TOC 
Network Working GroupP. Saint-Andre
Internet-DraftJabber Software Foundation
Expires: October 27, 2004A. Houri
 IBM
 J. Hildebrand
 Jabber, Inc.
 April 28, 2004

Interoperability between the Extensible Messaging and Presence Protocol (XMPP) and Session Initiation Protocol (SIP) Extensions for Instant Messaging and Presence

draft-saintandre-xmpp-simple-01

Status of this Memo

This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts.

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This Internet-Draft will expire on October 27, 2004.

Copyright Notice

Copyright (C) The Internet Society (2004). All Rights Reserved.

Abstract

This document defines a bi-directional protocol mapping for use by gateways that enable the exchange of instant messages and presence information between systems that implement the Extensible Messaging and Presence Protocol (XMPP) and those that implement the basic extensions to the Session Initiation Protocol (SIP) for instant messaging and presence.



Table of Contents

1.  Introduction
    1.1  Architectural Assumptions
    1.2  Terminology
2.  Addresses
    2.1  Overview
    2.2  XMPP to SIP
    2.3  SIP to XMPP
3.  Instant Messages
    3.1  Overview
    3.2  XMPP to SIP
    3.3  SIP to XMPP
4.  Presence Notifications
    4.1  Overview
    4.2  XMPP to SIP
    4.3  SIP to XMPP
5.  Presence Subscriptions
    5.1  Overview
    5.2  XMPP to SIP
    5.3  SIP to XMPP
6.  IANA Considerations
7.  Security Considerations
8.  Open Issues
9.  Acknowledgements
§.  Normative References
§.  Informative References
§  Authors' Addresses
A.  Revision History
    A.1  Changes from draft-saintandre-xmpp-simple-00
§  Intellectual Property and Copyright Statements




 TOC 

1. Introduction

In order to help ensure interoperability between instant messaging and presence systems that conform to the requirements of RFC 2779Day, M., Aggarwal, S. and J. Vincent, Instant Messaging / Presence Protocol Requirements, February 2000.[IMP-REQS], it is important to clearly define mappings between such protocols. Within the IETF, work has proceeded on two such protocols:

One approach to helping ensure interoperability between such protocols is to map each protocol to the abstract semantics described in [CPIM]Peterson, J., Common Profile for Instant Messaging (CPIM), August 2003. and [CPP]Peterson, J., Common Profile for Presence (CPP), August 2003.; that is the approach taken by [SIMPLE-CPIM]Rosenberg, J. and B. Campbell, CPIM Mapping of SIMPLE Presence and Instant Messaging, June 2002. and [XMPP-CPIM]Saint-Andre, P., Mapping the Extensible Messaging and Presence Protocol (XMPP) to Common Presence and Instant Messaging (CPIM), March 2004.. Another approach is that taken by [DRAFT-UMPP]Mahy, R., A Unified Proposal for Server-to-Server Presence and Instant Messaging, February 2004.. The approach taken in this document is to directly map semantics from one protocol to another (i.e., from SIP to XMPP and vice-versa), mainly for use by gateways between systems that implement one or the other of these protocols. (These approaches describe concepts that are complementary in many ways, and a future draft may provide a more substantive merger between them.)

The mappings specified in this document cover three main areas:

1.1 Architectural Assumptions

This document assumes that the mapping between protocols will most likely occur by means of a gateway between an XMPP network and a SIP network being used for instant messaging and presence. Such a gateway is a dedicated translator between the XMPP and SIP protocols. Although such a gateway could use the [CPIM]Peterson, J., Common Profile for Instant Messaging (CPIM), August 2003. and [CPP]Peterson, J., Common Profile for Presence (CPP), August 2003. specifications to define the common formats into which the protocols are translated for purposes of interworking (as specified in [SIMPLE-CPIM]Rosenberg, J. and B. Campbell, CPIM Mapping of SIMPLE Presence and Instant Messaging, June 2002. and [XMPP-CPIM]Saint-Andre, P., Mapping the Extensible Messaging and Presence Protocol (XMPP) to Common Presence and Instant Messaging (CPIM), March 2004.), this document assumes that a gateway will translate directly from one protocol to the other. Naturally, a gateway need not be a distinct entity on the network and may be co-resident with an XMPP server or a SIMPLE "server" (although there is no such thing as a SIMPLE server, we use the term here to refer to a SIP proxy, redirect, or registrar server that supports the SIP extensions for instant messaging and/or presence). Within this document, we refer to a gateway from an XMPP network to a SIP network being used for instant messaging and presence as an "XMPP-SIMPLE gateway", and we refer to a gateway from a SIP network being used for instant messaging and presence to an XMPP network as a "SIMPLE-XMPP gateway".

1.2 Terminology

The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119Bradner, S., Key words for use in RFCs to Indicate Requirement Levels, March 1997.[TERMS].



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2. Addresses

2.1 Overview

The address formats used to identify XMPP entities are different from those used to identify SIP entities. The XMPP address format is specified in [XMPP-CORE]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Core, April 2004.; as specified in [XMPP-IM]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence, April 2004., instant messaging and presence applications of XMPP must also support 'im:' and 'pres:' URIs as specified in [CPIM]Peterson, J., Common Profile for Instant Messaging (CPIM), August 2003. and [CPP]Peterson, J., Common Profile for Presence (CPP), August 2003. respectively, although such support may simply involve leaving resolution of such addresses up to an XMPP server. The SIP address format for instant messaging is specified in [SIP-IM]Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C. and D. Gurle, Session Initiation Protocol (SIP) Extension for Instant Messaging, December 2002.; it may use either 'sip:' or 'sips:' URIs as specified in [SIP]Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, SIP: Session Initiation Protocol, June 2002. or an 'im:' URI as specified in [CPIM]Peterson, J., Common Profile for Instant Messaging (CPIM), August 2003.. The SIP address format for presence is specified in [SIP-PRES]Rosenberg, J., A Presence Event Package for the Session Initiation Protocol (SIP), January 2003.; it may use either 'sip:' or 'sips:' URIs as specified in [SIP]Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, SIP: Session Initiation Protocol, June 2002. or a 'pres:' URI as specified in [CPP]Peterson, J., Common Profile for Presence (CPP), August 2003..

In this document we describe mappings for addresses of the form <user@domain> only, ignoring any protocol-specific extensions such as XMPP resource identifiers or SIP telephone numbers and passwords. In addition, we have ruled the mapping of domain names as out of scope for now since that is a matter for the Domain Name System; specifically, the issue for interworking between SIP and XMPP relates to the translation of fully internationalized domain names (which the SIP address format does not allow, but which the XMPP address format does allow via [IDNA]Faltstrom, P., Hoffman, P. and A. Costello, Internationalizing Domain Names in Applications (IDNA), March 2003.) into non-internationalized domain names. Therefore, in the following sections we discuss local-part addresses only (these are called variously "usernames", "instant inboxes", "presentities", and "node identifiers" in the protocols at issue).

The sip:/sips:, im:/pres:, and XMPP address schemes allow different sets of characters. In some cases, characters allowed in one scheme are disallowed in others; these characters must be mapped appropriately in order to ensure interoperable communications across systems. The table below summarizes our findings regarding the complement of allowable US-ASCII characters in each addressing scheme when compared individually to the other schemes.

Table 1: Partial complements of allowable US-ASCII characters

   +----------+----------+-----------+-------+
   |          | SIP/SIPS |  IM/PRES  |  XMPP |
   +----------+----------+-----------+-------+
   | SIP/SIPS |  N/A     |  (),;     |  &'/  |
   +----------+----------+-----------+-------+
   | IM/PRES  | #%^`{|}  |  N/A      |  &'/  |
   +----------+----------+-----------+-------+
   | XMPP     |  none    |  none     |  N/A  |
   +----------+----------+-----------+-------+
          

Note: Each cell shows US-ASCII characters that are disallowed in the column protocol but allowed in the row protocol; e.g., the last cell of the third row shows that the characters &'/ are allowed in sip:/sips: URIs but disallowed in XMPP addresses.

The table below is another way of looking at the same issue, since it shows the intersection of allowable US-ASCII characters in each addressing scheme when compared individually to the other schemes.

Table 2: Partial intersections of allowable US-ASCII characters

   +-------------------+------------------+----------------------+
   |  SIP/SIPS & XMPP  |  IM/PRES & XMPP  |  SIP/SIPS & IM/PRES  |
   +-------------------+------------------+----------------------+
   |  a-z  A-Z  0-9    |  a-z  A-Z  0-9   |  a-z  A-Z  0-9       |
   |  !$()*+,-.;=?_~   |  !#$%*+-.=?^_`   |  !$*+-.=?_~          |
   |  %hexhex          |  {|}~            |                      |
   +-------------------+------------------+----------------------+
          

Therefore the following US-ASCII characters are allowed in all three addressing schemes (i.e., the intersection of all three sets of allowable characters):

   a-z  A-Z  0-9  ! $ * + - . = ?
          

In addition to the US-ASCII characters described above, many non-US-ASCII (specifically, UTF-8) characters are allowed in XMPP addresses but not allowed in sip:/sips: or im:/pres: URIs, since XMPP allows internationalized local-part addresses. A straightforward mapping of these characters to US-ASCII characters is provided in Section 2.2.5 of [URL-GUIDE]Masinter, L., Alvestrand, H., Zigmond, D. and R. Petke, Guidelines for new URL Schemes, November 1999., namely to encode unsafe octets using the %hexhex encoding.

2.2 XMPP to SIP

The following is a high-level algorithm for mapping an XMPP address to a sip:, sips:, im:, or pres: URI:

  1. Split XMPP address into node identifier (local-part; mapping described in remaining steps), domain identifier (hostname; mapping is out of scope), and resource identifier (specifier for particular device or connection; discard this for cross-system interoperability)
  2. Apply Nodeprep profile of [STRINGPREP]Hoffman, P. and M. Blanchet, Preparation of Internationalized Strings ("STRINGPREP"), December 2002. (as specified in [XMPP-CORE]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Core, April 2004.) for canonicalization (OPTIONAL)
  3. Translate #26; to &, #27; to ', and #2f; to / respectively
  4. For each byte, if the byte is not in the set -A-Za-z0-9!$*.?_~+= then change to %hexhex
  5. Combine resulting local-part with mapped hostname to form local@domain address
  6. Prepend with 'im:' scheme (for XMPP <message/> stanzas) or 'pres:' scheme (for XMPP <presence/> stanzas) if foreign domain supports these (discovered via [SRV]Gulbrandsen, A., Vixie, P. and L. Esibov, A DNS RR for specifying the location of services (DNS SRV), February 2000. lookup as specified in [XMPP-IM]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence, April 2004.), else prepend with 'sip:' or 'sips:' scheme according to local service policy

2.3 SIP to XMPP

The following is a high-level algorithm for mapping a sip:, sips:, im:, or pres: URI to an XMPP address:

  1. Remove URI scheme
  2. Split at the first '@' character into local-part and hostname (mapping the latter is out of scope)
  3. Translate %hexhex to equivalent octets
  4. Treat result as a UTF-8 string
  5. Translate & to #26;, ' to #27;, and / to #2f respectively
  6. Apply Nodeprep profile of [STRINGPREP]Hoffman, P. and M. Blanchet, Preparation of Internationalized Strings ("STRINGPREP"), December 2002. (as specified in [XMPP-CORE]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Core, April 2004.) for canonicalization (OPTIONAL)
  7. Recombine local-part with mapped hostname to form local@domain address


 TOC 

3. Instant Messages

3.1 Overview

Both XMPP and IM-aware SIP systems enable entities (often but not necessarily human users) to send "instant messages" to other entities. The term "instant message" usually refers to messages sent between two entities 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 message:

This document addresses single messages only, since they form the "lowest common denominator" for instant messaging on the Internet. It is likely that future versions of this document will address chat messages as well, especially once the SIMPLE WG completes its work on one-to-one message sessions (a likely candidate for finalization is [MSRP]Campbell, B., Rosenberg, J., Sparks, R. and P. Kyzivat, The Message Session Relay Protocol, March 2004.).

Instant messaging using XMPP message stanzas of type "normal" is specified in [XMPP-IM]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence, April 2004.. Instant messaging using SIP requests of type MESSAGE (often called "pager-model" messaging) is specified in [SIP-IM]Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C. and D. Gurle, Session Initiation Protocol (SIP) Extension for Instant Messaging, December 2002..

As described in [XMPP-IM]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence, April 2004., a single instant message is an XML <message/> stanza of type "normal" sent over an XML stream (since "normal" is the default for the 'type' attribute of the <message/> stanza, the attribute is often omitted). In this document we will assume that such a message is sent from an XMPP client to an XMPP server over an XML stream negotiated between the client and the server, and that the client is controlled by a human user (this is a simplifying assumption introduced for explanatory purposes only; the XMPP sender could be a bot-controlled client, a component such as a workflow application, a server, etc.). Continuing the tradition of Shakespeare examples in XMPP documentation, we will say that the XMPP user has an XMPP address of <juliet@example.com>.

As described in [SIP-IM]Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C. and D. Gurle, Session Initiation Protocol (SIP) Extension for Instant Messaging, December 2002., a single instant message is a SIP MESSAGE request sent from a SIP user agent to an intended recipient who is most generally referenced by an Instant Message URI of the form "im:user@domain" but who may be referenced by a SIP or SIPS URI of the form "sip:user@domain" or "sips:user@domain". Here again we introduce the simplifying assumption that the user agent is controlled by a human user, whom we shall dub <romeo@example.net>.

3.2 XMPP to SIP

When Juliet wants to send an instant message to Romeo, she interacts with her XMPP client, which generates an XMPP <message/> stanza. The syntax of the <message/> stanza, including required and optional elements and attributes, is defined in [XMPP-IM]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence, April 2004.. The following is an example of such a stanza:

Example: XMPP user sends message:

|  <message from='juliet@example.com/balcony'
|           to='romeo@example.net'>
|    <body>Art thou not Romeo, and a Montague?</body>
|  </message>
          

Upon receiving such a stanza, the XMPP server to which Juliet has connected either delivers it to a local recipient (if the hostname in the 'to' attribute matches one of the hostnames serviced by the XMPP server) or attempts to route it to the foreign 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 [XMPP-IM]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence, April 2004., the XMPP server needs to determine the identity of the foreign domain, which it does by performing one or more [SRV]Gulbrandsen, A., Vixie, P. and L. Esibov, A DNS RR for specifying the location of services (DNS SRV), February 2000. lookups. For message stanzas, the order of lookups recommended by [XMPP-IM]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence, April 2004. is to first try the "_xmpp-server" service as specified in [XMPP-CORE]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Core, April 2004. and to then try the "_im" service as specified in [IMP-SRV]Peterson, J., Address Resolution for Instant Messaging and Presence, October 2003.. 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 foreign 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 has determined that the foreign domain is serviced by a SIMPLE server, it must determine how to proceed. We here assume that the XMPP server contains or has available to it an XMPP-SIMPLE gateway. The XMPP server would then deliver the message stanza to the XMPP-SIMPLE gateway.

The XMPP-SIMPLE gateway is then responsible for translating the XMPP message stanza into a SIP MESSAGE request from the XMPP user to the SIP user:

Example: XMPP user sends message (SIP transformation):

|  MESSAGE sip:romeo@example.net SIP/2.0
|  Via: SIP/2.0/TCP julietpc.example.com;branch=z9hG4bK776sgdkse
|  Max-Forwards: 70
|  From: sip:juliet@example.com;tag=49583
|  To: sip:romeo@example.net
|  Call-ID: Hr0zny9l3@example.com
|  CSeq: 1 MESSAGE
|  Content-Type: text/plain
|  Content-Length: 37
|
|  Art thou not Romeo, and a Montague?
          

The mapping of XMPP syntax elements to SIP syntax elements SHOULD be as follows (mappings for elements not mentioned are undefined):

Table 3: Message syntax mapping from XMPP to SIP

   +-----------------------------+--------------------------+
   |  XMPP Element or Attribute  |  SIP Header or Contents  |
   +-----------------------------+--------------------------+
   |  <body/>                    |  body of MESSAGE         |
   |  <subject/>                 |  Subject                 |
   |  <thread/>                  |  Call-ID                 |
   |  from                       |  From                    |
   |  id                         |  CSeq [1]                |
   |  to                         |  To                      |
   |  type                       |  (no mapping)            |
   |  xml:lang                   |  Content-Language        |
   +-----------------------------+--------------------------+
          

Note the following regarding these mappings:

  1. According to Section 8.1.1.5 of RFC 3261, a SIP CSeq header must be expressible as a 32-bit unsigned integer. However, the only restriction on the XMPP 'id' attribute is that it shall conform to the XML NMTOKEN datatype. If an XMPP 'id' attribute is not expressible as a 32-bit unsigned integer, it SHOULD NOT be mapped.

3.3 SIP to XMPP

When Romeo wants to send an instant message to Juliet, he interacts with his SIP user agent, which generates a SIP MESSAGE request. The syntax of the MESSAGE request is defined in [SIP-IM]Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C. and D. Gurle, Session Initiation Protocol (SIP) Extension for Instant Messaging, December 2002.. The following is an example of such a request:

Example: SIP user sends message:

|  MESSAGE sip:juliet@example.com SIP/2.0
|  Via: SIP/2.0/TCP romeopc.example.com;branch=eskdgs677Kb4Ghz9
|  Max-Forwards: 70
|  From: sip:romeo@example.net;tag=38594
|  To: sip:juliet@example.com
|  Call-ID: M4spr4vdu@example.net
|  CSeq: 1 MESSAGE
|  Content-Type: text/plain
|  Content-Length: 26
|
|  Neither, fair saint, if either thee dislike.
          

Section 5 of [SIP-IM]Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C. and D. Gurle, Session Initiation Protocol (SIP) Extension for Instant Messaging, December 2002. stipulates that a SIP User Agent presented with an im: URI should resolve it to a sip: or sips: URI. Therefore we assume that the To header of a request received by a SIMPLE-XMPP gateway will contain a sip: or sips: URI. The gateway SHOULD resolve that address to an im: URI for SIP MESSAGE requests, then follow the rules in [IMP-SRV]Peterson, J., Address Resolution for Instant Messaging and Presence, October 2003. regarding the "_im" SRV service for the target domain contained in the To header. If SRV address resolution fails for the "_im" service, the gateway MAY attempt a lookup for the "_xmpp-server" service as specified in [XMPP-CORE]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Core, April 2004. or MAY return an error to the sender (the SIP "502 Bad Gateway" error seems most appropriate). If SRV address resolution succeeds, the gateway is responsible for translating the request into an XMPP message stanza from the SIP user to the XMPP user and returning a SIP "200 OK" message to the sender:

Example: SIP user sends message (XMPP transformation):

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

The mapping of SIP syntax elements to XMPP syntax elements SHOULD be as follows (mappings for elements not mentioned are undefined):

Table 4: Message syntax mapping from SIP to XMPP

   +--------------------------+-----------------------------+
   |  SIP Header or Contents  |  XMPP Element or Attribute  |
   +--------------------------+-----------------------------+
   |  Call-ID                 |  <thread/>                  |
   |  Content-Language        |  xml:lang                   |
   |  CSeq                    |  id                         |
   |  From                    |  from                       |
   |  Subject                 |  <subject/>                 |
   |  To                      |  to                         |
   |  body of MESSAGE         |  <body/>                    |
   +--------------------------+-----------------------------+
          


 TOC 

4. Presence Notifications

4.1 Overview

Both XMPP and presence-aware SIP systems enable entities (often but not necessarily human users) to send presence notifications to other entities. At a minimum, the term "presence" refers to information about an entity's availability for communication on a network (on/off), often supplemented by information that further specifies the entity's communications context (e.g., "do not disturb"). Some systems and protocols extend this notion even further and refer to any relatively ephemeral information about an entity as a kind of presence; categories of such "extended presence" include geographical location (e.g., GPS coordinates), user mood (e.g., grumpy), user activity (e.g., walking), and ambient environment (e.g., noisy). In this document, we focus on the "least common denominator" of network availability only, although future revisions of this document may address broader notions of presence. (Presence subscriptions are described in the following section.)

Presence using XMPP presence stanzas of type "available" or "unavailable" is specified in [XMPP-IM]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence, April 2004.. SIP presence using a SIP event package for presence is specified in [SIP-PRES]Rosenberg, J., A Presence Event Package for the Session Initiation Protocol (SIP), January 2003..

As described in [XMPP-IM]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence, April 2004., presence information about an entity is communicated by means of an XML <presence/> stanza sent over an XML stream. In this document we will assume that such a presence stanza is sent from an XMPP client to an XMPP server over an XML stream negotiated between the client and the server, and that the client is controlled by a human user (again, this is a simplifying assumption introduced for explanatory purposes only). In general, XMPP presence is sent by the user to the user's server and then broadcasted to all entities who are subscribed to the user's presence information.

As described in [SIP-PRES]Rosenberg, J., A Presence Event Package for the Session Initiation Protocol (SIP), January 2003., presence information about an entity is communicated by means of a SIP NOTIFY event sent from a SIP user agent to an intended recipient who is most generally referenced by an Instant Message URI of the form "pres:user@domain" but who may be referenced by a SIP or SIPS URI of the form "sip:user@domain" or "sips:user@domain". Here again we introduce the simplifying assumption that the user agent is controlled by a human user.

4.2 XMPP to SIP

When Juliet interacts with her XMPP client to modify her presence information (or when her client automatically updates her presence information, e.g. via an "auto-away" feature), her client generates an XMPP <presence/> stanza. The syntax of the <presence/> stanza, including required and optional elements and attributes, is defined in [XMPP-IM]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence, April 2004.. The following is an example of such a stanza:

Example: XMPP user sends presence notification:

|  <presence from='juliet@example.com/balcony'/>
          

Upon receiving such a stanza, the XMPP server to which Juliet has connected broadcasts it to all subscribers who are authorized to receive presence notifications from Juliet. For each subscriber, broadcasting the presence notification involves either delivering it to a local recipient (if the hostname in the subscriber's address matches one of the hostnames serviced by the XMPP server) or attempting to route it to the foreign domain that services the hostname in the subscriber's address. Naturally, in this document we assume that the hostname is a SIP presence service hosted by a separate server. As specified in [XMPP-IM]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence, April 2004., the XMPP server needs to determine the identity of the foreign domain, which it does by performing one or more [SRV]Gulbrandsen, A., Vixie, P. and L. Esibov, A DNS RR for specifying the location of services (DNS SRV), February 2000. lookups. For presence stanzas, the order of lookups recommended by [XMPP-IM]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence, April 2004. is to first try the "_xmpp-server" service as specified in [XMPP-CORE]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Core, April 2004. and to then try the "_pres" service as specified in [IMP-SRV]Peterson, J., Address Resolution for Instant Messaging and Presence, October 2003.. Here we assume that the first lookup will fail but that the second lookup will succeed and return a resolution "_pres._simple.example.net.", since we have already assumed that the example.net hostname is running a SIP presence service. (Note: The XMPP server may have previously determined that the foreign 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 has determined that the foreign domain is serviced by a SIMPLE server, it must determine how to proceed. We here assume that the XMPP server contains or has available to it an XMPP-SIMPLE gateway. The XMPP server would then deliver the presence stanza to the XMPP-SIMPLE gateway.

The XMPP-SIMPLE gateway is then responsible for translating the XMPP presence stanza into a SIP NOTIFY request and included PIDF document from the XMPP user to the SIP user:

Example: XMPP user sends presence notification (SIP transformation):

|  NOTIFY sip:romeo@example.net SIP/2.0
|  Via: SIP/2.0/TCP simple.example.net;branch=z9hG4bKna998sk
|  From: <sip:juliet@example.com>;tag=ffd2
|  To: <sip:romeo@example.net>;tag=xfg9
|  Call-ID: j4s0h4vny@example.com
|  Event: presence
|  Subscription-State: active;expires=599
|  Max-Forwards: 70
|  CSeq: 8775 NOTIFY
|  Contact: sip:simple.example.net
|  Content-Type: application/cpim-pidf+xml
|  Content-Length: 192
|
|  <?xml version='1.0' encoding='UTF-8'?>
|  <presence xmlns='urn:ietf:params:xml:ns:pidf'
|            entity='pres:juliet@example.com'>
|    <tuple id='balcony'>
|      <status>
|        <basic>open</basic>
|      </status>
|    </tuple>
|  </presence>
          

The mapping of XMPP syntax elements to SIP syntax elements SHOULD be as follows (mappings for elements not mentioned are undefined):

Table 5: Presence syntax mapping from XMPP to SIP

   +-----------------------------+---------------------------+
   |  XMPP Element or Attribute  |  SIP Header or PIDF Data  |
   +-----------------------------+---------------------------+
   |  <priority/>                |  PIDF priority for tuple  |
   |  <show/>                    |  TBD                      |
   |  <status/>                  |  TBD                      |
   |  from                       |  From                     |
   |  id                         |  CSeq [1]                 |
   |  to                         |  To                       |
   |  type                       |  basic status [2]         |
   |  xml:lang                   |  Content-Language         |
   |  ---                        |  "Event: presence"        |
   +-----------------------------+---------------------------+
          

Note the following regarding these mappings:

  1. According to Section 8.1.1.5 of [SIP]Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, SIP: Session Initiation Protocol, June 2002., a SIP CSeq header must be expressible as a 32-bit unsigned integer. However, the only restriction on the XMPP 'id' attribute is that it shall conform to the XML NMTOKEN datatype. If an XMPP 'id' attribute is not expressible as a 32-bit unsigned integer, it SHOULD NOT be mapped.
  2. Only a presence stanza which lacks a 'type' attribute or whose 'type' attribute has a value of "unavailable" SHOULD be mapped by an XMPP-SIMPLE gateway to a SIP NOTIFY request. Because the lack of a 'type' attribute indicates that an XMPP entity is available for communications, the gateway SHOULD map that information to a PIDF <basic/> status of "open". Because a 'type' attribute with a value of "unavailable" indicates that an XMPP entity is not available for communications, the gateway SHOULD map that information to a PIDF <basic/> status of "closed".

4.3 SIP to XMPP

When Romeo changes his presence, his SIP user agent generates a SIP NOTIFY request. The syntax of the NOTIFY request is defined in [SIP-PRES]Rosenberg, J., A Presence Event Package for the Session Initiation Protocol (SIP), January 2003.. The following is an example of such a request:

Example: SIP user sends presence notification:

|  NOTIFY sip:juliet@example.com SIP/2.0
|  Via: SIP/2.0/TCP simple.example.net;branch=z9hG4bKna998sk
|  From: <sip:romeo@example.net>;tag=ffd2
|  To: <sip:juliet@example.com>;tag=xfg9
|  Call-ID: j0sj4sv1m@example.net
|  Event: presence
|  Subscription-State: active;expires=499
|  Max-Forwards: 70
|  CSeq: 8775 NOTIFY
|  Contact: sip:simple.example.net
|  Content-Type: application/cpim-pidf+xml
|  Content-Length: 193
|
|  <?xml version='1.0' encoding='UTF-8'?>
|  <presence xmlns='urn:ietf:params:xml:ns:pidf'
|            entity='pres:romeo@example.net'>
|    <tuple id='orchard'>
|      <status>
|        <basic>closed</basic>
|      </status>
|    </tuple>
|  </presence>
          

Upon receiving such a request, a SIMPLE-XMPP gateway is responsible for translating it into an XMPP presence stanza from the SIP user to the XMPP user:

Example: SIP user sends presence notification (XMPP transformation):

|  <presence from='romeo@example.net'
|            to='juliet@example.com/balcony'
|            type='unavailable'/>
          

The mapping of SIP syntax elements to XMPP syntax elements SHOULD be as follows (mappings for elements not mentioned are undefined):

Table 6: Presence syntax mapping from SIP to XMPP

   +---------------------------+-----------------------------+
   |  SIP Header or PIDF Data  |  XMPP Element or Attribute  |
   +---------------------------+-----------------------------+
   |  basic status             |  type [1]                   |
   |  Content-Language         |  xml:lang                   |
   |  CSeq                     |  id                         |
   |  From                     |  from                       |
   |  priority for tuple       |  <priority/>                |
   |  To                       |  to                         |
   |  body of MESSAGE          |  <body/>                    |
   +---------------------------+-----------------------------+
          

Note the following regarding these mappings:

  1. A PIDF basic status of "open" SHOULD be mapped to no 'type' attribute, and a PIDF basic status of "closed" SHOULD be mapped to a 'type' attribute whose value is "unavailable".


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5. Presence Subscriptions

5.1 Overview

Both XMPP and presence-aware SIP systems enable entities (often but not necessarily human users) to subscribe to the presence of other entities. XMPP presence subscriptions are specified in [XMPP-IM]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence, April 2004.. Presence subscriptions using a SIP event package for presence are specified in [SIP-PRES]Rosenberg, J., A Presence Event Package for the Session Initiation Protocol (SIP), January 2003..

As described in [XMPP-IM]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence, April 2004., XMPP presence subscriptions are managed using XMPP presence stanzas of type "subscribe", "subscribed", "unsubscribe", and "unsubscribed". The main subscription states are "none" (neither the user nor the contact is subscribed to the other's presence information), "from" (the user has a subscription from the contact), "to" (the user has a subscription to the contact's presence information), and "both" (both user and contact are subscribed to each other's presence information).

As described in [SIP-PRES]Rosenberg, J., A Presence Event Package for the Session Initiation Protocol (SIP), January 2003., SIP presence subscriptions are managed through the use of SIP SUBSCRIBE events sent from a SIP user agent to an intended recipient who is most generally referenced by an Instant Message URI of the form "pres:user@domain" but who may be referenced by a SIP or SIPS URI of the form "sip:user@domain" or "sips:user@domain".

The subscription models underlying XMPP and SIP are quite different. For instance, XMPP presence subscriptions are long-lived (indeed permanent if not explicitly cancelled), whereas SIP presence subscriptions are short-lived (the default time to live of a SIP presence subscription is 3600 seconds, as specified in Section 6.4 of [SIP-PRES]Rosenberg, J., A Presence Event Package for the Session Initiation Protocol (SIP), January 2003.). These differences are addressed below.

5.2 XMPP to SIP

An XMPP user initiates a subscription by sending a subscription request to another entity (conventionally called a "contact"), which the contact either accepts or declines. If the contact accepts the request, the user will have a subscription to the contact's presence information until (1) the user unsubscribes or (2) the contact cancels the subscription. The subscription request is encapsulated in a presence stanza of type "subscribe":

Example: XMPP user subscribes to SIP contact:

|  <presence from='juliet@example.com'
|            to='romeo@example.net'
|            type='subscribe'/>
          

Upon receiving such a stanza, the XMPP server to which Juliet has connected needs to determine the identity of the foreign domain, which it does by performing one or more [SRV]Gulbrandsen, A., Vixie, P. and L. Esibov, A DNS RR for specifying the location of services (DNS SRV), February 2000. lookups. For presence stanzas, the order of lookups recommended by [XMPP-IM]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence, April 2004. is to first try the "_xmpp-server" service as specified in [XMPP-CORE]Saint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Core, April 2004. and to then try the "_pres" service as specified in [IMP-SRV]Peterson, J., Address Resolution for Instant Messaging and Presence, October 2003.. Here we assume that the first lookup will fail but that the second lookup will succeed and return a resolution "_pres._simple.example.net.", since we have already assumed that the example.net hostname is running a SIP presence service.

Once the XMPP server has determined that the foreign domain is serviced by a SIMPLE server, it must determine how to proceed. We here assume that the XMPP server contains or has available to it an XMPP-SIMPLE gateway. The XMPP server would then deliver the presence stanza to the XMPP-SIMPLE gateway.

The XMPP-SIMPLE gateway is then responsible for translating the XMPP subscription request into a SIP SUBSCRIBE request from the XMPP user to the SIP user:

Example: XMPP user subscribes to SIP contact (SIP transformation):

|  SUBSCRIBE sip:romeo@example.net SIP/2.0
|  Via: SIP/2.0/TCP simple.example.net;branch=z9hG4bKna998sk
|  From: <sip:juliet@example.com>;tag=ffd2
|  To: <sip:romeo@example.net>;tag=xfg9
|  Call-ID: 1h4t3s1p@example.com
|  Event: presence
|  Max-Forwards: 70
|  CSeq: 8776 SUBSCRIBE
|  Contact: sip:simple.example.net
|  Accept: application/cpim-pidf+xml
|  Expires: 3600
|  Content-Length: 0
          

Note: It is the responsibility of the XMPP-SIMPLE gateway to set the value of the Expires header and to renew the subscription accordingly so that the subscription appears to be permanent to the XMPP user.

At any time after subscribing, the XMPP user may unsubscribe from the contact's presence. This is done by sending a presence stanza of type "unsubscribe":

Example: XMPP user unsubscribes from SIP contact:

|  <presence from='juliet@example.com'
|            to='romeo@example.net'
|            type='unsubscribe'/>
          

The XMPP-SIMPLE gateway is responsible for translating the unsubscribe command into a SIP SUBSCRIBE request with the Expires header set to a value of zero:

Example: XMPP user unsubscribes from SIP contact (SIP transformation):

|  SUBSCRIBE sip:romeo@example.net SIP/2.0
|  Via: SIP/2.0/TCP simple.example.net;branch=z9hG4bKna998sk
|  From: <sip:juliet@example.com>;tag=ffd2
|  To: <sip:romeo@example.net>;tag=xfg9
|  Call-ID: 1ckm32@example.com
|  Event: presence
|  Max-Forwards: 70
|  CSeq: 18776 SUBSCRIBE
|  Contact: sip:simple.example.net
|  Accept: application/cpim-pidf+xml
|  Expires: 0
|  Content-Length: 0
          

5.3 SIP to XMPP

A SIP user initiates a subscription to a contact's presence information by sending a SIP SUBSCRIBE request to the contact. The following is an example of such a request:

Example: SIP user subscribes to XMPP contact:

|  SUBSCRIBE sip:juliet@example.com SIP/2.0
|  Via: SIP/2.0/TCP simple.example.net;branch=z9hG4bKna998sk
|  From: <sip:romeo@example.net>;tag=ffd2
|  To: <sip:juliet@example.com>;tag=xfg9
|  Call-ID: 4wcm0n@example.net
|  Event: presence
|  Max-Forwards: 70
|  CSeq: 17263 NOTIFY
|  Contact: sip:simple.example.net
|  Accept: application/cpim-pidf+xml
|  Content-Length: 0
          

Upon receiving such a request, a SIMPLE-XMPP gateway is responsible for translating it into an XMPP subscription request from the SIP user to the XMPP user:

Example: SIP user subscribes to XMPP contact (XMPP transformation):

|  <presence from='romeo@example.net'
|            to='juliet@example.com'
|            type='subscribe'/>
          

Notice that the Expires header was not included in the SUBSCRIBE request; this means that the default value of 3600 (i.e., 3600 seconds = 1 hour) applies.

It is the responsibility of the SIMPLE-XMPP gateway to properly handle the difference between short-lived SIP presence subscriptions and long-lived XMPP presence subscriptions. The gateway has two options when the SIP user's subscription expires:

Which of these the SIMPLE-XMPP gateway chooses is up to the implementation.

At any time, the SIP user may cancel the subscription by sending a SUBSCRIBE request whose Expires header is set to a value of zero:

Example: SIP user cancels subscription:

|  SUBSCRIBE sip:juliet@example.com SIP/2.0
|  Via: SIP/2.0/TCP simple.example.net;branch=z9hG4bKna998sk
|  From: <sip:romeo@example.net>;tag=ffd2
|  To: <sip:juliet@example.com>;tag=xfg9
|  Call-ID: 1tsn1ce@example.net
|  Event: presence
|  Max-Forwards: 70
|  CSeq: 17987 NOTIFY
|  Contact: sip:simple.example.net
|  Expires: 0
|  Accept: application/cpim-pidf+xml
|  Content-Length: 0
          

Upon receiving such a request, a SIMPLE-XMPP gateway is responsible for translating it into an XMPP presence stanza of type "unsubscribe" from the SIP user to the XMPP user:

Example: SIP user cancels subscription (XMPP transformation):

|  <presence from='romeo@example.net'
|            to='juliet@example.com'
|            type='unsubscribe'/>
          


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6. IANA Considerations

This document requires no action on the part of the IANA.



 TOC 

7. Security Considerations

Detailed security considerations for instant messaging and presence protocols are given in [IMP-REQS]Day, M., Aggarwal, S. and J. Vincent, Instant Messaging / Presence Protocol Requirements, February 2000., specifically in Sections 5.1 through 5.4. Detailed security considerations for XMPP are given in XMPP CoreSaint-Andre, P., Extensible Messaging and Presence Protocol (XMPP): Core, April 2004.[XMPP-CORE]. Detailed security considerations for SIP-based messaging are given in [SIP-IM]Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C. and D. Gurle, Session Initiation Protocol (SIP) Extension for Instant Messaging, December 2002. and for SIP-based presence are given in [SIP-PRES]Rosenberg, J., A Presence Event Package for the Session Initiation Protocol (SIP), January 2003. (see also the security considerations for the Session Initiation Protocol given in [SIP]Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, SIP: Session Initiation Protocol, June 2002.).

This document specifies methods for exchanging instant messages and presence information through a gateway that translates between SIP and XMPP. Such a gateway MUST be compliant with the minimum security requirements of the instant messaging and presence protocols for which it translates (i.e., SIP and XMPP). The introduction of gateways to the security model of instant messaging and presence specified in [IMP-REQS]Day, M., Aggarwal, S. and J. Vincent, Instant Messaging / Presence Protocol Requirements, February 2000. introduces some new risks. In particular, end-to-end security properties (especially confidentiality and integrity) between instant messaging and presence user agents that interface through a SIMPLE-XMPP gateway can be provided only if common formats are supported. Specification of those common formats is out of scope for this document, although it is recommended to use [MSGFMT]Atkins, D. and G. Klyne, Common Presence and Instant Messaging: Message Format, January 2003. for instant messages and [PIDF]Fujimoto, S., Sugano, H., Klyne, G., Bateman, A., Carr, W. and J. Peterson, CPIM Presence Information Data Format, May 2003. for presence.

[IMP-REQS]Day, M., Aggarwal, S. and J. Vincent, Instant Messaging / Presence Protocol Requirements, February 2000. requires that conformant technologies shall include methods for blocking communications from unwanted addresses. Such blocking is the responsibility of conformant technology (e.g., XMPP or SIP) and is out of scope for this memo.



 TOC 

8. Open Issues

This document has made certain simplifying assumptions and has ruled a number of issues out of scope for now. Future revisions of this document will attempt to address these issues, which include:



 TOC 

9. Acknowledgements

The authors wish to thank Nathaniel Borenstein and Rohan Mahy for suggestions and encouragement. Thanks also to Daniel-Constantin Mierla for earlier work on SIMPLE-XMPP interworking.



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10. References



 TOC 

10.1 Normative References

[IMP-SRV] Peterson, J., "Address Resolution for Instant Messaging and Presence", draft-ietf-impp-srv-04 (work in progress), October 2003.
[PIDF] Fujimoto, S., Sugano, H., Klyne, G., Bateman, A., Carr, W. and J. Peterson, "CPIM Presence Information Data Format", draft-ietf-impp-cpim-pidf-08 (work in progress), May 2003.
[SIP] 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.
[SIP-IM] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C. and D. Gurle, "Session Initiation Protocol (SIP) Extension for Instant Messaging", RFC 3428, December 2002.
[SIP-PRES] Rosenberg, J., "A Presence Event Package for the Session Initiation Protocol (SIP)", draft-ietf-simple-presence-10 (work in progress), January 2003.
[SRV] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000.
[STRINGPREP] Hoffman, P. and M. Blanchet, "Preparation of Internationalized Strings ("STRINGPREP")", RFC 3454, December 2002.
[TERMS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[URL-GUIDE] Masinter, L., Alvestrand, H., Zigmond, D. and R. Petke, "Guidelines for new URL Schemes", RFC 2718, November 1999.
[XMPP-CORE] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Core", draft-ietf-xmpp-core-23 (work in progress), April 2004.
[XMPP-IM] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence", draft-ietf-xmpp-im-22 (work in progress), April 2004.


 TOC 

10.2 Informative References

[CPIM] Peterson, J., "Common Profile for Instant Messaging (CPIM)", draft-ietf-impp-im-04 (work in progress), August 2003.
[CPP] Peterson, J., "Common Profile for Presence (CPP)", draft-ietf-impp-pres-04 (work in progress), August 2003.
[DRAFT-UMPP] Mahy, R., "A Unified Proposal for Server-to-Server Presence and Instant Messaging", draft-mahy-impp-unified-proposal-00 (work in progress), February 2004.
[IDNA] Faltstrom, P., Hoffman, P. and A. Costello, "Internationalizing Domain Names in Applications (IDNA)", RFC 3490, March 2003.
[IMP-MODEL] Day, M., Rosenberg, J. and H. Sugano, "A Model for Presence and Instant Messaging", RFC 2778, February 2000.
[IMP-REQS] Day, M., Aggarwal, S. and J. Vincent, "Instant Messaging / Presence Protocol Requirements", RFC 2779, February 2000.
[IRC] Oikarinen, J. and D. Reed, "Internet Relay Chat Protocol", RFC 1459, May 1993.
[MSGFMT] Atkins, D. and G. Klyne, "Common Presence and Instant Messaging: Message Format", draft-ietf-impp-cpim-msgfmt-08 (work in progress), January 2003.
[MSRP] Campbell, B., Rosenberg, J., Sparks, R. and P. Kyzivat, "The Message Session Relay Protocol", draft-ietf-simple-message-sessions-04 (work in progress), March 2004.
[SIMPLE-CPIM] Rosenberg, J. and B. Campbell, "CPIM Mapping of SIMPLE Presence and Instant Messaging", draft-ietf-simple-cpim-mapping-01 (work in progress), June 2002.
[XMPP-CPIM] Saint-Andre, P., "Mapping the Extensible Messaging and Presence Protocol (XMPP) to Common Presence and Instant Messaging (CPIM)", draft-ietf-xmpp-cpim-04 (work in progress), March 2004.


 TOC 

Authors' Addresses

  Peter Saint-Andre
  Jabber Software Foundation
  
  Avshalom Houri
  IBM
  
  Joe Hildebrand
  Jabber, Inc.


 TOC 

Appendix A. Revision History

Note to RFC Editor: please remove this entire appendix, and the corresponding entries in the table of contents, prior to publication.

A.1 Changes from draft-saintandre-xmpp-simple-00



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Intellectual Property Statement

Full Copyright Statement

Acknowledgment