P2PSIP Working Group                                            D. BryanInternet-Draft                                         Cogent Force, LLCIntended status: Informational                               P. MatthewsExpires: October 23, 2016                                 Alcatel-Lucent                                                                 E. Shim                                           Samsung Electronics Co., Ltd.                                                               D. Willis                                                       Softarmor Systems                                                              S. Dawkins                                                            Huawei (USA)                                                          April 21, 2016             Concepts and Terminology for Peer to Peer SIP                     draft-ietf-p2psip-concepts-09Abstract   This document defines concepts and terminology for the 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.  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   and the RELOAD protocol and SIP usage document defined by the working   group.Status of This Memo   This Internet-Draft is submitted in full conformance with the   provisions of BCP 78 and BCP 79.   Internet-Drafts are working documents of the Internet Engineering   Task Force (IETF).  Note that other groups may also distribute   working documents as Internet-Drafts.  The list of current Internet-   Drafts is at http://datatracker.ietf.org/drafts/current/.   Internet-Drafts are draft documents valid for a maximum of six months   and may be updated, replaced, or obsoleted by other documents at any   time.  It is inappropriate to use Internet-Drafts as reference   material or to cite them other than as "work in progress."   This Internet-Draft will expire on October 23, 2016.Bryan, et al.           Expires October 23, 2016                [Page 1]Internet-Draft       P2PSIP Concepts and Terminology          April 2016Copyright 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  . . . . . . . . . . . . . . . . . . . . . . . . .   2   2.  High-Level Description  . . . . . . . . . . . . . . . . . . .   3     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  . . . . . . . . . . . . . . . . . . . . . . .   5     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.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16   8.  Informative References  . . . . . . . . . . . . . . . . . . .  16   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  181.  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 isBryan, et al.           Expires October 23, 2016                [Page 2]Internet-Draft       P2PSIP Concepts and Terminology          April 2016   independent of the host or hosts where the user can be contacted,   while a Contact URI indicates the host where the user can be   contacted.   In the common SIP-using architectures that we refer to as   "Conventional SIP" or "Client/Server SIP", there is a relatively   fixed hierarchy of SIP routing proxies and SIP user agents.  To   deliver a SIP INVITE to the host or hosts at which the user can be   contacted, a SIP UA follows the procedures specified in [RFC3263] to   determine the IP address of a SIP proxy, and then sends the INVITE to   that proxy.  The proxy will then, in turn, deliver the SIP INVITE to   the hosts where the user can be contacted.   This document gives a high-level description of an alternative   solution to this problem.  In this alternative solution, the   relatively fixed hierarchy of Client/Server SIP is replaced by a   peer-to-peer overlay network.  In this peer-to-peer overlay network,   the various AoR to Contact URI mappings are not centralized at proxy/   registrar nodes but are instead distributed amongst the peers in the   overlay.   The details of this alternative solution are specified by the RELOAD   protocol [RFC6940], which defines a mechanism to distribute using a   Distributed Hash Table (DHT) and specifies the wire protocol,   security, and authentication mechanisms needed to convey this   information.  This 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 when   possible design decisions were made that allow RELOAD to be used in   other instances where a DHT is desirable, but only when making such   decisions did not add undue complexity to the RELOAD protocol.  The   RELOAD sip draft [I-D.ietf-p2psip-sip] specifies how RELOAD is used   with the SIP protocol to enable a distributed, server-less SIP   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 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.Bryan, et al.           Expires October 23, 2016                [Page 3]Internet-Draft       P2PSIP Concepts and Terminology          April 2016   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 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 or   store information.  Clients access the services of the RELOAD   protocol by connecting to a peer which performs operations on the   behalf of the client.  Note that in RELOAD there is no distinct   client protocol.  Instead, a client 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 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.Bryan, et al.           Expires October 23, 2016                [Page 4]Internet-Draft       P2PSIP Concepts and Terminology          April 2016   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.2.5.  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 zero configuration   techniques such as multicast DNS and DNS Service Discovery   [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 DNS-SD and mDNS for   bootstrapping of P2PSIP overlays.Bryan, et al.           Expires October 23, 2016                [Page 5]Internet-Draft       P2PSIP Concepts and Terminology          April 20162.6.  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 a 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.           Expires October 23, 2016                [Page 6]Internet-Draft       P2PSIP Concepts and Terminology          April 2016                                                  --->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: P2PSIP Overlay Reference Model   Here, the large perimeter depicted by "#" represents a stylized view   of the Overlay (the actual connections could be a mesh, a ring, or   some other structure).  Around the periphery of the Overlay   rectangle, we have a number of Peers.  Each peer is labeled with its   coupled SIP entity -- for example, "Proxy Peer P" means that peer P   which is coupled with a SIP proxy.  In some cases, a peer or client   might be coupled with two or more SIP entities.  In this diagram we   have a PSTN gateway coupled with peer "G", three peers ("D", "E" and   "F") which are each coupled with a UA, a peer "P" which is coupledBryan, et al.           Expires October 23, 2016                [Page 7]Internet-Draft       P2PSIP Concepts and Terminology          April 2016   with a SIP proxy, an ordinary peer "Q" with no SIP capabilities, and   one peer "R" which is coupled with a SIP Redirector.  Note that   because these are all Peers, each is responsible for storing Resource   Records and transporting messages around the Overlay.   To the left, two of the peers ("D" and "E") are behind network   address translators (NATs).  These peers are included in the P2PSIP   overlay and thus participate in storing resource records and routing   messages, despite being behind the NATs.   On the right side, we have a client "C", which uses the RELOAD   Protocol to communicate with Proxy Peer "Q".  The Client "C" uses   RELOAD to obtain information from the overlay, but has not inserted   itself into the overlay, and therefore does not participate in   routing messages or storing information.   Below the Overlay, we have a conventional SIP UA "A" which is not   part of the Overlay, either directly as a peer or indirectly as a   client.  It does not speak the RELOAD P2PSIP protocol, and is not   participating in the overlay as either a Peer nor Client.  Instead,   it uses SIP to interact with the Overlay via an adapter peer or peers   which 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, then processes the SIP requests   as appropriate (proxying or redirecting towards the next-hop).  Note   that proxy operation is bidirectional - the proxy may be forwarding a   request from an ordinary SIP device to the Overlay, or from the   P2PSIP overlay to an ordinary SIP device.   The PSTN Gateway at peer "G" provides a similar sort of adaptation to   and from the public switched telephone network (PSTN).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 theBryan, et al.           Expires October 23, 2016                [Page 8]Internet-Draft       P2PSIP Concepts and Terminology          April 2016      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" 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.  At present, these protocols include [RFC6940],      [I-D.ietf-p2psip-sip], [I-D.ietf-p2psip-diagnostics], [RFC7374]      and [RFC7363].   User:  A human that interacts with the overlay through SIP UAs      located on peers and clients (and perhaps other ways).      The following terms are defined here only within the scope of      P2PSIP.  These terms may have conflicting definitions in other      bodies of literature.  Some earlier versions of this document      prefixed each term with "P2PSIP" to clarify the term's scope.      This prefixing has been eliminated from the text; however the      scoping still applies.   Overlay Name:  A human-friendly name that identifies a specific      P2PSIP Overlay.  This is in the format of (a portion of) a URI,      but may or may not have a related record in the DNS.   Peer:  A node participating in a P2PSIP Overlay that provides storage      and transport services to other nodes in that P2PSIP Overlay.      Each Peer has a unique identifier, known as a Peer-ID, within the      Overlay.  Each Peer may be coupled to one or more SIP entities.      Within the Overlay, the peer is capable of performing several      different operations, including: joining and leaving the overlay,      transporting SIP messages within the overlay, storing information      on behalf of the overlay, putting information into the overlay,      and getting information from the overlay.Bryan, et al.           Expires October 23, 2016                [Page 9]Internet-Draft       P2PSIP Concepts and Terminology          April 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 participating in a P2PSIP Overlay but that 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 which is used as a key for storing and retrieving      data about the resource.  One way to generate a Resource-ID is by      applying a mapping function to some other unique name (e.g., User      Name or Service Name) for the resource.  The Resource-ID is used      by the distributed database algorithm to determine the peer or      peers that are responsible for storing the data for the overlay.   Resource Record:  A block of data, stored using distributed database      mechanism of the Overlay, that includes information relevant to a      specific resource.  We presume that there may be multiple types of      resource records.  Some may hold data about Users, and others may      hold data about Services, and the working group may define other      types.  The types, usages, and formats of the records are a      question for future study.   Responsible Peer  The Peer that is responsible for storing the      Resource Record for a Resource.  In the literature, the term "Root      Peer" is also used for this concept.Bryan, et al.           Expires October 23, 2016               [Page 10]Internet-Draft       P2PSIP Concepts and Terminology          April 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 [RFC6940] protocol.   Client Protocol:  The protocol spoken between Clients and Peers.  In      P2PSIP and RELOAD, this is the same protocol syntactically as the      Peer Protocol.  The only difference is that Clients are not      routing messages or routing information, and have not (or can not)      insert themselves into the overlay.   Peer Protocol Connection / P2PSIP Client Protocol     Connection:      The TLS, 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 which helps the Joining Peer      join the Overlay.  The choice of the admitting peer may depend on      the joining peer (e.g., depend on the joining peer's Peer-ID).      For example, the admitting peer might be chosen as the peer which      is "closest" in the logical structure of the overlay to the future      position of the joining peer.  The selection of the admitting peer      is typically done by the bootstrap peer.  It is allowable for the      bootstrap peer to select itself as the admitting peer.   Bootstrap Server:  A network node used by Joining Peers to locate a      Bootstrap Peer.  A Bootstrap Server may act as a proxy for      messages between the Joining Peer and the Bootstrap Peer.  The      Bootstrap Server itself is typically a stable host with a DNS name      that is somehow communicated (for example, through configuration,      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.   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 presentBryan, et al.           Expires October 23, 2016               [Page 11]Internet-Draft       P2PSIP Concepts and Terminology          April 2016      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.  Discussion5.1.  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 users   SIP AoR).  Full details of how this is implemented using RELOAD are   provided in [I-D.ietf-p2psip-sip]   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].   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 nameBryan, et al.           Expires October 23, 2016               [Page 12]Internet-Draft       P2PSIP Concepts and Terminology          April 2016   using some function defined by the distributed database algorithm   used by the overlay.   A class of algorithms known as Distributed Hash Tables 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 document.   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,   which 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.           Expires October 23, 2016               [Page 13]Internet-Draft       P2PSIP Concepts and Terminology          April 2016         Peer B           Peer Z           Peer A         |                    |                   |         |                    |         Store(U@Y)|         |                    |<------------------|         |                    |Store-Resp(OK)     |         |                    |------------------>|         |                    |                   |         |Fetch(U)            |                   |         |------------------->|                   |         |     Fetch-Resp(U@Y)|                   |         |<-------------------|                   |         |                    |                   |          (RELOAD IS USED TO ESTABLISH CONNECTION)         |                    |                   |         | SIP INVITE(To:U)   |                   |         |--------------------------------------->|         |                    |                   |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 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 will uniquely   identify the peer within the overlay, while the credentials show that   the peer is allowed to join the overlay.   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   specifies how this information may be obtained, along with   credentials and a Node-ID, from an offline enrollment server.Bryan, et al.           Expires October 23, 2016               [Page 14]Internet-Draft       P2PSIP Concepts and Terminology          April 2016   Once the configuration information is obtained, RELOAD specifies a   mechanism whereby a peer may obtain a multicast-bootstrap address in   the configuration file, and can broadcast to this address to attempt   to locate a bootstrap peer.  Additionally, the peer may store   previous peers it has seen and attempt to use these as bootstrap   peers, or 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 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   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 which 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.   For SIP devices, another way to realize this functionality is for a   Peer to behave as a [RFC3261] proxy/registrar.  SIP devices then use   standard SIP mechanisms to add, update, and remove registrations and   to send SIP messages to peers 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.Bryan, et al.           Expires October 23, 2016               [Page 15]Internet-Draft       P2PSIP Concepts and Terminology          April 20165.6.  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      |       |__________________________|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.  IANA Considerations   This document has no actions for IANA.8.  Informative References   [Chord]    Singh, K., Stoica, I., Morris, R., Karger, D., Kaashock,              M., Dabek, F., and H. Balakrishman, "Chord: A scalable              peer-to-peer lookup protocol for internet applications",              IEEE/ACM Transactions on Neworking Volume 11 Issue 1, pp.              17-32, Feb. 2003, August 2001.              Copy available at http://pdos.csail.mit.edu/chord/papers/              paper-ton.pdf   [I-D.ietf-p2psip-diagnostics]              Song, H., Xingfeng, J., Even, R., Bryan, D., and Y. Sun,              "P2P Overlay Diagnostics", draft-ietf-p2psip-              diagnostics-22 (work in progress), March 2016.Bryan, et al.           Expires October 23, 2016               [Page 16]Internet-Draft       P2PSIP Concepts and Terminology          April 2016   [I-D.ietf-p2psip-sip]              Jennings, C., Lowekamp, B., Rescorla, E., Baset, S.,              Schulzrinne, H., and T. Schmidt, "A SIP Usage for RELOAD",              draft-ietf-p2psip-sip-20 (work in progress), April 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>.Bryan, et al.           Expires October 23, 2016               [Page 17]Internet-Draft       P2PSIP Concepts and Terminology          April 2016   [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>.   [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>.Authors' Addresses   David A. Bryan   Cogent Force, LLC   Cedar Park, TX, Texas   USA   Email: dbryan@ethernot.org   Philip Matthews   Alcatel-Lucent   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.comBryan, et al.           Expires October 23, 2016               [Page 18]Internet-Draft       P2PSIP Concepts and Terminology          April 2016   Dean Willis   Softarmor Systems   3100 Independence Pkwy #311-164   Plano, Texas  75075   USA   Phone: +1 214 504 1987   Email: dean.willis@softarmor.com   Spencer Dawkins   Huawei Technologies (USA)   Phone: +1 214 755 3870   Email: spencerdawkins.ietf@gmail.comBryan, et al.           Expires October 23, 2016               [Page 19]