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rfc:rfc3922

Network Working Group P. Saint-Andre Request for Comments: 3922 Jabber Software Foundation Category: Standards Track October 2004

  Mapping the Extensible Messaging and Presence Protocol (XMPP) to
            Common Presence and Instant Messaging (CPIM)

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2004).

Abstract

 This memo describes a mapping between the Extensible Messaging and
 Presence Protocol (XMPP) and the Common Presence and Instant
 Messaging (CPIM) specifications.

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
 2.  Approach . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
 3.  Address Mapping  . . . . . . . . . . . . . . . . . . . . . . .  4
 4.  Syntax Mapping of Instant Messages . . . . . . . . . . . . . .  5
 5.  Syntax Mapping of Presence Information . . . . . . . . . . . . 13
 6.  XMPP-CPIM Gateway as Presence Service  . . . . . . . . . . . . 26
 7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 31
 8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 33
 Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 34

Saint-Andre Standards Track [Page 1] RFC 3922 XMPP to CPIM October 2004

1. Introduction

1.1. Overview

 The Instant Messaging and Presence (IMPP) Working Group has defined
 an abstract framework for interoperability among instant messaging
 (IM) and presence systems that are compliant with [IMP-REQS].  This
 framework is commonly called Common Presence and Instant Messaging or
 "CPIM".  The CPIM family of specifications include a Common Profile
 for Instant Messaging [CPIM] (also called CPIM), a Common Profile for
 Presence [CPP], a CPIM Message Format [MSGFMT], and a Common Presence
 Information Data Format [PIDF].  (Note: To prevent confusion, Common
 Presence and Instant Messaging is referred to herein collectively as
 "the CPIM specifications", whereas the Common Profile for Instant
 Messaging is referred to as "CPIM".)
 This memo describes how the Extensible Messaging and Presence
 Protocol ([XMPP-CORE], [XMPP-IM]) maps to the abstract model
 contained in the CPIM specifications, mainly for the purpose of
 establishing gateways between XMPP services and non-XMPP services
 that conform to [IMP-REQS].  Such a gateway, referred to herein as an
 "XMPP-CPIM gateway", may be established to interpret the protocols of
 one service and translate them into the protocols of the other
 service.  We can visualize this relationship as follows:
   +-------------+        +-------------+        +------------+
   |             |        |             |        |            |
   |    XMPP     |        |  XMPP-CPIM  |        |  Non-XMPP  |
   |   Service   | <----> |   Gateway   | <----> |  Service   |
   |             |        |             |        |            |
   +-------------+        +-------------+        +------------+
 This memo defines a mapping for use by a gateway that translates
 between XMPP and a non-XMPP protocol via the CPIM specifications.
 Such a gateway is not an intermediate hop on a network of non-XMPP
 servers (whose native formats may or may not be defined by the CPIM
 specifications), but a dedicated translator between XMPP and a
 non-XMPP protocol, where the CPIM specifications define the common
 formats into which the protocols are translated for purposes of
 interworking.
 The mapping defined herein applies to instant messages and presence
 information that are not encrypted or signed for end-to-end security.
 For information about secure communications to or from an XMPP
 service through an XMPP-CPIM gateway, refer to [XMPP-E2E].

Saint-Andre Standards Track [Page 2] RFC 3922 XMPP to CPIM October 2004

1.2. Terminology

 This memo inherits vocabulary defined in [IMP-MODEL].  Terms such as
 CLOSED, INSTANT INBOX, INSTANT MESSAGE, OPEN , PRESENCE SERVICE,
 PRESENTITY, SUBSCRIPTION, and WATCHER are used in the same meaning as
 defined therein.
 This memo also inherits vocabulary defined in [XMPP-CORE].  Terms
 such as ENTITY, NODE IDENTIFIER, DOMAIN IDENTIFIER, RESOURCE
 IDENTIFIER, MESSAGE STANZA, and PRESENCE STANZA are used in the same
 meaning as defined therein.

1.3. Conventions Used in this Document

 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
 [TERMS].

2. Approach

 XMPP and CPIM are distinctly foreign technologies.  Therefore, care
 must be taken in mapping between XMPP and the abstract syntax defined
 by the CPIM specifications.
 At root, XMPP is a data transport protocol for streaming XML elements
 (called "stanzas") between any two endpoints on the network; message
 and presence stanzas are two of the core data elements defined in
 XMPP and are often used to exchange instant messages and presence
 information between IM users (although the inherent extensibility of
 XML enables applications to use the general semantics of these stanza
 types for other purposes).  XMPP is not based on [MIME]; instead,
 [XMPP-CORE] defines XML schemas for both message and presence stanzas
 (for example, the <body/> child of a message stanza contains XML
 character data that is usually intended to be read by a human user).
 The CPIM specifications provide common formats for instant messaging
 and presence through two [MIME] content-types: "Message/CPIM" for
 messages ([MSGFMT]) and "application/pidf+xml" for presence ([PIDF]).
 The syntax of "Message/CPIM" objects is similar to but stricter than
 that defined in [RFC2822], and provides the ability to include
 arbitrary MIME media types [MIMETYPES].  By contrast, each
 "application/pidf+xml" object is a complete XML document whose
 structure is defined by an XML schema.

Saint-Andre Standards Track [Page 3] RFC 3922 XMPP to CPIM October 2004

 The approach taken herein is to specify mappings from XMPP elements
 and attributes to the headers and MIME formats defined by [MSGFMT]
 and [PIDF] in order to comply with the semantics defined by [CPIM]
 and [CPP].  Naturally, mappings in the opposite direction are
 provided as well.

3. Address Mapping

3.1. Overview

 Address mapping may be required since the address formats used to
 identify XMPP entities (specified in [XMPP-CORE]) are different from
 those used to identify instant inboxes (the im: URI scheme specified
 in [CPIM]) and presentities (the pres: URI scheme specified in
 [CPP]).  In particular, different characters are allowed in im: and
 pres: URIs than are allowed in XMPP addresses:
 o  The following [US-ASCII] characters are allowed in im:/pres: URIs
    but not in XMPP addresses: #26; (&), #27; ('), and #2f; (/).
 o  Many non-US-ASCII (specifically, UTF-8) characters are allowed in
    XMPP addresses but not allowed in im:/pres: URIs, since XMPP
    allows internationalized local-part addresses.
 Note: In this document we discuss characters allowed in local-part
 addresses only (i.e., we have ruled the mapping of domain names as
 out of scope for the initial version of this document, since it is a
 matter for the Domain Name System and the translation of fully
 internationalized domain names).

3.2. XMPP to CPIM

 The following is a high-level algorithm for mapping an XMPP address
 to an 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] (as specified in
     [XMPP-CORE]) 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 as described in Section 2.2.5 of
     [URL-GUIDE]

Saint-Andre Standards Track [Page 4] RFC 3922 XMPP to CPIM October 2004

 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)

3.3. CPIM to XMPP

 The following is a high-level algorithm for mapping an 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 as described in Section
     2.2.5 of [URL-GUIDE]
 4.  Treat result as a UTF-8 string
 5.  Translate & to #26;, ' to #27;, and / to #2f respectively
 6.  Apply Nodeprep profile of [STRINGPREP] (as specified in
     [XMPP-CORE]) for canonicalization (OPTIONAL)
 7.  Recombine local-part with mapped hostname to form local@domain
     address

4. Syntax Mapping of Instant Messages

 This section describes how a gateway SHOULD map instant messages
 between an XMPP service and a non-XMPP service using a "Message/CPIM"
 object as the bearer of encapsulated text content in order to comply
 with the instant messaging semantics defined by [CPIM].

4.1. Message Syntax Mapping from XMPP to CPIM Specifications

 This section defines the mapping of syntax primitives from XMPP
 message stanzas to "Message/CPIM" objects with encapsulated text
 content.
 Note: As specified in [MIME], the default Content-type of a MIME
 object is "Content-type: text/plain; charset=us-ascii".  Because XMPP
 uses the [UTF-8] character encoding exclusively, the encapsulated
 MIME object generated by an XMPP-CPIM gateway MUST set the

Saint-Andre Standards Track [Page 5] RFC 3922 XMPP to CPIM October 2004

 "Content-type" MUST be set to "text/plain" and the charset MUST be
 set to "utf-8".

4.1.1. From Address

 The 'from' attribute of an XMPP message stanza maps to the 'From'
 header of a "Message/CPIM" object.  In XMPP, the sender's server
 stamps or validates the "from" address and sets its value to the full
 <user@host/resource> negotiated between client and server during
 authentication and resource binding as defined in [XMPP-CORE].  Thus
 an XMPP-CPIM gateway will receive from the sender's XMPP server a
 message stanza containing a "from" address of the form
 <user@host/resource>.  To map the 'from' attribute of an XMPP message
 stanza to the 'From' header of a "Message/CPIM" object, the gateway
 MUST remove the resource identifier, MUST append the "im:"  Instant
 Messaging URI scheme to the front of the address, and MAY include a
 CPIM "Formal-name" for the sender (if known).
 Example: From Address Mapping
 XMPP 'from' attribute
   <message from='juliet@example.com/balcony'>
     ...
   </message>
 CPIM 'From' header
   From: Juliet Capulet <im:juliet@example.com>

4.1.2. To Address

 The 'to' attribute of an XMPP message stanza maps to the 'To' header
 of a "Message/CPIM" object.  In XMPP, the sender SHOULD include a
 'to' attribute on a message stanza, and MUST include it if the
 message is intended for delivery to another user.  Thus an XMPP-CPIM
 gateway will receive from the sender's XMPP server a message stanza
 containing a "to" address of the form <user@host> or
 <user@host/resource>.  To map the 'to' attribute of an XMPP message
 stanza to the 'To' header of a "Message/CPIM" object, the gateway
 MUST remove the resource identifier (if included), MUST append the
 "im:" Instant Messaging URI scheme to the front of the address, and
 MAY include a CPIM "Formal-name" for the recipient (if known).

Saint-Andre Standards Track [Page 6] RFC 3922 XMPP to CPIM October 2004

 Example: To Address Mapping
 XMPP 'to' attribute
   <message to='romeo@example.net/orchard'>
     ...
   </message>
 CPIM 'To' header
   To: Romeo Montague <im:romeo@example.net>

4.1.3. Stanza ID

 An XMPP message stanza MAY possess an 'id' attribute, which is used
 by the sending application for the purpose of tracking stanzas and is
 not a globally-unique identifier such as is defined by the MIME
 Content-ID header.  Because the XMPP 'id' attribute does not have the
 same meaning as the MIME Content-ID header, it SHOULD NOT be mapped
 to that header; however, if the 'id' is known to be unique (e.g., if
 it is generated to be unique by the XMPP server and that fact is
 known by the XMPP-CPIM gateway), then it SHOULD be so mapped.

4.1.4. Message Type

 An XMPP message stanza MAY possess a 'type' attribute, which is used
 by the sending application to capture the conversational context of
 the message.  There is no mapping of an XMPP 'type' attribute to a
 "Message/CPIM" header, common MIME features, or encapsulated text
 content.  Therefore if an XMPP stanza received by an XMPP-CPIM
 gateway possesses a 'type' attribute, the gateway SHOULD ignore the
 value provided.

4.1.5. Message Thread

 An XMPP message stanza MAY contain a <thread/> child element to
 specify the conversation thread in which the message is situated.
 There is no mapping of an XMPP <thread/> element to a "Message/CPIM"
 header, common MIME features, or encapsulated text content. Therefore
 if an XMPP message stanza received by an XMPP-CPIM gateway contains a
 <thread/> child element, the gateway SHOULD ignore the value
 provided.

4.1.6. Message Subject

 An XMPP message stanza MAY include a <subject/> child element.  If
 included, it maps to the 'Subject' header of a "Message/CPIM" object.
 To map the XMPP <subject/> element to the 'Subject' header of a
 "Message/CPIM" object, the gateway SHOULD simply map the XML
 character data of the XMPP <subject/> element to the value of the

Saint-Andre Standards Track [Page 7] RFC 3922 XMPP to CPIM October 2004

 'Subject' header.  The <subject/> element MAY include an 'xml:lang'
 attribute specifying the language in which the subject is written. If
 an 'xml:lang' attribute is provided, it MUST be mapped by including
 ';lang=tag' after the header name and colon, where 'tag' is the value
 of the 'xml:lang' attribute.
 Example: Subject Mapping
 XMPP <subject/> element
   <subject>Hi!</subject>
   <subject xml:lang='cz'>Ahoj!</subject>
 CPIM 'Subject' header
   Subject: Hi!
   Subject:;lang=cz Ahoj!

4.1.7. Message Body

 The <body/> child element of an XMPP message stanza is used to
 provide the primary meaning of the message.  The XML character data
 of the XMPP <body/> element maps to the encapsulated text message
 content.
 Example: Message Body
 XMPP message <body/>
   <message>
     <body>Wherefore art thou, Romeo?</body>
   </message>
 Encapsulated MIME text content
   Content-type: text/plain; charset=utf-8
   Content-ID: <123456789@example.net>
   Wherefore art thou, Romeo?

4.1.8. Message Extensions

 As defined in [XMPP-CORE], an XMPP message stanza may contain
 "extended" content in any namespace in order to supplement or extend
 the semantics of the core message stanza.  With the exception of
 extended information qualified by the
 'urn:ietf:params:xml:ns:xmpp-e2e' namespace as defined in [XMPP-E2E],
 an XMPP-CPIM gateway SHOULD ignore such information and not pass it
 through the gateway to the intended recipient.  No mapping for such
 information is defined.

Saint-Andre Standards Track [Page 8] RFC 3922 XMPP to CPIM October 2004

4.1.9. Gateway-Generated CPIM Syntax

 CPIM specifies the existence of "Message/CPIM" headers in addition to
 those described above, but there is no exact analogue for those
 headers in the core XMPP specifications.  These include:
 o  cc -- specifies the address of an entity that is to receive a
    "courtesy copy" of the message (i.e., a non-primary addressee)
 o  DateTime -- specifies the datetime at which the message was sent
 o  NS -- specifies the namespace of a feature extension
 o  Require -- specifies mandatory-to-recognize features
 An XMPP-CPIM gateway MAY independently generate such headers based on
 its own information (e.g., the datetime at which it received a
 message stanza from an XMPP entity) or based on data encoded in
 non-core XMPP extensions, but rules for doing so are out of scope for
 this memo.

4.2. Message Syntax Mapping from CPIM Specifications to XMPP

 This section defines the mapping of syntax primitives from
 "Message/CPIM" objects with encapsualted text content to XMPP message
 stanzas.

4.2.1. From Address

 The 'From' header of a "Message/CPIM" object maps to the 'from'
 attribute of an XMPP message stanza.  To map the CPIM 'From' header
 to the XMPP 'from' attribute, the gateway MUST remove the "im:"
 Instant Messaging URI scheme from the front of the address and MUST
 remove the CPIM "Formal-name" (if provided).
 Example: From Address Mapping
 CPIM 'From' header
   From: Romeo Montague <im:romeo@example.net>
 XMPP 'from' attribute
   <message from='romeo@example.net'>
     ...
   </message>

4.2.2. To Address

 The 'To' header of a "Message/CPIM" object maps to the 'to' attribute
 of an XMPP message stanza.  To map the CPIM 'To' header to the XMPP
 'to' attribute, the gateway MUST remove the "im:" Instant Messaging
 URI scheme from the front of the address and MUST remove the CPIM

Saint-Andre Standards Track [Page 9] RFC 3922 XMPP to CPIM October 2004

 "Formal-name" (if provided).  If the gateway possesses knowledge of
 the resource identifier in use by the XMPP entity, the gateway MAY
 append the resource identifier to the address.
 Example: To Address Mapping
 CPIM 'To' header
   To: Juliet Capulet <im:juliet@example.com>
 XMPP 'to' attribute
   <message to='juliet@example.com/balcony'>
     ...
   </message>

4.2.3. Courtesy Copy

 The core XMPP specification does not include syntax for specifying a
 "courtesy copy" (non-primary addressee) for a message stanza.
 Therefore, if an XMPP-CPIM gateway receives a "Message/CPIM" object
 that contains a 'cc' header, it SHOULD NOT pass the information
 contained in that header on to the XMPP recipient.

4.2.4. DateTime Header

 The core XMPP specification does not include syntax for specifying
 the datetime at which a message stanza was sent.  Therefore, if an
 XMPP-CPIM gateway receives a "Message/CPIM" object that contains a
 'DateTime' header, it SHOULD NOT pass the information contained in
 that header on to the XMPP recipient.

4.2.5. Message Subject

 The 'Subject' header of a "Message/CPIM" object maps to the
 <subject/> child element of an XMPP message stanza.  To map the CPIM
 'Subject' header to the XMPP <subject/> element, the gateway SHOULD
 simply map the value of the 'Subject' header to the XML character
 data of the XMPP <subject/> element.  The 'Subject' header MAY
 specify the "lang" in which the subject is written.  If "lang"
 information is provided, it MUST be mapped to the 'xml:lang'
 attribute of the <subject/> element, where the value of the
 'xml:lang' attribute is the "tag" value supplied in the string
 ';lang=tag' included after the CPIM 'Subject' header name and colon.

Saint-Andre Standards Track [Page 10] RFC 3922 XMPP to CPIM October 2004

 Example: Subject Mapping
 CPIM 'Subject' header
   Subject: Hi!
   Subject:;lang=cz Ahoj!
 XMPP <subject/> element
   <subject>Hi!</subject>
   <subject xml:lang='cz'>Ahoj!</subject>

4.2.6. Header Extensions

 "Message/CPIM" objects MAY include an optional 'NS' header to specify
 the namespace of a feature extension.  An XMPP-CPIM gateway MUST NOT
 pass such headers through to the XMPP recipient, and no mapping for
 such headers is defined.

4.2.7. Require Header

 "Message/CPIM" objects MAY include an optional 'Require' header to
 specify mandatory-to-recognize features.  In general, such a header
 would be included by the non-XMPP sending application to (1) insist
 that the receiving application needs to understand functionality
 specified by a particular header or (2) indicate that some non-header
 semantics need to be implemented by the receiving application in
 order to understand the contents of the message (e.g.,
 "Locale.MustRenderKanji").  Because the mandatory-to-recognize
 features would be required of the XMPP receiving application rather
 than the XMPP-CPIM gateway itself, the gateway cannot properly handle
 the 'Require' header without detailed knowledge about the
 capabilities of the XMPP receiving application.  Therefore, it seems
 appropriate that the XMPP-CPIM gateway SHOULD return a warning or
 error to the non-XMPP sending application if it includes one or more
 'Require' headers in a "Message/CPIM" object; the exact nature of the
 warning or error will depend on the nature of the non-XMPP technology
 used by the foreign system, and is not defined herein.  Furthermore,
 any mapping of the 'Require' header into XMPP or an XMPP extension is
 left up to the implementation or to a future specification.

4.2.8. MIME Content-ID

 XMPP does not include an element or attribute that captures a
 globally unique ID as is defined for the Content-ID MIME header as
 specified in [MIME].  If an XMPP-CPIM gateway receives a MIME object
 that includes a Content-ID, it MAY provide the Content-ID as the
 value of the message stanza's 'id' attribute, but this is OPTIONAL.

Saint-Andre Standards Track [Page 11] RFC 3922 XMPP to CPIM October 2004

 Example: Content-ID for Encapsulated Object
 MIME header
   Content-ID: <123456789@example.net>
 XMPP 'id' attribute (OPTIONAL)
   <message id='123456789@example.net'>
     ...
   </message>

4.2.9. Message Body

 If the Content-type of an encapsulated MIME object is "text/plain",
 then the encapsulated text message content maps to the XML character
 data of the <body/> child element of an XMPP message stanza.
 Example: Message Body
 Encapsulated MIME text content
   Content-type: text/plain; charset=utf-8
   Content-ID: <123456789@example.net>
   Wherefore art thou?
 XMPP message <body/>
   <message id='123456789@example.net'>
     <body>Wherefore art thou?</body>
   </message>
 If the Content-Type is not "text/plain", the XMPP-CPIM gateway MAY
 map the content to an XMPP extension but MUST NOT map it to the
 <body/> child of the XMPP message stanza, which is allowed to contain
 XML character data only.  The only exception to this rule is a
 multi-part MIME object of the kind specified in [XMPP-E2E], which is
 to be mapped as described in that memo.
 If the charset is "US-ASCII" or "UTF-8", the gateway MUST map the
 "Message/CPIM" object; otherwise it SHOULD NOT.

4.2.10. Gateway-Generated XMPP Syntax

 XMPP specifies the existence of a 'type' attribute for XMPP message
 stanzas, which enables the sender to define the conversational
 context of the message.  There is no exact analogue for this
 attribute in CPIM.  An XMPP-CPIM gateway MAY independently generate
 the 'type' attribute based on its own information, but this is
 OPTIONAL and rules for doing so are out of scope for this memo.

Saint-Andre Standards Track [Page 12] RFC 3922 XMPP to CPIM October 2004

5. Syntax Mapping of Presence Information

 This section describes how a gateway SHOULD map presence information
 between an XMPP service and a non-XMPP service using a "Message/CPIM"
 object as the bearer of an encapsulated [PIDF] object in order to
 comply with the presence semantics defined by [CPP].

5.1. Presence Syntax Mapping from XMPP to CPIM Specifications

 This section defines the mapping of syntax primitives from XMPP
 presence stanzas to "Message/CPIM" objects with encapsulated
 "application/pidf+xml" objects.
 Note: As specified in [MIME], the default Content-type of a MIME
 object is "Content-type: text/plain; charset=us-ascii".  Because XMPP
 uses the [UTF-8] character encoding exclusively and because PIDF
 specifies the "application/pidf+xml" MIME type, the encapsulated MIME
 object generated by an XMPP-CPIM gateway for presence information
 MUST set the 'Content-type' header for that object.  The
 "Content-type" MUST be set to "application/pidf+xml" and the charset
 MUST be set to "utf-8".

5.1.1. From Address

 The 'from' attribute of an XMPP presence stanza maps to the 'From'
 header of a "Message/CPIM" object.  In XMPP, the sender's server
 stamps or validates the "from" address and sets its value to the
 <user@host/resource> negotiated between client and server during
 authenticating and resource binding as defined in [XMPP-CORE].  Thus
 an XMPP-CPIM gateway will receive from the sender's XMPP server a
 presence stanza containing a "from" address of the form
 <user@host/resource>.  To map the 'from' attribute of an XMPP
 presence stanza to the 'From' header of a "Message/CPIM" object, the
 gateway MUST remove the resource identifier, MUST append the "im:"
 Instant Messaging URI scheme to the front of the address, and MAY
 include a CPIM "Formal-name" for the sender (if known).
 Example: From Address Mapping
 XMPP 'from' attribute
   <presence from='juliet@example.com/balcony'>
     ...
   </presence>
 CPIM 'From' header
   From: Juliet Capulet <im:juliet@example.com>

Saint-Andre Standards Track [Page 13] RFC 3922 XMPP to CPIM October 2004

 In addition, the 'from' attribute of an XMPP presence stanza maps to
 the 'entity' attribute of a PIDF <presence/> root element.  To map
 the XMPP 'from' attribute to the PIDF 'entity' attribute, the gateway
 MUST remove the resource identifier and MUST append the "pres:"
 Instant Messaging URI scheme to the front of the address.
 Example: From Address Mapping (PIDF)
 XMPP 'from' attribute
   <presence from='juliet@example.com/balcony'>
     ...
   </presence>
 PIDF 'entity' attribute
   <presence entity='pres:juliet@example.com'>
     ...
   </presence>
 Finally, an XMPP-CPIM gateway SHOULD map the resource identifier of
 the XMPP address contained in the XMPP 'from' attribute to the 'id'
 attribute of the PIDF <tuple/> child element.
 Example: Resource Identifier Mapping
 XMPP 'from' attribute
   <presence from='juliet@example.com/balcony'>
     ...
   </presence>
 PIDF 'id' for <tuple/>
   <presence entity='pres:juliet@example.com'>
     <tuple id='balcony'>
       ...
     </tuple>
   </presence>

5.1.2. To Address

 The 'to' attribute of an XMPP presence stanza maps to the 'To' header
 of a "Message/CPIM" object.  In XMPP, the sender MAY include a 'to'
 attribute on a presence stanza, and MUST include it if the presence
 stanza is intended for delivery directly to another user (presence
 stanzas intended for broadcasting are stamped with a 'to' address by
 the sender's server).  Thus an XMPP-CPIM gateway will receive from
 the sender's XMPP server a presence stanza containing a "to" address
 of the form <user@host> or <user@host/resource>.  To map the 'to'
 attribute of an XMPP presence stanza to the 'To' header of a
 "Message/CPIM" object, the gateway MUST remove the resource

Saint-Andre Standards Track [Page 14] RFC 3922 XMPP to CPIM October 2004

 identifier (if included), MUST append the "im:" Instant Messaging URI
 scheme to the front of the address, and MAY include a CPIM
 "Formal-name" for the recipient (if known).
 Example: To Address Mapping
 XMPP 'to' attribute
   <presence to='romeo@example.net/orchard'>
     ...
   </presence>
 CPIM 'To' header
   To: Romeo Montague <im:romeo@example.net>

5.1.3. Stanza ID

 An XMPP presence stanza MAY possess an 'id' attribute, which is used
 by the sending application for the purpose of tracking stanzas and is
 not a globally-unique identifier such as is defined by the MIME
 Content-ID header.  Because the XMPP 'id' attribute does not have the
 same meaning as the MIME Content-ID header, it SHOULD NOT be mapped
 to that header; however, if the 'id' is known to be unique (e.g., if
 it is generated to be unique by the XMPP server and that fact is
 known by the XMPP-CPIM gateway), then it SHOULD be so mapped.

5.1.4. Presence Type

 An XMPP presence stanza MAY possess a 'type' attribute.  If no 'type'
 attribute is included, the presence stanza indicates that the sender
 is available; this state maps to the PIDF basic presence type of
 OPEN.  If the 'type' attribute has a value of "unavailable", the
 presence stanza indicates that the sender is no longer available;
 this state maps to the PIDF basic presence type of CLOSED.  Thus both
 the absence of a 'type' attribute and a 'type' attribute set to a
 value of "unavailable" correspond to the [CPP] "notify operation".
 All other presence types are used to manage presence subscriptions or
 probe for current presence; mappings for these other presence types
 are defined under XMPP-CPIM Gateway as Presence Service (Section 6).
 Example: Available Presence
 XMPP available presence
   <presence from='juliet@example.com/balcony'/>
 PIDF basic presence (OPEN)
   <?xml version='1.0' encoding='UTF-8'?>
   <presence xmlns='urn:ietf:params:xml:ns:pidf'
             entity='pres:juliet@example.com'>

Saint-Andre Standards Track [Page 15] RFC 3922 XMPP to CPIM October 2004

     <tuple id='balcony'>
       <status>
         <basic>open</basic>
       </status>
     </tuple>
   </presence>
 Example: Unavailable Presence
 XMPP unavailable presence
   <presence from='juliet@example.com/balcony' type='unavailable'/>
 PIDF basic presence (CLOSED)
   <?xml version='1.0' encoding='UTF-8'?>
   <presence xmlns='urn:ietf:params:xml:ns:pidf'
             entity='pres:romeo@example.net'>
     <tuple id='balcony'>
       <status>
         <basic>closed</basic>
       </status>
     </tuple>
   </presence>

5.1.5. Show Element

 The <show/> child element of an XMPP presence stanza provides
 additional information about the sender's availability.  The XML
 character data of the XMPP <show/> element maps to extended <status/>
 content in PIDF.  The defined values of the <show/> element are
 'away', 'chat', 'dnd', and 'xa'; as soon as values are specified for
 extended status states in the 'urn:ietf:params:xml:ns:pidf:im'
 namespace, the XMPP values will be mapped to the PIDF values.
 Example: Show Element
 XMPP <show/> element
   <presence from='juliet@example.com/balcony'>
     <show>away</show>
   </presence>
 PIDF extended presence information
   <?xml version='1.0' encoding='UTF-8'?>
   <presence xmlns='urn:ietf:params:xml:ns:pidf'
             xmlns:im='urn:ietf:params:xml:ns:pidf:im'
             entity='pres:juliet@example.com'>
     <tuple id='balcony'>
       <status>
         <basic>open</basic>

Saint-Andre Standards Track [Page 16] RFC 3922 XMPP to CPIM October 2004

         <im:im>away</im:im>
       </status>
     </tuple>
   </presence>

5.1.6. Status Element

 The <status/> child element of an XMPP presence stanza provides a
 user-defined, natural-language description of the sender's detailed
 availability state.  The XMPP <status/> element maps to the PIDF
 <note/> child of the PIDF <tuple/> element.
 Example: Status Element
 XMPP <status/> element
   <presence from='juliet@example.com/balcony'>
     <show>away</show>
     <status>retired to the chamber</status>
   </presence>
 PIDF <note/> element
   <?xml version='1.0' encoding='UTF-8'?>
   <presence xmlns='urn:ietf:params:xml:ns:pidf'
             xmlns:im='urn:ietf:params:xml:ns:pidf:im'
             entity='pres:juliet@example.com'>
     <tuple id='balcony'>
       <status>
         <basic>open</basic>
         <im:im>away</im:im>
       </status>
       <note>retired to the chamber</note>
     </tuple>
   </presence>

5.1.7. Presence Priority

 An XMPP presence stanza MAY contain a <priority/> child element whose
 value is an integer between -128 and +127.  The value of this element
 MAY be mapped to the 'priority' attribute of the <contact/> child of
 the PIDF <tuple/> element.  If the value of the XMPP <priority/>
 element is negative, an XMPP-CPIM gateway MUST NOT map the value. The
 range of allowable values for the PIDF 'priority' attribute is any
 decimal number from zero to one inclusive, with a maximum of three
 decimal places.  If an XMPP-CPIM gateway maps these values, it SHOULD
 treat XMPP <priority>0</priority> as PIDF priority='0' and XMPP
 <priority>127</priority> as PIDF priority='1', mapping intermediate
 values appropriately so that they are unique (e.g., XMPP priority 1
 to PIDF priority 0.007, XMPP priority 2 to PIDF priority 0.015, and

Saint-Andre Standards Track [Page 17] RFC 3922 XMPP to CPIM October 2004

 so on up through mapping XMPP priority 126 to PIDF priority 0.992;
 note that this is an example only, and that the exact mapping shall
 be determined by the XMPP-CPIM gateway).
 Example: Presence Priority
 XMPP <status/> element
   <presence from='juliet@example.com/balcony'>
     <priority>13</priority>
   </presence>
 PIDF <note/> element
   <?xml version='1.0' encoding='UTF-8'?>
   <presence xmlns='urn:ietf:params:xml:ns:pidf'
             entity='pres:juliet@example.com'>
     <tuple id='balcony'>
       ...
       <contact priority='0.102'>im:juliet@example.com</contact>
     </tuple>
   </presence>

5.1.8. Presence Extensions

 As defined in [XMPP-CORE], an XMPP presence stanza may contain
 "extended" content in any namespace in order to supplement or extend
 the semantics of the core presence stanza.  With the exception of
 extended information qualified by the
 'urn:ietf:params:xml:ns:xmpp-e2e' namespace as defined in [XMPP-E2E],
 an XMPP-CPIM gateway SHOULD ignore such information and not pass it
 through the gateway to the intended recipient.  No mapping for such
 information is defined.

5.1.9. Gateway-Generated CPIM and PIDF Syntax

5.1.9.1. CPIM Message Headers

 CPIM specifies the existence of "Message/CPIM" headers in addition to
 those described above, but there is no exact analogue for those
 headers in the core XMPP specifications.  These include:
 o  cc -- specifies the address of an entity that is to receive a
    "courtesy copy" of the presence information (i.e., a non-primary
    addressee)
 o  DateTime -- specifies the datetime at which the presence
    information was sent
 o  NS -- specifies the namespace of a feature extension

Saint-Andre Standards Track [Page 18] RFC 3922 XMPP to CPIM October 2004

 o  Subject -- specifies the subject or topic of the encapsulated
    "Message/CPIM" object
 o  Require -- specifies mandatory-to-recognize features
 An XMPP-CPIM gateway MAY independently generate such headers based on
 its own information (e.g., the datetime at which it received a
 presence stanza from an XMPP entity) or based on data encoded in
 non-core XMPP extensions, but rules for doing so are out of scope for
 this memo.

5.1.9.2. PIDF Elements

 PIDF specifies the existence of XML elements in addition to those
 described above, but there is no exact analogue for those XML
 elements in the core XMPP specifications.  These include:
 o  <contact/> -- specifies an address (e.g., an im:, tel:, or mailto:
    URI) at which one may communicate with the presentity; an
    XMPP-CPIM gateway MAY include this element, in which case it
    SHOULD set its value to the <user@host> of the XMPP sender,
    prepended by the "im:" Instant Messaging URI scheme.
 o  <timestamp/> -- specifies the datetime at which the presence
    information was sent; an XMPP-CPIM gateway MAY independently
    generate this element based on its own information (e.g., the
    datetime at which it received the presence stanza from an XMPP
    entity) or based on data encoded in non-core XMPP extensions, but
    rules for doing so are out of scope for this memo.

5.2. Presence Syntax Mapping from CPIM Specifications to XMPP

 This section defines the mapping of syntax primitives from
 "Message/CPIM" objects with encapsulated "application/pidf+xml"
 objects to XMPP presence stanzas.
 Note: An XMPP-CPIM gateway MUST NOT map to an XMPP presence stanza a
 "Message/CPIM" object whose encapsulated MIME object has a
 Content-type other than "application/pidf+xml" (with the exception of
 multi-part MIME objects as specified in [XMPP-E2E]).

5.2.1. From Address

 The 'From' header of a "Message/CPIM" object maps to the <user@host>
 portion of the 'from' attribute of an XMPP presence stanza, and the
 'id' attribute of the PIDF <tuple/> child element maps to the
 resource identifier portion XMPP 'from' attribute.  Therefore, to map
 the CPIM and PIDF information to the XMPP 'from' attribute, the

Saint-Andre Standards Track [Page 19] RFC 3922 XMPP to CPIM October 2004

 gateway MUST remove the "im:" Instant Messaging URI scheme from the
 front of the address and MUST remove the CPIM "Formal-name" (if
 provided) in order to generate the <user@host> portion of the XMPP
 'from' attribute, then add a '/' character followed by the value of
 the PIDF <tuple/> element's 'id' attribute.
 Example: From Address Mapping
 CPIM 'From' header
   From: Romeo Montague <im:romeo@example.net>
 XMPP 'from' attribute
   <presence from='romeo@example.net'>
     ...
   </presence>
 Example: Resource Identifier Mapping
 XMPP 'from' attribute
   <presence from='juliet@example.com/balcony'>
     ...
   </presence>
 PIDF 'id' for <tuple/>
   <presence entity='pres:juliet@example.com'>
     <tuple id='balcony'>
       ...
     </tuple>
   </presence>

5.2.2. To Address

 The 'To' header of a "Message/CPIM" object maps to the 'to' attribute
 of an XMPP presence stanza.  To map the CPIM 'To' header to the XMPP
 'to' attribute, the gateway MUST remove the "im:" Instant Messaging
 URI scheme from the front of the address and MUST remove the CPIM
 "Formal-name" (if provided).  If the gateway possesses knowledge of
 the resource identifier in use by the XMPP entity, the gateway MAY
 append the resource identifier to the address.

Saint-Andre Standards Track [Page 20] RFC 3922 XMPP to CPIM October 2004

 Example: To Address Mapping
 CPIM 'To' header
   To: Juliet Capulet <im:juliet@example.com>
 XMPP 'to' attribute
   <presence to='juliet@example.com/balcony'>
     ...
   </presence>

5.2.3. Courtesy Copy

 The core XMPP specification does not include syntax for specifying a
 "courtesy copy" (non-primary addressee) for a presence stanza.
 Therefore, if an XMPP-CPIM gateway receives a "Message/CPIM" object
 with encapsulated PIDF object that contains a 'cc' header, it SHOULD
 NOT pass the information contained in that header on to the XMPP
 recipient.

5.2.4. DateTime Header

 The core XMPP specification does not include syntax for specifying
 the datetime at which a presence stanza was sent.  Therefore, if an
 XMPP-CPIM gateway receives a "Message/CPIM" object with encapsulated
 PIDF object that contains a 'DateTime' header, it SHOULD NOT pass the
 information contained in that header on to the XMPP recipient.

5.2.5. Subject Header

 An XMPP presence stanza contains no information that can be mapped to
 the 'Subject' header of a "Message/CPIM" object.  Therefore, if an
 XMPP-CPIM gateway receives a "Message/CPIM" object with encapsulated
 PIDF object that contains a 'Subject' header, it SHOULD NOT pass the
 information contained in that header on to the XMPP recipient.

5.2.6. Header Extensions

 "Message/CPIM" objects MAY include an optional 'NS' header to specify
 the namespace of a feature extension.  An XMPP-CPIM gateway MUST NOT
 pass such headers through to the XMPP recipient, and no mapping for
 such headers is defined.

5.2.7. Require Header

 "Message/CPIM" objects MAY include an optional 'Require' header to
 specify mandatory-to-recognize features.  An XMPP-CPIM gateway MUST
 NOT pass such headers through to the XMPP recipient, and no mapping
 for such headers is defined.

Saint-Andre Standards Track [Page 21] RFC 3922 XMPP to CPIM October 2004

5.2.8. MIME Content-ID

 XMPP does not include an element or attribute that captures a
 globally unique ID as is defined for the Content-ID MIME header as
 specified in [MIME].  If an XMPP-CPIM gateway receives a MIME object
 that includes a Content-ID, it MAY provide the Content-ID as the
 value of the presence stanza's 'id' attribute, but this is OPTIONAL.
 Example: Content-ID for Encapsulated Object
 MIME header
   Content-ID: <123456789@example.net>
 XMPP 'id' attribute (OPTIONAL)
   <presence id='123456789@example.net'>
     ...
   </presence>

5.2.9. Basic Presence Status

 The basic presence status types defined in PIDF are OPEN and CLOSED.
 The PIDF basic presence status of OPEN maps to an XMPP presence
 stanza that possesses no 'type' attribute (indicating default
 availability).  The PIDF basic presence status of CLOSED maps to an
 XMPP presence stanza that possesses a 'type' attribute with a value
 of "unavailable".
 Example: OPEN Presence
 PIDF basic presence (OPEN)
   <?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>open</basic>
       </status>
     </tuple>
   </presence>
 XMPP available presence
   <presence from='romeo@example.net/orchard'/>

Saint-Andre Standards Track [Page 22] RFC 3922 XMPP to CPIM October 2004

 Example: CLOSED Presence
 PIDF basic presence (CLOSED)
   <?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>
 XMPP unavailable presence
   <presence from='romeo@example.net/orchard'
             type='unavailable'/>

5.2.10. Extended Status Information

 PIDF documents may contain extended <status/> content.  As of this
 writing there are no pre-defined extended status states that can be
 mapped to the defined values of the XMPP <show/> element ('away',
 'chat', 'dnd', and 'xa').  Once PIDF extensions for such extended
 status states are defined within the Internet Standards Process, a
 gateway SHOULD map those extensions; however, any such mapping is out
 of scope for this memo, since the relevant PIDF extensions have not
 yet been defined.
 Example: Extended Status Information (provisional)
 PIDF extended presence information
   <?xml version='1.0' encoding='UTF-8'?>
   <presence xmlns='urn:ietf:params:xml:ns:pidf'
             xmlns:im='urn:ietf:params:xml:ns:pidf:im'
             entity='pres:romeo@example.net'>
     <tuple id='orchard'>
       <status>
         <basic>open</basic>
         <im:im>busy</im:im>
       </status>
     </tuple>
   </presence>
 XMPP <show/> element
   <presence from='romeo@example.net/orchard'>
     <show>dnd</show>
   </presence>

Saint-Andre Standards Track [Page 23] RFC 3922 XMPP to CPIM October 2004

5.2.11. Note Element

 A PIDF <tuple/> element may contain a <note/> child that provides a
 user-defined, natural-language description of the sender's detailed
 availability state.  The PIDF <note/> element maps to the XMPP
 <status/> element.
 Example: Note Element
 PIDF <note/> element
   <?xml version='1.0' encoding='UTF-8'?>
   <presence xmlns='urn:ietf:params:xml:ns:pidf'
             xmlns:im='urn:ietf:params:xml:ns:pidf:im'
             entity='pres:romeo@example.net'>
     <tuple id='orchard'>
       <status>
         <basic>open</basic>
         <im:im>busy</im:im>
       </status>
       <note>Wooing Juliet</note>
     </tuple>
   </presence>
 XMPP <status/> element
   <presence from='romeo@example.net/orchard'>
     <show>dnd</show>
     <status>Wooing Juliet</status>
   </presence>
 A PIDF document with zero tuples MAY contain one or more <note/>
 elements as direct children of the PIDF <presence/> element.  There
 is no mapping of such a PIDF document to an XMPP presence stanza; an
 entity on the non-XMPP side of an XMPP-CPIM gateway SHOULD NOT send
 such a PIDF document to an XMPP recipient if possible, and an
 XMPP-CPIM gateway MUST NOT map such a PIDF document to an XMPP
 presence stanza (see Zero Resources (Section 6.3.2)).

5.2.12. Contact Element

 A PIDF document may contain a <contact/> element specifying the URI
 of an address at which the principal can be contacted (e.g., an im:,
 tel:, or mailto: URI).  The core XMPP specification does not include
 syntax for specifying the URI of a contact address, since the contact
 address is implicit in the 'from' attribute of the XMPP presence
 stanza.  Therefore, if an XMPP-CPIM gateway receives a "Message/CPIM"
 object with encapsulated PIDF object that contains a <contact/>

Saint-Andre Standards Track [Page 24] RFC 3922 XMPP to CPIM October 2004

 element, it SHOULD NOT pass the XML character data of the <contact/>
 element on to the XMPP recipient.  (However, see Inclusion of
 Complete PIDF Document (Section 5.2.15) below.)
 Example: PIDF Contact Element
 PIDF <contact/> element
   <?xml version='1.0' encoding='UTF-8'?>
   <presence xmlns='urn:ietf:params:xml:ns:pidf'
             entity='pres:romeo@example.net'>
     <tuple id='orchard'>
       ...
       <contact>im:romeo@example.net</contact>
     </tuple>
   </presence>
 XMPP presence stanza
   <presence from='romeo@example.net/orchard'/>

5.2.13. Presence Priority

 The <contact/> child of the PIDF <tuple/> element MAY possess a
 'priority' attribute whose value is a decimal number between zero and
 one (with a maximum of three decimal places).  The value of this
 attribute MAY be mapped to the <priority/> child element of an XMPP
 presence stanza.  An XMPP-CPIM gateway MUST NOT map PIDF priority
 values to negative values of the XMPP <priority/> element.  If an
 XMPP-CPIM gateway maps these values, it SHOULD treat PIDF
 priority='0' as XMPP <priority>0</priority> and PIDF priority='1' as
 <priority>127</priority>, mapping intermediate values appropriately
 so that they are unique (e.g., PIDF priorities between 0.001 and
 0.007 to XMPP priority 1, PIDF priorities between 0.008 and 0.015 to
 XMPP priority 2, and so on up through mapping PIDF priorities between
 0.992 and 0.999 to XMPP priority 126; note that this is an example
 only, and that the exact mapping shall be determined by the XMPP-CPIM
 gateway).

5.2.14. Timestamp Element

 The core XMPP specification does not include syntax for specifying
 the datetime or timestamp at which a presence stanza was sent.
 Therefore, if an XMPP-CPIM gateway receives a "Message/CPIM" object
 with encapsulated PIDF object that contains a <timestamp/> element,
 it SHOULD NOT pass the XML character data of the <timestamp/> element
 on to the XMPP recipient.

Saint-Andre Standards Track [Page 25] RFC 3922 XMPP to CPIM October 2004

5.2.15. Inclusion of Complete PIDF Document

 Certain PIDF elements do not map to XMPP presence stanza syntax
 (e.g., the XML character data of the <contact/> element).  However,
 an XMPP client may be able to handle such information by parsing a
 native PIDF document.  To make this possible, an XMPP-CPIM gateway
 MAY include the complete PIDF document as a child element of the
 presence stanza, as described in [XMPP-PIDF].  If an XMPP client does
 not understand this extended data, it naturally MUST ignore it.

6. XMPP-CPIM Gateway as Presence Service

 [CPP] defines semantics for an abstract presence service.  An
 XMPP-CPIM gateway MAY function as such a presence service, and if so
 an XMPP entity can use defined XMPP syntax to interact with the
 gateway's presence service.  Because [PIDF] does not specify syntax
 for semantic operations such as subscribe, this section defines only
 the XMPP interactions with the presence service offered by an
 XMPP-CPIM gateway, not the translation of such XMPP syntax into PIDF.
 (Note: Detailed information about XMPP presence services can be found
 in [XMPP-IM]; as much as possible, an XMPP-CPIM gateway SHOULD
 implement the syntax, semantics, and server business rules defined
 therein.)

6.1. Requesting a Subscription

 If an XMPP entity wants to subscribe to the presence information of a
 non-XMPP presentity through an XMPP-CPIM gateway, it MUST send a
 presence stanza of type "subscribe" to the target presentity.  The
 syntax mapping is as follows:
 o  The XMPP 'from' attribute (user@host) MUST be mapped to the CPP
    "watcher parameter" field (pres:user@host).  The XMPP-CPIM gateway
    MUST append the "pres:" Presence URI scheme to the front of the
    address.
 o  The XMPP 'to' attribute (user@host) MUST be mapped to the CPP
    "target parameter" field (pres:user@host).  The XMPP-CPIM gateway
    MUST append the "pres:" Presence URI scheme to the front of the
    address.
 o  There is no XMPP mapping for the CPP "duration parameter", since
    XMPP subscriptions are active until they have been explicitly
    "unsubscribed".
 o  The XMPP 'id' attribute SHOULD be mapped to the CPP "TransID"
    field.

Saint-Andre Standards Track [Page 26] RFC 3922 XMPP to CPIM October 2004

 If the target presentity approves the subscription request (through
 whatever protocol it uses to interact with the gateway), the
 XMPP-CPIM gateway MUST return a presence stanza of type "subscribed"
 to the XMPP entity and notify the XMPP entity of the target's current
 available presence.  Thereafter, until the subscription is cancelled,
 the gateway MUST notify the subscribing XMPP entity every time the
 target's presence information changes.
 If the target presentity denies the subscription request, the
 XMPP-CPIM gateway MUST return a presence stanza of type
 "unsubscribed" to the XMPP entity and MUST NOT invoke the notify
 operation.
 In addition to the approval and denial cases, one of the following
 exceptions may occur:
 o  The target parameter (XMPP "to" address) does not refer to a valid
    presentity; if this exception occurs, the XMPP-CPIM gateway MUST
    return an <item-not-found/> stanza error to the XMPP entity.
 o  Access control rules do not permit the entity to subscribe to the
    target; if this exception occurs, the XMPP-CPIM gateway MUST
    return a <forbidden/> stanza error to the XMPP entity.
 o  There exists a pre-existing subscription or in-progress subscribe
    operation between the XMPP entity and the target presentity; if
    this exception occurs, the XMPP-CPIM gateway SHOULD return a
    <conflict/> stanza error to the XMPP entity.
 XMPP services assume that a subscription is active until it is
 explicitly terminated.  However, non-XMPP services may implement
 subscriptions of limited duration, which must be periodically
 refreshed in order to mimic the permanence of XMPP subscriptions.
 Therefore, an XMPP-to-CPIM gateway may need to send such refreshes to
 the non-XMPP entity on behalf of the XMPP entity to that the
 subscription does not expire.  Whether such refreshes are necessary
 depends on the native protocol implemented by the CPIM-aware non-XMPP
 service to which the gateway is translating.

6.2. Receiving a Subscription Request

 If a non-XMPP presentity wants to subscribe to the presence
 information of an XMPP entity through an XMPP-CPIM gateway, it MUST
 use whatever protocol it uses to interact with the gateway in order
 to request the subscription; subject to local access rules, the
 gateway MUST then send a presence stanza of type "subscribe" to the
 XMPP entity from the non-XMPP watcher.  The syntax mapping is as
 follows:

Saint-Andre Standards Track [Page 27] RFC 3922 XMPP to CPIM October 2004

 o  The CPP "watcher parameter" field (pres:user@host) MUST be mapped
    to the XMPP 'from' attribute (user@host).  The XMPP-CPIM gateway
    MUST remove the "pres:" Presence URI scheme from the front of the
    address.
 o  The CPP "target parameter" field (pres:user@host) MUST be mapped
    to the XMPP 'to' attribute (user@host).  The XMPP-CPIM gateway
    MUST remove the "pres:" Presence URI scheme from the front of the
    address.
 o  There is no XMPP mapping for the CPP "duration parameter", since
    XMPP subscriptions are active until they have been explicitly
    "unsubscribed".
 o  The CPP "TransID" field SHOULD be mapped to the XMPP 'id'
    attribute.
 If the target XMPP entity approves the subscription request, it MUST
 send a presence stanza of type "subscribed" to the watcher
 presentity.  The XMPP-CPIM gateway MUST then notify the watcher
 presentity of the target XMPP entity's current available presence.
 Thereafter, until the subscription is cancelled, the gateway MUST
 notify the watcher presentity every time the target's presence
 information changes.
 If the target XMPP entity denies the subscription request, it MUST
 send a presence stanza of type "unsubscribed" to the watcher
 presentity.  The XMPP-CPIM gateway MUST NOT invoke the notify
 operation.
 In addition to the approval and denial cases, one of the following
 exceptions MAY occur:
 o  The target parameter (XMPP "to" address) does not refer to a valid
    XMPP entity
 o  Access control rules do not permit the watcher presentity to
    subscribe to the target XMPP entity
 o  There exists a pre-existing subscription or in-progress subscribe
    operation between the watcher presentity and the target XMPP
    entity
 If any of these exceptions occurs, the XMPP-CPIM gateway MUST inform
 the watcher presentity of failure.

Saint-Andre Standards Track [Page 28] RFC 3922 XMPP to CPIM October 2004

 XMPP services assume that a subscription is active until it is
 explicitly terminated.  With the exception of handling duration
 parameters whose value is zero, handling duration parameters will be
 highly dependent on the implementation and requirements of the
 XMPP-CPIM gateway.  Since there are no explicit requirements for
 supporting a "duration parameter" specified in either [IMP-MODEL] or
 [IMP-REQS], duration parameter mapping is a local issue that falls
 outside the scope of this memo.  However, an XMPP-CPIM gateway MAY
 keep track of the duration parameter if received from an entity on
 the non-XMPP service and delete the subscription after that duration
 parameter expires.

6.3. The Notify Operation

 An XMPP-CPIM gateway invokes the CPP "notify operation" whenever the
 presence information associated with an XMPP entity or CPP presentity
 changes and there are subscribers to that information on the other
 side of the gateway.  The syntax mapping for presence information
 related to a notify operation is defined under Mapping for Presence
 (Section 5).

6.3.1. Multiple Resources

 Semantically, PIDF contains the notion of multiple presence "tuples".
 Normally, a PIDF document will contain at least one tuple but MAY
 contain more than one tuple (or zero tuples, for which see next
 section).  In the terminology of XMPP, each tuple would map to
 presence information for a separate resource.  However, XMPP does not
 include the ability to send presence information about more than one
 resource at a time, since the resource that generates the presence
 information is contained in the 'from' address of a presence stanza.
 Therefore, an XMPP-CPIM gateway that acts as a presence service
 SHOULD split a PIDF document that contains multiple tuples into
 multiple XMPP presence stanzas, and SHOULD generate only one PIDF
 document (with multiple tuples) if an XMPP user currently has
 multiple connected resources.
 In the interest of not multiplying XMPP stanzas beyond necessity, an
 XMPP-CPIM gateway SHOULD generate an XMPP presence stanza only if the
 presence information contained in a PIDF tuple communicates a change
 in the availability status of the device or application associated
 with that tuple ID.
 In the interest of complying with the PIDF recommendation to provide
 information about multiple "resources" in multiple tuples rather than
 in multiple PIDF documents, an XMPP-CPIM gateway SHOULD include

Saint-Andre Standards Track [Page 29] RFC 3922 XMPP to CPIM October 2004

 information about all of an XMPP user's resources in one PIDF
 document (with one tuple for each resource), even if the availability
 status of only one resource has changed.

6.3.2. Zero Resources

 A PIDF document may contain zero tuples.  For example:
 PIDF Document with Zero Tuples
   <presence entity='pres:juliet@example.com'
             xmlns='urn:ietf:params:xml:ns:pidf'/>
 Because (1) the 'entity' attribute of a PIDF <presence/> element maps
 to the <user@host> portion of an XMPP address and (2) the 'id'
 attribute of a PIDF <tuple/> element maps to the resource identifier
 portion of an XMPP address, a PIDF document that contains zero tuples
 would provide presence information about a <user@host> rather than a
 <user@host/resource> when mapped to XMPP.  Although the notion of
 presence notifications about a mere user rather than one of the
 user's resources is nearly meaningless in the XMPP context, an
 XMPP-CPIM gateway SHOULD map a PIDF document with zero tuples to an
 XMPP presence stanza whose 'from' address is the user@host of the
 non-XMPP entity.  However, an XMPP-CPIM gateway MUST NOT generate a
 PIDF document with zero <tuple/> children when receiving a presence
 stanza from an XMPP entity (i.e., all PIDF documents communicated by
 the gateway to a non-XMPP service MUST contain at least one <tuple/>
 element).

6.4. Unsubscribing

 If an XMPP entity wants to unsubscribe from the presence of a
 non-XMPP presentity through an XMPP-CPIM gateway, it MUST send a
 presence stanza of type "unsubscribe" to the target presentity.  The
 syntax mapping is as follows:
 o  The XMPP 'from' attribute (user@host) MUST be mapped to the CPP
    "watcher parameter" field (pres:user@host).  The XMPP-CPIM gateway
    MUST append the "pres:" Presence URI scheme to the front of the
    address.
 o  The XMPP 'to' attribute (user@host) MUST be mapped to the CPP
    "target parameter" field (pres:user@host).  The XMPP-CPIM gateway
    MUST append the "pres:" Presence URI scheme to the front of the
    address.
 o  The CPP "duration parameter" MUST be set to zero.

Saint-Andre Standards Track [Page 30] RFC 3922 XMPP to CPIM October 2004

 o  The XMPP 'id' attribute SHOULD be mapped to the CPP "TransID"
    field.
 If the target parameter (XMPP "to" address) does not refer to a valid
 presentity, the XMPP-CPIM gateway MUST return an <item-not-found/>
 stanza error to the XMPP entity.
 Upon receiving the presence stanza of type "unsubscribe" from the
 XMPP entity, the XMPP-CPIM gateway MUST NOT send further presence
 notifications to the XMPP entity.

6.5. Cancelling a Subscription

 If an XMPP entity wants to cancel a non-XMPP presentity's
 subscription to the entity's presence through an XMPP-CPIM gateway,
 it MUST send a presence stanza of type "unsubscribed" to the target
 presentity.  The syntax mapping is as follows:
 o  The XMPP 'from' attribute (user@host) MUST be mapped to the CPP
    "watcher parameter" field (pres:user@host).  The XMPP-CPIM gateway
    MUST add the "pres:" Presence URI scheme to the front of the
    address.
 o  The XMPP 'to' attribute (user@host) MUST be mapped to the CPP
    "target parameter" field (pres:user@host).  The XMPP-CPIM gateway
    MUST add the "pres:" Presence URI scheme to the front of the
    address.
 o  The CPP "duration parameter" MUST be set to zero.
 o  The XMPP 'id' attribute SHOULD be mapped to the CPP "TransID"
    field.
 Upon receiving the presence stanza of type "unsubscribed" from the
 XMPP entity, the XMPP-CPIM gateway MUST NOT send further presence
 notifications to the watcher presentity.

7. Security Considerations

 Detailed security considerations for instant messaging and presence
 protocols are given in [IMP-REQS], specifically in Sections 5.1
 through 5.4.
 This document specifies methods for exchanging instant messages and
 presence information through a gateway that implements [CPIM] and
 [CPP].  Such a gateway MUST be compliant with the minimum security
 requirements of the instant messaging and presence protocols with
 which it interfaces.  The introduction of gateways to the security
 model of instant messaging and presence in RFC 2779 also introduces

Saint-Andre Standards Track [Page 31] RFC 3922 XMPP to CPIM October 2004

 some new risks.  In particular, end-to-end security properties
 (especially confidentiality and integrity) between instant messaging
 and presence user agents that interface through an XMPP-CPIM gateway
 can be provided only if common formats are supported; these formats
 are specified fully in [XMPP-E2E].

8. References

8.1. Normative References

 [CPIM]       Peterson, J., "Common Profile for Instant Messaging
              (CPIM)", RFC 3860, August 2004.
 [CPP]        Peterson, J., "Common Profile for Presence (CPP)", RFC
              3859, August 2004.
 [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., Mohr, G., and J. Vincent,
              "Instant Messaging / Presence Protocol Requirements",
              RFC 2779, February 2000.
 [MIME]       Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.
 [MSGFMT]     Klyne, G. and D. Atkins, "Common Presence and Instant
              Messaging (CPIM): Message Format", RFC 3862, August
              2004.
 [PIDF]       Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr,
              W., and J. Peterson, "Presence Information Data Format
              (PIDF)", RFC 3863, August 2004.
 [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.

Saint-Andre Standards Track [Page 32] RFC 3922 XMPP to CPIM October 2004

 [US-ASCII]   Cerf, V., "ASCII format for network interchange", RFC
              20, October 1969.
 [UTF-8]      Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.
 [XMPP-CORE]  Saint-Andre, P., Ed., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 3920, October 2004.
 [XMPP-E2E]   Saint-Andre, P., Ed., "End-to-End Signing and Object
              Encryption in the Extensible Messaging and Presence
              Protocol (XMPP)", RFC 3923, October 2004.
 [XMPP-IM]    Saint-Andre (ed.), P., "Extensible Messaging and
              Presence Protocol (XMPP): Instant Messaging and
              Presence", RFC 3921, October 2004.

8.2. Informative References

 [RFC2822]    Resnick, P., Ed., "Internet Message Format", RFC 2822,
              April 2001.
 [MIMETYPES]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              November 1996.
 [XMPP-PIDF]  Saint-Andre, P., "Transporting Presence Information
              Data/Format (PIDF) over the Extensible Messaging and
              Presence Protocol (XMPP)", Work in Progress, February
              2004.

Author's Address

 Peter Saint-Andre
 Jabber Software Foundation
 EMail: stpeter@jabber.org

Saint-Andre Standards Track [Page 33] RFC 3922 XMPP to CPIM October 2004

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 retain all their rights.
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Saint-Andre Standards Track [Page 34]

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