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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                                             Pingtel Corp.
Intended status: Standards Track                           M. Huelsemann
Expires: December 19, 2008                                 D. Alexeitsev
                                                        Deutsche Telekom
                                                           June 17, 2008


         Call Completion for Session Initiation Protocol (SIP)
                  draft-ietf-bliss-call-completion-02

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   This Internet-Draft will expire on December 19, 2008.

Abstract

   The features "call completion on busy subscriber" and "call
   completion on no reply" allow the calling party of a failed call to
   be notified when the called party becomes available to receive a
   call.  This document describes an architecture for implementing these
   features in the Session Initiation Protocol: "Call completion"
   implementations at 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.




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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements terminology . . . . . . . . . . . . . . . . . . .  3
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     4.1.  General  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     4.2.  Differences from SS  . . . . . . . . . . . . . . . . . . .  6
   5.  Detailed Description of the Call-Completion Mechanism  . . . .  7
     5.1.  Caller's Call-Completion Agent . . . . . . . . . . . . . .  8
     5.2.  Callee's Call-Completion Monitor . . . . . . . . . . . . .  9
     5.3.  The Original Call Is Made  . . . . . . . . . . . . . . . . 10
     5.4.  Call-Completion Is Activated . . . . . . . . . . . . . . . 10
     5.5.  The Call-Completion Request Is Queued  . . . . . . . . . . 12
     5.6.  Call-Completion Is Invoked . . . . . . . . . . . . . . . . 12
     5.7.  The caller is busy on receipt of the CC recall . . . . . . 13
     5.8.  Data Provided in the Call-Completion Event Package . . . . 14
   6.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   7.  Call Completion Event Package  . . . . . . . . . . . . . . . . 19
     7.1.  Event Package Name . . . . . . . . . . . . . . . . . . . . 19
     7.2.  Event Package Parameters . . . . . . . . . . . . . . . . . 19
     7.3.  SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . . . 19
     7.4.  Subscribe Duration . . . . . . . . . . . . . . . . . . . . 20
     7.5.  NOTIFY Bodies  . . . . . . . . . . . . . . . . . . . . . . 20
     7.6.  Subscriber Generation of SUBSCRIBE Requests  . . . . . . . 21
     7.7.  Notifier Processing of SUBSCRIBE Requests  . . . . . . . . 21
     7.8.  Notifier Generation of NOTIFY Requests . . . . . . . . . . 22
     7.9.  Subscriber Processing of NOTIFY Requests . . . . . . . . . 22
     7.10. Handling of Forked Requests  . . . . . . . . . . . . . . . 22
     7.11. Rate of Notifications  . . . . . . . . . . . . . . . . . . 23
     7.12. State Agents . . . . . . . . . . . . . . . . . . . . . . . 23
   8.  Call-completion information format . . . . . . . . . . . . . . 23
     8.1.  call-completion-state  . . . . . . . . . . . . . . . . . . 24
     8.2.  service-retention  . . . . . . . . . . . . . . . . . . . . 24
     8.3.  cc-URI . . . . . . . . . . . . . . . . . . . . . . . . . . 24
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 24
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 25
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 25
   12. Normative References . . . . . . . . . . . . . . . . . . . . . 25
   Appendix A.  Example Caller's Agent  . . . . . . . . . . . . . . . 26
   Appendix B.  Example Callee's Monitor  . . . . . . . . . . . . . . 26
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
   Intellectual Property and Copyright Statements . . . . . . . . . . 28








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1.  Introduction

   The call-completion architecture is an end-to-end design driven by
   the interactions between two types of agents, a "caller's agent"
   which operates on behalf of the original caller, and a "callee's
   monitor", which operates on behalf of the original callee.  The two
   agents are associated with the two UAs, though they 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 callee, the two agents are independent
   of each other.

   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.

   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.


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
   terminologies:

   CC, or call completion: a service which allows a call which failed to
   reach a desired destination user to be automatically retried at a
   later time when the destination appears to be available.  Martin: we
   should avoid the expression 'retry' because it's not a trying, but a
   notification when it's possible to recall.  Therefore for me the
   description from the abstract seems to be more appropriate: to be
   notified when the called party becomes available to receive a call.



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

   CC request: (1) the indication by the caller to the caller's agent
   that the caller desires CC for a failed original call, (2) the
   indication transmitted from the caller's agent to the callee's
   monitor of the desire for CC processing, (3) the entry in the
   callee's monitor queue representing the original call and the
   caller's request for CC processing.  Martin: At least for 1) and 2)
   there may be a need for a differentiation in future.  Perhaps we
   should use differnet terms: 1) request 2) subscription 3) entry ?
   Other suggestions?

   CCBS, or Completion of Calls to Busy Subscriber: 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.

   CCBS/CCNR 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 determines that the callee is available.

   CC recall: (1) the action of the callee's monitor selecting a
   particular CC request as one that should initiate a CC call, (2) the
   indication from the caller's agent to the caller that it is now
   possible to initiate a CC call, (3) the indication by the caller to
   initiate a CC call.  Martin: IMO 3) belongs more to the 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.

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

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




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   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.

   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.

   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 call-completion service; if
   supported, call-completion 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.  Overview

4.1.  General

   The call-completion architecture augments each caller's UA (or UAC)
   which wishes to be able to use the call-completion features with 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.

   In order to request call-completion, the caller's agent subscribes to
   the call-completion event package (as defined in chapter 6) of the
   callee's monitor.  This subscription is used to coordinate with the
   monitor (and indirectly with other caller's agents and other callee's
   monitors) to implement the call-completion features.

   When the caller's UA makes a call to a callee that fails (e.g.,
   because the callee was busy or the callee did not answer), and the
   caller wishes to use CC to contact the callee later, the caller
   instructs the caller's agent to activate the CC feature.

   The caller's agent sends a SUBSCRIBE request for the call-completion
   event package to the original destination URI of the call.  This



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   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.

   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.  This
   call is identified as a call-completion call so it can be given
   precedence 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 top CC agent
   in 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 top CC agent in the queue the CC
   monitor shall perform the callee monitoring for this top 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, possibly
   with the possibility to update the subscription.

4.2.  Differences from SS

   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.

   SIP call completion does not fundamentally distinguish "call
   completion on no reply" (CCNR) from "call completion on busy
   subscriber" (CCBS), because the network does not need to make a
   distinction, and given the potential complexity of SIP routing,
   agents in the network may not be able to.  However, SIP call
   completion operations MAY carry an 'm' parameter to label call
   completion actions as to the original cause of the failure.



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   Currently, the only defined values are "BS" and "NR".  Other values
   may be defined in the future, and SIP call completion operations
   should succeed even if the 'm' parameter is omitted.  However, in
   these latter cases, interoperation with SS7 is likely to be impaired.

   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 and the callee's UA or
   service needs 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.

   JRE: How does "caller's UA or service" relate to "caller's agent",
   and how does "callee's UA or service" relate to "callee's monitor"?

   Dale: The caller's UA or the caller's service implements the caller's
   agent.  Similar for the callee.  What I was trying to emphasize is
   that while the caller's and callee's "end" of the service path must
   explicitly implement CC, any transit networks in between do not.

   MH: Is it possible to express this with something like 'instances on
   the originating resp. terminating side', in difference to
   'intermediate instances'?  Otherwise I think we have to explain the
   term 'service' more deeply.

   Due to flexibility needed to support legacy systems that are not
   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.  Detailed Description of the Call-Completion Mechanism

   Note: Many of the actions of call completion can be taken with the
   actor possessing greater or lesser information about the particular
   situation, depending on the specific messages involved.  This
   complexity is due to a number of factors: the complexities of SIP



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   forking, interoperation with other networks (specifically SS7), the
   availability and use of optional SIP features to assist CC, and any
   future enhancements of CC (either in SIP or SS7).  Thus, in some of
   the following descriptions, several alternative actions are presented
   based on the information that the actor possesses.  CC
   implementations MUST properly choose between the described
   alternatives, and MAY augment those alternatives based on any
   additional information they possess.

5.1.  Caller's Call-Completion Agent

   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".

   An agent may be an integrated part of the UA, a separate end-system,
   integrated with a proxy serving the UA, or a function of an
   application server that provides these services to many UAs.

   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 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 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 final responses and any HERFP
   provisional responses.

   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 call.

   The agent can communicate to the UA(s) that CC recall is in progress
   and to inquire if the relevant calling user is available for the CC
   call.

   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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5.2.  Callee's Call-Completion Monitor

   The call-completion architecture 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".

   A monitor may be an integrated part of the UA, a separate end-system,
   integrated with a proxy serving the UA, or a function of an
   application server that provides these services to many UAs.

   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 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.

   The monitor monitors calls made to the UA(s) in order to determine
   their callers and (potentially) their final response statuses.

   The monitor may supply the callee's UAS(s) with Call-Info header
   values for final responses and any HERFP provisional responses.

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

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

   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.  In a system with
   rich presence information, the presence information may directly
   provide this status.  In a more restricted system, this determination
   MAY 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=CR") 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.



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   The CC monitor MAY provide a URI to which the CC agent can subscribe
   for call completion.  When applicable, the CC URI SHALL be sent to
   the CC agent in the Call Info header of an appropriate response
   message to the initial INVITE, according to the following scheme:

   Call-Info:monitor-URI;purpose=call-completion;m=XX Note that this
   particular format for the information in a response has not been
   fixed yet.

   The 'm' parameter defines the "mode" of call completion and can
   currently have the values 'BS' for CCBS and 'NR' for CCNR.  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).

   Issue: How does the CC monitor inform the CC agent if CC retention is
   supported or not?  MH: service retention parameter

5.3.  The Original Call Is Made

   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.
   E.g., a call that is not answered by the callee user may connect to
   the callee user's voicemail server.  Eventually, the INVITE fails, or
   the resulting dialog(s) are terminated.

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

   Note that the caller's UA may not receive any response from any of
   the callee's UA(s), as the final response returned to the caller's UA
   may have been from a fork that reached a UA that was not the
   callee's.

5.4.  Call-Completion Is Activated

   The calling user indicates to the caller's agent that he wishes to
   invoke call-completion services on the recent call.  Note that from



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   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".

   Question: At this point, it seems that the best choice is that the
   caller's agent need not determine what type of CC is being requested
   (CCNR vs. CCBS), as (1) it cannot determine this from the INVITE
   final response, (2) it would be a burden to make the calling user to
   specify it, and (3) the callee's monitor can determine this from the
   responses returned by the callee's UAs.  MH: Dependent if 'm'
   parameter is included in the response.

   The caller's agent subscribes to the call-completion event package
   using the request URI of the original call.  This SUBSCRIBE should be
   routed in much the same way as the original INVITE, but ultimately
   being routed not to the callee's UAs but to the callee's monitor.
   The Event header of the subscribe specifies the call-completion event
   package with a parameter call_id={Call-Id of the original call}.

   Question: Should the specification of the original call be done in
   the SUBSCRIBE body rather than in an event-param?

   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.

   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 callee's
   monitor must be prepared to receive SUBSCRIBEs regarding original
   calls that it has no knowledge of, and should respond 404 (Not Found)
   to such SUBSCRIBEs.  The monitor may apply additional restrictions as
   to 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 call-completion event package returns various information to the
   caller's agent, but the vital datum is that it contains an indication
   whether the callee's monitor has chosen the caller's agent to perform
   the next CC call to the callee.  This datum is initially false.



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5.5.  The Call-Completion Request Is Queued

   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 caller's agent becomes
   unwilling to initiate the CC call (e.g., because the calling user has
   deactivated CC or because the caller's UA becomes busy), the caller's
   agent must terminate or suspend the subscription(s).  (Currently, no
   method of suspending a subscription is defined.)  If the caller's
   agent later becomes willing again to initiate CC for the original
   call, it may resume the suspended subscription(s) or initiate new
   one(s).

   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.)

5.6.  Call-Completion Is Invoked

   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 (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.

   The callee's monitor changes the "call completion active" datum for
   the chosen caller's agent from false to true.  This triggers a
   notification for the agent's subscription.

   The agent receives the notification with the CC active datum set to
   true.  It then terminates or suspends all other CC subscriptions 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).

   If the calling user is not available, the caller's agent indicates
   this to the callee's monitor by terminating the CC subscription.

   If the calling user is available, the caller's agent causes the
   caller's UA to initiate a call to the request URI (which is expected
   to be routed to the callee's UA(s)).



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   Question: Should the callee's monitor supply a URI which should be
   used in the CC call?  This seems like it would be more reliable, as
   the monitor is probably "for" a particular callee URI, and it has no
   information about the destinations of any other forks of the original
   call.

   Question: The CC must be marked in some way as a CC call in order for
   the callee's monitor to know that the CC activation is being acted
   upon by the caller's agent.  And the marking must include the
   original Call-Id to allow correlation with the original call.
   Possibilities for a marking are a special URI-parameter on the
   request URI or a special header.

   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
   will withdraw 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
   will also withdraw CC activation, but the original call will never
   again be selected for CC activation (and in practice, can be deleted
   from the monitor's records).

   Question: Is that last statement true?  Can a call appear to succeed
   from the monitor's point of view but fail from the calling user's
   point of view?

   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.

5.7.  The caller is busy on receipt of the CC recall

   If the caller is found to be busy previous to or on receipt of the CC
   recall, then the CC request shall be suspended until the caller
   becomes not busy again.  The CC agent shall send a PUBLISH request
   informing about the PIDF state 'closed' to the CC monitor, with the
   URI of the caller in the From header and the To header.  The Request-
   URI of this PUBLISH request contains the monitor URI as received in
   the Call-Info header of the 486 response.

   If a queue entry is suspended, it is stepped over during CC
   processing at the CC monitor.




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   When the caller is no longer busy, then the CC request shall be
   resumed.  The CC agent shall send a PUBLISH request informing about
   the PIDF state 'open' to the CC monitor, with the URI of the caller
   in the From header and the To header.  The Request-URI of this
   PUBLISH request contains the monitor URI as received in the Call-Info
   header of the 486 response.  When a CC request becomes resumed, then,
   if the callee becomes not busy again and there is no entry in the CC
   queue which is currently being processed, the CC monitor shall
   process the destination B queue as described above.

   If the processing of a CC request results in suspending that CC
   request, the monitor shall stop the recall timer and attempt to
   process the next CC request in the queue.

   In case of the CC agent had 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.

5.8.  Data Provided in the Call-Completion Event Package

   Question: What format should the event package data be presented in?
   This draft proposes a simple attribute-value format.  We might also
   consider yet another XML format.

   The only necessary information to be provided by the call-completion
   event package is the CC activation datum, whose value is false
   (meaning that this CC request has not been chosen for activation) or
   true (meaning that it has).

   Question: If we decide to let the callee's monitor provide the
   request URI for the CC call, that request URI should probably be a
   mandatory datum as well.

   The event package may provide information about the callee's
   monitor's policy.  In particular, the PSTN CC feature 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 has a caller-queuing facility, we 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, such
   as number of callers ahead of this caller and expected wait time.  In
   that case, this data should probably not trigger a notification every
   time it changes, but rather at suitable time increments.

   Need to revise



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6.  Examples

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


        Caller                     Callee
    sip:123@a.com              sip:456@b.com
      |                          |
      | INVITE sip:456@b.com     |
      |------------------------->|
      |                          |
      | 487                      |
      | Call-Info: <sip:456@b.com;monitor>;purpose=call-completion;m=NR
      |<-------------------------|
      |                          |
      | SUBSCRIBE sip:456@b.com;monitor;id=xxxx;m=NR
      | Request-Disposition: parallel, no-cancel
      |------------------------->|
      |                          |
      | 200                      |
      |<-------------------------|
      |                          |
      | SUBSCRIBE sip:456@b.com;id=xxxx;m=NR
      | Request-Disposition: parallel, no-cancel
      |------------------------->|
      |                          |
      | 482                      |
      |<-------------------------|
      |                          |
      | NOTIFY sip:123@a.com     |
      |<-------------------------|
      |                          |
      | INVITE sip:456@b.com;id=xxxx;m=NR
      |------------------------->|
      |                          |

   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 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 is the where the caller's agent
   should subscribe for call-completion processing.  Ideally, it is a



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   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 because it specifies "Event: call-completion".

   CC activation is done by sending a SUBSCRIBE to any known monitor
   URIs.  These can be provided by the Call-Info header in the response
   to the INVITE.  These can also be provided by any HERFP provisional
   responses that are received.  (More about that below.)  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 the URIs the 'id' and (if possible) 'm'
   parameters.

   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.

   Eventually, this caller is selected for CC, and is informed of this
   via a NOTIFY.  This NOTIFY carries a URI to which the CC completion
   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 'id'
   and (if possible) 'm' parameters, to specify CC processing.

   Complication: The caller's agent cannot remember the URIs returned in
   the Call-Info header and the call-completion NOTIFY.

   The caller's agent uses 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.

   Complication: Multiple callees are contacted in a deep forking
   structure.

   The caller's agent attempts to reach these by including a SUBSCRIBE
   to the original request-URI.




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   Complication: Depending on the request-URI to reach all callee's
   monitors is not reliable in deep forking structures.

   The best solution I know of is to use the proposed 130 response code
   (draft-mahy-sipping-herfp-fix) to allow callee UAs or their governing
   proxies to directly carry callee's monitor URIs back to the caller.
   The body of the 130 is a sipfrag which is the failure response of the
   original call at that callee.  In this application, most of the
   apparatus described in the referenced draft is ignored, the body
   sipfrag is only examined for a Call-Info header with purpose=call-
   completion.  If it is found, the URI is added to the list of URIs to
   be contacted for CC activation.

   (While the proposed 130 response code and the proposed 199 response
   code (draft-holmberg-sipping-199) are generated in nearly identical
   circumstances, they have different purposes and only the 130 carries
   the final response as its body.  If both purposes are to be served,
   the two proposals should be merged.)

   Fortunately, the 130 response proposal has good upward-compatibility
   -- only the callee UA/proxy and the caller UA need to understand its
   semantics, and if a caller UA that does not understand 130 receives
   one, it will just discard it.

   An example call flow is:


        Caller              Proxy               Callee           Callee
      sip:123@a.com       b.com         sip:456@b.com   sip:789@b.com
      |                   |                   |                   |
      | INVITE sip:abc@b.com                  |                   |
      |------------------>|                   |                   |
      |                   | INVITE sip:456@b.com                  |
      |                   |------------------>|                   |
      |                   |                   |                   |
      |                   | 487               |                   |
      |                   | Call-Info: <sip:456@b.com;monitor>    |
      |                   |            ;purpose=call-completion;m=NR
      |                   |<------------------|                   |
      |                   |                   |                   |
      |                   | 130 carrying:     |                   |
      |                   | 487               |                   |
      |                   | Call-Info: <sip:456@b.com;monitor>    |
      |                   |            ;purpose=call-completion;m=NR
      |<--------------------------------------|                   |
      |                   |                   |                   |
      |                   | INVITE sip:789@b.com                  |
      |                   |-------------------------------------->|



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      |                   |                   |                   |
      |                   | 487               |                   |
      |                   | Call-Info: <sip:789@b.com;monitor>    |
      |                   |            ;purpose=call-completion;m=NR
      |                   |<--------------------------------------|
      |                   |                   |                   |
      |                   | 130 carrying:     |                   |
      |                   | 487               |                   |
      |                   | Call-Info: <sip:789@b.com;monitor>    |
      |                   |            ;purpose=call-completion;m=NR
      |<----------------------------------------------------------|
      |                   |                   |                   |
      | 487               |                   |                   |
      | Call-Info: <sip:789@b.com;monitor>;purpose=call-completion;m=NR
      |<------------------|                   |                   |
      |                   |                   |                   |
      | SUBSCRIBE sip:456@b.com;monitor;id=xxxx;m=NR              |
      | Request-Disposition: parallel, no-cancel                  |
      |-------------------------------------->|                   |
      |                   |                   |                   |
      | 200               |                   |                   |
      |<--------------------------------------|                   |
      |                   |                   |                   |
      | SUBSCRIBE sip:789@b.com;monitor;id=xxxx;m=NR              |
      | Request-Disposition: parallel, no-cancel                  |
      |---------------------------------------------------------->|
      |                   |                   |                   |
      | 200               |                   |                   |
      |<----------------------------------------------------------|
      |                   |                   |                   |
      | SUBSCRIBE sip:abc@b.com;id=xxxx;m=NR  |                   |
      | Request-Disposition: parallel, no-cancel                  |
      |------------------>|                   |                   |
      |                   |                   |                   |
      |                   | SUBSCRIBE sip:456@b.com;id=xxxx;m=NR  |
      |                   | Request-Disposition: parallel, no-cancel
      |                   |------------------>|                   |
      |                   |                   |                   |
      |                   | 482               |                   |
      |                   |<------------------|                   |
      |                   |                   |                   |
      |                   | SUBSCRIBE sip:789@b.com;id=xxxx;m=NR  |
      |                   | Request-Disposition: parallel, no-cancel
      |                   |-------------------------------------->|
      |                   |                   |                   |
      |                   | 482               |                   |
      |                   |<--------------------------------------|
      |                   |                   |                   |



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      | 482               |                   |                   |
      |<------------------|                   |                   |
      |                   |                   |                   |
      | NOTIFY sip:123@a.com                  |                   |
      |<--------------------------------------|                   |
      |                   |                   |                   |
      | INVITE sip:456@b.com;id=xxxx;m=NR     |                   |
      |-------------------------------------->|                   |
      |                   |                   |                   |


7.  Call Completion Event Package

   This section specifies the call-completion event package, in
   accordance with section 4.4 of [RFC3265].

7.1.  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.

7.2.  Event Package Parameters

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

7.3.  SUBSCRIBE Bodies

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

   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)



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   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.
   (The other parts of the From header values are not considered, as
   they do not have canonical representations or a defined equality
   comparison.)

   John: Is this intended to be deleted from the final document?  If so,
   fine.  If not, I am not sure this captures the true reason for
   leaving out the tag, say.

   Dale: I had not considered that the tag must obviously be left out.
   I was more concerned with the display-name, of which there are many
   formats, and there are no clear rules for comparison or canonical
   representation.

7.4.  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 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.

7.5.  NOTIFY Bodies

   [RFC3265] requires package definitions to describe the allowed set if
   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



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   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.

7.6.  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 MAY imply the usage of call-
   completion service specific timers.  An example of such an
   implementation can be found in 3GPP TS 24.642.

   Issue: timers have to be specified, a timer section has to be added.

7.7.  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.

   [Need to insert text about handling if there is no call to be
   completed.]

   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).




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   The call-completion information can be sensitive.  Therefore, all
   subscriptions SHOULD be handled with consideration of the issues
   discussed in Section 9.

7.8.  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 "call-completion-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 "call-completion-state"
   parameter.

   When the callee's monitor selects the request for CC recall, the
   notifier MUST send a NOTIFY to the subscription of the selected
   request.  This NOTIFY MUST contain the "call-completion-state"
   parameter set to "ready".

   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
   the subscription of the selected request.  This NOTIFY MUST contain
   the "call-completion-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
   "retention-option" parameter.

7.9.  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 (e.g., 3GPP TS 24.642).

7.10.  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.



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7.11.  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 two notifications in any ten-second interval.

   John: A common situation might be a successful subscription to CCBS,
   resulting in an initial NOTIFY "queued", and then the callee becomes
   free a couple seconds later, resulting a second NOTIFY "ready".
   Similarly following resume.  It does not seem reasonable to suppress
   or delay that second NOTIFY.  I think the recommendation is too
   severe.  Perhaps NOTIFYs in response to SUBSCRIBEs need to be
   excluded from this.

   Dale: I'm quite willing to modify this prescription.  I only included
   it because RFC 3265 says that there must be a prescription; I believe
   that if a monitor generate NOTIFYs only in response to relevant
   events, its NOTIFY rate will be low enough.

   However each of in the two cases you mention, there are only two
   NOTIFYs, so the prescription as stated cannot be violation, since a
   violation requires 3 NOTIFYs.

7.12.  State Agents

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


8.  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,
   (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-



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   header

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

8.1.  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 = "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.

8.2.  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 = "service-retention" HCOLON "true"

8.3.  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.

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


9.  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 is to a considerable
   degree protected by the necessity of presenting the Call-Id of a



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   recent call to the callee in order to obtain information.

   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.


10.  IANA Considerations

   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

   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


11.  Acknowledgments

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


12.  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.

   [RFC4235]  Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE-



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              Initiated Dialog Event Package for the Session Initiation
              Protocol (SIP)", RFC 4235, November 2005.


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).  Since this does not give the
   agent access to the Call-Info headers of responses, the agent is
   limited to using the original request URI for making CC requests.

   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 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 monitor does not supply Call-Info header values.  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).




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   The callees cannot indicate to the monitor when they wish to receive
   CC calls.

   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): 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
   Pingtel Corp.
   10 North Ave.
   Burlington, MA,   01803
   US

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


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

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


   Denis Alexeitsev
   Deutsche Telekom
   Heinrich-Hertz-Strasse 3-7
   Darmstadt,   64307
   Germany

   Phone: +4961516282773
   Email: d.alexeitsev@telekom.de










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Full Copyright Statement

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