Network Working Group P. Saint-Andre
Internet-Draft Jabber Software Foundation
Expires: October 27, 2004 A. 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
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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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Architectural Assumptions . . . . . . . . . . . . . . . . 3
1.2 Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Addresses . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 XMPP to SIP . . . . . . . . . . . . . . . . . . . . . . . 6
2.3 SIP to XMPP . . . . . . . . . . . . . . . . . . . . . . . 6
3. Instant Messages . . . . . . . . . . . . . . . . . . . . . . . 6
3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2 XMPP to SIP . . . . . . . . . . . . . . . . . . . . . . . 7
3.3 SIP to XMPP . . . . . . . . . . . . . . . . . . . . . . . 9
4. Presence Notifications . . . . . . . . . . . . . . . . . . . . 11
4.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.2 XMPP to SIP . . . . . . . . . . . . . . . . . . . . . . . 12
4.3 SIP to XMPP . . . . . . . . . . . . . . . . . . . . . . . 14
5. Presence Subscriptions . . . . . . . . . . . . . . . . . . . . 16
5.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.2 XMPP to SIP . . . . . . . . . . . . . . . . . . . . . . . 17
5.3 SIP to XMPP . . . . . . . . . . . . . . . . . . . . . . . 19
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
7. Security Considerations . . . . . . . . . . . . . . . . . . . 20
8. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 21
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
10.1 Normative References . . . . . . . . . . . . . . . . . . . . 21
10.2 Informative References . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 23
A. Revision History . . . . . . . . . . . . . . . . . . . . . . . 24
A.1 Changes from draft-saintandre-xmpp-simple-00 . . . . . . . 24
Intellectual Property and Copyright Statements . . . . . . . . 25
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1. Introduction
In order to help ensure interoperability between instant messaging
and presence systems that conform to the requirements of RFC 2779
[IMP-REQS], it is important to clearly define mappings between such
protocols. Within the IETF, work has proceeded on two such
protocols:
o Various extensions to the Session Initiation Protocol ([SIP]) for
instant messaging and presence, work on which has been formalized
under the SIP for Instant Messaging and Presence Leveraging
Extensions (SIMPLE) Working Group
o The Extensible Messaging and Presence Protocol (XMPP), which
consists of a formalization of the core XML streaming protocols
developed originally by the Jabber open-source community
One approach to helping ensure interoperability between such
protocols is to map each protocol to the abstract semantics described
in [CPIM] and [CPP]; that is the approach taken by [SIMPLE-CPIM] and
[XMPP-CPIM]. Another approach is that taken by [DRAFT-UMPP]. 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:
o Mapping of addresses
o Mapping of instant messages
o Mapping of presence notifications
o Mapping of presence subscriptions
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] and [CPP] specifications
to define the common formats into which the protocols are translated
for purposes of interworking (as specified in [SIMPLE-CPIM] and
[XMPP-CPIM]), 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,
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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
2119 [TERMS].
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]; as specified in [XMPP-IM], instant
messaging and presence applications of XMPP must also support 'im:'
and 'pres:' URIs as specified in [CPIM] and [CPP] 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]; it may use either 'sip:' or
'sips:' URIs as specified in [SIP] or an 'im:' URI as specified in
[CPIM]. The SIP address format for presence is specified in
[SIP-PRES]; it may use either 'sip:' or 'sips:' URIs as specified in
[SIP] or a 'pres:' URI as specified in [CPP].
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]) 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
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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], namely to encode unsafe octets using
the %hexhex encoding.
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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] (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
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] lookup as specified in
[XMPP-IM]), 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] (as specified in
[XMPP-CORE]) for canonicalization (OPTIONAL)
7. Recombine local-part with mapped hostname to form local@domain
address
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
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request). Generally there are three kinds of instant message:
o Single messages, which are sent from the sender to the recipient
outside the context of any one-to-one chat session or multi-user
text conference.
o Chat messages, which are sent from the sender to the recipient in
the context of a "message session" between the two entities.
o Groupchat messages, which are sent from a sender to multiple
recipients in the context of a text conference (along the lines of
[IRC]).
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]).
Instant messaging using XMPP message stanzas of type "normal" is
specified in [XMPP-IM]. Instant messaging using SIP requests of type
MESSAGE (often called "pager-model" messaging) is specified in
[SIP-IM].
As described in [XMPP-IM], 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], 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
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syntax of the <message/> stanza, including required and optional
elements and attributes, is defined in [XMPP-IM]. 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], the
XMPP server needs to determine the identity of the foreign domain,
which it does by performing one or more [SRV] lookups. For message
stanzas, the order of lookups recommended by [XMPP-IM] is to first
try the "_xmpp-server" service as specified in [XMPP-CORE] and to
then try the "_im" service as specified in [IMP-SRV]. 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:
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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]. The following
is an example of such a request:
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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] 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] 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] 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):
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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/> |
+--------------------------+-----------------------------+
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]. SIP presence using a SIP
event package for presence is specified in [SIP-PRES].
As described in [XMPP-IM], 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.
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As described in [SIP-PRES], 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]. 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], the XMPP server needs to
determine the identity of the foreign domain, which it does by
performing one or more [SRV] lookups. For presence stanzas, the
order of lookups recommended by [XMPP-IM] is to first try the
"_xmpp-server" service as specified in [XMPP-CORE] and to then try
the "_pres" service as specified in [IMP-SRV]. 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
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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):
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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], 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]. The following is an example of such a request:
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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):
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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".
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].
Presence subscriptions using a SIP event package for presence are
specified in [SIP-PRES].
As described in [XMPP-IM], 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], 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
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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]). 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] lookups. For presence
stanzas, the order of lookups recommended by [XMPP-IM] is to first
try the "_xmpp-server" service as specified in [XMPP-CORE] and to
then try the "_pres" service as specified in [IMP-SRV]. 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:
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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
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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:
o Send an XMPP presence stanza of type "unsubscribe" to the XMPP
contact; this honors the SIP semantic but will seem rather odd to
the XMPP contact.
o Send a SIP NOTIFY request to the SIP user containing a PIDF
document specifying that the XMPP contact now has a basic status
of closed; this violates the letter of the SIP semantic but will
seem more natural to the XMPP contact.
Which of these the SIMPLE-XMPP gateway chooses is up to the
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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'/>
6. IANA Considerations
This document requires no action on the part of the IANA.
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. Detailed security considerations for XMPP are given in
XMPP Core [XMPP-CORE]. Detailed security considerations for
SIP-based messaging are given in [SIP-IM] and for SIP-based presence
are given in [SIP-PRES] (see also the security considerations for the
Session Initiation Protocol given in [SIP]).
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
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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] 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] for instant messages and [PIDF] for presence.
[IMP-REQS] 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.
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:
o Chat messages / instant messaging sessions
o End-to-end object encryption using common formats (e.g., PIDF for
presence)
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.
10. References
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.
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[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.
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.
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[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.
Authors' Addresses
Peter Saint-Andre
Jabber Software Foundation
Avshalom Houri
IBM
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Joe Hildebrand
Jabber, Inc.
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
o Defined handling of presence subscriptions.
o Specified SRV lookups from SIP to XMPP.
o Clarified syntax mappings.
o Added communications blocking to security considerations.
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