draft-ietf-bliss-call-completion-14:
Call Completion for Session Initiation Protocol (SIP)
Author(s): Denis Alexeitsev, Roland Jesske, Martin Huelsemann, Dale Worley
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....
bliss D. Worley
Internet-Draft Avaya Inc.
Updates: 3261 (if approved) M. Huelsemann
Intended status: Standards Track R. Jesske
Expires: June 1, 2012 Deutsche Telekom
D. Alexeitsev
TeleFLASH
November 29, 2011
Call Completion for Session Initiation Protocol (SIP)
draft-ietf-bliss-call-completion-14
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 June 1, 2012.
Copyright Notice
Copyright (c) 2011 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 . . . . . . . . . . . . . . . 7
4.2. Call-completion procedures . . . . . . . . . . . . . . . . 9
4.3. Automatic redial as a fallback . . . . . . . . . . . . . . 11
4.4. Differences from SS7 . . . . . . . . . . . . . . . . . . . 11
5. Call-completion queue model . . . . . . . . . . . . . . . . . 12
6. Caller's agent behavior . . . . . . . . . . . . . . . . . . . 13
6.1. Receiving the CC possible indication . . . . . . . . . . . 13
6.2. Subscribing to CC . . . . . . . . . . . . . . . . . . . . 13
6.3. Receiving a CC recall notification . . . . . . . . . . . . 14
6.4. Initiating a CC call . . . . . . . . . . . . . . . . . . . 14
6.5. Suspending CC . . . . . . . . . . . . . . . . . . . . . . 15
6.6. Resuming CC . . . . . . . . . . . . . . . . . . . . . . . 15
7. Callee's monitor behavior . . . . . . . . . . . . . . . . . . 15
7.1. Sending the CC possible indication . . . . . . . . . . . . 15
7.2. Receiving a CC subscription . . . . . . . . . . . . . . . 16
7.3. Sending a CC notification . . . . . . . . . . . . . . . . 17
7.4. Receiving a CC call . . . . . . . . . . . . . . . . . . . 18
7.5. Receiving a CC suspension . . . . . . . . . . . . . . . . 18
7.6. Receiving a CC resumption . . . . . . . . . . . . . . . . 18
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
9. Call-completion event package . . . . . . . . . . . . . . . . 22
9.1. Event package name . . . . . . . . . . . . . . . . . . . . 22
9.2. Event package parameters . . . . . . . . . . . . . . . . . 22
9.3. SUBSCRIBE bodies . . . . . . . . . . . . . . . . . . . . . 22
9.4. Subscribe duration . . . . . . . . . . . . . . . . . . . . 23
9.5. NOTIFY bodies . . . . . . . . . . . . . . . . . . . . . . 23
9.6. Subscriber generation of SUBSCRIBE requests . . . . . . . 24
9.7. Notifier processing of SUBSCRIBE requests . . . . . . . . 24
9.8. Notifier generation of NOTIFY requests . . . . . . . . . . 24
9.9. Subscriber processing of NOTIFY requests . . . . . . . . . 25
9.10. Handling of forked requests . . . . . . . . . . . . . . . 25
9.11. Rate of notifications . . . . . . . . . . . . . . . . . . 25
9.12. State agents . . . . . . . . . . . . . . . . . . . . . . . 25
10. Call-completion information format . . . . . . . . . . . . . . 25
10.1. Call-completion status . . . . . . . . . . . . . . . . . . 26
10.2. Call-completion service-retention indication . . . . . . . 26
10.3. Call-completion URI . . . . . . . . . . . . . . . . . . . 26
11. Security considerations . . . . . . . . . . . . . . . . . . . 27
12. IANA considerations . . . . . . . . . . . . . . . . . . . . . 27
12.1. SIP event package registration for call-completion . . . . 28
12.2. MIME registration for application/call-completion . . . . 28
12.3. SIP/SIPS URI parameter 'm' . . . . . . . . . . . . . . . . 29
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12.4. Call-Completion purpose parameter value . . . . . . . . . 29
12.5. 'm' header parameter for Call-Info . . . . . . . . . . . . 30
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 30
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
14.1. Normative References . . . . . . . . . . . . . . . . . . . 30
14.2. Informative References . . . . . . . . . . . . . . . . . . 31
Appendix A. Example Caller's Agent . . . . . . . . . . . . . . . 31
Appendix B. Example Callee's Monitor . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 32
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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 [RFC 2119].
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.
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.
Suspended CC request: a CC request which is temporarily not to be
selected for CC recall.
4. Solution
4.1. Call-completion architecture
The call-completion architecture augments each caller's UA (or UAC)
which wishes 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) which wishes to be the target
of the call-completion features with a "call-completion monitor"
(also written as "callee's monitor").
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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 they
are associated with the two UAs, also may be provided as services by
the endpoints' home proxies or other network elements. Though it is
expected that a UA that implements call completion will have both
types of 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 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 monitors calls made from the 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 information necessary to invoke call completion
feature.
A callee's 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 may supply the callee's UAS(s) with Call-Info header
field values for provisional and final responses.
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.
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 [RFC3265], [RFC 4235] and [RFC3515], as
described in the Appendixes.
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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 estimated to not have been successful. The caller's
agent monitors the status of the UA(s) to determine when they are
available to be used for a CC recall. The caller's agent can
communicate to the UA(s) that a CC recall is in progress and to
inquire if the relevant caller is available for the CC recall. The
caller's agent can order the UA(s) at which the relevant caller is
available to generate a CC call to the callee.
The callee's monitor has a method of monitoring the status of the
UA(s) and/or their users to determine when they are "available" for a
CC call, that is, in a suitable state to receive a CC call. This can
be achieved by monitoring calls made to the UA(s) in order to
determine their callers and (potentially) their final response
statuses. In a system with rich presence information, the presence
information may directly provide this status. In a more restricted
system, this determination can depend on the mode of the CC call in
question, which is provided by the 'm' parameter. E.g., a UA is
considered available for CCBS ("m=BS") when it is not busy, but a UA
is considered available for CCNR ("m=NR") when it becomes not busy
after being busy with an established call.
The callee's monitor maintains information about the set of INVITEs
received by the UA(s) that may not have been considered successful 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 messages to the initial INVITE
and forwards them to the caller. 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 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
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reply".
In order to request call-completion, the caller's agent subscribes to
the call-completion event package at the callee's monitor. This
subscription is used to coordinate with the callee's monitor (and
indirectly with other caller's agents and other callee's monitors) to
implement the call-completion features.
The caller's agent sends a SUBSCRIBE request for the call-completion
event package to 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). This
SUBSCRIBE reaches the callee's monitor. 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,
which indicate the callers waiting for the CC recall.
The subscriptions generated by the SUBSCRIBE requests represent 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 [RFC3265].
When the callee's monitor judges that the callee and/or callee's UA
is available for call-completion, the callee's monitor selects a
caller to execute the CC call 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 the CC recall.
When the caller's agent receives this update, it initiates the CC
recall by calling the caller's UA or otherwise tests whether the
caller is available to initiate the CC call. If the caller is
available, the caller's agent directs the caller's UA to make the
call to the callee again (CC call). This call is marked as a CC call
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 CC agent suspends the CC request at the CC
monitor by sending an appropriate PUBLISH request to the callee's
monitor. Once the caller becomes available for CC again, the CC
agent resumes the CC request by sending another appropriate PUBLISH
request to the callee's monitor. On the receipt of the suspension
request from the caller's agent at the top of the queue, the callee's
monitor shall perform 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,
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the callee's monitor shall perform the callee monitoring for this
caller's agent.
When the call completion 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. Note that the caller's agent can refresh the
subscription.
4.3. Automatic redial as a fallback
Automatic redial is a simple end-to-end design. An automatic redial
scenario is described in [RFC 5359], section 2.17. This solution is
based on the usage of the dialog event package. When the callee is
busy when the call arrives, the caller subscribes to the callee's
call state. The callee's UA sends a notification when the callee's
call state changes. This means the caller is also nofified when the
callee's call state changes to 'terminated'. The caller is alerted,
then the caller's UA starts a call establishment to the callee again.
If several callers have subscribed to a busy callee's call state,
they will be notified at the same time that the call state has
changed to 'terminated'. The problem of this solution is, that it
might happen that several recalls are started at the same time. This
means it is a heuristic approach with no guarantee of sucess.
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.
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.
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
cannot be successfully executed.
5. Call-completion queue model
The callee's monitor manages CC for a single URI. This URI is likely
to be a published AOR, or more likely "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, which is either "available" (for
recall) or "not-available" (for recall). It is not visible via
subscriptions.
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".
The callee's monitor may receive PIDF bodies [RFC3863] via PUBLISH
requests directed at its URI. These PUBLISH requests are expected to
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be sent by subscribers to suspend and resume their CC requests. 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'; 'status' of 'closed' sets the
availability state of the CCE to 'not-available'.
The callee's monitor MUST select for recall only CC requests whose
CCE's have availability state 'available', and for which the callee
appears to be available when considering the 'm' value of the CCE.
Within that constraint, the callee's monitor's selections are
determined by its 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 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.
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.
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
by the Call-Info header field in provisional and final responses to
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. The 'm' parameter
SHOULD have the value of the 'm' parameter received in the Call-Info
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header field, if a Call-Info header field was received with 'm'
parameter in responses to the original INVITE.
To minimize redundant subscriptions, these SUBSCRIBEs SHOULD have the
same Call-Id, that is, to be presented as forks of the same
transaction, if the caller's agent is capable of doing so.
The SUBSCRIBE SHOULD have header fields to optimize its routing. In
particular, it SHOULD contain "Request-Disposition: parallel, no-
cancel", 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 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.
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 [RFC3265] or suspends the
subscription(s) as specified in subclause 6.5.
6.3. Receiving a CC recall notification
When receiving a NOTIFY with the cc-state set to 'ready', the
caller's agent SHOULD terminate the subscription in accordance with
[RFC3265] or suspend all other CC subscriptions (at other callee's
monitors) for this original call, and it SHOULD suspend all
subscriptions to CC for all other original calls, 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'
parameter received 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 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
respectively callee's monitor.
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6.5. Suspending CC
If the caller is not available for the CC recall, or if determined by
the caller's agent's suspension policy, 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 send a PUBLISH request to each callee's monitor,
giving the PIDF state 'closed' for the caller's identity as
presentity.
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. If a queue entry is suspended,
it is stepped over during CC processing at the callee's monitor.
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 send to each callee's monitor a
PUBLISH request informing about the PIDF state 'open' but otherwise
be constructed as same as the suspend PUBLISH request.
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
perscribe 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 and at least
one non-100 provisional response message to the initial INVITE and
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when forwarding it to the caller. The Call-Info header field values
defined in this specification positively indicates that CC is
available for this failed fork of the call.
The callee's monitor SHOULD insert a URI in the Call-Info header
field where the caller's agent should subscribe for call-completion
processing. Ideally, it is a globally-routable URI for the callee's
monitor. In practice, it may be the callee's AOR, and the SUBSCRIBE
will be routed to the callee's monitor only because it specifies
"Event: call-completion".
When applicable, the Call-Info header 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 no-response, the "m=BS"
parameter indicates that it failed due to busy subscriber, and the
"m=NL" parameter indicates that it failed due to not registered
subscriber. The 'm' parameter is useful for PSTN interworking and
assessing presence information in the callee's monitor. It is
possible that other values will be defined in future. It is also
allowed to omit the 'm' parameter entirely. Implementations MUST
accept CC operations in which the 'm' parameter is missing or has an
unknown value, and execute them at its best in their environment
(which is likely to be a degraded service, especially in
interoperation 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) serviced
by the callee's monitor and any URIs that the callee's monitor
provides in Call-Info header fields. The SUBSCRIBEs SHALL be
processed in accordance with the procedures defined in [RFC3265].
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 with 482 (Merged Request) to all but one fork. The callee's
monitor may apply additional 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
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monitor becomes aware of a subscription which cannot be selected for
a cc recall, it SHOULD terminate the subscription in accordance with
[RFC3265].
7.3. Sending a CC notification
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 call-completion event package returns various
information to the caller's agent, but the vital datum is that it
contains is the cc-state, which in the beginning is 'queued'. The
notification SHOULD also contain a URI which can be used for
suspension requests. 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".
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
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 SHALL 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].
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7.4. Receiving a CC call
The callee's UA(s) and any associated proxies may give the CC call
precedence over non-CC calls. The callee's monitor supervises the
receiving of the CC call. Upon arrival of the CC call the recall
timer SHALL 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". Similarly, if the CC call fails, 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 SHOULD terminate the relevant subscription in accordance with
[RFC3265].
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.
7.5. Receiving a CC suspension
The monitor may receive PUBLISH requests to suspend or resume CC
requests. 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 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 SHALL stop the recall
timer and SHALL ensure that the request is set to a 'queued' state,
and then the callee's monitor SHALL 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 6.6, and if the callee is not busy and
there is no entry in the CC queue which is currently being processed,
the callee's monitor SHALL process the queue as described in
subclause 7.3 above.
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8. Examples
A basic flow, with only the most significant messages shown, is this:
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Caller Callee
sip:123@a.com sip:456@b.com
| |
| INVITE sip:456@b.com | [original call]
| From: sip:123@a.com |
|------------------------->|
| |
| 487 |
| Call-Info:<sip:456@z.b.com>;purpose=call-completion;m=NR
|<-------------------------|
| |
| SUBSCRIBE sip:456@z.b.com;m=NR [initial SUBSCRIBE]
| From: sip:123@a.com |
| Contact: sip:123@y.a.com |
| Request-Disposition: parallel, no-cancel
| Call-Id: abcd-efgh |
| Event: call-completion |
|------------------------->|
| |
| 200 |
|<-------------------------|
| |
| NOTIFY sip:123@y.a.com | [initial NOTIFY]
| Body: status: queued |
|<-------------------------|
| |
| SUBSCRIBE sip:456@b.com;m=NR [another init. SUB.]
| From: sip:foo@example.com|
| Request-Disposition: parallel, no-cancel
| Call-Id: abcd-efgh |
| Event: call-completion |
|------------------------->|
| |
| 482 | [duplicate SUB. 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, 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'
parameters (if possible), to specify CC processing.
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Finally the subscription for the CC request is terminated by the
callee's monitor.
9. Call-completion event package
This section specifies the call-completion event package, in
accordance with section 4.4 of [RFC3265]. The call-completion event
package has the media type "application/call-completion".
Note that if the callee has a caller-queuing facility, the callee's
monitor may want to treat the call-completion queue as part of the
queuing facility, and include in the event package information
regarding the state of the queue. How this information is conveyed
is left for further standardization.
9.1. Event package name
The SIP Events specification requires package definitions to specify
the name of their package or template-package. The name of this
package is "call-completion". This value appears in the Event and
Allow-events header fields.
9.2. Event package parameters
No package-specific Event header field parameters are defined for
this event package.
9.3. SUBSCRIBE bodies
[RFC3265] requires package definitions to define the usage, if any,
of bodies in SUBSCRIBE requests.
The SUBSCRIBE request MAY contain an Accept header field. If no such
header field is present, it has a default value of "application/
call-completion". If the header field is present, it MUST include
"application/call-completion".
A SUBSCRIBE request for a call-completion package MAY contain a body.
This body defines a filter to be applied to the subscription. Filter
documents are not specified in this document, and may be the subject
of future standardization activity.
A SUBSCRIBE request requests call-completion information regarding
calls recently made from the same caller to the callee UA(s) serviced
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.
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9.4. Subscribe duration
[RFC3265] requires package definitions to define a default value for
subscription durations, and to discuss reasonable choices for
durations when they are explicitly specified.
If a SUBSCRIBE does not explicitly request a duration, the default
requested duration is 3600 seconds, as that is the highest service
duration timer value recommended for the call completion services in
ETSI [ETS300.356-18] and ITU-T [ITU-T.Q.733]. It is RECOMMENDED that
subscribers request, and that notifiers grant, a subscription time of
at least 3600 seconds.
If a notifier can determine that, according to its policy, after
certain duration the requested subscription cannot proceed to "ready"
state, it SHOULD reduce the granted subscription time to that
duration. If a notifier can determine that, according to its policy,
the requested subscription cannot proceed to "ready" state, it should
refuse the subscription. For example, in many cases when resuming a
subscription the granted duration will be less than 3600 seconds.
9.5. NOTIFY bodies
[RFC3265] requires package definitions to describe the allowed set of
body types in NOTIFY requests, and to specify the default value to be
used when there is no Accept header field in the SUBSCRIBE request.
A NOTIFY for a call-completion package MUST contain a body that
describes the call-completion states.
As described in [RFC3265], the NOTIFY message will contain bodies
that describe the state of the subscribed resource. This body is in
a format listed in the Accept header field of the SUBSCRIBE, or in a
package-specific default format if the Accept header field was
omitted from the SUBSCRIBE.
In this event package, the body of the notification contains a call-
completion document. All subscribers and notifiers MUST support the
"application/call-completion" data format described in section 8.
The SUBSCRIBE request MAY contain an Accept header field. If no such
header field is present, it has a default value of "application/
call-completion". If the header field is present, it MUST include
"application/call-completion". Of course, the notifications
generated by the server MUST be in one of the formats specified in
the Accept header field in the SUBSCRIBE request.
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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 MAY imply the usage of a call-
completion service specific timer as described in section 9.4.
9.7. Notifier processing of SUBSCRIBE requests
Upon receiving a subscription refresh, the notifier MUST set the
"expires" parameter of the Subscription-State header field to 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. If a subscription is not successful
because an error has occurred that prevents and will continue to
prevent the call-completion service (long term denial), the notifier
MUST send a 403 Forbidden response to the subscriber.
A notifier MAY receive multiple forks of the same SUBSCRIBE.
(Multiple forks are, as always, identified by having the same
Call-Id.) In such a case, the notifier SHOULD reject all but one of
the SUBSCRIBEs with a 482 Merged Request response unless some other
failure response applies.
The call-completion information can be sensitive. Therefore, all
subscriptions SHOULD be handled with consideration of the issues
discussed in section 11.
9.8. Notifier generation of NOTIFY requests
Notifiers MUST generate NOTIFY requests when a call-completion
service condition occurs at the terminating side that needs to be
sent towards the originating side. A NOTIFY that is sent with non-
zero expiration MUST contain the "cc-state" parameter. The
parameter's value MUST be "queued" if the call-completion request
represented by the subscription is not at this time selected by the
callee's monitor for CC recall, and the parameter's value MUST be
"ready" if the request is at this time 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.
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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 "cc-state" parameter set to "queued".
If the call-completion subscription was successful and the retention
option is supported at the callee, the NOTIFY MUST contain the "cc-
service-retention" parameter.
9.9. Subscriber processing of NOTIFY requests
The subscriber processing of NOTIFY requests MAY trigger additional
CC service procedures, as described in this document and possibly in
other documents.
9.10. Handling of forked requests
Forked requests are expected to be common for the call-completion
event type. The subscriber MUST be prepared to process NOTIFY
requests from multiple notifiers and to coordinate its processing of
the information obtained from them in accordance with the procedures
in this document.
9.11. Rate of notifications
The call completion service typically involves a single notification
per notifier and per subscription but MAY involve several
notifications separated by a call completion call that failed due to
a busy 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 "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
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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, (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 = 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
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
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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 (name-addr / addr-spec) *(SEMI generic-
param)
11. Security considerations
The use of the CC facility allows the caller's agent to determine
some status information regarding the callee. The information is
confined to a busy/not-busy indication, and in legitimate use, will
be subscribed to stereotyped ways that limit the disclosure of status
information:
1. When a subscriber is selected for CC, a call should arrive
promptly for the callee, or the subscription should be
terminated. This expectation may be violated by a race condition
between selection of the subscription for CC and the caller
becoming unavailable, but it should be rare that a single
subscription will exhibit the race 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
party).
3. Subscriptions should be initiated only shortly after failed
incoming calls.
4. Most of the time, a callee should have no queued subscriptions.
Violations of these expectations should be detected by the callee's
monitor and reported as possible attempts at privacy violation.
The CC facility may enhance the effectiveness of SPIT by the
following technique: The caller makes calls to a group of 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.
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 [RFC3265]. 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 inXXXX
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]
(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
[RFC 2119] 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", RFC3261,
June 2002.
[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC3265, June 2002.
[RFC3515] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC3515, April 2003.
[RFC3863] Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr,
W., and J. Peterson, "Presence Information Data Format
(PIDF)", RFC3863, August 2004.
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[RFC3969] Camarillo, G., "The Internet Assigned Number Authority
(IANA) Uniform Resource Identifier (URI) Parameter
Registry for the Session Initiation Protocol (SIP)",
BCP 99, RFC3969, December 2004.
[RFC 4235] Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE-
Initiated Dialog Event Package for the Session Initiation
Protocol (SIP)", RFC 4235, November 2005.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC5234, January 2008.
14.2. Informative References
[ETS300.356-18]
"Completion of Calls to Busy Subscriber (CCBS)
supplementary service", February 1995.
[ETS300.359-1]
"Completion of Calls to Busy Subscriber (CCBS)
supplementary service; Digital Subscriber Signalling
System No. one (DSS1) protocol", November 1995.
[ITU-T.Q.733]
"DESCRIPTION FOR CALL COMPLETION SUPPLEMENTARY SERVICES
USING SS No. 7", February 1995.
[RFC 5359] 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
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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).
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
Avaya Inc.
600 Technology Park Dr.
Billerica, MA, 01821
US
Phone: +1 978 671 3465
Email: dworley@avaya.com
URI: http://www.avaya.com
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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
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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