Network Working Group A. Niemi Request for Comments: 5264 M. Lonnfors Category: Standards Track Nokia E. Leppanen Individual September 2008
Publication of Partial Presence Information
Status of This Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Abstract
The Session Initiation Protocol (SIP) Extension for Event State Publication describes a mechanism with which a presence user agent is able to publish presence information to a presence agent. Using the Presence Information Data Format (PIDF), each presence publication contains full state, regardless of how much of that information has actually changed since the previous update. As a consequence, updating a sizeable presence document with small changes bears a considerable overhead and is therefore inefficient. Especially with low bandwidth and high latency links, this can constitute a considerable burden to the system. This memo defines a solution that aids in reducing the impact of those constraints and increases transport efficiency by introducing a mechanism that allows for publication of partial presence information.
The Session Initiation Protocol (SIP) Extension for Event State Publication [RFC3903] allows Presence User Agents ('PUA') to publish presence information of a user ('presentity'). The Presence Agent (PA) collects publications from one or several presence user agents, and generates the composite event state of the presentity.
The baseline format for presence information is defined in the Presence Information Data Format (PIDF) [RFC3863] and is by default used in presence publication. The PIDF uses Extensible Markup Language (XML) [W3C.REC-xml], and groups data into elements called tuples. In addition, [RFC4479], [RFC4480], [RFC4481], [RFC4482], and [RFC5196] define extension elements that provide various additional features to PIDF.
Presence publication by default uses the PIDF document format, and each publication contains full state, regardless of how much of the presence information has actually changed since the previous update. As a consequence, updating a sizeable presence document especially with small changes bears a considerable overhead and is therefore inefficient. Publication of information over low bandwidth and high latency links further exacerbates this inefficiency.
This memo specifies a mechanism with which the PUA is after an initial full state publication able to publish only those parts of the presence document that have changed since the previous update. This is accomplished using the partial PIDF [RFC5262] document format
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to communicate a set of presence document changes to the PA, who then applies the changes in sequence to its version of the presence document.
This memo is structured in the following way: Section 3 gives an overview of the partial publication mechanism, Section 4 includes the detailed specification, Section 5 includes discussion of security considerations, and Section 6 includes examples of partial publication.
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, BCP 14 [RFC2119], and indicate requirement levels for compliant implementations.
This document makes use of the vocabulary defined in the Model for Presence and Instant Messaging [RFC2778], the Event State Publication Extension to SIP [RFC3903], and the PIDF Extension for Partial Presence [RFC5262].
This section introduces the baseline functionality for presence publication, and gives an overview of the partial publication mechanism. This section is informational in nature. It does not contain any normative statements.
Event State Publication is specified in [RFC3903].
The publication of presence information consists of a presence user agent sending a SIP PUBLISH request [RFC3903] targeted to the address-of-record of the presentity, and serviced by a presence agent or compositor. The body of the PUBLISH request carries full event state in the form of a presence document.
The compositor processes the PUBLISH request and stores the presence information. It also assigns an entity-tag that is used to identify the publication. This entity-tag is returned to the PUA in the response to the PUBLISH request.
The PUA uses the entity-tag in the following PUBLISH request for identifying the publication that the request is meant to refresh, modify or remove. Presence information is stored in an initial
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publication, and maintained using the refreshing and modifying publications. Presence information disappears either by explicitly removing it or when it meets its expiration time.
The partial publication mechanism enables the PUA to update only parts of its presence information, namely those sections of the presence document that have changed. The initial publication always carries full state. However, successive modifying publications to this initial presence state can communicate state deltas, i.e., one or more changes to the presence information since the previous update. Versioning of these partial publications is necessary to guarantee that the changes are applied in the correct order. The PUBLISH method [RFC3903] already accomplishes this using entity-tags and conditional requests, which guarantee correct ordering of publication updates.
Note that the partial PIDF format [RFC5262] contains the 'version' attribute that could be used for versioning as well. However, we chose not to introduce an additional versioning mechanism to partial publish, since that would only add ambiguity and a potentially undefined error case if the two versioning mechanisms were to somehow contradict.
To initialize its publication of presence information, the PUA first publishes a full state initial publication. The consequent modifying publications can carry either state deltas or full state. Both initial and modifying partial presence publications are accomplished using the 'application/pidf-diff+xml' content type [RFC5262], with the former using the <pidf-full> root element, and the latter using the <pidf-diff> or <pidf-full> root elements, respectively.
While the <pidf-full> encapsulates a regular PIDF document, the <pidf-diff> can contain one or more operations for adding new elements or attributes (<add> elements), replacing elements or attributes whose content has changed (<replace> elements), or indications of removal of certain elements or attributes (<remove> elements). The PUA is free to decide the granularity by which changes in presence information are communicated to the composer. It may very well happen that there are enough changes to be communicated that it is more efficient to send a full state publication instead of a set of state deltas.
When the presence compositor receives a partial publication, it applies the included patch operations in sequence. The resulting changed (or patched) presence document is then submitted to the composition logic in the same manner as with a full state presence
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publication. Similarly, any changes to the publication expiration apply to the full, patched presence publication. In other words, there is no possibility to roll back to an earlier version, except by submitting a full state publication.
The entities supporting the partial publication extension described in this document MUST support the 'application/pidf-diff+xml' content type defined in the partial PIDF format [RFC5262], in addition to the baseline 'application/pidf+xml' content type defined in [RFC3863].
Listing the partial PIDF content type in the Accept header field of a SIP response is an explicit indication of support for the partial publication mechanism. The PUA can learn server support either as a result of an explicit query, i.e., in a response to an OPTIONS request, or by trial-and-error, i.e., after a 415 error response is returned to an attempted partial publication.
Whenever a PUA decides to begin publication of partial presence information, it first needs to make an initial publication. This initial publication always carries full state. After the initial publication, presence information can be updated using modifying publications; the modifications can carry state deltas as well as full state. Finally, the publication can be terminated by explicit removal, or by expiration.
Both the initial and modifying publications make use of the partial presence document format [RFC5262], and all follow the normal rules for creating publications, as defined in RFC 3903 [RFC3903], Section 4.
If the initial PUBLISH request returns a 415 (Unsupported Media Type), it means that the compositor did not understand the partial publication format. In this case, the PUA MUST follow normal procedures for handling a 400-class response, as specified in Section 8.1.3.5 of [RFC3261]. Specifically, the PUA SHOULD retry the publication using the default PIDF content type, namely 'application/ pidf+xml'. In addition, to find out a priori whether a specific presence compositor supports partial presence publication, the PUA MAY use the OPTIONS method, as described in [RFC3261].
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To construct a full-state publication, the PUA uses the following process:
o The Content-Type header field in the PUBLISH request MUST be set to the value 'application/pidf-diff+xml'.
o The document in the body of the request is populated with a <pidf- full> root element that includes the 'entity' attribute set to identify the presentity.
o Under the <pidf-full> root element exists all of the children of a PIDF [RFC3863] <presence> element. This document contains the full state of which the PUA is aware, and MAY include elements from any extension namespace.
To construct a partial publication, the following process is followed:
o The Content-Type header field in the PUBLISH request MUST be set to the value 'application/pidf-diff+xml'.
o The document in the body of the request is populated with a <pidf- diff> root element that includes the 'entity' attribute identifying the presentity.
o Under the <pidf-diff> root element exists a set of patch operations that communicate the changes to the presentity's presence information. These operations MUST be constructed in sequence, and as defined in the partial PIDF format [RFC5262].
The PUA is free to decide the granularity by which changes in the presentity's presence information are communicated to the presence compositor. In order to reduce unnecessary network traffic, the PUA SHOULD batch several patch operations in a single PUBLISH request.
A reasonable granularity might be to batch state changes resulting from related UI events together in a single PUBLISH request. For example, when the user sets their status to "Away", several things including freetext notes, service availability, and activities might change as a result.
If the size of the delta state becomes more than the size of the full state, the PUA SHOULD instead send a modifying publication carrying full state, unless this size comparison is not possible.
To an implementation that generates state deltas directly out of its internal events, it may not be trivial to determine the size of the corresponding full state.
For each resource, the compositor maintains a record for each of the publications. These are indexed using the entity-tag of the publications.
Processing of publications generally follows the guidelines set in [RFC3903]. In addition, processing PUBLISH requests that contain 'application/pidf-diff+xml' require some extra processing that is dependant on whether the request contains full or partial state.
If the value of the Content-Type header field is 'application/ pidf-diff+xml', and the document therein contains a <pidf-full> root element, the publication contains full presence information, and the next step applies:
o The compositor MUST take the received presence document under the <pidf-full> as the local presence document, replacing any previous publications.
If any errors are encountered before the entire publication is completely processed, the compositor MUST reject the request with a 500 (Server Internal Error) response, and revert back to its original, locally stored presence information.
If the value of the Content-Type header field is 'application/ pidf-diff+xml', and the document in the body contains a <pidf-diff> root element, the publication contains partial presence information (state delta), and the next steps apply:
o If the publication containing the <pidf-diff> root element is a modifying publication (i.e., contains an If-Match header field with a valid entity-tag), the compositor MUST apply the included patch operations in sequence against its locally stored presence document.
o Else, the publication is an initial publication, for which only <pidf-full> is allowed. Therefore, the publication MUST be rejected with an appropriate error response, such as a 400 (Invalid Partial Publication).
If a publication carrying partial presence information expires without the PUA refreshing it, the compositor MUST clear the entire, full state publication.
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This means that the compositor does not keep a record of the applied patches, and consequently (unlike some versioning systems), the compositor does not roll back to an earlier version if a particular partial publication were to expire.
If the compositor encounters errors while processing the 'application/pidf-diff+xml' document, it MUST reject the request with a 400 (Bad Request) response. In addition, the compositor MAY include diagnostics information in the body of the response, using an appropriate error condition element defined in Section 5.1. of [RFC5261].
If any other errors are encountered before the entire partial publication is completely processed, including all of the patch operations in the 'application/pidf-diff+xml' body, the compositor MUST reject the request with a 500 (Server Internal Error) response, and revert back to its original, locally stored presence information.
This specification relies on protocol behavior defined in [RFC3903]. General security considerations related to Event State Publication are extensively discussed in that specification and all the identified security considerations apply to this document in entirety. In addition, this specification adds no new security considerations.
The following message flow (Figure 1) shows an example of a presence system that applies the partial publication mechanism.
First, the PUA sends an initial publication that contains full state. In return, it receives a 200 OK response containing an entity-tag. This entity-tag serves as a reference with which the initial full state can be updated using partial publications containing state deltas.
Then at some point the resource state changes, and the PUA assembles these changes into a set of patch operations. It then sends a modifying publication containing the patch operations, using the entity-tag as a reference to the publication against which the patches are to be applied. The compositor applies the received patch operations to its local presence document in sequence, and returns a 200 OK, which includes a new entity-tag.
<p:add sel="presence/note" pos="before"><tuple id="ert4773"> <status> <basic>open</basic> </status> <contact priority="0.4">mailto:pep@example.com</contact> <note xml:lang="en">This is a new tuple inserted between the last tuple and note element</note> </tuple>
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3903] Niemi, A., "Session Initiation Protocol (SIP) Extension for Event State Publication", RFC 3903, October 2004.
[RFC3863] Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr, W., and J. Peterson, "Presence Information Data Format (PIDF)", RFC 3863, August 2004.
[RFC3261] 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.
[RFC5262] Lonnfors, M., Costa-Requena, J., Leppanen, E., and H. Khartabil, "Presence Information Data Format (PIDF) Extension for Partial Presence", RFC 5262, September 2008.
[RFC5261] Urpalainen, J., "An Extensible Markup Language (XML) Patch Operations Framework Utilizing XML Path Language (XPath) Selectors", RFC 5261, September 2008.
[RFC2778] Day, M., Rosenberg, J., and H. Sugano, "A Model for Presence and Instant Messaging", RFC 2778, February 2000.
[RFC4479] Rosenberg, J., "A Data Model for Presence", RFC 4479, July 2006.
[RFC4480] Schulzrinne, H., Gurbani, V., Kyzivat, P., and J. Rosenberg, "RPID: Rich Presence Extensions to the Presence Information Data Format (PIDF)", RFC 4480, July 2006.
[RFC4481] Schulzrinne, H., "Timed Presence Extensions to the Presence Information Data Format (PIDF) to Indicate Status Information for Past and Future Time Intervals", RFC 4481, July 2006.
[RFC4482] Schulzrinne, H., "CIPID: Contact Information for the Presence Information Data Format", RFC 4482, July 2006.
[RFC5196] Lonnfors, M. and K. Kiss, "Session Initiation Protocol (SIP) User Agent Capability Extension to Presence Information Data Format (PIDF)", RFC 5196, September 2008.
[W3C.REC-xml] Bray, T., Paoli, J., Sperberg-McQueen, C., and E. Maler, "Extensible Markup Language (XML) 1.0 (2nd ed)", W3C REC-xml, October 2000, <http://www.w3.org/TR/REC-xml>.
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Authors' Addresses
Aki Niemi Nokia P.O. Box 407 NOKIA GROUP, FIN 00045 Finland
Phone: +358 71 8008000 EMail: aki.niemi@nokia.com
Mikko Lonnfors Nokia Itamerenkatu 11-13 Helsinki Finland
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