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Versions: (draft-ietf-simple-presencelist-package) 00 01 02 03 04 05 06 07 RFC 4662

Network Working Group                                        A. B. Roach
Internet-Draft                                              J. Rosenberg
Expires: August 18, 2003                                     B. Campbell
                                                             dynamicsoft
                                                       February 17, 2003


  A Session Initiation Protocol (SIP) Event Notification Extension for
                              Collections
                    draft-ietf-simple-event-list-00

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
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at http://
   www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on August 18, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   This document presents an extension to the Session Initiation
   Protocol (SIP)-Specific Event Notification mechanism for subscribing
   to a homogenous collection of event packages.  Instead of the
   subscriber sending a SUBSCRIBE for each resource individually, the
   subscriber can subscribe to an entire collection, and then receive
   notifications when the state of any of the resources in the
   collection changes.





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Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Overview of Operation  . . . . . . . . . . . . . . . . . . .   4
   3.   Operation of List Subscriptions  . . . . . . . . . . . . . .   6
   3.1  Negotiation of Support for Resource Lists  . . . . . . . . .   6
   3.2  Event Header Parameters  . . . . . . . . . . . . . . . . . .   7
   3.3  SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . . . .   7
   3.4  Subscription Duration  . . . . . . . . . . . . . . . . . . .   7
   3.5  NOTIFY Bodies  . . . . . . . . . . . . . . . . . . . . . . .   7
   3.6  Notifier Processing of SUBSCRIBE Requests  . . . . . . . . .   7
   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/related to Convey Aggregate State  . . . . .  12
   4.1  XML Syntax . . . . . . . . . . . . . . . . . . . . . . . . .  12
   4.2  List Attributes  . . . . . . . . . . . . . . . . . . . . . .  13
   4.3  Resource Attributes  . . . . . . . . . . . . . . . . . . . .  13
   4.4  Instance Attributes  . . . . . . . . . . . . . . . . . . . .  14
   4.5  Constructing Coherent Resource State . . . . . . . . . . . .  15
   5.   Example  . . . . . . . . . . . . . . . . . . . . . . . . . .  17
   6.   Security Considerations  . . . . . . . . . . . . . . . . . .  30
   6.1  Authentication . . . . . . . . . . . . . . . . . . . . . . .  30
   6.2  Risks of Improper Aggregation  . . . . . . . . . . . . . . .  30
   6.3  Signing and Sealing  . . . . . . . . . . . . . . . . . . . .  30
   7.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .  32
   7.1  New SIP Option Tag: eventlist  . . . . . . . . . . . . . . .  32
   7.2  New MIME type for Resource List Meta-Information . . . . . .  32
   8.   Open Issues  . . . . . . . . . . . . . . . . . . . . . . . .  34
        Normative References . . . . . . . . . . . . . . . . . . . .  35
        Non-Normative References . . . . . . . . . . . . . . . . . .  36
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  36
        Full Copyright Statement . . . . . . . . . . . . . . . . . .  38
















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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 in which bandwidth is limited, such as wireless
   networks, subscribing to each resource individually is problematic.
   The specific problems are:

   o  Doing so 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 an extension that
   allows for requesting and conveying notifications for collections of
   resources.  A resource list is identified by a URI and it represents
   a list of zero or more URIs.  Each of these URIs is an identifier for
   an individual resource for which the subscriber wants to receive
   information.  In many cases, the URI will be a SIP URI [1]; however,
   the use of other schemes (such as pres:) is also forseen.

   The notifier for the collection is called a "resource list server",
   or RLS.  In order to determine the state of the entire list, the RLS
   will act as if it has typically generated a subscription to each
   resource in the list.

   The resource list is not restricted to be inside the domain of the
   subscriber.  Similarly, the resources in the list are not
   contstrained to be in the domain of the resource list server.




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2. Overview of Operation

   This section provides an overview of the typical mode of operation of
   this extension.  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 list could be manipulated through a web page, through a voice
   response system, or through some other protocol.  The specific means
   by which the list is created and maintained is outside of the scope
   of this specification.

   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 could subscribe
   to any non-local resources specified by the list resource.  (Note
   that such a subscription will typically be SIP subscription; however,
   any mechanism for determining such information may be employed).

   Since the RLS is acting on behalf of the user, it will need a
   mechanism for authenticating the user so that appropriate policy can
   be applied.  In the simplest case, the user can provide credentials
   to the RLS ahead of time; this requires a trust relationship between
   the user and RLS.  Additional approaches can be formulated using the
   mechanisms defined in "Private Extensions to the Session Initiation
   Protocol (SIP) for Asserted Identity within Trusted Networks" [6] and
   "Enhancements for Authenticated Identity Management in the Session
   Initiation Protocol (SIP)" [7].

   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.

   The list notifications contain a body of type multipart/related.  The
   root section of the multipart/related content is an XML document that
   provides meta-information about each resource present in the list.



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   The remaining sections contain the actual state information for each
   resource.


            Joe               RLS             User A            User B
             |                 |                 |                 |
             |(1) SUBSCRIBE    |                 |                 |
             |---------------->|                 |                 |
             |(2) 200 OK       |                 |                 |
             |<----------------|                 |                 |
             |(3) NOTIFY       |                 |                 |
             |<----------------|                 |                 |
             |(4) 200 OK       |                 |                 |
             |---------------->|                 |                 |
             |                 |(5) SUBSCRIBE a  |                 |
             |                 |---------------->|                 |
             |                 |(6) SUBSCRIBE b  |                 |
             |                 |---------------------------------->|
             |                 |(7) 200 OK       |                 |
             |                 |<----------------|                 |
             |                 |(8) 200 OK       |                 |
             |                 |<----------------------------------|
             |                 |(9) NOTIFY       |                 |
             |                 |<----------------|                 |
             |                 |(10) 200 OK      |                 |
             |                 |---------------->|                 |
             |(11) NOTIFY      |                 |                 |
             | a's state       |                 |                 |
             |<----------------|                 |                 |
             |(12) 200 OK      |                 |                 |
             |---------------->|                 |                 |
             |                 |(13) NOTIFY      |                 |
             |                 |<----------------------------------|
             |                 |(14) 200 OK      |                 |
             |                 |---------------------------------->|
             |(15) NOTIFY      |                 |                 |
             | b's state       |                 |                 |
             |<----------------|                 |                 |
             |(16) 200 OK      |                 |                 |
             |---------------->|                 |                 |

   As an example, consider a resource list with two resources,
   sip:userA@a.com and sip:userB@b.com.  A typical flow for a
   subscription to this resource list is shown above.







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3. Operation of List Subscriptions

   A list subscription acts, in many ways, like an event template
   package.  In particular, any single list subscription MUST be
   homogenous with respect to the underlying event package.  In other
   words, a single list subscription cannot contain subscriptions to
   different kinds of event packages.

   The key difference between a list subscription and templates in
   general is that support for list subscriptions indicates support for
   arbitrary nesting of list subscriptions.  In other words, elements
   within the list may be atomic elements, or they may be lists
   themselves.

   The consequence of this is that subscription to a URI that represents
   a list actually results in subscription to a tree of resources.  The
   leaf nodes of this tree are subscriptions of the event type given in
   the "Event" header; all other nodes in the tree are list
   subscriptions that are serviced as described in this section and its
   subsections.

   It is important to keep in mind that, although the overall system is
   specified in terms that imply that such subscriptions are literal SIP
   subscriptions on separate servers, servers are free to implement in
   any way that provides equivalent functionality as viewed from the
   interface towards the subscriber.  In particular, it is expected that
   notifiers for list subscriptions will often have direct information
   about some of the resources in the list; they will presumably not
   send SUBSCRIBE requests to themselves in order to service such list
   subscriptions.

3.1 Negotiation of Support for Resource Lists

   This specification uses the SIP option tag mechanism for negotiating
   support of the extension defined herein.  Refer to RFC3261 [1] for
   the normative description of processing of the "Supported" and
   "Require" headers and the 421 (Extension Required) response code.

   Any client that supports the event list extension will include an
   option tag of "eventlist" in a "Supported" header of every SUBSCRIBE
   message for a subscription for which it is willing to process a list.
   If the subscription is made to a URI that represents a list, the RLS
   will include "eventlist" in a "Require" header of the response to the
   SUBSCRIBE, and in all NOTIFY messages within that subscription.  Note
   that including "eventlist" in a "Require" header in a SUBSCRIBE
   request serves no purpose, and is consequently NOT RECOMMENDED.

   As described in RFC3265 [2], a subscription to a particular state of



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   a resource is identified by the Request-URI and the event package
   used.  Because it is quite reasonable for an RLS to contain a
   resource that is inherantly a list (e.g.
   "sip:buddylist@example.com"), it is perfectly reasonable to expect
   that RLSes will return a 421 (Extension Required) response code if
   the "eventlist" option tag is not indicated in a request to subscribe
   to that resource.  Because of the forgoing situation, this
   specification relaxes the "NOT RECOMMENED" provision described in
   RFC3261 [1], section 8.2.4.

3.2 Event Header Parameters

   This specification does not define any parameters on the Event
   header.  Any parameters that are present on the Event header,
   however, are applied to the underlying leaf subscriptions.  If a new
   subscription is generated to learn about this resource state, this
   generally means that such parameters are propagated to any SUBSCRIBE
   messages generated for the resources in the list.

3.3 SUBSCRIBE Bodies

   The SUBSCRIBE message MAY contain a body whose purpose is to define
   filters on the individual leaf subscriptions.  If a new subscription
   is generated to learn about this resource state, this means that such
   bodies are propagated to any SUBSCRIBE messages generated for the
   resources in the list.

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.

3.5 NOTIFY Bodies

   An implementation compliant to this specification MUST support the
   multipart/related and application/rlmi+xml MIME types.  These types
   MUST be included in an Accept header sent in SUBSCRIBE message, in
   addition to any other types supported by the client.

3.6 Notifier Processing of SUBSCRIBE Requests

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




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   Once the subscriber is authenticated, the RLS performs authorization
   per its local policy.  In many cases, 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, this mode of operation is not inherent in the
   use of resource lists, and a notifier can use any authorization
   policy it chooses.

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
   generated.

   See Section 4 for a definition of the syntax used to convey resource
   lists.  For the purposes of the following discussion, it is important
   to know that the overall list contains one or more resources, and
   that the resources contains one or more instances of the resource.
   Each instance of the resource has a state associated with it
   (pending, active, or terminating), representing the state of the
   subscription.

   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.

   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 "state"
   attribute is set to "terminated", and the "reason" attribute is
   copied from the "reason" parameter from the "Subscription-State" in
   the NOTIFY received from the resource.

   If a SUBSCRIBE to a resource is refused with a response code that
   cannot be recovered from, that resource is still present in resource
   list NOTIFY messages as appropriate.  The resource will be reported
   with a a "state" of "terminated," and a "reason" parmeter of
   "rejected".

   Note that the forgoing explanation is given in term of a back-end
   subscription to resources in the list.  In practice, such
   subscriptions may not actually exist.  In these cases, the RLS sets
   the "state" and "reason" attributes in such as was as would be
   consistent with such back-end subscriptions.



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   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.  For any resource for which state information is not
   known, the corresponding "uri" attribute will be set appropriately,
   and no <instance> elements will be present for the resource.

   For an initial notification, sections corresponding to resources for
   which the resource list notifier does have state will be populated
   with appropriate 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 full meta-information about the list of
   resources.  The RLS will also include state information for all
   resources in the list for which the RLS has state.  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.

   Because the notifications delivered by this mechanism have a tendency
   to be large, implementors are directed towards "A Mechanism for
   Content Indirection in Session Initiation Protocol (SIP) Messages"
   [8].

3.8 Subscriber Processing of NOTIFY Requests

   Notifications for a resource list can convey information about a
   subset of the list elements.  This means that an explicit algorithm
   needs to be defined in order to construct coherent and consistent
   state.

   The XML document present in the root of the multipart/related
   document contains a <resource> element for each resource in the list.
   Each <resource> element contains a URI which uniquely identifies the
   resource to which that section corresponds.  When a NOTIFY arrives,
   it can contain full or partial state (as indicate by the "fullState"
   attribute of the top-level <list> element).  If full state is
   indicated, then the recipient replaces all state associated with the
   list with the entities in the NOTIFY body.  If full state is not



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   indicated, the recipient 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 4.5 for more
   information.

   Note that the underlying event package may have its own 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 subscriptions to event lists, 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
   RFC 3265[2].

   If the resource list server generates subscriptions to packages that
   allow forking, and multiple subscriptions are established as the
   result of sending a single SUBSCRIBE message, the RLS maintains both
   subscriptions (as described in [2]), and includes the resource state
   information for all the subscriptions.  This will result in having
   multiple <instance> elements in the corresponding <resource> element
   in the root part of the multipart/related body.  See section Section
   4 and its subsections for details.

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.

   The usage of an 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



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   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
   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 (see [6] and [7]) to provide
   an authenticated identity to any downstream servers.  It is even
   conceivable that the subscriber may provide an authenticated ID in
   its original subscribe request for use by the RLS for the dual
   purpose of local authentication and use in any generated SUBSCRIBE
   messages.





































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4. Using multipart/related to Convey Aggregate State

   In order to convey the state of multiple resources, the list template
   package uses the "multipart/related" mime type.  The syntax for
   multipart/related is defined in "The MIME Multipart/Related Content-
   type" [4].

4.1 XML Syntax

   The root document of the multipart/related body is always a Resource
   List Meta-Information (RLMI) document.  It is of type "application/
   rlmi+xml".  This document containes the metadata for the resources
   contained in the notification.  The schema for this XML document is
   given below.


   <?xml version="1.0" encoding="UTF-8"?>
   <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
     <xs:element name="instance">
       <xs:attribute name="id" type="xs:string use="required""/>
       <xs:attribute name="state" type="xs:string" use="required"/>
       <xs:attribute name="reason" type="xs:string" use="optional"/>
       <xs:attribute name="cid" type="xs:string use="optional""/>
     </xs:element>
     <xs:element name="resource">
       <xs:complexType>
         <xs:element ref="instance" minOccurs="0" maxOccurs="unbounded"/>
         <xs:attribute name="uri" type="xs:string" use="required"/>
         <xs:attribute name="name" type="xs:string" use="optional"/>
       </xs:complexType>
     </xs:element>
     <xs:element name="list">
       <xs:complexType>
         <xs:element ref="resource" minOccurs="0" maxOccurs="unbounded"/>
         <xs:attribute name="uri" type="xs:string" use="required"/>
         <xs:attribute name="version" type="xs:unsignedInt" use="required"/>
         <xs:attribute name="fullState" type="xs:boolean" use="required"/>
         <xs:attribute name="name" type="xs:string" use="optional"/>
       </xs:complexType>
     </xs:element>
   </xs:schema>


   An example of a document formatted using this schema follows.







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   <list uri="sip:adam-friends@lists.example.com" version="7"
         name="Buddy List" fullState="true">
     <resource uri="sip:bob@example.com" name="Bob Smith">
       <instance id="juwigmtboe" state="active" cid="12345.aaa@example.com"/>
     </resource>
     <resource uri="sip:dave@example.com" name="Dave Jones">
       <instance id="hqzsuxtfyq" state="active" cid="12345.aab@example.com"/>
     </resource>
     <resource uri="sip:jim@example.com" name="Jim">
       <instance id="oflzxqzuvg" state="terminated" reason="rejected" />
     </resource>
     <resource uri="sip:ed@example.com" name="Ed">
       <instance id="grqhzsppxb" state="pending"/>
     </resource>
   </list>


4.2 List Attributes

   The <list> element present in a list notification MUST contain three
   attributes.

   The first mandatory <list> attribute is "uri", which contains the uri
   that corresponds to the list.  Typically, this is the URI to which
   the SUBSCRIBE request was sent.

   The second mandatory <list> attribute 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 third mandatory attribute is "fullState".  The "fullState"
   attribute indicates whether the NOTIFY message contains information
   for every resource in the collection.  If it does, the value of the
   attribute is "true" (or "1"); otherwise, it is "false" (or "0").
   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.

   The optional "name" attribute can contain a human readable
   description or name for the resource list.  This attribute is
   somewhat analogous to the "Display Name" present in the SIP name-addr
   element.

4.3 Resource Attributes

   The resource list contains one <resource> element for each resource



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   being reported in the notification.  These resource elements contain
   attributes that identify meta-data assocated with each resource.

   The "uri" attribute identifies the resource to which the <resource>
   element corresponds.  Typically, this will be a SIP URI which, if
   subscribed to, would return the state of the resource.  This
   attribute must be present.

   The optional "name" attribute can contain a human readable
   description or name for the resource.  This attribute is somewhat
   analogous to the "Display Name" present in the SIP name-addr element.

4.4 Instance Attributes

   Each resource element contains one or more instance elements.  These
   instance elements are used to represent a single notifier for the
   resource.  For event packages that allow forking, multiple
   subscriptions corresponding to a single resource are represented by
   including multiple instance elements in the corresponding resource
   element.  For subscriptions in which forking does not occur, only one
   instance will be present in the resource.

   The "id" attribute contains an opaque string used to uniquely
   identify the instance of the resource.  The "id" attribute is unique
   only within the context of a resource.  Composition of this string is
   an implementation decision.  One viable method for constructing the
   instance id would be copying the "From:" header "tag" parameter from
   the NOTIFY message received from the instance of the resource.  Any
   other mechanism for generating this string is equally valid, as long
   as uniqueness within the resource is assured.

   The "state" attribute contains the subscription state for the
   identified instance of the resource.  This attribute contains one of
   the values "active", "pending", or "terminated".  The meanings for
   these values are as defined for the "Subscription-State" header in
   SIP Events [2].

   If the "state" attribute indicates "terminated", then a "reason"
   attribute MUST also be present.  This "reason" attribute has the same
   values and meanings as given for the "reason" parameter on the
   "Subscription-State" header in SIP Events [2].  Note that the reason
   is included for informational purposes; the list subscriber is not
   expected to take any automated actions based on the reason value.

   Finally, the "cid" attribute, which must be present if the "state"
   attribute is "active", identifies the section within the multipart/
   related body that contains the actual resource state.  This state is
   expressed in the content type defined by the event package for



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   conveying state.  This cid is Content-ID the corresponding section.

   Note that the expiration durations of the underlying subscriptions
   (if any) are not propagated into the aggregated state in any way.

4.5 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 "uri" attribute on each <resource> element.  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 resource state 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.
   This value is used when comparing versions of partial notifications
   for a resource.

   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 <list> attribute "fullState", 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" element.

   First, a check is made to ensure that no list notifications have been
   lost.  The value of the local version number (the "version" attribute
   of the <list> element) is compared to the version number of the new
   document.

   o  If the value in the new document is exactly one higher than the
      local version number, the local version number is increased by
      one, and the document is processed, as described below.

   o  If the version 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, and the document is processed as described
      below.  Further, if the notification does not contain full state
      (as indicated by the "fullState" attribute of the <list> element),
      the list subscriber SHOULD generate a refresh request to trigger a
      full state notification.

   o  If the version in the document is less than or equal to the local
      version, the document is discarded without any further processing.



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   If the resource list document contains partial state, the
   notification 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, as described in the definition of the event
   package.  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.








































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5. Example

   This section gives an example callflow.  It is not normative.  If a
   conflict arises between this callflow and the normative behavior
   described in this or any other document, the normative descriptions
   are to be followed.

   In this particular example, we request a subscription to a nested
   presence list.  The subscriber's address-of-record is
   "sip:adam@example.com", and the name of the nested list resource that
   we are subscribing to is called "sip:adam-buddies@pres.example.com".
   The underlying event package is "presence", described by [5].

   1.   We initate the subscription by sending a SUBSCRIBE message to
        our local RLS.  (There is no reason that the RLS we contact has
        to be in our domain, of course).  Note that we must advertise
        support for application/rlmi+xml and multipart/related because
        we support the eventlist extension, and we must advertise
        application/cpim-pidf+xml because we are requesting a
        subscription to a list.

   Terminal -> Local RLS

   SUBSCRIBE sip:adam-buddies@pres.example.com SIP/2.0
   Via: SIP/2.0/TCP terminal.example.com;branch=z9hG4bKwYb6QREiCL
   Max-Forwards: 70
   To: <sip:adam-buddies@pres.example.com>
   From: <sip:adam@example.com>;tag=ie4hbb8t
   Call-ID: cdB34qLToC@terminal.example.com
   CSeq: 322723822 SUBSCRIBE
   Contact: <sip:terminal.example.com>
   Event: presence
   Expires: 7200
   Supported: eventlist
   Accept: application/cpim-pidf+xml
   Accept: application/rlmi+xml
   Accept: multipart/related
   Accept: multipart/signed
   Accept: multipart/encrypted
   Content-Length: 0



   2.   The Local RLS completes the SUBSCRIBE transaction.  Note that
        authentication and authorization would normally take place at
        this point in the callflow.  Those steps are omitted for
        brevity.




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   Local RLS -> Terminal

   SIP/2.0 200 OK
   Via: SIP/2.0/TCP terminal.example.com;branch=z9hG4bKwYb6QREiCL
   To: <sip:adam-buddies@pres.example.com>;tag=zpNctbZq
   From: <sip:adam@example.com>;tag=ie4hbb8t
   Call-ID: cdB34qLToC@terminal.example.com
   CSeq: 322723822 SUBSCRIBE
   Contact: <sip:pres.example.com>
   Expires: 7200
   Require: eventlist
   Content-Length: 0



   3.   As is required by SIP Events [2], the RLS sends a NOTIFY
        immediately upon accepting the subscription.  In this example,
        we are asserting that the local RLS is also an authority for
        presence information for the users in the "example.com" domain.
        The NOTIFY contains an RLMI document describing the entire buddy
        list (initial notifies require full state), as well as presence
        information for the users about which it already knows.  Note
        that, since the RLS has not yet retrieved information for some
        of the entries on the list, those <resource> elements contain no
        <instance> elements.

   Local RLS -> Terminal

   NOTIFY sip:terminal.example.com SIP/2.0
   Via: SIP/2.0/TCP pres.example.com;branch=z9hG4bKMgRenTETmm
   Max-Forwards: 70
   From: <sip:adam-buddies@pres.example.com>;tag=zpNctbZq
   To: <sip:adam@example.com>;tag=ie4hbb8t
   Call-ID: cdB34qLToC@terminal.example.com
   CSeq: 997935768 NOTIFY
   Contact: <sip:pres.example.com>
   Event: presence
   Subscription-State: active;expires=7200
   Require: eventlist
   Content-Type: multipart/related;type="application/rlmi+xml";
       start="<nXYxAE@pres.example.com>";boundary="50UBfW7LSCVLtggUPe5z"
   Content-Length: 1560

   --50UBfW7LSCVLtggUPe5z
   Content-Transfer-Encoding: 8bit
   Content-ID: <nXYxAE@pres.example.com>
   Content-Type: application/rlmi+xml;charset="UTF-8"




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   <?xml version="1.0" encoding="UTF-8"?>
   <list uri="sip:adam-friends@pres.example.com" version="1"
         name="Buddy List at COM" fullState="true">
     <resource uri="sip:bob@example.com" name="Bob Smith">
       <instance id="juwigmtboe" state="active" cid="bUZBsM@pres.example.com"/>
     </resource>
     <resource uri="sip:dave@example.com" name="Dave Jones">
       <instance id="hqzsuxtfyq" state="active" cid="ZvSvkz@pres.example.com"/>
     </resource>
     <resource uri="sip:ed@example.net" name="Ed at NET" />
     <resource uri="sip:adam-friends@example.org" name="My Friends at ORG" />
   </list>

   --50UBfW7LSCVLtggUPe5z
   Content-Transfer-Encoding: 8bit
   Content-ID: <bUZBsM@pres.example.com>
   Content-Type: application/cpim-pidf+xml;charset="UTF-8"

   <?xml version="1.0" encoding="UTF-8"?>
   <presence xmlns="urn:ietf:params:xml:ns:cpim-pidf"
       entity="sip:bob@example.com">
     <tuple id="sg89ae">
       <status>
         <basic>open</basic>
       </status>
       <contact priority="1.0">sip:bob@example.com</contact>
     </tuple>
   </presence>

   --50UBfW7LSCVLtggUPe5z
   Content-Transfer-Encoding: 8bit
   Content-ID: <ZvSvkz@pres.example.com>
   Content-Type: application/cpim-pidf+xml;charset="UTF-8"

   <?xml version="1.0" encoding="UTF-8"?>
   <presence xmlns="urn:ietf:params:xml:ns:cpim-pidf"
       entity="sip:dave@example.com">
     <tuple id="slie74">
       <status>
         <basic>closed</basic>
       </status>
     </tuple>
   </presence>

   --50UBfW7LSCVLtggUPe5z--






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   4.   The terminal completes the transaction.

   Terminal -> Local RLS

   SIP/2.0 200 OK
   Via: SIP/2.0/TCP pres.example.com;branch=z9hG4bKMgRenTETmm
   From: <sip:adam-buddies@pres.example.com>;tag=zpNctbZq
   To: <sip:adam@example.com>;tag=ie4hbb8t
   Call-ID: cdB34qLToC@terminal.example.com
   CSeq: 997935768 NOTIFY
   Contact: <sip:terminal.example.com>
   Content-Length: 0



   5.   In order to service the subscription, the local RLS subscribes
        to the state of the resources.  In this step, the RLS attempts
        to subscribe to the presence state of the resource
        "sip:ed@example.com".  Since the local RLS knows how to receive
        notifications for list subscriptions, it includes the
        "Supported: eventlist" header in its request.  Although the
        linkage between this subscription and the one sent by the
        terminal is left up to the application, this message
        demonstrates some reasonable behavior by including "Accept"
        headers for all of the body types it knows the subscriber
        (Terminal) supports.  This is safe to do, since the local RLS
        will only pass these formats through to the subscriber, and does
        not need to actually understand them.























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   Local RLS -> Presence Server in example.net

   SUBSCRIBE sip:ed@example.net SIP/2.0
   Via: SIP/2.0/TCP pres.example.com;branch=z9hG4bKMEyGjdG1LH
   Max-Forwards: 70
   To: <sip:ed@example.net>
   From: <sip:pres.example.com>;tag=aM5icQu9
   Call-ID: Ugwz5ARxNw@pres.example.com
   CSeq: 870936068 SUBSCRIBE
   Contact: <sip:pres.example.com>
   Event: presence
   Expires: 3600
   Supported: eventlist
   Accept: application/cpim-pidf+xml
   Accept: application/rlmi+xml
   Accept: multipart/related
   Accept: multipart/signed
   Accept: multipart/encrypted
   Content-Length: 0



   6.   The Presence Server in example.net completes the SUBSCRIBE
        transaction.  Note that authentication and would normally take
        place at this point in the callflow.  Those steps are omitted
        for brevity.

   Presence Server in example.net -> Local RLS

   SIP/2.0 200 OK
   Via: SIP/2.0/TCP pres.example.com;branch=z9hG4bKMEyGjdG1LH
   To: <sip:ed@example.net>;tag=e45TmHTh
   From: <sip:pres.example.com>;tag=aM5icQu9
   Call-ID: Ugwz5ARxNw@pres.example.com
   CSeq: 870936068 SUBSCRIBE
   Contact: <sip:example.net>
   Event: presence
   Expires: 3600
   Content-Length: 0



   7.   In this example, we assume that the server at example.net
        doesn't have enough authorization information to reject or
        accept our subscription.  The initial notify, therefore,
        contains a "Subscription-State" of "pending".  Presumably, the
        party responsible for accepting or denying authorization for the
        resource is notified of this change; however, those steps are



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        not included in this call flow for brevity.

   Presence Server in example.net -> Local RLS

   NOTIFY sip:pres.example.com SIP/2.0
   Via: SIP/2.0/TCP pres.example.com;branch=z9hG4bKfwpklPxmrW
   Max-Forwards: 70
   From: <sip:ed@example.net>;tag=e45TmHTh
   To: <sip:pres.example.com>;tag=aM5icQu9
   Call-ID: Ugwz5ARxNw@pres.example.com
   CSeq: 1002640632 NOTIFY
   Contact: <sip:example.net>
   Subscription-State: pending;expires=3600
   Event: presence
   Require: eventlist
   Content-Length: 0



   8.   The local RLS completes the NOTIFY transaction.  Note that, at
        this point, the Local RLS has new information to report to the
        subscriber.  Whether it chooses to report the information
        immediately or spool it up for later delivery is completely up
        to the application.  For this example, we assume that the RLS
        will wait for a short period of time before doing so, in order
        to allow the subscriptions it sent out sufficient time to
        provide useful data.

   Local RLS -> Presence Server in example.net

   SIP/2.0 200 OK
   Via: SIP/2.0/TCP pres.example.com;branch=z9hG4bKfwpklPxmrW
   From: <sip:ed@example.net>;tag=e45TmHTh
   To: <sip:pres.example.com>;tag=aM5icQu9
   Call-ID: Ugwz5ARxNw@pres.example.com
   CSeq: 1002640632 NOTIFY
   Contact: <sip:pres.example.com>
   Event: presence
   Content-Length: 0



   9.   The Local RLS subscribes to the state of the other non-local
        resource.







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   Local RLS -> RLS in example.org

   SUBSCRIBE sip:adam-friends@example.org SIP/2.0
   Via: SIP/2.0/TCP pres.example.com;branch=z9hG4bKFSrAF8CZFL
   Max-Forwards: 70
   To: <sip:adam-friends@example.org>
   From: <sip:pres.example.com>;tag=a12eztNf
   Call-ID: kBq5XhtZLN@pres.example.com
   CSeq: 980774491 SUBSCRIBE
   Contact: <sip:pres.example.com>
   Event: presence
   Expires: 3600
   Supported: eventlist
   Accept: application/cpim-pidf+xml
   Accept: application/rlmi+xml
   Accept: multipart/related
   Accept: multipart/signed
   Accept: multipart/encrypted
   Content-Length: 0



   10.  The RLS in example.org completes the SUBSCRIBE transaction.
        Note that authentication and would normally take place at this
        point in the callflow.  Those steps are omitted for brevity.

   RLS in example.org -> Local RLS

   SIP/2.0 200 OK
   Via: SIP/2.0/TCP pres.example.com;branch=z9hG4bKFSrAF8CZFL
   To: <sip:adam-friends@example.org>;tag=JenZ40P3
   From: <sip:pres.example.com>;tag=a12eztNf
   Call-ID: kBq5XhtZLN@pres.example.com
   CSeq: 980774491 SUBSCRIBE
   Contact: <sip:example.org>
   Event: presence
   Expires: 3600
   Content-Length: 0



   11.  In this example, we are asserting that the RLS in example.org is
        also an authority for presence information for the users in the
        "example.org" domain.  The NOTIFY contains an RLMI document
        describing the contained buddy list, as well as presence
        information for those users.  In this particular case, the RLS
        in example.org has chosen to PGP sign [11] the body of the
        NOTIFY message.



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   RLS in example.org -> Local RLS

   NOTIFY sip:pres.example.com SIP/2.0
   Via: SIP/2.0/TCP pres.example.com;branch=z9hG4bKmGL1nyZfQI
   Max-Forwards: 70
   From: <sip:adam-friends@example.org>;tag=JenZ40P3
   To: <sip:pres.example.com>;tag=a12eztNf
   Call-ID: kBq5XhtZLN@pres.example.com
   CSeq: 294444656 NOTIFY
   Contact: <sip:example.org>
   Event: presence
   Subscription-State: pending
   Require: eventlist
   Content-Type: multipart/signed;protocol="application/pgp-signature";
       micalc="pgp-md5";boundary="l3WMZaaL8NpQWGnQ4mlU"
   Content-Length: 2038

   --l3WMZaaL8NpQWGnQ4mlU
   Content-Transfer-Encoding: 8bit
   Content-ID: <ZPvJHL@example.org>
   Content-Type: multipart/related;type="application/rlmi+xml";
       start="<Cvjpeo@example.org>";boundary="tuLLl3lDyPZX0GMr2YOo"

   --tuLLl3lDyPZX0GMr2YOo
   Content-Transfer-Encoding: 8bit
   Content-ID: <Cvjpeo@example.org>
   Content-Type: application/rlmi+xml;charset="UTF-8"

   <?xml version="1.0" encoding="UTF-8"?>
   <list uri="sip:adam-friends@example.org" version="1"
         name="Buddy List at ORG" fullState="true">
     <resource uri="sip:joe@example.org" name="Joe Thomas">
       <instance id="1" state="active" cid="mrEakg@example.org"/>
     </resource>
     <resource uri="sip:mark@example.org" name="Mark Edwards">
       <instance id="1" state="active" cid="KKMDmv@example.org"/>
     </resource>
   </list>

   --tuLLl3lDyPZX0GMr2YOo
   Content-Transfer-Encoding: 8bit
   Content-ID: <mrEakg@example.org>
   Content-Type: application/cpim-pidf+xml;charset="UTF-8"

   <?xml version="1.0" encoding="UTF-8"?>
   <presence xmlns="urn:ietf:params:xml:ns:cpim-pidf"
       entity="sip:joe@example.org">
     <tuple id="7823a4">



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       <status>
         <basic>open</basic>
       </status>
       <contact priority="1.0">sip:joe@example.org</contact>
     </tuple>
   </presence>

   --tuLLl3lDyPZX0GMr2YOo
   Content-Transfer-Encoding: 8bit
   Content-ID: <KKMDmv@example.org>
   Content-Type: application/cpim-pidf+xml;charset="UTF-8"

   <?xml version="1.0" encoding="UTF-8"?>
   <presence xmlns="urn:ietf:params:xml:ns:cpim-pidf"
       entity="sip:mark@example.org">
     <tuple id="398075">
       <status>
         <basic>closed</basic>
       </status>
     </tuple>
   </presence>

   --tuLLl3lDyPZX0GMr2YOo--

   --l3WMZaaL8NpQWGnQ4mlU
   Content-Transfer-Encoding: 8bit
   Content-ID: <K9LB7k@example.org>
   Content-Type: application/pgp-signature

   -----BEGIN PGP MESSAGE-----
   Version: 2.6.2

   iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC//
   jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq
   uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn
   HOxEa44b+EI=
   =ndaj
   -----END PGP MESSAGE-----

   --l3WMZaaL8NpQWGnQ4mlU--



   12.  The Local RLS completes the NOTIFY transaction.







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   Local RLS -> RLS in example.org

   SIP/2.0 200 OK
   Via: SIP/2.0/TCP pres.example.com;branch=z9hG4bKmGL1nyZfQI
   From: <sip:adam-friends@example.org>;tag=JenZ40P3
   To: <sip:pres.example.com>;tag=a12eztNf
   Call-ID: kBq5XhtZLN@pres.example.com
   CSeq: 294444656 NOTIFY
   Contact: <sip:pres.example.com>
   Event: presence
   Content-Length: 0



   13.  At this point, the Local RLS decides it has collected enough
        additional information to warrant sending a new notification to
        the user.  Although sending a full notification would be
        perfectly acceptable, the RLS decides to send a partial
        notification instead.  The RLMI document contains only
        information for the updated resources, as indicated by setting
        the "fullState" parameter to "false".

   Local RLS -> Terminal

   NOTIFY sip:terminal.example.com SIP/2.0
   Via: SIP/2.0/TCP pres.example.com;branch=z9hG4bK4EPlfSFQK1
   Max-Forwards: 70
   From: <sip:adam-buddies@pres.example.com>;tag=zpNctbZq
   To: <sip:adam@example.com>;tag=ie4hbb8t
   Call-ID: cdB34qLToC@terminal.example.com
   CSeq: 997935769 NOTIFY
   Contact: <sip:pres.example.com>
   Event: presence
   Subscription-State: active;expires=7200
   Require: eventlist
   Content-Type: multipart/related;type="application/rlmi+xml";
       start="<2BEI83@pres.example.com>";boundary="TfZxoxgAvLqgj4wRWPDL"
   Content-Length: 2862

   --TfZxoxgAvLqgj4wRWPDL
   Content-Transfer-Encoding: 8bit
   Content-ID: <2BEI83@pres.example.com>
   Content-Type: application/rlmi+xml;charset="UTF-8"

   <?xml version="1.0" encoding="UTF-8"?>
   <list uri="sip:adam-friends@pres.example.com" version="2"
         name="Buddy List at COM" fullState="false">
     <resource uri="sip:ed@example.net" name="Ed at NET">



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       <instance id="sdlkmeopdf" state="pending"/>
     </resource>
     <resource uri="sip:adam-friends@example.org" name="My Friends at ORG">
       <instance id="cmpqweitlp" state="active" cid="1KQhyE@pres.example.com"/>
     </resource>
   </list>

   --TfZxoxgAvLqgj4wRWPDL
   Content-Transfer-Encoding: 8bit
   Content-ID: <1KQhyE@pres.example.com>
   Content-Type: multipart/signed;protocol="application/pgp-signature";
       micalc="pgp-md5";boundary="l3WMZaaL8NpQWGnQ4mlU"

   --l3WMZaaL8NpQWGnQ4mlU
   Content-Transfer-Encoding: 8bit
   Content-ID: <ZPvJHL@example.org>
   Content-Type: multipart/related;type="application/rlmi+xml";
       start="<Cvjpeo@example.org>";boundary="tuLLl3lDyPZX0GMr2YOo"

   --tuLLl3lDyPZX0GMr2YOo
   Content-Transfer-Encoding: 8bit
   Content-ID: <Cvjpeo@example.org>
   Content-Type: application/rlmi+xml;charset="UTF-8"

   <?xml version="1.0" encoding="UTF-8"?>
   <list uri="sip:adam-friends@example.org" version="1"
         name="Buddy List at ORG" fullState="true">
     <resource uri="sip:joe@example.org" name="Joe Thomas">
       <instance id="1" state="active" cid="mrEakg@example.org"/>
     </resource>
     <resource uri="sip:mark@example.org" name="Mark Edwards">
       <instance id="1" state="active" cid="KKMDmv@example.org"/>
     </resource>
   </list>

   --tuLLl3lDyPZX0GMr2YOo
   Content-Transfer-Encoding: 8bit
   Content-ID: <mrEakg@example.org>
   Content-Type: application/cpim-pidf+xml;charset="UTF-8"

   <?xml version="1.0" encoding="UTF-8"?>
   <presence xmlns="urn:ietf:params:xml:ns:cpim-pidf"
       entity="sip:joe@example.org">
     <tuple id="7823a4">
       <status>
         <basic>open</basic>
       </status>
       <contact priority="1.0">sip:joe@example.org</contact>



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     </tuple>
   </presence>

   --tuLLl3lDyPZX0GMr2YOo
   Content-Transfer-Encoding: 8bit
   Content-ID: <KKMDmv@example.org>
   Content-Type: application/cpim-pidf+xml;charset="UTF-8"

   <?xml version="1.0" encoding="UTF-8"?>
   <presence xmlns="urn:ietf:params:xml:ns:cpim-pidf"
       entity="sip:mark@example.org">
     <tuple id="398075">
       <status>
         <basic>closed</basic>
       </status>
     </tuple>
   </presence>

   --tuLLl3lDyPZX0GMr2YOo--

   --l3WMZaaL8NpQWGnQ4mlU
   Content-Transfer-Encoding: 8bit
   Content-ID: <K9LB7k@example.org>
   Content-Type: application/pgp-signature

   -----BEGIN PGP MESSAGE-----
   Version: 2.6.2

   iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC//
   jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq
   uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn
   HOxEa44b+EI=
   =ndaj
   -----END PGP MESSAGE-----

   --l3WMZaaL8NpQWGnQ4mlU--

   --TfZxoxgAvLqgj4wRWPDL--



   14.  The terminal completes the NOTIFY transaction.









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   Terminal -> Local RLS

   SIP/2.0 200 OK
   Via: SIP/2.0/TCP pres.example.com;branch=z9hG4bK4EPlfSFQK1
   From: <sip:adam-buddies@pres.example.com>;tag=zpNctbZq
   To: <sip:adam@example.com>;tag=ie4hbb8t
   Call-ID: cdB34qLToC@terminal.example.com
   CSeq: 997935769 NOTIFY
   Contact: <sip:terminal.example.com>
   Content-Length: 0









































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6. Security Considerations

6.1 Authentication

   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
   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 [6][7] to provide an
   authenticated identity to the PA.

6.2 Risks of Improper Aggregation

   A resource list server typically serves information to multiple
   subscribers at once.  In many cases, resources may be present in
   several lists; additionally, it is quite possible that resource list
   servers will have two users subscribe to the same list.

   In these cases, misguided RLS implementations may attempt to minimize
   network load by maintaining only one subscription to a resource in a
   list, and presenting the result of such a subscription to more than
   one user.  Of course, doing so circumvents any authorization policy
   that the resource maintains.  It is important to keep in mind that
   authorization is often much more than a simple binary "allowed/not
   allowed" decision; resources may render very different -- and even
   conflicting -- resource states, depending on the identity of the
   subscribing user.

   Implementations MUST NOT attempt to perform this type of optimization
   unless adequate access to complete authorizaton policy can be
   guaranteed.  Note that this is a very difficult problem to solve
   correctly; even in the cases that such access is beleived possible,
   this mode of operation is NOT RECOMMENDED.

6.3 Signing and Sealing

   Implementors should keep in mind that any section of the MIME body



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   may be signed and/or encrypted as necessary.  Resource List Servers
   should take care not to modify any MIME bodies they receive from
   other notifiers, and should not generally rely on being able to read
   them.

   In order to facilitate security, resource list servers SHOULD pass
   along indication for support of "multipart/signed" and "multipart/
   encrypted" content types, if the subscriber includes them in the
   initial SUBSCRIBE message.  Not doing so may actually result in
   resources refusling to divulge state (if notifier policy requires
   encryption, but the RLS fails to convey support), or subscribers
   discarding valid state (if subscriber policy requires a signature,
   but the RLS fails to convey support).

   Note that actual implemetation of encryption and signing by the RLS
   is not necessary to be able to pass through signed and/or encrypted
   bodies.


































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7. IANA Considerations

7.1 New SIP Option Tag: eventlist

   Option Tag Name: eventlist

   Description: Extension to allow subscriptions to collections of
      resources

   Published specification: RFC xxxx [[Note to RFC editor: replace xxxx
      with the RFC number of this document when published]]


7.2 New MIME type for Resource List Meta-Information

   MIME Media Type Name: application

   MIME subtype name: rlmi+xml

   Required parameters: None

   Optional parameters: charset

      See RFC 3023 [10] for a discussion of the charset parameter on
      XML-derived MIME types.  Since this MIME type is used exclusively
      in SIP, the use of UTF-8 encoding is strongly encouraged.

   Encoding considerations: 8-bit text

   Security considerations: Security considerations specific to uses of
      this this MIME type are discussed in RFC xxxx [[Note to RFC
      editor: replace xxxx with the RFC number of this document when
      published]].  RFC 1874 [9] and RFC 3023 [10] discuss security
      issues common to all uses of XML.

   Interoperability considerations: The use of this MIME body is
      intended to be generally interoperable.  No unique considerations
      have been identified.

   Published specification: RFC xxxx [[Note to RFC editor: replace xxxx
      with the RFC number of this document when published]]

   Applications which use this media type: This media type is used to
      convey metadata for the state of collections of resources within a
      Session Initiation Protocol (SIP) subscription.

   Additional information:




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      Magic Number(s): None.

      File Extension(s): None.

      Macintosh File Type Code(s): None.

      Object Identifier(s) or OID(s): None.

   Intended usage: Limited Use

   Other Information/General Comment: None.

   Person to contact for further information:

      Name: Adam Roach

      E-Mail: adam@dynamicsoft.com

      Author/Change Controller: The specification of this MIME type is a
         work product of the SIMPLE working group, and was authored by
         Adam Roach, Jonathan Rosenberg, and Ben Campbell.  The IETF has
         change control over its specification.





























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8. Open Issues

   o  RFC 3265 allows event packages to include bodies in their
      SUBSCRIBE requests to filter notifications.  It would be natural
      to merely copy the body from the SUBSCRIBE in a list subscription
      to any SUBSCRIBE messages generated to additional entities, so
      that they would take the appropriate actions.  Would that be
      correct behavior, or do we need to define some other way to handle
      this (e.g.  including the filter documents as part of the
      provisioned data that is considered part of the list).









































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Normative References

   [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]  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
        1993.

   [4]  Levinson, E., "The MIME Multipart/Related Content-type", RFC
        2387, August 1998.



































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Non-Normative References

   [5]   Rosenberg, J., "Session Initiation Protocol (SIP) Extensions
         for Presence", draft-ietf-simple-presence-07 (work in
         progress), May 2002.

   [6]   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.

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

   [8]   Olson, S., "A Mechanism for Content Indirection in Session
         Initiation Protocol (SIP) Messages", draft-ietf-sip-content-
         indirect-mech-01 (work in progress), November 2002.

   [9]   Levinson, E., "SGML Media Types", RFC 1874, December 1995.

   [10]  Murata, M., St. Laurent, S. and D. Kohn, "XML Media Types", RFC
         3023, January 2001.

   [11]  Elkins, M., Del Torto, D., Levien, R. and T. Roessler, "MIME
         Security with OpenPGP", RFC 3156, August 2001.

   [12]  Galvin, J., Murphy, S., Crocker, S. and N. Freed, "Security
         Multiparts for MIME: Multipart/Signed and Multipart/Encrypted",
         RFC 1847, October 1995.


Authors' Addresses

   Adam Roach
   dynamicsoft
   5100 Tennyson Pkwy
   Suite 1200
   Plano, TX  75024
   US

   EMail: adam@dynamicsoft.com









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

   EMail: jdrosen@dynamicsoft.com


   Ben Campbell
   dynamicsoft
   5100 Tennyson Pkwy
   Suite 1200
   Plano, TX  75024
   US

   EMail: bcampbell@dynamicsoft.com

































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Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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