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rfc:rfc7890

Internet Engineering Task Force (IETF) D. Bryan Request for Comments: 7890 Cogent Force, LLC Category: Informational P. Matthews ISSN: 2070-1721 Nokia

                                                               E. Shim
                                         Samsung Electronics Co., Ltd.
                                                             D. Willis
                                                     Softarmor Systems
                                                            S. Dawkins
                                                          Huawei (USA)
                                                             June 2016
       Concepts and Terminology for Peer-to-Peer SIP (P2PSIP)

Abstract

 This document defines concepts and terminology for using 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 mechanisms may 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 problems addressed by the P2PSIP working
 group, the REsource LOcation And Discovery (RELOAD) protocol, and the
 SIP usage document defined by the working group.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7890.

Bryan, et al. Informational [Page 1] RFC 7890 P2PSIP Concepts and Terminology June 2016

Copyright Notice

 Copyright (c) 2016 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   3
 2.  High-Level Description  . . . . . . . . . . . . . . . . . . .   4
   2.1.  Services  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.2.  Clients . . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.3.  Relationship between P2PSIP and RELOAD  . . . . . . . . .   5
   2.4.  Relationship between P2PSIP and SIP . . . . . . . . . . .   5
   2.5.  Relationship between P2PSIP and Other AoR-Dereferencing
         Approaches  . . . . . . . . . . . . . . . . . . . . . . .   6
   2.6.  NAT Issues  . . . . . . . . . . . . . . . . . . . . . . .   6
 3.  Reference Model . . . . . . . . . . . . . . . . . . . . . . .   6
 4.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   8
 5.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .  12
   5.1.  The Distributed Database Function . . . . . . . . . . . .  12
   5.2.  Using the Distributed Database Function . . . . . . . . .  13
   5.3.  NAT Traversal . . . . . . . . . . . . . . . . . . . . . .  14
   5.4.  Locating and Joining an Overlay . . . . . . . . . . . . .  14
   5.5.  Clients and Connecting Unmodified SIP Devices . . . . . .  15
   5.6.  Architecture  . . . . . . . . . . . . . . . . . . . . . .  16
 6.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
 7.  Informative References  . . . . . . . . . . . . . . . . . . .  16
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

Bryan, et al. Informational [Page 2] RFC 7890 P2PSIP Concepts and Terminology June 2016

1. Background

 One of the fundamental problems in multimedia communication between
 Internet nodes is the rendezvous problem, or 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 mappings of AoRs to Contact URIs are not centralized at
 proxy/registrar nodes but are instead distributed amongst the peers
 in the overlay.
 The details of this alternative solution are specified by the RELOAD
 protocol [RFC6940], which defines a mechanism for distribution using
 a Distributed Hash Table (DHT) and specifies the wire protocol,
 security, and authentication mechanisms needed to convey this
 information.  This DHT protocol was designed specifically with the
 purpose of enabling a distributed SIP registrar in mind.  While
 designing the protocol, other applications were considered, and then
 design decisions were made that allow RELOAD to be used in other
 instances where a DHT is desirable, but only when such decisions did
 not add undue complexity to the RELOAD protocol.  The RELOAD SIP
 document [P2PSIP] specifies how RELOAD is used with the SIP protocol
 to enable a distributed, server-less SIP solution.

Bryan, et al. Informational [Page 3] RFC 7890 P2PSIP Concepts and Terminology June 2016

2. High-Level Description

 A Peer-to-Peer SIP (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
 nodes in the Overlay collectively run a distributed database
 algorithm.  This distributed database algorithm allows data to be
 stored on nodes and retrieved in an efficient manner.  It may also
 ensure that a copy of a data item is stored on more than one node, so
 that the loss of a node 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 both distributed storage
 and distributed message-routing services, allowing these functions to
 be implemented across the Overlay.  Additionally, the RELOAD protocol
 offers a simplistic discovery mechanism specific to the Traversal
 Using Relays around NAT (TURN) [RFC5766] protocol used for NAT
 traversal.  Individual Peers may also offer other services as an
 enhancement to P2PSIP functionality (for example, to support
 voicemail) or to support other applications beyond SIP.  To support
 these additional services, Peers may need to store additional
 information in the Overlay.  [RFC7374] describes the mechanism used
 in P2PSIP for resource discovery.

2.2. Clients

 An Overlay may or may not also include one or more nodes called
 "Clients".  Clients are supported in the RELOAD protocol as peers
 that have not joined the Overlay, and therefore do not route messages

Bryan, et al. Informational [Page 4] RFC 7890 P2PSIP Concepts and Terminology June 2016

 or store information.  Clients access the services of the RELOAD
 protocol by connecting to a Peer that performs operations on the
 behalf of the Client.  Note that in RELOAD, there is no distinct
 client protocol.  Instead, a Client connects using the same protocol,
 but never joins the Overlay as a Peer.  For more information, see
 [RFC6940].
 A special Peer may also be a member of the P2PSIP Overlay and may
 present the functionality of one or all of a SIP registrar, proxy, or
 redirect server to conventional SIP devices (i.e., unmodified SIP
 user agent (UA) or client).  In this way, existing, unmodified SIP
 clients may connect to the P2PSIP network.  Note that in the context
 of P2PSIP, the unmodified SIP client is also sometimes referred to as
 a "client".  These unmodified SIP devices do not speak the RELOAD
 protocol, and this is a distinct concept from the notion of "Client"
 discussed in the previous paragraph.

2.3. Relationship between P2PSIP and RELOAD

 The RELOAD protocol defined by the P2PSIP working group implements a
 DHT primarily for use by server-less, peer-to-peer SIP deployments.
 However, the RELOAD protocol could be used for other applications as
 well.  As such, a "P2PSIP" deployment is generally assumed to be a
 use of RELOAD to implement distributed SIP, but it is possible that
 RELOAD is used as a mechanism to distribute other applications,
 completely unrelated to SIP.

2.4. Relationship between P2PSIP and SIP

 Since P2PSIP is about peer-to-peer networks for real-time
 communication, it is expected that most Peers and Clients will be
 coupled with SIP 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,
 the Peer or Client portion of the node 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.  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 any direct SIP capabilities.

Bryan, et al. Informational [Page 5] RFC 7890 P2PSIP Concepts and Terminology June 2016

2.5. Relationship between P2PSIP and Other AoR-Dereferencing Approaches

 As noted above, the fundamental task of P2PSIP is to turn an AoR into
 a Contact.  This task might be approached using zero configuration
 techniques such as multicast DNS (mDNS) and DNS Service Discovery
 (DNS-SD) [RFC6762] [RFC6763], 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 mDNS and DNS-SD for the
 bootstrapping of P2PSIP overlays.

2.6. NAT Issues

 Network Address Translators (NATs) are impediments to establishing
 and maintaining peer-to-peer networks, since NATs hinder direct
 communication between nodes.  Some peer-to-peer network architectures
 avoid this problem by insisting that all nodes exist in the same
 address space.  However, RELOAD provides capabilities that allow
 nodes to be located in multiple address spaces interconnected by
 NATs, to allow RELOAD messages to traverse NATs, and to assist in
 transmitting application-level messages (for example, SIP messages)
 across NATs.

3. Reference Model

 The following diagram shows a P2PSIP Overlay consisting of a 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 an ordinary
 SIP proxy as well.  The figure is not intended to cover all possible
 architecture variations, but simply to show a deployment with many
 common P2PSIP elements.

Bryan, et al. Informational [Page 6] RFC 7890 P2PSIP Concepts and Terminology June 2016

  1. –>PSTN

+——+ N +——+ +———+ /

   |      |    A     |      |     | Gateway |-/
   |  UA  |####T#####|  UA  |#####|   Peer  |########
   | Peer |    N     | Peer |     |    G    |       #   RELOAD
   |  E   |    A     |  F   |     +---------+       #   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                                    # RELOAD
      #        T                                    # P2PSIP
      #        N    +-------+        +-------+      # Protocol
      #        A    |       |        |       |      #
      #########T####| Proxy |########| Redir |#######
               N    | Peer  |        | Peer  |
               A    |   P   |        |   R   |
               T    +-------+        +-------+
                      |                 /
                      | SIP            /
                \__/  /               /
                 /\  / ______________/ SIP
                /  \/ /
               / UA \/
              /______\
              SIP UA A
               Figure 1: 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
 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
 Public Switched Telephone Network (PSTN) gateway coupled with Peer
 "G", three Peers ("D", "E", and "F") that are each coupled with a UA,
 a Peer "P" that is coupled with a SIP proxy, an ordinary Peer "Q"

Bryan, et al. Informational [Page 7] RFC 7890 P2PSIP Concepts and Terminology June 2016

 with no SIP capabilities, and one Peer "R" that 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" that is not part
 of the Overlay, either directly as a Peer or indirectly as a Client.
 It does not speak the RELOAD P2PSIP protocol and is not participating
 in the Overlay as a Peer or a Client.  Instead, it uses SIP to
 interact with the Overlay via an adapter Peer or Peers that
 communicate with the 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 protocol to interact with
 the routing knowledge of the Overlay, and 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 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 that 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

Bryan, et al. Informational [Page 8] RFC 7890 P2PSIP Concepts and Terminology June 2016

    networks are overlay networks because they run on top of the
    Internet.  Dial-up Internet is an overlay upon the telephone
    network.
 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
    containing audio, video, data, or anything in digital format is
    very common, and real-time data, such as telephony traffic, is
    also exchanged using P2P technology.  A P2P Network may also be
    called a "P2P Overlay", a "P2P Overlay Network", or a "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.  At present, these protocols include [RFC6940],
    [RFC7363], [RFC7374], [RFC7851] and [P2PSIP].
 User:  A human that interacts with the Overlay through SIP UAs
    located on Peers and Clients (and perhaps in 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 draft 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.

Bryan, et al. Informational [Page 9] RFC 7890 P2PSIP Concepts and Terminology June 2016

 Node-ID:  Information that uniquely identifies each Node within a
    given Overlay.  This value is not human-friendly -- in a DHT
    approach, this is a numeric value in the hash space.  These Node-
    IDs are completely independent of the identifier of any user of a
    user agent associated with a Peer.
 Client:  A node that participates in a P2PSIP Overlay but does not
    store information or forward messages.  A Client can also be
    thought of as a peer that has not joined the Overlay.  Clients can
    store and retrieve information from the Overlay.
 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.  Not all Peers and Clients will offer the
    same set of services, and P2PSIP provides service discovery
    mechanisms to locate services.
 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 that 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 data for the Overlay.
 Resource Record:  A block of data, stored using the 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.

Bryan, et al. Informational [Page 10] RFC 7890 P2PSIP Concepts and Terminology June 2016

 Peer Protocol:  The protocol spoken between P2PSIP Overlay Peers to
    share information and organize the P2PSIP Overlay Network.  In
    P2PSIP, this is implemented using the RELOAD protocol [RFC6940].
 Client Protocol:  The protocol spoken between Clients and Peers.  In
    P2PSIP and RELOAD, this is syntactically the same protocol as the
    Peer Protocol.  The only difference is that Clients are not
    routing messages or routing information, and have not (or cannot)
    insert themselves into the Overlay.
 Peer Protocol Connection / P2PSIP Client Protocol Connection:
    The Transport Layer Security (TLS), Datagram Transport Layer
    Security (DTLS), TCP, UDP, or other transport-layer protocol
    connection over which the RELOAD Peer Protocol messages are
    transported.
 Neighbors:  The set of P2PSIP Peers that 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 that 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,
    specification on a web page, or using DHCP) 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.

Bryan, et al. Informational [Page 11] RFC 7890 P2PSIP Concepts and Terminology June 2016

 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.

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 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.
 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 can include things like the full name of the
 user and the location of the UAs that the user is using (the user's
 SIP AoR).  Full details of how this is implemented using RELOAD are
 provided in [P2PSIP].
 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.  In RELOAD, users
 are issued certificates, which in the case of centrally signed
 certificates, identify the User Name as well as a certain number of
 Resource-IDs where the user may store their information.  For more
 information, see [RFC6940].
 The P2PSIP suite of protocols also standardizes information about how
 to locate 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.  Service discovery for P2PSIP is
 defined in [RFC7374].

Bryan, et al. Informational [Page 12] RFC 7890 P2PSIP Concepts and Terminology June 2016

 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.
 A class of algorithms known as Distributed Hash Tables (DHTs) are one
 way to implement the distributed database.  The RELOAD protocol is
 extensible and allows many different DHTs to be implemented, but
 specifies a mandatory-to-implement DHT in the form of a modified
 Chord DHT.  For more information, see [Chord].

5.2. Using the Distributed Database Function

 While there are a number of ways the distributed database described
 in the previous section can be used to establish multimedia sessions
 using SIP, the basic mechanism defined in the RELOAD protocol and SIP
 usage is summarized below.  This is a very simplistic overview.  For
 more detailed information, please see the RELOAD protocol [RFC6940].
 Contact information for a user is stored in the resource record for
 that user.  Assume that a user is using a device, here called "Peer
 A", that serves as the contact point for this user.  The user adds
 contact information to this resource record, as authorized by the
 RELOAD certificate mechanism.  The resource record itself is stored
 with Peer Z in the network, where Peer Z is chosen by the particular
 distributed database algorithm in use by the Overlay.
 When the SIP entity coupled with Peer B 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 A, and uses the
 Overlay to establish a connection to Peer A, including any
 appropriate NAT traversal (the details of which are not shown).
 Note that RELOAD is used only to establish the 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
 "Fetch(U)" illustrates the distributed database operation of
 retrieving the resource record for user U.  Note that the messages
 between the 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.

Bryan, et al. Informational [Page 13] RFC 7890 P2PSIP Concepts and Terminology June 2016

       Peer B           Peer Z           Peer A
       |                    |                   |
       |                    |         Store(U@A)|
       |                    |<------------------|
       |                    |Store-Resp(OK)     |
       |                    |------------------>|
       |                    |                   |
       |Fetch(U)            |                   |
       |------------------->|                   |
       |     Fetch-Resp(U@A)|                   |
       |<-------------------|                   |
       |                    |                   |
        (RELOAD IS USED TO ESTABLISH CONNECTION)
       |                    |                   |
       | SIP INVITE(To:U)   |                   |
       |--------------------------------------->|
       |                    |                   |
      Figure 2: SIP Exchange Using Distributed Database Function

5.3. NAT Traversal

 NAT traversal in P2PSIP using RELOAD treats all Peers as equal and
 establishes a partial mesh of connections between them.  Messages
 from one Peer to another are 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.  Existing
 connections, along with the Interactive Connectivity Establishment
 (ICE) NAT traversal techniques [RFC5245], are used to establish new
 connections between Peers, and also to allow the applications running
 on Peers to establish a connection to communicate with one another.

5.4. Locating and Joining an Overlay

 Before a Peer can attempt to join a P2PSIP Overlay, it must first
 obtain a Node-ID, configuration information, and optionally a set of
 credentials.  The Node-ID is an identifier that uniquely identifies
 the Peer within the Overlay, while the credentials show that the Peer
 is allowed to join the Overlay.

Bryan, et al. Informational [Page 14] RFC 7890 P2PSIP Concepts and Terminology June 2016

 The P2PSIP WG does not impose a particular mechanism for how the
 Peer-ID and the credentials are obtained, but the RELOAD protocol
 does specify the format for the configuration information and how
 this information may be obtained, along with credentials and a
 Node-ID, from an offline enrollment server.
 Once the configuration information is obtained, RELOAD specifies a
 mechanism whereby a Peer may obtain a multicast-bootstrap address in
 the configuration file and broadcast to this address to attempt
 locating a Bootstrap Peer.  Additionally, the Peer may store previous
 Peers it has seen and attempt using these as Bootstrap Peers, or it
 may obtain an address for a Bootstrap Peer by some other mechanism.
 For more information, see the RELOAD protocol.
 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 the Admitting Peer will often depend
 on the Joining Peer -- 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
 Peer.
 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 by 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. Clients and Connecting Unmodified SIP Devices

 As mentioned above, in RELOAD, from the perspective of the protocol,
 Clients are simply peers that do not store information, do not route
 messages, and have not inserted themselves into the Overlay.  The
 same protocol is used for the actual message exchanged.  Note that
 while the protocol is the 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 capable of processing such messages or
 understanding a DHT.

Bryan, et al. Informational [Page 15] RFC 7890 P2PSIP Concepts and Terminology June 2016

 For SIP devices, another way to realize this functionality is for a
 Peer to behave as a proxy/registrar as specified in [RFC3261].  SIP
 devices then use standard SIP mechanisms to add, update, and remove
 registrations and to send SIP messages to Peers and other Clients.
 The authors here refer to these devices simply as a "SIP UA", not a
 "P2PSIP Client", to distinguish it from the concept described above.

5.6. Architecture

 The architecture adopted by RELOAD to implement P2PSIP is shown
 below.  An application (for example, SIP 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      |
                    |__________________________|
            Figure 3: Architecture for Implementing P2PSIP

6. Security Considerations

 This specification is an overview of existing specifications and does
 not introduce any security considerations on its own.  Please refer
 to the security considerations of the respective specifications,
 particularly the RELOAD protocol specification ([RFC6940]) for
 further details.

7. Informative References

 [Chord]    Stoica, I., Morris, R., Liben-Nowell, D., Karger, D.,
            Kaashoek, M., Dabek, F., and H. Balakrishnan, "Chord: A
            scalable peer-to-peer lookup protocol for internet
            applications", IEEE/ACM Transactions on Networking,
            Volume 11, Issue 1, pp. 17-32,
            DOI 10.1109/TNET.2002.808407, February 2003.
 [P2PSIP]   Jennings, C., Lowekamp, B., Rescorla, E., Baset, S.,
            Schulzrinne, H., and T. Schmidt, "A SIP Usage for RELOAD",
            Work in Progress, draft-ietf-p2psip-sip-21, April 2016.

Bryan, et al. Informational [Page 16] RFC 7890 P2PSIP Concepts and Terminology June 2016

 [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
            "Dynamic Updates in the Domain Name System (DNS UPDATE)",
            RFC 2136, DOI 10.17487/RFC2136, April 1997,
            <http://www.rfc-editor.org/info/rfc2136>.
 [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
            A., Peterson, J., Sparks, R., Handley, M., and E.
            Schooler, "SIP: Session Initiation Protocol", RFC 3261,
            DOI 10.17487/RFC3261, June 2002,
            <http://www.rfc-editor.org/info/rfc3261>.
 [RFC3263]  Rosenberg, J. and H. Schulzrinne, "Session Initiation
            Protocol (SIP): Locating SIP Servers", RFC 3263,
            DOI 10.17487/RFC3263, June 2002,
            <http://www.rfc-editor.org/info/rfc3263>.
 [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifier (URI): Generic Syntax", STD 66,
            RFC 3986, DOI 10.17487/RFC3986, January 2005,
            <http://www.rfc-editor.org/info/rfc3986>.
 [RFC4795]  Aboba, B., Thaler, D., and L. Esibov, "Link-local
            Multicast Name Resolution (LLMNR)", RFC 4795,
            DOI 10.17487/RFC4795, January 2007,
            <http://www.rfc-editor.org/info/rfc4795>.
 [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
            (ICE): A Protocol for Network Address Translator (NAT)
            Traversal for Offer/Answer Protocols", RFC 5245,
            DOI 10.17487/RFC5245, April 2010,
            <http://www.rfc-editor.org/info/rfc5245>.
 [RFC5766]  Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
            Relays around NAT (TURN): Relay Extensions to Session
            Traversal Utilities for NAT (STUN)", RFC 5766,
            DOI 10.17487/RFC5766, April 2010,
            <http://www.rfc-editor.org/info/rfc5766>.
 [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
            DOI 10.17487/RFC6762, February 2013,
            <http://www.rfc-editor.org/info/rfc6762>.
 [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
            Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
            <http://www.rfc-editor.org/info/rfc6763>.

Bryan, et al. Informational [Page 17] RFC 7890 P2PSIP Concepts and Terminology June 2016

 [RFC6940]  Jennings, C., Lowekamp, B., Ed., Rescorla, E., Baset, S.,
            and H. Schulzrinne, "REsource LOcation And Discovery
            (RELOAD) Base Protocol", RFC 6940, DOI 10.17487/RFC6940,
            January 2014, <http://www.rfc-editor.org/info/rfc6940>.
 [RFC7363]  Maenpaa, J. and G. Camarillo, "Self-Tuning Distributed
            Hash Table (DHT) for REsource LOcation And Discovery
            (RELOAD)", RFC 7363, DOI 10.17487/RFC7363, September 2014,
            <http://www.rfc-editor.org/info/rfc7363>.
 [RFC7374]  Maenpaa, J. and G. Camarillo, "Service Discovery Usage for
            REsource LOcation And Discovery (RELOAD)", RFC 7374,
            DOI 10.17487/RFC7374, October 2014,
            <http://www.rfc-editor.org/info/rfc7374>.
 [RFC7851]  Song, H., Jiang, X., Even, R., Bryan, D., and Y. Sun,
            "Peer-to-Peer (P2P) Overlay Diagnostics", RFC 7851,
            DOI 10.17487/RFC7851, May 2016,
            <http://www.rfc-editor.org/info/rfc7851>.

Bryan, et al. Informational [Page 18] RFC 7890 P2PSIP Concepts and Terminology June 2016

Authors' Addresses

 David A. Bryan
 Cogent Force, LLC
 Cedar Park, Texas
 United States
 Email: dbryan@ethernot.org
 Philip Matthews
 Nokia
 600 March Road
 Ottawa, Ontario  K2K 2E6
 Canada
 Phone: +1 613 784 3139
 Email: philip_matthews@magma.ca
 Eunsoo Shim
 Samsung Electronics Co., Ltd.
 San 14, Nongseo-dong, Giheung-gu
 Yongin-si, Gyeonggi-do  446-712
 South Korea
 Email: eunsooshim@gmail.com
 Dean Willis
 Softarmor Systems
 3100 Independence Pkwy #311-164
 Plano, Texas  75075
 United States
 Phone: +1 214 504 1987
 Email: dean.willis@softarmor.com
 Spencer Dawkins
 Huawei Technologies (USA)
 Phone: +1 214 755 3870
 Email: spencerdawkins.ietf@gmail.com

Bryan, et al. Informational [Page 19]

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