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Versions: (draft-poetzl-bliss-call-completion) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 RFC 6910

bliss                                                          D. Worley
Internet-Draft                                     Nortel Networks Corp.
Intended status: Standards Track                           M. Huelsemann
Expires: April 15, 2011                                        R. Jesske
                                                           D. Alexeitsev
                                                        Deutsche Telekom
                                                        October 12, 2010


         Call Completion for Session Initiation Protocol (SIP)
                  draft-ietf-bliss-call-completion-07

Abstract

   The call completion features allow the calling user of a failed call
   to be notified when the called user becomes available to receive a
   call.

   For the realization of a basic solution without queueing call-
   completion requests, this document references the usage of the the
   dialog event package (defined in RFC 4235) as described as 'automatic
   redial' in (RFC 5359, SIP Service Examples).

   For the realization of a more comprehensive solution with queueing
   call-completion requests, this document introduces an architecture
   for implementing these features in the Session Initiation Protocol:
   "Call completion" implementations associated with the caller's and
   callee's endpoints cooperate to place the caller's request for call
   completion into a queue at the callee's endpoint, and, when a
   caller's request is ready to be serviced, re-attempt the original,
   failed call.

   The deployment of a certain SIP call-completion solution is also
   dependent on the needed level of interoperability with existing call-
   completion solutions in other networks.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any



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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 15, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
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   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.




















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Requirements terminology . . . . . . . . . . . . . . . . . . .  5
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Solution . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Call-Completion architecture . . . . . . . . . . . . . . .  7
     4.2.  Call-Completion procedures . . . . . . . . . . . . . . . .  9
     4.3.  Automatic redial as a fallback . . . . . . . . . . . . . . 11
     4.4.  Differences from SS7 . . . . . . . . . . . . . . . . . . . 11
   5.  Call-Completion Queue model  . . . . . . . . . . . . . . . . . 12
   6.  Caller's Call-Completion Agent behaviour . . . . . . . . . . . 13
     6.1.  Receiving the CC possible indication . . . . . . . . . . . 13
     6.2.  Subscribing to CC  . . . . . . . . . . . . . . . . . . . . 13
     6.3.  Receiving a CC notification  . . . . . . . . . . . . . . . 14
     6.4.  Initiating a CC call . . . . . . . . . . . . . . . . . . . 14
     6.5.  Suspending CC  . . . . . . . . . . . . . . . . . . . . . . 14
     6.6.  Resuming CC  . . . . . . . . . . . . . . . . . . . . . . . 15
   7.  Callee's Call-Completion Monitor behaviour . . . . . . . . . . 15
     7.1.  Sending the CC possible indication . . . . . . . . . . . . 15
     7.2.  Receiving a CC subscription  . . . . . . . . . . . . . . . 16
     7.3.  Sending a CC notification  . . . . . . . . . . . . . . . . 16
     7.4.  Receiving a CC call  . . . . . . . . . . . . . . . . . . . 17
     7.5.  Receiving a CC suspension  . . . . . . . . . . . . . . . . 17
     7.6.  Receiving a CC resumption  . . . . . . . . . . . . . . . . 18
   8.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   9.  Call Completion Event Package  . . . . . . . . . . . . . . . . 21
     9.1.  Event Package Name . . . . . . . . . . . . . . . . . . . . 21
     9.2.  Event Package Parameters . . . . . . . . . . . . . . . . . 21
     9.3.  SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . . . 21
     9.4.  Subscribe Duration . . . . . . . . . . . . . . . . . . . . 22
     9.5.  NOTIFY Bodies  . . . . . . . . . . . . . . . . . . . . . . 22
     9.6.  Subscriber Generation of SUBSCRIBE Requests  . . . . . . . 22
     9.7.  Notifier Processing of SUBSCRIBE Requests  . . . . . . . . 23
     9.8.  Notifier Generation of NOTIFY Requests . . . . . . . . . . 23
     9.9.  Subscriber Processing of NOTIFY Requests . . . . . . . . . 24
     9.10. Handling of Forked Requests  . . . . . . . . . . . . . . . 24
     9.11. Rate of Notifications  . . . . . . . . . . . . . . . . . . 24
     9.12. State Agents . . . . . . . . . . . . . . . . . . . . . . . 24
   10. Call-completion information format . . . . . . . . . . . . . . 24
     10.1. call-completion-state  . . . . . . . . . . . . . . . . . . 25
     10.2. service-retention  . . . . . . . . . . . . . . . . . . . . 25
     10.3. cc-URI . . . . . . . . . . . . . . . . . . . . . . . . . . 25
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 26
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 26
     12.1. SIP Event Package Registration for call-completion . . . . 26
     12.2. MIME Registration for application/call-completion  . . . . 27
     12.3. SIP/SIPS URI parameter 'm' . . . . . . . . . . . . . . . . 27



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     12.4. 'purpose=call-completion' header parameter for
           Call-Info  . . . . . . . . . . . . . . . . . . . . . . . . 27
     12.5. 'm' header parameter for Call-Info . . . . . . . . . . . . 28
   13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
   14. Normative References . . . . . . . . . . . . . . . . . . . . . 28
   Appendix A.  Example Caller's Agent  . . . . . . . . . . . . . . . 29
   Appendix B.  Example Callee's Monitor  . . . . . . . . . . . . . . 29
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30











































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1.  Introduction

   The call completion (CC) feature allows the caller of a failed call
   to have the call completed without having to make a new call attempt
   when the callee becomes available.  When the caller requests the CC
   feature, the callee will be monitored for becoming available.  When
   the callee becomes available he will be given a certain time for
   initiating a call himself.  If the callee does not initiate a new
   call within this time, then the caller will be recalled.  When the
   caller accepts the CC recall then a CC call to the callee will be
   automatically started.  If several callers have requested the CC
   feature on the same callee, they will be recalled in a predefined
   order, which is usually the order in which they have requested the CC
   feature.

   This draft defines the following CC features:

   Call Completion on Busy Subscriber (CCBS): The callee is busy.  The
   caller is recalled after the callee is not busy any longer.

   Call Completion on No Reply (CCNR): The callee does not answer the
   call.  The caller is recalled after the callee has completed a new
   call.

   Call Completion on Not Logged-in (CCNL): The callee is not
   registered.  The caller is recalled after the callee has registered
   again.


2.  Requirements 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 [RFC2119].

   This document uses terms from [RFC3261].


3.  Terminology

   For the purpose of this service, we provide the following
   terminology:

   CC, or call completion: a service which allows a caller who failed to
   reach a desired destination user to be notified when the called party
   becomes available to receive a call.

   CC possible indication: the data in responses to the INVITE of the



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   original call which indicate that CC is available for this call.

   CC indicator: a iniction in the CC call INVITE used to prioritize the
   call at the destination.

   CC activation: the indication by the caller to the caller's agent
   that the caller desires CC for a failed original call; this implies
   an indication transmitted from the caller's agent to the callee's
   monitor of the desire for CC processing.

   CC request: the entry in the callee's monitor queue representing the
   original call and the caller's request for CC processing.

   CCBS, or Completion of Calls to Busy Subscriber: a CC service when
   the initial failure was that the destination UA was busy.

   CCNR, or Completion of Calls on No Reply: a CC service when the
   initial failure was that the destination UA was not answered.

   CCNL, or Completion of Call on Not Logged-in: a CC service when the
   initial failure was that the destination UA was not registered.

   CCBS/CCNR/CCNL service duration timer, or CC service duration timer:
   maximum time a CC request may remain active within the network.

   CC call: a call from the caller to the callee, triggered by the CC
   service when it has determined that the callee is available.

   CC recall: the action of the callee's monitor selecting a particular
   CC request as one that should initiate a CC call, resulting in an
   indication from the caller's agent to the caller that it is now
   possible to initiate a CC call.

   CC recall events: event notifications of event package "call-
   completion", sent by the callee's monitor to the caller's agent to
   inform it of the status of its CC request.

   CC queue: a buffer at the callee' monitor which stores incoming calls
   which have failed or may have failed.  Note: This buffer may or may
   not be organized as a queue.  The use of the term "queue" is by
   analogy with SS7 usage.

   Caller, calling user, originator, or CC user: the initiator of the
   original call and the CC request.  The user on whose behalf the CC
   call is made.

   Callee, called user, destination, or CC target: a destination of the
   original call, and a target of the CC call.



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   Caller's agent, or agent: a component which makes CC requests and
   responds to CC recall events on behalf of originating user(s)/UA(s),
   analogous to the originating local exchange's role in SS7 CC.

   Callee's monitor, or monitor: a component which implements the call-
   completion queue for destination user(s)/UA(s), and performs the
   associated tasks, including sending CC recall events, analogous to
   the destination local exchange's role in SS7 CC.

   Failed call: a call which does not reach a desired callee, from the
   caller's point of view.  Note that a failed call may be successful
   from the SIP point of view; e.g., if the call reached the callee's
   voicemail, but the caller desired to speak to the callee in person,
   the INVITE receives a 200 response, but the caller considers the call
   to have failed.

   Original call: the initial call which failed to reach a desired
   destination.

   Suspended CC request: a CC request which is temporarily not to be
   selected for CC recall.

   Retain option: a characteristic of the CC service; if supported, CC
   calls which again encounter a busy callee will not be queued again,
   but the position of the caller's entry in the queue is retained


4.  Solution

4.1.  Call-Completion architecture

   The call-completion architecture augments each caller's UA (or UAC)
   which wishes to be able to use the call-completion features with a
   "call-completion agent" (also written as "CC agent", "agent", or
   "caller's agent").

   It augments each callee's UA (or UAS) which wishes to be able to be
   the target of the call-completion features with a "call-completion
   monitor" (also written as "CC monitor", "monitor", or "callee's
   monitor").

   The agent and monitor functions can be integrated into the respective
   UAs, be independent end-systems, or be provided by a centralized
   application server.  The two functions, though they are associated
   with the two UAs, also may be provided as services by the endpoints'
   home proxies or other network elements.  Though it is expected that a
   UA that "implements call completion" will have both types of agents
   so that it can participate in call completion as both caller and



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   callee, the two agents are independent of each other.

   An agent may service more than one UA as a collective group if it is
   common that a caller or population of users will be shared between
   the UAs, and especially if the UAs share an AOR.  The agent monitors
   calls made from the UA(s) in order to determine their destinations
   and (potentially) their final response statuses, and the Call-Info
   headers of provisional and final responses.

   A monitor may service more than one UA as a collective group if it is
   common that a callee or population of users will be shared between
   the UAs, and especially if the UAs share an AOR.  The monitor may
   supply the callee's UAS(s) with Call-Info header values for
   provisional and final responses.

   The callees using the UA(s) may be able to indicate to the monitor
   when they wish to receive CC calls.

   In order to allow flexibility and innovation, most of the interaction
   between the caller's agent and the caller-user(s) and the caller's
   UA(s) is out of the scope of this document.  Similarly, most of the
   interaction between the callee's monitor and the callee-user(s) and
   the callee's UA(s) is out of the scope of this document, as is also
   the policy by which the callee's monitor arbitrates between multiple
   call-completion requests.

   The caller's agent must be capable of performing a number of
   functions relative to the UA(s).  The method by which it does so is
   outside the scope of this document, but an example method is
   described in Appendix A.  The callee's monitor must be capable of
   performing a number of functions relative to the UA(s).  The method
   by which it does so is outside the scope of this document, but an
   example method is described in Appendix B.

   As a proof of concept, simple agents and monitors can be devised that
   interact with users and UAs entirely through standard SIP mechanisms
   [RFC3265], [RFC4235] and [RFC3515], as described in the Appendixes.

   The callers using the UA(s) can indicate to the agent when they wish
   to avail themselves of CC for a recently-made call which failed to
   reach their chosen destination.  The agent monitors the status of the
   UA(s) to determine when they are available to be used for a CC
   recall.  The agent can communicate to the UA(s) that a CC recall is
   in progress and to inquire if the relevant calling user is available
   for the CC recall.  The agent can order the UA(s) at which the
   relevant calling user is available to generate a CC call to the
   callee.




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   The monitor has a method of monitoring the status of the UA(s) and/or
   their users to determine when they are "available" for a CC call,
   that is, in a suitable state to receive a CC call.  This can be
   achieved by monitoring calls made to the UA(s) in order to determine
   their callers and (potentially) their final response statuses.  In a
   system with rich presence information, the presence information may
   directly provide this status.  In a more restricted system, this
   determination can depend on the mode of the CC call in question,
   which is provided by the 'm' parameter.  E.g., a UA is considered
   available for CCBS ("m=BS") when it is not busy, but a UA is
   considered available for CCNR ("m=NR") when it becomes not busy after
   being busy with an established call.

   The monitor maintains information about the set of INVITEs that have
   been received by the UA(s) that may not have been considered
   successful by the calling user.  In practice, the monitor may remove
   knowledge about an incoming dialog from its set if its CC policy
   establishes that the dialog is no longer eligible for CC requests.

4.2.  Call-Completion procedures

   The caller's UA sends an INVITE to a request URI.  One or more forks
   of this request reach one or more of the callee's UAs.  By
   hypothesis, none of the callee's UAs returns a success response, as
   otherwise, call completion services would not be needed for this
   call.

   However, the caller's INVITE might succeed at some other UA that the
   calling user considers insufficient to satisfy his needs.
   Eventually, the INVITE fails, or the resulting dialog(s) are
   terminated.

   If the call-completion feature is available, the callee's monitor
   inserts a Call-Info header with its URI and with "purpose=call-
   completion" in an appropriate non-100 provisional or final response
   message to the initial INVITE and forwards it to the caller.

   The calling user indicates to the caller's agent that he wishes to
   invoke call-completion services on the recent call.  Note that from
   the SIP point of view, the INVITE may be successful, but from the
   user's point of view, the call may be unsuccessful.  E.g., the call
   may have connected to the callee's voicemail, which would return a
   200 status to the INVITE but from the caller's point of view is "no
   reply".

   In order to request call-completion, the caller's agent subscribes to
   the call-completion event package of the callee's monitor.  This
   subscription is used to coordinate with the monitor (and indirectly



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   with other caller's agents and other callee's monitors) to implement
   the call-completion features.

   The caller's agent sends a SUBSCRIBE request for the call-completion
   event package to all known monitor URIs and to the original
   destination URI of the call, which are provided by a Call-Info header
   in provisional and final responses to the INVITE.  This SUBSCRIBE
   reaches the callee's monitor.  The callee's monitor uses the
   existence of the subscription to know that the caller is interested
   in using the CC feature in regard to the specified original call.
   The monitor keeps a list or queue of failed calls to the callee, and
   of the caller's agent's subscriptions, which indicate the callers
   that are waiting to use the CC features.

   When the callee's monitor judges that the callee and/or callee's UA
   is available for call-completion, the callee's monitor selects
   (usually) one request to be the next caller to execute call-
   completion to the callee.  The callee's monitor sends a call-
   completion event update to the selected caller's agent's
   subscription, telling it to begin execution of call-completion (CC
   recall).

   When the caller's agent receives this update, it calls the caller's
   UA or otherwise tests whether the caller is available to take
   advantage of call-completion.  If the caller is available, the agent
   directs the caller's UA to make again the call to the callee (CC
   call).  This call is marked as a CC call by adding a specific SIP URI
   parameter, so it can be given precedence by the monitor in reaching
   the callee's UA.

   If the caller is not available on the receipt of the "ready for
   recall" notification, the CC agent suspends the CC request at the CC
   monitor.  The CC agent resumes the CC request once the caller becomes
   available for CC again.  On the receipt of the suspension from the CC
   agent at the top of the queue, the CC monitor shall perform the
   callee monitoring for the next not suspended CC agent in the queue.
   On the receipt of the resume from the previously suspended CC agent
   that was at the top of the queue the CC monitor shall perform the
   callee monitoring for this CC agent.

   When the call completion call fails there are two possible options:
   the CC feature has to be activated again, or CC remains activated and
   the original CC request retains its position in the queue (retain
   option), optionally with the possibility to update the subscription.







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4.3.  Automatic redial as a fallback

   Automatic redial is a simple end-to-end design.  An automatic redial
   scenario is described in [RFC5359], Section 2.17.  This solution is
   based on the usage of the dialog event package.  When the callee is
   busy when the call arrives, the caller subscribes to the callee's
   call state.  The callee's UA sends a notification when the callee's
   call state changes.  This means the caller is also nofified when the
   callee's call state changes to 'terminated'.  The caller is alerted,
   then the caller's UA starts a call establishment to the callee again.
   If several callers have subscribed to a busy callee's call state,
   they will be notified at the same time that the call state has
   changed to 'terminated'.  The problem of this solution is, that it
   might occur that several CC recalls are started at the same time.
   This means it is a heuristic approach with no guarantee in the order
   of sucessfull recalls.

   There is no interaction between call completion and automatic redial,
   as there is a difference in the behaviour of the callee's monitor and
   the caller when using the dialog event package for receiving dialog
   information or for aggregating a call completion state.

4.4.  Differences from SS7

   SIP call completion differs in some ways from the CCBS and CCNR
   features of SS7 (which is used in the PSTN).  For ease of
   understanding, we enumerate some of the differences here.

   Due to the complex forking situations that are possible in SIP, a
   call may "fail" from the point of view of the user and yet have a
   "success" response from SIP's point of view.  (This can happen even
   in simple situations: e.g., a call to a busy user that fails over to
   his voicemail receives a SIP success response, even though the caller
   may consider it "busy subscriber".)  Thus, the calling user must be
   able to invoke call completion even when the original call appeared
   to succeed.  To support this, the caller's agent (and to a lesser
   degree the callee's monitor) must record successful calls as well as
   unsuccessful calls.

   In SIP, only the caller's UA or service system on the originating
   side and the callee's UA or service system on the terminating side
   need specifically to support call completion in order that call
   completion work successfully between the UAs.  Intermediate SIP
   systems (proxies or B2BUAs) do not need specifically to implement
   call completion; they only need to be transparent to the usual range
   of SIP messages.

   Due to flexibility needed to support legacy systems that are not



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   optimized to support call completion, there are a larger number of
   situations in SIP where call completion services are offered but
   eventually cannot be successfully executed.


5.  Call-Completion Queue model

   The callee's monitor manages CC for a single URI.  This URI is likely
   to be a published AOR, or more likely "an AOR without its voicemail",
   but it may be as narrowly scoped as a single UA's contact URI.  The
   monitor manages a dynamic set of entities which represent calls
   eligible for completion ("CECEs" for short).  When an INVITE reaches
   the URI but does not result in a confirmed dialog, a CECE is created
   which carries the From-URI of the INVITE.  (If a CECE already exists
   carrying the From-URI, no additional CECE is created, however its
   lifetime timer is reset to 0.)  A CECE is removed from the set after
   its lifetime timer exceeds a particular time limit.

   A subset of the CECEs are organized into a queue.  Each CECE has an
   availability state, which is either "available for recall" or "not
   available for recall".  This availability state is only significant
   for CECEs in the queue.  It is not visible via subscriptions.  Each
   CECE has a recall state which is visible via subscriptions.

   The recall state is either "queued" or "ready".  This recall state is
   only significant for CECEs in the queue.  CC subscriptions arrive at
   the the monitor by being addressed to the managed URI.  The CECEs
   being subscribed to are identified by the Request URI.

   When a CECE is destroyed, any subscriptions to its state are
   terminated.

   The monitor maintains "call-completion" subscriptions to all of the
   CECEs.  (And it knows the identities of all the CECEs, since it
   creates them.)  When a CECE becomes the target of a subscription, the
   CECE is added to the queue (if it is not already there), and is given
   the availability state "available" and recall state "queued".

   When a CECE loses its last subscription, the CECE is removed from the
   queue (though it remains in the set of CECEs).

   The monitor may receive PIDF bodies (RFC 3863[RFC3863]), via PUBLISH
   requests directed at its URI.  These PUBLISH requests are expected to
   be sent by subscribers to suspend and resume their CC requests.  A
   PIDF body contains an 'entity' attribute of the 'presence' element
   which is the From-URI of the original call in question, which
   therefore identifies a CECE.  Receipt of a PUBLISH with 'status' of
   'open' sets the availability state of the CECE to 'available';



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   'status' of 'closed' sets the availability state of the CECE to 'not-
   available'.

   The monitor uses the following algorithm to select callers for
   recall: When the callee is available for recall, and there is at
   least one CECE in the queue with availability state 'available', then
   the monitor selects from these CECEs the one that has resided in the
   queue the longest.  The CECE's recall state is changed from 'queued'
   to 'ready', which triggers a notification on the CECE's subscription.

   When the monitor discovers that an CC INVITE has arrived containing a
   From-URI of a CECE, it deletes the CECE from the set.  However if
   that INVITE fails, it may cause the creation of another CECE with the
   same From-URI, or if the retain option is supported, the CECE is
   retained in the set.

   If the monitor discovers after a time interval that no INVITE arrives
   containing the From-URI of the CECE, it deletes the CECE.  This may
   cause another caller to be selected for recall.

   If the subscriber suspends its request by sending a PUBLISH with
   status 'closed', the CECE becomes not-available, and the monitor
   changes the CECE's recall state to 'queued'.  This may cause another
   CECE (that has been in the queue for less time) to be selected for
   recall.


6.  Caller's Call-Completion Agent behaviour

6.1.  Receiving the CC possible indication

   The caller's agent MUST record the From URI and MAY record the final
   request status that the caller's UA received and the contents of
   Call-Info headers of provisional and final responses.

6.2.  Subscribing to CC

   For CC activation the agent MUST send a SUBSCRIBE to all known
   monitor URIs.  These can be provided by the Call-Info header in
   provisional and final responses to the INVITE.  Additionally, the
   caller's agent MUST include the original request-URI in its set of
   monitor URIs, because the call may have forked to additional callees
   whose responses the caller has not seen.  A SUBSCRIBE to the request-
   URI alone is used in cases where the caller's agent has not received
   or cannot remember any monitor URI.  The caller's agent MAY add an
   'm' parameter to these URIs.  The 'm' parameter SHOULD have the value
   of the 'm' parameter in the Call-Info header, if a Call-Info header
   was received.



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   To minimize redundant subscriptions, these SUBSCRIBEs SHOULD have the
   same Call-Id, that is, be presented as forks of the same transaction,
   if the caller's agent is capable of doing so.

   The SUBSCRIBE should have headers to optimize its routing.  In
   particular, it SHOULD contain "Request-Disposition: parallel, no-
   cancel", and an Accept-Contact header to eliminate callee UAs that
   are not acceptable to the calling user.

   If the caller's agent becomes unwilling to initiate the CC call
   (e.g., because the calling user has deactivated CC), the caller's
   agent terminates the subscription(s).

6.3.  Receiving a CC notification

   When receiving a CC notification with the cc-state set to 'ready',
   the caller's agent SHOULD terminate or suspend all other CC
   subscriptions (at other monitors) for this original call, and all CC
   subscriptions for all other original calls, in order to prevent any
   other CC requests from this caller from being activated.  The agent
   then determines whether the calling user is available for the CC
   call, usually by calling the caller's UA(s).

6.4.  Initiating a CC call

   If the calling user is available, the caller's agent causes the
   caller's UA to generate a new INVITE to the URI specified in the
   NOTIFY.  The caller MAY add the 'm' parameters (if possible), to
   specify CC processing and prioritize the CC call.  If the caller's
   agent cannot remember the URIs returned in the Call-Info header and
   the call-completion NOTIFY or if there was no such URI, it SHOULD use
   the request-URI of the original INVITE.  This may not provide ideal
   routing, but in simple cases it is likely to reach the desired
   callee/callee's monitor.

6.5.  Suspending CC

   If the caller is found to be busy previous to or on receipt of the CC
   recall, then the CC request is suspended by the CC agent until the
   caller becomes not busy again.  To suspend the CC request, the CC
   agent SHALL send a PUBLISH request to each CC monitor, giving the
   PIDF state 'closed' for the caller's identity as presentity.

   Each PUBLISH SHOULD be sent to the URI as received in the NOTIFY, or
   within the corresponding SUBSCRIBE dialog, or if that is not
   possible, to the corresponding monitor URI as received in the Call-
   Info header, or if one is not available, the Contact address of the
   subscription.  If a queue entry is suspended, it is stepped over



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   during CC processing at the CC monitor.

6.6.  Resuming CC

   When the caller is no longer busy, then the CC request is resumed by
   the CC agent.  To resume a CC request, the CC agent SHALL send to
   each CC monitor a PUBLISH request informing about the PIDF state
   'open' but otherwise constructed as the suspend PUBLISH request.
   Each PUBLISH SHOULD be sent to the URI as received in the NOTIFY, or
   within the corresponding SUBSCRIBE dialog, or if that is not
   possible, to the corresponding monitor URI as received in the Call-
   Info header, or if one is not available, the Contact address of the
   subscription.

   In the case where the CC agent has sent several CC suspension
   requests to different CC monitors and the caller becomes not busy
   again, the CC agent shall send a CC resumption request to each CC
   monitor for which there is a suspended CC request.


7.  Callee's Call-Completion Monitor behaviour

7.1.  Sending the CC possible indication

   The callee's monitor MUST record the From URI and MAY record the
   final request status(es) returned by the callee's UA(s).

   If the callee's monitor wants to enable the caller to make use of the
   CC service, it inserts a Call-Info header with "purpose=call-
   completion" in an appropriate response message to the initial INVITE
   and forwards it to the caller.  The Call-Info header positively
   indicates that CC is available for this failed fork of the call.

   The callee's monitor SHOULD insert a URI in the Call-Info header
   where the caller's agent should subscribe for call-completion
   processing.  Ideally, it is a globally-routable URI for the callee's
   monitor.  In practice, it may be the callee's AOR, and the SUBSCRIBE
   will be routed to the callee's monitor only because it specifies
   "Event: call-completion".

   When applicable, the Call-Info header MUST be set up according to the
   following scheme:

   Call-Info:monitor-URI;purpose=call-completion;m=XX

   The 'm' parameter defines the "mode" of call completion.  The "m=NR"
   parameter indicates that it failed due to no-response, the "m=BS"
   parameter indicates that it failed due to busy subscriber, and the



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   "m=NL" parameter indicates that it failed due to not registered
   subscriber.  The 'm' parameter is useful for PSTN interworking and
   assessing presence information in the callee's monitor.  It is
   possible that other values will be defined in future.  It is also
   allowed to omit the 'm' parameter entirely.  Implementations MUST
   accept CC operations in which the 'm' parameter is missing or has an
   unknown value, and perform them as well as is possible in their
   environment (which is likely to be with degraded service, especially
   in interoperation with SS7).

7.2.  Receiving a CC subscription

   The monitor MUST be prepared to receive SUBSCRIBEs for the call-
   completion event package directed to the URIs of UA(s) serviced by
   the monitor and any URIs that the monitor provides for use in Call-
   Info headers.

   The callee's monitor(s) that receive the SUBSCRIBE establish
   subscriptions.  These subscriptions represent the caller's agent's
   request for call-completion services.  The callee's monitor MUST be
   prepared to receive multiple forks of a single SUBSCRIBE, and should
   respond 482 (Merged Request) to all but one fork.  The monitor may
   apply additional restrictions as which caller's agents may subscribe.

   The caller's agent MUST be prepared to receive multiple responses to
   the SUBSCRIBE and to have multiple subscriptions established.  The
   agent must also be prepared to have the SUBSCRIBE fail, in which
   case, CC cannot be invoked for this original call.

   _The continuation of the caller's agent's subscription indicates that
   the caller's agent is prepared to initiate the CC call when it is
   selected by the callee's monitor.  If the callee's monitor becomes
   aware that, according to its policy, the original call referenced by
   a subscription will never be selected for call-completion, it SHOULD
   terminate the subscription and respond to any attempt to start a new
   subscription for that original call with 404._

7.3.  Sending a CC notification

   When the cc-state of the agent's request changes, the monitor MUST
   send a NOTIFY for a call-completion event to the agent.  The call-
   completion event package returns various information to the caller's
   agent, but the vital datum is that it contains an indication about
   the cc-state, which in the beginning is 'queued'.  The notification
   SHOULD also contain a URI which can be used for suspension requestst.
   Ideally, it is a globally-routable URI for the callee's monitor.  In
   practice, it may be the callee's AOR, and the SUBSCRIBE will be
   routed to the callee's monitor only because it specifies "Event:



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   call-completion".

   The call-completion event package provides information about the
   callee's monitor's policy.  In particular, like in the PSTN, the
   "'cc-service-retention" datum gives an indication of the "service
   retention" attribute, which indicates whether the CC request can be
   continued to a later time if the call-completion call fails due to
   the callee's UA(s) being busy.  If the callee's monitor supports the
   service-retention option, the monitor SHOULD include the cc-service-
   retention parameter.

   The callee's monitor has a policy regarding when and how it selects
   CC requests to be activated.  This policy may take into account the
   type of the requests (e. g.  CCNR vs. CCBS), the state of the
   callee's UA(s), the order in which the original calls arrived, and
   any previous CC attempts for the same original call.  Usually the
   callee's monitor will choose only one CC request for activation at a
   time, but if the callee's UA(s) can support multiple calls, it may
   choose more than one.

   When the callee's monitor changes the "call completion active" datum
   for the chosen caller's agent from false to true, the monitor MUST
   send a notification for the agent's subscription with the CC-state
   set to 'ready'.  The notification SHOULD also contain also a URI
   which should be used in the CC call.  In practice, this may be the
   AOR of the callee.

7.4.  Receiving a CC call

   The callee's UA(s) and any associated proxies may give the CC call
   precedence over non-CC calls.  The callee's monitor supervises the
   receiving of the CC call.  If the CC call does not arrive at the
   callee's UA(s) promptly, the monitor should withdraw the CC
   activation from the caller's agent by changing the value of its CC
   active datum to false.  Similarly, if the CC call fails, the monitor
   will withdraw CC activation.  Depending on its policy, the same
   original call may be selected again for CC activation at a later
   time.  If the CC call succeeds, the monitor should withdraw CC
   activation.

   Once the CC call has failed, or if it has succeeded, once the CC call
   has been terminated, the callee's monitor's policy MAY select another
   CC request for activation.

7.5.  Receiving a CC suspension

   If the processing of a CC request results in suspending that CC
   request, the monitor SHALL stop the recall timer and attempt to



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   process the next CC request in the queue.

7.6.  Receiving a CC resumption

   When a CC request becomes resumed, then, if the callee is not busy
   and there is no entry in the CC queue which is currently being
   processed, the CC monitor SHALL process the queue as described above.


8.  Examples

   A basic flow, with only the most significant messages shown, is this:







































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      Caller                     Callee
      sip:123@a.com              sip:456@b.com
        |                          |
        | INVITE sip:456@b.com     |                [original call]
        | From: sip:123@a.com      |
        |------------------------->|
        |                          |
        | 487                      |
        | Call-Info:<sip:456@z.b.com>;purpose=call-completion;m=NR
        |<-------------------------|
        |                          |
        | SUBSCRIBE sip:456@z.b.com;m=NR  [initial SUBSCRIBE]
        | From: sip:123@a.com      |
        | Contact: sip:123@y.a.com |
        | Request-Disposition: parallel, no-cancel
        | Call-Id: abcd-efgh       |
        | Event: call-completion   |
        |------------------------->|
        |                          |
        | 200                      |
        |<-------------------------|
        |                          |
        | NOTIFY sip:123@y.a.com   |            [initial NOTIFY]
        | Body:  status: queued    |
        |<-------------------------|
        |                          |
        | SUBSCRIBE sip:456@b.com;m=NR          [another init. SUB.]
        | From: sip:foo@example.com|
        | Request-Disposition: parallel, no-cancel
        | Call-Id: abcd-efgh       |
        | Event: call-completion   |
        |------------------------->|
        |                          |
        | 482                      |           [duplicate SUB. rejected]
        |<-------------------------|
        |                          |
        | NOTIFY sip:123@y.a.com   |             [CC invoked]
        | Body:  status: ready     |
        |        URI: sip:recall@z.b.com
        |<-------------------------|
        |                          |
        | INVITE sip:recall@z.b.com;m=NR            [CC recall]
        | From: sip:foo@example.com|
        |------------------------->|
        |                          |

   The original call is an ordinary INVITE.  It fails due to no-response
   (ring-no-answer).  In this case, the callee's governing proxy



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   generates a 487 response because the proxy canceled the INVITE to the
   UA when it rang too long without an answer.  The 487 response carries
   a Call-Info header with "purpose=call-completion".  The Call- Info
   header positively indicates that CC is available for this failed fork
   of the call.  The "m=NR" parameter indicates that it failed due to
   no-response, which is useful for PSTN interworking and assessing
   presence information in the callee's monitor.

   The URI in the Call-Info header (<sip:456@z.b.com>) is the where the
   caller's agent should subscribe for call-completion processing.
   Ideally, it is a globally-routable URI for the callee's monitor.  In
   practice, it may be the callee's AOR, and the SUBSCRIBE will be
   routed to the callee's monitor only because it specifies "Event:
   call-completion".

   CC activation is done by sending a SUBSCRIBE to all known monitor
   URIs.  These can be provided by the Call-Info header in the response
   to the INVITE.

   Additionally, the caller's agent needs to include the original
   request-URI in its set of monitor URIs, because the call may have
   forked to additional callees whose responses the caller has not seen.
   (A SUBSCRIBE to the request-URI alone is used in cases where the
   caller's agent has not received or cannot remember any monitor URI.)

   The caller's agent adds to these URIs an 'm' parameter (if possible).
   In this case, the caller's agent forks the SUBSCRIBE to two
   destinations, with appropriate Request-Disposition.  The first
   SUBSCRIBE is to the URI from Call-Info.

   The second SUBSCRIBE is to the original request-URI, and reaches the
   same callee's monitor.  Because it has the same Call-Id as the
   SUBSCRIBE that has already reached the monitor, the monitor rejects
   it with a 482, thus avoiding redundant subscriptions.

   The initial NOTIFY for the successful SUBSCRIBE has "state: queued"
   in its body.  Eventually, this caller is selected for CC, and is
   informed of this via a NOTIFY containing "state: ready".  This NOTIFY
   carries a URI to which the CC INVITE should be sent.  In practice,
   this may be the AOR of the callee.

   The caller generates a new INVITE to the URI specified in the NOTIFY,
   or if there was no such URI or if the caller's agent cannot remember
   it, it may use the original request-URI.  The caller adds the 'm'
   parameters (if possible), to specify CC processing.






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9.  Call Completion Event Package

   This section specifies the call-completion event package, in
   accordance with section 4.4 of [RFC3265].  The call-completion event
   package has the media type "application/call-completion".

   Note that if the callee has a caller-queuing facility, the callee's
   monitor may want to treat the call-completion queue as part of the
   queuing facility, and include in the event package information
   regarding the state of the queue.  How this information is conveyed
   is left for further standardization.

9.1.  Event Package Name

   The SIP Events specification requires package definitions to specify
   the name of their package or template-package.  The name of this
   package is "call-completion".  This value appears in the Event and
   Allow-events header fields.

9.2.  Event Package Parameters

   No package-specific Event header parameters are defined for this
   event package.

9.3.  SUBSCRIBE Bodies

   [RFC3265] requires package definitions to define the usage, if any,
   of bodies in SUBSCRIBE requests.

   The SUBSCRIBE request MAY contain an Accept header field.  If no such
   header field is present, it has a default value of "application/
   call-completion".  If the header field is present, it MUST include
   "application/call-completion".

   A SUBSCRIBE request for a call-completion package MAY contain a body.
   This body defines a filter to be applied to the subscription.  Filter
   documents are not specified in this document, and may be the subject
   of future standardization activity.

   A SUBSCRIBE request requests call-completion information regarding
   calls recently made from the same originator to the destination UA(s)
   serviced by the notifier.  Calls are defined to be "from the same
   originator" if the URI-part of the From header value in the INVITE is
   the same as the URI-part of the From header value in the SUBSCRIBE.







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9.4.  Subscribe Duration

   [RFC3265] requires package definitions to define a default value for
   subscription durations, and to discuss reasonable choices for
   durations when they are explicitly specified.

   If a SUBSCRIBE does not explicitly request a duration, the default
   requested duration is 3600 seconds, as that is the highest service
   duration timer value recommended for the call completion services in
   ETSI and ITU-T.  It is RECOMMENDED that subscribers request, and that
   notifiers grant, a subscription time of at least 3600 seconds.

   If a notifier can determine that, according to its policy, after
   certain duration the requested subscription cannot proceed to "ready"
   state, it SHOULD reduce the granted subscription time to that
   duration.  If a notifier can determine that, according to its policy,
   the requested subscription cannot proceed to "ready" state, it should
   refuse the subscription.  For example, in many cases when resuming a
   subscription the granted duration will be less than 3600 seconds.

9.5.  NOTIFY Bodies

   [RFC3265] requires package definitions to describe the allowed set of
   body types in NOTIFY requests, and to specify the default value to be
   used when there is no Accept header field in the SUBSCRIBE request.
   A NOTIFY for a call-completion package MUST contain a body that
   describes the call-completion states.

   As described in [RFC3265], the NOTIFY message will contain bodies
   that describe the state of the subscribed resource.  This body is in
   a format listed in the Accept header field of the SUBSCRIBE, or in a
   package-specific default format if the Accept header field was
   omitted from the SUBSCRIBE.

   In this event package, the body of the notification contains a call-
   completion document.  All subscribers and notifiers MUST support the
   "application/call-completion" data format described in section 8.
   The SUBSCRIBE request MAY contain an Accept header field.  If no such
   header field is present, it has a default value of "application/
   call-completion".  If the header field is present, it MUST include
   "application/call-completion".  Of course, the notifications
   generated by the server MUST be in one of the formats specified in
   the Accept header field in the SUBSCRIBE request.

9.6.  Subscriber Generation of SUBSCRIBE Requests

   Subscribers MUST generate SUBSCRIBE requests when they want to
   subscribe to the call-completion event package at the terminating



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   side in order to receive call-completion notifications.  The
   generation of SUBSCRIBE requests MAY imply the usage of a call-
   completion service specific timer as described in section 9.4.

9.7.  Notifier Processing of SUBSCRIBE Requests

   Upon receiving a subscription refresh, the notifier MUST set the
   "expires" parameter of the Subscription-State header to the current
   remaining duration of the subscription regardless of the value
   received in the Expires header (if present) of the subscription
   refresh.

   If a subscription is not successful because the call-completion queue
   has reached the maximum allowed number of entries (short term
   denial), the notifier MUST send a 480 Temporarily Unavailable
   response to the subscriber.  If a subscription is not successful
   because an error has occurred that prevents and will continue to
   prevent the call-completion service (long term denial), the notifier
   MUST send a 403 Forbidden response to the subscriber.

   A notifier MAY receive multiple forks of the same SUBSCRIBE.
   (Multiple forks are, as always, identified by having the same
   Call-Id.)  In such a case, the notifier SHOULD reject all but one of
   the SUBSCRIBEs with a 482 Merged Request response unless some other
   failure response applies.

   The call-completion information can be sensitive.  Therefore, all
   subscriptions SHOULD be handled with consideration of the issues
   discussed in Section 11.

9.8.  Notifier Generation of NOTIFY Requests

   Notifiers MUST generate NOTIFY requests when a call-completion
   service condition occurs at the terminating side that needs to be
   sent towards the originating side.  A NOTIFY that is sent with non-
   zero expiration MUST contain the "cc-state" parameter.  The
   parameter's value MUST be "queued" if the call-completion request
   represented by the subscription is not at this time selected by the
   monitor for CC recall, and the parameter's value MUST be "ready" if
   the request is at this time selected by the monitor for CC recall.

   A NOTIFY sent with a zero expiration (e.g., as a confirmation of a
   request to unsubscribe) MAY contain the "cc-state" parameter.

   When the callee's monitor withdraws selection of the request for CC
   recall (e.g., because the agent has not initiated the CC recall
   INVITE promptly, or because the agent has suspended the request from
   being considered for CC recall), the notifier MUST send a NOTIFY to



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   the subscription of the selected request.  This NOTIFY MUST contain
   the "cc-state" parameter set to "queued".

   If the call-completion subscription was successful and the retention
   option is supported at the callee, the NOTIFY MUST contain the "cc-
   service-retention" parameter.

9.9.  Subscriber Processing of NOTIFY Requests

   The subscriber processing of NOTIFY requests MAY trigger additional
   CC service procedures, as described in this document and possibly in
   other documents.

9.10.  Handling of Forked Requests

   Forked requests are expected to be common for the call-completion
   event type.  The subscriber MUST be prepared to process NOTIFY
   requests from multiple notifiers and to coordinate its processing of
   the information obtained from them in accordance with the procedures
   in this document.

9.11.  Rate of Notifications

   The call completion service typically involves a single notification
   per notifier and per subscription but MAY involve several
   notifications separated by a call completion call that failed due to
   a busy call completion target.  Typically, notifications will be
   separated by at least tens of seconds.  Notifiers SHOULD NOT generate
   more than three notifications for one subscription in any ten-second
   interval.  Since it is important to to avoid useless re-calls, a
   notifier SHOULD send state changes to "queued" promptly.  Thus, a
   notifier SHOULD NOT send a state change to "ready" as the third
   notification in a ten-second interval, as that would make it
   impossible to promptly send a further state change to "queued".

9.12.  State Agents

   State agents have no defined role in the handling of the call-
   completion package.


10.  Call-completion information format

   The following syntax specification uses the Augmented Backus-Naur
   Form (ABNF) as described in RFC 2234.  The formal syntax for the
   application/call-completion MIME type is described below.  In
   general, the call-completion body is to be interpreted in the same
   way as SIP headers: (1) the names of the lines are case-insensitive,



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   (2) the lines can be continued over line boundaries if the succeeding
   lines start with horizontal white space, (3) lines with unknown names
   are to be ignored, and (4) names starting with "X-" are reserved for
   non-standardized uses.  Two lines with the same name MUST NOT be
   present, except when specifically permitted.

   call-completion = +(cc-header CRLF)

   cc-header = cc-state / cc-service-retention / cc-URI / extension-
   header

   The above rules whose names start with "cc-" are described below.
   Other rules are described in RFC 3261.

10.1.  call-completion-state

   The cc-state line indicates the call-completion status of a
   particular user with an entry in a call-completion queue.  It MUST be
   present unless the expiration time indicated in the NOTIFY is zero.

   cc-state = "cc-state" HCOLON ( "queued" / "ready" )

   The value "queued" indicates that a user's entry in the call-
   completion queue is not selected for CC recall.  The value "ready"
   indicates that a user's entry in the call-completion queue has been
   selected for CC recall.

10.2.  service-retention

   The service-retention line indicates the support of the retain
   option.  The retain option, if supported at the callee, will maintain
   the entry in the call-completion queue, if a call-completion call has
   failed due to destination busy condition.  If present, this parameter
   indicates that the retain option is supported, otherwise it is not
   supported.  This parameter MAY be present in NOTIFY requests.

   cc-service-retention = "cc-service-retention" HCOLON "true"

10.3.  cc-URI

   The cc-URI line provides a URI (possibly in the form of a name-addr)
   which the agent SHOULD use as the request-URI of the CC recall
   INVITE.  It SHOULD NOT be provided and MUST be ignored unless the cc-
   state value is "ready".  The URI SHOULD be globally routable.  The
   syntax provides for generic-params in the value, but this document
   defines no such parameters.  Parameters that are not understood by
   the subscriber MUST be ignored.




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   cc-URI = "cc-URI" HCOLON (name-addr / addr-spec) *(SEMI generic-
   param)


11.  Security Considerations

   The use of the CC facility allows the caller's agent to determine
   some status information regarding the callee.  The information is
   confined to a busy/not-busy indication, and in legitimate use, > will
   be subscribed to stereotyped ways that limit the disclosure of status
   information:

   1.  When a subscriber is selected for CC, a call should arrive
       promptly for the callee, or the subscription should be
       terminated.  This expectation may be violated by a race condition
       between selection of the subscription for CC and the caller
       becoming unavailable, but it should be rare that a single
       subscription will exhibit the race more than once.

   2.  Subscriptions should not remain suspended for longer than the
       expected duration of a call (a call by the caller to a third
       party).

   3.  Subscriptions should be initiated only shortly after failed
       incoming calls.

   4.  Most of the time, a callee should have no queued subscriptions.
       Violations of these expectations should be detected by the
       callee's monitor and reported as possible attempts at privacy
       violation.

   The CC facility may enhance the effectiveness of SPIT by the
   following technique: The caller makes calls to a group of targets.
   The caller then requests CC for the calls that do not connect to the
   targets.  The CC calls resulting are probably more likely to reach
   the targets than original calls to a further group of targets.


12.  IANA Considerations

12.1.  SIP Event Package Registration for call-completion

   This specification registers an event package, based on the
   registration procedures defined in .  The followings is the
   information required for such a registration:

   Package Name: call-completion




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   Package or Template-Package: This is a package.

   Published Document: RFC XXXX (Note for RFC Editor: Please fill in
   XXXX with the RFC number of this specification).

   Person to Contact: Martin Huelsemann, martin.huelsemann@telekom.de

12.2.  MIME Registration for application/call-completion

   MIME media type name: application

   MIME subtype name: call-completion

   Required parameters: none.

   Optional parameters: none.

   to be defined

12.3.  SIP/SIPS URI parameter 'm'

   This RFC adds a URI parameter 'm'.  It is used to identify that an
   INVITE request is a CC call, or to further identify that a SUBSCRIBE
   request is for the call-completion event package.  The parameter may
   have a value that describes the type of the call-completion
   operation, as described in this RFC.  This adds a row to the registry
   SIP/SIPS URI Parameters:

   Parameter Name Predefined Values Reference

   ------------------------ ------------------------- --------------

   m Yes [this RFC]

12.4.  'purpose=call-completion' header parameter for Call-Info

   This RFC adds a new predefined value "call-completion" for the
   "purpose" header parameter of the Call-Info header.  This modifies
   the registry Header Field Parameters and Parameter Values by adding
   this RFC as a Reference to the line for Header Field "Call-Info" and
   Parameter Name "purpose":

   Header Field Parameter Name Predefined Values Reference

   --------------------- ---------------------------
   ------------------------- ---------

   Call-Info purpose Yes [ RFC3261][RFC5367][this RFC]



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12.5.  'm' header parameter for Call-Info

   This RFC adds a new header parameter 'm' of the Call-Info header.
   This adds a row to the registry Header Field Parameters and Parameter
   Values:

   Header Field Parameter Name Predefined Values Reference

   --------------------- ---------------------------
   ------------------------- ---------

   Call-Info m Yes [this RFC]


13.  Acknowledgements

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).


14.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3261]  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.

   [RFC3265]  Roach, A., "Session Initiation Protocol (SIP)-Specific
              Event Notification", RFC 3265, June 2002.

   [RFC3515]  Sparks, R., "The Session Initiation Protocol (SIP) Refer
              Method", RFC 3515, April 2003.

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

   [RFC4235]  Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE-
              Initiated Dialog Event Package for the Session Initiation
              Protocol (SIP)", RFC 4235, November 2005.

   [RFC5359]  Johnston, A., Sparks, R., Cunningham, C., Donovan, S., and
              K. Summers, "Session Initiation Protocol Service
              Examples", BCP 144, RFC 5359, October 2008.




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Appendix A.  Example Caller's Agent

   This section outlines how an autonomous caller's agent can operate
   using only standard SIP features to interact with the caller's UA.
   This example is suitable only as a learning aid, as its performance
   is poor.

   The agent monitors calls made from the UA(s) by subscribing to the
   dialog event package of the UA(s).

   The UA(s) or their proxy routes calls made with either of two special
   dial sequences to the agent, which interprets the INVITEs as
   indications to make a CC request with BS or NR or NL mode for the
   latest call made by the UA.

   The agent monitors the status of the UA(s) for availability to be
   used for a CC call by examining the dialog events.

   The agent indicates to the UA(s) that CC recall is in progress by
   making call to the UA(s).  If the UA is answered, the agent assumes
   that the caller is available and plays pre-recorded audio to indicate
   that CC recall is in progress.

   After playing the pre-recorded audio, the caller's agent uses REFER
   to order the UA to make the CC call to the callee.


Appendix B.  Example Callee's Monitor

   This section outlines how an autonomous callee's monitor can operate
   using only standard SIP features to interact with the callee's UA.
   This example is suitable only as a learning aid, as its performance
   is poor.

   The monitor monitors calls made to the UA(s) by subscribing to the
   UA(s) dialog events.  This enables it to determine their Call-Id's
   and their final response statuses.

   The proxy for the UA(s) routes to the monitor any SUBSCRIBEs for the
   call-completion event package directed to the URIs serviced by the
   UA(s).

   The monitor monitors the status of the UA(s) to determine when they
   are in a suitable state to receive a CC call by watching the busy/
   not-busy status of the UA(s): e.g. a UA is available for CCBS when it
   is not busy, but a UA is available for CCNR when it becomes not busy
   after being busy with an established call.




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Authors' Addresses

   Dale R. Worley
   Nortel Networks Corp.
   600 Technology Park Dr.
   Billerica, MA,   01821
   US

   Phone: +1 781 229 0533 x173
   Email: dworley@nortel.com
   URI:   URI: http://www.nortel.com


   Martin Huelsemann
   Deutsche Telekom
   Heinrich-Hertz-Strasse 3-7
   Darmstadt,   64307
   Germany

   Phone: +4961516282765
   Email: martin.huelsemann@telekom.de


   Roland Jesske
   Deutsche Telekom
   Heinrich-Hertz-Strasse 3-7
   Darmstadt,   64307
   Germany

   Phone: +4961516282766
   Email: r.jesske@telekom.de


   Denis Alexeitsev
   Deutsche Telekom
   Friedrich-Ebert-Allee
   Bonn  53113
   Germany

   Phone: +49-228-18112010
   Email: d.alexeitsev@telekom.de










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