarios-00 (now expired)?  There was some
   interest in doing so in previous versions, but no conclusion was
   reached.

8.  Informative References

   [Chord]    Singh, K., Stoica, I., Morris, R., Handley, Karger, D., Kaashock,
              M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3263]  Rosenberg, J. Dabek, F., and H. Schulzrinne, "Session Initiation
              Protocol (SIP): Locating SIP Servers", RFC 3263,
              June 2002.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

11.2.  Informative References

   [I-D.bryan-p2psip-reload] Balakrishman, "Chord: A scalable
              peer-to-peer lookup protocol for internet applications",
              IEEE/ACM Transactions on Neworking Volume 11 Issue 1, pp.
              17-32, Feb. 2003.

              Copy available at
              http://pdos.csail.mit.edu/chord/papers/paper-ton.pdf

   [I-D.ietf-p2psip-base]
              Jennings, C., Lowekamp, B., Rescorla, E., Baset, S., and
              H. Schulzrinne, "REsource LOcation And Discovery
              (RELOAD)", draft-bryan-p2psip-reload-04 (work in
              progress), June 2008.

   [I-D.camarillo-hip-bone]
              Camarillo, G., Nikander, P., and J. Hautakorpi, "HIP BONE:
              Host Identity Protocol (HIP) Based Overlay Networking
              Environment", draft-camarillo-hip-bone-01 (work in
              progress), February 2008.

   [I-D.iab-nat-traversal-considerations]
              Rosenberg, J., "Considerations for Selection of Techniques
              for NAT Traversal",
              draft-iab-nat-traversal-considerations-00 (RELOAD)
              Base Protocol", draft-ietf-p2psip-base-11 (work in
              progress), October 2005.

   [I-D.ietf-behave-rfc3489bis]
              Rosenberg, J., Mahy, 2010.

   [I-D.ietf-p2psip-diagnostics]
              Yongchao, S., Jiang, X., Even, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for (NAT) (STUN)",
              draft-ietf-behave-rfc3489bis-16 Bryan, "P2PSIP
              Overlay Diagnostics", draft-ietf-p2psip-diagnostics-04
              (work in progress), July 2008.

   [I-D.ietf-mmusic-ice]
              Rosenberg, 2010.

   [I-D.ietf-p2psip-self-tuning]
              Maenpaa, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address  Translator (NAT)
              Traversal Camarillo, G., and J. Hautakorpi, "A Self-
              tuning Distributed Hash Table (DHT) for Offer/Answer Protocols",
              draft-ietf-mmusic-ice-19 REsource LOcation
              And Discovery (RELOAD)", draft-ietf-p2psip-self-tuning-02
              (work in progress), October 2007.

   [I-D.ietf-sipping-nat-scenarios]
              Boulton, C., Rosenberg, J., July 2010.

   [I-D.ietf-p2psip-service-discovery]
              Maenpaa, J. and G. Camarillo, "Best
              Current Practices for NAT Traversal for SIP",
              draft-ietf-sipping-nat-scenarios-08 (work in progress),
              April 2008.

   [I-D.jiang-p2psip-sep]
              Jiang, X. and H. Zhang, "Service Extensible P2P Peer
              Protocol", draft-jiang-p2psip-sep-01 (work in progress),
              February 2008.

   [I-D.li-p2psip-node-types]
              Wang, Y., "Different types of nodes in P2PSIP",
              draft-li-p2psip-node-types-00 (work in progress),
              December 2007.

   [I-D.matthews-p2psip-id-loc]
              Cooper, E., Johnston, A., and P. Matthews, "An ID/Locator
              Architecture Discovery Usage for P2PSIP", draft-matthews-p2psip-id-loc-01
              REsource LOcation And Discovery (RELOAD)",
              draft-ietf-p2psip-service-discovery-01 (work in progress), February 2008.

   [I-D.pascual-p2psip-clients]
              Pascual, V., Matuszewski, M., Shim,
              July 2010.

   [I-D.ietf-p2psip-sip]
              Jennings, C., Lowekamp, B., Rescorla, E., Zhang, H., and S.
              Yongchao, "P2PSIP Clients",
              draft-pascual-p2psip-clients-01 (work in progress),
              February 2008.

   [I-D.zheng-p2psip-client-protocol]
              Yongchao, Baset, S., Jiang, X., Zhang, H., and
              H. Deng, "P2PSIP
              Client Protocol", draft-zheng-p2psip-client-protocol-01 Schulzrinne, "A SIP Usage for RELOAD",
              draft-ietf-p2psip-sip-05 (work in progress), February 2008. July 2010.

   [RFC2136]  Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, April 1997.

   [RFC3424]  Daigle, L.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and IAB, "IAB Considerations for UNilateral
              Self-Address Fixing (UNSAF) Across Network Address
              Translation", E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3424, November 3261,
              June 2002.

   [RFC4485]

   [RFC3263]  Rosenberg, J. and H. Schulzrinne, "Guidelines for Authors
              of Extensions to the Session "Session Initiation
              Protocol (SIP)", (SIP): Locating SIP Servers", RFC 3263,
              June 2002.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 4485, May 2006. 3986, January 2005.

   [RFC4795]  Aboba, B., Thaler, D., and L. Esibov, "Link-local
              Multicast Name Resolution (LLMNR)", RFC 4795,
              January 2007.

   [Using-an-External-DHT]
              Singh, K.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              April 2010.

   [RFC5766]  Mahy, R., Matthews, P., and H. Schulzrinne, "Using an External DHT as a
              SIP Location Service",  Columbia University Computer
              Science Dept. Tech Report 388).

              Copy available at http://mice.cs.columbia.edu/
              getTechreport.php?techreportID=388/ J. Rosenberg, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.

Authors' Addresses

   David A. Bryan
   SIPeerior Technologies
   3000 Easter Circle
   Cogent Force, LLC
   Williamsburg, Virginia  23188
   USA

   Phone: +1 757 565 0101 571 314 0256
   Email: bryan@sipeerior.com bryan@ethernot.org

   Philip Matthews
   Unaffiliated
   Alcatel-Lucent
   600 March Road
   Ottawa, Ontario  K2K 2E6
   Canada

   Phone: +1 613 592 4343 x224 784 3139
   Email: philip_matthews@magma.ca

   Eunsoo Shim
   Locus Telecommunications
   111 Sylvan Avenue
   Englewood Cliffs,
   Avaya, Inc.
   233 Mt. Airy Road
   Basking Ridge, New Jersey  07632  07920
   USA

   Phone: unlisted

   Email: eunsooshim@gmail.com
   Dean Willis
   Softarmor Systems
   3100 Independence Pkwy #311-164
   Plano, Texas  75075
   USA

   Phone: unlisted +1 214 504 1987
   Email: dean.willis@softarmor.com

   Spencer Dawkins
   Huawei Technologies (USA)

   Phone: +1 214 755 3870
   Email: spencer@wonderhamster.org

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