SIMPLE                                                      J. Rosenberg
Internet-Draft                                                S. Donovan
Intended status: Standards Track                              K. McMurry
Expires: May 15, 2008                                              Cisco
                                                       November 12, 2007


            Optimizing Federated Presence with View Sharing
                 draft-rosenberg-simple-view-sharing-00

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
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   This Internet-Draft will expire on May 15, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   Presence federation refers to the exchange of presence information
   between systems.  One of the primary challenges in presence
   federation is scale.  With a large number of watchers in one domain
   obtaining presence for many presentities in another, the amount of
   notification traffic is large.  This document describes an extension
   to the Session Initiation Protocol (SIP) event framework, called view
   sharing.  View sharing can substantially reduce the amount of



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   traffic, but requires a certain level of trust between domains.  View
   sharing allows the amount of presence traffic between domains to
   achieve the theoretical lower bound on information exchange in any
   presence system.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  4
   3.  RLS Behavior . . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  On Receipt of a Resource List Subscription Request . . . .  7
       3.1.1.  Authentication and Authorization . . . . . . . . . . .  7
       3.1.2.  No Active Back-End Subscription  . . . . . . . . . . .  7
       3.1.3.  Active Back-End Subscription . . . . . . . . . . . . .  8
     3.2.  Processing NOTIFY Requests . . . . . . . . . . . . . . . .  9
       3.2.1.  Processing ACL-Infos . . . . . . . . . . . . . . . . .  9
       3.2.2.  Processing Presence Documents  . . . . . . . . . . . . 10
       3.2.3.  Processing Back-End Terminations . . . . . . . . . . . 11
   4.  Presence Agent Behavior  . . . . . . . . . . . . . . . . . . . 11
     4.1.  Authentication and Authorization . . . . . . . . . . . . . 11
     4.2.  Processing Initial SUBSCRIBE Requests  . . . . . . . . . . 12
     4.3.  SUBSCRIBE Refreshes  . . . . . . . . . . . . . . . . . . . 12
     4.4.  Policy Changes . . . . . . . . . . . . . . . . . . . . . . 13
     4.5.  Presence State Changes . . . . . . . . . . . . . . . . . . 14
   5.  ACL Format . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     5.1.  Document Structure and Semantics . . . . . . . . . . . . . 15
     5.2.  Trust Considerations when Construcing ACLs . . . . . . . . 16
     5.3.  Example Documents  . . . . . . . . . . . . . . . . . . . . 17
     5.4.  Rule Determination Algorithm . . . . . . . . . . . . . . . 18
     5.5.  XML Schema . . . . . . . . . . . . . . . . . . . . . . . . 20
   6.  Performance Analysis . . . . . . . . . . . . . . . . . . . . . 20
   7.  Requirements Analysis  . . . . . . . . . . . . . . . . . . . . 21
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 23
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
     9.1.  MIME Type Registration . . . . . . . . . . . . . . . . . . 24
     9.2.  URN Sub-Namespace Registration . . . . . . . . . . . . . . 25
     9.3.  Schema Registration  . . . . . . . . . . . . . . . . . . . 26
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 26
     10.2. Informative References . . . . . . . . . . . . . . . . . . 27
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28
   Intellectual Property and Copyright Statements . . . . . . . . . . 29








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1.  Introduction

   Presence refers to the ability, willingness and desire to communicate
   across differing devices, mediums and services [RFC2778].  Presence
   is described using presence documents [RFC3863] [RFC4479], exchanged
   using a SIP-based event package [RFC3856].

   Presence federation refers to the interconnection of disparate
   systems for the purposes of sharing presence information.  This
   interconnection involves passing of subscriptions from one system to
   another, and then the passing of notifications in the opposite
   direction.

   [I-D.ietf-simple-interdomain-scaling-analysis] has analyzed the
   amount of traffic, in terms of messages and in terms of bytes, which
   flow between systems in various federated use cases.  These numbers
   demonstrate that presence traffic can be a substantial source of
   overhead.  The root cause of this scale challenge is the so-called
   multiplicative effect of presence data.  If there are N users, each
   of which have B buddies on their buddy list, and each buddy changes
   state L times per hour, the amount of notification traffic is
   proportional to N*B*L. For example, in the case of two extremely
   large public IM providers that federate with each other (each with 20
   million users), [I-D.ietf-simple-interdomain-scaling-analysis] shows
   that the amount of traffic due to these steady state notifications is
   18.4 billion messages per day, an astoundingly large number.
   Overhead for subscription maintenance and refreshes brings the total
   to 25.6 billion per day.

   The overhead for SIP-based presence can be reduced using SIP
   optimizations.  In particular, [I-D.ietf-sip-subnot-etags] can reduce
   the amount of traffic due to refreshes and polls.  However, this
   optimization targets the overhead, and doesn't address the core
   scaling problem - the multiplicative effect of presence data.

   For this reason, there is a clear need to improve the scale of SIMPLE
   in federated envrionments.
   [I-D.houri-sipping-presence-scaling-requirements] has laid out a set
   of requirements for optimizations.  The essence of these requirements
   are that the extension should improve performance, while being
   backwards compatible and supporting the privacy and policy
   requirements of users.

   This document defines a mechanism called view sharing in support of
   those requirements.  The key idea with view sharing is that when
   there are many watchers in a domain to a single presentity in another
   domain, each of which is actually going to get the exact same
   presence document, the domain of the watchers extends a single



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   subscription to the domain of the presentity, and the domain of the
   presentity sends a single copy of the presence document back to the
   domain of the watcher.  Assuming a symmetrical system whereby the
   average buddies per watcher is B and the average number of watchers
   for a user is also B, this optimization can reduce the overall
   subscription overhead and notification traffic by a factor of B.


2.  Overview of Operation

   The extensions works in the environment shown in Figure 1.  The
   environment assumes two domains.  There are some number of watchers
   (W1 - W3) in the domain on the left, which we call the watching
   domain.  All of those watchers are interested in the presence of a
   single presentity P1 in the domain on the right, which we call the
   serving domain.  The watchers all make use of a resource list server
   (RLS) [RFC4662] which stores their buddy lists and performs the buddy
   list expansion.  Consequently, when each watcher subscribes to their
   buddy list on the RLS, in absence of any optimizations, the RLS will
   generate three separate subscriptions to P1, each of which reaches
   the presence server in the serving domain.


                                        .
                +--------------+        .     +--------------+
                |              |        .     |              |
                |              |  SUB   .     |              |
                |              | -------.---> |   Presence   |
                |     RLS      |  NOT   .     |   Server     |
                |              | <------.---- |              |
                |              |        .     |              |
                |              |        .     |              |
                +--------------+        .     +--------------+
                 ^      ^     ^         .            ^
          List   |      |     |         .            | PUB
          SUB    |      |     |         .            |
                 |      |     |         .            |
             +----+  +----+  +----+     .          +----+
             |    |  |    |  |    |     .          |    |
             | W1 |  | W2 |  | W3 |     .          | P1 |
             |    |  |    |  |    |     .          |    |
             +----+  +----+  +----+     .          +----+
                                        .
                                        .
                                        .
                Watching                .        Serving
                Domain                  .        Domain
                                        .



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                        Figure 1: Deployment Model

   Of course, in practice each domain will act as both a watching domain
   and a serving domain, thus implementing both sides of the system.

   The initial SUBSCRIBE generated by the RLS includes a SIP option tag
   "view-share" in the Supported header field, indicating that the RLS
   supports the view sharing extension.  If the presence server also
   supports the extension, it makes use of it and includes an indication
   of this fact in the Require header field in the SUBSCRIBE response
   and in NOTIFY requests it generates.

   View sharing requires a level of trust between the two domains.
   Consequently, the connection between them utilizes TLS with mutual
   authentication.  The presence server verifies that the certificate
   presented in the mutual authentication matches the domain of the
   watcher.

   If this is the first subscription from domain 1 for that particular
   presentity, the presence server accepts the subscription (assuming
   the watcher is authorized of course).  The notifications sent to the
   RLS include two separate pieces of state.  One is the actual presence
   state for the presentity.  The other is an Access Control List (ACL)
   document.  This document describes the set of other watchers from the
   originating domain, if any, who are authorized to see exactly the
   same presence document - in other words, the set of users that share
   the same view.  Should one of those watchers seek the presence of
   that presentity, the RLS from the originating domain does not need to
   generate a back-end subscription; rather, it just uses the presence
   document it is receiving from the original subscription, and passes
   it to both watchers.  The ACL can also list users in the originating
   domain that are authorized to subscribe to that presentity, but who
   will end up receiving a different view.  Should one of those watchers
   subscribe, the RLS knows that it must perform a back-end subscription
   to obtain that view.  The ACL can also list watchers in the
   originating domain that are not authorized at all, in which case the
   RLS could immediately reject their subscriptions.  Finally, if the
   ACL says nothing about a particular watcher, it means that the
   presence server has elected to say nothing about what view this
   watcher will receive.  Consequently, the RLS must perform a back-end
   subscription should that watcher subscribe to the presentity.

   Other subsequent subscriptions to the same presentity from the same
   originating domain are processed in a similar way.  However, the
   presence server in the serving domain will keep track of the set of
   subscriptions to the same presentity from the same RLS which are to
   receive the same view.  When a presence notification is to be sent,
   instead of sending it on all subscriptions, the notification is sent



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   on just a single subscription.

   Should the authorization policies in the serving domain change, an
   updated ACL is sent, informing the watching domain of the new
   policies.  This may require the watching domain to extend a back-end
   subscription to obtain a view, or may change the view an existing
   watcher is receiving, and so on.

   The ACL allows the serving domain a great deal of flexibility in the
   level of trust it imparts to the watching domain.  The following are
   all possible approaches that the serving domain can utilize:

   Minimal Trust:  When a watcher subscribes to a presentity, the ACL
      generated for that subscription includes only that watcher, along
      with an identifier for their view.  Consequently, for each watcher
      in domain 1 there will be a backend subscription to domain 2.
      However, should multiple watchers share the same view, the
      presence server in domain 2 sends a single presence document on
      one of the subscriptions, and the RLS uses this for all of the
      other watchers with the same view.  With this approach, domain 2
      never discloses the list of authorized watchers ahead of time, and
      it has full knowledge of each watcher that is subscribed.
      However, it gets the performance benefits of reducing the amount
      of notification traffic.

   Partial Trust:  When a watcher subscribes to a presentity, the ACL
      generated for that subscription includes that watcher and all
      other watchers authorized for that same view.  Consequently, there
      will only be one backend subscription from the RLS to the presence
      server for each view.  However, the full set of authorized
      watchers is not disclosed ahead of time, only those that will get
      the same view.  With partial trust, the presence server will not
      know the full set of watchers currently subscribed.

   Full Trust:  When a watcher subscribes to a presentity, the ACL
      generated for that subscription includes that watcher and all
      other watchers that are authorized for that view, and all other
      views, along with a rule that says that all other watchers get
      rejected.  In this case, as with partial trust, there is only one
      backend subscription from the RLS to the presence server for each
      view.  The full set of watchers is disclosed ahead of time as
      well.  The presence server will not know the full set of watchers
      currently subscribed.








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3.  RLS Behavior

   This section defines the procedures that are to be followed by the
   RLS.  It is important to note that, even though this specification
   defines an extension to the SIP events framework, that extension is
   only useful for the back-end subscriptions generated by an RLS.  The
   extension defined here is not applicable or useful for individual
   users generating subscriptions.  Indeed, if it were utilized by
   individual users, it has the potential for violations of user
   privacy.  See Section 8 for a discussion.

3.1.  On Receipt of a Resource List Subscription Request

   When the RLS receives a subscription to a resource list which
   includes some presentity P in another domain, it follows the rules
   defined here.

3.1.1.  Authentication and Authorization

   First, the RLS MUST check a configured list of peer domains for which
   this extension is to be applied.  Because of the potential privacy
   disclosures involved in unauthorized use of this facility, it can
   only be used between pairs of domains which have a pre-arranged
   agreement to utilize it.  If the domain of the presentity P matches
   one of the configured list of peer domains, the RLS is permitted to
   utilize this extension.  If not, the extension MUST NOT be used.

   Next, the RLS MUST send the SUBSCRIBE request over a mutually
   authenticated TLS connection.  The RLS MUST check that the
   subjectAltName in the certificate of its peer contains a domain name
   that is a match for the domain of the URI of the presentity.  If they
   are not a match, view sharing cannot be utilized for this
   subscription.

   The procedures followed by the RLS after this point depend on whether
   the RLS already has a backend subscription to the presentity that is
   in the active state, and for which an ACL has been received.

3.1.2.  No Active Back-End Subscription

   The RLS MUST generate a back-end subscription to obtain the state of
   the presentity.  The RLS MUST include a Supported header field in the
   request with the option tag "view-share".  The Accept header field
   MUST be present in the request and MUST include the "application/
   aclinfo+xml" MIME type amongst the list of supported types.

   Note that it is possible that two watchers, in a short period of
   time, both subscribe to their resource lists, both of which include



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   presentity P. This will cause the RLS to generate two back-end
   subscriptions at around the same time.  The RLS is forced to generate
   the second back-end subscription because it doesn't have an active
   back-end subscription that has yet generated an ACL.  Once both
   subscriptions become active and generate ACLs, if the watchers are
   receiving the same view and both ACLs contain both watchers, the RLS
   SHOULD terminate one of the back-end subscriptions.

3.1.3.  Active Back-End Subscription

   In this case, the RLS already has at least one back-end subscription
   to the target presentity P, and it has received at least one ACL for
   that presentity.  It has received a resource list subscription from
   watcher W which includes presentity P. Based on the procedures of
   Section 3.2.1, the RLS will keep, for each presentity, the list of
   the most recent ACLs received on each back-end subscription currently
   in place.  This is called the current ACL list.

   For each ACL Ai in the current ACL list, the RLS performs the rule
   determination algorithm of Section 5.4 to compute the rule ID for the
   watcher W. This represents the view that the watcher is supposed to
   receive.

   Next, the RLS goes through all subscriptions it currently has for
   presentity P. For each one, it takes the identity of the watcher for
   that actual subscription.  The identity for the watcher for that
   actual subscription is equal to the asserted identity included in the
   back-end subscription.  For example, if SIP Identity [RFC4474] is
   utilized, this would be the URI present in the From header field of
   the back-end SUBSCRIBE.  Call the watcher identity for each
   subscription Wj.

   Next, the RLS computes the rule determination algorithm of
   Section 5.4 to compute the rule ID Rj for the watcher Wj on each
   subscription j.  This represents the rule ID for the view being
   delivered on that subscription.

   Then, processing depends on the values of R and Rj:

   o  If R is null, it means that the ACL doesn't specify the view for
      this watcher.  The RLS MUST generate a back-end subscription to
      presentity P, and MUST use watcher W as the identity in the back-
      end subscription.

   o  If R is not null, but the associated rule is blocked, it means
      that the watcher will be rejected.  The RLS SHOULD NOT perform
      another back-end subscription, and must act as if it had created a
      back-end subscription which was rejected.



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   o  If R was not null, and there is at least one subscription j for
      which Rj = R, it means that subscription j is already generating
      notifications for the view that watcher W is supposed to receive.
      In that case, the RLS SHOULD NOT generate a back-end subscription
      for P on behalf of W. Rather, it should treat the existing back
      end subscription j as if it were the back-end subscription for W,
      and follow the guidelines of RFC 4662 [RFC4662] based on that.
      Subscription j is called the generating subscription for watcher
      W, and the actual watcher associated with subscription j, Wj, is
      called the generating watcher Wgen for watcher W.

   o  If R was not null, but there is no subscription j for which Rj=R,
      it means that the RLS is not yet receiving the view that watcher W
      requires.  The RLS MUST generate a back-end subscription to
      presentity P, and MUST use watcher W as the identity in the back-
      end subscription.

3.2.  Processing NOTIFY Requests

   If a NOTIFY request arrives with a Require header field that includes
   the "view-share" option tag, it MUST be processed according to the
   rules of this specification.

3.2.1.  Processing ACL-Infos

   If the contents of the NOTIFY are of type "application/aclinfo+xml",
   the subscriber processes the body as described here.

   For each presentity that the RLS has at least one back-end
   subscription for, the RLS MUST remember the most recent aclinfo
   received on each back-end subscription.  This is called the current
   ACL list for the presentity.  This set of aclinfo is used in the
   processing of subscription requests, as described in Section 3.1.3.

   The serving domain can change policies at any time.  When it does, it
   sends an updated ACL on one or more subscriptions.  The RLS MUST
   store this ACL.  Furthermore, the ACL might impact the views being
   received by watchers, and may impact the state of the back-end
   subscriptions.

   The RLS computes the set of watchers Wi which have a resource list
   subscription that includes the presentity P for whom an updated ACL
   has just been received.  For each Wi, it performs the view
   determination algorithm (see Section 5.4 on the current ACL set.  Let
   Ri be the view associated with watcher Wi.  If Ri has not changed
   from prior to the receipt of the new ACL, no action is taken.
   However, if it has changed, the RLS takes the set of current back-end
   subscriptions, and for each subscription j, computes the view



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   determination algorithm for its associated watcher Wj, to produce Rj.
   The action to take depends on what has changed:

   o  If Ri is now null, it means that the serving domain has changed
      the views associated with watcher Wi, and this new view is not
      known to the RLS.  The RLS MUST generate a new back-end
      subscription on behalf of watcher Wi for presentity P to obtain
      this view.

   o  If Ri is now a blocked rule, it means that the serving domain has
      now blocked Wi from obtaining the presence of the presentity.  The
      RLS must act as if it had a back-end subscription on behalf of
      watcher Wi which was terminated.

   o  If Ri is not null and not blocked, and if there is an Rj which
      matches the new Ri, it means that the serving domain has changed
      the views associated with watcher Wi, and changed them to another
      view already being received by the RLS.  The RLS MUST treat this
      back-end subscription j as if it were the back-end subscription to
      presentity P for watcher Wi.  If the most recent presence document
      received on this back-end subscription is not a semantic match for
      the presence document most recently delivered to Wi for presentity
      P, the RLS MUST send this most recent presence document to watcher
      Wi.

   o  If Ri is not null and not blocked, but there is no Rj which
      matches the new Ri, it means that the serving domain has changed
      the views associated with watcher Wi, and this new view is not one
      currently being delivered to the RLS.  The RLS MUST generate a new
      back-end subscription on behalf of watcher Wi for presentity P to
      obtain this view.

   Furthermore, if there are now two back-end subscriptions j1 and j2
   for which Aj1 = Aj2, the RLS SHOULD terminate one of those two
   subscriptions.  Two ACL documents are considered equal if they
   enumerate the same set of rules with the same members for each rule.

3.2.2.  Processing Presence Documents

   If the contents of the NOTIFY is a presence document, the RLS follows
   the procedures defined here.

   Let Wj be the watcher on the subscription j on which the presence
   document was just received.  Let Rj be the results of running the
   rule determination algorithm on Wj using the current ACL set.  Next,
   the RLS takes the set of watchers Wi which have presentity P on their
   buddy lists.  The RLS then runs the rule determination algorithm on
   each Wi using the current ACL set, producting Ri for each watcher Wi.



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   For each Ri that is equal to Rj, the RLS MUST utilize the presence
   document just received as if the back-end subscription j was in fact
   for watcher Wi.  This will typically cause that presence document to
   be sent in a NOTIFY request to each such watcher, though each watcher
   may have some kind of filtering policy which causes the RLS to modify
   the document prior to delivery.

3.2.3.  Processing Back-End Terminations

   If the NOTIFY request from the serving domain terminates the back-end
   subscription, it may be because the watcher Wj associated with that
   subscription is no longer permitted to view the presence of the
   presentity.

   The ACL associated with the subscription MUST be removed from the
   current ACL set.  The procedures of Section 3.2.1 MUST be performed
   to adjust back-end subscriptions, if needed.


4.  Presence Agent Behavior

   When a presence agent receives a SUBSCRIBE request containing a
   Supported header with the value "view-share", and it wishes to
   utilize view sharing for this subscription, it follows the procedures
   defined here.

4.1.  Authentication and Authorization

   First, the presence agent MUST have received the SUBSCRIBE request
   over a mutually authenticated TLS connection.  If it had not, view
   sharing cannot be utilized for this subscription.  The presence agent
   MUST check that the subjectAltName in the certificate of its peer
   contains a domain name that is a match for the domain of the URI of
   the watcher.  If they are not a match, view sharing cannot be
   utilized for this subscription.

   Assuming they are a match, the presence agent MUST check a configured
   list of peer domains for which this extension is to be applied.
   Because of the potential privacy disclosures involved in unauthorized
   use of this facility, it can only be used between pairs of domains
   which have a pre-arranged agreement to utilize it.  If the domain of
   the watcher W matches one of the configured list of peer domains, the
   presence agent is permitted to utilize this extension.  If not, the
   extension MUST NOT be used.







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4.2.  Processing Initial SUBSCRIBE Requests

   First, the subscription is processed as it normally would be,
   including authorization and policy around the presence document to be
   delivered to the watcher.  Furthermore, if the presence agent wishes
   to utilize view sharing for this subscription, it MUST include a
   Require header field in the first NOTIFY request (and indeed any
   subsequent ones) it sends confirming this subscription, and that
   NOTIFY MUST contain the "view-share" option tag.

   Furthermore, the initial state sent by the presence agent MUST
   include an ACL document.  It is formatted according to the rules and
   considerations in Section 5.

   The initial state sent by the presence agent might include an actual
   presence document.  In particular, a presence document MUST be sent
   if one of the following is true:

   o  There is only one subscription from the watching domain to this
      presentity that has the view associated with the watcher.

   o  There is more than one subscription from the watching domain to
      this presentity with the same view, but the User-Agent header
      field in the request differs between them.

   If one of these conditions is not true, the presence agent SHOULD NOT
   send an initial presence document on this subscription.

      OPEN ISSUE: This bit about user agent is trying to address the
      case where there are a multiplicity of RLS in the originating
      domain, each of which serves a different subset of the watcher
      population.  If two watchers get the same view, but are served by
      different RLS, presence state must be sent to each.  There needs
      to be some way to indicate to the presence agent that
      subscriptions come from different RLS.  Other choices besides
      User-Agent are the Via header field or the TLS certificate.  There
      are pros and cons to each choice.  Possibly a separate header
      field for this?

   If an ACL and a presence document are to be delivered, they MUST be
   delivered in a separate NOTIFY request (unless the subscriber
   indicated support for multipart, in which case the content MAY be
   included in a single NOTIFY with mulitpart content).

4.3.  SUBSCRIBE Refreshes

   When the presence agent receives a SUBSCRIBE refresh, it MUST send
   the most recent ACL document, and if presence documents are being



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   sent for this subscription, the most recent presence document.

4.4.  Policy Changes

   There are several different types of policy changes that can occur:

   o  If the definitions for a particular rule change, the presence
      agent MUST assign a new rule ID for that rule.  For each
      subscription to a presentity which contained that rule, the
      presence agent MUST send an updated ACL which includes a rule with
      this new rule ID.

   o  If some watcher W was previously associated with rule X and is now
      associated with rule Y, the presence agent checks if it has any
      subscriptions from watcher W. If it does, it MUST send an updated
      ACL on that subscription.  Based on the rules in Section 5, this
      ACL will contain rule Y and will at least include W amongst the
      list of members.  Furthermore, if there were subscriptions from
      other watchers, but the presence agent had previously sent an ACL
      on the subscription which was a match for W, it MUST send an
      updated ACL on that subscription.  This updated ACL MAY omit a
      statement about rule Y or MAY include it.  However, the updated
      ACL MUST NOT claim that watcher W will receive rule X.

   o  If some watcher W was previously associated with rule X and is now
      blocked, the presence agent checks if it has any subscriptions
      from watcher W. If it does, it MUST terminate the back-end
      subscription.  If it doesn't, but it has a subscription from some
      other watcher which had included a rule that was a match for W,
      the presence agent MUST send an updated ACL on that subscription.
      This updated ACL MAY omit any statement about watcher W, or MAY
      include them as part of a blocked rule in that ACL.

   o  If some watcher W was previously blocked and is now permitted and
      associated with some rule X, the presence agent checks if it had
      any subscriptions from some other watcher which included a blocked
      rule that matched watcher W. If it had, it MUST send an updated
      ACL on that subscription.  That updated ACL MAY omit any statement
      about watcher W, or MAY indicate that watcher W is now associated
      with rule X.

   Of course, a policy change will also potentially alter the presence
   documents that are associated with a view.  If so, the presence agent
   MUST send an updated document on a subscription if one of the
   following is true:

   o  There is only one subscription from the watching domain to this
      presentity that has the view associated with the watcher.



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   o  There is more than one subscription from the watching domain to
      this presentity with the same view, but the User-Agent header
      field in the request differs between them.

   If neither is true, the presence agent MUST select one subscription
   amongst the several which share the same presentity, view, and User-
   Agent header field, and sent an updated notification on that
   subscription.  The choice of subscriptions is arbitrary and MAY
   change for each notification.

4.5.  Presence State Changes

   If the state of some presentity changes, the presence agent may need
   to send an updated notification on a subscription.  The presence
   agent MUST send an update on a subscription if one of the following
   is true:

   o  There is only one subscription from the watching domain to this
      presentity that has the view associated with the watcher.

   o  There is more than one subscription from the watching domain to
      this presentity with the same view, but the User-Agent header
      field in the request differs between them.

   If neither is true, the presence agent MUST select one subscription
   amongst the several which share the same presentity, view, and User-
   Agent header field, and sent an updated notification on that
   subscription.  The choice of subscriptions is arbitrary and MAY
   change for each notification.


5.  ACL Format

   An ACL document is an XML [W3C.REC-xml-20001006] document that MUST
   be well-formed and MUST be valid according to schemas, including
   extension schemas, available to the validater and applicable to the
   XML document.  ACL documents MUST be based on XML 1.0 and MUST be
   encoded using UTF-8.  This specification makes use of XML namespaces
   for identifying ACL documents and document fragments.  The namespace
   URI for elements defined by this specification is a URN [RFC2141],
   using the namespace identifier 'ietf' defined by RFC 2648 [RFC2648]
   and extended by RFC 3688 [RFC3688].  This URN is:

      urn:ietf:params:xml:ns:aclinfo







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5.1.  Document Structure and Semantics

   An ACL document informs a watching domain of the set of views that
   can be received by that domain, and associates specific watchers with
   specific views.  It is very important to understand that the ACL
   document does not convey the actual processing that will be applied
   by the serving domain.  It does not indicate, for example, whether
   geolocation is present in a presence document, or which rich presence
   [RFC4480] data elements will be conveyed.  It merely provides
   grouping - indicating which watchers from the watching domain will
   receive the same view.

   Each ACL document starts with the enclosing root element <acl-list>.
   This contains the list of rules defined by the ACL.  Each rule is
   represented by the <rule> element.  Each rule represents a specific
   view, which is generated by the presence server based on its
   authorization, composition and filtering policies.  Each rule is
   associated with a rule ID, which is a mandatory attribute of the
   <rule> element.  This ID is scoped within a single presentity.  That
   is, the IDs for two rules for different presentities are unrelated.

   The <rule> element also contains an optional "blocked" boolean
   attribute.  If "true", it means that the rule specifies that the
   associated set of watchers will be rejected, should they subscribe.
   This can be used by the watching domain to avoid performing back-end
   subscriptions to users which will only be blocked anyway.

   Each <rule> contains the set of users that will receive the
   corresponding view.  This can be described by an enumerated set or by
   a default.  If it is an enumerated set, the <rule> is followed by a
   sequence of <member> elements, each of which contains a SIP URI for
   the watcher that will receive that view.

   The default view is specified by including a single child element for
   <rule> - <other>.  The default view applies to all watchers except
   those enumerated by other rules.  For this reason, an ACL document
   which contains a default view MUST include the rule IDs and
   associated members for all other views that are delivered to
   watchers.  For example, consider a presentity that has three views.
   View 1 is delivered to watchers A and B. View 2 is delivered to
   watcher C. View 3 is delivered to everyone else.  An ACL document
   that includes the default view must also include views 1 and 2 with
   watchers A, B, and C.

   In contrast, an ACL document that does not include a default does not
   need to include all views, and it does not need to include all
   members for a particular view.  Using the example above, it is valid
   to include an ACL document which includes only view 1 with watcher 1.



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   If two URI are present within <member> elements within the same
   <rule>, it represents a contract by the presence server that both
   users MUST get the same view.  Formally, if the presence server were
   to receive a subscription from each watcher, both subscriptions would
   be accepted or both would be rejected, and if accepted, each
   subscription would receive semantically identical presence documents
   at approximately the same time.

   Even if two users will receive the same view, a presence server MAY
   assign each to a different view ID.  There is no requirement that two
   unique views actually contain different presence data.  The only
   requirement is that, if two users are listed within the same rule,
   that they do in fact receive the same view.

   An ACL document delivered in a subscription from watcher W MUST
   include the view associated with watcher W and MUST include watcher W
   explicitly in a <member> element or implicitly by presence of an
   <other> element.

5.2.  Trust Considerations when Construcing ACLs

   The semantics above give very little guidance about what a presence
   server should include in an ACL.  The amount of information to convey
   depends on the level of trust between the watching and serving
   domains.

   Optimal performance is achieved when the ACL document for a
   presentity includes all views that the server might ever deliver, and
   includes all members for each view, including any defaults and
   blocked rules.  However, this informs the watching domain of the set
   of allowed and blocked watchers, and associated groupings amongst
   watchers.

   Slightly worse performance is achieved when the ACL document for a
   presentity sent in a subscription from watcher W includes only a
   single view - the one for watcher W - along with the full set of
   watchers which will also receive that view, assuming it is not the
   default view.  If the view is the default view, the document can
   include just watcher W. This approach will cause back-end
   subscriptions from every watcher that will receive the default, but
   it discloses less information to the watching domain.  In particular,
   the full set and number of views is never known by the watching
   domain.  The fact that a view is default is never known by the
   watching domain.  The full set of users that are permitted to view
   the presence of the presentity is never disclosed to the watching
   domain.  The performance of this approach is still reasonably good
   when the default rule is blocked.  However it is much less effective
   when the default is not blocked, and many watchers receive the



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   default.

   Another choice for construction of ACL documents is to include, in a
   subscription from watcher W, a single rule containing the rule ID for
   the view that watcher W will receive, along with a single member - W.
   This approach will still result in a back-end subscription from each
   watcher.  However, a single notification is sent for each view,
   rather than one per watcher.  The benefit of this construction is
   that it provides the watching domain no additional information about
   the authorization policies of the presentity than if this extension
   were not in use at all.

5.3.  Example Documents

   The example document in Figure 2 shows the case when there is maximum
   trust between domains.  The full set of watchers, include a blocked
   default, is included.


<?xml version="1.0" encoding="utf-8"?>
<!-- Created with Liquid XML Studio 1.0.6.0 (http://www.liquid-technologies.com) -->
<acl-list xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <rule id="6228">
    <member>sip:user1@example.com</member>
    <member>sip:user2@example.com</member>
    <member>sip:user3@example.com</member>
    <member>sip:user4@example.com</member>
    <member>sip:user5@example.com</member>
  </rule>
  <rule id="3584">
    <member>sip:user6@example.com</member>
  </rule>
  <rule id="1735">
    <member>sip:user7@example.com</member>
    <member>sip:user8@example.com</member>
    <member>sip:user9@example.comm</member>
    <member>sip:user10@example.com</member>
    <member>sip:user11@example.com</member>
  </rule>
  <rule blocked="true" id="9433">
    <other />
  </rule>
</acl-list>

                   Figure 2: Example with Maximum Trust

   The example in Figure 3 shows a moderate level of trust.  This ACL
   only shows the view associated with the watcher user1.



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<?xml version="1.0" encoding="utf-8"?>
<!-- Created with Liquid XML Studio 1.0.6.0 (http://www.liquid-technologies.com) -->
<acl-list xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <rule id="6228">
    <member>sip:user1@example.com</member>
    <member>sip:user2@example.com</member>
    <member>sip:user3@example.com</member>
    <member>sip:user4@example.com</member>
    <member>sip:user5@example.com</member>
  </rule>
</acl-list>

                   Figure 3: Example with Partial Trust

   The example in Figure 4 shows the minimal level of trust.  This ACL
   would be sent in a subscription to user1.


<?xml version="1.0" encoding="utf-8"?>
<!-- Created with Liquid XML Studio 1.0.6.0 (http://www.liquid-technologies.com) -->
<acl-list xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <rule id="6228">
    <member>sip:user1@example.com</member>
  </rule>
</acl-list>

                   Figure 4: Example with Minimal Trust

5.4.  Rule Determination Algorithm

   Several steps in the processing of the ACL require that the RLS in
   the watching domain execute the rule determination algorithm for
   watcher W on an ACL set.  This algorithm is a simple algorithm which
   takes, as input, a watcher W with a given SIP URI, and a set of ACL
   documents Ai, and returns as output, a rule ID R, which is the rule
   ID for the view that, according to the set of ACLs, watcher W should
   receive.

   The algorithm proceeds as follows.  First, each Ai is matched to W.
   ACL Ai is a match for watcher W if:

   o  ACL Ai contains a <member> tag whose URI is a match, based on URI
      equality, for W, or

   o  none of the <member> tags in Ai contain a URI that is a match,
      based on URI equality, for W, but there is an <other> element in
      Ai




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   If no ACL Ai matched, the algorithm returns a null result.

   For each ACL Ai that matches based on the rules above, take the id of
   the enclosing <rule> element that contained the <member> or <other>
   element that caused the match.  For ACL Ai, this is rule Ri.  For
   example, consider the following ACL:


   <?xml version="1.0" encoding="UTF-8"?>
   <acl-list "xmlns=urn:ietf:params:xml:ns:aclinfo">
       <rule id="1">
         <member>sip:user1@example.com</member>
         <member>sip:user2@example.com</member>
       </rule>
       <rule id="2">
          <member>sip:user3@example.com</member>
       </rule>
       <rule id="3">
         <other/>
       </rule>
    </acl-list>

   If this document is A1, and the watcher is sip:user3@example.com, the
   associated rule R1 is 2.  If the watcher is sip:user1@example.com or
   sip:user2@example.com, the rule R1 is 1.  If the watcher is anyone
   else from example.com, such as sip:user4@example.com, the rule R1 is
   3.

   If all Ri are equal, denote R = Ri.  Thus, R is the rule ID
   associated with this watcher.  Normally, all Ri will be equal.
   However, during transient periods of changes in authorization state,
   it is possible that inconsistent ACL documents exist.  In that case,
   R is assigned the value Ri from the ACL Ai which is the most recently
   received amonst all ACL.

















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5.5.  XML Schema



<?xml version="1.0" encoding="utf-8"?>
<!-- Created with Liquid XML Studio 1.0.6.0 (http://www.liquid-technologies.com) -->
<xs:schema elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="acl-list">
    <xs:complexType>
      <xs:sequence minOccurs="1" maxOccurs="unbounded">
        <xs:element name="rule">
          <xs:complexType>
            <xs:choice>
              <xs:element name="other" />
              <xs:sequence minOccurs="1" maxOccurs="unbounded">
                <xs:element name="member" type="xs:anyURI" />
              </xs:sequence>
            </xs:choice>
            <xs:attribute name="id" type="xs:integer" use="required" />
            <xs:attribute default="false" name="blocked" type="xs:boolean" use="optional" />
          </xs:complexType>
        </xs:element>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>


6.  Performance Analysis

   This section considers the performance improvement of the mechanism
   when it is maximally exercised.  In that case, the full ACL,
   including blocked senders, is returned in the first subscription to a
   presentity.  This analysis assumes there is a single, monolithic
   presence server serving each domain.

   The optimizations improve ramp-up, steady state, and termination
   message loads.  IN particular, each of those loads, without the
   optimization described here, is proportional to C04, the total number
   of federated presentities per watcher.  If we assume symmetry, such
   that the number of federated presentities per watcher is equal to the
   number of watchers per federated presentity, then each of the load
   figures is reduced by C04.  That is, the system behaves identically
   to the case where there is a single watcher per federated presentity,
   and assuming symmetric, the same as if there is a single federated
   presentity per watcher - e.g., C04 = 1.

   Consider then the very large network peering model in



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   [I-D.ietf-simple-interdomain-scaling-analysis].  In this model, the
   assumption is two large peering domains with 20 million users each,
   with a value of 10 for C04.  With this optimization, the number of
   steady state notifications due to presence state changes drops from
   18.4 billion per day to 1.84 billion per day.  The number of messages
   per second overall is reduced from 654,167 per second to 65,417 per
   second.  Still a big number, of course, but it can't actually get
   much smaller.

   Indeed, it can be readily shown that, assuming the federated domains
   do not actually share raw presence inputs and the actual policies
   that govern operation of their servers, no protocol can do better
   (constants, such as mesage size and the need for protocol responses
   and acknowledgements aside).  Consider a domain with N presentities.
   Each presentity changes state P times per hour.  Every time the state
   changes, the domain applies its authorization and composition
   policies.  The resulting presence document cannot be known to the
   watching domain.  Thus, there must be at least one message from the
   serving to watching domain, per view, in order to inform it of that
   view.  This means that the steady state rate of messages can never be
   better than N*P, and this is exactly the factor governing the rate of
   messages when this optimization is applied.


7.  Requirements Analysis

   This section analyzes the requirements in
   [I-D.houri-sipping-presence-scaling-requirements] to show how they
   are met by the mechanism proposed here.

   REQ-001:  The solution should not hinder the ability of existing
      SIMPLE clients and/or servers from peering with a domain or client
      implementing the solution.  No changes may be required of existing
      servers to interoperate.  This requirement is met by usage of the
      Supported and Require mechanisms and SIP which negotiate its
      usage.

   REQ-002:  It does NOT constrain any existing RFC functional or
      security requirements for presence.  The mechanism does not change
      anything that is possible without it.  It does, however, introduce
      new privacy considerations, described below in Section 8.

   REQ-003:  Systems that are not using the new additions to the
      protocol should operate at the same level as they do today.  This
      requirement is met by usage of the Supported and Require
      mechanisms in SIP.





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   REQ-004:  The solution does not limit the ability for presentities to
      present different views of presence to different watchers.  This
      requirement is met by usage of the ACL document, which allows the
      serving domain to associate a watcher with any view it likes, and
      to change it over time.

   REQ-005:  The solution does not restrict the ability of a presentity
      to obtain its list of watchers.  The mechanism does allow a
      presence server to know the list of watchers, at the expense of
      non-optimal performance.  In particular, it will receive a
      subscription from each watcher.  However, it only generates one
      notification per view on presence changes.  The fully optimized
      solution will result in a loss of knowledge of the set of
      watchers.  However, it is a policy decision at the presence agent
      about whether it would like to make this tradeoff.

   REQ-006:  The solution MUST NOT create any new or make worse any
      existing privacy holes.  This requirement is met, but only when
      carefully provisioned.  See Section 8.

   REQ-007:  It is highly desirable for any presence system (intra or
      inter-domain) to scale linearly as number of watchers and
      presentities increase linearly.  When the most optimal technique
      is used, there is always one subscription per view per presentity,
      independent of the number of watchers in the remote domain or the
      number of averages buddies per buddy list.  Since the number of
      views is not proportional to the number of users, the total
      traffic volume in a domain is linear with its number of
      presentities, and is independent of the number of users in the
      peering domain.

   REQ-008:  The solution SHOULD NOT require significantly more state in
      order to implement the solution.  The mechanism requires storage
      of the ACL, which has a size exactly equal to the number of
      subscriptions that would be required if the extension were not in
      place.  Thus the memory usage is not worsened compared to the
      baseline.

   REQ-009:  It MUST be able to scale to tens of millions of concurrent
      users in each domain and in each peer domain.  The analysis in
      Section 6 shows that, when fully utilized, this mechanism is the
      best that can possibly be achieved in any system that does not
      actually share policies and raw presence data.

   REQ-010:  It MUST support a very high level of watcher/presentity
      intersections in various intersection models.  The mechanism is
      optimized for this case.




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   REQ-011:  Protocol changes MUST NOT prohibit optimizations in
      different deployment models esp. where there is a high level of
      cross subscriptions between the domains.  Since standard SIP
      techniques are utilized to negotiate the extension, other
      mechansims can be defined in the future.

   REQ-012:  New functionalities and extensions to the presence protocol
      SHOULD take into account scalability with respect to the number of
      messages, state size and management and processing load.  That is
      exactly what this extension targets.

   REQ-013:  The solution SHOULD allow for arbitrary federation
      topologies including direct peering and intermediary routing.  The
      mechanism is optimized for direct peering.  It can work in
      intermediary routing cases as well.


8.  Security Considerations

   The principal question with the specification is whether it alters
   the privacy characteristics of a non-optimized federated system.

   Consider first the case where the serving domain is using the minimal
   trust model.  In that case, the ACL provided to the watching
   information does not carry any information that the watching domain
   doesn't already know.  It merely points out when two watchers share
   the same view.  This is something that the watching domain could have
   already ascertained by comparing presence documents delivered to each
   watcher.  The ACL makes this task easier, but nonetheless the
   watching domain could have already ascertained it.  Consequently,
   there is no change whatsoever in the level of privacy afforded by the
   optimization when this mode is used.

   However, when an ACL is provided that includes other users besides
   the actual watcher, this provides additional information to the
   watching domain.  This is, however, information that the watching
   domain could find out anyway.  If it generated a subscription from
   each of its users to the presentity it would be able to determine who
   from its domain is allowed to subscribe and what view they would
   receive.  This would be an expensive operation to be sure, but it is
   possible.  Consequently, the optimization doesn't really provide
   anything new to the originating domain, even in this case.

   However, there is an attack possible when the information is divulged
   to an end user.  Consider a watching domain that doesn't actually
   implement this extension at all.  A user within the domain uses a
   client that generates a subscription to a presentity in a remote
   domain.  This subscription uses an outbound proxy in the watching



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   domain.  The outbound proxy is just a proxy, and therefore doesn't
   remove or modify the Supported header field in the request.  The
   serving domain accepts the subscription and sends an ACL that
   contains the full set of watchers that are permitted in the
   originating domain.  The original watcher now knows the set of other
   authorized buddies within their own domain, and what views they will
   see.  While this is information that the domain overall would have
   access to, it is not information an end user would normally have
   access to.  Consequently, this is a more serious privacy violation.

   It is for this reason that this specification requires that both
   sides of the federated link be explicitly provisioned to utilize this
   optimization.  In the attack above, the watching domain would not
   have set up a peering relationship with the serving domain.  If it
   had, it would have an RLS and would not have permitted the user to
   directly subscribe in this way.  Thus, when the subscription is
   received by the serving domain, it will find that it has no agreement
   with the originating domain, and would not utilize view sharing.
   This thwarts the attack.

   This remedy is not optimal because it requires on provisioning to
   prevent.  There does not appear to be any easy cryptographic means to
   prevent it, however.


9.  IANA Considerations

   There are several IANA considerations associated with this
   specification.

9.1.  MIME Type Registration

   This specification requests the registration of a new MIME type
   according to the procedures of RFC 2048 [RFC2048] and guidelines in
   RFC 3023 [RFC3023].

      MIME media type name: application

      MIME subtype name: aclinfo+xml

      Mandatory parameters: none

      Optional parameters: Same as charset parameter application/xml as
      specified in RFC 3023 [RFC3023].







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      Encoding considerations: Same as encoding considerations of
      application/xml as specified in RFC 3023 [RFC3023].

      Security considerations: See Section 10 of RFC 3023 [RFC3023] and
      Section 8 of RFC XXXX [[NOTE TO IANA/RFC-EDITOR: Please replace
      XXXX with the RFC number of this specification]].

      Interoperability considerations: none.

      Published specification: RFC XXXX [[NOTE TO IANA/RFC-EDITOR:
      Please replace XXXX with the RFC number of this specification]]

      Applications which use this media type: This document type has
      been used to support subscriptions to lists of users [RFC4662] for
      SIP-based presence [RFC3856].

      Additional Information:

         Magic Number: None

         File Extension: .acl

         Macintosh file type code: "TEXT"

         Personal and email address for further information: Jonathan
         Rosenberg, jdrosen@jdrosen.net

         Intended usage: COMMON

         Author/Change controller: The IETF.

9.2.  URN Sub-Namespace Registration

   This section registers a new XML namespace, as per the guidelines in
   RFC 3688 [RFC3688].

      URI: The URI for this namespace is urn:ietf:params:xml:ns:aclinfo.

      Registrant Contact: IETF, SIMPLE working group, (simple@ietf.org),
      Jonathan Rosenberg (jdrosen@jdrosen.net).

      XML:









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             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=iso-8859-1"/>
               <title>ACL Info Namespace</title>
             </head>
             <body>
               <h1>Namespace for ACL Info</h1>
               <h2>urn:ietf:params:xml:ns:aclinfo</h2>
               <p>See <a href="[URL of published RFC]">RFCXXXX [NOTE
TO IANA/RFC-EDITOR: Please replace XXXX with the RFC number of this
specification.]</a>.</p>
             </body>
             </html>
             END

9.3.  Schema Registration

   This section registers an XML schema per the procedures in [RFC3688].

      URI: urn:ietf:params:xml:schema:aclinfo

      Registrant Contact: IETF, SIMPLE working group, (simple@ietf.org),
      Jonathan Rosenberg (jdrosen@jdrosen.net).

      The XML for this schema can be found as the sole content of
      Section 5.5.


10.  References

10.1.  Normative References

   [RFC4662]  Roach, A., Campbell, B., and J. Rosenberg, "A Session
              Initiation Protocol (SIP) Event Notification Extension for
              Resource Lists", RFC 4662, August 2006.

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474, August 2006.

   [RFC2141]  Moats, R., "URN Syntax", RFC 2141, May 1997.

   [RFC2648]  Moats, R., "A URN Namespace for IETF Documents", RFC 2648,



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              August 1999.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              January 2004.

   [RFC2048]  Freed, N., Klensin, J., and J. Postel, "Multipurpose
              Internet Mail Extensions (MIME) Part Four: Registration
              Procedures", BCP 13, RFC 2048, November 1996.

   [RFC3023]  Murata, M., St. Laurent, S., and D. Kohn, "XML Media
              Types", RFC 3023, January 2001.

   [W3C.REC-xml-20001006]
              Maler, E., Paoli, J., Bray, T., and C. Sperberg-McQueen,
              "Extensible Markup Language (XML) 1.0 (Second Edition)",
              World Wide Web Consortium FirstEdition REC-xml-20001006,
              October 2000,
              <http://www.w3.org/TR/2000/REC-xml-20001006>.

10.2.  Informative References

   [RFC2778]  Day, M., Rosenberg, J., and H. Sugano, "A Model for
              Presence and Instant Messaging", RFC 2778, February 2000.

   [RFC3863]  Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr,
              W., and J. Peterson, "Presence Information Data Format
              (PIDF)", RFC 3863, August 2004.

   [RFC4479]  Rosenberg, J., "A Data Model for Presence", RFC 4479,
              July 2006.

   [RFC3856]  Rosenberg, J., "A Presence Event Package for the Session
              Initiation Protocol (SIP)", RFC 3856, August 2004.

   [RFC4480]  Schulzrinne, H., Gurbani, V., Kyzivat, P., and J.
              Rosenberg, "RPID: Rich Presence Extensions to the Presence
              Information Data Format (PIDF)", RFC 4480, July 2006.

   [I-D.ietf-simple-interdomain-scaling-analysis]
              Houri, A., "Presence Interdomain Scaling Analysis for SIP/
              SIMPLE", draft-ietf-simple-interdomain-scaling-analysis-01
              (work in progress), July 2007.

   [I-D.ietf-sip-subnot-etags]
              Niemi, A., "An Extension to Session Initiation Protocol
              (SIP) Events for Conditional  Event Notification",
              draft-ietf-sip-subnot-etags-00 (work in progress),
              May 2007.



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   [I-D.houri-sipping-presence-scaling-requirements]
              Houri, A., "Scaling Requirements for Presence in SIP/
              SIMPLE",
              draft-houri-sipping-presence-scaling-requirements-00 (work
              in progress), July 2007.


Authors' Addresses

   Jonathan Rosenberg
   Cisco
   Edison, NJ
   US

   Phone: +1 973 952-5000
   Email: jdrosen@cisco.com
   URI:   http://www.jdrosen.net


   Steve Donovan
   Cisco
   Richardson, TX
   US

   Email: stdonova@cisco.com


   Kathleen McMurry
   Cisco
   Richardson, TX
   US

   Email: kmcmurry@cisco.com


















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Full Copyright Statement

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