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RFC 4662 - A Session Initiation Protocol (SIP) Event Notificatio


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Network Working Group                                        A. B. Roach
Request for Comments: 4662                                   B. Campbell
Category: Standards Track                               Estacado Systems
                                                            J. Rosenberg
                                                           Cisco Systems
                                                             August 2006

   A Session Initiation Protocol (SIP) Event Notification Extension
                           for Resource Lists

Status of This Memo

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

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

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

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................4
   3. Overview of Operation ...........................................4
   4. Operation of List Subscriptions .................................5
      4.1. Negotiation of Support for Resource Lists ..................6
      4.2. Subscription Duration ......................................7
      4.3. NOTIFY Bodies ..............................................7
      4.4. RLS Processing of SUBSCRIBE Requests .......................7
      4.5. RLS Generation of NOTIFY Requests ..........................7
      4.6. Subscriber Processing of NOTIFY Requests ...................9
      4.7. Handling of Forked Requests ...............................10
      4.8. Rate of Notifications .....................................10
   5. Using multipart/related to Convey Aggregate State ..............10
      5.1. XML Syntax ................................................11
      5.2. List Attributes ...........................................13
      5.3. Resource Attributes .......................................14
      5.4. Name Attributes ...........................................14
      5.5. Instance Attributes .......................................14
      5.6. Constructing Coherent Resource State ......................16
           5.6.1. Processing Full State Notifications ................17
           5.6.2. Processing Partial State Notifications .............17
   6. Example ........................................................18
   7. Security Considerations ........................................31
      7.1. Authentication ............................................31
           7.1.1. RLS and Subscriber in the Same Domain ..............31
           7.1.2. RLS and Subscriber in Different Domains ............32
      7.2. Risks of Improper Aggregation .............................33
      7.3. Signing and Sealing .......................................33
      7.4. Infinite Loops ............................................34
   8. IANA Considerations ............................................34
      8.1. New SIP Option Tag: eventlist .............................34
      8.2. New MIME type for Resource List Meta-Information ..........34
      8.3. URN Sub-Namespace .........................................35
   9. Acknowledgements ...............................................36
   10. References ....................................................36
      10.1. Normative References .....................................36
      10.2. Informative References ...................................37

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 the subscriber
   generating a SUBSCRIBE request for the resource, which is processed
   by a notifier that represents the resource.

   In many cases, a subscriber has a list of resources they are
   interested in.  Without some aggregating mechanism, this will require
   the subscriber to generate a SUBSCRIBE request for each resource
   about which they want information.  For environments in which
   bandwidth is limited, such as wireless networks, subscribing to each
   resource individually is problematic.  Some 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 a long-lived subscription state.  This means that the
      subscriber may need to generate SUBSCRIBE refreshes 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.

   To solve these problems, this specification defines an extension to
   RFC 3265 [2] that allows for requesting and conveying notifications
   for lists 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 used to identify the
   resource list will be a SIP URI [1]; however, the use of other
   schemes (such as pres: [10]) is also foreseen.

   The notifier for the list 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 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 constrained
   to be in the domain of the resource list server.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [5].

   The following terms are used throughout the remainder of this
   document.

   Back-End Subscription:  Any subscription (SIP or otherwise) that an
      RLS creates to learn of the state of a resource.  An RLS will
      create back-end subscriptions to learn of the state of a resource
      about which the RLS is not an authority.  For back-end
      subscriptions, RLSes act as a subscriber.

   List Subscription:  A subscription to a resource list.  In list
      subscriptions, RLSes act as the notifier.

   Resource:  A resource is any logical entity that has a state or
      states that can be subscribed to.  Resources are identified by
      URIs.

   Resource List:  A list of zero or more resources that can have their
      individual states subscribed to with a single subscription.

   RLMI:  Resource List Meta-Information.  RLMI is a document that
      describes the state of the virtual subscriptions associated with a
      list subscription.

   RLS:  Resource List Server.  RLSes accept subscriptions to resource
      lists and send notifications to update subscribers of the state of
      the resources in a resource list.

   Virtual Subscription:  A Virtual Subscription is a logical construct
      within an RLS that represents subscriptions to the resources in a
      resource list.  For each list subscription it services, an RLS
      creates at least one virtual subscription for every resource in
      the resource list being subscribed to.  In some cases, such as
      when the RLS is not the authority for the state of the resource,
      this virtual subscription will be associated with a back-end
      subscription.  In other cases, such as when the RLS is the
      authority for the state of the resource, the virtual subscription
      will not have a corresponding back-end subscription.

3.  Overview of Operation

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

   When users wish to subscribe to the resource of a list of resources,
   they can use the mechanisms described in this specification.  The
   first step is the creation of 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.  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 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 subscriptions to non-local resources may or may not be SIP
   subscriptions; any mechanism for determining such information may be
   employed.  This document uses the term "back-end subscription" to
   refer to such a subscription, regardless of whether SIP is used to
   establish and service it.

   As the state of resources in the list change, the RLS generates
   notifications to the list subscribers.  The RLS can, at its
   discretion, buffer notifications of resource changes 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.
   The remaining sections contain the actual state information for each
   resource.

4.  Operation of List Subscriptions

   The event list extension acts, in many ways, like an event template
   package.  In particular, any single list subscription must be
   homogeneous with respect to the underlying event package.  In other
   words, a single list subscription can apply only one event package to
   all the resources in the resource list.

   Note that it is perfectly valid for an RLS to allow multiple
   subscriptions to the same list to use differing 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 several virtual subscriptions to a tree of
   resources.  The leaf nodes of this tree are virtual subscriptions of
   the event type given in the "Event" header field; all other nodes in
   the tree are list subscriptions that are serviced as described in
   this section and its subsections.

   Keep in mind that these virtual subscriptions are not literal SIP
   subscriptions (although they may result in SIP subscriptions,
   depending on the RLS implementation).

4.1.  Negotiation of Support for Resource Lists

   This specification uses the SIP option tag mechanism for negotiating
   support for the extension defined herein.  Refer to RFC 3261 [1] for
   the normative description of processing of the "Supported" and
   "Require" header fields and the 421 (Extension Required) response
   code.

      A non-normative description of the implications of the use of
      option tags follows.
      Any client that supports the event list extension will include an
      option tag of "eventlist" in a "Supported" header field 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 field of the response to the SUBSCRIBE, and in all NOTIFY
      messages within that subscription.

      Use of "Require: eventlist" in NOTIFY messages is applied by the
      notifier to satisfy the RFC 3261 requirement that a UAC MUST
      insert a Require header field into a request if the UAC wishes to
      insist that a UAS understand an extension in order to process the
      request.  Because the NOTIFY would not be usable without applying
      the eventlist option, the notifier is obligated to include it.

   Including "eventlist" in a "Require" header field in a SUBSCRIBE
   request serves no purpose except to break interoperability in certain
   cases, and is consequently NOT RECOMMENDED.

   Sending of "Supported: eventlist" in a NOTIFY message is meaningless
   and silly.  Implementations SHOULD NOT include "Supported: eventlist"
   in any requests except for SUBSCRIBE.

   There is nothing in a SIP URI that indicates whether it represents a
   list of resources or a single resource.  Therefore, if a subscriber
   sends a request to a URI that represents a list resource but does not
   include a Supported header field listing the "eventlist" token, the
   notifier will typically return a 421 (Extension Required) response
   code.  RFC 3261 [1] advises that servers avoid returning a 421 and
   instead attempt to process the request without the extension.
   However, in this case, the URI fundamentally represents a list
   resource, and therefore the subscription cannot proceed without this
   extension.

4.2.  Subscription Duration

   Since the primary benefit of the resource list server is to reduce
   the overall messaging volume to a subscriber, it is RECOMMENDED that
   the subscription duration to a list be reasonably long.  The default,
   when no duration is specified, is taken from the underlying event
   package.  Of course, the standard techniques [2] can be used to
   increase or reduce this amount.

4.3.  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 a SUBSCRIBE message, in
   addition to any other types supported by the client (including any
   types required by the event package being used).

4.4.  RLS Processing of SUBSCRIBE Requests

   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 an RLS can use any authorization policy it
   chooses.

4.5.  RLS 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
   differentiators between various RLS implementations is the means by
   which they aggregate, rate-limit, or optimize the way in which

   notifications are generated.  As a baseline behavior, the RLS MAY
   generate a NOTIFY to the RLS subscriber whenever the state of any
   resource on the list changes.

   It is important to understand that any given subscription is a
   subscription either to a single resource or to a list of resources.
   This nature (single resource versus list of resources) cannot change
   during the duration of a single subscription.  In particular, this
   means that RLSes MUST NOT send NOTIFY messages that do not contain
   RLMI for a subscription if they have previously sent NOTIFY messages
   in that subscription containing RLMI.  Similarly, RLSes MUST NOT send
   NOTIFY messages that do contain RLMI for a subscription if they have
   previously sent NOTIFY messages in that subscription which do not.

      List representations necessarily contain RLMI documents for two
      reasons.  Importantly, they identify the resource to which the
      event state corresponds.  Many state syntaxes do not fully
      identify the resource to which the state applies, or they may
      identify the resource in a different way than it is represented in
      the list; for example, PIDF documents may contain resource URIs
      that are not identical to the URI used to retrieve them.  Further,
      RLMI documents serve to disambiguate multiple instances of a
      single resource.

   See Section 5 for a detailed definition of the syntax used to convey
   the state of resource lists.  For the purposes of the following
   discussion, it is important to know that the overall list contains
   zero or more resources, and that the resources contain zero or more
   instances.  Each instance has a state associated with it (pending,
   active, or terminating) representing the state of the virtual
   subscription.

   Notifications contain a multipart document, the first part of which
   always contains meta-information about the list (e.g., membership,
   state of the virtual subscription to the resource).  Remaining parts
   are used to convey the actual state of the resources listed in the
   meta-information.

   The "state" attribute of each instance of a resource in the
   meta-information is set according to the state of the virtual
   subscription.  The meanings of the "state" attribute are described in
   RFC 3265 [2].

   If an instance of a resource was previously reported to the
   subscriber but is no longer available (i.e., the virtual subscription
   to that instance has been terminated), the resource list server
   SHOULD include that resource instance in the meta-information in the
   first NOTIFY message sent to the subscriber following the instance's

   unavailability.  The RLS MAY continue to do so for future
   notifications.

   When sending information for a terminated resource instance, the RLS
   indicates a state of "terminated" and an appropriate reason value.
   Valid reason values and their meanings are described in RFC 3265 [2].
   If the RLS will attempt to recover the resource state again at some
   point in the future (e.g., when the reason in the meta-information is
   "probation"), then the instance of the resource SHOULD remain in the
   meta-information until the instance state is available, or until the
   RLS gives up on making such state available.

   When the first SUBSCRIBE message for a particular subscription is
   received by an RLS, the RLS will often not know state information for
   all 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 RLS 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 co-located 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 include an RLMI document in which the full state is
   indicated.  The RLS SHOULD also include state information for all
   resources in the list for which the RLS has state, subject to policy
   restrictions.  This allows the subscriber to refresh their state, and
   to recover from lost notifications.

4.6.  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 some or all of the
   resources in the list.  Each <resource> element contains a URI that
   uniquely identifies the resource to which that section corresponds.
   When a NOTIFY arrives, it can contain full or partial state (as
   indicated 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 indicated, the recipient of the NOTIFY

   updates information for each identified resource.  Information for
   any resources that are not identified in the NOTIFY is not changed,
   even if they were indicated in previous NOTIFY messages.  See
   Section 5.6 for more information.

      When full state is indicated, note that it applies only to the
      RLMI document in which it occurs.  In particular, one of the
      <resource> elements in the document may in turn refer to another
      list of resources.  Any such sub-lists will be detailed in their
      own RLMI documents, which may or may not have full state
      indicated.

      Further 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 list does not change their
      partial notification semantics).

      Finally, note that as a consequence of the way in which resource
      list subscriptions work, polling of resource state may not be
      particularly useful.  While such polls will retrieve the resource
      list, they will not necessarily contain state for some or all of
      the resources on the list.

4.7.  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 to a list MUST NOT install multiple
   subscriptions when the initial request is forked.  If multiple
   responses are received, they are handled using the techniques
   described in Section 4.4.9 of RFC 3265 [2].

4.8.  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.

5.  Using multipart/related to Convey Aggregate State

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

5.1.  XML Syntax

   The root document of the multipart/related body MUST be a Resource
   List Meta-Information (RLMI) document.  It is of the type
   "application/rlmi+xml".  This document contains the meta-information
   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 targetNamespace="urn:ietf:params:xml:ns:rlmi"
              elementFormDefault="qualified"
              xmlns="urn:ietf:params:xml:ns:rlmi"
              xmlns:xs="http://www.w3.org/2001/XMLSchema">
   <xs:import namespace="http://www.w3.org/XML/1998/namespace"
              schemaLocation="http://www.w3.org/2001/xml.xsd"/>
     <xs:element name="list">
       <xs:complexType>
         <xs:sequence>
           <xs:element ref="name" minOccurs="0"
                       maxOccurs="unbounded" />
           <xs:element ref="resource" minOccurs="0"
                       maxOccurs="unbounded" />
         </xs:sequence>
         <xs:attribute name="uri" type="xs:anyURI" use="required" />
         <xs:attribute name="version" type="xs:unsignedInt"
                       use="required" />
         <xs:attribute name="fullState" type="xs:boolean"
                       use="required" />
         <xs:attribute name="cid" type="xs:string" use="optional" />
         <xs:anyAttribute processContents="lax" />
       </xs:complexType>
     </xs:element>
     <xs:element name="resource">
       <xs:complexType>
         <xs:sequence>
           <xs:element ref="name" minOccurs="0"
                       maxOccurs="unbounded" />
           <xs:element ref="instance" minOccurs="0"
                       maxOccurs="unbounded" />
         </xs:sequence>
         <xs:attribute name="uri" type="xs:anyURI" use="required" />
         <xs:anyAttribute processContents="lax" />
       </xs:complexType>
     </xs:element>
     <xs:element name="instance">
       <xs:complexType>
         <xs:sequence>
           <xs:any minOccurs="0" maxOccurs="unbounded"

                   processContents="lax" />
         </xs:sequence>
         <xs:attribute name="id" type="xs:string" use="required" />
         <xs:attribute name="state" use="required">
           <xs:simpleType>
             <xs:restriction base="xs:string">
               <xs:enumeration value="active" />
               <xs:enumeration value="pending" />
               <xs:enumeration value="terminated" />
             </xs:restriction>
           </xs:simpleType>
         </xs:attribute>
         <xs:attribute name="reason" type="xs:string"
                       use="optional" />
         <xs:attribute name="cid" type="xs:string" use="optional" />
         <xs:anyAttribute processContents="lax" />
       </xs:complexType>
     </xs:element>
     <xs:element name="name">
       <xs:complexType>
         <xs:simpleContent>
           <xs:extension base="xs:string">
             <xs:attribute ref="xml:lang" use="optional"/>
           </xs:extension>
         </xs:simpleContent>
       </xs:complexType>
     </xs:element>
   </xs:schema>

   An example of a document formatted using this schema follows.

   <?xml version="1.0"?>
   <list xmlns="urn:ietf:params:xml:ns:rlmi"
         uri="sip:adam-friends@lists.vancouver.example.com"
         version="7" fullState="true">
     <name xml:lang="en">Buddy List</name>
     <name xml:lang="fr">Liste d'amis</name>
     <resource uri="sip:bob@vancouver.example.com">
       <name>Bob Smith</name>
       <instance id="juwigmtboe" state="active"
                 cid="12345.aaa@vancouver.example.com"/>
     </resource>
     <resource uri="sip:dave@vancouver.example.com">
       <name>Dave Jones</name>
       <instance id="hqzsuxtfyq" state="active"
                 cid="12345.aab@vancouver.example.com"/>
     </resource>
     <resource uri="sip:jim@vancouver.example.com">

       <name>Jim</name>
       <instance id="oflzxqzuvg" state="terminated"
                 reason="rejected" />
     </resource>
     <resource uri="sip:ed@vancouver.example.com">
       <name>Ed</name>
       <instance id="grqhzsppxb" state="pending"/>
     </resource>
   </list>

5.2.  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, 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 list.  If it does, the value of the
   attribute is "true" (or "1"); otherwise, it is "false" (or "0").  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.

   Finally, <list> elements MAY contain a "cid" attribute.  If present,
   the "cid" attribute identifies a section within the multipart/related
   body that contains aggregate state information for the resources
   contained in the list.  The definition of such aggregate information
   is outside the scope of this document and will be defined on a per-
   package basis, as needed.  The cid attribute is the Content-ID for
   the corresponding section in the multipart body.

   The cid attribute MUST refer only to top-level parts of the
   multipart/related document for which the RLMI document in which it
   appears is the root.  See Section 5.5 for an example.

5.3.  Resource Attributes

   The resource list contains one <resource> element for each resource
   being reported in the notification.  These resource elements contain
   attributes that identify meta-data associated with each resource.

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

5.4.  Name Attributes

   Each list and resource element contains zero or more name elements.
   These name elements contain human-readable descriptions or names for
   the resource list or resource.  The contents of these elements are
   somewhat analogous to the "Display Name" present in the SIP name-addr
   element.

   Name elements optionally contain the standard XML "xml:lang"
   attribute.  The "xml:lang" attribute, if present, specifies the
   language of the human-readable name.  If this attribute is present,
   it MUST contain a valid language tag.  Language tags are defined in
   RFC 3066 [6].  The language tag assists applications in determining
   which of potentially several name elements should be rendered to the
   user.

5.5.  Instance Attributes

   Each resource element contains zero or more instance elements.  These
   instance elements are used to represent a single notifier for the
   resource.  For event packages that allow forking, multiple virtual
   subscriptions may exist for a given resource.  Multiple virtual
   subscriptions are represented as multiple instance elements in the
   corresponding resource element.  For subscriptions in which forking
   does not occur, at most one instance will be present for a given
   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.  Construction of this string
   is an implementation decision.  Any mechanism for generating this
   string is 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 field
   in RFC 3265 [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 those given for the "reason" parameter on the
   "Subscription-State" header field in RFC 3265 [2].  Note that the
   "reason" attribute 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 conveying state.  The cid attribute is the Content-ID for the
   corresponding section in the multipart body.

   The cid attribute MUST refer only to top-level parts of the
   multipart/related document for which the RLMI document in which it
   appears is the root.

      For example, consider a multipart/related document containing
      three parts; we'll label these parts A, B, and C.  Part A is type
      application/rlmi+xml, part B is type multipart/related, and part C
      is type application/pidf+xml.  Part B is in turn a document
      containing three parts: D, E, and F.  Part D is of type
      application/rlmi+xml, and parts E and F are of type
      application/pidf+xml.

       +-------------------------------------------+
       | Top Level Document: multipart/related     |
       |                                           |
       | +---------------------------------------+ |
       | | Part A: application/rlmi+xml          | |
       | +---------------------------------------+ |
       | | Part B: multipart/related             | |
       | |                                       | |
       | | +-----------------------------------+ | |
       | | | Part D: application/rlmi+xml      | | |
       | | +-----------------------------------+ | |
       | | | Part E: application/pidf+xml      | | |
       | | +-----------------------------------+ | |
       | | | Part F: application/pidf+xml      | | |
       | | +-----------------------------------+ | |
       | |                                       | |
       | +---------------------------------------+ |
       | | Part C: application/pidf+xml          | |
       | +---------------------------------------+ |
       |                                           |
       +-------------------------------------------+

      Any "cid" attributes in document A must refer only to parts B or
      C.  Referring to parts D, E, or F would be illegal.  Similarly,
      any "cid" attributes in document D must refer only to parts E or
      F.  Referring to any other parts would be illegal.
      Also note that the subscription durations of any back-end
      subscriptions are not propagated into the meta-information state
      in any way.

5.6.  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 corresponding 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.

5.6.1.  Processing Full State Notifications

   If a notification contains full state, indicated by the <list>
   attribute "fullState" set to "true", the notification is used to
   update the table.  A check is first made to ensure that the "version"
   attribute of the <list> attribute in the received message is greater
   than the local version number.  If not, the received document is
   discarded without any further processing.  Otherwise, the contents of
   the resource-list table are flushed and repopulated from the contents
   of the document.  A new row in the table is created for each
   "resource" element.

5.6.2.  Processing Partial State Notifications

   If a notification contains partial state, indicated by the <list>
   attribute "fullState" set to "false", 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.  The list subscriber SHOULD also 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.

   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.

6.  Example

   This section gives an example call flow.  It is not normative.  If a
   conflict arises between this call flow 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@vancouver.example.com", and the name of the nested list
   resource that we are subscribing to is called
   "sip:adam-buddies@pres.vancouver.example.com".  The underlying event
   package is "presence", described by [8].

   In this example, the RLS has information to service some of the
   resources on the list, but must consult other servers to retrieve
   information for others.  The implementation of the RLS in this
   example uses the SIP SUBSCRIBE/NOTIFY mechanism to retrieve such
   information.

   Terminal   pres.vancouver.example.com   pres.stockholm.example.org
     |                |        pres.dallas.example.net  |
   1 |---SUBSCRIBE--->|                |                |
   2 |<-----200-------|                |                |
   3 |<----NOTIFY-----|                |                |
   4 |------200------>|                |                |
   5 |                |---SUBSCRIBE--->|                |
   6 |                |<-----200-------|                |
   7 |                |<----NOTIFY-----|                |
   8 |                |------200------>|                |
   9 |                |------------SUBSCRIBE----------->|
   10|                |<--------------200---------------|
   11|                |<-------------NOTIFY-------------|
   12|                |---------------200-------------->|
   13|<----NOTIFY-----|                |                |
   14|------200------>|                |                |

   1.   We initiate 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 that we must advertise
        application/pidf+xml because we are requesting a subscription to
        presence.

   Terminal -> Local RLS

   SUBSCRIBE sip:adam-buddies@pres.vancouver.example.com SIP/2.0
   Via: SIP/2.0/TCP terminal.vancouver.example.com;
     branch=z9hG4bKwYb6QREiCL
   Max-Forwards: 70
   To: <sip:adam-buddies@pres.vancouver.example.com>
   From: <sip:adam@vancouver.example.com>;tag=ie4hbb8t
   Call-ID: cdB34qLToC@terminal.vancouver.example.com
   CSeq: 322723822 SUBSCRIBE
   Contact: <sip:terminal.vancouver.example.com>
   Event: presence
   Expires: 7200
   Supported: eventlist
   Accept: application/pidf+xml
   Accept: application/rlmi+xml
   Accept: multipart/related
   Accept: multipart/signed
   Accept: application/pkcs7-mime
   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 call flow.  Those steps are omitted for
        brevity.

   Local RLS -> Terminal

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

   3.   As is required by RFC 3265 [2], the RLS sends a NOTIFY
        immediately upon accepting the subscription.  In this example,
        we are assuming that the local RLS is also an authority for
        presence information for the users in the
        "vancouver.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.vancouver.example.com SIP/2.0
   Via: SIP/2.0/TCP pres.vancouver.example.com;
     branch=z9hG4bKMgRenTETmm
   Max-Forwards: 70
   From: <sip:adam-buddies@pres.vancouver.example.com>;tag=zpNctbZq
   To: <sip:adam@vancouver.example.com>;tag=ie4hbb8t
   Call-ID: cdB34qLToC@terminal.vancouver.example.com
   CSeq: 997935768 NOTIFY
   Contact: <sip:pres.vancouver.example.com>
   Event: presence
   Subscription-State: active;expires=7200
   Require: eventlist
   Content-Type: multipart/related;type="application/rlmi+xml";
       start="<nXYxAE@pres.vancouver.example.com>";
       boundary="50UBfW7LSCVLtggUPe5z"
   Content-Length: 1560

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

   <?xml version="1.0" encoding="UTF-8"?>
   <list xmlns="urn:ietf:params:xml:ns:rlmi"
         uri="sip:adam-friends@pres.vancouver.example.com"
         version="1" fullState="true">
     <name xml:lang="en">Buddy List at COM</name>
     <name xml:lang="de">Liste der Freunde an COM</name>
     <resource uri="sip:bob@vancouver.example.com"">
       <name>Bob Smith</name>
       <instance id="juwigmtboe" state="active"
                 cid="bUZBsM@pres.vancouver.example.com"/>
     </resource>
     <resource uri="sip:dave@vancouver.example.com">
       <name>Dave Jones</name>
       <instance id="hqzsuxtfyq" state="active"
                 cid="ZvSvkz@pres.vancouver.example.com"/>
     </resource>
     <resource uri="sip:ed@dallas.example.net">
       <name>Ed at NET</name>
     </resource>
     <resource uri="sip:adam-friends@stockholm.example.org">
       <name xml:lang="en">My Friends at ORG</name>
       <name xml:lang="de">Meine Freunde an ORG</name>
     </resource>
   </list>

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

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

   --50UBfW7LSCVLtggUPe5z
   Content-Transfer-Encoding: binary

   Content-ID: <ZvSvkz@pres.vancouver.example.com>
   Content-Type: application/pidf+xml;charset="UTF-8"

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

   --50UBfW7LSCVLtggUPe5z--

   4.   The terminal completes the transaction.

   Terminal -> Local RLS

   SIP/2.0 200 OK
   Via: SIP/2.0/TCP pres.vancouver.example.com;
     branch=z9hG4bKMgRenTETmm
   From: <sip:adam-buddies@pres.vancouver.example.com>;tag=zpNctbZq
   To: <sip:adam@vancouver.example.com>;tag=ie4hbb8t
   Call-ID: cdB34qLToC@terminal.vancouver.example.com
   CSeq: 997935768 NOTIFY
   Contact: <sip:terminal.vancouver.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@dallas.example.net".  Since the local RLS knows how to
        receive notifications for list subscriptions, it includes the
        "Supported: eventlist" header field 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"
        header fields for all 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.

   Local RLS -> Presence Server in dallas.example.net

   SUBSCRIBE sip:ed@dallas.example.net SIP/2.0
   Via: SIP/2.0/TCP pres.vancouver.example.com;
     branch=z9hG4bKMEyGjdG1LH

   Max-Forwards: 70
   To: <sip:ed@dallas.example.net>
   From: <sip:adam@vancouver.example.com>;tag=aM5icQu9
   Call-ID: Ugwz5ARxNw@pres.vancouver.example.com
   CSeq: 870936068 SUBSCRIBE
   Contact: <sip:pres.vancouver.example.com>
   Identity: Tm8sIHRoaXMgaXNuJ3QgYSByZWFsIGNlcnQuIFlvdSBvn
             Zpb3VzbHkgaGF2ZSB0aW1lIHRvIGtpbGwuIEkKc3VnZ2V
             zdCBodHRwOi8vd3d3LmhvbWVzdGFycnVubmVyLmNvbS8K
   Identity-Info: https://vancouver.example.com/cert
   Event: presence
   Expires: 3600
   Supported: eventlist
   Accept: application/pidf+xml
   Accept: application/rlmi+xml
   Accept: multipart/related
   Accept: multipart/signed
   Accept: application/pkcs7-mime
   Content-Length: 0

   6.   The Presence Server in dallas.example.net completes the
        SUBSCRIBE transaction.  Note that authentication would normally
        take place at this point in the call flow.  This step is omitted
        for brevity.

   Presence Server in dallas.example.net -> Local RLS

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

   7.   In this example, we assume that the server at dallas.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
        not included in this call flow for brevity.

   Presence Server in dallas.example.net -> Local RLS

   NOTIFY sip:pres.vancouver.example.com SIP/2.0
   Via: SIP/2.0/TCP pres.dallas.example.net;
     branch=z9hG4bKfwpklPxmrW
   Max-Forwards: 70
   From: <sip:ed@dallas.example.net>;tag=e45TmHTh
   To: <sip:adam@vancouver.example.com>;tag=aM5icQu9
   Call-ID: Ugwz5ARxNw@pres.vancouver.example.com
   CSeq: 1002640632 NOTIFY
   Contact: <sip:dallas.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 dallas.example.net

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

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

   Local RLS -> RLS in stockholm.example.org

   SUBSCRIBE sip:adam-friends@stockholm.example.org SIP/2.0
   Via: SIP/2.0/TCP pres.vancouver.example.com;
     branch=z9hG4bKFSrAF8CZFL
   Max-Forwards: 70
   To: <sip:adam-friends@stockholm.example.org>
   From: <sip:adam@vancouver.example.com>;tag=a12eztNf

   Call-ID: kBq5XhtZLN@pres.vancouver.example.com
   CSeq: 980774491 SUBSCRIBE
   Contact: <sip:pres.vancouver.example.com>
   Identity: Tm90IGEgcmVhbCBzaWduYXR1cmUsIGVpdGhlci4gQ2VydGFp
             bmx5IHlvdSBoYXZlIGJldHRlcgp0aGluZ3MgdG8gYmUgZG9p
             bmcuIEhhdmUgeW91IGZpbmlzaGVkIHlvdXIgUkxTIHlldD8K
   Identity-Info: https://vancouver.example.com/cert
   Event: presence
   Expires: 3600
   Supported: eventlist
   Accept: application/pidf+xml
   Accept: application/rlmi+xml
   Accept: multipart/related
   Accept: multipart/signed
   Accept: application/pkcs7-mime
   Content-Length: 0

   10.  The RLS in stockholm.example.org completes the SUBSCRIBE
        transaction.  Note that authentication would normally take place
        at this point in the call flow.  This step is omitted for
        brevity.

   RLS in stockholm.example.org -> Local RLS

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

   11.  In this example, we assume that the RLS in stockholm.example.org
        is also an authority for presence information for the users in
        the "stockholm.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 stockholm.example.org has chosen to sign [14] the
        body of the NOTIFY message.  As described in RFC 3851, signing
        is performed by creating a multipart/signed document that has
        two parts.  The first part is the document to be signed (in this
        example, the multipart/related document that describes the list
        resource states), while the second part is the actual signature.

   RLS in stockholm.example.org -> Local RLS

   NOTIFY sip:pres.vancouver.example.com SIP/2.0
   Via: SIP/2.0/TCP pres.stockholm.example.org;
     branch=z9hG4bKmGL1nyZfQI
   Max-Forwards: 70
   From: <sip:adam-friends@stockholm.example.org>;tag=JenZ40P3
   To: <sip:adam@vancouver.example.com>;tag=a12eztNf
   Call-ID: kBq5XhtZLN@pres.vancouver.example.com
   CSeq: 294444656 NOTIFY
   Contact: <sip:stockholm.example.org>
   Event: presence
   Subscription-State: active;expires=3600
   Require: eventlist
   Content-Type: multipart/signed;
       protocol="application/pkcs7-signature";
       micalg=sha1;boundary="l3WMZaaL8NpQWGnQ4mlU"
   Content-Length: 2038

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

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

   <?xml version="1.0" encoding="UTF-8"?>
   <list xmlns="urn:ietf:params:xml:ns:rlmi"
         uri="sip:adam-friends@stockholm.example.org" version="1"
         fullState="true">
     <name xml:lang="en">Buddy List at COM</name>
     <name xml:lang="de">Liste der Freunde an COM</name>
     <resource uri="sip:joe@stockholm.example.org">
       <name>Joe Thomas</name>
       <instance id="1" state="active"
                 cid="mrEakg@stockholm.example.org"/>
     </resource>
     <resource uri="sip:mark@stockholm.example.org">
       <name>Mark Edwards</name>
       <instance id="1" state="active"
                 cid="KKMDmv@stockholm.example.org"/>
     </resource>
   </list>

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

   <?xml version="1.0" encoding="UTF-8"?>
   <presence xmlns="urn:ietf:params:xml:ns:pidf"
       entity="sip:joe@stockholm.example.org">
     <tuple id="x823a4">
       <status>
         <basic>open</basic>
       </status>
       <contact priority="1.0">sip:joe@stockholm.example.org</contact>
     </tuple>
   </presence>

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

   <?xml version="1.0" encoding="UTF-8"?>
   <presence xmlns="urn:ietf:params:xml:ns:pidf"
       entity="sip:mark@stockholm.example.org">
     <tuple id="z98075">
       <status>
         <basic>closed</basic>
       </status>
     </tuple>
   </presence>

   --tuLLl3lDyPZX0GMr2YOo--

   --l3WMZaaL8NpQWGnQ4mlU
   Content-Transfer-Encoding: binary
   Content-ID: <K9LB7k@stockholm.example.org>
   Content-Type: application/pkcs7-signature

   [PKCS #7 signature here]

   --l3WMZaaL8NpQWGnQ4mlU--

   12.  The Local RLS completes the NOTIFY transaction.

   Local RLS -> RLS in stockholm.example.org

   SIP/2.0 200 OK
   Via: SIP/2.0/TCP pres.stockholm.example.org;
     branch=z9hG4bKmGL1nyZfQI
   From: <sip:adam-friends@stockholm.example.org>;tag=JenZ40P3
   To: <sip:adam@vancouver.example.com>;tag=a12eztNf
   Call-ID: kBq5XhtZLN@pres.vancouver.example.com
   CSeq: 294444656 NOTIFY
   Contact: <sip:pres.vancouver.example.com>
   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".  To avoid corrupting the
        S/MIME signature on the data received from the RLS in
        stockholm.example.org, the local RLS copies the entire
        multipart/signed body as-is into the notification that it sends.

   Local RLS -> Terminal

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

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

   <?xml version="1.0" encoding="UTF-8"?>
   <list xmlns="urn:ietf:params:xml:ns:rlmi"
         uri="sip:adam-friends@pres.vancouver.example.com" version="2"
         fullState="false">
     <name xml:lang="en">Buddy List at COM</name>
     <name xml:lang="de">Liste der Freunde an COM</name>
     <resource uri="sip:ed@dallas.example.net">
       <name>Ed at NET</name>
       <instance id="sdlkmeopdf" state="pending"/>
     </resource>
     <resource uri="sip:adam-friends@stockholm.example.org">
       <name xml:lang="en">My Friends at ORG</name>
       <name xml:lang="de">Meine Freunde an ORG</name>
       <instance id="cmpqweitlp" state="active"
                 cid="1KQhyE@pres.vancouver.example.com"/>
     </resource>
   </list>

   --TfZxoxgAvLqgj4wRWPDL
   Content-Transfer-Encoding: binary
   Content-ID: <1KQhyE@pres.vancouver.example.com>
   Content-Type: multipart/signed;
       protocol="application/pkcs7-signature";
       micalg=sha1;boundary="l3WMZaaL8NpQWGnQ4mlU"

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

   --tuLLl3lDyPZX0GMr2YOo
   Content-Transfer-Encoding: binary
   Content-ID: <Cvjpeo@stockholm.example.org>
   Content-Type: application/rlmi+xml;charset="UTF-8"
   <?xml version="1.0" encoding="UTF-8"?>
   <list xmlns="urn:ietf:params:xml:ns:rlmi"
         uri="sip:adam-friends@stockholm.example.org" version="1"
         fullState="true">
     <name xml:lang="en">Buddy List at ORG</name>
     <name xml:lang="de">Liste der Freunde an ORG</name>
     <resource uri="sip:joe@stockholm.example.org">
       <name>Joe Thomas</name>
       <instance id="1" state="active"
                 cid="mrEakg@stockholm.example.org"/>
     </resource>
     <resource uri="sip:mark@stockholm.example.org">

       <name>Mark Edwards</name>
       <instance id="1" state="active"
                 cid="KKMDmv@stockholm.example.org"/>
     </resource>
   </list>

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

   <?xml version="1.0" encoding="UTF-8"?>
   <presence xmlns="urn:ietf:params:xml:ns:pidf"
       entity="sip:joe@stockholm.example.org">
     <tuple id="x823a4">
       <status>
         <basic>open</basic>
       </status>
       <contact priority="1.0">sip:joe@stockholm.example.org</contact>
     </tuple>
   </presence>

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

   <?xml version="1.0" encoding="UTF-8"?>
   <presence xmlns="urn:ietf:params:xml:ns:pidf"
       entity="sip:mark@stockholm.example.org">
     <tuple id="z98075">
       <status>
         <basic>closed</basic>
       </status>
     </tuple>
   </presence>
   --tuLLl3lDyPZX0GMr2YOo--

   --l3WMZaaL8NpQWGnQ4mlU
   Content-Transfer-Encoding: binary
   Content-ID: <K9LB7k@stockholm.example.org>
   Content-Type: application/pkcs7-signature

   [PKCS #7 signature here]

   --l3WMZaaL8NpQWGnQ4mlU--

   --TfZxoxgAvLqgj4wRWPDL--

   14.  The terminal completes the NOTIFY transaction.

   Terminal -> Local RLS

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

7.  Security Considerations

   Note that the mechanisms for obtaining state information for
   resources in a list are generally left to the RLS implementor.  Some
   of the security issues below are specific to the circumstance in
   which a SIP back-end subscription is used for such a purpose.  Non-
   SIP mechanisms for obtaining state information of resources in a list
   will typically have their own security issues associated with doing
   so; however, exhaustively enumerating such access methods is not
   possible in this document.  Implementors using such mechanisms must
   analyze their chosen access methods for relevant security issues.

7.1.  Authentication

   If back-end subscriptions are required to retrieve resource state
   information, the end user is no longer the direct subscriber to the
   state of the resource.  This means that direct authentication of the
   user is no longer possible.

7.1.1.  RLS and Subscriber in the Same Domain

   It is expected that the most common deployment of RLSes entails that
   the subscribers to the RLS will be in the same domain as the RLS.
   When this is the case, the RLS then has the ability to act as an
   authentication service.  The role of authentication service is
   defined in "Enhancements for Authenticated Identity Management in the
   Session Initiation Protocol (SIP)" [7].

   At a high level, under this system, the RLS authenticates the
   subscriber and then includes an "Identity" header field in all of the
   back-end subscriptions performed on behalf of that authenticated
   user.  This "Identity" header field cryptographically asserts that
   the request has been authorized to be made on behalf of the user
   indicated in the "From" header field.

   Because the ability to authenticate requests is central to the proper
   functioning of the network, any RLS that uses SIP back-end
   subscriptions to acquire information about the resources in a
   resource list MUST be able to act as an authentication service as
   defined in [7], provided that local administrative policy allows it
   to do so.

      In other words, all RLS implementations that support back-end SIP
      subscriptions also must include the ability to be configured to
      act as an authentication service.  Whether any given administrator
      chooses to activate such a feature is completely up to them.  Of
      course, lacking the ability to act as an identity server, any RLS
      so configured will behave as described in the following section,
      since it is effectively acting as if it were in a different domain
      than the user.

7.1.2.  RLS and Subscriber in Different Domains

   In the general case, the SIP Authenticated Identity extensions do not
   provide a means for the RLS to securely assert that subscriptions are
   being performed on the end user's behalf.  Specifically, when the
   subscriber and the RLS are in different domains, the RLS will have no
   means by which it can vouch for the user's identity.  Mechanisms by
   which back-end subscriptions in such circumstances can be
   authenticated are left for future study.

   Until such general solutions are developed, RLSes that are in a
   different domain than the subscriber on whose behalf they are
   creating back-end subscriptions SHOULD subscribe to the resources
   using their own identity.  By doing so, the RLS will generally obtain
   only the resource information that is made publicly available.

   Absent such general solutions, implementations of subscriber user
   agents MAY attempt direct subscriptions to resources in the resource
   list when subscribing to an RLS outside of their domain (either
   directly or by way of another resource list subscription).  The
   resources to be subscribed to will be those indicated in the "uri"
   attribute of the <resource> elements present in the RLMI document
   returned by the RLS.  Directly subscribing to the resources allows
   proper authentication of the user to take place, which will generally
   authorize them to receive more complete state information.
   Implementations that choose to perform such direct subscriptions
   SHOULD use the data retrieved instead of any information about the
   resource obtained via the list subscription.

7.2.  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 back-end 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 notifier for the resource maintains.
   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.

   To prevent the transmission of event information to anyone other than
   the intended recipient, implementations MUST NOT present the result
   of one back-end subscription to more than one user, unless:

   a.  The RLS has adequate access to the complete authorization policy
       associated with the resource to which the back-end subscription
       has been made, AND

   b.  The RLS can and has determined that presenting the information to
       more than one user does not violate such policy.

   Note that this is a very difficult problem to solve correctly.  Even
   in the cases where such access is believed possible, this mode of
   operation is NOT RECOMMENDED.

7.3.  Signing and Sealing

   Implementors should keep in mind that any section of the MIME body
   may be signed and/or encrypted as necessary.  Resource List Servers
   should take care not to modify any MIME bodies they receive from any
   back-end subscriptions, 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 "application/
   pkcs7-mime" content types to any SIP back-end subscriptions, if the
   subscriber includes them in the initial SUBSCRIBE message.  Not doing
   so may actually result in resources refusing 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 implementation of encryption and signing by the RLS
   is not necessary to be able to pass through signed and/or encrypted
   bodies.

7.4.  Infinite Loops

   One risk introduced by the ability to nest resource lists is the
   possibility of creating lists that ultimately contain themselves as a
   sub-list.  Detection and handling of such a case is trivial when the
   RLS services all the virtual subscriptions internally.  When back-end
   subscriptions are created to service virtual subscriptions, however,
   detection of such situations becomes a more difficult problem.

   Implementors of RLSes that create back-end subscriptions MUST
   implement safeguards to prevent such nestings from creating an
   infinite loop of subscriptions.  Typically, such mechanisms will
   require support in the back-end subscription protocol.  In
   particular, applying filters to the back-end subscriptions can be an
   effective way to preclude such problems.

8.  IANA Considerations

8.1.  New SIP Option Tag: eventlist

   This section defines a new option tag for the registry established by
   Section 27.1 of RFC 3261[1].

   Option Tag Name:  eventlist

   Description:  Extension to allow subscriptions to lists of resources.

   Published specification:  RFC 4662

8.2.  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 [12] 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 MIME type are discussed in RFC 4662.  RFC 1874 [11] and RFC
      3023 [12] 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 4662

   Applications that use this media type:  This media type is used to
      convey meta-information for the state of lists of resources within
      a Session Initiation Protocol (SIP) subscription.

   Additional information:
      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@estacado.net
      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.

8.3.  URN Sub-Namespace

   URI:  urn:ietf:params:xml:ns:rlmi

   Description:  This is the XML namespace URI for XML elements defined
      by RFC 4662 to describe information about subscriptions when such
      subscriptions are aggregated within a single SIP subscription.  It
      is used in the application/rlmi+xml body type.

   Registrant Contact:
      Name:  Adam Roach
      E-Mail:  adam@estacado.net
      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.

   XML:
      BEGIN
        <?xml version="1.0"?>
        <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
            "http://www.w3.org/TR/xhtml-basic/xhtml-basic10.dtd">
        <html xmlns="http://www.w3.org/1999/xhtml">
        <head>
          <meta http-equiv="content-type"
             content="text/html;charset=utf-8"/>
          <title>Namespace for SIP Event Resource List
                 Meta-Information</title>
        </head>
        <body>
          <h1>Namespace for SIP Event Resource List
              Meta-Information</h1>
          <h2>application/rlmi+xml</h2>
          <p>See <a href="[http://www.rfc-editor.org/rfc/rfc4662.txt]">
             RFC4662</a>.</p>
        </body>
        </html>
      END

9.  Acknowledgements

   Thanks to Sean Olson for a review of and corrections to the usage of
   XML in this protocol.

   Thanks also to Hisham Khartabil, Paul Kyzivat, Keith Drage, and
   Robert Sparks for their careful reviews of and comments on this
   document.

10.  References

10.1.  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. B., "Session Initiation Protocol (SIP)-Specific Event
        Notification", RFC 3265, June 2002.

   [3]   Freed, N. and N. Borenstein, "Multipurpose Internet Mail
        Extensions (MIME) Part One: Format of Internet Message Bodies",
        RFC 2045, November 1996.

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

   [5]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [6]  Alvestrand, H., "Tags for the Identification of Languages", BCP
        47, RFC 3066, January 2001.

   [7]  Peterson, J. and C. Jennings, "Enhancements for Authenticated
        Identity Management in the Session Initiation Protocol (SIP)",
        RFC 4474, August 2006.

10.2.  Informative References

   [8]   Rosenberg, J., "A Presence Event Package for the Session
         Initiation Protocol (SIP)", RFC 3856, August 2004.

   [9]   Burger, E., "A Mechanism for Content Indirection in Session
         Initiation Protocol (SIP) Messages", RFC 4483, May 2006.

   [10]  Peterson, J., "Common Profile for Presence (CPP)", RFC 3859,
         August 2004.

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

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

   [13]  Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
         (S/MIME) Version 3.1 Message Specification", RFC 3851, July
         2004.

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

   [15]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

Authors' Addresses

   Adam Roach
   Estacado Systems
   US

   EMail: adam@estacado.net

   Ben Campbell
   Estacado Systems
   US

   EMail: ben@estacado.net

   Jonathan Rosenberg
   Cisco Systems
   600 Lanidex Plaza
   Parsippany, NJ  07054-2711
   US

   EMail: jdrosen@cisco.com

Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

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   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
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