    Internet Engineering Task Force (IETF)                    P. Saint-Andre
    Request for Comments: 7573                                          &yet
    Category: Standards Track                                      S. Loreto
    ISSN: 2070-1721                                                 Ericsson
                                                                   June 2015

       Interworking between the Session Initiation Protocol (SIP) and the
               Extensible Messaging and Presence Protocol (XMPP):
                         One-to-One Text Chat Sessions

    Abstract

       This document defines a bidirectional 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 is an Internet Standards Track document.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       Internet Standards is available in Section 2 of RFC 5741.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       http://www.rfc-editor.org/info/rfc7573.

    Copyright Notice

       Copyright (c) 2015 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.

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    Table of Contents

       1. Introduction ....................................................2
       2. Intended Audience ...............................................3
       3. Terminology .....................................................4
       4. XMPP to MSRP ....................................................4
       5. MSRP to XMPP ....................................................9
       6. Composing Events ...............................................13
          6.1. Use of the Gone Chat State ................................14
       7. Delivery Reports ...............................................15
       8. Message Size ...................................................17
       9. Internationalization Considerations ............................18
       10. Security Considerations .......................................18
       11. References ....................................................18
          11.1. Normative References .....................................18
          11.2. Informative References ...................................19
       Acknowledgements ..................................................20
       Authors' Addresses ................................................20

    1.  Introduction

       Both the Session Initiation Protocol (SIP) [RFC3261] and the
       Extensible Messaging and Presence Protocol (XMPP) [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 bidirectional protocol mappings.

       The architectural assumptions underlying such protocol mappings are
       provided in [RFC7247], including mapping of addresses and error
       conditions.  This document specifies mappings for one-to-one text
       chat sessions (sometimes called "session-mode" messaging); in
       particular, this document specifies mappings between XMPP messages of
       type "chat" and the Message Session Relay Protocol (MSRP) [RFC4975],
       which is commonly used in SIP-based systems for chat functionality
       (although note that MSRP is not conjoined to SIP, and can be used by
       non-SIP technologies).  Mappings for single instant messages and
       groupchat are provided in [RFC7572] and [GROUPCHAT].

       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  MSRP chat sessions using the SIP INVITE and SEND request types are
          specified in [RFC4975].

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       In SIP-based systems that use MSRP, a chat session is formally
       negotiated (just as any other session type is negotiated when using
       SIP).  By contrast, a one-to-one chat "session" in XMPP is an
       informal construct and is not formally negotiated: a user simply
       sends a message of type "chat" to a contact, the contact then replies
       to the message, and the sum total of such messages exchanged during a
       defined period of time is considered to be a chat session (ideally
       tied together using an XMPP <thread/> element as described in
       Section 5.1 of [RFC6121]).  To overcome the disparity between these
       approaches, a gateway that wishes to map between SIP/MSRP and XMPP
       for one-to-one chat sessions needs to maintain some additional state,
       as described below.

    2.  Intended Audience

       The documents in this series are intended for use by software
       developers who have an existing system based on one of these
       technologies (e.g., SIP) and who would like to enable communication
       from that existing system to systems based on the other technology
       (e.g., XMPP).  We assume that readers are familiar with the core
       specifications for both SIP [RFC3261] and XMPP [RFC6120], with the
       base document for this series [RFC7247], and with the following chat-
       related specifications:

       o  "The Message Session Relay Protocol (MSRP)" [RFC4975]

       o  "Extensible Messaging and Presence Protocol (XMPP): Instant
          Messaging and Presence" [RFC6121]

       o  "Indication of Message Composition for Instant Messaging"
          [RFC3994]

       o  "Chat State Notifications" [XEP-0085]

       Note well that not all protocol-compliant messages are shown (such as
       SIP 100 TRYING messages), in order to focus the reader on the
       essential aspects of the protocol flows.

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    3.  Terminology

       A number of terms used here are explained in [RFC3261], [RFC4975],
       [RFC6120], and [RFC6121].

       In flow diagrams, SIP/MSRP traffic is shown using arrows such as
       "***>" whereas XMPP traffic is shown using arrows such as "...>".

       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].

    4.  XMPP 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 [RFC7572].  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.

       XMPP            XMPP        XMPP-to-MSRP        SIP             SIP
       User           Server         Gateway          Server          User
        |               |               |               |               |
        | (F1) XMPP     |               |               |               |
        | message       |               |               |               |
        |..............>|               |               |               |
        |               | (F2) XMPP     |               |               |
        |               | message       |               |               |
        |               |..............>|               |               |
        |               |               | (F3) SIP      |               |
        |               |               | INVITE        |               |
        |               |               |**************>|               |
        |               |               |               | (F4) SIP      |
        |               |               |               | INVITE        |
        |               |               |               |**************>|
        |               |               |               | (F5) SIP      |
        |               |               |               | 200 OK        |
        |               |               |               |<**************|
        |               |               | (F6) SIP      |               |
        |               |               | 200 OK        |               |
        |               |               |<**************|               |
        |               |               | (F7) SIP ACK  |               |
        |               |               |**************>|               |

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        |               |               |               | (F8) SIP ACK  |
        |               |               |               |**************>|
        |               |               | (F9) MSRP SEND                |
        |               |               |******************************>|
        .               .               .               .               .
        .               .               .               .               .
        |               |               | (F10) MSRP SEND               |
        |               |               |<******************************|
        |               | (F11) XMPP    |               |               |
        |               | message       |               |               |
        |               |<..............|               |               |
        | (F12) XMPP    |               |               |               |
        | message       |               |               |               |
        |<..............|               |               |               |
        .               .               .               .               .
        .               .               .               .               .
        |               |               |               | (F13) SIP BYE |
        |               |               |               |<**************|
        |               |               | (F14) SIP BYE |               |
        |               |               |<**************|               |
        |               |               | (F15) SIP     |               |
        |               |               | 200 OK        |               |
        |               |               |**************>|               |
        |               |               |               | (F16) SIP     |
        |               |               |               | 200 OK        |
        |               |               |               |**************>|

                      Figure 1: XMPP to MSRP Order of Events

       The mapping of XMPP syntax to SIP syntax MUST be as specified in
       [RFC7572].

       First, the XMPP user would generate an XMPP chat message.

       Example 1: Juliet Sends XMPP Message (F1)

       | <message from='juliet@example.com/yn0cl4bnw0yr3vym'
       |          to='romeo@example.net'
       |          id='a786hjs2'
       |          type='chat'>
       |   <thread>29377446-0CBB-4296-8958-590D79094C50</thread>
       |   <body>Art thou not Romeo, and a Montague?</body>
       | </message>

       Upon receiving such a message stanza, the XMPP server needs to
       determine the identity of the domainpart in the 'to' address, which
       it does by following the procedures explained in Section 5 of
       [RFC7247].  If the domain is a SIP domain, the XMPP server will hand

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       off the message stanza to an XMPP-to-SIP gateway or connection
       manager that natively communicates with MSRP-aware SIP servers.

       The XMPP-to-SIP gateway at the XMPP server would then initiate an
       MSRP session with Romeo on Juliet's behalf (since there is no
       reliable way for the gateway to determine whether Romeo's client
       supports MSRP, if that is not the case then MSRP session initiation
       might result in an error).

       Example 2: Gateway Starts SIP Session on Behalf of Juliet (F3)

       | INVITE sip:romeo@example.net SIP/2.0
       | To: <sip:romeo@example.net>
       | From: <sip:juliet@example.com>
       | Contact: <sip:juliet@example.com>;gr=yn0cl4bnw0yr3vym
       | Subject: Open chat with Juliet?
       | Call-ID: 29377446-0CBB-4296-8958-590D79094C50
       | CSeq: 1 INVITE
       | Content-Type: application/sdp
       |
       | c=IN IP4 x2s.example.com
       | m=message 7654 TCP/MSRP *
       | a=accept-types:text/plain
       | a=path:msrp://x2s.example.com:7654/jshA7weztas;tcp

       Here we assume that Romeo's client supports MSRP and that Romeo
       accepts the MSRP session request.

       Example 3: Romeo Accepts Session Request (F5)

       | SIP/2.0 200 OK
       | From: <sip:juliet@example.com>
       | To: <sip:romeo@example.net>
       | Contact: <sip:romeo@example.net>;gr=dr4hcr0st3lup4c
       | Call-ID: 29377446-0CBB-4296-8958-590D79094C50
       | CSeq: 1 INVITE
       | Content-Type: application/sdp
       |
       | c=IN IP4 s2x.example.net
       | m=message 12763 TCP/MSRP *
       | a=accept-types:text/plain
       | a=path:msrp://s2x.example.net:12763/kjhd37s2s20w2a;tcp

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

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       Example 4: Gateway Sends ACK to Romeo (F7)

       | ACK sip:juliet@example.com SIP/2.0
       | To: <sip:romeo@example.net>;gr=dr4hcr0st3lup4c
       | From: <sip:juliet@example.com>
       | Contact: <sip:juliet@example.com>;gr=yn0cl4bnw0yr3vym
       | Call-ID: 29377446-0CBB-4296-8958-590D79094C50
       | CSeq: 2 ACK

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

       Example 5: Gateway Maps XMPP Message to MSRP (F9)

       | MSRP a786hjs2 SEND
       | From-Path: msrp://x2s.example.com:7654/jshA7weztas;tcp
       | To-Path: msrp://s2x.example.net:12763/kjhd37s2s20w2a;tcp
       | Message-ID: 54C6F4F1-A39C-47D6-8718-FA65B3D0414A
       | 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 client.

       Example 6: Romeo Sends Reply (F10)

       | MSRP di2fs53v SEND
       | To-Path: msrp://x2s.example.com:7654/jshA7weztas;tcp
       | From-Path: msrp://s2x.example.net:12763/kjhd37s2s20w2a;tcp
       | Message-ID: 6480C096-937A-46E7-BF9D-1353706B60AA
       | Byte-Range: 1-25/25
       | Failure-Report: no
       | Content-Type: text/plain
       |
       | Neither, fair saint, if either thee dislike.
       | -------di2fs53v$

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

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       Example 7: Gateway Maps MSRP Message to XMPP (F11)

       | <message from='romeo@example.net/dr4hcr0st3lup4c'
       |          to='juliet@example.com/yn0cl4bnw0yr3vym'
       |          id='di2fs53v'
       |          type='chat'>
       |   <thread>29377446-0CBB-4296-8958-590D79094C50</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.

       Example 8: Romeo Terminates Chat Session (F13)

       | BYE juliet@example.com sip: SIP/2.0
       | From: <sip:juliet@example.com>;tag=786
       | To: <sip:romeo@example.net>;tag=087js
       | Call-ID: 29377446-0CBB-4296-8958-590D79094C50
       | CSeq: 3 BYE
       | Content-Length: 0

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

       Example 9: Gateway Acknowledges Termination (F15)

       | SIP/2.0 200 OK
       | From: <sip:juliet@example.com>;tag=786
       | To: <sip:romeo@example.net>;tag=087js
       | Call-ID: 29377446-0CBB-4296-8958-590D79094C50
       | CSeq: 3 BYE
       | Content-Length: 0

       Because there is no formal session on the XMPP side, there is no
       corresponding communication from the gateway to the XMPP user.
       However, it is reasonable for the gateway to send a "gone" chat state
       notification [XEP-0085], as described under Section 6.1.

       In addition, there is no explicit method defined in [RFC6121] for an
       XMPP user to formally terminate a chat session, so a gateway would
       need to listen for a "gone" chat state notification from the XMPP
       user or institute a timer that considers the XMPP informal chat
       session to be ended after some amount of time has elapsed ([XEP-0085]
       suggests generating a "gone" chat state if a user has not interacted
       with the chat session interface, system, or device for a relatively
       long period of time, e.g., 10 minutes).

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    5.  MSRP to XMPP

       When an MSRP client sends messages through a gateway to an XMPP
       client, the order of events is as follows.

       SIP             SIP         MSRP-to-XMPP       XMPP            XMPP
       User           Server         Gateway         Server           User
        |               |               |               |               |
        | (F17) SIP     |               |               |               |
        | INVITE        |               |               |               |
        |**************>|               |               |               |
        |               | (F18) SIP     |               |               |
        |               | INVITE        |               |               |
        |               |**************>|               |               |
        |               | (F19) SIP     |               |               |
        |               | 200 OK        |               |               |
        |               |<**************|               |               |
        | (F20) SIP     |               |               |               |
        | 200 OK        |               |               |               |
        |<**************|               |               |               |
        | (F21) SIP ACK |               |               |               |
        |**************>|               |               |               |
        |               | (F22) SIP ACK |               |               |
        |               |**************>|               |               |
        | (F23) MSRP SEND               |               |               |
        |******************************>|               |               |
        |               |               | (F24) XMPP    |               |
        |               |               | message       |               |
        |               |               |..............>|               |
        |               |               |               | (F25) XMPP    |
        |               |               |               | message       |
        |               |               |               |..............>|
        .               .               .               .               .
        .               .               .               .               .
        |               |               |               | (F26) XMPP    |
        |               |               |               | message       |
        |               |               |               |<..............|
        |               |               | (F27) XMPP    |               |
        |               |               | message       |               |
        |               |               |<..............|               |
        | (F28) MSRP SEND               |               |               |
        |<******************************|               |               |
        .               .               .               .               .
        .               .               .               .               .
        |               |               |               |               |
        |               |               |               |               |
        | (F29) SIP BYE |               |               |               |
        |**************>|               |               |               |

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        |               | (F30) SIP BYE |               |               |
        |               |**************>|               |               |
        |               | (F31) SIP     |               |               |
        |               | 200 OK        |               |               |
        |               |<**************|               |               |
        | (F32) SIP     |               |               |               |
        | 200 OK        |               |               |               |
        |<**************|               |               |               |

                      Figure 2: MSRP to XMPP Order of Events

       The mapping of SIP syntax to XMPP syntax MUST be as specified in
       [RFC7572].

       The protocol flow begins when Romeo starts a chat session with
       Juliet.

       Example 10: Romeo Starts Chat Session (F17)

       | INVITE sip:juliet@example.com SIP/2.0
       | From: <sip:romeo@example.net>
       | To: <sip:juliet@example.com>
       | Contact: <sip:romeo@example.net>;gr=dr4hcr0st3lup4c
       | Subject: Open chat with Romeo?
       | Call-ID: F6989A8C-DE8A-4E21-8E07-F0898304796F
       | CSeq: 1 INVITE
       | Content-Type: application/sdp
       |
       | c=IN IP4 s2x.example.net
       | m=message 7313 TCP/MSRP *
       | a=accept-types:text/plain
       | a=path:msrp://s2x.example.net:7313/ansp71weztas;tcp

       Upon receiving the INVITE, the SIP (MSRP) server needs to determine
       the identity of the domain portion of the Request-URI or To header,
       which it does by following the procedures explained in Section 5 of
       [RFC7247].  If the domain is an XMPP domain, the SIP server will hand
       off the INVITE to an associated MSRP-to-XMPP gateway or connection
       manager that natively communicates with XMPP servers.

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       Example 11: Gateway Accepts Session on Juliet's Behalf (F19)

       | SIP/2.0 200 OK
       | From: <sip:romeo@example.net>;gr=dr4hcr0st3lup4c
       | To: <sip:juliet@example.com>
       | Contact: <sip:juliet@example.com>;gr=yn0cl4bnw0yr3vym
       | Call-ID: F6989A8C-DE8A-4E21-8E07-F0898304796F
       | CSeq: 1 INVITE
       | Content-Type: application/sdp
       |
       | c=IN IP4 x2s.example.com
       | m=message 8763 TCP/MSRP *
       | a=accept-types:text/plain
       | a=path:msrp://x2s.example.com:8763/lkjh37s2s20w2a;tcp

       Example 12: Romeo Sends ACK (F21)

       | ACK sip:juliet@example.com SIP/2.0
       | To: <sip:juliet@example.com>;gr=yn0cl4bnw0yr3vym
       | From: <sip:romeo@example.net>
       | Contact: <sip:romeo@example.net>;gr=dr4hcr0st3lup4c
       | Call-ID: F6989A8C-DE8A-4E21-8E07-F0898304796F
       | CSeq: 2 ACK

       Example 13: Romeo Sends Message (F23)

       | MSRP ad49kswow SEND
       | To-Path: msrp://x2s.example.com:8763/lkjh37s2s20w2a;tcp
       | From-Path: msrp://s2x.example.net:7313/ansp71weztas;tcp
       | Message-ID: 676FDB92-7852-443A-8005-2A1B9FE44F4E
       | Byte-Range: 1-32/32
       | Failure-Report: no
       | Content-Type: text/plain
       |
       | I take thee at thy word ...
       | -------ad49kswow$

       Example 14: MSRP-to-XMPP Gateway Maps MSRP Message to XMPP (F24)

       | <message from='romeo@example.net'
       |          to='juliet@example.com'
       |          id='ad49kswow'
       |          type='chat'>
       |   <thread>F6989A8C-DE8A-4E21-8E07-F0898304796F</thread>
       |   <body>I take thee at thy word ...</body>
       | </message>

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       Example 15: Juliet Sends Reply (F26)

       | <message from='juliet@example.com'
       |          to='romeo@example.net'
       |          id='ms53b7z9'
       |          type='chat'>
       |   <thread>29377446-0CBB-4296-8958-590D79094C50</thread>
       |   <body>What man art thou ...?</body>
       | </message>

       Example 16: Gateway Maps XMPP Message to MSRP (F28)

       | MSRP ms53b7z9 SEND
       | To-Path: msrp://s2x.example.net:7313/jshA7weztas;tcp
       | From-Path: msrp://x2s.example.com:8763/lkjh37s2s20w2a;tcp
       | Message-ID: 17EBA17B-94C0-463B-AD84-DE405C4C9D41
       | Byte-Range: 1-25/25
       | Failure-Report: no
       | Content-Type: text/plain
       |
       | What man art thou ...?
       | -------ms53b7z9$

       Example 17: Romeo Terminates Chat Session (F29)

       | BYE juliet@example.com sip: SIP/2.0
       | To: <sip:juliet@example.com>;gr=yn0cl4bnw0yr3vym
       | From: <sip:romeo@example.net>
       | Contact: <sip:romeo@example.net>;gr=dr4hcr0st3lup4c
       | Call-ID: F6989A8C-DE8A-4E21-8E07-F0898304796F
       | CSeq: 3 BYE
       | Content-Length: 0

       Example 18: Gateway Acknowledges Termination of Session on Behalf of
       Juliet (F31)

       | SIP/2.0 200 OK
       | To: <sip:juliet@example.com>;gr=yn0cl4bnw0yr3vym
       | From: <sip:romeo@example.net>
       | Contact: <sip:romeo@example.net>;gr=dr4hcr0st3lup4c
       | Call-ID: F6989A8C-DE8A-4E21-8E07-F0898304796F
       | CSeq: 3 BYE

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    6.  Composing Events

       Both XMPP and MSRP enable a client to receive notifications when a
       person's conversation partner is composing an instant message within
       the context of a chat session.

       For XMPP, the Chat State Notifications specification [XEP-0085]
       defines five states: active, inactive, gone, composing, and paused.
       Some of these states are related to the act of message composition
       (composing, paused), whereas others are related to the sender's
       involvement with the chat session (active, inactive, gone).  Note
       that the "gone" chat state is not to be confused with the <gone/>
       stanza error condition defined in [RFC6120].

       For MSRP (and, in general, for so-called SIP for Instant Messaging
       and Presence Leveraging Extensions (SIMPLE) systems), the Indication
       of Message Composition for Instant Messaging specification [RFC3994]
       defines two states: idle and active.  Here the idle state indicates
       that the sender is not actively composing a message, and the active
       state indicates that the sender is indeed actively composing a
       message (the sending client simply toggles between the two states).

       Because XEP-0085 states can represent information that is not
       captured in RFC 3994, gateways can either (a) map only the composing-
       related states or (b) map all the XEP-0085 states.

       The following mappings are suggested.

       Table 3: Mapping of SIP/SIMPLE isComposing Events to XMPP Chat states

       +-------------------+--------------------+
       | isComposing Event |  Chat State        |
       +-------------------+--------------------+
       | active            |  composing         |
       | idle              |  active            |
       +-------------------+--------------------+

       Table 4: Mapping of XMPP Chat States to SIP/SIMPLE isComposing Events

       +-------------------+--------------------+
       | Chat State        | isComposing Event  |
       +-------------------+--------------------+
       | active            |  idle              |
       | inactive          |  idle              |
       | gone              |  none (Section 6.1)|
       | composing         |  active            |
       | paused            |  idle              |
       +-------------------+--------------------+

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       The XMPP Chat State Notifications specification [XEP-0085] allows the
       sending of "standalone notifications" outside the context of a
       message, theoretically even before any messages are exchanged;
       although a gateway could thus send an <active/> notification to the
       XMPP user when the SIP user accepts or initiates a chat session
       (i.e., after F6 in Section 4 or after F22 in Section 5), this usage
       might be unexpected by XMPP clients as a way to signal the beginning
       of an informal chat session.

    6.1.  Use of the Gone Chat State

       Although there is no direct mapping for the "gone" chat state to an
       isComposing event, receipt of the "gone" state at an XMPP-to-MSRP
       gateway can serve as a trigger for terminating the formal chat
       session within MSRP, i.e., for sending a SIP BYE for the session from
       the XMPP-to-MSRP gateway to the SIP user.  The following examples
       illustrate this indirect mapping (which would arise if, for example,
       the XMPP user were to send a "gone" chat state notification after
       step F12 in Figure 1 or step F28 in Figure 2; in either of these
       cases, the session would be terminated by the XMPP user instead of by
       the SIP user, as currently shown in Figures 1 and 2).

       Example 19: Juliet Sends Gone Chat State

       | <message from='juliet@example.com'
       |          id='nx62f197'
       |          to='romeo@example.net'
       |          type='chat'>
       |   <thread>29377446-0CBB-4296-8958-590D79094C50</thread>
       |   <gone xmlns='http://jabber.org/protocol/chatstates'/>
       | </message>

       Example 20: XMPP-to-MSRP Gateway Maps Gone Chat State to SIP BYE

       | BYE romeo@example.net sip: SIP/2.0
       | From: <sip:juliet@example.com>;tag=786
       | To: <sip:romeo@example.net>;tag=087js
       | Call-ID: 29377446-0CBB-4296-8958-590D79094C50
       | CSeq: 3 BYE
       | Content-Length: 0

       Similarly, receipt of a SIP BYE message at an MSRP-to-XMPP gateway
       can serve as a trigger for sending a "gone" chat state notification
       to the XMPP user.  The following examples illustrate this indirect
       mapping (which would occur after step F14 in Figure 1 or step F30 in
       Figure 2).

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       Example 21: Romeo Terminates Chat Session

       | BYE juliet@example.com sip: SIP/2.0
       | To: <sip:juliet@example.com>;gr=yn0cl4bnw0yr3vym
       | From: <sip:romeo@example.net>
       | Contact: <sip:romeo@example.net>;gr=dr4hcr0st3lup4c
       | Call-ID: F6989A8C-DE8A-4E21-8E07-F0898304796F
       | CSeq: 3 BYE
       | Content-Length: 0

       Example 22: MSRP-to-XMPP Gateway Generates Gone Chat State

       | <message from='romeo@example.net'
       |          id='hs61v397'
       |          to='juliet@example.com'
       |          type='chat'>
       |   <thread>F6989A8C-DE8A-4E21-8E07-F0898304796F</thread>
       |   <gone xmlns='http://jabber.org/protocol/chatstates'/>
       | </message>

       To enable these uses, gateways that support chat state notifications
       MUST support the "gone" state (which is merely recommended, not
       required, by [XEP-0085]).

       It is also reasonable for gateways to implement timers that
       automatically trigger a "gone" chat state if the XMPP user has not
       sent a message within the "session" for a given amount of time
       ([XEP-0085] suggests generating a "gone" chat state if a user has not
       interacted with the chat session interface, system, or device for a
       relatively long period of time, e.g., 10 minutes).

    7.  Delivery Reports

       Both XMPP and MSRP enable a client to receive notifications when a
       message has been received by the intended recipient.

       For XMPP, the Message Receipts specification [XEP-0184] defines a
       method and XML namespace for requesting and returning indications
       that a message has been received by a client controlled by the
       intended recipient.

       For MSRP, a native reporting feature is included, in the form of
       REPORT chunks (see Sections 7.1.2 and 7.1.3 of [RFC4975]).

       An XMPP Message Receipts element of <request
       xmlns='urn:xmpp:receipts'/> is to be mapped to an MSRP Success-Report
       header field with a value of "yes", and an XMPP Message Receipts

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       element of <received xmlns='urn:xmpp:receipts'/> is to be mapped to
       an MSRP REPORT request.

       A Success-Report header field with a value of "yes" in an MSRP SEND
       request is to be mapped to an XMPP Message Receipts element of
       <request xmlns='urn:xmpp:receipts'/>, and an MSRP REPORT request is
       to be mapped to an XMPP message containing only a Message Receipts
       element of <received xmlns='urn:xmpp:receipts'/>.

       Because the XMPP Message Receipts specification does not support
       failure reports, there is no mapping for the MSRP Failure-Report
       header field and gateways SHOULD set that header field to "no".

       Examples follow.

       First, the XMPP user sends a message containing a request for
       delivery notification.

       Example 23: Juliet Sends XMPP Message with Receipt Request

       | <message from='juliet@example.com'
       |          id='bf9m36d5'
       |          to='romeo@example.net'
       |          type='chat'>
       |   <thread>29377446-0CBB-4296-8958-590D79094C50</thread>
       |   <body>What man art thou ...?</body>
       |   <request xmlns='urn:xmpp:receipts'/>
       | </message>

       Example 24: Gateway Maps XMPP Message to MSRP

       | MSRP bf9m36d5 SEND
       | To-Path: msrp://s2x.example.net:7313/jshA7weztas;tcp
       | From-Path: msrp://x2s.example.com:8763/lkjh37s2s20w2a;tcp
       | Message-ID: 6187CF9B-317A-41DA-BB6A-5E48A9C794EF
       | Byte-Range: 1-25/25
       | Success-Report: yes
       | Failure-Report: no
       | Content-Type: text/plain
       |
       | What man art thou ...?
       | -------bf9m36d5$

       Next, the recipient returns a report.

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       Example 25: Romeo Returns MSRP Receipt

       | MSRP hx74g336 REPORT
       | To-Path: msrp://x2s.example.com:8763/lkjh37s2s20w2a;tcp
       | From-Path: msrp://s2x.example.net:7313/jshA7weztas;tcp
       | Message-ID: 6187CF9B-317A-41DA-BB6A-5E48A9C794EF
       | Byte-Range: 1-106/106
       | Status: 000 200 OK
       | -------hx74g336$

       Example 26: MSRP-to-XMPP Gateway Maps Receipt to XMPP

       | <message from='romeo@example.net'
       |          id='hx74g336'
       |          to='juliet@example.com'>
       |   <received xmlns='urn:xmpp:receipts' id='87652491'/>
       | </message>

    8.  Message Size

       Unlike page-mode messaging [RFC3428] (which specifies that the size
       of a MESSAGE request is not allowed to exceed 1300 bytes), session-
       mode messaging [RFC4975] can be used to send larger messages.  MSRP
       includes a chunking mechanism such that larger messages can be broken
       up into multiple MSRP SEND requests.  Because the MSRP gateway at an
       XMPP service acts as an MSRP endpoint, it is responsible for
       receiving chunked messages and reconstructing them into a single
       message for delivery toward the XMPP recipient.  (Naturally,
       implementations need to be careful about accepting very large
       messages; see Section 14.5 of [RFC4975].)

       Although there is no hard limit on the size of an XMPP stanza, in
       practice, most XMPP services (at least on the public Internet) are
       configured with a maximum stanza size in order to help prevent
       denial-of-service attacks.  As specified in Section 13.12 of
       [RFC6120], this maximum is not allowed to be less than 10,000 bytes.

       The administrators of an XMPP service need to ensure that the
       associated MSRP gateway is configured with the same or smaller
       maximum MSRP message size as the maximum XMPP stanza size; this
       enables the gateway to return an appropriate value for the Session
       Description Protocol (SDP) "max-size" attribute (see Section 8.6 of
       [RFC4975]) and to properly handle incoming messages larger than the
       configured limits.

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       If an MSRP-to-XMPP gateway implementation receives an MSRP message
       that exceeds its configured limit as just described, it MUST return
       an MSRP 413 error (e.g., in response to the first SEND request whose
       Byte-Range header field indicates a byte total exceeding the limit).

    9.  Internationalization Considerations

       Relevant discussion of internationalized text in messages can be
       found in [RFC7572].

    10.  Security Considerations

       Detailed security considerations are given in the following
       documents:

       o  For instant messaging protocols in general, see [RFC2779]

       o  For MSRP chat, see [RFC4975]; for when SIP is used to negotiate
          MSRP sessions, see [RFC3261]

       o  For XMPP-based instant messaging, see [RFC6121] and also [RFC6120]

       o  For SIP-XMPP interworking in general, see [RFC7247]

       o  For end-to-end encryption of instant messages, see [RFC7572]

    11.  References

    11.1.  Normative References

       [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
                   Requirement Levels", BCP 14, RFC 2119,
                   DOI 10.17487/RFC2119, March 1997,
                   <http://www.rfc-editor.org/info/rfc2119>.

       [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>.

       [RFC3994]   Schulzrinne, H., "Indication of Message Composition for
                   Instant Messaging", RFC 3994, DOI 10.17487/RFC3994,
                   January 2005, <http://www.rfc-editor.org/info/rfc3994>.

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    RFC 7573               SIP-XMPP Interworking: Chat             June 2015

       [RFC4975]   Campbell, B., Ed., Mahy, R., Ed., and C. Jennings, Ed.,
                   "The Message Session Relay Protocol (MSRP)", RFC 4975,
                   DOI 10.17487/RFC4975, September 2007,
                   <http://www.rfc-editor.org/info/rfc4975>.

       [RFC6120]   Saint-Andre, P., "Extensible Messaging and Presence
                   Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
                   March 2011, <http://www.rfc-editor.org/info/rfc6120>.

       [RFC6121]   Saint-Andre, P., "Extensible Messaging and Presence
                   Protocol (XMPP): Instant Messaging and Presence",
                   RFC 6121, DOI 10.17487/RFC6121, March 2011,
                   <http://www.rfc-editor.org/info/rfc6121>.

       [RFC7247]   Saint-Andre, P., Houri, A., and J. Hildebrand,
                   "Interworking between the Session Initiation Protocol
                   (SIP) and the Extensible Messaging and Presence Protocol
                   (XMPP): Architecture, Addresses, and Error Handling",
                   RFC 7247, DOI 10.17487/RFC7247, May 2014,
                   <http://www.rfc-editor.org/info/rfc7247>.

       [RFC7572]   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", RFC 7572,
                   DOI 10.17487/RFC7572, June 2015,
                   <http://www.rfc-editor.org/info/rfc7572>.

       [XEP-0085] Saint-Andre, P. and D. Smith, "Chat State Notifications",
                  XSF XEP 0085, September 2009.

       [XEP-0184] Saint-Andre, P. and J. Hildebrand, "Message Delivery
                  Receipts", XSF XEP 0184, March 2011.

    11.2.  Informative References

       [GROUPCHAT] Saint-Andre, P., Corretge, S., and S. Loreto,
                   "Interworking between the Session Initiation Protocol
                   (SIP) and the Extensible Messaging and Presence Protocol
                   (XMPP): Groupchat", Work in Progress,
                   draft-ietf-stox-groupchat-11, March 2015.

       [RFC2779]   Day, M., Aggarwal, S., Mohr, G., and J. Vincent, "Instant
                   Messaging / Presence Protocol Requirements", RFC 2779,
                   DOI 10.17487/RFC2779, February 2000,
                   <http://www.rfc-editor.org/info/rfc2779>.

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       [RFC3428]   Campbell, B., Ed., Rosenberg, J., Schulzrinne, H.,
                   Huitema, C., and D. Gurle, "Session Initiation Protocol
                   (SIP) Extension for Instant Messaging", RFC 3428,
                   DOI 10.17487/RFC3428, December 2002,
                   <http://www.rfc-editor.org/info/rfc3428>.

    Acknowledgements

       Special thanks to Eddy Gavita and Nazin Hossain for coauthoring an
       early draft version of this document.

       Thanks to Mary Barnes, Ben Campbell, Dave Crocker, Adrian Georgescu,
       Philipp Hancke, Saul Ibarra Corretge, Tory Patnoe, and Matt Ryan for
       their feedback.

       Stephen Farrell, Brian Haberman, Joel Jaeggli, Barry Leiba, Kathleen
       Moriarty, and Pete Resnick provided helpful input during IESG review.

       The authors gratefully acknowledge the assistance of Markus Isomaki
       and Yana Stamcheva as the working group chairs and Gonzalo Camarillo
       and Alissa Cooper as the sponsoring Area Directors.

       Peter Saint-Andre wishes to acknowledge Cisco Systems, Inc., for
       employing him during his work on earlier draft versions of this
       document.

    Authors' Addresses

       Peter Saint-Andre
       &yet

       EMail: peter@andyet.com
       URI:   https://andyet.com/

       Salvatore Loreto
       Ericsson
       Hirsalantie 11
       Jorvas  02420
       Finland

       EMail: Salvatore.Loreto@ericsson.com

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