| Network Working Group | P. Saint-Andre |
| Internet-Draft | Cisco Systems, Inc. |
| Intended status: Standards Track | A. Houri |
| Expires: March 09, 2014 | IBM |
| J. Hildebrand | |
| Cisco Systems, Inc. | |
| September 05, 2013 |
Interworking between the Session Initiation Protocol (SIP) and the Extensible Messaging and Presence Protocol (XMPP): Presence
draft-ietf-stox-presence-04
This document defines a bi-directional protocol mapping for the exchange of presence information between the Session Initiation Protocol (SIP) and the Extensible Messaging and Presence Protocol (XMPP).
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
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Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved.
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In order to help ensure interworking between presence systems that conform to the instant message / presence requirements [RFC2779], it is important to clearly define protocol mappings between such systems. Within the IETF, work has proceeded on two presence technologies:
One approach to helping ensure interworking between these protocols is to map each protocol to the abstract semantics described in [RFC3860]; that is the approach taken by both [RFC3922] and [I-D.ietf-simple-cpim-mapping]. The approach taken in this document is to directly map semantics from one protocol to another (i.e., from SIP/SIMPLE to XMPP and vice-versa).
The architectural assumptions underlying such direct mappings are provided in [I-D.ietf-stox-core], including mapping of addresses and error conditions. The mappings specified in this document cover basic presence functionality. Mapping of more advanced functionality (e.g., so-called "rich presence") is out of scope for this document.
SIP and XMPP differ significantly in their presence subscription models, since SIP subscriptions are short-lived (requiring relatively frequent refreshes even during a presence session) whereas XMPP subscriptions last across presence sessions until they are explicitly cancelled. This document provides suggestions for bridging the gap between these very different models.
The discussion venue for this document is the mailing list of the STOX WG; visit https://www.ietf.org/mailman/listinfo/stox for subscription information and discussion archives.
A number of terms used here are explained in [RFC3261], [RFC3856], [RFC6120], and [RFC6121].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
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 [RFC6121]. Presence subscriptions using a SIP event package for presence are specified in [RFC3856].
As described in [RFC6121], 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 [RFC3856], 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 a Presence URI of the form <pres:user@domain> but who might be referenced by a SIP or SIPS URI of the form <sip:user@domain> or <sips:user@domain>.
The subscription models underlying XMPP and SIP are quite different. For instance, XMPP presence subscriptions are long-lived (indeed permanent if not explicitly cancelled), whereas SIP presence subscriptions are short-lived (the default time-to-live of a SIP presence subscription is 3600 seconds, as specified in Section 6.4 of [RFC3856]). These differences are addressed below.
An XMPP user (e.g., juliet@example.com) initiates a subscription by sending a subscription request to another entity (e.g., romeo@example.net), and the other entity (conventionally called a "contact") either accepts or declines the request. 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 1: 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 the user has connected needs to determine the identity of the foreign domain, which it does by following the procedures discussed in [I-D.ietf-stox-core]. Here we assume that the XMPP server has determined the foreign domain is serviced by a SIMPLE server, that it contains or has available to it an XMPP-SIMPLE gateway or connection manager (which enables it to speak natively to SIMPLE servers), and that it hands off the presence stanza to the XMPP-SIMPLE gateway.
The XMPP-SIMPLE gateway is then responsible for translating the XMPP subscription request into a SIP SUBSCRIBE request from the XMPP user to the SIP user:
Example 2: XMPP user subscribes to SIP contact (SIP transformation):
| SUBSCRIBE sip:romeo@example.net SIP/2.0
| Via: SIP/2.0/TCP x2s.example.com;branch=z9hG4bKna998sk
| From: <sip:juliet@example.com>;tag=ffd2
| Call-ID: l04th3s1p@example.com
| Event: presence
| Max-Forwards: 70
| CSeq: 123 SUBSCRIBE
| Contact: <sip:x2s.example.com;transport=tcp>
| Accept: application/pidf+xml
| Expires: 3600
| Content-Length: 0
The SIP user then SHOULD send a response indicating acceptance of the subscription request:
Example 3: SIP accepts subscription request:
| SIP/2.0 200 OK
| Via: SIP/2.0/TCP s2x.example.net;branch=z9hG4bKna998sk
| From: <sip:romeo@example.net>;tag=ffd2
| To: <sip:juliet@example.com>;tag=j89d
| Call-ID: l04th3s1p@example.com
| CSeq: 234 SUBSCRIBE
| Contact: <sip:simple.example.net;transport=tcp>
| Expires: 3600
| Content-Length: 0
In accordance with [RFC6665], the XMPP-SIMPLE gateway SHOULD consider the subscription state to be "neutral" until it receives a NOTIFY message. Therefore the SIP user or SIP-XMPP gateway at the SIP user's domain SHOULD immediately send a NOTIFY message containing a "Subscription-State" header whose value contains the string "active" (see Section 4).
Example 4: SIP user sends presence notification:
| NOTIFY sip:192.0.2.1 SIP/2.0
| Via: SIP/2.0/TCP simple.example.net;branch=z9hG4bKna998sk
| From: <sip:romeo@example.net>;tag=yt66
| To: <sip:juliet@example.com>;tag=bi54
| Call-ID: l04th3s1p@example.com
| Event: presence
| Subscription-State: active;expires=499
| Max-Forwards: 70
| CSeq: 8775 NOTIFY
| Contact: <sip:simple.example.net;transport=tcp>
| Content-Type: application/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='ID-orchard'>
| <status>
| <basic>open</basic>
| <show xmlns='jabber:client'>away</show>
| </status>
| </tuple>
| </presence>
In response, the SIMPLE-XMPP gateway would send a 200 OK (not shown here since it is not translated into an XMPP stanza).
Upon receiving the first NOTIFY with a subscription state of active, the XMPP-SIMPLE gateway MUST generate a presence stanza of type "subscribed":
Example 5: XMPP user receives acknowledgement from SIP contact:
| <presence from='romeo@example.net'
| to='juliet@example.com'
| type='subscribed'/>
As described under Section 4, the gateway MUST also generate a presence notification to the XMPP user:
Example 6: XMPP user receives presence notification from SIP contact:
| <presence from='romeo@example.net/orchard'
| to='juliet@example.com'/>
It is the responsibility of the XMPP-SIMPLE gateway to set the value of the "Expires" header and to periodically renew the subscription on the SIMPLE side of the gateway so that the subscription appears to be permanent to the XMPP user (e.g., the XMPP-SIMPLE gateway SHOULD send a new SUBSCRIBE request to the SIP user whenever the XMPP user sends initial presence to its XMPP server, i.e., upon initiating a presence session with the XMPP server). See the Security Considerations [sec] of this document for important information and requirements regarding the security implications of this functionality.
At any time after subscribing, the XMPP user can unsubscribe from the contact's presence. This is done by sending a presence stanza of type "unsubscribe":
Example 7: 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 8: XMPP user unsubscribes from SIP contact (SIP transformation):
| SUBSCRIBE sip:romeo@example.net SIP/2.0
| Via: SIP/2.0/TCP s2x.example.net;branch=z9hG4bKna998sk
| From: <sip:juliet@example.com>;tag=j89d
| Call-ID: 1ckm32@example.com
| Event: presence
| Max-Forwards: 70
| CSeq: 789 SUBSCRIBE
| Contact: <sip:x2s.example.com;transport=tcp>
| Accept: application/pidf+xml
| Expires: 0
| Content-Length: 0
Upon sending the transformed unsubscribe, the XMPP-SIMPLE gateway SHOULD send a presence stanza of type "unsubscribed" to the XMPP user:
Example 9: XMPP user receives unsubscribed notification:
| <presence from='romeo@example.net'
| to='juliet@example.com'
| type='unsubscribed'/>
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 10: SIP user subscribes to XMPP contact:
| SUBSCRIBE sip:juliet@example.com SIP/2.0
| Via: SIP/2.0/TCP s2x.example.net;branch=z9hG4bKna998sk
| From: <sip:romeo@example.net>;tag=xfg9
| Call-ID: 4wcm0n@example.net
| Event: presence
| Max-Forwards: 70
| CSeq: 263 SUBSCRIBE
| Contact: <sip:simple.example.net;transport=tcp>
| Accept: application/pidf+xml
| Content-Length: 0
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.
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 11: SIP user subscribes to XMPP contact (XMPP transformation):
| <presence from='romeo@example.net'
| to='juliet@example.com'
| type='subscribe'/>
In accordance with [RFC6121], the XMPP user's server MUST deliver the presence subscription request to the XMPP user (or, if a subscription already exists in the XMPP user's roster, discard the subscribe request).
If the XMPP user approves the subscription request, the XMPP server then MUST return a presence stanza of type "subscribed" from the XMPP user to the SIP user; if a subscription already exists, the XMPP server SHOULD auto-reply with a presence stanza of type "subscribed". In any case, if the SIMPLE-XMPP gateway receives a presence stanza of type "subscribed" from the XMPP user, it SHOULD silently discard the stanza.
If the XMPP user declines the subscription request, the XMPP server then MUST return a presence stanza of type "unsubscribed" from the XMPP user to the SIP user and the XMPP-SIMPLE gateway MUST transform that stanza into an empty SIP NOTIFY message with a Subscription-State of "terminated" and a reason of "rejected":
Example 12: SIP subscription request rejected:
| NOTIFY sip:192.0.2.2 SIP/2.0
| Via: SIP/2.0/TCP s2x.example.net;branch=z9hG4bKna998sk
| From: <sip:juliet@example.com>;tag=ur93
| To: <sip:romeo@example.net>;tag=pq72
| Call-ID: 4wcm0n@example.net
| Event: presence
| Subscription-State: terminated;reason=rejected
| Max-Forwards: 70
| CSeq: 232 NOTIFY
| Contact: <sip:x2s.example.com;transport=tcp>
| Content-Type: application/pidf+xml
| Content-Length: 0
For as long as a SIP user is online and interested in receiving presence notifications from the XMPP users, the user's SIP user agent is responsible for periodically refreshing the subscription by sending an updated SUBSCRIBE request with an appropriate value for the Expires header. In response, the SIMPLE-XMPP gateway MUST send a SIP NOTIFY to the user agent (per [RFC6665]; if the gateway has meaningful information about the availability state of the XMPP user then the NOTIFY MUST communicate that information (e.g., by including a PIDF body [RFC3863] with the relevant data), whereas if the gateway does not have meaningful information about the availability state of the XMPP user then the NOTIFY MUST be empty as allowed by [RFC6665].
Once the SIP user goes offline at the end of a presence session, it is the responsibility of the SIMPLE-XMPP gateway to properly handle the difference between short-lived SIP presence subscriptions and long-lived XMPP presence subscriptions. The gateway has two options when the SIP user's subscription expires:
Which of these options the SIMPLE-XMPP gateway chooses is up to the implementation.
If the implementation chooses the first option, the protocol generated would be as follows:
Example 13: SIP subscription expires (treated as temporary by gateway):
| <presence from='romeo@example.net'
| to='juliet@example.com'
| type='unsubscribe'/>
If the implementation chooses the second option, the protocol generated would be as follows:
Example 14: SIP subscription expires (treated as long-lived by gateway):
| NOTIFY sip:192.0.2.2 SIP/2.0
| Via: SIP/2.0/TCP s2x.example.net;branch=z9hG4bKna998sk
| From: <sip:juliet@example.com>;tag=ur93
| To: <sip:romeo@example.net>;tag=pq72
| Call-ID: j4s0h4vny@example.com
| Event: presence
| Subscription-State: terminated;reason=timeout
| Max-Forwards: 70
| CSeq: 232 NOTIFY
| Contact: <sip:x2s.example.com;transport=tcp>
| Content-Type: application/pidf+xml
| Content-Length: 194
|
| <?xml version='1.0' encoding='UTF-8'?>
| <presence xmlns='urn:ietf:params:xml:ns:pidf'
| entity='pres:juliet@example.com'>
| <tuple id='ID-balcony'>
| <status>
| <basic>closed</basic>
| </status>
| </tuple>
| </presence>
Example 15: SIP subscription expires (treated as long-lived by gateway):
| <presence from='romeo@example.net'
| to='juliet@example.com'
| type='unavailable'/>
At any time, the SIP user can cancel the subscription by sending a SUBSCRIBE message whose "Expires" header is set to a value of zero ("0"):
Example 16: 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=yt66
| Call-ID: 1tsn1ce@example.net
| Event: presence
| Max-Forwards: 70
| CSeq: 8775 SUBSCRIBE
| Contact: <sip:simple.example.net;transport=tcp>
| Expires: 0
| Content-Length: 0
As above, upon receiving such a request, a SIMPLE-XMPP gateway is responsible for doing one of the following:
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 documents might address broader notions of presence, including extended presence.
[RFC6121] defines how XMPP presence stanzas can indicate availability (via absence of a 'type' attribute) or lack of availability (via a 'type' attribute with a value of "unavailable"). SIP presence using a SIP event package for presence is specified in [RFC3856].
As described in [RFC6121], presence information about an entity is communicated by means of an XML <presence/> stanza sent over an XML stream. In this document we will assume that such a presence stanza is sent from an XMPP client to an XMPP server over an XML stream negotiated between the client and the server, and that the client is controlled by a human user (again, this is a simplifying assumption introduced for explanatory purposes only). In general, XMPP presence is sent by the user to the user's server and then broadcasted to all entities who are subscribed to the user's presence information.
As described in [RFC3856], 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 Presence URI of the form <pres:user@domain> but who might 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.
This document addresses basic presence or network availability only, not the various extensions to SIP and XMPP for "rich presence", such as [RFC4480], [XEP-0107], and [XEP-0108].
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 [RFC6121]. The following is an example of such a stanza:
Example 17: 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 (this is similar to the SIP NOTIFY method). 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 [RFC6121], the XMPP server needs to determine the identity of the foreign domain, which it does by performing one or more DNS SRV lookups [RFC2782]. For presence stanzas, the order of lookups recommended by [RFC6121] is to first try the "_xmpp-server" service as specified in [RFC6120] and to then try the "_pres" service as specified in [RFC3861]. Here we assume that the first lookup will fail but that the second lookup will succeed and return a resolution "_pres._simple.example.net.", since we have already assumed that the example.net hostname is running a SIP presence service. (Note: The XMPP server might have previously determined that the foreign domain is a SIMPLE server, e.g., when it sent a SIP SUBSCRIBE to the SIP user when Juliet sent initial presence to the XMPP 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 needs to determine how to proceed. We here assume that the XMPP server contains or has available to it an XMPP-SIMPLE gateway. The XMPP server would then deliver the presence stanza to the XMPP-SIMPLE gateway.
The XMPP-SIMPLE gateway is then responsible for translating the XMPP presence stanza into a SIP NOTIFY request and included PIDF document from the XMPP user to the SIP user.
Example 18: XMPP user sends presence notification (SIP transformation):
| NOTIFY sip:192.0.2.2 SIP/2.0
| Via: SIP/2.0/TCP x2s.example.com;branch=z9hG4bKna998sk
| From: <sip:juliet@example.com>;tag=gh19
| To: <sip:romeo@example.net>;tag=yt66
| Contact: <sip:juliet@example.com>;gr=balcony
| Call-ID: j4s0h4vny@example.com
| Event: presence
| Subscription-State: active;expires=599
| Max-Forwards: 70
| CSeq: 157 NOTIFY
| Contact: <sip:x2s.example.com;transport=tcp>
| Content-Type: application/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='ID-balcony'>
| <status>
| <basic>open</basic>
| <show xmlns='jabber:client'>away</show>
| </status>
| </tuple>
| </presence>
The mapping of XMPP syntax elements to SIP syntax elements SHOULD be as shown in the following table. (Mappings for elements not mentioned are undefined.)
Table 1: Presence syntax mapping from XMPP to SIP
+-----------------------------+---------------------------+
| XMPP Element or Attribute | SIP Header or PIDF Data |
+-----------------------------+---------------------------+
| <presence/> stanza | "Event: presence" (1) |
| XMPP resource identifer | tuple 'id' attribute (2) |
| from | From |
| id | CSeq (3) |
| to | To |
| type | basic status (4) (5) |
| xml:lang | Content-Language |
| <priority/> | priority for tuple (6) |
| <show/> | no mapping (7) |
| <status/> | <note/> |
+-----------------------------+---------------------------+
Note the following regarding these mappings:
When Romeo changes his presence, his SIP user agent generates a SIP NOTIFY request for any active subscriptions. The syntax of the NOTIFY request is defined in [RFC3856]. The following is an example of such a request:
Example 19: SIP user sends presence notification:
| NOTIFY sip:192.0.2.1 SIP/2.0
| Via: SIP/2.0/TCP simple.example.net;branch=z9hG4bKna998sk
| From: <sip:romeo@example.net>;tag=yt66
| To: <sip:juliet@example.com>;tag=bi54
| Contact: <sip:romeo@example.net>;gr=orchard
| Call-ID: j0sj4sv1m@example.net
| Event: presence
| Subscription-State: active;expires=499
| Max-Forwards: 70
| CSeq: 8775 NOTIFY
| Contact: <sip:simple.example.net;transport=tcp>
| Content-Type: application/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='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 20: 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 shown in the following table. (Mappings for elements not mentioned are undefined.)
Table 2: 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 (2) |
| From | from |
| priority for tuple | <priority/> (3) |
| To | to |
| <note/> | <status/> |
| <show/> | <show/> (4) |
+---------------------------+-----------------------------+
Note the following regarding these mappings:
Both SIP and XMPP provide methods for requesting presence information about another entity.
In XMPP, a request for presence information is completed by sending a presence stanza of type "probe":
Example 21: XMPP server sends presence probe on behalf of XMPP user:
| <presence from='juliet@example.com/chamber'
| to='romeo@example.net'
| type='probe'/>
Note: As described in [RFC6121], presence probes are used by XMPP servers to request presence on behalf of XMPP users; XMPP clients are discouraged from sending presence probes since retrieving presence is a service that servers provide.
An XMPP-SIMPLE gateway would transform the presence probe into its SIP equivalent, which is a SUBSCRIBE request with an Expires header value of zero:
Example 22: Presence probe (SIP transformation):
| SUBSCRIBE sip:romeo@example.net SIP/2.0
| Via: SIP/2.0/TCP x2s.example.com;branch=z9hG4bKna998sk
| From: <sip:juliet@example.com>;tag=ffd2
| Call-ID: l04th3s1p@example.com
| Event: presence
| Max-Forwards: 70
| CSeq: 123 SUBSCRIBE
| Contact: <sip:x2s.example.com;transport=tcp>
| Accept: application/pidf+xml
| Expires: 0
| Content-Length: 0
As described in [RFC3856], this cancels any subscription but causes a NOTIFY to be sent to the subscriber, just as a presence probe does (the transformation rules for presence notifications have been previously described in this document).
In SIP, a request for presence information is effectively completed by sending a SUBSCRIBE with an Expires header value of zero:
Example 23: SIP user sends presence request:
| SUBSCRIBE sip:juliet@example.com SIP/2.0
| Via: SIP/2.0/TCP simple.example.net;branch=z9hG4bKna998sk
| From: <sip:romeo@example.net>;tag=yt66
| Call-ID: 1tsn1ce@example.net
| Event: presence
| Max-Forwards: 70
| CSeq: 8775 SUBSCRIBE
| Contact: <sip:simple.example.net;transport=tcp>
| Expires: 0
| Content-Length: 0
When honoring the long-lived semantics of an XMPP presence subscription, a SIMPLE-XMPP gateway SHOULD translate such a SIP request into a presence stanza of type 'probe' if it does not already have presence information about the subscribee:
Example 24: SIP user requests XMPP presence (XMPP transformation):
| <presence from='romeo@example.net'
| to='juliet@example.com'
| type='probe'/>
This document makes no requests of IANA.
Detailed security considerations for presence protocols are given in [RFC2779], for SIP-based presence in [RFC3856] (see also [RFC3261]), and for XMPP-based presence in [RFC6121] (see also [RFC6120]).
The mismatch between long-lived XMPP presence subscriptions and short-lived SIP presence subscriptions introduces the possibility of an amplification attack launched from the XMPP network against a SIP presence server. To help prevent such an attack, access to an XMPP-SIMPLE gateway that is hosted on the XMPP network SHOULD be restricted to XMPP users associated with a single domain or trust realm (e.g., a gateway hosted at simple.example.com ought to allow only users within the example.com domain to access the gateway, not users within example.org, example.net, or any other domain); if a SIP presence server receives communications through an XMPP-SIMPLE gateway from users who are not associated with a domain that is so related to the hostname of the gateway, it MAY (based on local service provisioning) refuse to service such users or refuse to communicate with the gateway. Furthermore, whenever an XMPP-SIMPLE gateway seeks to refresh an XMPP user's long-lived subscription to a SIP user's presence, it MUST first send an XMPP <presence/> stanza of type "probe" from the address of the gateway to the "bare JID" (user@domain.tld) of the XMPP user, to which the user's XMPP server MUST respond in accordance with [RFC6121]; however, the administrator of an XMPP-SIMPLE gateway MAY (based on local service provisioning) exempt "known good" XMPP servers from this check (e.g., the XMPP server associated with the XMPP-SIMPLE gateway as described above).
The authors wish to thank the following individuals for their feedback: Chris Christou, Fabio Forno, Adrian Georgescu, Philipp Hancke, Saul Ibarra Corretge, Markus Isomaki, Paul Kyzivat, Salvatore Loreto, Michael Lundberg, Daniel-Constantin Mierla, and Tory Patnoe.
Some text in this document was borrowed from [RFC3922].