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Versions: 00 01 02 03 04 05 06 draft-ietf-stox-chat

Network Working Group                                     P. Saint-Andre
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Informational                                 E. Gavita
Expires: April 18, 2013                                       N. Hossain
                                                               S. Loreto
                                                                Ericsson
                                                        October 15, 2012


   Interworking between the Session Initiation Protocol (SIP) and the
Extensible Messaging and Presence Protocol (XMPP): One-to-One Text Chat
                   draft-saintandre-sip-xmpp-chat-04

Abstract

   This document defines a bi-directional protocol mapping for the
   exchange of instant messages in the context of a one-to-one chat
   session between a user of the Session Initiation Protocol (SIP) and a
   user of the Extensible Messaging and Presence Protocol (XMPP).
   Specifically for SIP text chat, this document specifies a mapping to
   the Message Session Relay Protocol (MSRP).

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 April 18, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   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.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
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   Without obtaining an adequate license from the person(s) controlling
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   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.


































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.4.  Formal and Informal Sessions . . . . . . . . . . . . . . .  5
     1.5.  Gateway Heuristics . . . . . . . . . . . . . . . . . . . .  6
     1.6.  Connection Maintenance . . . . . . . . . . . . . . . . . .  6
     1.7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . .  7
     1.8.  Discussion Venue . . . . . . . . . . . . . . . . . . . . .  7
   2.  XMPP Formal Chat Session to MSRP . . . . . . . . . . . . . . .  7
     2.1.  Initiating a Formal Session  . . . . . . . . . . . . . . .  8
     2.2.  Accepting a Formal Session . . . . . . . . . . . . . . . . 11
     2.3.  Exchanging Messages  . . . . . . . . . . . . . . . . . . . 13
     2.4.  Terminating a Formal Session . . . . . . . . . . . . . . . 16
     2.5.  Cancelling the Negotiation . . . . . . . . . . . . . . . . 17
     2.6.  Rejecting a Formal Session . . . . . . . . . . . . . . . . 19
   3.  XMPP Informal Session to MSRP  . . . . . . . . . . . . . . . . 20
   4.  MSRP to XMPP Formal Session  . . . . . . . . . . . . . . . . . 24
     4.1.  Initiating a Session . . . . . . . . . . . . . . . . . . . 25
     4.2.  Accepting a Session  . . . . . . . . . . . . . . . . . . . 28
     4.3.  Completing the Transaction . . . . . . . . . . . . . . . . 29
     4.4.  Exchanging Messages  . . . . . . . . . . . . . . . . . . . 29
     4.5.  Terminating a Session  . . . . . . . . . . . . . . . . . . 31
     4.6.  Cancelling the Transaction . . . . . . . . . . . . . . . . 33
     4.7.  Rejecting the Transaction  . . . . . . . . . . . . . . . . 34
     4.8.  Session Negotiation Fails  . . . . . . . . . . . . . . . . 34
   5.  MSRP to XMPP Informal Session  . . . . . . . . . . . . . . . . 35
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 38
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 38
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 38
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 38
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 39
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 40
















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1.  Introduction

1.1.  Overview

   Both the Session Initiation Protocol [RFC3261] and the Extensible
   Messaging and Presence Protocol [RFC6120] can be used for the purpose
   of one-to-one text chat over the Internet.  To ensure interworking
   between these technologies, it is important to define bi-directional
   protocol mappings.

   The architectural assumptions underlying such protocol mappings are
   provided in [I-D.saintandre-sip-xmpp-core], including mapping of
   addresses and error conditions.  Mappings for single instant messages
   (sometimes called "pager-mode" messaging) are provided in
   [I-D.saintandre-sip-xmpp-im].  This document specifies mappings for
   one-to-one text chat sessions (sometimes called "session-mode"
   messaging); in particular, this document specifies mappings between
   XMPP and the Message Session Relay Protocol [RFC4975].  Mapping of
   multi-user text chat (sometimes called "groupchat") is out of scope
   for this document.

1.2.  Terminology

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

1.3.  Scope

   Both XMPP and SIP/SIMPLE technologies enable end users to send
   "instant messages" to other end users.  The term "instant message"
   usually refers to messages sent between two end users for delivery in
   close to real time (rather than messages that are stored and
   forwarded to the intended recipient upon request).  Generally, there
   are three kinds of instant messages:

   1.  Single messages, which are sent from the sender to the recipient
       outside the context of any one-to-one chat session or multi-user
       text conference.  The message is immediately delivered and not
       stored in an inbox.  In XMPP a single message is a <message/>
       stanza of type "normal" as specified in [RFC6121].  In SIP/SIMPLE
       a single message is sent via the MESSAGE method as specified in
       [RFC3428].
   2.  One-to-one chat messages, which are sent from the sender to the
       recipient (i.e., one-to-one) in the context of a "chat session"
       between the two entities.  In XMPP a chat message is a <message/>
       stanza of type "chat".  In SIP/SIMPLE a chat message is sent



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       using an MSRP session as specified in [RFC4975].
   3.  Groupchat messages, which are sent from a sender to multiple
       recipients (i.e., two or more) in the context of a "multi-user
       chat session", "text conference", or "chatroom".  In XMPP a
       groupchat message is a <message/> stanza of type "groupchat" that
       is reflected from the sender to multiple recipients by a multi-
       user chat service, as defined in [XEP-0045].  In SIP/SIMPLE a
       groupchat message is reflected from the sender to multiple
       recipients by a conference server that uses MSRP to handle
       groupchat sessions, as defined in [MSRP-MULTI].

   This document covers only scenario #2 for converting XMPP messages of
   type "chat" to and from their corresponding SIP INVITE and MSRP
   message types on the SIP/SIMPLE side.

   As in [I-D.saintandre-sip-xmpp-im] and related documents, the
   approach taken here is to directly map syntax and semantics from one
   protocol to another.  The mapping described herein depends on the
   protocols defined in the following specifications:

   o  XMPP chat sessions using message stanzas of type "chat" are
      specified in [RFC6121].
   o  A method for formally negotiating an XMPP chat session is
      specified in the Stanza Session Negotiation extension to XMPP
      [XEP-0155].
   o  SIP-based chat sessions using the SIP INVITE and SEND request
      types are specified in [RFC4975].

1.4.  Formal and Informal Sessions

   The traditional model for a one-to-one chat "session" in XMPP is for
   a user to simply send a message of type "chat" to a contact, without
   any formal negotiation of session parameters; the contact would then
   reply to the message, and the sum total of such messages exchanged
   during a defined period of time can be considered a chat session.
   This informal approach to chat sessions in XMPP can be mapped both to
   SIP pager-mode messaging using the SIP MESSAGE method (as documented
   in [I-D.saintandre-sip-xmpp-core]) and to an MSRP chat session.  How
   a gateway chooses to map the XMPP chat session to the SIP side is a
   matter of the implementation, although guidelines are provided under
   Section 1.5.

   However, in XMPP it is also possible to formally request a chat
   session and negotiate its parameters (e.g., security, privacy,
   message logging) before beginning the session and exchanging
   messages.  The protocol for doing so is defined in [XEP-0155].  In
   this case, the XMPP chat session SHOULD be translated into an MSRP
   session.



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   This document covers the mapping of both informal and formally-
   negotiated XMPP chat sessions into MSRP sessions, and from MSRP
   sessions into XMPP informal and formal sessions.

1.5.  Gateway Heuristics

   When a gateway receives a chat message or chat session request
   intended for a recipient that is registered with the gateway itself
   or has an account on a local service, it SHOULD adhere to the
   following process in determining whether to (1) initiate a formal
   chat session with the recipient, (2) initiate an informal chat
   session with the recipient, or (3) return an error to the sender.

   1.  If the gateway has knowledge of the recipient's online endpoints
       (available resources in XMPP or registered UAs in SIP), then it
       SHOULD discover the capabilities of those endpoints.
   2.  If the gateway determines that one or more of the endpoints
       supports formal chat sessions, it SHOULD initiate a formal chat
       session with one of those endpoints (deciding among the endpoints
       based on presence information or communications priority).
   3.  If the gateway determines that none of the endpoints supports
       formal chat sessions, it SHOULD initiate an informal chat session
       with one of those endpoints (deciding among the endpoints based
       on presence information or communications priority).
   4.  If the gateway does not know if the recipient has any online
       endpoints, it SHOULD return an appropriate error to the sender.

   The methods by which a gateway determines support for various
   capabilities are protocol-specific.  For XMPP a gateway SHOULD use
   the Service Discovery extension defined in [XEP-0030] or, if it
   receives presence information from the XMPP endpoint, use the Entity
   Capabilities extension defined in [XEP-0115].  For MSRP a gateway
   SHOULD use the Session Description Protocol defined in [RFC4566] in
   conjunction with a high-level protocol that provides a capability
   query, such as the SIP OPTIONS request defined in [RFC3261].

1.6.  Connection Maintenance

   XMPP makes use of long-lived TCP connections.  If mobility affecting
   Layer 3 causes a dropped connection, the connection must be re-
   established.  If mobility does not preserve the IP address, the TCP
   connection will be dropped.  Any TLS session and SASL associations
   must be re-established if the TCP connection is dropped.  XMPP binds
   directly to TCP in the core specification, so the TCP session must
   remain open for the entire duration of the chat session.  The XMPP
   Standards Foundation does define protocol extensions enabling
   transport of XMPP traffic over HTTP (refer to [XEP-0124] and
   [XEP-0206]), so that individual messages are carried using HTTP and



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   are more robust in environments such as mobile networks, allowing for
   better recovery if a TCP session is broken.

   SIMPLE is similar when using MSRP.  The Message Session Relay
   Protocol [RFC4975] is a protocol for transmitting instant messages
   (IM) in the context of a session.  The protocol specification
   describes how the session can be negotiated and established with an
   offer or answer (see [RFC3264]) using the Session Description
   Protocol [RFC4566].  In SIMPLE, this exchange is carried using SIP as
   the signaling protocol.  After the TCP connection is established, if
   it fails for any reason, then an MSRP endpoint MAY choose to re-
   create such a session using a new SDP exchange in a SIP re-INVITE.
   SIMPLE also uses the MESSAGE request type for transporting instant
   messaging outside the context of a session.  The MESSAGE request is
   sent inside the signaling path without establishing any dedicated
   connection.

1.7.  Acknowledgements

   Some text in this document was borrowed from
   [I-D.saintandre-sip-xmpp-core] and from [XEP-0155].

1.8.  Discussion Venue

   The authors welcome discussion and comments related to the topics
   presented in this document.  The preferred forum is the
   <sip-xmpp@xmpp.org> mailing list, for which archives and subscription
   information are available at
   <http://mail.jabber.org/mailman/listinfo/sip-xmpp>.


2.  XMPP Formal Chat Session to MSRP

   This section describes how to map an XMPP "formal session" to an MSRP
   session.

   The XMPP formal session is based on the protocol described in
   [XEP-0155], which enables the initiation, renegotiation, and
   termination of a formal chat session on the XMPP side.  This approach
   maps to the semantic of the SIP INVITE and BYE methods.











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   XMPP User                      GW                      SIP User
       |                          |                          |
       |(F1) (XMPP) Stanza session request                   |
       |------------------------->|                          |
       |                          |(F2) (SIP) INVITE         |
       |                          |------------------------->|
       |                          |(F3) (SIP) 200 OK         |
       |                          |<-------------------------|
       |(F4) (XMPP) Stanza session acceptance                |
       |<-------------------------|                          |
       |                          |(F5) (SIP) ACK            |
       |                          |------------------------->|
       |(F6) (XMPP) Stanza session completion                |
       |------------------------->|                          |
       |(F7) (XMPP) A chat message                           |
       |------------------------->|                          |
       |                          |(F8) (MSRP) SEND          |
       |                          |------------------------->|
       |                          |(F9) (MSRP) A reply       |
       |(F10) (XMPP) A reply      |                          |
       |<-------------------------|                          |
       |                          |                          |
       .                          .                          .
       .                          .                          .
       .                          .                          .
       |                          |                          |
       |(F11) (XMPP) Stanza session termination              |
       |------------------------->|                          |
       |                          |(F12) (SIP) BYE           |
       |                          |------------------------->|
       |                          |(F13) (SIP) 200 OK        |
       |                          |<-------------------------|
       |(F14) (XMPP) Termination acknowledgment              |
       |<-------------------------|                          |

2.1.  Initiating a Formal Session

   When the XMPP user ("Juliet") wants to initiate a negotiated session
   with a SIP user ("Romeo"), she sends a <message/> stanza to Romeo
   containing a <feature/> child qualified by the
   'http://jabber.org/protocol/feature-neg' namespace.  The <message/>
   stanza must not contain a <body/> child (as specified in [RFC6121]),
   since that child element is used for human-readable text.  The
   <message/> stanza type should be "normal".  The stanza MUST contain a
   <thread/> element for tracking purposes (where the newly-generated
   ThreadID is unique to the proposed session).  The encapsulated data
   form MUST contain a FORM_TYPE field whose type is "hidden" and whose
   value is "urn:xmpp:ssn"; it must also contain a boolean field named



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   "accept".

   The XMPP user may request a session with a specific resource of the
   contact.  However in this document the resource identifier will be
   ignored and discarded for cross-system interworking.

   Example: (F1) Juliet starts a formal session

   <message from='juliet@example.com'
            to='romeo@example.net'
            type='normal'>
     <thread>711609sa</thread>
     <feature xmlns='http://jabber.org/protocol/feature-neg'>
       <x xmlns='jabber:x:data' type='form'>
         <title>Open chat with Juliet?</title>
         <field var='FORM_TYPE' type='hidden'>
           <value>urn:xmpp:ssn</value>
         </field>
         <field label='Accept this session?'
                type='boolean'
                var='accept'>
           <value>true</value>
           <required/>
         </field>
         <field label='Primary written language of the chat'
                type='list-single'
                var='language'>
           <value>en</value>
           <option label='English'><value>en</value></option>
           <option label='Italiano'><value>it</value></option>
        </field>
        <field label='XHTML formatting'
               type='list-single'
               var='http://jabber.org/protocol/xhtml-im'>
           <value>may</value>
           <option label='Allow XHTML formatting'>
             <value>may</value>
           </option>
           <option label='Disallow XHTML formatting'>
             <value>mustnot</value>
           </option>
         </field>
       </x>
     </feature>
   </message>

   Upon receiving such a session request, the XMPP server to which
   Juliet has authenticated attempts to deliver the request to a local



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   user or attempts to route the request to the remote domain that
   services the hostname in the 'to' attribute.  Naturally, in this
   document we assume that the hostname in the 'to' attribute is an IM-
   aware SIP service hosted by a separate server.

   As specified in [RFC6121], the XMPP server needs to determine the
   identity of the remote domain, which it does by performing one or
   more [RFC2782] lookups.  For message stanzas, the order of lookups
   recommended by [RFC6121] is to first try the "_xmpp-server" service
   as specified in [RFC6120] and to then try the "_im" service as
   specified in [RFC3861].  Here we assume that the first lookup will
   fail but that the second lookup will succeed and return a resolution
   "_im._simple.example.net.", since we have already assumed that the
   example.net hostname is running a SIP instant messaging service.
   (Note: The XMPP server may have previously determined that the remote
   domain is a SIMPLE server, in which case it would not need to perform
   the SRV lookups; the caching of such information is a matter of
   implementation and local service policy, and is therefore out of
   scope for this document.)

   Once the XMPP server (example.com) has determined that the remote
   domain is serviced by a SIMPLE server, it hands the XMPP message off
   to its local XMPP-to-SIP gateway (x2s.example.com), which transforms
   the message into SIP syntax and routes it to the remote SIMPLE server
   (example.net).

   Example: (F2) Juliet starts a formal session (SIP transformation)

   INVITE sip:romeo@example.net SIP/2.0
   To: <sip:romeo@example.net>
   From: <sip:juliet@example.com>;tag=786
   Subject: Open chat with Juliet?
   Call-ID: 711609sa
   Content-Type: application/sdp

   c=IN IP4 x2s.example.com
   m=message 7654 TCP/MSRP *
   a=accept-types:text/plain
   a=lang:en
   a=lang:it
   a=path:msrp://x2s.example.com:7654/jshA7weztas;tcp

   Here the Session Description Protocol offer specifies the MSRP-aware
   XMPP-to-SIP gateway on the XMPP side as well as other particulars of
   the session.






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      There is no direct mapping for the MSRP URIs.  In fact MSRP URIs
      identify a session of instant messages at a particular device;
      they are ephemeral and have no meaning outside the scope of that
      session.  The authority component of the MSRP URI MUST contain the
      XMPP-to-SIP gateway hostname or numeric IP address and an explicit
      port number.

      Native XMPP messages as described in [RFC6121] supports text
      (i.e., UTF-8) only.  However, there exists an XMPP extension for
      XHTML-formatted messges, as defined by the XHTML-IM integration
      set specified in [XEP-0071].  Unless the use of XHTML-formatted
      messages is supported by the endpoints or negotiated during
      session establishment, the "accept-types" attribute that follows
      an MSRP media line SHOULD indicate "text/plain" as the only media-
      type that is acceptable to the endpoint; if XHTML is supported or
      negotiated, the "accept-types" attribute MAY also indicate a
      media-type of "text/html".  (Note: The XHTML-IM integration set
      supports only a subset of XHTML formatting; it is the
      responsibility of a gateway to map between full XHTML and
      XHTML-IM.)

   As specified in [I-D.saintandre-sip-xmpp-core], the mapping of XMPP
   syntax elements to SIP and SDP syntax elements SHOULD be as shown in
   the following table.  (Mappings for elements not mentioned are
   undefined.)

   Table 1: Message syntax mapping from XMPP to SIP/SDP

   +-----------------------------+-----------------------------+
   |  XMPP Element or Attribute  |  SIP Header or SDP Contents |
   +-----------------------------+-----------------------------+
   |  <thread/>                  |  Call-ID                    |
   |  from                       |  From                       |
   |  to                         |  To                         |
   |  <title/>                   |  Subject                    |
   |  xml:lang                   |  a=lang:<language tag>      |
   |  -                          |  a=accept-types:text/plain  |
   +-----------------------------+-----------------------------+

2.2.  Accepting a Formal Session

   Here we assume that the SIP user agent that receives the SIP
   invitation (containing an offered session description that includes a
   session of MSRP) accepts the invitation and includes an answer
   session description that acknowledges the choice of media.






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   Example: (F3) Romeo accepts the request

   SIP/2.0 200 OK
   To: <sip:romeo@example.net>;tag=087js
   From: <sip:juliet@example.com>;tag=786
   Call-ID: 711609sa
   Content-Type: application/sdp

   c=IN IP4 example.net
   m=message 12763 TCP/MSRP *
   a=accept-types:text/plain
   a=lang:it
   a=path:msrp://s2x.example.net:12763/kjhd37s2s20w2a;tcp

   Upon receiving such a response, the SIMPLE server or associated SIP-
   to-XMPP gateway SHOULD remember that this is a response to a SIP
   transaction related to an XMPP-SIP translation, based on the SIP
   Call-ID (which is functionally equivalent to the XMPP <thread/>).
   The SIP-to-XMPP gateway is responsible for translating the response
   into an XMPP message stanza and routing it from the SIP user to the
   XMPP server or associated XMPP-to-SIP gateway.

   The SIP-to-XMPP gateway MUST include in the response translation
   values for all the fields that the XMPP request indicated are
   required.

   Example: (F4) Romeo accepts the request (XMPP translation)

   <message from='romeo@example.net'
            to='juliet@example.com'
            type='normal'>
     <thread>711609sa</thread>
     <feature xmlns='http://jabber.org/protocol/feature-neg'>
       <x xmlns='jabber:x:data' type='submit'>
         <field var='FORM_TYPE'>
           <value>urn:xmpp:ssn</value>
         </field>
         <field var='accept'><value>true</value></field>
         <field var='language'><value>it</value></field>
       </x>
     </feature>
   </message>

   The SIP-to-XMPP gateway MUST also send a SIP ACK to the SIP user.







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   Example: (F5) Gateway sends ACK to Romeo's UA

   ACK sip:romeo@example.net SIP/2.0
   To: <sip:romeo@example.net>;tag=087js
   From: <sip:juliet@example.com>;tag=786
   Call-ID: 711609sa

   If Romeo accepted the session, Juliet MUST either complete or cancel
   the stanza session negotiation.  The user's client SHOULD verify that
   the selected values of the fields are acceptable before completing
   the stanza session negotiation -- and confirming that the session is
   open -- by replying with the form 'type' attribute set to 'result'.
   The form MUST contain the FORM_TYPE field and the "accept" field set
   to "1" or "true".

   Example: (F6) Juliet completes negotiation

   <message from='juliet@example.com'
            to='romeo@example.net'
            type='normal'>
     <thread>711609sa</thread>
     <feature xmlns='http://jabber.org/protocol/feature-neg'>
       <x xmlns='jabber:x:data' type='result'>
         <field var='FORM_TYPE'>
           <value>urn:xmpp:ssn</value>
         </field>
         <field var='accept'><value>true</value></field>
       </x>
     </feature>
   </message>

   Upon receiving such a stanza completing the session negotiation, the
   XMPP server MUST NOT send any confirmation to the SIP side; instead,
   it MUST route the acceptance to the SIMPLE server or associated SIP-
   to-XMPP gateway.

   The session is now open and the parties can proceed to exchanging
   messages.

2.3.  Exchanging Messages

   Once the session is created, the endpoints can exchange an unbounded
   number of messages.

   The XMPP 'id' attribute is not required in the protocol and there is
   no way to enforce its use for messages.  It is RECOMMENDED to include
   it as a negotiable item in the SSN negotiation, via the "message-ids"
   field.  However, it is possible that the 'id' will not be present



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   within the <message/> stanza; in this case the XMPP-to-SIP gateway
   MUST generate a new unique Message-ID.

   If the XMPP user has not explicitly requested message receipts during
   the negotiation, it is RECOMMENDED that the SIP-to-XMPP gateway shall
   insert a Failure-Report header field value of "no" during the
   creation of a SEND request.  The XMPP user can include a request for
   message receipts using the Message Receipts XMPP protocol extension
   [XEP-0184]; use of this extension can be negotiated via the
   "urn:xmpp:receipts" field during SSN negotiation.

   The mapping of XMPP syntax elements to MSRP syntax elements SHOULD be
   as shown in the following table.  (Mappings for elements not
   mentioned are undefined.)

   Table 2: Message syntax mapping from XMPP Message to MSRP

   +-----------------------------+-----------------------------+
   |  XMPP Element or Attribute  |  MSRP Header                |
   +-----------------------------+-----------------------------+
   |  to                         |  To-Path                    |
   |  from                       |  From-Path                  |
   |  <body/>                    |  body of the SEND request   |
   |  -                          |  Content-Type: text/plain   |
   |  id                         |  Message-ID                 |
   +-----------------------------+-----------------------------+

   The following examples show an exchange of messages.

   Example: (F7) Juliet sends an XMPP message

   <message from='juliet@example.com'
            to='romeo@example.net'
            type='chat'>
     <thread>711609sa</thread>
     <body>Art thou not Romeo, and a Montague?</body>
   </message>














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   Example: (F8) Gateway transforms XMPP message to MSRP

   MSRP a786hjs2 SEND
   To-Path: msrp://s2x.example.net:12763/kjhd37s2s20w2a;tcp
   From-Path: msrp://x2s.example.com:7654/jshA7weztas;tcp
   Message-ID: 87652491
   Byte-Range: 1-25/25
   Content-Type: text/plain

   Art thou not Romeo, and a Montague?
   -------a786hjs2$

   Upon receiving the SEND request, if the request either contains a
   Failure-Report header field value of "yes" or does not contain a
   Failure-Report header at all, Romeo's client MUST immediately
   generate and send a response.

   MSRP d93kswow 200 OK
   To-Path: msrp://x2s.example.com:7654/jshA7weztas;tcp
   From-Path: msrp://s2x.example.net:12763/kjhd37s2s20w2a;tcp
   -------d93kswow$

   Romeo can then send a reply using his MSRP user agent.

   Example: (F9) Romeo sends a reply

   MSRP a786hjs2 SEND
   To-Path: msrp://x2s.example.com:7654/jshA7weztas;tcp
   From-Path: msrp://s2x.example.net:12763/kjhd37s2s20w2a;tcp
   Message-ID: 87652491
   Byte-Range: 1-25/25
   Content-Type: text/plain

   Neither, fair saint, if either thee dislike.
   -------a786hjs2$

   The SIP-to-XMPP gateway would then transform that message into
   appropriate XMPP syntax for routing to the intended recipient.

   Example: (F10) Gateway transforms MSRP message to XMPP

   <message from='romeo@example.net'
            to='juliet@example.com'
            type='chat'>
     <thread>711609sa</thread>
     <body>Neither, fair saint, if either thee dislike.</body>
   </message>




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   Note: The size of the XML character data of an XMPP message is not
   limited by the protocol, but is sometimes limited in deployment.
   However messages sent using MSRP can be delivered in several SEND
   requests, so when the XMPP-to-SIP gateway receives a message longer
   then 2048, it is STRONGLY RECOMMENDED it delivers this message using
   as few chunks (at least 2048 octets long) as possible.

2.4.  Terminating a Formal Session

   If Juliet decides to terminate the negotiated chat session, her
   client sends a <message/> stanza to Romeo containing a data form of
   type "submit".  The <message/> stanza MUST contain a <thread/>
   element with the same XML character data as the original initiation
   request.  The data form containing a boolean field named "terminate"
   set to a value of "1" or "true".

   Example: (F11) Juliet terminates the chat session

   <message from='juliet@example.com'
            to='romeo@example.net'
            type='normal'>
     <thread>711609sa</thread>
     <feature xmlns='http://jabber.org/protocol/feature-neg'>
       <x xmlns='jabber:x:data' type='submit'>
         <field var='FORM_TYPE'>
           <value>urn:xmpp:ssn</value>
         </field>
         <field var='terminate'>
           <value>1</value>
         </field>
       </x>
     </feature>
   </message>

   Upon receiving such stanza terminating the chat session, the XMPP-to-
   SIP gateway terminates the SIP session by sending a SIP BYE to tear
   down the MSRP session with Romeo's client.  Romeo's SIP client then
   responds with a 200 OK.

   Example: (F12) Juliet terminates the chat session (SIP translation)

   BYE romeo@example.net sip: SIP/2.0
   Max-Forwards: 70
   From: <sip:juliet@example.com>;tag=786
   To: <sip:romeo@example.net>;tag=087js
   Call-ID: 711609sa
   Cseq: 1 BYE
   Content-Length: 0



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   Example: (F13) Romeo acknowledges termination

   SIP/2.0 200 OK
   From: <sip:juliet@example.com>;tag=786
   To: <sip:romeo@example.net>;tag=087js
   Call-ID: 711609sa
   CSeq: 1 BYE
   Content-Length: 0

   Upon receiving the 200 OK, the SIP-to-XMPP gateway acknowledges the
   termination of the chat session on the XMPP side by sending a
   <message/> containing a data form of type "result", and the value of
   the "terminate" field set to "1" or "true".  The client must mirror
   the <thread/> value it received.

   Example: (F14) Romeo acknowledges termination (XMPP translation)

   <message from='romeo@example.net'
            to='juliet@example.com'
            type='normal'>
     <thread>711609sa</thread>
     <feature xmlns='http://jabber.org/protocol/feature-neg'>
       <x xmlns='jabber:x:data' type='result'>
         <field var='FORM_TYPE'>
           <value>urn:xmpp:ssn</value>
         </field>
         <field var='terminate'>
           <value>1</value>
         </field>
       </x>
     </feature>
   </message>

2.5.  Cancelling the Negotiation

   If Romeo accepted the session but Juliet decides to cancel the stanza
   session negotiation, the flow is as follows.














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   XMPP User                      GW                      SIP User
       |                          |                          |
       |(F1) (XMPP) Stanza session request                   |
       |------------------------->|                          |
       |                          |(F2) (SIP) INVITE         |
       |                          |------------------------->|
       |                          |(F3) (SIP) 200 OK         |
       |                          |<-------------------------|
       |(F4) (XMPP) Stanza session acceptance                |
       |<-------------------------|                          |
       |                          |(F5) (SIP) ACK            |
       |                          |------------------------->|
       |(F6) (XMPP) Stanza session cancelling                |
       |------------------------->|                          |
       |                          |(F7) (SIP) BYE            |
       |                          |------------------------->|
       |                          |(F8) (SIP) 200 OK         |
       |                          |<-------------------------|

   That is, Juliet's client shall reply with a data form containing the
   FORM_TYPE field and the "accept" field set to "0" or "false":

   Example: (F6) User cancels stanza session negotiation

   <message from='juliet@example.com'
            to='romeo@example.net'
            type='normal'>
     <thread>711609sa</thread>
     <feature xmlns='http://jabber.org/protocol/feature-neg'>
       <x xmlns='jabber:x:data' type='result'>
         <field var='FORM_TYPE'>
           <value>urn:xmpp:ssn</value>
         </field>
         <field var='accept'>
           <value>0</value>
         </field>
       </x>
     </feature>
   </message>

   Upon receiving such stanza cancelling the session negotiation, the
   XMPP-to-SIP gateway MUST send a SIP BYE.  Once the XMPP-to-SIP
   gateway receives the 200 OK, the internal session data is removed and
   the session is officially cancelled also in the SIP-to-XMPP gateway.

   If the SIP user had sent any messages to XMPP while awaiting
   confirmation of the session, the SIP-to-XMPP gateway MUST return them
   to the SIP user with an appropriate error.



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2.6.  Rejecting a Formal Session

   A common scenario occurs when the SIP UA is currently unwilling or
   unable to accept a formal session, in which case the flow is as
   follows.

   XMPP User                      GW                      SIP User
       |                          |                          |
       |(F1) (XMPP) Stanza session request                   |
       |------------------------->|                          |
       |                          |(F2) (SIP) INVITE         |
       |                          |------------------------->|
       |                          |(F3) (SIP) 4xx/6xx        |
       |                          |<-------------------------|
       |(F4) (XMPP) Stanza session decline                   |
       |<-------------------------|                          |

   Here the SIP UA declining an offer contained in an INVITE SHOULD
   return a 4xx or a 6xx response, such as 406 Not Acceptable or 603
   Decline.  Such a response SHOULD include a Warning header field value
   explaining why the offer was rejected.

   Example: (F3) SIP user declines offer and specifies reason (SIP)

   SIP/2.0 603 Decline
   From: <sip:juliet@example.com>;tag=786
   To: <sip:romeo@example.net>;tag=087js
   Call-ID: 711609sa
   Warning: I'm busy!
   Content-Length: 0

   Upon receiving the error response for the SIP INVITE, the XMPP-to-SIP
   gateway shall send a "Session Reject" message back to the XMPP
   Client.  This message contains a data form that MUST contain the
   FORM_TYPE field and the "accept" field set to "0" or "false".  It is
   RECOMENDED that the form does not contain any other field even if the
   request indicated they are required.  The client MAY include a reason
   in the <body/> child of the <message/> stanza.  The content of the
   Warning header field present in the SIP response SHOULD be mapped to
   a "reason" field in the data form.  If the Warning header is not
   present then the descriptive phrase of the SIP response can be used.










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   Example: (F4) SIP user declines offer and specifies reason (XMPP
   translation)

   <message from='romeo@example.net'
            to='juliet@example.com'
            type='normal'>
     <thread>711609sa</thread>
     <feature xmlns='http://jabber.org/protocol/feature-neg'>
       <x xmlns='jabber:x:data' type='submit'>
         <field var='FORM_TYPE'>
           <value>urn:xmpp:ssn</value>
         </field>
         <field var='accept'>
           <value>0</value>
         </field>
         <field var='reason'>
           <value>I&apos;m busy!</value>
         </field>
       </x>
     </feature>
   </message>


3.  XMPP Informal Session to MSRP

   In XMPP, the "informal session" approach is to simply send someone a
   <message/> of type "chat" without starting any session negotiation
   ahead of time (as described in [RFC6121]).  The XMPP "informal
   session" approach maps very well into a SIP MESSAGE request, as
   described in [I-D.saintandre-sip-xmpp-core].  However, the XMPP
   informal session approach can also be mapped to MSRP if the XMPP-to-
   SIP gateway maintains additional state.

   The order of events is as follows.

















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   XMPP User                      GW                      SIP User
       |                          |                          |
       |(F1) (XMPP) Chat message  |                          |
       |------------------------->|                          |
       |                          |(F2) (SIP) INVITE         |
       |                          |------------------------->|
       |                          |(F3) (SIP) 200 OK         |
       |                          |<-------------------------|
       |                          |(F4) (SIP) ACK            |
       |                          |------------------------->|
       |                          |(F5) (MSRP) SEND          |
       |                          |------------------------->|
       |                          |(F6) (MSRP) A reply       |
       |                          |<-------------------------|
       |(F7) (XMPP) A reply       |                          |
       |<-------------------------|                          |
       |                          |                          |
       .                          .                          .
       .                          .                          .
       .                          .                          .
       |                          |                          |
       |                          |(F8) (SIP) BYE            |
       |                          |<-------------------------|
       |                          |(F9) (SIP) 200 OK         |
       |                          |------------------------->|
       |                          |                          |

   First the XMPP user would generate an XMPP chat message.

   Example: (F1) Juliet sends an XMPP message

   <message from='juliet@example.com'
            to='romeo@example.net'
            type='chat'>
     <thread>711609sa</thread>
     <body>Art thou not Romeo, and a Montague?</body>
   </message>

   The local SIP-to-XMPP gateway at the SIMPLE server would then
   determine if Romeo supports MSRP.  If so, the SIP-to-XMPP gateway
   would initiate an MSRP session with Romeo on Juliet's behalf.










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   Example: (F2) Gateway starts a formal session on behalf of Juliet

   INVITE sip:romeo@example.net SIP/2.0
   To: <sip:romeo@example.net>
   From: <sip:juliet@example.com>;tag=786
   Subject: Open chat with Juliet?
   Call-ID: 711609sa
   Content-Type: application/sdp

   c=IN IP4 x2s.example.com
   m=message 7654 TCP/MSRP *
   a=accept-types:text/plain
   a=lang:en
   a=lang:it
   a=path:msrp://x2s.example.com:7654/jshA7weztas;tcp

   Here we assume that Romeo accepts the MSRP session request.

   Example: (F3) Romeo accepts the request

   SIP/2.0 200 OK
   To: <sip:romeo@example.net>;tag=087js
   From: <sip:juliet@example.com>;tag=786
   Call-ID: 711609sa
   Content-Type: application/sdp

   c=IN IP4 s2x.example.net
   m=message 12763 TCP/MSRP *
   a=accept-types:text/plain
   a=lang:it
   a=path:msrp://s2x.example.net:12763/kjhd37s2s20w2a;tcp

   The XMPP-to-SIP gateway then acks the session acceptance on behalf of
   Juliet.

   Example: (F4) Gateway sends ACK to Romeo's UA

   ACK sip:romeo@example.net SIP/2.0
   To: <sip:romeo@example.net>;tag=087js
   From: <sip:juliet@example.com>;tag=786
   Call-ID: 711609sa

   The XMPP-to-SIP gateway then transforms the original XMPP chat
   message into MSRP.







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   Example: (F5) Gateway transforms XMPP message to MSRP

   MSRP a786hjs2 SEND
   From-Path: msrp://x2s.example.com:7654/jshA7weztas;tcp
   To-Path: msrp://s2x.example.net:12763/kjhd37s2s20w2a;tcp
   Message-ID: 87652491
   Byte-Range: 1-25/25
   Content-Type: text/plain

   Art thou not Romeo, and a Montague?
   -------a786hjs2$

   Romeo can then send a reply using his MSRP user agent.

   Example: (F6) Romeo sends a reply

   MSRP a786hjs2 SEND
   To-Path: msrp://x2s.example.com:7654/jshA7weztas;tcp
   From-Path: msrp://s2x.example.net:12763/kjhd37s2s20w2a;tcp
   Message-ID: 87652491
   Byte-Range: 1-25/25
   Failure-Report: no
   Content-Type: text/plain

   Neither, fair saint, if either thee dislike.
   -------a786hjs2$

   Note: As previously described, if the users have not negotiated the
   use message receipts, it is RECOMMENDED that the SIP-to-XMPP gateway
   shall insert a Failure-Report header field value of "no" during the
   creation of a SEND request.

   The SIP-to-XMPP gateway would then transform that message into
   appropriate XMPP syntax for routing to the intended recipient.

   Example: (F7) Gateway transforms MSRP message to XMPP

   <message from='romeo@example.net'
            to='juliet@example.com'
            type='chat'>
     <thread>711609sa</thread>
     <body>Neither, fair saint, if either thee dislike.</body>
   </message>

   When the MSRP user wishes to end the chat session, the user's MSRP
   client sends a SIP BYE.





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   Example: (F8) Romeo terminates the chat session

   BYE juliet@example.com sip: SIP/2.0
   Max-Forwards: 70
   From: <sip:romeo@example.net>;tag=087js
   To: <sip:juliet@example.com>;tag=786
   Call-ID: 711609sa
   Cseq: 1 BYE
   Content-Length: 0

   The BYE is then acknowledged by the XMPP-to-SIP gateway.

   Example: (F9) Gateway acknowledges termination

   SIP/2.0 200 OK
   From: <sip:juliet@example.com>;tag=786
   To: <sip:romeo@example.net>;tag=087js
   Call-ID: 711609sa
   CSeq: 1 BYE
   Content-Length: 0


4.  MSRP to XMPP Formal Session

   Unlike the XMPP protocol, the MSRP protocol offers only one way to
   initiate a chat session, typically using the Session Description
   Protocol [RFC4566] via the SIP offer/answer mechanism (see
   [RFC3264]).

   The order of events is as follows.





















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   SIP User                     GW                     XMPP User
      |                         |                          |
      |(F1)(SIP) INVITE         |                          |
      |------------------------>|                          |
      |                         |(F2)(XMPP) Stanza session request
      |                         |------------------------->|
      |                         |(F3)(XMPP) Stanza session acceptance
      |                         |<-------------------------|
      |(F4)(SIP) 200 OK         |                          |
      |<------------------------|                          |
      |(F5)(SIP) ACK            |                          |
      |------------------------>|                          |
      |                         |(F6)(XMPP) Stanza session completion
      |                         |------------------------->|
      |(F7)(MSRP) SEND          |                          |
      |------------------------>|                          |
      |                         |(F8)(XMPP) A chat message |
      |                         |------------------------->|
      |                         |(F9)(XMPP) A reply        |
      |                         |<-------------------------|
      |(F10)(MSRP) SEND         |                          |
      |<------------------------|                          |
      |                         |                          |
      .                         .                          .
      .                         .                          .
      .                         .                          .
      |                         |                          |
      |(F11)(SIP) BYE           |                          |
      |------------------------>|                          |
      |                         |(F12)(XMPP) Stanza session termination
      |                         |------------------------->|
      |                         |(F13)(XMPP) Termination acknowledgment
      |                         |<-------------------------|
      |(F14)(SIP) 200 OK        |                          |
      |<------------------------|                          |

4.1.  Initiating a Session

   When Romeo wants to start an MSRP message session with Juliet, he
   first has to start the SIP session by sending out a SIP INVITE
   request containing an offered session description that includes an
   MSRP media line accompanied by a mandatory "path" attribute and
   corresponding URIs.  The MSRP media line is also accompanied by an
   "accept-types" attribute used to specify the only media-types
   acceptable for Romeo (i.e., text/plain and/or text/html).

   Note: In addition to plain text messages, MSRP is able to carry
   arbitrary (binary) Multipurpose Internet Mail Extensions [RFC2045]



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   compliant content, such as images or video clips.  Disposition of
   media types other than text/plain and text/html is out of scope for
   this specification and is a matter of implementation.

   Example: (F1) SIP user starts the session

   INVITE sip:juliet@example.com SIP/2.0
   To: <sip:juliet@example.com>
   From: <sip:romeo@example.net>;tag=576
   Subject: Open chat with Romeo?
   Call-ID: 742507no
   Content-Type: application/sdp

   c=IN IP4 s2x.example.net
   m=message 7313 TCP/MSRP *
   a=accept-types:text/plain
   a=lang:en
   a=lang:it
   a=path:msrp://s2x.example.net:7313/ansp71weztas;tcp

   Upon receiving the INVITE, the SIP-to-XMPP gateway needs to determine
   the identity of the remote domain, which it does by performing one or
   more DNS SRV lookups [RFC2782].  The SIP-to-XMPP gateway SHOULD
   resolve the address present in the To header of the INVITE to an im
   URI, then follow the rules in [RFC3861] regarding the "_im" SRV
   service for the target domain contained in the To header.  If SRV
   address resolution fails for the "_im" service, the SIP-to-XMPP
   gateway MAY attempt a lookup for the "_xmpp-server" service as
   specified in [RFC6120] or MAY return an error to the sender (i.e. 502
   Bad Gateway).

   If SRV address resolution succeeds, the SIP-to-XMPP gateway is
   responsible for translating the request into an XMPP message stanza
   to initiate a negotiated session from the SIP user to the XMPP user.

















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   Example: (F2) SIP user starts the session (XMPP transformation)

   <message from='romeo@example.net'
            to='juliet@example.com'
            type='normal'>
     <thread>742507no</thread>
     <feature xmlns='http://jabber.org/protocol/feature-neg'>
       <x xmlns='jabber:x:data' type='form'>
         <title>Open chat with Romeo?</title>
         <field var='FORM_TYPE' type='hidden'>
           <value>urn:xmpp:ssn</value>
         </field>
         <field label='Accept this session?' type='form' var='accept'>
           <value>true</value>
           <required/>
         </field>
         <field label='Primary written language of the chat'
                  type='list-single'
                  var='language'>
           <value>en</value>
           <option label='English'><value>en</value></option>
           <option label='Italiano'><value>it</value></option>
         </field>
       </x>
     </feature>
   </message>

   The mapping of SIP and SDP syntax elements to XMPP syntax elements
   SHOULD be as shown in the following table.  (Mappings for elements
   not mentioned in the foregoing table are undefined.)

   Table 3: Message syntax mapping from SIP to XMPP

   +-----------------------------+-----------------------------+
   |  SIP Header or SDP Contents |  XMPP Element or Attribute  |
   +-----------------------------+-----------------------------+
   |  Call-ID                    |  <thread/>                  |
   |  From                       |  from                       |
   |  To                         |  to                         |
   |  Subject                    |  <title/>                   |
   |  accept-types               |  -                          |
   |  a=lang                     |  xml:lang                   |
   |  To                         |  to                         |
   +-----------------------------+-----------------------------+

   See previous note regarding negotiation and use of the XHTML-IM
   integration set for XHTML-formatted messages (i.e., the "text/html"
   accept-type).



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4.2.  Accepting a Session

   If the request is accepted then Juliet's client MUST include all the
   fields that were marked as required in the request message.

   In the example below, we assume that Juliet accepts the session and
   specifies that she prefers to speak Italian with Romeo.

   Example: (F3) Juliet accepts session and specifies parameters

   <message from='juliet@example.com'
            to='romeo@example.net'
            type='normal'>
     <thread>742507no</thread>
     <feature xmlns='http://jabber.org/protocol/feature-neg'>
       <x xmlns='jabber:x:data' type='submit'>
         <field var='FORM_TYPE'>
           <value>urn:xmpp:ssn</value>
         </field>
         <field var='accept'><value>true</value></field>
         <field var='language'><value>it</value></field>
       </x>
     </feature>
   </message>

   Upon receiving such a response, the SIP-to-XMPP gateway SHOULD
   remember that this is a response to a stanza related to an SIP-XMPP
   translation, based on the SIP Call-ID.  The SIP-to-XMPP gateway is
   responsible for translating the response into a SIP response and
   delivering it from the XMPP user back to the SIP user.

   Example: (F4) Juliet accepts session (SIP translation)

   SIP/2.0 200 OK
   To: <sip:juliet@example.com>;tag=534
   From: <sip:romeo@example.net>;tag=576
   Call-ID: 742507no
   Content-Type: application/sdp

   c=IN IP4 x2s.example.com
   m=message 8763 TCP/MSRP *
   a=accept-types:text/plain
   a=lang:it
   a=path:msrp://x2s.example.com:8763/lkjh37s2s20w2a;tcp







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4.3.  Completing the Transaction

   In this case, the 200 OK is routed back and is received by Romeo's
   UA.  Finally, Romeo's client sends an acknowledgment message, ACK, to
   Juliet's client to confirm the reception of the final response (200
   OK).

   Example: (F5) Romeo sends ACK

   ACK sip:juliet@example.com SIP/2.0
   To: <sip:juliet@example.com>;tag=534
   From: <sip:romeo@example.net>;tag=576
   Call-ID: 742507no

   Upon receiving the ACK, the SIP-to-XMPP gateway SHOULD remember this
   is an acknowledgment to an XMPP formal session.  The SIP-to-XMPP
   gateway is responsible for translating the acknowledgment into a
   confirmation stanza, without inserting other content (e.g. a <body/>
   element cannot be inserted).

   Example: (F6) Romeo sends ACK (XMPP translation)

   <message from='romeo@example.net'
            to='juliet@example.com'
            type='normal'>
     <thread>742507no</thread>
     <feature xmlns='http://jabber.org/protocol/feature-neg'>
       <x xmlns='jabber:x:data' type='result'>
         <field var='FORM_TYPE'>
           <value>urn:xmpp:ssn</value>
         </field>
         <field var='accept'>
           <value>true</value>
         </field>
       </x>
     </feature>
   </message>

4.4.  Exchanging Messages

   When Romeo wants to send a message, he creates an MSRP SEND request
   that contains the message.









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   Example: (F7) Romeo sends a message

   MSRP ad49kswow SEND
   To-Path: msrp://x2s.example.com:8763/lkjh37s2s20w2a;tcp
   From-Path: msrp://s2x.example.net:7313/ansp71weztas;tcp
   Message-ID: 44921zaqwsx
   Byte-Range: 1-32/32
   Failure-Report: no
   Content-Type: text/plain

   I take thee at thy word ...
   -------ad49kswow$

   Upon receiving the MSRP SEND request, the SIP-to-XMPP gateway SHOULD
   remember that the message is for an XMPP user.  The SIP-to-XMPP
   gateway is responsible for translating the MSRP SEND request into an
   XMPP message stanza.

   Example: (F8) Romeo sends a message (XMPP translation)

   <message from='romeo@example.net'
            to='juliet@example.com'
            type='chat'>
     <thread>742507no</thread>
     <body>I take thee at thy word ...</body>
   </message>

   The mapping of MSRP syntax elements to XMPP syntax elements SHOULD be
   as shown in the following table.  (Mappings for elements not
   mentioned are undefined.)

   Table 4: Message syntax mapping from MSRP Message to XMPP

   +-----------------------------+-----------------------------+
   | MSRP Header                 |  XMPP Element or Attribute  |
   +-----------------------------+-----------------------------+
   |  To-Path                    |  to                         |
   |  From-Path                  |  from                       |
   |  body of the SEND request   |  <body/>                    |
   |  Content-Type: text/plain   |  -                          |
   |  Message-ID                 |  id                         |
   +-----------------------------+-----------------------------+

   Upon receiving the chat message, Juliet can send a reply.







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   Example: (F9) Juliet sends a reply

   <message from='juliet@example.com'
            to='romeo@example.net'
            type='chat'>
     <thread>711609sa</thread>
     <body>What man art thou ...?</body>
   </message>

   Example: (F10) Gateway transforms XMPP message to MSRP

   MSRP a786hjs2 SEND
   From-Path: msrp://x2s.example.com:8763/lkjh37s2s20w2a;tcp
   To-Path: msrp://s2x.example.net:7313/jshA7weztas;tcp
   Message-ID: 87652491
   Byte-Range: 1-25/25
   Failure-Report: no
   Content-Type: text/plain

   What man art thou ...?
   -------a786hjs2$

4.5.  Terminating a Session

   When Romeo wants to terminate the session, he is required to send a
   SIP BYE request.

   Example: (F11) Romeo terminates the session

   BYE juliet@example.com sip: SIP/2.0
   Max-Forwards: 70
   From: <sip:romeo@example.net>;tag=576
   To: <sip:juliet@example.com>;tag=534
   Call-ID: 742507no
   Cseq: 1 BYE
   Content-Length: 0

   Upon receiving the SIP BYE request, the XMPP-to-SIP gateway SHOULD
   translate the request to a <message/> stanza containing a data form
   of type "submit".  The <message/> element MUST contain a <thread/>
   element with the same XML character data as the original initiation
   request.  The data form containing a boolean field named "terminate"
   should be set to a value of "1" or "true".








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   Example: (F12) Romeo terminates the session (XMPP translation)

   <message from='romeo@example.net'
            to='juliet@example.com'
            type='normal'>
     <thread>742507no</thread>
     <feature xmlns='http://jabber.org/protocol/feature-neg'>
       <x xmlns='jabber:x:data' type='submit'>
         <field var='FORM_TYPE'>
           <value>urn:xmpp:ssn</value>
         </field>
         <field var='terminate'>
           <value>1</value>
         </field>
       </x>
     </feature>
   </message>

   Juliet explicitly acknowledges the termination of the chat session on
   the XMPP side by sending a <message/> containing a data form of type
   "result", and the value of the "terminate" field set to "1" or
   "true".  The client MUST mirror the <thread/> value it received.

   Example: (F13) Juliet acknowledges the termination of the session

   <message from='juliet@example.com'
            to='romeo@example.net'
            type='normal'>
     <thread>742507no</thread>
     <feature xmlns='http://jabber.org/protocol/feature-neg'>
       <x xmlns='jabber:x:data' type='result'>
         <field var='FORM_TYPE'>
           <value>urn:xmpp:ssn</value>
         </field>
         <field var='terminate'>
           <value>1</value>
         </field>
       </x>
     </feature>
   </message>

   Upon receiving the acknowledgment message, the XMPP-to-SIP gateway
   SHOULD translate it to a SIP answer 200 OK.








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   Example: (F14) Juliet acknowledges the termination of the session
   (SIP translation)

   SIP/2.0 200 OK
   From: <sip:romeo@example.net>;tag=576
   To: <sip:juliet@example.com>;tag=534
   Call-ID: 742507no
   CSeq: 1 BYE

4.6.  Cancelling the Transaction

   SIP User                    GW                     XMPP User
      |                        |                          |
      |(F1)(SIP) INVITE        |                          |
      |----------------------->|                          |
      |                        |(F2)(XMPP) Stanza session request
      |                        |------------------------->|
      |(F3)(SIP) CANCEL        |                          |
      |----------------------->|                          |
      |                        |(F4)(XMPP) Stanza session termination
      |                        |------------------------->|
      |                        |(F5)(XMPP) Stanza acknowledgment
      |                        |            session termination
      |                        |<-------------------------|
      |(F6)(SIP) 200 OK        |                          |
      |<-----------------------|                          |

   A common scenario occurs when the SIP user issues an invitation to
   set up a chat session with an XMPP user and immediately after the SIP
   invitation is sent, the SIP user decides to cancel it.  The SIP-to-
   XMPP gateway will receive the CANCEL request and using the Call-ID,
   To, From and CSeq (sequence number only) header field values as a
   guide, will issue an XMPP stanza session termination request to the
   XMPP user to cancel the XMPP formal session (assuming that it was
   already set up).  Once the XMPP-to-SIP gateway receives an ACK stanza
   message for the session termination, the XMPP-to-SIP gateway will
   respond with a status of 200 (OK) back to the SIP user.  It is
   important to note that if the SIP session transaction does not exist,
   the XMPP-to-SIP gateway will return a status of 481 (Transaction Does
   Not Exist) back to the SIP User.











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4.7.  Rejecting the Transaction

   SIP User                        GW                     XMPP User
       |                           |                          |
       |(F1)(SIP) INVITE           |                          |
       |-------------------------->|                          |
       |                           |(F2)(XMPP) Stanza session request
       |                           |------------------------->|
       |                           |(F3)(XMPP) Stanza session decline
       |                           |<-------------------------|
       |(F4)(SIP) 4xx/6xx          |                          |
       |<--------------------------|                          |

   Another common scenario occurs when the XMPP UA is currently not
   willing or able to accept a formal session request.  The XMPP UA
   SHOULD decline the invitation.  The data form MUST contain the
   FORM_TYPE field and the "accept" field set to "0" or "false".  It is
   RECOMMENDED that the form does not contain any other fields even if
   the request indicated they are required.

   Example: (F3) User declines offer

   <message from='juliet@example.com'
            to='romeo@example.net'
            type='normal'>
     <thread>742507no</thread>
     <feature xmlns='http://jabber.org/protocol/feature-neg'>
       <x xmlns='jabber:x:data' type='submit'>
         <field var='FORM_TYPE'>
           <value>urn:xmpp:ssn</value>
         </field>
         <field var='accept'><value>0</value></field>
       </x>
     </feature>
   </message>

   Upon receiving the declined response for the XMPP formal session
   request, the XMPP-to-SIP gateway SHOULD return a 4xx or a 6xx SIP
   response back to the SIP client.

4.8.  Session Negotiation Fails

   If the XMPP recipient of a formal session request does not support
   stanza session negotiation as specified in [XEP-0155], it will return
   an XMPP <service-unavailable/> stanza error.  Upon receiving this
   error from the XMPP recipient, the XMPP-to-SIP gateway SHOULD return
   a 501 SIP response back to the SIP sender.




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5.  MSRP to XMPP Informal Session

   When an MSRP client sends messages through a gateway to an XMPP
   client that does not support formal sessinos, the order of events is
   as follows.

   SIP User                     GW                     XMPP User
      |                         |                          |
      |(F1)(SIP) INVITE         |                          |
      |------------------------>|                          |
      |(F2)(SIP) 200 OK         |                          |
      |<------------------------|                          |
      |(F3)(SIP) ACK            |                          |
      |------------------------>|                          |
      |(F4)(MSRP) SEND          |                          |
      |------------------------>|                          |
      |                         |(F5)(XMPP) A chat message |
      |                         |------------------------->|
      |                         |(F6)(XMPP) A reply        |
      |                         |<-------------------------|
      |                         |                          |
      |(F7)(MSRP) SEND          |                          |
      |<------------------------|                          |
      |                         |                          |
      .                         .                          .
      .                         .                          .
      .                         .                          .
      |                         |                          |
      |(F8)(SIP) BYE            |                          |
      |------------------------>|                          |
      |(F9)(SIP) 200 OK         |                          |
      |<------------------------|                          |
      |                         |                          |


















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   Example: (F1) SIP user starts the session

   INVITE sip:juliet@example.com SIP/2.0
   To: <sip:juliet@example.com>
   From: <sip:romeo@example.net>;tag=576
   Subject: Open chat with Romeo?
   Call-ID: 742507no
   Content-Type: application/sdp

   c=IN IP4 s2x.example.net
   m=message 7313 TCP/MSRP *
   a=accept-types:text/plain
   a=lang:en
   a=lang:it
   a=path:msrp://s2x.example.net:7313/ansp71weztas;tcp

   Example: (F2) Gateway accepts session on Juliet's behalf

   SIP/2.0 200 OK
   To: <sip:juliet@example.com>;tag=534
   From: <sip:romeo@example.net>;tag=576
   Call-ID: 742507no
   Content-Type: application/sdp

   c=IN IP4 x2s.example.com
   m=message 8763 TCP/MSRP *
   a=accept-types:text/plain
   a=lang:it
   a=path:msrp://x2s.example.com:8763/lkjh37s2s20w2a;tcp

   Example: (F3) Romeo sends ACK

   ACK sip:juliet@example.com SIP/2.0
   To: <sip:juliet@example.com>;tag=534
   From: <sip:romeo@example.net>;tag=576
   Call-ID: 742507no















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   Example: (F4) Romeo sends a message

   MSRP ad49kswow SEND
   To-Path: msrp://x2s.example.com:8763/lkjh37s2s20w2a;tcp
   From-Path: msrp://s2x.example.net:7313/ansp71weztas;tcp
   Message-ID: 44921zaqwsx
   Byte-Range: 1-32/32
   Failure-Report: no
   Content-Type: text/plain

   I take thee at thy word ...
   -------ad49kswow$

   Example: (F5) Romeo sends a message (XMPP translation)

   <message from='romeo@example.net'
            to='juliet@example.com'
            type='chat'>
     <thread>742507no</thread>
     <body>I take thee at thy word ...</body>
   </message>

   Example: (F6) Juliet sends a reply

   <message from='juliet@example.com'
            to='romeo@example.net'
            type='chat'>
     <thread>711609sa</thread>
     <body>What man art thou ...?</body>
   </message>

   Example: (F8) Gateway transforms XMPP message to MSRP

   MSRP a786hjs2 SEND
   To-Path: msrp://s2x.example.net:7313/jshA7weztas;tcp
   From-Path: msrp://x2s.example.com:8763/lkjh37s2s20w2a;tcp
   Message-ID: 87652491
   Byte-Range: 1-25/25
   Failure-Report: no
   Content-Type: text/plain

   What man art thou ...?
   -------a786hjs2$








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   Example: (F9) Romeo terminates the session

   BYE juliet@example.com sip: SIP/2.0
   Max-Forwards: 70
   From: <sip:romeo@example.net>;tag=576
   To: <sip:juliet@example.com>;tag=534
   Call-ID: 742507no
   Cseq: 1 BYE
   Content-Length: 0

   Example: (F10) Gateway acknowledges the termination of the session on
   behalf of XMPP user

   SIP/2.0 200 OK
   From: <sip:romeo@example.net>;tag=576
   To: <sip:juliet@example.com>;tag=534
   Call-ID: 742507no
   CSeq: 1 BYE


6.  Security Considerations

   To follow.


7.  IANA Considerations

   This document requests no actions of IANA.


8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [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.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.

   [RFC3861]  Peterson, J., "Address Resolution for Instant Messaging
              and Presence", RFC 3861, August 2004.



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   [RFC4975]  Campbell, B., Mahy, R., and C. Jennings, "The Message
              Session Relay Protocol (MSRP)", RFC 4975, September 2007.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, March 2011.

   [RFC6121]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Instant Messaging and Presence",
              RFC 6121, March 2011.

   [XEP-0155]
              Paterson, I. and P. Saint-Andre, "Stanza Session
              Negotiation", XSF XEP 0155, January 2008.

8.2.  Informative References

   [I-D.saintandre-sip-xmpp-core]
              Saint-Andre, P., Houri, A., and J. Hildebrand,
              "Interworking between the Session Initiation Protocol
              (SIP) and the Extensible Messaging and Presence Protocol
              (XMPP): Core", draft-saintandre-sip-xmpp-core-02 (work in
              progress), October 2012.

   [I-D.saintandre-sip-xmpp-im]
              Saint-Andre, P., Houri, A., and J. Hildebrand,
              "Interworking between the Session Initiation Protocol
              (SIP) and the Extensible Messaging and Presence Protocol
              (XMPP): Instant Messaging",
              draft-saintandre-sip-xmpp-im-01 (work in progress),
              March 2009.

   [MSRP-MULTI]
              Niemi, A., Garcia-Martin, M., and G. Sandbakken, "Multi-
              party Instant Message (IM) Sessions Using the Message
              Session Relay  Protocol (MSRP)", draft-ietf-simple-chat-16
              (work in progress), August 2012.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [RFC3428]  Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C.,
              and D. Gurle, "Session Initiation Protocol (SIP) Extension
              for Instant Messaging", RFC 3428, December 2002.



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   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [XEP-0030]
              Hildebrand, J., Millard, P., Eatmon, R., and P. Saint-
              Andre, "Service Discovery", XSF XEP 0030, June 2008.

   [XEP-0045]
              Saint-Andre, P., "Multi-User Chat", XSF XEP 0045,
              April 2007.

   [XEP-0071]
              Saint-Andre, P., "XHTML-IM", XSF XEP 0071, August 2007.

   [XEP-0115]
              Hildebrand, J., Saint-Andre, P., Troncon, R., and J.
              Konieczny, "Entity Capabilities", XSF XEP 0115,
              February 2008.

   [XEP-0124]
              Paterson, I., Smith, D., and P. Saint-Andre,
              "Bidirectional-streams Over Synchronous HTTP (BOSH)", XSF
              XEP 0124, October 2008.

   [XEP-0184]
              Saint-Andre, P. and J. Hildebrand, "Message Receipts", XSF
              XEP 0184, September 2007.

   [XEP-0206]
              Paterson, I., "XMPP Over BOSH", XSF XEP 0206,
              October 2008.


Authors' Addresses

   Peter Saint-Andre
   Cisco Systems, Inc.
   1899 Wynkoop Street, Suite 600
   Denver, CO  80202
   USA

   Phone: +1-303-308-3282
   Email: psaintan@cisco.com








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   Eddy Gavita
   Ericsson
   Decarie Boulevard
   Town of Mount Royal, Quebec
   Canada

   Email: eddy.gavita@ericsson.com


   Nazin Hossain
   Ericsson
   Decarie Boulevard
   Town of Mount Royal, Quebec
   Canada

   Email: Nazin.Hossain@ericsson.com


   Salvatore Loreto
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: Salvatore.Loreto@ericsson.com


























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