P2PSIP Working Group                                            D. Bryan
Internet-Draft                           College of William and Mary and
Intended status: Informational                    SIPeerior Technologies
Expires: December 3, 2007 May 18, 2008                                        P. Matthews
                                                                 E. Shim
                                                Locus Telecommunications
                                                               D. Willis
                                                       November 15, 2007

             Concepts and Terminology for Peer to Peer SIP

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This document defines concepts and terminology for use of 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 that might be implemented using a
   distributed hash table or other distributed data mechanism with
   similar external properties.  This document includes a high-level
   view of the functional relationships between the network elements
   defined herein, a conceptual model of operations, and an outline of
   the related open problems being addressed by the P2PSIP working
   group.  As this document matures, it is expected to define the
   general framework for P2PSIP.

Table of Contents

   1.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  4

   2.  High Level Description . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Services . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.2.  Clients  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.3.  Protocol . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.4.  Relationship of Peer and Client Protocols  . . . . . . . .  6
     2.5.  Relationship Between P2PSIP and SIP  . . . . . . . . . . .  6
     2.6.  Relationship Between P2PSIP and Other AoR
           Dereferencing Approaches . . . . . . . . . . . . . . . . .  6
     2.7.  NAT Issues . . . . . . . . . . . . . . . . . . . . . . . .  7

   3.  Reference Model  . . . . . . . . . . . . . . . . . . . . . . .  7

   4.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  9

   5.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     5.1.  The Distributed Database Function  . . . . . . . . . . . . 13
     5.2.  Using the Distributed Database Function  . . . . . . . . . 15
     5.3.  NAT Traversal  . . . . . . . . . . . . . . . . . . . . . . 18
     5.4.  Locating and Joining an Overlay  . . . . . . . . . . . . . 20
     5.5.  Possible Client Behavior . . . . . . . . . . . . . . . . . 21
     5.6.  Interacting with non-P2PSIP entities . . . . . . . . . . . 22
     5.7.  Architecture . . . . . . . . . . . . . . . . . . . . . . . 23

   6.  Additional Questions . . . . . . . . . . . . . . . . . . . . . 23 24
     6.1.  Selecting between Multiple Peers offering the Same
           Service  . . . . . . . . . . . . . . . . . . . . . . . . . 23 24
     6.2.  Visibility of Messages to Intermediate Peers . . . . . . . 23 24
     6.3.  Using C/S SIP and P2PSIP Simultaneously in a Single UA . . 23 25
     6.4.  Clients, Peers, and Services . . . . . . . . . . . . . . . 24 25
     6.5.  Relationships of Domains to Overlays . . . . . . . . . . . 24
     6.6.  Protocol Layering  . . . . . . . . . . . . . . . . . . . . 24 25

   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 25 26

   9.  Changes in This Version  . . . . . . . . . . . . . . . . . . . 25 26

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

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

   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28 29
   Intellectual Property and Copyright Statements . . . . . . . . . . 30

1.  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

   The details of this alternative solution are currently being worked
   out in the P2PSIP working group.  This document describes the basic
   concepts of such a peer-to-peer overlay, and lists the open questions
   that still need to be resolved.  As the work proceeds, it is expected
   that this document will develop into a high-level architecture
   document for the solution.

2.  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 network overlay
   locations.  This provides for the mapping of Addresses of Record
   (AoRs) to Contact URIs, thereby providing the "location server"
   function of [RFC3261].  A P2PSIP 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 P2PSIP 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.

2.1.  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.  It is expected that individual
   peers may also offer other services.  Some of these additional
   services (for example, a STUN server service
   [I-D.ietf-behave-rfc3489bis]) may be required to allow the overlay to
   form and operate, while others (for example, a voicemail service) may
   be enhancements to the basic P2PSIP functionality.

   To allow peers to offer these additional services, the distributed
   database may need to store information about services.  For example,
   it may need to store information about which peers offer which
   services, and perhaps what sort of capacity each peer has for
   delivering each listed service.

2.2.  Clients

   An overlay may or may not also include one or more nodes called
   P2PSIP Clients.  The role of a client in the P2PSIP model is still
   under discussion, with a number of suggestions for roles being put
   forth, and some arguing that clients are not needed at all.  However,
   if they exist, then people agree that they will also be able to store
   and retrieve information from the overlay.  Section 5.5 discusses the
   possible roles of a client in more detail.

2.3.  Protocol

   Peers in an overlay need to speak some protocol between themselves to
   maintain the overlay and to store and retrieve data.  Until a better
   name is found, this protocol has been dubbed the P2PSIP Peer
   Protocol.  The details of this protocol are still very much under
   debate: some have suggested that the protocol should be SIP with some
   extensions, while others have suggested that it should be an entirely
   new protocol.

2.4.  Relationship of Peer and Client Protocols

   If the P2PSIP model also contains clients, then a protocol is needed
   for client - peer communication.  Until a better name is found, this
   protocol has been dubbed the P2PSIP Client Protocol.  The details of
   this protocol are also very much under debate.  However, if the
   client protocol exists, then it is agreed that it should be a logical
   subset of the peer protocol.  In other words, the syntax of the peer
   and client protocols may be completely different, but any operation
   supported by client protocol is also supported by the peer protocol.
   This implies that clients cannot do anything that peers cannot also

2.5.  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.  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 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 have no SIP function whatsoever.

2.6.  Relationship Between P2PSIP and Other AoR Dereferencing Approaches

   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.

2.7.  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 that
   peers can live in multiple address spaces interconnected by NATs.
   This implies that Peer Protocol connections must be able to traverse
   NATs.  It also means that the peers must collectively provide a
   distributed transport function that allows a peer to send 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.

3.  Reference Model

   The following diagram shows a P2PSIP Overlay consisting of a number
   of P2PSIP Peers and Peers, one P2PSIP Client.  It also shows 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.

     +------+    N     +------+     +---------+  /
     |      |    A     |      |     | Gateway |-/
     |  UA  |####T#####|  UA  |#####|   Peer  |########
     | Peer |    N     | Peer |     |    G    |       #   P2PSIP
     |  E   |    A     |  F   |     +---------+       #   Client
     |      |    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
        #        T                                    # Peer
        #        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 P2PSIP Overlay (the actual connections could be a mesh, a
   ring, or some other structure).  Around the periphery of the P2PSIP
   Overlay rectangle, we have a number of P2PSIP 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", and one peer
   "R" which is coupled with a SIP Redirector.  Note that because these
   are all P2PSIP Peers, each is responsible for storing P2PSIP Resource
   Records and transporting messages around the P2PSIP 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.

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

   On the right side, we have a P2PSIP client "C", which uses the P2PSIP
   Client Protocol depicted by "=" to communicate with Proxy Peer "Q".
   The P2PSIP 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 5.5).

   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 P2PSIP Overlay.  Each accepts standard SIP requests
   and resolves the next-hop by using the P2PSIP overlay Peer Protocol
   to interact with the routing knowledge of the P2PSIP 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 P2PSIP overlay, or from the P2PSIP overlay to an ordinary SIP

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

4.  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 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 include the P2PSIP Peer
      Protocol and may include a P2PSIP Client Protocol (see definitions

   P2PSIP Overlay:  A P2PSIP Overlay is an association, collection, or
      federation of nodes that provides SIP registration, SIP message
      transport, and similar functions using a P2P organization, as
      defined by "P2P Network" above.

   P2PSIP 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.

   P2PSIP Peer:  A node participating in a P2PSIP Overlay that provides
      storage and transport services to other nodes in that P2PSIP
      Overlay.  Each P2PSIP Peer has a unique identifier, known as a
      Peer-ID, within the P2PSIP Overlay.  Each P2PSIP Peer may be
      coupled to one or more SIP entities.  Within the P2PSIP 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.

   P2PSIP Peer-ID:  Information that uniquely identifies each P2PSIP
      Peer within a given P2PSIP 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).

   P2PSIP Client:  A node participating in a P2PSIP Overlay that is less
      capable than a P2PSIP Peer in some way.  The role of a P2PSIP
      Client is still under debate, with a number of competing
      proposals, and some have suggested removing the concept entirely
      (see the discussion on this later in the document).  If clients
      exist, then 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:  A human that interacts with the overlay through SIP UAs
      located on peers and clients (and perhaps other ways).

   P2PSIP 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.

   P2PSIP 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.

   P2PSIP Service Name:  A unique, human-friendly, name for a service.

   P2PSIP Resource:  Anything about which information can be stored in
      the overlay.  Both Users and Services are examples of Resources.

   P2PSIP 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.

   P2PSIP Resource Record:  A block of data, stored using distributed
      database mechanism of the P2PSIP 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.

   P2PSIP 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.

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

   P2PSIP Client Protocol:  The protocol spoken between P2PSIP Clients
      and P2PSIP 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).

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

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

   P2PSIP Joining Peer:  A node that is attempting to become a P2PSIP
      Peer in a particular P2PSIP Overlay.

   P2PSIP Bootstrap Peer:  A P2PSIP Peer in the P2PSIP Overlay that is
      the first point of contact for a P2PSIP Joining Peer.  It selects
      the peer that will serve as the P2PSIP Admitting Peer and helps
      the joining peer contact the admitting peer.

   P2PSIP Admitting Peer:  A P2PSIP Peer in the P2PSIP Overlay which
      helps the P2PSIP Joining Peer join the Overlay.  The choice of the
      admitting peer may depend on the joining peer (e.g., depend the
      joining peer's P2PSIP 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.

   P2PSIP Bootstrap Server:  A network node used by P2PSIP Joining Peers
      to locate a P2PSIP Bootstrap Peer.  Typically, a P2PSIP Bootstrap
      Server acts as a proxy for messages between the P2PSIP Joining
      Peer and the P2PSIP Bootstrap Peer.  The P2PSIP 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 P2PSIP Bootstrap Server is NOT
      required to be a peer or client, though it may be if desired.

   P2PSIP Peer Admission:  The act of admitting a node (the "P2PSIP
      Joining Peer") into a P2PSIP Overlay as a P2PSIP 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.

   P2PSIP 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 P2PSIP Resource-ID as a key.

5.  Discussion

5.1.  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

   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

   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

   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.

5.2.  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

5.3.  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 of connections between them.  Messages from one peer to
   another are then routed along the edges in the mesh of connections
   until they reach their destination.  To make the routing efficient
   and to avoid the use of standard Internet routing protocols, the
   partial mesh is organized in a structured manner.  If the structure
   is based on any one of a number of common DHT algorithms, then the
   maximum number of hops between any two peers is log N, where N is the
   number of peers in the overlay.

   The first approach is significantly more efficient than the second in
   overlays with large numbers of peers.  However, the first approach
   assumes there 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 the case where every peer is behind
   a distinct NAT.

   The second approach, while less efficient in overlays with larger
   numbers of peers, is efficient in smaller overlays and can be made to
   work in many use cases where the first approach fails.

   Both of these approaches assume a method of setting up Peer Protocol
   connections between peers.  Many such methods exist; the now expired
   [I-D.iab-nat-traversal-considerations] is an attempt to give a fairly
   comprehensive list along with a discussion of their pros and cons.
   After a consideration of the various techniques, the P2PSIP working
   group has decided to select the Unilateral Self-Address Fixing method
   [RFC3424] of NAT Traversal, and in particular the ICE
   [I-D.ietf-mmusic-ice] implementation of this approach.

   The above discussion covers NAT traversal for Peer Protocol
   connections.  For Client Protocol connections, the approach depends
   on the role adopted for clients and we defer the discussion on that
   point until the role becomes clearer.

   In addition to Peer Protocol and Client Protocol messages, a P2PSIP
   Overlay must also provide a solution to the NAT Traversal problem for
   SIP messages.  If it does not, there is no reliable way for a peer
   behind one NAT to send a SIP INVITE to a peer behind another NAT.
   One way to solve this problem is to transport SIP messages along Peer
   and Client Protocol connections: this could be done either by
   encapsulating the SIP messages inside Peer and Client Protocol
   messages or by multiplexing SIP with the Peer (resp.Client) Protocol
   on a Peer (resp. Client) Protocol connection.

   Finally, it should be noted that the NAT traversal problem for media
   connections signaled using SIP is outside the scope of the P2PSIP
   working group.  As discussed in [I-D.ietf-sipping-nat-scenarios], the
   current recommendation is to use ICE.

5.4.  Locating and Joining an Overlay

   Before a peer can attempt to join a P2PSIP overlay, it must first
   obtain a Peer-ID and optionally a set of credentials.  The Peer-ID is
   an identifier that will uniquely identify the peer within the
   overlay, while the credentials show that the peer is allowed to join
   the overlay.

   The P2PSIP WG will not standardize how the peer-ID and the
   credentials 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

   Once a peer (the "joining peer") has a peer-ID and optionally a set
   of credentials, it can attempt to join 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 the joining peer.  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 of different ways:

   o  By remembering 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 to
      locate 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 in
   finding a bootstrap peer.

   The job of the bootstrap peer is simple: refer the joining peer to a
   peer (called the "admitting peer") that will help the joining peer
   join the network.  The choice of admitting peer will often depend on
   the joining node - for example, the admitting peer may be a peer that
   will become a neighbor of the joining peer in the overlay.  It is
   possible that the bootstrap peer might also serve as the admitting

   The admitting peer will help the joining peer learn about other peers
   in the overlay and establish connections to them as appropriate.  The
   admitting peer and/or the other peers in the overlay will also do
   whatever else is required to help the joining peer become a fully-
   functional peer.  The details of how this is done will depend on the
   distributed database algorithm used in the overlay.

   At various stages in this process, the joining peer may be asked to
   present its credentials to show that it is authorized to join the
   overlay.  Similarly, the various peers contacted may be asked to
   present their credentials so the joining peer can verify that it is
   really joining the overlay it wants to.

5.5.  Possible Client Behavior

   As mentioned above, a number of people have proposed a second type of
   P2PSIP entity, known as a "P2PSIP client".  The question of whether
   the concept of a "client" is needed and, if it is needed, its exact
   nature, is still very much under debate.  This section presents some
   of the alternatives that have been suggested for the possible role of
   a client.

   In one approach, a client interacts with the P2PSIP overlay through
   an associated peer (or 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 to other peers or clients.  Through interactions
   with its associated peer, a client can insert, modify, examine, and
   remove resource records.  A client can also send SIP messages to its
   associated peer for routing through the overlay.  In this approach, a
   client is a node that wants to take advantage of the overlay, but is
   unable or unwilling to contribute resources back to the overlay.

   One way to realize this alternative is for a peer to behave as a
   [RFC3261] proxy/registrar.  Clients then use standard SIP mechanisms
   to add, update, and remove registrations and to send SIP messages to
   peers and other clients.  If this is done, there is no need for a
   separate Client Protocol and no need for P2PSIP to define a distinct
   "P2PSIP Client" concept.

   In a second alternative, a client behaves in a way similar to the way
   described in first alternative, except that it does store resource
   records.  In essence, the client contributes its storage capacity to
   its associated peer.  A peer which needs to store a resource record
   may elect to store this on one or more of its associated clients
   instead, thus boosting its effective storage capacity.

   In a third alternative, a client acts almost the same as a peer,
   except that it does not store any resource records.  In this
   alternative, a client has a "peer-ID" and joins the overlay in the
   same way as a peer, perhaps establishing the same network of
   connections that a peer would.  Clients participate in the
   distributed database algorithm, and can help in transporting messages
   to other peers and clients.  However, the distributed database
   algorithm does not assign resource records to clients.  The role of a
   client in this model has been described as "a peer with bad memory".

   Another way to look at this distinction is that a client is simply a
   peer that is not currently offering some or all services to the
   overlay, possibly due to a run-time decision about available
   resources such as bandwidth or storage capacity.  With this approach,
   the distinction between client and peer becomes much less distinct,
   and probably eliminates the requirement to have two distinct terms
   for the roles.  Rather, we might speak in terms such as "high-
   function" vs "low-function" peers.  This approach would also seem to
   eliminate the requirement for a distinct P2PSIP Client Protocol.

   It has also been proposed that the client role could be fulfilled by
   conventional SIP UAs served by a peer that is also acting as a proxy/
   registrar.  While this might fulfill the requirement, the authors
   contend that such as device is a "SIP UA", not a "P2PSIP Client" as
   defined in this document, and that exclusively using SIP UAs in this
   role eliminates the need for P2PSIP Clients and P2PSIP Client
   Protocol from the architecture.

5.6.  Interacting with non-P2PSIP entities

   It is possible for network nodes that are not peers or clients to
   interact with a P2PSIP overlay.  Such nodes would do this through
   mechanisms not defined by the P2PSIP working group provided they can
   find a peer or client that supports that mechanism and which will do
   any related P2PSIP operations necessary.  In this section, we briefly
   describe two ways this might be done.  (Note that these are just
   examples and the descriptions here are not recommendations).

   One example is a peer that also acts as a standard SIP proxy and
   registrar.  SIP UAs can interact with it using mechanisms defined in
   [RFC3261].  The peer inserts registrations for users learned from
   these UAs into the distributed database, and retrieves contact
   information when proxying INVITE messages.

   Another example is a peer that has a fully-qualified domain name
   (FQDN) that matches the name of the overlay and acts as a SIP proxy
   for calls coming into the overlay.  A SIP INVITE addressed to
   "user@overlay-name" arrives at the peer (using the mechanisms in
   [RFC3263]) and this peer then looks up the user in the distributed
   database and proxies the call onto it.

6.  Additional Questions

   This section lists some additional questions that the proposed P2PSIP
   Working Group may need to consider

5.7.  Architecture

   There has been much debate in the process of defining the
   Peer and Client protocols.

6.1.  Selecting between Multiple Peers offering the Same Service

   If a group over what an appropriate
   architecture for P2PSIP network contains two or more peers that offer should be.  Currently, the same
   service, then how does a peer or client that wishes to use group is
   investigating architectures that
   service select the peer to use?  This question comes up in involve a number
   of contexts:

   o  When two or more peers are willing to serve as a STUN Relay, how
      do we select a peer P2P layer that is close in distinct
   from the netpath sense and applications that run on the overlay.
        |                          |
        |    SIP, other apps...    |
        |       ___________________|
        |      |   P2P Layer       |
        |     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 and access the existing transport layer protocols
   (e.g., TCP, UDP, etc.) directly.

   A notable feature of this architecture is that it envisions the use
   of protocols other than SIP in the overlay.  Though the working group
   is primarily focused on the use of SIP in peer-to-peer overlays, this
   architecture envisions a future in which other protocols can play a

   The group initially considered another architecture.  In this
   alternative architecture, the Peer Protocol was defined as an
   extension to SIP.  That is, that the necessary operations for forming
   and maintaining the overlay and for storing and retrieving resource
   records in the distributed database were defined as extensions to
   SIP.  Each peer in the overlay was viewed as a SIP proxy that would
   forward the overlay maintenance and distributed database query
   messages (expressed in SIP) on behalf of other peers.
   [I-D.bryan-p2psip-dsip] presents a detailed design, and
   [I-D.zangrilli-p2psip-whysip] argues for this general approach.

   This architecture was eventually rejected by the working group for
   the following reasons:

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

   o  In SIP, proxies are assumed to be trusted parties.  Relying on the
      peers to route the message as proxies exposes the SIP messages to
      attacks from untrusted proxies that SIP's design does not
      anticipate.  A design that does not allow the peers to modify the
      SIP message and ideally prevents them from reading it is

   o  SIP was seen as a "heavy-weight" protocol for this task.  SIP uses
      a text-based encoding which is very flexible, but leads to both
      large messages and slow processing times at proxies.  This was
      seen to be a poor match for P2PSIP, where a distributed database
      lookup operation requires O(log N) peers to receive, process and
      forward the message.

   More discussion on this alternate approach and why it was rejected
   can be found on the P2PSIP mailing list in a thread that started on
   20 March 2007.

6.  Additional Questions

   This section lists some additional questions that the proposed P2PSIP
   Working Group may need to consider in the process of defining the
   Peer and Client protocols.

6.1.  Selecting between Multiple Peers offering the Same Service

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

   o  When two or more peers are willing to serve as a STUN Relay, how
      do we select a peer that is close in the netpath sense and is
      otherwise appropriate for the call?

   o  When two or more peers are willing to serve as PSTN gateways, how
      do we select an appropriate gateway for a call that is both
      netpath efficient and provides good quality or inexpensive PSTN

   It has been suggested that, at least initially, the working group
   should restrict itself to defining a mechanism that can return a list
   of peers offering a service and not define the mechanism for
   selecting a peer from that list.

6.2.  Visibility of Messages to Intermediate Peers

   When transporting SIP messages through the overlay, are the headers
   and/or bodies of the SIP messages visible to the peers that the
   messages happen to pass through?  If they are, what types of security
   risks does this pose in the presence of peers that have been
   compromised in some way?

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

   If a given UA is capable of operating in both P2PSIP and conventional
   SIP modalities (especially simultaneously), is it possible for it to
   use and respond to the same AOR using both conventional and P2PSIP?
   An example of such a topology might 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 that both conventional SIP users and P2PSIP users
   (within the overlay or a federation thereof) would be able to contact
   the user without necessarily traversing some sort of gateway.  Is
   this something that we want to make work?

6.4.  Clients, Peers, and Services

   1.  Do all peers providing routing, storage, and all other services,
       or do only some peers provide certain services?

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

   3.  Do we need clients as a discrete class, or do SIP UAs and/or low-
       function peers completely satisfy the requirements?

   4.  How we can we describe the capacity of a peer for delivering a
       given service?

6.5.  Relationships of Domains to Overlays

   1.  Can there be names from more than one domain in a single overlay?

   2.  Can there be names from one domain in more than a single overlay?
       If so, how do we route Client/Server SIP requests to the right

   3.  Can the domain of an AoR be in more than one overlay?

   4.  Should we have a "default overlay" to search for peers in many

6.6.  Protocol Layering

   It is possible to define the overlay operations as extensions to SIP
   as proposed in [I-D.bryan-p2psip-dsip] or using a lower level
   transport protocol (or distinct P2P protocol layer below SIP) as
   described in [I-D.matthews-p2psip-hip-hop], [I-D.bryan-p2psip-reload]
   and [I-D.marocco-p2psip-xpp-pcan].  Which is the better or more
   appropriate model?  It seems that this requires analysis along
   several axes, presumably described below in descending order of

   1.  Functionality: Which approach more completely meets the
       functional requirements implicit in the conceptual model?

   2.  Simplicity: Ease of implementation.  Which approach can be more
       readily turned into running code?

   3.  Generality: Do we have a requirement to only provide P2P support
       for SIP operations, or is it likely that other applications will
       need this functionality?  If we make SIP work for P2P, will this
       increase the pressure on SIP to serve as a general transport
       protocol because it solves the rendezvous problem using P2P,
       despite the warnings of [RFC4485] about using SIP as a transport

   4.  Efficiency: Which approach produces the lesser loading on the
       transport layer, the peers themselves, and other infrastructure?

7.  Security Considerations

   Building a P2PSIP system has many security considerations, many of
   which we have only begun to consider.  We anticipate that the
   protocol documents describing the actual protocols will deal more
   thoroughly with security topics.

   One critical security issue that will need to be addressed is
   providing for the privacy and integrity of SIP messages being routed
   by peer nodes, when those peer nodes might well be hostile.  This is
   a departure from Client/Server SIP, where the proxies are generally
   operated by enterprises or service providers with whom the users of
   SIP UAs have a trust relationship.

8.  IANA Considerations

   This document presently raises no IANA considerations.

9.  Changes in This Version

   1.  Revised "Open Questions" to reflect current discussion.

   2.  Resolved conflict between "services provided by overlay" and
       "named services provided by peers" by calling all overlay-level
       operations "functions".  Thus, we would now speak of an overlay
       providing a "distributed transport function".

   3.  Resolved open issue "Does P2PSIP provide a distributed location
       function or an alternative mechanism to RFC 3263?  The answer
       seems to be both, but what is the relationship between these?" by
       documenting that each overlay provides an alternative to
       [RFC3263] within that overlay, but that [RFC3263] is used in the
       conventional manner between overlays.

   4.  Revised abstract to include SIP message routing within the scope.

   5.  Added brief mention of peer's capacity for services offered in
       overview section on distributed database.

   6.  Revised definition of P2PSIP Service.

   7.  Revised abstract and high level discussion.

   8.  Added discussion of proposed peer models and relationship to SIP

   9.  Revised reference model diagram to clarify client behavior.

10.  Acknowledgements

   This document draws heavily from the contributions of many
   participants in the P2PSIP Mailing List but the authors are
   especially grateful for the support of Spencer Dawkins, Cullen
   Jennings, and Henning Schulzrinne, all of whom spent time on phone
   calls about this document or provided text.  In addition, Spencer
   contributed the Reference Model figure.

11.  References

11.1.  Normative References

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3263]  Rosenberg, J. 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

              Bryan, D., "dSIP: A P2P Approach to SIP Registration and
              Resource Location", draft-bryan-p2psip-dsip-00 (work in
              progress), February 2007.

              Bryan, D., "REsource LOcation And Discovery (RELOAD)",
              draft-bryan-p2psip-reload-01 (work in progress),
              July 2007.

              Rosenberg, J., "Considerations for Selection of Techniques
              for NAT Traversal",
              draft-iab-nat-traversal-considerations-00 (work in
              progress), October 2005.

              Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for (NAT) (STUN)", draft-ietf-behave-rfc3489bis-06
              draft-ietf-behave-rfc3489bis-12 (work in progress), March
              November 2007.

              Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address  Translator (NAT)
              Traversal for Offer/Answer Protocols",
              draft-ietf-mmusic-ice-19 (work in progress), June October 2007.

              Boulton, C., "Best Current Practices for NAT Traversal for
              SIP", draft-ietf-sipping-nat-scenarios-06 draft-ietf-sipping-nat-scenarios-07 (work in
              progress), March July 2007.

              Marocco, E. and E. Ivov, "XPP Extensions for Implementing
              a Passive P2PSIP Overlay Network based on  the CAN
              Distributed Hash Table", draft-marocco-p2psip-xpp-pcan-00
              (work in progress), June 2007.

              Cooper, E., "A Distributed Transport Function in P2PSIP
              using HIP for Multi-Hop Overlay  Routing",
              draft-matthews-p2psip-hip-hop-00 (work in progress),
              June 2007.

              Zangrilli, M. and B. Lowekamp, "Why SIP should be used for
              encoding the P2PSIP Peer Protocol.",
              draft-zangrilli-p2psip-whysip-00 (work in progress),
              March 2007.

   [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. and IAB, "IAB Considerations for UNilateral
              Self-Address Fixing (UNSAF) Across Network Address
              Translation", RFC 3424, November 2002.

   [RFC4485]  Rosenberg, J. and H. Schulzrinne, "Guidelines for Authors
              of Extensions to the Session Initiation Protocol (SIP)",
              RFC 4485, May 2006.

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

              Singh, K. 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/

Authors' Addresses

   David A. Bryan
   College of William and Mary and SIPeerior Technologies
   3000 Easter Circle
   Williamsburg, Virginia  23188

   Phone: +1 757 565 0101
   Email: bryan@sipeerior.com

   Philip Matthews
   1135 Innovation Drive
   Ottawa, Ontario  K2K 3G7

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

   Eunsoo Shim
   Locus Telecommunications
   111 Sylvan Avenue
   Englewood Cliffs, New Jersey  07632

   Phone: unlisted
   Email: eunsooshim@gmail.com

   Dean Willis
   3100 Independence Pkwy #311-164
   Plano, Texas  75075

   Phone: unlisted
   Email: dean.willis@softarmor.com

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