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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                           Ariadne Internet Services, Inc.
Intended status: Standards Track                           M. Huelsemann
Expires: August 15, 2013                                       R. Jesske
                                                        Deutsche Telekom
                                                           D. Alexeitsev
                                                               TeleFLASH
                                                       February 11, 2013


         Call Completion for Session Initiation Protocol (SIP)
                  draft-ietf-bliss-call-completion-19

Abstract

   The call completion feature defined in this specification allows the
   caller of a failed call to be notified when the callee becomes
   available to receive a call.

   For the realization of a basic solution without queuing, this
   document references the usage of the dialog event package (RFC 4235)
   that is described as 'automatic redial' in the SIP Service Examples
   (RFC 5359).

   For the realization of a more comprehensive solution with queuing,
   this document introduces an architecture for implementing these
   features in the Session Initiation Protocol where "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 of the original, failed call is made.

   The architecture is designed to interoperate well 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
   time.  It is inappropriate to use Internet-Drafts as reference



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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 15, 2013.

Copyright Notice

   Copyright (c) 2013 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
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

































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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 . . . . . . . . . . . . . . .  8
     4.2.  Call-completion procedures . . . . . . . . . . . . . . . .  9
     4.3.  Automatic redial as a fallback . . . . . . . . . . . . . . 12
     4.4.  Differences from SS7 . . . . . . . . . . . . . . . . . . . 12
   5.  Call-completion queue model  . . . . . . . . . . . . . . . . . 13
   6.  Caller's agent behavior  . . . . . . . . . . . . . . . . . . . 14
     6.1.  Receiving the CC possible indication . . . . . . . . . . . 14
     6.2.  Subscribing to CC  . . . . . . . . . . . . . . . . . . . . 14
     6.3.  Receiving a CC recall notification . . . . . . . . . . . . 16
     6.4.  Initiating a CC call . . . . . . . . . . . . . . . . . . . 16
     6.5.  Suspending CC  . . . . . . . . . . . . . . . . . . . . . . 16
     6.6.  Resuming CC  . . . . . . . . . . . . . . . . . . . . . . . 17
   7.  Callee's monitor behavior  . . . . . . . . . . . . . . . . . . 17
     7.1.  Sending the CC possible indication . . . . . . . . . . . . 17
     7.2.  Receiving a CC subscription  . . . . . . . . . . . . . . . 18
     7.3.  Sending a CC notification  . . . . . . . . . . . . . . . . 19
     7.4.  Receiving a CC call  . . . . . . . . . . . . . . . . . . . 20
     7.5.  Receiving a CC suspension  . . . . . . . . . . . . . . . . 20
     7.6.  Receiving a CC resumption  . . . . . . . . . . . . . . . . 21
   8.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
   9.  Call-completion event package  . . . . . . . . . . . . . . . . 26
     9.1.  Event package name . . . . . . . . . . . . . . . . . . . . 26
     9.2.  Event package parameters . . . . . . . . . . . . . . . . . 26
     9.3.  SUBSCRIBE bodies . . . . . . . . . . . . . . . . . . . . . 26
     9.4.  Subscribe duration . . . . . . . . . . . . . . . . . . . . 27
     9.5.  NOTIFY bodies  . . . . . . . . . . . . . . . . . . . . . . 27
     9.6.  Subscriber generation of SUBSCRIBE requests  . . . . . . . 28
     9.7.  Notifier processing of SUBSCRIBE requests  . . . . . . . . 28
     9.8.  Notifier generation of NOTIFY requests . . . . . . . . . . 28
     9.9.  Subscriber processing of NOTIFY requests . . . . . . . . . 29
     9.10. Handling of forked requests  . . . . . . . . . . . . . . . 29
     9.11. Rate of notifications  . . . . . . . . . . . . . . . . . . 29
     9.12. State agents . . . . . . . . . . . . . . . . . . . . . . . 30
   10. Call-completion information format . . . . . . . . . . . . . . 30
     10.1. Call-completion status . . . . . . . . . . . . . . . . . . 30
     10.2. Call-completion service-retention indication . . . . . . . 30
     10.3. Call-completion URI  . . . . . . . . . . . . . . . . . . . 31
   11. Security considerations  . . . . . . . . . . . . . . . . . . . 31
   12. IANA considerations  . . . . . . . . . . . . . . . . . . . . . 32
     12.1. SIP event package registration for call-completion . . . . 33
     12.2. MIME registration for application/call-completion  . . . . 33
     12.3. SIP/SIPS URI parameter 'm' . . . . . . . . . . . . . . . . 34



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     12.4. Call-Completion purpose parameter value  . . . . . . . . . 34
     12.5. 'm' header parameter for Call-Info . . . . . . . . . . . . 35
   13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 35
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 35
     14.2. Informative References . . . . . . . . . . . . . . . . . . 36
   Appendix A.  Example Caller's Agent  . . . . . . . . . . . . . . . 37
   Appendix B.  Example Callee's Monitor  . . . . . . . . . . . . . . 37
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38










































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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
   while guessing when the callee becomes available.  When the caller
   requests the use of the CC feature, the callee will be monitored for
   its availability.  When the callee becomes available the callee will
   be given a certain timeframe for initiating a call.  If the callee
   does not initiate a new call within this timeframe, then the caller
   will be recalled.  When the caller accepts the CC recall then a CC
   call to the callee will automatically start.  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:

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

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



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   Callee's monitor: a logical 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.

   Caller's agent: a logical 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.

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

   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.

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

   CCNR, or Call Completion on No Reply: a CC service when the initial
   failure was that the destination UA did not answer.

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

   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 indicator: an indication in the CC call INVITE used to prioritize
   the call at the destination.

   CC possible indication: the data in responses to the INVITE of the
   original call which indicate that CC is available for the call.

   CC recall: the action of the callee's monitor selecting a particular
   CC request for initiation of 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 recall timer: maximum time the callee's monitor will wait for the
   caller's response to a CC recall.




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   CC request: the entry in the callee's monitor queue representing the
   caller's request for CC processing, that is, the caller's call-
   completion subscription.

   CC service duration timer: maximum time a CC request may remain
   active within the network.

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

   CCE, or call-completion entity: the representation of a CC request,
   or equivalently, an existing call-completion subscription within a
   callee's monitor's queue

   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.

   Notifier: the user agent that generates NOTIFY requests for the
   purpose of notifying subscribers of the callee's availibility; for
   the CC service this is task of the callee's monitor.

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

   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.
   Note that SIP CC always operates with the retain option active; a
   failed CC call does not cause the CC request to lose its position in
   the queue.

   Subscriber: the user agent that receives NOTIFY requests with
   information of the callee's availibility; for the CC service this is
   task of the caller's agent.

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


4.  Solution





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4.1.  Call-completion architecture

   The call-completion architecture augments each caller's UA (or UAC)
   wishing to use the call-completion features with a "call-completion
   agent" (also written as "caller's agent").

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

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

   A caller's agent may service more than one UA as a collective group
   if a caller or population of users will be shared between the UAs,
   and especially if the UAs share an AOR.

   The caller's agent monitors calls made from the caller's UA(s) in
   order to determine their destinations and (potentially) their final
   response statuses, and the Call-Info header fields of provisional and
   final responses for to invoke the call completion feature.

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

   The callee's monitor also instantiates a presence server used to
   monitor caller's availability for CC recall.

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

   In order to allow flexibility and innovation, most of the interaction
   between the caller's agent, 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, the callee(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.



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   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 caller's agents and callee's monitors
   can be devised that interact with users and UAs entirely through
   standard SIP mechanisms [RFC6665], [RFC4235] and [RFC3515], as
   described in the Appendixes.

   The callers using the UA(s) can indicate to the caller's agent when
   they wish to avail themselves of CC for a recently-made call which
   the callers determined to unsuccessful.  The caller's agent monitors
   the status of the caller's UA(s) to determine when they are available
   to be used for a CC recall.  The caller's agent can communicate to
   the caller's UA(s) that a CC recall is in progress and to inquire if
   the relevant caller is available for the CC recall.

   The callee's monitor may utilize several methods to monitor the
   status of the callee's UA(s) and/or their users for availability to
   receive a CC call.  This can be achieved through monitoring calls
   made to the callee's UA(s) to determine their caller's and their
   final response' statuses.  And 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 URI
   '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 callee's monitor maintains information about the set of INVITEs
   received by the callee's UA(s) considered unsuccessful by the caller.
   In practice, the callee's monitor may remove knowledge about an
   incoming dialog from its set if local policy at the callee's monitor
   establishes that the dialog is no longer eligible for CC activations.

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.  If the call-
   completion feature is available, the callee's monitor (note there can
   be a monitor for each of the callee's UAs) inserts a Call-Info header
   field with its URI and with "purpose=call-completion" in appropriate
   non-100 provisional or final response to the initial INVITE and



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   forwards them to the caller.  The provisional response SHOULD be sent
   reliably, if the INVITE contained a Supported header field with the
   option tag 100rel.  On receipt of a non-100 provisional or a final
   response with the indication that the call-completion feature is
   available, the calling user can invoke the CC feature.

   The caller 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 have been successful, but from the
   user's point of view, the call may have been 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 receive information necessary for the caller to complete
   the call at the callee, the caller's agent subscribes to the call-
   completion event package at the callee's monitor.

   The possibility of the caller to complete the call at the callee is
   also known as the call-completion state (cc-state) of the caller.
   The cc-states comprehend the values 'queued' and 'ready' (for call-
   completion).

   In order to receive information from all destinations where the
   callee will be reachable, the caller's agent sends a SUBSCRIBE
   request for the call-completion event package to the original
   destination URI of the call and to all known callee's monitor URIs
   (which are provided by Call-Info header fields in provisional and
   final responses to the INVITE).  The callee's monitor uses the
   subscription as an indication that caller is interested in using the
   CC feature with regard to the specified callee.  The callee's monitor
   keeps a list or queue of the caller's agent's subscriptions,
   representing the requests from the caller's agent to the callee's
   monitors for call-completion services.  These subscriptions are
   created, refreshed, and terminated according to the procedures of
   [RFC6665].

   Upon receiving a SUBSCRIBE request from the caller's agent, the
   callee's monitor instantiates a presence state for the caller's UA
   which can be modified by the caller's UA to indicate its availability
   for CC call.  The status at the presence upon instantiation is
   "open".

   When the callee's monitor determines the callee and/or callee's UA
   available for a CC call, it selects a caller to execute the CC call
   and sends a call-completion event update (''cc-state: ready'') via a
   NOTIFY request to the selected caller's agent's subscription, telling
   it to begin the CC call to the callee's UA.



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   When the caller's agent receives this update, it initiates a CC
   recall by calling the caller's UA, and then starts the CC call to the
   callee's UA, using 3rd party call control procedures in accordance
   with [RFC3725].  The caller's agent can also check by other means
   whether the caller is available to initiate the CC call to the
   callee's UA.  If the caller is available, the caller's agent directs
   the caller's UA to initiate the CC call to the callee's UA.

   The caller's agent marks the CC call as such by adding a specific SIP
   URI parameter to the Request-URI, so it can be given precedence by
   the callee's monitor in reaching the callee's UA.

   If the caller is not available on the receipt of the "ready for
   recall" notification, the caller's agent suspends the CC request at
   the callee's monitor by sending a PUBLISH request containing presence
   information to the callee's monitor's presence server, informing
   about the presence status 'closed'.  Once the caller becomes
   available for a CC call again, the caller's agent resumes the CC
   request by sending another PUBLISH request to the callee's monitor,
   informing about the presence status 'open'.

   On the receipt of the suspension request, the callee's monitor
   performs the monitoring for the next non-suspended CC request in the
   queue.  On the receipt of the resume from the previously suspended
   caller's agent that was at the top of the queue, the callee's monitor
   performs the callee monitoring for this caller's agent.

   When the CC call fails there are two possible options: the CC feature
   has to be activated again by caller's agent subscribing to callee's
   monitor, or CC remains activated and the original CC request retains
   its position in the queue if retain option is supported.

   The retain option (see section 3) determines the callee's monitor's
   behavior when a CC call fails.  If the retain option is supported, CC
   remains activated and the original CC request retains its position in
   the queue.  Otherwise the CC feature is deactivated, and the caller's
   agent would have to subscribe again to reactivate it.

   A monitor that supports the retain option provides the cc-service-
   retention header in its call-completion events.  A caller's agent
   that also supports the retain option uses the presence of this header
   to know not to generate a new CC request after a failed CC call.

   Monitors not supporting the retain option do not provide the cc-
   service-retention header.  A failed CC call causes the CC request to
   be deleted from the queue, and these monitors will terminate the
   corresponding caller's agent's subscription to inform that agent that
   its CC request is no longer in the queue.  A caller's agent that does



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   not support the retain option can also terminate its subscription
   when a CC call fails, so it is possible that both the caller's agent
   and the monitor may be signalling the termination of the subscription
   concurrently.  This is a normal SIP Events [ [RFC6665]] scenario.
   After the subscription is terminated, the caller's agent may create a
   new subscription (as described in section 6.2) to reactivate the CC
   feature for the original call.

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 notified 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 happen that several recalls are started at the same time.  This
   means it is a heuristic approach with no guarantee of success.

   There is no interaction between call completion and automatic redial,
   as there is a difference in the behavior 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.

   As there is no equivalent to the forking mechnism in SS7, in the PSTN
   calls can be clearly differentiated between successful or
   unsuccessful.  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
   caller must be able to invoke call completion even when the original
   call appeared to succeed.  To support this, the caller's agent must
   record successful calls as well as unsuccessful calls.

   In SIP, only the caller's UA or service system on the originating



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   side and the callee's UA or service system on the terminating side
   need to support call completion for call completion to work
   successfully between the UAs.  Intermediate SIP systems (proxies or
   B2BUAs) do not need to implement call completion; they only need to
   be transparent to the usual range of SIP messages.  In the PSTN also
   intermediate nodes like media gateway controllers have to implement
   the call completion service.


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 "non-voicemail AOR", but it may
   be as narrowly scoped as a single UA's contact URI.  The callee's
   monitor manages a dynamic set of call-completion entities (called
   "CCEs") which represent CC requests, or equivalently, the existing
   incoming call-completion subscriptions.  This set is also called a
   queue, because a queue data structure often aids in implementing the
   callee's monitor's policies for selecting CCEs for CC recall.

   Each CCE has an availability state, determined through caller's
   presence status at the callee's monitor.  A presence status of
   ''open'' represents CCE's availability state of 'available' and a
   presence status of "closed" represents CCE's availability state of
   'unavailable'.

   Each CCE has a recall state which is visible via subscriptions.  The
   recall state is either "queued" or "ready".

   Each CCE carries the From URI of the SUBSCRIBE request that caused
   its creation.

   CC subscriptions arrive at the callee's monitor by addressing the
   URIs the callee's monitor returns in Call-Info header fields.  The
   request URI of the SUBSCRIBE request determines the queue to which
   the resulting CCE is added.  The resulting subscription reports the
   status of the queue.  The base event data is the status of all the
   CCEs in the queue, but the data returned by each subscription is
   filtered to report only the status of that subscription's CCE.
   (Further standardization may define means for obtaining more
   comprehensive information about a queue.)

   When a CCE is created, it is given the availability state "available"
   and recall state "queued".

   When the callee's monitor receives PIDF bodies [RFC3863] via PUBLISH
   requests [ [RFC3903]], these PUBLISH requests are expected to be sent
   by subscribers to indirectly suspend and resume their CC requests by



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   modifying its CCE availability state.  A CCE is identified by the
   request-URI (if it was taken from a call-completion event
   notification which identifies the CCE) or the From URI of the request
   (matching the From URI recorded in the CCE).  Receipt of a PUBLISH
   with 'status' of 'open' sets the availability state of the CCE to
   'available' (resume); 'status' of 'closed' sets the availability
   state of the CCE to 'not-available' (suspend).

   A CC request is eligible for recall only when its CCE's availability
   state is "available" and the "m" value of the CCE also indicates an
   available state.  The callee's monitor MUST NOT select for recall any
   CC requests that fail to meet those criteria.  Within that
   constraint, the callee's monitor's selections are determined by its
   local policy.  Often, a callee's monitor will choose the acceptable
   CCE that has been in the queue the longest.  When the callee's
   monitor has selected a CCE for recall, it changes the CCE's recall
   state from 'queued' to 'ready', which triggers a notification on the
   CCE's subscription.

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

   The caller's presence status at the callee's monitor is terminated
   when the caller completes its CC call or when the caller's agent's
   subscription at the callee's monitor is terminated.


6.  Caller's agent behavior

6.1.  Receiving the CC possible indication

   The caller's agent MUST record the From URI and SHOULD record the
   final request status that the caller's UA received along with the
   contents of Call-Info header fields of provisional and final
   responses.

   Note that receiving a CC possible indication also depends on the
   aggregation of final responses by proxies, in case of 4xx responses
   some 4xx responses are more likely to be sent to the caller.

6.2.  Subscribing to CC

   For CC activation the caller's agent MUST send a SUBSCRIBE to all
   known callee's monitor URIs.  A callee's monitor URI may be provided
   in the Call-Info header field in provisional and final responses to



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   the INVITE sent back by the callee's monitor(s).  Additionally, the
   caller's agent SHOULD include the original request-URI that it sent
   the original INVITE to, in its set of callee's monitor URIs, when it
   is unclear if the call has forked to additional callees whose
   responses the caller has not seen.  A SUBSCRIBE to the original
   request-URI alone is used in cases where the caller's agent has not
   received or does not remember any callee's monitor URI.  The caller's
   agent SHOULD add an 'm' parameter to these URIs in order to indicate
   the desired call-completion proceeding at the callee's monitor.  The
   'm' parameter SHOULD have the value of the 'm' parameter received in
   the Call-Info header field of the responses to the original INVITE.

   To minimize redundant subscriptions, these SUBSCRIBEs SHOULD be
   presented as forks of the same transaction as defined by section
   8.2.2.2 of [RFC3261], if the caller's agent is capable of doing so.

   The agent MUST NOT maintain more than one CC request for a single
   caller and directed to a single original destination URI.  If a
   caller requests CC a second time for the same destination URI, the
   agent MUST consolidate the new request with the existing CC request
   by either reusing the existing CC subscriptions or terminating and
   then recreating them.  For this purpose, equality of callers is
   determined by comparing caller's AORs and equality of destination
   URIs is determined by comparing them per [RFC3261] section 19.1.4.

   When generating these SUBSCRIBEs, the From URI MUST be the caller's
   AOR.  The To URI SHOULD be the destination URI of the original call
   (if the agent knows that and can insert it into the To header), and
   otherwise MUST be the request-URI of the SUBSCRIBE.

   The SUBSCRIBE SHOULD have header fields to optimize its routing.  In
   particular, it SHOULD contain "Request-Disposition: parallel", and an
   Accept-Contact header field to eliminate callee UAs that are not
   acceptable to the caller.

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

   If the caller's agent no longer wants to initiate the CC call (e.g.,
   because the caller has deactivated CC), the caller's agent terminates
   the subscription in accordance with [RFC6665] or suspends the
   subscription(s) as specified in subclause 6.5.






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6.3.  Receiving a CC recall notification

   When receiving a NOTIFY with the cc-state set to 'ready', the
   caller's agent SHOULD suspend all other subscriptions to CC, by
   following the step in section 6.5, in order to prevent any other CC
   requests from this caller to receive CC recalls.  The caller's agent
   starts the CC recall to the caller by confirming that the caller
   would be able to initiate a CC call, e.g. by calling the caller's
   UA(s).

6.4.  Initiating a CC call

   If the caller is available for the CC call and willing to initiate
   the CC call, the caller's agent causes the caller's UA to generate a
   new INVITE towards the callee.  The caller's UA MAY add a 'm' URI
   parameter with the value of the 'm' parameter received in Call-Info
   header in the response to original INVITE, in order to specify his
   preferences in CC processing and to prioritize the CC call.  The
   INVITE SHOULD be addressed to the URI specified in the cc-URI of the
   NOTIFY, or if not available it SHOULD use the URI in the Call-Info
   header field of the response to the original INVITE, or if not
   available it MAY use the request-URI of the original INVITE, if this
   URI was recorded.  Note that the latter choice 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 not available for the CC recall, the CC request
   SHALL be suspended by the caller's agent until the caller becomes
   available again, or if the conditions relevant to the caller's
   agent's local policy for suspensions have changed.  To suspend the CC
   request, the caller's agent SHALL publish the caller's presence state
   by sending a PUBLISH request to each callee's monitor where the
   presence server for CC resides in accordance with the procedures
   described in [RFC3903] , giving the PIDF state 'closed' for the
   caller's identity as presentity.  The PUBLISH request SHOULD contain
   an Expires header field with a value that corresponds to the current
   value of the remaining CC subscription duration.

   Each PUBLISH SHOULD be sent to the CC URI as received in the NOTIFY,
   or within the corresponding SUBSCRIBE dialog, or if that is not
   possible, to the corresponding callee's monitor URI received in the
   Call-Info header field of the NOTIFY, or if one is not available, the
   Contact address of the subscription.






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6.6.  Resuming CC

   When the caller is no longer busy, or if the conditions relevant to
   the caller's agent's suspension policy have changed, then the CC
   request SHALL be resumed by the caller's agent.  To resume a CC
   request, the caller's agent SHALL publish the Caller's presence state
   by sending a PUBLISH request to each callee's monitor a PUBLISH
   request in accordance with the procedures described in [RFC3903] ,
   informing about the PIDF state 'open' but otherwise be constructed as
   same as the suspend PUBLISH request.  These PUBLISH requests are sent
   to presence server that are instantiated at a CC monitor.

   In the case where the caller's agent has sent several CC suspension
   requests to different callee's monitors and the caller becomes
   available again, as determined by the caller's agent's local policy
   about resumption the caller's agent MAY send a PUBLISH to resume a CC
   request to each callee's monitor for which there is a suspended CC
   request.  Note that the caller's agent's policy about resumption may
   prescribe a manual resumption and thus a suspended CC request should
   not be automatically resumed.


7.  Callee's monitor behavior

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 MUST insert a Call-Info header field with
   "purpose=call-completion" in the final response message (e.g. in a
   486 to enable call-completion due to busy subscriber) and at least
   one non-100 provisional response message (e.g. in a 180 due to no
   response) to the initial INVITE when forwarding it to the caller.
   The non-100 provisional response message SHOULD be sent reliably if
   the INVITE contained a Supported header field with the option tag
   100rel.  The Call-Info header field values defined in this
   specification positively indicates that CC is available for the
   failed fork of the call.

   The callee's monitor SHOULD insert a URI in the Call-Info header
   field where the caller's agent should subscribe for call-completion.
   Ideally, it is a globally-routable URI [RFC5627] 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".




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   In order to enable call-completion, the Call-Info header field 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 lack of response, the
   "m=BS" parameter indicates that it failed due to busy subscriber, and
   the "m=NL" parameter indicates that it failed due to non registered
   subscriber (no devices are registered for the AoR contacted).  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
   execute them at its best in their environment (which is likely to be
   a degraded service, especially when interoperating with SS7).

7.2.  Receiving a CC subscription

   The callee's monitor MUST be prepared to receive SUBSCRIBEs for the
   call-completion event package directed to the URIs of UA(s) that it
   is servicing and any URIs that the callee's monitor provides in Call-
   Info header fields.  The SUBSCRIBEs MUST be processed in accordance
   with the procedures defined in [RFC6665].

   The callee's monitor(s) that receive the SUBSCRIBE establish
   subscriptions.  These subscriptions represent the caller's agent's
   request for call-completion services.

   If the monitor receives two or more SUBSCRIBEs that have the same
   Call-Id header field value and the monitor considers the request-URIs
   of the received SUBSCRIBEs to request the status of the same set of
   UAs, then they are redundant forks of one SUBSCRIBE request, and the
   monitor SHOULD reject all but one of the requests with 482 (Merged
   Request) responses.

   The monitor MAY determine that an incoming CC SUBSCRIBE is a
   duplicate of an existing CC subscription if: (1) the Call-Id header
   field values are different, (2) the From URIs (i.e., the caller's
   AORs) are the same (per [RFC3261] section 19.1.4), (3) the To URIs
   (which should be the request-URI of the original call) have the same
   user and hostpart components, and (4) the monitor considers the
   request-URIs of the received SUBSCRIBEs to request the status of the
   same set of UAs.

   If the monitor determines that a new subscription is a duplicate of
   an existing subscription, it MAY terminate the existing subscription



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   in accordance with the procedures defined in [RFC6665].  In any case,
   it MUST establish the new subscription.

   The callee's monitor may apply restrictions as to which caller's
   agents may subscribe.

   The continuation of the caller's agent's subscription indicates to
   the callee's monitor that the caller's agent is prepared to initiate
   the CC call if it is selected for the 'ready' state.  If the callee's
   monitor becomes aware of a subscription which cannot be selected for
   a CC recall, it SHOULD terminate the subscription in accordance with
   [RFC6665].

7.3.  Sending a CC notification

   The call-completion event package returns various information to the
   caller's agent, but the vital datum it contains is the cc-state of
   the caller's agent's CC request in the CC queue, which in the
   beginning is 'queued'.  When the cc-state of the agent's request
   changes, the callee's monitor MUST send a NOTIFY for a call-
   completion event to the caller's agent.  The notification SHOULD also
   contain a URI which can be used for suspension requests.  Ideally, it
   is a globally-routable URI [RFC5627] 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".

   The call-completion event package provides limited 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 CC call fails due to the callee's
   UA(s) being busy.  If the callee's monitor supports the service-
   retention option, the callee's monitor SHOULD include the cc-service-
   retention parameter.

   The callee's monitor has a policy regarding when and how it selects
   CC requests for the recall.  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 CC requests arrived, the
   length of time the CC requests have been active, and any previous
   attempts of CC activations for the same original call.  Usually the
   callee's monitor will choose only one CC request for the recall at a
   time, but if the callee's UA(s) can support multiple calls, it may
   choose more than one.  Usually the callee's monitor will choose the
   oldest active request.

   When the callee's monitor changes the state datum for the chosen



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   subscription from "queued" to "ready", the callee's monitor MUST send
   a NOTIFY for the caller's agent's subscription with the cc-state set
   to 'ready' (recall notification).  The NOTIFY SHOULD also contain in
   the cc-URI a URI to be used in the CC call.  In practice, this may be
   the AOR of the callee.

   Upon sending the recall notification the callee's monitor MUST start
   a recall timer.  It is RECOMMENDED to use a value between 10 and 20
   seconds, which corresponds to the recommendation for the call
   completion services in ETSI [ETS300.356-18] and ITU-T [ITU-T.Q.733].

7.4.  Receiving a CC call

   The callee's UA(s) and the callee's monitor may give the CC call
   precedence over non-CC calls by evaluating the presence of the 'm'
   URI parameter and the From header of the INVITE request.  The
   callee's monitor supervises the receiving of the CC call.  Upon
   arrival of the CC call the recall timer MUST be stopped.  If the CC
   call does not arrive at the callee's UA(s) before the expiry of the
   recall timer, the callee's monitor SHOULD stop processing the recall
   and change the value of the cc-state datum to "queued" if it supports
   the retain option and terminate the subscription along with the queue
   if it doesn't support the retain option.  Similarly, if the CC call
   is not accepted, the callee's monitor will stop the CC recall
   processing.  Depending on its policy, the same original call may be
   selected again for a CC recall at a later time.  If the CC call
   succeeds, the callee's monitor MUST terminate the relevant
   subscription in accordance with [RFC6665], and MUST remove any
   associated presence event state used for suspend and resume for the
   caller of the CC call.

   Once the CC call has been terminated, successfully or unsuccessfully,
   the callee's monitor's policy MAY select another CC request for a
   recall according to the callee's monitor's policy.  Note that
   according to the callee's monitor's policy several recalls may be
   processed at the same time.

7.5.  Receiving a CC suspension

   The monitor may receive PUBLISH requests to suspend CC requests from
   caller's agent as described in section 6.5.  The PUBLISH requests may
   be received via the URI it manages, any URI that it inserts into a
   Call-Info header, any contact URI it uses as a notifier for "call-
   completion" events, or any URI it returns as the "URI" line of the
   call-completion event packages.

   The receipt of the PUBLISH request initiates a presence event state
   for the caller's identity at the presence server functionality of the



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   callee's monitor as specified in [RFC3903] , together with a logical
   presence server if this has not been done before for another call.

   Note: The presence server may initiate a presence event state for the
   caller's identity at the receipt of SUBSCRIBE request as well,
   dependent on the implementation.

   The monitor SHOULD identify the addressed CCE by the request-URI of
   the PUBLISH request, or if that is not possible, by the From URI.

   If the processing of a CC request results in suspending that CC
   request by receiving a PUBLISH request from caller's agent as
   described in section 6.5, the callee's monitor MUST stop the recall
   timer and MUST ensure that the request is set to a 'queued' state,
   and then the callee's monitor MUST attempt to process another CC
   request in the queue according to the callee's monitor's local
   policy.

7.6.  Receiving a CC resumption

   When a CC request becomes resumed by receiving a PUBLISH request from
   caller's agent as described in section 6.6, the presence event state
   for the caller's identity at the presence server functionality of the
   CC monitor MUST be modified as described in [RFC3903].  If the callee
   is not busy and there is no entry in the CC queue which is currently
   being processed, the callee's monitor MUST process the queue as
   described in section 7.3 above.


8.  Examples

   A basic call flow, with only the most significant messages of a call-
   completion activation and invocation shown, is as follows (please
   note this is an example and there may be variations in the failure
   responses):
















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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
         | 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
         | Call-Id: abcd-efgh       |
         | Event: call-completion   |
         |------------------------->|
         |                          |
         | 482                      |         [duplicate SUB. rej.]
         |<-------------------------|
         |                          |
         | 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 call]
         | From: sip:foo@example.com|
         |------------------------->|
         |                          |
         | NOTIFY sip:123@y.a.com   |         [CC terminated]
         | Expires = 0              |
         |<-------------------------|



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   The original call is an ordinary INVITE.  It fails due to no-response
   (ring-no-answer).  In this case, the callee's governing proxy
   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 field with "purpose=call-completion".  The Call-
   Info header field 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 field (<sip:456@z.b.com>) is 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 is activated by sending a SUBSCRIBE to all known callee's monitor
   URIs.  These can be provided by the Call-Info header field in the
   response to the INVITE.

   Additionally, the caller's agent needs to include the original
   request-URI in its set of callee's 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 callee's
   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 as defined by section 8.2.2.2 of [RFC3261], 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 callee's monitor, the callee's
   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 INVITE for CC call 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'



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   parameters (if possible), to specify CC processing.

   Finally the subscription for the CC request is terminated by the
   callee's monitor.

   Another flow, with only the most significant messages of call-
   completion suspension and resumption shown, is as follows:












































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       Caller                     Callee
       sip:123@a.com              sip:456@b.com
         |                          |
         | NOTIFY sip:123@y.a.com   |      [CC notification, caller not
         | Body:  status: ready     |      available for CC recall]
         |        URI: sip:recall@z.b.com
         |<-------------------------|
         |                          |
         | 200                      |
         |------------------------->|
         |                          |
         | PUBLISH sip:456@z.b.com  |      [non-availibility for recall
         | From: sip:123@a.com      |       is published]
         | Contact: sip:123@y.a.com |
         | Event: presence
         | Content-Type: 'app/pidf' |
         | Body: status=closed
         |------------------------->|
         |                          |
         | 200                      |
         |<-------------------------|
         |                          |
         |                          |      [caller becomes available
         |                          |       again]
         |                          |
         | PUBLISH sip:456@z.b.com  |      [availibility for recall
         | From: sip:123@a.com      |       is published]
         | Contact: sip:123@y.a.com |
         | Event: presence
         | Content-Type: 'app/pidf' |
         | Body: status=open
         |------------------------->|
         |                          |
         | 200                      |
         |<-------------------------|
         |                          |

   The caller is selected for CC, and is informed of this via a NOTIFY
   request containing "state: ready".  At this time, the caller is not
   available for the CC recall.

   For updating his presence event state at the presence server
   functionality at the callee, the caller generates a PUBLISH request
   to the CC URI as received in the NOTIFY, or within the corresponding
   SUBSCRIBE dialog, or if that is not possible, to the corresponding
   callee's monitor URI received in the Call-Info header field of the
   NOTIFY, or if one is not available, the Contact address of the
   subscription, informing about the PIDF state 'closed' .



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   When the caller is again available for the CC recall, he caller
   updates his presence event state at the presence server functionality
   at the callee by generating a PUBLISH request informing about the
   PIDF state 'open', but otherwise constructed as same as the suspend
   PUBLISH request.


9.  Call-completion event package

   This section specifies the call-completion event package, in
   accordance with section 4.4 of [RFC6665].  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 field parameters are defined for
   this event package.

9.3.  SUBSCRIBE bodies

   [RFC6665] 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 caller to the callee UA(s) serviced



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   by the notifier.  Calls are defined to be "from the same caller" if
   the URI-part of the From header field value in the INVITE is the same
   as the URI-part of the From header field value in the SUBSCRIBE.

9.4.  Subscribe duration

   [RFC6665] 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 [ETS300.356-18] and ITU-T [ITU-T.Q.733].  As because of the
   subscription duration no explicit timer is needed, and the
   subscription duration can be seen as an equivalent to the SS7 service
   duration timer, this specification refers to the subscription
   duration also as the service duration timer.  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 a
   certain duration the requested subscription can not any more 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 can never proceed to "ready"
   state, it should refuse the subscription.

9.5.  NOTIFY bodies

   [RFC6665] 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 [RFC6665], 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 10.
   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



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   "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
   side in order to receive call-completion notifications.  The
   generation of SUBSCRIBE requests can 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 field to a value
   not higher than the current remaining duration of the subscription
   regardless of the value received in the Expires header field (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, possibly with a Retry-after header field
   in accordance with the notifier's policy.  If a subscription is not
   successful because a condition 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, as
   defined by section 8.2.2.2 of [RFC3261].  In such a case, the
   notifier MUST 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 security
   considerations discussed in section 11, in particular for verifying
   the identity of the subscriber.

9.8.  Notifier generation of NOTIFY requests

   Notifiers MUST generate NOTIFY requests when the CC request's state
   changes to 'queued' or to 'ready (for call-completion)'.  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 callee's monitor for CC recall, and the
   parameter's value MUST be "ready" if the request is at this time



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   selected by the callee's 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 the selection of the request for
   the CC recall (e.g., because the caller's agent has not initiated the
   CC recall INVITE before the CC recall timer expires, or because the
   agent has suspended the request from being considered for CC recall),
   the notifier MUST send a NOTIFY to 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 retain
   option is supported at the callee, the NOTIFY MUST contain the "cc-
   service-retention" parameter.

9.9.  Subscriber processing of NOTIFY requests

   When receiving a NOTIFY requests with the cc-state set to 'ready',
   subscribers SHOULD suspend all other CC subscriptions for the
   original call at other notifiers.  The receipt of a NOTIFY request
   with the cc-state set to 'ready' by the subscriber will also cause an
   interaction with the instances at the subscribers side hat are
   responsible for starting the CC recall.

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 that notifies about the change to
   'ready (for call-completion)', but MAY involve several notifications
   about the change to the 'ready' state, separated by a call completion
   call that failed due to a busy callee .  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 avoid useless recalls,
   a notifier SHOULD send state changes to "queued" from "ready"
   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



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   "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 [RFC5234].  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,
   (2) the lines can be continued over line boundaries if the succeeding
   lines start with horizontal white space, and (3) lines with unknown
   names are to be ignored.  The header lines defined in this document
   can occur at most once in any given call-completion document.

   call-completion = 1*(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 [RFC3261].

10.1.  Call-completion status

   The cc-state line indicates the CC status of a particular user with
   an entry in a CC 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 subscriber'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.  Call-completion service-retention indication

   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 CC queue, if a CC call has failed due to callee busy
   condition.  If present, this parameter indicates that the retain
   option is supported, otherwise it is not supported.  This parameter



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   MAY be present in NOTIFY requests.

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

10.3.  Call-completion 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
   and the suspend/resume PUBLISH.  It SHOULD be provided in all
   NOTIFYs.  The URI SHOULD be globally routable and SHOULD uniquely
   identify the CCE in question.  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 retained
   with the URI.

   cc-URI = "cc-URI" HCOLON addr-spec


11.  Security considerations

   The CC facility allows the caller's agent to determine some status
   information regarding the callee.  This information intrinsically
   diminishes the privacy of the callee; in order to protect
   sufficiently the privacy of the callee, the overall amount of
   disclosure must be limited, and the amount of disclosure to any
   single caller must be limited.

   Of course, if a caller is not permitted to call the callee, that
   caller should not be allowed to establish a CC subscription.  Callers
   that are particularly sensitive about their privacy may reject all CC
   subscriptions.  But in the ordinary case, the optimal protection is
   to permit any caller to subscribe, but prevent any caller from
   subscribing for too long, or too often, or in a pattern that does not
   reveal to the callee (through CC calls) that the subscriptions are
   taking place.

   In legitimate use, CC event subscriptions will be made in 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 condition 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



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       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 Spam over Internet
   Telephony (SPIT) by the following technique: The caller makes calls
   to a group of callees.  The caller then requests CC for the calls
   that do not connect to the callees.  The CC calls resulting are
   probably more likely to reach the callees than original calls to a
   further group of targets.

   In order to prevent Denial of Service (DoS) attacks and manipulations
   of the call-completion queue by suspending other call-completion
   entries than the own, a mechanism to correlate the identity of the
   original caller and the generator of the SUBSCRIBE and PUBLISH
   request is needed.  The RECOMMENDED mechanism to authenticate the
   identity of the originator of call-completion relevant requests is
   the SIP Identity mechanism [RFC4474] .  Alternatively, CC agents and
   monitors within an administrative domain or federation of domains MAY
   use the mechanism described in [RFC3325] to authenticate their
   identity with a P-Asserted-Identity header field.

   Furthermore, the use of presence server functionality to suspend or
   resume SHOULD be limited to a caller which has an active queue in the
   callee's monitor.  This can be achieved first by monitoring and
   logging incoming call to the callee and the destination where CC
   indication was sent, then to ensure subscription to the call
   completion package is permitted only within a short timeframe after
   the initial call failed and to only accept PUBLISH request to the
   presence server functionality if there is an active queue for the
   caller in question.

   Note that regarding the authentication/authorization/billing logic
   subject to operator policy CC calls or subscriptions do not differ
   from other basic calls or event subscriptions.


12.  IANA considerations







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12.1.  SIP event package registration for call-completion

   This specification registers an event package, based on the
   registration procedures defined in [RFC6665].  The followings is the
   information required for such a registration:

   Package Name: call-completion

   Is this registration for a Template-Package: No.

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

   Person and e-mail to contact for further information: 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.

   Encoding considerations: Consists of lines of UTF-8-encoded
   characters, ended with CR-LF

   Security considerations: There are no security considerations
   internal to the media type.  Its typical usage involves the security
   considerations described in RFC XXXX

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

   Interoperability considerations: See RFC XXXX (Note for RFC Editor:
   Please fill in XXXX with the RFC number of this specification).

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

   Applications that use this media type: the implementations of the
   call-completion features of the Session Initiation Protocol

   Additional information:

   Magic number(s): none



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   File extension(s): not expected to be stored in files

   Macintosh file type code(s): not expected to be stored in files

   Person & email address to contact for further information: Martin
   Huelsemann, martin.huelsemann@telekom.de

   Intended usage: LIMITED USE

   Restrictions on usage: none

   Author/Change controller: the IETF

12.3.  SIP/SIPS URI parameter 'm'

   This specification defines one new SIP/SIPS URI parameter 'm' as per
   the registry created by [RFC3969].  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 specification.

   Name of the Parameter: m

   Predefined Values : yes

   RFC Reference : [RFC XXXX]

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

12.4.  Call-Completion purpose parameter value

   This specification adds a new predefined value "call-completion" for
   the "purpose" header field parameter of the Call-Info header field.
   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: Call-Info

   Parameter Name: purpose

   Predefined Values: yes

   Reference: [RFC3261].[RFC5367][[RFC XXXX]]

   (Note for RFC Editor: Please fill in XXXX with the RFC number of this



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   specification)

12.5.  'm' header parameter for Call-Info

   This specification extends [RFC3261] to add a new header field
   parameter 'm' to the Call-Info header field.  This adds a row to the
   registry header field parameters and parameter values:

   Header Field: Call-Info

   Parameter Name: m

   Predefined Values: yes

   Reference: [RFC XXXX]

   This RFC predefines the values 'BS', 'NR' and 'NL' .

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


13.  Acknowledgements

   Thanks to Paul Kyzivat, John Elwell, Keith Drage, Andrew Hutton,
   Thomas Stach, Dennis Luebbers and Christer Holmberg who provided
   helpful comments, feedback and suggestions.


14.  References

14.1.  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.

   [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)", RFC 3863, August 2004.




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   [RFC3903]  Niemi, A., "Session Initiation Protocol (SIP) Extension
              for Event State Publication", RFC 3903, October 2004.

   [RFC3969]  Camarillo, G., "The Internet Assigned Number Authority
              (IANA) Uniform Resource Identifier (URI) Parameter
              Registry for the Session Initiation Protocol (SIP)",
              BCP 99, RFC 3969, December 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.

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

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5367]  Camarillo, G., Roach, A., and O. Levin, "Subscriptions to
              Request-Contained Resource Lists in the Session Initiation
              Protocol (SIP)", RFC 5367, October 2008.

   [RFC5627]  Rosenberg, J., "Obtaining and Using Globally Routable User
              Agent URIs (GRUUs) in the Session Initiation Protocol
              (SIP)", RFC 5627, October 2009.

   [RFC6665]  Roach, A., "SIP-Specific Event Notification", RFC 6665,
              July 2012.

14.2.  Informative References

   [ETS300.356-18]
              "Completion of Calls to Busy Subscriber (CCBS)
              supplementary service", February 1995.

   [ITU-T.Q.733]
              "DESCRIPTION FOR CALL COMPLETION SUPPLEMENTARY SERVICES
              USING SS No. 7", February 1995.

   [RFC3325]  Jennings, C., Peterson, J., and M. Watson, "Private
              Extensions to the Session Initiation Protocol (SIP) for
              Asserted Identity within Trusted Networks", RFC 3325,
              November 2002.

   [RFC3725]  Rosenberg, J., Peterson, J., Schulzrinne, H., and G.
              Camarillo, "Best Current Practices for Third Party Call
              Control (3pcc) in the Session Initiation Protocol (SIP)",



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              BCP 85, RFC 3725, April 2004.

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


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



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   The callee's 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.


Authors' Addresses

   Dale R. Worley
   Ariadne Internet Services, Inc.
   738 Main St.
   Waltham, MA,   02451
   US

   Phone: +1 781 647 9199
   Email: worley@ariadne.com
   URI:   http://


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

   Phone: +4961515812765
   Email: martin.huelsemann@telekom.de
   URI:   http://www.telekom.de


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

   Phone: +4961515812766
   Email: r.jesske@telekom.de
   URI:   http://www.telekom.de











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   Denis Alexeitsev
   TeleFLASH
   Mainzer Landstrasse 47
   Frankfurt  60329
   Germany

   Phone: +49-69-257-378-230
   Email: alexeitsev@teleflash.com
   URI:   http://www.teleflash.com










































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