Network Working Group                                      A. Niemi, Ed.
Request for Comments: 3903                                         Nokia
Category: Standards Track                                   October 2004


              Session Initiation Protocol (SIP) Extension
                      for Event State Publication

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   This document describes an extension to the Session Initiation
   Protocol (SIP) for publishing event state used within the SIP Events
   framework.  The first application of this extension is for the
   publication of presence information.

   The mechanism described in this document can be extended to support
   publication of any event state for which there exists an appropriate
   event package.  It is not intended to be a general-purpose mechanism
   for transport of arbitrary data, as there are better-suited
   mechanisms for this purpose.

Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.   Definitions and Document Conventions . . . . . . . . . . . .   3
   3.   Overall Operation  . . . . . . . . . . . . . . . . . . . . .   4
   4.   Constructing PUBLISH Requests  . . . . . . . . . . . . . . .   5
        4.1.  Identification of Published Event State. . . . . . . .   6
        4.2.  Creating Initial Publication . . . . . . . . . . . . .   7
        4.3.  Refreshing Event State . . . . . . . . . . . . . . . .   8
        4.4.  Modifying Event State  . . . . . . . . . . . . . . . .   9
        4.5.  Removing Event State . . . . . . . . . . . . . . . . .   9
   5.   Processing PUBLISH Responses . . . . . . . . . . . . . . . .  10
   6.   Processing PUBLISH Requests  . . . . . . . . . . . . . . . .  10
   7.   Processing OPTIONS Requests  . . . . . . . . . . . . . . . .  13
   8.   Use of Entity-tags in PUBLISH  . . . . . . . . . . . . . . .  13



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        8.1.  General Notes. . . . . . . . . . . . . . . . . . . . .  13
        8.2.  Client Usage . . . . . . . . . . . . . . . . . . . . .  14
        8.3.  Server Usage . . . . . . . . . . . . . . . . . . . . .  14
   9.   Controlling the Rate of Publication  . . . . . . . . . . . .  15
   10.  Considerations for Event Packages using PUBLISH  . . . . . .  15
        10.1. PUBLISH Bodies . . . . . . . . . . . . . . . . . . . .  16
        10.2. PUBLISH Response Bodies. . . . . . . . . . . . . . . .  16
        10.3. Multiple Sources for Event State . . . . . . . . . . .  16
        10.4. Event State Segmentation . . . . . . . . . . . . . . .  17
        10.5. Rate of Publication. . . . . . . . . . . . . . . . . .  17
   11.  Protocol Element Definitions . . . . . . . . . . . . . . . .  17
        11.1. New Methods. . . . . . . . . . . . . . . . . . . . . .  17
              11.1.1. PUBLISH Method . . . . . . . . . . . . . . . .  17
        11.2. New Response Codes . . . . . . . . . . . . . . . . . .  19
              11.2.1. "412 Conditional Request Failed" Response Code  19
        11.3. New Header Fields  . . . . . . . . . . . . . . . . . .  20
              11.3.1. "SIP-ETag" Header Field  . . . . . . . . . . .  20
              11.3.2. "SIP-If-Match" Header Field  . . . . . . . . .  20
   12.  Augmented BNF Definitions  . . . . . . . . . . . . . . . . .  21
   13.  IANA Considerations  . . . . . . . . . . . . . . . . . . . .  21
        13.1. Methods  . . . . . . . . . . . . . . . . . . . . . . .  21
        13.2. Response Codes . . . . . . . . . . . . . . . . . . . .  21
        13.3. Header Field Names . . . . . . . . . . . . . . . . . .  21
   14.  Security Considerations  . . . . . . . . . . . . . . . . . .  22
        14.1. Access Control . . . . . . . . . . . . . . . . . . . .  22
        14.2. Denial of Service Attacks  . . . . . . . . . . . . . .  22
        14.3. Replay Attacks . . . . . . . . . . . . . . . . . . . .  22
        14.4. Man in the Middle Attacks  . . . . . . . . . . . . . .  23
        14.5. Confidentiality  . . . . . . . . . . . . . . . . . . .  23
   15.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . .  24
   16.  Contributors . . . . . . . . . . . . . . . . . . . . . . . .  29
   17.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  30
   18.  References . . . . . . . . . . . . . . . . . . . . . . . . .  30
        18.1. Normative References . . . . . . . . . . . . . . . . .  30
        18.2. Informative References . . . . . . . . . . . . . . . .  31
   Author's Address. . . . . . . . . . . . . . . . . . . . . . . . .  31
   Full Copyright Statement. . . . . . . . . . . . . . . . . . . . .  32

1.  Introduction

   This specification provides a framework for the publication of event
   state from a user agent to an entity that is responsible for
   compositing this event state and distributing it to interested
   parties through the SIP Events [1] framework.







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   In addition to defining an event publication framework, this
   specification defines a concrete usage of that framework for the
   publication of presence state [2] by a presence user agent [3] to a
   presence compositor, which has a tightly coupled relationship with
   the presence agent [1].

   The requirements and model for presence publication are documented in
   [10].  This specification will address each of those requirements.

   The mechanism described in this document can be extended to support
   publication of any event state for which there exists an appropriate
   event package as defined in [1].  For instance, an application of SIP
   events for message waiting indications [11] might choose to collect
   the statuses of voice-mail boxes across a set of user agents using
   the PUBLISH mechanism.  The compositor in such an application would
   then be responsible for collecting and distributing this state to the
   subscribers of the event package.

   Each application that makes use of the PUBLISH mechanism in the
   publication of event state will need to adhere to the guidelines set
   in Section 10.  The mechanism described in this document is not
   intended to be a general-purpose mechanism for transport of arbitrary
   data, as there are better-suited mechanisms for this purpose.

2.  Definitions and Document Conventions

   In addition to the definitions of RFC 2778 [3], RFC 3265 [1], and RFC
   3261 [4], this document introduces some new concepts:

   Event State: State information for a resource, associated with an
      event package and an address-of-record.

   Event Publication Agent (EPA): The User Agent Client (UAC) that
      issues PUBLISH requests to publish event state.

   Event State Compositor (ESC): The User Agent Server (UAS) that
      processes PUBLISH requests, and is responsible for compositing
      event state into a complete, composite event state of a resource.

   Presence Compositor: A type of Event State Compositor that is
      responsible for compositing presence state for a presentity.

   Publication: The act of an EPA sending a PUBLISH request to an ESC to
      publish event state.

   Event Hard State: The steady-state or default event state of a
      resource, which the ESC may use in the absence of, or in addition
      to, soft state publications.



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   Event Soft State: Event state published by an EPA using the PUBLISH
      mechanism.  A protocol element (i.e., an entity-tag) is used to
      identify a specific soft state entity at the ESC.  Soft state has
      a defined lifetime and will expire after a negotiated amount of
      time.

   The 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 BCP 14, RFC 2119 [5]
   and indicate requirement levels for compliant implementations.

      Indented passages such as this one are used in this document to
      provide additional information and clarifying text.  They do not
      contain descriptions of normative protocol behavior.

3.  Overall Operation

   This document defines a new SIP method, PUBLISH, for publishing event
   state.  PUBLISH is similar to REGISTER in that it allows a user to
   create, modify, and remove state in another entity which manages this
   state on behalf of the user.  Addressing a PUBLISH request is
   identical to addressing a SUBSCRIBE request.  The Request-URI of a
   PUBLISH request is populated with the address of the resource for
   which the user wishes to publish event state.  The user may in turn
   have multiple User Agents or endpoints that publish event state.
   Each endpoint may publish its own unique state, out of which the
   event state compositor generates the composite event state of the
   resource.  In addition to a particular resource, all published event
   state is associated with a specific event package.  Through a
   subscription to that event package, the user is able to discover the
   composite event state of all of the active publications.

   A User Agent Client (UAC) that publishes event state is labeled an
   Event Publication Agent (EPA).  For presence, this is the familiar
   Presence User Agent (PUA) role as defined in [2].  The entity that
   processes the PUBLISH request is known as an Event State Compositor
   (ESC).  For presence, this is the familiar Presence Agent (PA) role
   as defined in [2].

   PUBLISH requests create soft state in the ESC.  This event soft state
   has a defined lifetime and will expire after a negotiated amount of
   time, requiring the publication to be refreshed by subsequent PUBLISH
   requests.  There may also be event hard state provisioned for each
   resource for a particular event package.  This event state represents
   the resource state that is present at all times, and does not expire.
   The ESC may use event hard state in the absence of, or in addition
   to, event soft state provided through the PUBLISH mechanism.  Setting




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   this event hard state or configuring the ESC policy regarding the
   aggregation of different event state is out of the scope of this
   specification.

   The body of a PUBLISH request carries the published event state.  In
   response to every successful PUBLISH request, the ESC assigns an
   identifier to the publication in the form of an entity-tag.  This
   identifier is then used by the EPA in any subsequent PUBLISH request
   that modifies, refreshes or removes the event state of that
   publication.  When event state expires or is explicitly removed, the
   entity-tag associated with it becomes invalid.  A publication for an
   invalid entity-tag will naturally fail, and the EPA needs to start
   anew and resend its event state without referencing a previous
   entity-tag.

4.  Constructing PUBLISH Requests

   PUBLISH requests create, modify, and remove event state associated
   with an address-of-record.  A suitably authorized third party may
   also perform publication on behalf of a particular address-of-record.

   Except as noted, the construction of the PUBLISH request and the
   behavior of clients sending a PUBLISH request are identical to the
   general UAC behavior described in Section 8.1 and Section 17.1 of RFC
   3261 [4].

   If necessary, clients may probe for the support of PUBLISH using the
   OPTIONS request defined in SIP [4].  The presence of "PUBLISH" in the
   "Allow" header field in a response to an OPTIONS request indicates
   support for the PUBLISH method.  In addition, the "Allow-Events"
   header field indicates the supported event packages.

      Note that it is possible for the OPTIONS request to fork, and
      consequently return a response from a User Agent other than the
      ESC.  In that case, support for the PUBLISH method may not be
      appropriately represented for that particular Request-URI.

   A PUBLISH request does not establish a dialog.  A UAC MAY include a
   Route header field in a PUBLISH request based on a pre-existing route
   set as described in Section 8.1 of RFC 3261 [4].  The Record-Route
   header field has no meaning in PUBLISH requests or responses, and
   MUST be ignored if present.  In particular, the UAC MUST NOT create a
   new route set based on the presence or absence of a Record-Route
   header field in any response to a PUBLISH request.

   The PUBLISH request MAY contain a Contact header field, but including
   one in a PUBLISH request has no meaning in the event publication
   context and will be ignored by the ESC.  An EPA MAY send a PUBLISH



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   request within an existing dialog.  In that case, the request is
   received in the context of any media session or sessions associated
   with that dialog.

      Note that while sending a PUBLISH request within an existing
      dialog is not prohibited, it will typically not result in the
      expected behavior.  Unless the other end of the dialog is also an
      ESC, it will probably reject the request.

   EPAs MUST NOT send a new PUBLISH request (not a re-transmission) for
   the same Request-URI, until they have received a final response from
   the ESC for the previous one or the previous PUBLISH request has
   timed out.

4.1.  Identification of Published Event State

   Identification of published event state is provided by three pieces
   of information: Request-URI, event type, and (optionally) an entity-
   tag.