Network Working Group                                        A. B. Roach
Internet-Draft                                              J. Rosenberg
Expires: April 25, 2003                                      B. Campbell
                                                        October 25, 2002

     A Session Initiation Protocol (SIP) Event Template Package for

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 25, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.


   This document presents a Session Initiation Protocol (SIP) event
   package template for subscribing to a collection of event packages.
   Instead of the subscriber sending a SUBSCRIBE to each notifier
   individually, the subscriber can subscribe to their an entire
   collection, and then receive notifications when the state of any of
   the notifiers in the collection changes.

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Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Overview of Operation  . . . . . . . . . . . . . . . . . . .   4
   2.1  Recursive Application of the "list" Template . . . . . . . .   5
   3.   Template Event Package for "list"  . . . . . . . . . . . . .   7
   3.1  Event Package Name . . . . . . . . . . . . . . . . . . . . .   7
   3.2  Event Package Parameters . . . . . . . . . . . . . . . . . .   7
   3.3  SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . . . .   7
   3.4  Subscription Duration  . . . . . . . . . . . . . . . . . . .   7
   3.5  NOTIFY Bodies  . . . . . . . . . . . . . . . . . . . . . . .   8
   3.6  Notifier Processing of SUBSCRIBE Requests  . . . . . . . . .   8
   3.7  Notifier Generation of NOTIFY Requests . . . . . . . . . . .   8
   3.8  Subscriber Processing of NOTIFY Requests . . . . . . . . . .   9
   3.9  Handling of Forked Requests  . . . . . . . . . . . . . . . .  10
   3.10 Rate of Notifications  . . . . . . . . . . . . . . . . . . .  10
   3.11 State Agents . . . . . . . . . . . . . . . . . . . . . . . .  10
   4.   Using multipart/mixed to Convey Aggregate State  . . . . . .  12
   4.1  Preamble Headers . . . . . . . . . . . . . . . . . . . . . .  12
   4.2  Body-Part Headers  . . . . . . . . . . . . . . . . . . . . .  12
   4.3  Constructing Coherent Resource State . . . . . . . . . . . .  13
   4.4  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   4.5  Examples . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   5.   Security Considerations  . . . . . . . . . . . . . . . . . .  15
   6.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .  16
   7.   Open Issues  . . . . . . . . . . . . . . . . . . . . . . . .  17
        References . . . . . . . . . . . . . . . . . . . . . . . . .  18
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  18
        Full Copyright Statement . . . . . . . . . . . . . . . . . .  20

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1. Introduction

   The SIP-specific event notification mechanism [2] allows a user (the
   subscriber) to request to be notified of changes in the state of a
   particular resource.  This is accomplished by having the subscriber
   generate a SUBSCRIBE request for the resource, which is processed by
   a notifier that represents the resource.  In many cases, a subscriber
   has a collection of resources they are interested in.

   For environments where bandwidth is limited, such as a wireless
   network, subscribing to each resource individually is problematic.
   The specific problems are:

   o  It generates substantial message traffic, in the form of the
      initial SUBSCRIBE requests for each resource, and the refreshes of
      each individual subscription.

   o  The notifier may insist on low refresh intervals, in order to
      avoid long lived subscription state.  This means that the
      subscriber may need to generate subscriptions faster than it would
      like to, or has the capacity to.

   o  The notifier may generate NOTIFY requests more rapidly than the
      subscriber desires, causing NOTIFY traffic at a greater volume
      than is desired by the subscriber.

   o  If a subscriber has only intermittent connectivity, and generally
      polls for state rather than simply subscribing, the latency to
      obtain the state of the entire resource can be large.  The
      messaging required for each poll can also be substantial.

   To solve these problems, this specification defines a template event
   package for collections of resources.  A resource list is identified
   by a SIP URI [1], and it represents a list of zero or more URIs.
   Each of these URIs is the Request URI for an individual resource for
   which the subscriber wants to receive information.

   The notifier for the collection is called an "resource list server",
   or RLS.  In order to determine the state of the entire list, the RLS
   will typically generate a subscription to each element of the list.

   The resource list may exist within the domain of the subscriber, but
   it can also exist within a third party domain.

   The first section provides more detail on the operation of the RLS,
   and the second section defines the event package for resource list

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2. Overview of Operation

   This section provides an overview of the typical mode of operation of
   this event template package.  It is not normative.

   When a user wishes to subscribe to the resource of a list of
   resources, they create a resource list.  This resource list is
   represented by a SIP URI.  The list contains a set of URIs, each of
   which represents a resource for which the subscriber wants to receive
   information.  The resource list can exist in any domain.  Typically,
   the user who creates the list (and subsequently subscribes to it)
   will have a trust relationship with the domain that hosts the list.
   The specific means by which the list is created and maintained is
   outside of the scope of this specification.  The list could be
   manipulated through a web page, through a voice response system, or
   through some protocol.

   To learn the resource state of the set of elements on the list, the
   user sends a single SUBSCRIBE request targeted to the URI of the
   list.  This will be routed to an RLS for that URI.  The RLS acts as a
   notifier, authenticates the subscriber, and accepts the subscription.

   The RLS may have direct information about some or all of the
   resources specified by the list.  If it does not, it subscribes to
   the resource specified by the request URI, for the base event package
   to which ".list" has been appended.

   Since the RLS is acting on behalf of the user, it will provide the
   identity of the user in the From field.  If the resources require
   credentials in order to accept the subscription, the user will have
   had to provide them to the RLS ahead of time.  This requires a trust
   relationship between the user and RLS.

   As notifications arrive from individual resources, the RLS accepts
   them, extracts the resource information, and generates a notification
   to the subscriber.  The RLS can, at its discretion, buffer
   notifications that it receives, and send the resource information to
   the subscriber in batches, rather than individually.  This allows the
   RLS to provide rate limiting for the subscriber.

            Joe               RLS             User A            User B
             |                 |                 |                 |
             |(1) SUBSCRIBE    |                 |                 |
             | Event: foo.list |                 |                 |
             |---------------->|                 |                 |
             |(2) 200 OK       |                 |                 |

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             |<----------------|                 |                 |
             |(3) NOTIFY       |                 |                 |
             | Event: foo.list |                 |                 |
             |<----------------|                 |                 |
             |(4) 200 OK       |                 |                 |
             |---------------->|                 |                 |
             |                 |(5) SUBSCRIBE a  |                 |
             |                 | Event: foo      |                 |
             |                 |---------------->|                 |
             |                 |(6) SUBSCRIBE b  |                 |
             |                 | Event: foo      |                 |
             |                 |---------------------------------->|
             |                 |(7) 200 OK       |                 |
             |                 |<----------------|                 |
             |                 |(8) 200 OK       |                 |
             |                 |<----------------------------------|
             |                 |(9) NOTIFY       |                 |
             |                 | Event: foo      |                 |
             |                 |<----------------|                 |
             |                 |(10) 200 OK      |                 |
             |                 |---------------->|                 |
             |(11) NOTIFY      |                 |                 |
             | Event: foo.list |                 |                 |
             | a's state       |                 |                 |
             |<----------------|                 |                 |
             |(12) 200 OK      |                 |                 |
             |---------------->|                 |                 |
             |                 |(13) NOTIFY      |                 |
             |                 | Event: foo      |                 |
             |                 |<----------------------------------|
             |                 |(14) 200 OK      |                 |
             |                 |---------------------------------->|
             |(15) NOTIFY      |                 |                 |
             | Event: foo.list |                 |                 |
             | b's state       |                 |                 |
             |<----------------|                 |                 |
             |(16) 200 OK      |                 |                 |
             |---------------->|                 |                 |

   As an example, consider a resource list with two resources, and  A typical flow for a
   subscription to this resource list is shown in Figure 1.

2.1 Recursive Application of the "list" Template

   As described in [2], one of the key features of any template package
   is that it can be applied to any other defined package -- including
   other templates, and even itself.  For many templates, the arbitrary

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   recursive application of the template to itself may be of
   questionable value.  For the "list" template, however, there is
   significant utility that can be provided in this fashion.  Take the
   example of applying "list" to the "presence" event package [5].  A
   user may quite reasonably maintain several lists of users for whom
   they want to know presence information.

   TBD: Addtional motivating text here.

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3. Template Event Package for "list"

   The following subsections formally define the resource list event
   package, following the requirements defined by the SIP events
   framework [2].

3.1 Event Package Name

   The name of this event template package is "list".

   The following is the information needed to register this event
   package with IANA:

   Package Name: list

   Type: template

   Contact: Jonathan Rosenberg,

   Reference: RFC XXXX [[Note to RFC Editor: replace with the RFC number
      for this specification]]

3.2 Event Package Parameters

   This specification does not define any parameters in the Event header
   for this package.  Any parameters that are present on the Event
   header, however, are propigated to any SUBSCRIBE messages generated
   for the base package to which it has been applied.

3.3 SUBSCRIBE Bodies

   The SUBSCRIBE message MAY contain a body whose purpose is to define
   filters on the operation of the list.  These filters would include
   any rate limitation desired for the notifications, or any aggregation
   that is desired.  There is no default or mandatory body type defined
   for this purpose.

3.4 Subscription Duration

   Since the primary benefit of the resource list server is to reduce
   the overall messaging volume to a handset, it is RECOMMENDED that the
   subscription duration to a list be reasonably long.  The default,
   when no duration is specified, is two hours, which reduces the need
   to refresh too frequently.  Of course, the standard techniques [2]
   can be used to increase or reduce this amount.

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3.5 NOTIFY Bodies

   An implementation compliant to this specification MUST support the
   multipart/mixed type.  This allows a notification to contain multiple
   resource documents.

   The absence of an Accept header in the SUBSCRIBE indicates support
   for multipart/mixed and the content type(s) used by the base package.
   If an Accept header is present, these types MUST be included, in
   additional to any other types supported by the client.

3.6 Notifier Processing of SUBSCRIBE Requests

   All subscriptions for resourcce lists SHOULD be authenticated.  The
   use of the SIP HTTP Digest mechanism [1] over TLS is RECOMMENDED.

   Once authenticated, the subscription is authorized.  Typically, each
   resource list is associated with a particular user (the one who
   created it and manages the set of elements in it), and only that user
   will be allowed to subscribe.  Of course, there may be exceptions to
   this rule, and a notifier MAY use any authorization policy it

3.7 Notifier Generation of NOTIFY Requests

   This specification leaves the choice about how and when to generate
   NOTIFY requests at the discretion of the implementor.  One of the
   value propositions of the RLS is the means by which it aggregates,
   rate limits, or optimizes the way in which notifications are

   As a baseline behavior, if the RLS acts as a subscriber to determine
   the state of the resources on the resource list, it MAY generate a
   NOTIFY to the RLS subscriber whenever it receives a NOTIFY about a
   state change in one or more resources.  The body of the NOTIFY would
   merely be copied from that NOTIFY into the NOTIFY sent by the RLS to
   the subscriber.

   If a subscription to a resource is terminated by the notifier and the
   RLS is unable to re-establish the subscription by the recovery
   mechanisms described in SIP Events [2], that resource is still
   present in resource list NOTIFY messages as appropriate.  The
   "Subscription-State" body-header is set to "terminated", and the
   "reason" parameter is copied from the NOTIFY received from the

   If a SUBSCRIBE to a resource is refused with a response code that
   cannot be recovered from, that resource is still present in resource

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   list NOTIFY messages as appropriate.  The resource will be reported
   with a a "Subscription-State" value of "terminated," and a "reason"
   parmeter of "rejected".

   When the first SUBSCRIBE message for a particular subscription is
   received by a resource list notifier, the notifier will often not
   know state information for all of the resources specified by the
   resource list.  The NOTIFY message triggered from the initial
   SUBSCRIBE message will contain a multipart/mixed message, with one
   section for each resource in the list.  Sections corrsponding to
   resources for which no state information is yet available will
   contain zero-byte bodies.  The Resource-URI header will be populated,
   and the Subscription-State header will be set to "unknown".

   Sections corresponding to resources for which the resource list
   notifier does have state SHOULD be populated with correct data
   (subject, of course, to local policy decisions).  This will often
   occur if the resource list server is colocated with the server for
   one or more of the resources specified on the list.

   Immediate notifications triggered as a result of subsequent SUBSCRIBE
   messages SHOULD result in the full state of all resources to be
   present in the NOTIFY.  This allows the subscriber to refresh their
   state, and to recover from lost notifications.

   Note that a consequence of the way in which resource list
   subscriptions work, polling of resource state will often not be
   particularly useful.  While such polls will retrieve the resource
   list (and potentially even some of the states if a resource on the
   list is colocated with the resource list server), they will often not
   contain state for some or all of the resources on the list.

3.8 Subscriber Processing of NOTIFY Requests

   The SIP Events framework expects packages to specify how a subscriber
   processes NOTIFY requests in any package specific ways, and in
   particular, how it uses the NOTIFY requests to contruct a coherent
   view of the state of the subscribed resource.

   Notifications within this package can convey partial information;
   that is, they can indicate information about a subset of the state
   associated with the subscription.  This means that an explicit
   algorithm needs to be defined in order to construct coherent and
   consistent state.

   For this template package, each section of the multipart/mixed
   document contains a URI which uniquely identifies the resource to
   which that section corresponds.  When a NOTIFY arrives, the recipient

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   of the NOTIFY updates information for each identified resource.
   Information for any resources that are not identified in the NOTIFY
   are not changed, even if they were indicated in previous NOTIFY
   mesages.  See section Section 4.3 for more information.

   Note that the package to which the ".list" template has been applied
   may, in turn, have rules for compositing partial state notification.
   When processing data related to those packages, their rules apply
   (i.e.  the fact that they were reported as part of a collection does
   not change their partial notification semantics).

3.9 Handling of Forked Requests

   Forking makes little sense with this event package, since the whole
   idea is a centralization of the source of notifications.  Therefore,
   a subscriber MUST create just a single dialog as a result of a single
   subscription request, using the techniques described in [2].

3.10 Rate of Notifications

   One potential role of the RLS is to perform rate limitations on
   behalf of the subscriber.  As such, this specification does not
   mandate any particular rate limitation, and rather leaves that to the
   discretion of the implementation.

3.11 State Agents

   Effectively, a resource list server is nothing more than a state
   agent for the resource event type.  A separate event package is
   needed because of the differing body types which can be used in
   NOTIFY, and the need to construct complete state from the partial
   notifications.  Furthermore, there are differing values of the
   subscription interval, differing recommendations on rate limitation,
   and so on.

   The usage of the RLS does introduce some security considerations.
   The end user is no longer the direct subscriber to the state of the
   resource.  If the notifier for the resource demands end-to-end
   authentication, the RLS will need to be provided appropriate
   credentials to access those resources (e.g.  shared secrets for
   Digest authentication).  This requires a certain level of trust
   between the user and their RLS.  This specification does not describe
   any particular means of providing such credentials to the RLS (such
   as uploading a shared secret).  However, any such upload mechanism
   MUST ensure privacy of the key data; using HTTPS to fill out a form
   is a reasonable method.

   If the notifier for the resource is using a transitive trust model to

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   validate the subscriber, then this works well with the RLS concept.
   The RLS would authenticate the subscriber, and then MAY use the SIP
   extensions for network asserted identity [3][4] to provide an
   authenticated identity to the PA.

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4. Using multipart/mixed to Convey Aggregate State

   In order to convey the state of multiple resources, the list template
   package uses the "multipart/mixed" mime type.  The syntax for
   multipart/mixed is defined in MIME Part 1 [6].

   Because the document itself must contain several pieces of
   information that aren't conveyed by default, multipart/mixed bodies
   used for delivering collections of state have a few additional
   requirements beyond those describe in MIME.

4.1 Preamble Headers

   The preamble section of a multipart/mixed body used in a list
   notification MUST contain two headers.  Note that these headers
   follow the same syntax rules as defined for headers in SIP [1], with
   the distinction that the list of headers is not required to end with

   The first mandatory preamble header is "Version", which contains a
   number from 0 to 2^32-1.  This version number MUST be 0 for the first
   NOTIFY message sent within a subscription (typically in response to a
   SUBSCRIBE request), and MUST increase by exactly one for each
   subsequent NOTIFY sent within a subscription.

   The second mandatory preamble header is "State".  The "State" header
   indicates whether the NOTIFY message contains one section for each
   resource in the collection.  If it does, the value of the header is
   "full"; otherwise, it is "partial".  Note that the first NOTIFY sent
   in a subscription MUST contain full state, as must the first NOTIFY
   sent after receipt of a SUBSCRIBE request for the subscription.

4.2 Body-Part Headers

   Each body part of mime/multipart documents contains zero or more
   headers that convey information about the contents of that section.
   When used in list templates, each body part also MUST contain two
   addtional headers.

   The first mandatory body-part header is "Resource-URI".  This header
   contains the URI that uniquely identifies the resource whose state is
   contained in the body part.  Note that "Resource-URI" might also
   contain a name-addr syntax; this can be used to convey a human-
   readable description of the resource that is identified by the URI in
   the "Resource-URI" header (if the RLS has such a description).

   The second mandatory body-part header is "Subscription-State".  This
   header contains the "Subscription-State" for the individual resource

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   defined in this body-part.  The syntax and meaning for this header
   are defined in SIP Events [2].  In a resource list, however, the
   "expires" and "retry-after" parameters have no semantics associated
   with them, and the "reason" parameter is included for informational
   purposes only.  That is, the subscriber to a resource list does not
   take action based on the "reason" parameter in a body-part
   "Subscrtiption-State" header.

   In the simplest case (i.e.  the RLS issues literal SUBSCRIBE requests
   and receives literal NOTIFY requests), an RLS can copy the
   "Subscription-State" header from the NOTIFY received for that
   resource into the body-part headers.  The RLS MAY remove the
   "expires" and "retry-after" parameters from the "Subscription-State"
   header when it does so.

4.3 Constructing Coherent Resource State

   The resource list subscriber maintains a table for each resource
   list.  The table contains a row for each resource in the resource
   list.  Each row is indexed by the URI for that resource.  That URI is
   obtained from the "Resource-URI" body-part header.  The contents of
   each row contain the state of that resource as conveyed in the
   resource document.

   For resources that provide versioning information (which is mandated
   by [2] for any formats that allow partial notification), each row
   also contains a version number.  The version number of the row is
   initialized with the version specified in the first document
   received, as defined by the corrsponding event package.

   Each time a new document for a resource is received, the value of the
   local version number is compared to the version number of the new
   document.  If the value in the new document is one higher than the
   local version number, the local version number is increased by one,
   and the document is processed.  If the value in the document is more
   than one higher than the local version number, the local version
   number is set to the value in the new document, the document is
   processed, and the .list subscriber SHOULD generate a refresh request
   to trigger a full state notification.  If the value in the document
   is less than or equal to the local version, the document is discarded
   without processing.

   The processing of the resource list notification depends on whether
   it contains full or partial state.  If it contains full state,
   indicated by the value of the preable header "State", the contents of
   the resource-list table are flushed.  They are repopulated from the
   document.  A new row in the table is created for each "resource"

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   If the resource list document contains partial state, as indicated by
   the value of the preable header "State", the document is used to
   update the table.  For each resource listed in the document, the
   subscriber checks to see whether a row exists for that resource.
   This check is done by comparing the Resource-URI value with the URI
   associated with the row.  If the resource doesn't exist in the table,
   a row is added, and its state is set to the information from that
   "resource" element.  If the resource does exist, its state is updated
   to be the information from that "resource" element.  If a row is
   updated or created such that its state is now "terminated," that
   entry MAY be removed from the table at any time.

4.4 Syntax

   This section uses ABNF to define the additional syntactic elements
   required by this document.  It uses elements from the base SIP
   specification [1], the SIP Events document [2], and MIME [6].

   Preamble-Headers =  ( Version CRLF State *CRLF ) /
                       ( State CRLF Version *CRLF )

   Version          =  "Version" HCOLON 1*DIGIT

   State            =  "State" HCOLON ( "full" / "partial" )

   Bodypart-Headers =  * (   Resource-URI
                          / Subscription-State
                          / content
                          / description
                          / encoding
                          / id
                         ) CRLF

   Resource-URI     =  "Resource-URI" HCOLON ( name-addr / addr-spec )

   Further, this document defines an additional substate-value of
   "unknown" for the "Subscription-State" header.

4.5 Examples

   TBD: Add examples.

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5. Security Considerations

   This package does introduce some security considerations, which are
   discussed in Section Section 3.11.

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6. IANA Considerations

   This document defines a new Template Event Package, as described in
   [2].  The information necessary to register this Template Event
   Package is given in section Section 3.1.

   OPEN ISSUE: Do we need to register the headers we define for the
      preamble and body parts? It's not clear where we would do so.  Do
      we need to create a new registry?

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7. Open Issues

   o  Technically, MIME [6] talks about multipart/mixed representing
      ordered body parts, while multipart/parallel is used for unordered
      body parts.  The delivery of state of a collection of resources
      seems unordered; should we be using multipart/parallel instead of

   o  Technically, MIME [6] says that the preamble portion of multitype
      bodies is ignored.  However, this definition of multitype bodies
      was aimed more at mail delivery than use in other contexts.  We
      have the need to convey body-level information that applies to all
      parts of a multitype document, and have elected to use the
      preamble for this purpose.  Are there any problems with doing so?

   o  Similaraly,  MIME [6] also says that any headers appearing in body
      parts other than Content-* cannot be depended on.  The rationale
      for this restriction, however, is that intervening mail gateways
      may discard such headers.  Of course, this reasoning does not have
      any chance of applying to the use described in this document, so
      we reason that such headers *can* be relied upon in this context.
      Are there any flaws in this reasoning?

   o  Do the new headers require any additional IANA action? (section
      Section 6)

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   [1]  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.

   [2]  Roach, A., "Session Initiation Protocol (SIP)-Specific Event
        Notification", RFC 3265, June 2002.

   [3]  Watson, M., Peterson, J. and C. Jennings, "Private Extensions to
        the Session Initiation Protocol (SIP) for Asserted  Identity
        within Trusted Networks", draft-ietf-sip-asserted-identity-02
        (work in progress), August 2002.

   [4]  Peterson, J., "Enhancements for Authenticated Identity
        Management in the Session  Initiation Protocol (SIP)", draft-
        peterson-sip-identity-01 (work in progress), July 2002.

   [5]  Rosenberg, J., "Session Initiation Protocol (SIP) Extensions for
        Presence", draft-ietf-simple-presence-07 (work in progress), May

   [6]  Borenstein, N. and N. Freed, "MIME (Multipurpose Internet Mail
        Extensions) Part One: Mechanisms for Specifying and Describing
        the Format of Internet Message Bodies", RFC 1521, September

Authors' Addresses

   Adam Roach
   5100 Tennyson Pkwy
   Suite 1200
   Plano, TX  75024


   Jonathan Rosenberg
   72 Eagle Rock Ave.
   First Floor
   East Hanover, NJ  07936


Roach, et al.            Expires April 25, 2003                [Page 18]

Internet-Draft            List Event Template               October 2002

   Ben Campbell
   5100 Tennyson Pkwy
   Suite 1200
   Plano, TX  75024


Roach, et al.            Expires April 25, 2003                [Page 19]

Internet-Draft            List Event Template               October 2002

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