RFC 5263

Session Initiation Protocol (SIP) Extension for Partial Notification of Presence Information

📅 published September 2008, from ietf

RFC 5263: Session Initiation Protocol (SIP) Extension for Partial Notification of Presence Information 💖

Okay bestie, here's the header tea ☕. It's RFC 5263, a Standards Track doc from September 2008. The authors are M. Lonnfors, J. Costa-Requena and E. Leppanen (all from Nokia), plus H. Khartabil from Ericsson 🧚. The header doesn't list anything it obsoletes or updates, so there's nothing to track there. It's a brand new extension, no cap 🦋

This is Part 1 of 4 🎀

Status of This Memo ✨

This doc specifies an Internet standards track protocol for the Internet community, and it's asking everyone for discussion and suggestions for improvements 🫶 For the standardization state and status of this protocol, check the current edition of the "Internet Official Protocol Standards" (STD 1). And the distribution of this memo is unlimited, so share it wherever 📣

Abstract 🌸

Okay so, by default, presence sent through the presence event package for the Session Initiation Protocol (SIP) is represented in the Presence Information Data Format (PIDF) 💅 A PIDF document is made of a bunch of elements, and each one covers a different part of the presence being reported. The annoying bit is that if any subset of those elements changes, even just one tiny element, the whole thing gets resent with the full set of elements 😭

This memo adds an extension so only the presence data that actually changed gets delivered. Iconic 🔥

Table of Contents 📋

1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
3.  Introduction to the Partial Notification Mechanism . . . . . .  3
  3.1.  Basic Presence Agent Operation . . . . . . . . . . . . . .  3
  3.2.  Operation with Partial Notification  . . . . . . . . . . .  3
4.  Client and Server Operations . . . . . . . . . . . . . . . . .  4
  4.1.  Content-Type for Partial Notifications . . . . . . . . . .  4
  4.2.  Watcher Generation of SUBSCRIBE Requests . . . . . . . . .  4
  4.3.  Presence Agent Processing of SUBSCRIBE Requests  . . . . .  4
  4.4.  Presence Agent Generation of Partial Notifications . . . .  5
  4.5.  Watcher Processing of NOTIFY Requests  . . . . . . . . . .  6
5.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 13
8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
  8.1.  Normative References . . . . . . . . . . . . . . . . . . . 14
  8.2.  Informative References . . . . . . . . . . . . . . . . . . 14

1. Introduction 🦋

Okay so, the presence event package for SIP [3] lets users ('watchers') subscribe to other users' ('presentities') presence info 👀 That info is made of multiple pieces of data sent to the watcher. The presence document can get HUGE, because it can hold an arbitrary number of elements called presence tuples that carry the data 😩

As RFC 2778 [9] and the presence event package for SIP [3] say, a Presence Agent (PA) always delivers all the presence data that's been authorized for a given watcher in presence notifications. It does this no matter what actually changed since the last notification. Sending the complete presence info over low bandwidth and high latency links when only a bit of it changed can be a lot, bestie. It can drag down the whole presence service and give the watcher a bad vibe 💀

This document defines a partial notification approach. The presence server only delivers the parts of the presence info that changed compared to what was sent in the previous notifications. That means way less data moving across the network 🍓

This mechanism uses the presence event package for SIP [3] and a new MIME type for carrying partial Presence Information Data Format documents [2] 💜

2. Conventions 📝

In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in RFC 2119 [1]. They indicate requirement levels for compliant implementations 🎀

This document also uses the vocabulary defined in RFC 2778 [9], RFC 3265 [6], the presence event package for SIP [3], and the PIDF extension for Partial Presence [2] 🌈

3. Introduction to the Partial Notification Mechanism 🧚

This chapter gives a quick tour of the regular presence service and an overview of the partial notification solution. This section is informational in nature. It doesn't contain any normative statements, so relax 🫶

3.1. Basic Presence Agent Operation 🌸

Normally, a watcher sends a SIP SUBSCRIBE request aimed at the presentity. That request gets routed to the presence agent where the presentity's presence info lives 🏠 The SUBSCRIBE request can include an Accept header field that says which content types the watcher supports ✨

When the presence agent gets the SUBSCRIBE request, it checks the Accept header. If there's no Accept header saying what content types are supported, or if the Accept header contains the default PIDF content type, the PA generates presence notifications in the default PIDF format [5]. The PIDF document can hold one or multiple XML elements. It includes a set of elements defined in RFC 2778 [9], plus its extensions for representing presence info 🍓 That PIDF document rides along in the body of a NOTIFY request built as per RFC 3265 [6] 📦

During basic operation, the presence document always has the full state of the presentity's presence status. That's determined by the PA's local policy and authorization rules 💅

3.2. Operation with Partial Notification 🦋

The partial notification mechanism lets a watcher ask that, whenever possible, notifications only contain the presence info that actually changed. 🙌 A watcher who wants partial notifications under this document creates a SIP SUBSCRIBE request that looks a lot like a regular presence subscription. The difference is that this SUBSCRIBE request has an Accept header field whose value includes the "application/pidf-diff+xml" tag as well as the "application/pidf+xml" tag 💖

When the presence agent gets the subscription, it looks at the Accept header value. If "application/pidf-diff+xml" is there, the PA can decide to use the partial notifications mechanism from this memo 🤔 The PA then builds NOTIFY requests with the Content-Type header field set to "application/pidf-diff+xml". The first NOTIFY request that has presence info carries a full presence document. After that, the NOTIFY requests can carry partial presence documents. Section 4 has all the details 📖

4. Client and Server Operations 👑

Unless this document says otherwise, the regular watcher and presence agent behavior from the SIP presence event package [3] applies 🎀

4.1. Content-Type for Partial Notifications 🌈

Entities that support the partial notification extension in this document MUST support the 'application/pidf-diff+xml' content type, which is specified in the PIDF extension for partial presence [2]. No skipping this one, bestie 🚨

4.2. Watcher Generation of SUBSCRIBE Requests 💅

A SUBSCRIBE request can be used to negotiate the preferred content type for the notifications. The Accept header field does this job, as specified in RFC 3261 [4] ✨

If a watcher wants to let the presence agent send partial notifications, the watcher MUST include an Accept header field in its SUBSCRIBE request. The value of that Accept header field MUST contain 'application/pidf-diff+xml', in addition to 'application/pidf+xml', which the SIP presence event package [3] requires. The watcher MAY include a "q" parameter with each Accept value to show how much it prefers that value 🍓

4.3. Presence Agent Processing of SUBSCRIBE Requests 🧚

The presence agent gets subscriptions from watchers and generates notifications according to the SIP presence event package [3] 💌 If the watcher listed supported content types in the Accept header field of the SUBSCRIBE request, the presence agent compares those values with the ones it supports, then decides which one to use 🔥

If the watcher indicated preferred accept values with "q" parameters, the presence agent SHOULD base its decision on those preferences. The exception is when the presence agent's local policy says otherwise 🙈

Quick heads up: that last one is a SHOULD, so the PA follows the watcher's "q" preferences by default, unless its local policy overrides them. ✨

Part 1 of 4 is done, and the good stuff is coming! 🌈💜

4.4. Presence Agent Generation of Partial Notifications ✨

okay bestie, once a subscription is accepted and installed, the PA MUST deliver the full state of the presence information in the first partial notification that contains a presence document with a <pidf-full> root element 💖. if the presence agent decides to send notifications that include a presence document under this spec, it MUST build that presence document using the PIDF extension for Partial Presence [2] and MUST set the Content-Type header field to application/pidf-diff+xml 🎀.

when you're using the application/pidf-diff+xml MIME type, the PA MUST include a version attribute 🔢. for the first partial notification in a given subscription, the PA MUST set version to the value one (1) ✨. this version counter is scoped to the subscription and goes up by one in each partial notification. it only resets when that subscription is terminated, and it does NOT reset when the subscription gets refreshed 🦋.

when the PA makes a partial presence document, it SHOULD only include the presence info that has changed since the previous notifications 🧚. what counts as a "change" is up to the PA's local policy 💅.

the PA MUST build the partial presence document using this logic:

  • the PA MUST construct the presence information according to the PIDF extension for Partial Presence [2]. everything that was added to the presence document goes inside <add> elements, everything that was removed goes inside <remove> elements, and everything that changed gets listed under <replace> elements 🌸.
  • the PA MUST include a version attribute in the presence document. the PA MUST increase the version number by one compared to the earlier presence document that was successfully sent to the watcher for that subscription, in the PIDF extension for Partial Presence [2] format 🫶.

the PA MUST NOT send a new NOTIFY request carrying a partial notification for the same Request-URI until it has gotten a final response from the watcher for the previous one, or until the previous NOTIFY request has timed out 🙈.

when the PA receives a SUBSCRIBE request (a refresh or a termination) within the associated subscription, it SHOULD send a NOTIFY request that contains the full presence document 👑.

if the PA has been using some content type other than application/pidf-diff+xml in notifications within the existing subscription, and it changes over to sending partial notifications, the PA MUST deliver the full state of the presence information as the first partial notification, inside a presence document with a <pidf-full> root element 🌈.

4.5. Watcher Processing of NOTIFY Requests 💜

the watcher processes all NOTIFY requests that use the application/pidf+xml content type, as described in RFC 3856 [3] 💅.

when the watcher gets the first notification with an application/pidf-diff+xml MIME body, the watcher MUST set up an internal version counter for this subscription, using the value of the version included in the presence document 🔥. this version counter is scoped to the subscription. the watcher MUST also store the full presence document it received as its local copy 📦.

when the watcher gets a later application/pidf-diff+xml encoded presence document, the watcher MUST compare the received version attribute with its local version counter 👀. if the version attribute on the presence document is equal to or lower than the version number stored locally, that counts as a PA failure, and the watcher SHOULD throw the document away without processing it any further 💀. otherwise, the watcher MUST update its locally stored info using this logic:

  • if the root element of the presence document is <pidf-full>, the watcher must replace its local copy of the presence document with the presence document received in the application/pidf-diff+xml body, and set the internal version value to the version attribute's value from that presence document 🧸.
  • if the root element of the presence document is <pidf-diff> and the received version number is exactly one more than the local version counter, the watcher MUST apply the changes to its local copy of the full presence document, as indicated in the received application/pidf-diff+xml document, following the PIDF extension for Partial Presence [2]. the watcher MUST then increase the local version counter by one 🍓.
  • if the root element of the presence document is <pidf-diff> and the received version value is higher than the locally stored value by more than one, the watcher assumes that one or more NOTIFYs got lost 😭. the watcher SHOULD either refresh the subscription to get a full presence document, or terminate the subscription 🫠.

if the watcher runs into a processing error while handling the received application/pidf-diff+xml encoded presence document, go check out Section 5.1 of [8] 🚨. in that case, the watcher SHOULD renew the subscription 🔁. the watcher MAY also fall back to normal presence operations by not putting application/pidf-diff+xml in a new SUBSCRIBE request. honestly it's hardly reasonable to report this error back to the notifier, even if the error is actually on the notifier's side 🙃.

if the PA changes the content type it uses in notifications within the existing subscription, the watcher MUST throw out all the presence information it previously received from that particular presentity (except the local version counter) and process the new content as defined for that content type 🗑️. the local version counter MUST NOT be thrown out, because if the PA switches back to application/pidf-diff+xml, the MIME type version counter keeps climbing from its last value 📈✨.

5. Examples ✨

okay so this section is the fun part, bestie 🎀 it walks through a whole message flow showing the partial notifications mechanism actually working.

A watcher sends a SUBSCRIBE request saying it can handle both the default presence format ('application/pidf+xml) and the partial notification format ('application/pidf-diff+xml') in its Accept header 💅 It uses the "q" parameter to say which one it prefers, and here it's giving partial notifications all the way (q=1). The PA accepts the subscription and, going off that "q" value, picks partial notifications for its NOTIFY requests 🦋 The first NOTIFY carries the full state of the presence information. After that, every notification only includes the delta, meaning just what changed since the previous NOTIFY 🌸

    Watcher                   Presence Agent                  PUA
         | F1 SUBSCRIBE              |                         |
         |-------------------------->|                         |
         | F2 200 OK                 |                         |
         |<--------------------------|                         |
         | F3 NOTIFY                 |                         |
         |<--------------------------|                         |
         | F4 200 OK                 |                         |
         |-------------------------->|                         |
         |                           |                         |
         |                           |   Update presence       |
         |                           |<----------------------- |
         |                           |                         |
         | F5 NOTIFY                 |                         |
         |<--------------------------|                         |
         | F6 200 OK                 |                         |
         |-------------------------->|                         |

Message Details

F1 SUBSCRIBE watcher -> example.com server 🍓

SUBSCRIBE sip:resource@example.com SIP/2.0
Via: SIP/2.0/TCP watcherhost.example.com;
  branch=z9hG4bKnashds7
To: <sip:resource@example.com>
From: <sip:watcher@example.com> ;tag=xfg9
Call-ID: 2010@watcherhost.example.com
CSeq: 17766 SUBSCRIBE
Max-Forwards: 70
Event: presence
Accept: application/pidf+xml;q=0.3,
  application/pidf-diff+xml;q=1
Contact: <sip:user@watcherhost.example.com>
Expires: 3600
Content-Length: 0

The PA accepts the subscription and sends back a 200 OK response to the SUBSCRIBE request ✨

F2 200 OK example.com server -> watcher 🧚

SIP/2.0 200 OK
Via: SIP/2.0/TCP watcherhost.example.com;
  branch=z9hG4bKnashds7
  ;received=192.0.2.1
To: <sip:resource@example.com>;tag=ffd2
From: <sip:watcher@example.com>;tag=xfg9
Call-ID: 2010@watcherhost.example.com
CSeq: 17766 SUBSCRIBE
Event: presence
Expires: 3600
Contact: <sip:server.example.com>
Content-Length: 0

The PA looks at the "q" parameter in the Accept header of the SUBSCRIBE request (that's F1) and decides to go with partial notifications 🔥 It then builds the first NOTIFY, which holds the full presence document.

F3 NOTIFY example.com server -> watcher 💜

NOTIFY sip:user@watcherhost.example.com SIP/2.0
Via: SIP/2.0/TCP server.example.com;
  branch=z9hG4bKna998sk
To: <sip:watcher@example.com>;tag=xfg9
From: <sip:resource@example.com>;tag=ffd2
Call-ID: 2010@watcherhost.example.com
Event: presence
Subscription-State: active;expires=3599
Max-Forwards: 70
CSeq: 8775 NOTIFY
Contact: <sip:server.example.com>
Content-Type: application/pidf-diff+xml
Content-Length: ...

<?xml version="1.0" encoding="UTF-8"?>
  <p:pidf-full xmlns="urn:ietf:params:xml:ns:pidf"
         xmlns:p="urn:ietf:params:xml:ns:pidf-diff"
         xmlns:r="urn:ietf:params:xml:ns:pidf:rpid"
         xmlns:c="urn:ietf:params:xml:ns:pidf:caps"
         xmlns:cp="urn:ietf:params:xml:ns:pidf:cipid"
         xmlns:dm="urn:ietf:params:xml:ns:pidf:data-model"
         entity="sip:resource@example.com"
         version="1">

   <tuple id="sg89ae">
    <status>
     <basic>open</basic>
    </status>
    <c:servcaps>
     <c:audio>true</c:audio>
     <c:video>false</c:video>
     <c:message>true</c:message>
    </c:servcaps>
     <r:relationship><r:assistant/></r:relationship>
    <contact priority="0.8">tel:09012345678</contact>
   </tuple>

   <tuple id="cg231jcr">
    <status>
     <basic>open</basic>
    </status>
    <contact priority="1.0">im:res@example.com</contact>
   </tuple>

   <tuple id="r1230d">
    <status>
     <basic>closed</basic>
    </status>
    <cp:homepage>http://example.com/~res/</cp:homepage>
    <cp:icon>http://example.com/~res/icon.gif</cp:icon>
    <cp:card>http://example.com/~res/card.vcd</cp:card>
    <contact priority="0.9">sip:resource@example.com</contact>
   </tuple>

   <note xml:lang="en">Full state presence document</note>

   <dm:person id="fdkfj">
     <r:activities>
      <r:on-the-phone/>
      <r:busy/>
     </r:activities>
   </dm:person>
   <dm:device id="u00b40c7">
     <c:devcaps>
      <c:mobility>
       <c:supported>
        <c:mobile/>
       </c:supported>
      </c:mobility>
     </c:devcaps>
     <dm:deviceID>mac:xxx</dm:deviceID>
   </dm:device>

  </p:pidf-full>

F4 200 OK watcher -> example.com server 🫶

SIP/2.0 200 OK
Via: SIP/2.0/TCP server.example.com;
  branch=z9hG4bKna998sk
  ;received=192.0.2.2
To: <sip:watcher@example.com>;tag=xfg9
From: <sip:resource@example.com>;tag=ffd2
Call-ID: 2010@watcherhost.example.com
CSeq: 8775 NOTIFY
Content-Length: 0

At a later time, the presentity's presence information changes 👀 The PA generates a NOTIFY request that includes information about those changes.

F5 NOTIFY example.com server -> watcher 🌈

NOTIFY sip:user@watcherhost.example.com SIP/2.0
Via: SIP/2.0/TCP server.example.com;
  branch=z9hG4bKna998sl
To: <sip:watcher@example.com>;tag=xfg9
From: <sip:resource@example.com>;tag=ffd2
Call-ID: 2010@watcherhost.example.com
CSeq: 8776 NOTIFY
Event: presence
Subscription-State: active;expires=3543
Max-Forwards: 70
Contact: <sip:server.example.com>
Content-Type: application/pidf-diff+xml
Content-Length: ...

<?xml version="1.0" encoding="UTF-8"?>
<p:pidf-diff xmlns="urn:ietf:params:xml:ns:pidf"
             xmlns:p="urn:ietf:params:xml:ns:pidf-diff"
             xmlns:r="urn:ietf:params:xml:ns:pidf:rpid"
             xmlns:dm="urn:ietf:params:xml:ns:pidf:data-model"
        entity="sip:resource@example.com"
        version="2">

   <p:add sel="presence/note" pos="before"><tuple id="ert4773">
    <status>
     <basic>open</basic>
    </status>
    <contact priority="0.4">mailto:res@example.com</contact>
    <note xml:lang="en">This is a new tuple inserted
          between the last tuple and note element</note>
   </tuple>

   </p:add>

   <p:replace sel="*/tuple[@id='r1230d']/status/basic/text()"
    >open</p:replace>

   <p:remove sel="*/dm:person/r:activities/r:busy"/>

   <p:replace sel="*/tuple[@id='cg231jcr']/contact/@priority"
    >0.7</p:replace>

  </p:pidf-diff>

F6 200 OK watcher -> example.com server 🎀

SIP/2.0 200 OK
Via: SIP/2.0/TCP server.example.com;
  branch=z9hG4bKna998sl
 ;received=192.0.2.2
To: <sip:watcher@example.com>;tag=xfg9
From: <sip:resource@example.com>;tag=ffd2
Call-ID: 2010@watcherhost.example.com
CSeq: 8776 NOTIFY
Content-Length: 0

6. Security Considerations 🔐

okay bestie, this one is serious, so let's get into it 🙈

This specification relies on the presence event package for SIP [3]. Partial notifications can reveal information about what has changed compared to the previous notification. That can make it easier for an eavesdropper to figure out what kind of changes are happening in the presentity's presence information. Still, the same information can be found if the presence event package is used with baseline PIDF [5] 💭

A third party can inject a NOTIFY request with partial state that makes the watcher think it has missed a partial notification, so it asks for a new full presence document 😬 This is no worse than what we already have without this extension. A party that could do this could also send a NOTIFY with Subscription-State: terminated and get the same effect without knowing anything about the extension. Partial Notification doesn't make the situation any worse, and the protection mechanisms from the existing system apply to stopping this attack against the partial notification mechanism 🛡️

Presence-related security considerations are covered in depth in the presence event package for SIP [3], and every one of those security considerations applies to this document too 💅 The issues described there, including confidentiality, message integrity and authenticity, outbound authentication, replay prevention, Denial-of-Service (DoS) attacks against thirst parties and DoS attacks against servers, all apply here without any change 🚨

It is RECOMMENDED that TLS [7] be used between elements to provide hop-by-hop confidentially protection. Furthermore, S/MIME MAY be used for integrity and authenticity of SUBSCRIBE and NOTIFY requests. This is described in Section 23 of RFC 3261 🔒

7. Acknowledgments 🌸

The authors would like to thank Jari Urpalainen, Jyrki Aarnos, Jonathan Rosenberg, Dean Willis, Kriztian Kiss, Juha Kalliokulju, Miguel Garcia, Anders Kristensen, Yannis Pavlidis, Ben Cambell, Robert Sparks, and Tim Moran for their valuable comments 💖

8. References 📚

8.1. Normative References

  • [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
  • [2] Lonnfors, M., Khartabil, H., Leppanen, E., and J. Urpalainen, "Presence Information Data Format (PIDF) Extension for Partial Presence", RFC 5262, August 008.
  • [3] Rosenberg, J., "A Presence Event Package for the Session Initiation Protocol (SIP)", RFC 3856, August 2004.
  • [4] 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.
  • [5] Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr, W., and J. Peterson, "Presence Information Data Format (PIDF)", RFC 3863, August 2004.
  • [6] Roach, A., "SIP-Specific Event Notification", RFC 3265, June 2002.
  • [7] Dierks, T. and E. Rescorla, "The TLS Protocol Version 1.1", RFC 4346, April 2006.
  • [8] Urpalainen, J., "An Extensible Markup Language (XML) Patch Operations Framework Utilizing XML Path Language (XPath) Selectors", RFC 5261, September 2008.

8.2. Informative References ✨

Okay bestie, last stop on the reference tour! 🧚 There's just one informative reference here, and it's the big-picture model that the whole presence thing is built on.

[9] Day, M., Rosenberg, J., and H. Sugano, "A Model for Presence and Instant Messaging", RFC 2778, February 2000. 💜

Authors' Addresses 🎀

Here are the four amazing people who wrote this, with their contact info so you can slide into their inboxes if you have questions. 💌

Mikko Lonnfors
Nokia
P.O. Box 321
Helsinki
Finland

Phone: +358 71 8008000
EMail: mikko.lonnfors@nokia.com


Jose Costa-Requena
Nokia
P.O. Box 321
Helsinki
Finland

Phone: +358 71 8008000
EMail: jose.costa-requena@nokia.com


Eva Leppanen
Nokia
Lempaala
Finland

EMail: eva.leppanen@saunalahti.fi


Hisham Khartabil
Ericsson
Melbourne
Australia

Phone: +61 416 108 890
EMail: hisham.khartabil@gmail.com

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