Network Working Group                                            M. Rose
Internet-Draft                                    Invisible Worlds, Inc.
Expires: November 28, October 30, 2001                                       G. Klyne
                                                  Baltimore Technologies
                                                              D. Crocker
                                                  Brandenburg Consulting
                                                            May 30, 2001
                       The APEX Presence Service
                      draft-ietf-apex-presence-02
                      draft-ietf-apex-presence-03

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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   This Internet-Draft will expire on November 28, October 30, 2001.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   This memo describes the APEX presence service, addressed as the well-
   known endpoint "apex=presence".  The presence service is used to
   manage presence information for APEX endpoints.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Management of Presence Information . . . . . . . . . . . . . .  4
   2.1 Update of Presence Information . . . . . . . . . . . . . . . .  5
   2.2 Distribution of Presence Information . . . . . . . . . . . . .  7
   2.3 Distribution of Watcher Information  . . . . . . . . . . . . . 10
   3.  Format of Presence Entries . . . . . . . . . . . . . . . . . . 13
   4.  The Presence Service . . . . . . . . . . . . . . . . . . . . . 14
   4.1 Use of XML and MIME  . . . . . . . . . . . . . . . . . . . . . 15
   4.2 The Subscribe Operation  . . . . . . . . . . . . . . . . . . . 16
   4.3 The Watch Operation  . . . . . . . . . . . . . . . . . . . . . 18
   4.4 The Publish Operation  . . . . . . . . . . . . . . . . . . . . 20
   4.5 The Terminate Operation  . . . . . . . . . . . . . . . . . . . 22
   4.6 The Notify Operation . . . . . . . . . . . . . . . . . . . . . 23
   4.7 The Reply Operation  . . . . . . . . . . . . . . . . . . . . . 23
   5.  Registration: The Presence Service . . . . . . . . . . . . . . 24
   6.  The Presence Service DTD . . . . . . . . . . . . . . . . . . . 25
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 28
       References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 29
   A.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 30
   B.  Revision History . . . . . . . . . . . . . . . . . . . . . . . 31
   B.1 Changes from draft-ietf-apex-presence-02 . . . . . . . . . . . 31
   B.2 Changes from draft-ietf-apex-presence-01 . . . . . . . . . . . 31
   C.
   B.3 Changes from draft-ietf-apex-presence-00 . . . . . . . . . . . 32 31
       Full Copyright Statement . . . . . . . . . . . . . . . . . . . 33 32

1. Introduction

   This memo describes a presence service that is built upon the APEX
   [1] "relaying mesh".  The APEX presence service is used to manage
   presence information for APEX endpoints.

   APEX, at its core, provides a best-effort datagram service.  Within
   an administrative domain, all relays must be able to handle messages
   for any endpoint within that domain.  APEX services are logically
   defined as endpoints but given their ubiquitous semantics they do not
   necessarily need to be associated with a single physical endpoint.
   As such, they may be provisioned co-resident with each relay within
   an administrative domain, even though they are logically provided on
   top of the relaying mesh, i.e.,

      +----------+     +----------+    +----------+    +---------+
      |   APEX   |     |   APEX   |    |   APEX   |    |         |
      |  access  |     | presence |    |  report  |    |   ...   |
      | service  |     |  service |    | service  |    |         |
      +----------+     +----------+    +----------+    +---------+
           |                |               |               |
           |                |               |               |
   +----------------------------------------------------------------+
   |                                                                |
   |                            APEX core                           |
   |                                                                |
   +----------------------------------------------------------------+

   That is, applications communicate with an APEX service by exchanging
   data with a "well-known endpoint" (WKE).

   APEX applications communicate with the presence service by exchanging
   data with the well-known endpoint "apex=presence" in the
   corresponding administrative domain, e.g.,
   "apex=presence@example.com" is the endpoint associated with the
   presence service in the "example.com" administrative domain.

   Note that within a single administrative domain, the presence service
   makes use of the APEX access [3] service in order to determine if an
   originator is allowed to view or manage presence information.

2. Management of Presence Information

   Management of presence information falls into three categories:

   o  applications may update the presence entry information associated with
      an endpoint;

   o  applications may subscribe to receive presence information about
      associated with an endpoint; and,

   o  applications may find out who is subscribed to receive presence
      information.

   Each is now described in turn.

2.1 Update of Presence Information

   When an application wants to modify the presence entry information
   associated with an endpoint, it sends a publish operation to the
   service, e.g.,

       +-------+                  +-------+
       |       | -- data -------> |       |
       | appl. |                  | relay |
       |       | <--------- ok -- |       |
       +-------+                  +-------+

     C: <data content='#Content'>
            <originator identity='fred@example.com' />
            <recipient identity='apex=presence@example.com' />
            <data-content Name='Content'>
                <publish publisher='fred@example.com' transID='1'
                         timeStamp='14 May 2000 13:30:00 -0800'>
                    <presence publisher='fred@example.com'
                           lastUpdate='14 May 2000 13:02:00 -0800'
                           publisherInfo='http://www.example.com/fred/'>
                        <tuple
                          destination='apex:fred/appl=im@example.com'
                          availableUntil='14 May 2000 14:02:00 -0800' />
                        <tuple destination='mailto:fred@flintstone.com'
                          availableUntil='31 Dec 2525 23:59:59 -0800' />
                    </presence>
                </publish>
            </data-content>
        </data>
     S: <ok />

   Note that this example uses the "subaddress" convention specified in
   Section 2.2 of [1] (e.g., "fred/appl=im") to denote multiplexing of
   traffic for a particular endpoint.  Of course, popular applications
   may have their own URI method assigned to them (e.g.,
   "im:fred@example.com").

   The service immediately responds with a reply operation containing
   the same transaction-identifier, e.g.,

                                  +-------+                  +-------+
                                  |       | <------- data -- |       |
                                  | relay |                  | pres. |
                                  |       | -- ok ---------> |  svc. |
                                  +-------+                  +-------+

     C: <data content='#Content'>
            <originator identity='apex=presence@example.com' />
            <recipient identity='fred@example.com' />
            <data-content Name='Content'>
                <reply code='250' transID='1' />
            </data-content>
        </data>
     S: <ok />

2.2 Distribution of Presence Information

   When an application wants to (periodically) receive the presence
   entry
   information associated with an endpoint, it sends a subscribe
   operation to the service, e.g.,

       +-------+                  +-------+
       |       | -- data -------> |       |
       | appl. |                  | relay |
       |       | <--------- ok -- |       |
       +-------+                  +-------+

     C: <data content='#Content'>
            <originator identity='wilma@example.com' />
            <recipient identity='apex=presence@example.com' />
            <data-content Name='Content'>
                <subscribe publisher='fred@example.com' duration='86400'
                           transID='100' />
            </data-content>
        </data>
     S: <ok />
   The service immediately responds with a publish operation containing
   the same transaction-identifier, e.g.,

                                  +-------+                  +-------+
                                  |       | <------- data -- |       |
                                  | relay |                  | pres. |
                                  |       | -- ok ---------> |  svc. |
                                  +-------+                  +-------+

     C: <data content='#Content'>
            <originator identity='apex=presence@example.com' />
            <recipient identity='wilma@example.com' />
            <data-content Name='Content'>
                <publish publisher='fred@example.com' transID='100'
                         timeStamp='14 May 2000 13:30:00 -0800'>
                    <presence publisher='fred@example.com'
                           lastUpdate='14 May 2000 13:02:00 -0800'
                           publisherInfo='http://www.example.com/fred/'>
                        <tuple
                          destination='apex:fred/appl=im@example.com'
                          availableUntil='14 May 2000 14:02:00 -0800' />
                    </presence>
                </publish>
            </data-content>
        </data>
     S: <ok />

   Subsequently, for up to the specified "duration", the service sends
   new publish operations whenever there are any changes to the
   endpoint's presence information.  If the "duration" is zero-valued, a
   one time poll of the presence information is achieved; otherwise, at
   the end of the "duration", a terminate operation is sent.

   Note that Step 5 of Section 4.4 requires that the "lastUpdate"
   attribute of a presence entry be supplied in order to update that
   entry; accordingly, applications must successfully retrieve an
   publish entry prior to trying to update that entry.  This is usually
   accomplished by subscribing with a zero-valued duration.

   Either the subscriber or the service may cancel a subscription by
   sending a terminate operation, e.g.,

       +-------+                  +-------+
       |       | -- data -------> |       |
       | appl. |                  | relay |
       |       | <--------- ok -- |       |
       +-------+                  +-------+

     C: <data content='#Content'>
            <originator identity='wilma@example.com' />
            <recipient identity='apex=presence@example.com' />
            <data-content Name='Content'>
                <terminate transID='100' />
            </data-content>
        </data>
     S: <ok />

                                  +-------+                  +-------+
                                  |       | <------- data -- |       |
                                  | relay |                  | pres. |
                                  |       | -- ok ---------> |  svc. |
                                  +-------+                  +-------+

     C: <data content='#Content'>
            <originator identity='apex=presence@example.com' />
            <recipient identity='wilma@example.com' />
            <data-content Name='Content'>
                <reply code='250' transID='100' />
            </data-content>
        </data>
     S: <ok />

   or
                                  +-------+                  +-------+
                                  |       | <------- data -- |       |
                                  | relay |                  | pres. |
                                  |       | -- ok ---------> |  svc. |
                                  +-------+                  +-------+

     C: <data content='#Content'>
            <originator identity='apex=presence@example.com' />
            <recipient identity='wilma@example.com' />
            <data-content Name='Content'>
                <terminate transID='100' />
            </data-content>
        </data>
     S: <ok />

2.3 Distribution of Watcher Information

   When an application wants to (periodically) receive notices about
   endpoints that are subscribed to receive presence information, it
   sends a watch operation to the service, e.g.,

       +-------+                  +-------+
       |       | -- data -------> |       |
       | appl. |                  | relay |
       |       | <--------- ok -- |       |
       +-------+                  +-------+

     C: <data content='#Content'>
            <originator identity='fred@example.com' />
            <recipient identity='apex=presence@example.com' />
            <data-content Name='Content'>
                <watch publisher='fred@example.com' duration='86400'
                       transID='2' />
            </data-content>
        </data>
     S: <ok />

   The service immediately responds with a reply operation containing
   the same transaction-identifier, e.g.,

                                  +-------+                  +-------+
                                  |       | <------- data -- |       |
                                  | relay |                  | pres. |
                                  |       | -- ok ---------> |  svc. |
                                  +-------+                  +-------+

     C: <data content='#Content'>
            <originator identity='apex=presence@example.com' />
            <recipient identity='fred@example.com' />
            <data-content Name='Content'
                <reply code='250' transID='2' />
            </data-content>
        </data>
     S: <ok />
   For each current subscriber, the service immediately sends a notify
   operation containing the same transaction-identifier, e.g.,

                                  +-------+                  +-------+
                                  |       | <------- data -- |       |
                                  | relay |                  | pres. |
                                  |       | -- ok ---------> |  svc. |
                                  +-------+                  +-------+

     C: <data content='#Content'>
            <originator identity='apex=presence@example.com' />
            <recipient identity='fred@example.com' />
            <data-content Name='Content'>
                <notify subscriber='wilma@example.com' transID='2'
                        duration='86000' action='subscribe' />
            </data-content>
        </data>
     S: <ok />

   Subsequently, for up to the specified "duration", the service sends
   new notify operations whenever an application subscribes successfully
   or a subscription is terminated.  If the "duration" is zero-valued, a
   one time poll of the watcher information is achieved; otherwise, at
   the end of the "duration", a terminate operation is sent.

   Either the watcher or the service may cancel the request by sending a
   terminate operation, e.g.,

       +-------+                  +-------+
       |       | -- data -------> |       |
       | appl. |                  | relay |
       |       | <--------- ok -- |       |
       +-------+                  +-------+

     C: <data content='#Content'>
            <originator identity='fred@example.com' />
            <recipient identity='apex=presence@example.com' />
            <data-content Name='Content'>
                <terminate transID='2' />
            </data-content>
        </data>
     S: <ok />

                                  +-------+                  +-------+
                                  |       | <------- data -- |       |
                                  | relay |                  | pres. |
                                  |       | -- ok ---------> |  svc. |
                                  +-------+                  +-------+

     C: <data content='#Content'>
            <originator identity='apex=presence@example.com' />
            <recipient identity='fred@example.com' />
            <data-content Name='Content'>
                <reply code='250' transID='2' />
             </data-content>
        </data>
     S: <ok />

   or
                                  +-------+                  +-------+
                                  |       | <------- data -- |       |
                                  | relay |                  | pres. |
                                  |       | -- ok ---------> |  svc. |
                                  +-------+                  +-------+

     C: <data content='#Content'>
            <originator identity='apex=presence@example.com' />
            <recipient identity='fred@example.com' />
            <data-content Name='Content'>
                <terminate transID='2' />
             </data-content>
        </data>
     S: <ok />

3. Format of Presence Entries

   Each administrative domain is responsible for maintaining a "presence
   entry" for each of its endpoints (regardless of whether those
   endpoints are currently attached to the relaying mesh).

   Section 6 defines the syntax for presence entries.  Each presence
   entry has a "publisher" attribute, a "lastUpdate" attribute, a
   "publisherInfo" attribute, and contains one or more "tuple" elements:

   o  the "publisher" attribute specifies the endpoint associated with
      the presence entry;

   o  the "lastUpdate" attribute specifies the date and time that the
      service last updated the presence entry;

   o  the "publisherInfo" attribute specifies arbitrary information
      about the publisher (using a URI); and,

   o  each "tuple" element specifies information about an entity
      associated with the endpoint.

   Each "tuple" element has a "destination" attribute, an
   "availableUntil" attribute, a "tupleInfo" attribute, and contains
   zero or more "capability" elements:

   o  the "destination" attribute identifies the entity as a URI (e.g.,
      "apex:fred/appl=im@example.com" or "mailto:fred@flintstone.com");

   o  the "availableUntil" attribute specifies the latest date and time
      that the entity is capable of receiving messages;

   o  the "tupleInfo" attribute specifies arbitrary information about
      the entity (using a URI); and,

   o  each "capability" element contains a specification as to the kinds
      of content the entity is capable of receiving.

   Each "capability" element contains arbitrary character data formatted
   according to the standard indicated in the element's "baseline"
   attribute.

4. The Presence Service

   Section 5 contains the APEX service registration for the presence
   service:

   o  Within an administrative domain, the service is addressed using
      the well-known endpoint of "apex=presence".

   o  Section 6 defines the syntax of the operations exchanged with the
      service.

   o  A consumer of the service initiates communications by sending data
      containing the subscribe, watch, or publish operation.

   o  In addition to replying to these operations, the service may also
      initiate communications by sending data containing the terminate,
      publish, or notify operations.

   An implementation of the service must maintain information about both
   presence entries and in-progress operations in persistent storage.

   Consult Section 6.1.1 of [1] for a discussion on the properties of
   long-lived transaction-identifiers.

4.1 Use of XML and MIME

   Section 4.1 of [1] describes how arbitrary MIME content is exchanged
   as a BEEP [2] payload.  For example, to transmit:

       <data content='...'>
            <originator identity='apex=presence@example.com' />
           <recipient identity='fred@example.com' />
       </data>

   where "..." refers to: <reply code='250' transID='1' />

   then the corresponding BEEP message might look like this:

       C: MSG 1 1 . 42 1234
       C: Content-Type: multipart/related; boundary="boundary";
       C:               start="<1@example.com>";
       C:               type="application/beep+xml"
       C:
       C: --boundary
       C: Content-Type: application/beep+xml
       C: Content-ID: <1@example.com>
       C:
       C: <data content='cid:2@example.com'>
       C:     <originator identity='fred@example.com' />
       C:     <recipient identity='apex=presence@example.com' />
       C: </data>
       C: --boundary
       C: Content-Type: application/beep+xml
       C: Content-ID: <2@example.com>
       C:
       C: <reply code='250' transID='1' />
       C: --boundary--
       C: END

   or this:

       C: MSG 1 1 . 42 1234
       C: Content-Type: application/beep+xml
       C:
       C: <data content='#Content'>
       C:     <originator identity='fred@example.com' />
       C:     <recipient identity='apex=presence@example.com' />
       C:     <data-content Name='Content'>
       C:         <reply code='250' transID='1' />
       C:     </data-content>
       C: </data>
       C: END

4.2 The Subscribe Operation

   When an application wants to (periodically) receive the presence
   entry
   information associated with an endpoint, it sends a "subscribe"
   element to the service.

   The "subscribe" element has a "publisher" attribute, a "duration"
   attribute, a "transID" attribute, and no content:

   o  the "publisher" attribute specifies the endpoint associated with
      the presence entry;

   o  the "transID" attribute specifies the transaction-identifier
      associated with this operation; and,

   o  the "duration" attribute specifies the maximum number of seconds
      for which the originator is interested in receiving updated
      presence information.

   When the service receives a "subscribe" element, we refer to the
   "publisher" attribute of that element as the "subject", and the
   service performs these steps:

   1.  If the subject is outside of this administrative domain, a
       "reply" element having code 553 is sent to the originator.

   2.  If the subject does not refer to a valid endpoint, a "reply"
       element having code 550 is sent to the originator.

   3.  If the subject's access entry does not contain a
       "presence:subscribe" token for the originator, a "reply" element
       having code 537 is sent to the originator.

   4.  If the originator already has an in-progress subscribe operation
       for the subject, then the previous subscribe operation is
       silently terminated, and processing continues.

   5.  If the "transID" attribute refers to an in-progress subscribe or
       watch operation for the originator, a "reply" element having code
       555 is sent to the originator.

   6.  Otherwise:

       1.  A "publish" element, corresponding to the subject's presence
           entry, is immediately sent to the originator.

       2.  For each endpoint currently watching subscribers to the
           subject's presence information, a "notify" element is
           immediately as sent (c.f., Step 6.3 of Section 4.6).

       3.  For up to the amount of time indicated by the "duration"
           attribute of the "subscribe" element, if the subject's
           presence entry changes, an updated "presence" element is sent
           to the originator using the publish operation (Section 4.4).
           Finally, when the amount of time indicated by the "duration"
           attribute expires, a terminate operation (Section 4.5) is
           sent to the originator.

       Note that if the duration is zero-valued, then the subscribe
       operation is making a one-time poll of the presence information.
       Accordingly, Step 6.3 above does not occur.

   Regardless of whether a "publish" or "reply" element is sent to the
   originator, the "transID" attribute is identical to the value found
   in the "subscribe" element sent by the originator.

4.3 The Watch Operation

   When an application wants to (periodically) receive notices about
   endpoints that are subscribed to receive presence information, entry, it sends a
   "watch" element to the service.

   The "watch" element has a "publisher" attribute, a "duration"
   attribute, a "transID" attribute, and no content:

   o  the "publisher" attribute specifies the endpoint associated with
      the presence entry;

   o  the "transID" attribute specifies the transaction-identifier
      associated with this operation; and,

   o  the "duration" attribute specifies the maximum number of seconds
      for which the originator is interested in watching subscribers.

   When the service receives a "watch" element, we refer to the
   "publisher" attribute of that element as the "subject", and the
   service performs these steps:

   1.  If the subject is outside of this administrative domain, a
       "reply" element having code 553 is sent to the originator.

   2.  If the subject does not refer to a valid endpoint, a "reply"
       element having code 550 is sent to the originator.

   3.  If the subject's access entry does not contain a "presence:watch"
       token for the originator, a "reply" element having code 537 is
       sent to the originator.

   4.  If the originator already has an in-progress watch operation for
       the subject, then the previous watch operation is silently
       terminated, and processing continues.

   5.  If the "transID" attribute refers to an in-progress subscribe or
       watch operation for the originator, a "reply" element having code
       555 is sent to the originator.

   6.  Otherwise:

       1.  A "reply" element having code 250 is sent to the originator.

       2.  For each endpoint currently subscribing to the subject's
           presence information, a "notify" element is immediately sent
           to the originator (c.f., Section 4.6).

       3.  For up to the amount of time indicated by the "duration"
           attribute of the "watch" element, whenever a subscribe
           operation succeeds or a subscription is terminated, a
           "notify" element is sent to the originator.  Finally, when
           the amount of time indicated by the "duration" attribute
           expires, a terminate operation (Section 4.5) is sent to the
           originator.

       Note that if the duration is zero-valued, then the watch
       operation is making a one-time poll of the presence information.
       Accordingly, Step 6.3 above does not occur.

   Regardless of whether a "notify" or "reply" element is sent to the
   originator, the "transID" attribute is identical to the value found
   in the "presence" element sent by the originator.

4.4 The Publish Operation

   When an application wants to modify the presence entry associated
   with an endpoint, it sends a "publish" element to the service.  In
   addition, the service sends a "publish" element to endpoints that
   have subscribed to see presence information (c.f., Section 4.2).

   The "publish" element has a "publisher" attribute, a "transID"
   attribute, a "timeStamp" attribute, and contains a "presence"
   element:

   o  the "publisher" attribute specifies the endpoint to be associated
      with the presence entry;

   o  the "transID" attribute specifies the transaction-identifier
      associated with this operation;

   o  the "timeStamp" attribute specifies the application's notion of
      the current date and time; and,

   o  the "presence" element contains the desired presence entry for the
      endpoint.

   When the service sends a "publish" element, the "transID" attribute
   specifies the transaction-identifier associated with the subscribe
   operation that caused this "publish" element to be sent, and the
   "timeStamp" attribute specifies the service's notion of the current
   date and time.  No reply is sent by the receiving endpoint.

   When the service receives a "publish" element, we refer to the
   "publisher" attribute of that element as the "subject", and the
   service performs these steps:

   1.  If the "publisher" attribute of the "publish" element doesn't
       match the "publisher" attribute of the "presence" element
       contained in the "publish" element, a "reply" element having code
       503 is sent to the originator.

   2.  If the subject is outside of this administrative domain, a
       "reply" element having code 553 is sent to the originator.

   3.  If the subject does not refer to a valid endpoint, a "reply"
       element having code 550 is sent to the originator.

   4.  If the subject's access entry does not contain a
       "presence:publish" token for the originator, a "reply" element
       having code 537 is sent to the originator.

   5.  If the "lastUpdate" attribute of the "publish" element is not
       semantically identical to the "lastUpdate" attribute of the
       subject's presence entry, a "reply" element having code 555 is
       sent to the originator.  (This allows a simple mechanism for
       atomic updates.)

   6.  Otherwise:

       1.  The subject's presence entry is updated from the "publish"
           element.

       2.  The "lastUpdate" attribute of the presence entry is set to
           the service's notion of the current date and time.

       3.  A "reply" element having code 250 is sent to the originator.

   When sending the "reply" element, the "transID" attribute is
   identical to the value found in the "publish" element sent by the
   originator.

4.5 The Terminate Operation

   When an application no longer wishes to subscribe to presence
   information or to watch endpoints that are subscribed to receive
   presence information, it sends a "terminate" element to the service;
   similarly, when the service no longer considers an application to be
   subscribing or watching, a "terminate" element is sent to the
   application.

   The "terminate" element contains only a "transID" attribute that
   specifies the transaction-identifier associated an in-progress
   subscribe or watch operation.  Section 9.1 of [1] defines the syntax
   for the "terminate" element.

   When the service receives a "terminate" element, it performs these
   steps:

   1.  If the transaction-identifier does not refer to a previous
       subscribe or watch operation for the originator, an "error"
       element having code 550 is returned.

   2.  Otherwise, the previous subscribe or watch operation for the
       originator is terminated, and a "reply" element having code 250
       is sent to the originator.

   Note that following a terminate operation, the originator may receive
   further presence or watcher updates.  Although the service will send
   no further updates after processing a terminate operation and sending
   the reply operation, earlier updates may be in transit.

4.6 The Notify Operation

   The service sends a "notify" element to endpoints that are watching
   other endpoints subscribed to presence information (c.f., Section
   4.3).

   The "notify" element has a "subscriber" attribute, a "transID"
   attribute, a "duration" attribute, an "action" attribute, and no
   content:

   o  the "subscriber" attribute specifies the endpoint that is
      subscribed to presence information; and,

   o  the "transID" attribute specifies the transaction-identifier
      associated with the watch operation that caused this "notify"
      element to be sent;

   o  the "action" attribute specifies whether a subscription or its
      termination has occurred; and,

   o  if a subscription is being reported, the "duration" attribute
      specifies the requested duration of the subscription.

   No reply is sent by the receiving endpoint.

4.7 The Reply Operation

   While processing operations, the service may respond with a "reply"
   element.  Consult Sections 10.2 and 6.1.2 of [1], respectively, for
   the syntax and semantics of the reply operation.

5. Registration: The Presence Service

   Well-Known Endpoint: apex=presence

   Syntax of Messages Exchanged: c.f., Section 6

   Sequence of Messages Exchanged: c.f., Section 4

   Access Control Tokens: presence:subscribe, presence:watch,
      presence:publish

   Contact Information: c.f., the "Authors' Addresses" section of this
      memo

6. The Presence Service DTD

   <!--
     DTD for the APEX presence service, as of 2001-05-08

     Refer to this DTD as:

       <!ENTITY % APEXPRESENCE PUBLIC "-//IETF//DTD APEX PRESENCE//EN"
              "http://xml.resource.org/profiles/APEX/apex-presence.dtd">
       %APEXPRESENCE;
     -->

   <!ENTITY % APEXCORE PUBLIC "-//IETF//DTD APEX CORE//EN"
              "http://xml.resource.org/profiles/APEX/apex-core.dtd">
   %APEXCORE;

   <!--
     Synopsis of the APEX presence service

       service WKE: apex=presence

       message exchanges:

           consumer initiates    service replies
           ==================    ================
           subscribe             publish or reply
           terminate             reply
           watch                 reply
           publish               reply

           service initiates     consumer replies
           =================     ================
           terminate             (nothing)
           publish               (nothing)
           notify                (nothing)

       access control:

           token                 target
           ==================    ======
           presence:subscribe    for "publisher" of "subscribe" element
           presence:watch        for "publisher" of "watch" element
           presence:publish      for "publisher" of "publish" element
     -->
   <!ELEMENT subscribe   EMPTY>
   <!ATTLIST subscribe
             publisher   %ENDPOINT;        #REQUIRED
             transID     %UNIQID;          #REQUIRED
             duration    %SECONDS;         #REQUIRED>

   <!ELEMENT watch       EMPTY>
   <!ATTLIST watch
             publisher   %ENDPOINT;        #REQUIRED
             transID     %UNIQID;          #REQUIRED
             duration    %SECONDS;         #REQUIRED>

   <!-- publisher attributes must match in publish and presence -->
   <!ELEMENT publish     (presence)>
   <!ATTLIST publish
             publisher   %ENDPOINT;        #REQUIRED
             transID     %UNIQID;          #REQUIRED
             timeStamp   %TIMESTAMP;       #REQUIRED>

   <!ELEMENT notify      EMPTY>
   <!ATTLIST notify
             subscriber  %ENDPOINT;        #REQUIRED
             transID     %UNIQID;          #REQUIRED
             duration    %SECONDS;         "0"
             action     (subscribe|terminate)
                                           "subscribe">
   <!--
     presence entries
     -->

   <!ELEMENT presence    (tuple+)>
   <!ATTLIST presence
             publisher   %ENDPOINT;        #REQUIRED
             lastUpdate  %TIMESTAMP;       #REQUIRED
             publisherInfo
                         %URI;             "">

   <!ELEMENT tuple       (capability*)>
   <!ATTLIST tuple
             destination %URI;             #REQUIRED
             availableUntil
                         %TIMESTAMP;       #REQUIRED
             tupleInfo %URI;               "">

   <!-- e.g., baseline='rfc2533' -->
   <!ELEMENT capability (#PCDATA)>
   <!ATTLIST capability
             baseline    NMTOKEN           #REQUIRED>

7. Security Considerations

   Consult [1]'s Section 11 for a discussion of security issues.

References

   [1]  Rose, M., Klyne, G. and D. Crocker, "The Application Exchange
        Core", draft-ietf-apex-core-02 draft-ietf-apex-core-03 (work in progress), May June 2001.

   [2]  Rose, M., "The Blocks Extensible Exchange Protocol Core", RFC
        3080, March 2001.

   [3]  Rose, M., Klyne, G. and D. Crocker, "The APEX Access Service",
        draft-ietf-apex-access-02
        draft-ietf-apex-access-05 (work in progress), May June 2001.

Authors' Addresses

   Marshall T. Rose
   Invisible Worlds, Inc.
   131 Stony Circle
   Suite 500
   Santa Rosa, CA  95401
   US

   Phone: +1 707 578 2350
   EMail: mrose@invisible.net
   URI:   http://invisible.net/

   Graham Klyne
   Baltimore Technologies
   1310 Waterside
   Arlington Business Park
   Theale, Reading  RG7 4SA
   UK

   Phone: +44 118 903 8000
   EMail: gk@acm.org

   David H. Crocker
   Brandenburg Consulting
   675 Spruce Drive
   Sunnyvale, CA  94086
   US

   Phone: +1 408 246 8253
   EMail: dcrocker@brandenburg.com
   URI:   http://www.brandenburg.com/

Appendix A. Acknowledgements

   The authors gratefully acknowledge the contributions of: Neil Cook,
   Eric Dixon, Darren New, Scott Pead, and Bob Wyman.

Appendix B. Revision History

B.1 Changes from draft-ietf-apex-presence-02

   o  Re-organization previous version for consistency.

B.2 Changes from draft-ietf-apex-presence-01

   o  Grammar error in Security Considerations.

   o  Extraneous sentence in Step 6.2 of Section 4.3.

   o  Notifications are now sent when a subscription is terminated.

Appendix C.

B.3 Changes from draft-ietf-apex-presence-00

   o  Change "subaddress" convention from RFC 2846 to APEX's custom
      ABNF.

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