Internet DRAFT - draft-ietf-bliss-call-completion
draft-ietf-bliss-call-completion
bliss D. Worley
Internet-Draft Ariadne Internet Services, Inc.
Intended status: Standards Track M. Huelsemann
Expires: August 15, 2013 R. Jesske
Deutsche Telekom
D. Alexeitsev
TeleFLASH
February 11, 2013
Call Completion for Session Initiation Protocol (SIP)
draft-ietf-bliss-call-completion-19
Abstract
The call completion feature defined in this specification allows the
caller of a failed call to be notified when the callee becomes
available to receive a call.
For the realization of a basic solution without queuing, this
document references the usage of the dialog event package (RFC 4235)
that is described as 'automatic redial' in the SIP Service Examples
(RFC 5359).
For the realization of a more comprehensive solution with queuing,
this document introduces an architecture for implementing these
features in the Session Initiation Protocol where "call completion"
implementations associated with the caller's and callee's endpoints
cooperate to place the caller's request for call completion into a
queue at the callee's endpoint, and when a caller's request is ready
to be serviced, re-attempt of the original, failed call is made.
The architecture is designed to interoperate well with existing call-
completion solutions in other networks.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 15, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Requirements terminology . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Call-completion architecture . . . . . . . . . . . . . . . 8
4.2. Call-completion procedures . . . . . . . . . . . . . . . . 9
4.3. Automatic redial as a fallback . . . . . . . . . . . . . . 12
4.4. Differences from SS7 . . . . . . . . . . . . . . . . . . . 12
5. Call-completion queue model . . . . . . . . . . . . . . . . . 13
6. Caller's agent behavior . . . . . . . . . . . . . . . . . . . 14
6.1. Receiving the CC possible indication . . . . . . . . . . . 14
6.2. Subscribing to CC . . . . . . . . . . . . . . . . . . . . 14
6.3. Receiving a CC recall notification . . . . . . . . . . . . 16
6.4. Initiating a CC call . . . . . . . . . . . . . . . . . . . 16
6.5. Suspending CC . . . . . . . . . . . . . . . . . . . . . . 16
6.6. Resuming CC . . . . . . . . . . . . . . . . . . . . . . . 17
7. Callee's monitor behavior . . . . . . . . . . . . . . . . . . 17
7.1. Sending the CC possible indication . . . . . . . . . . . . 17
7.2. Receiving a CC subscription . . . . . . . . . . . . . . . 18
7.3. Sending a CC notification . . . . . . . . . . . . . . . . 19
7.4. Receiving a CC call . . . . . . . . . . . . . . . . . . . 20
7.5. Receiving a CC suspension . . . . . . . . . . . . . . . . 20
7.6. Receiving a CC resumption . . . . . . . . . . . . . . . . 21
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
9. Call-completion event package . . . . . . . . . . . . . . . . 26
9.1. Event package name . . . . . . . . . . . . . . . . . . . . 26
9.2. Event package parameters . . . . . . . . . . . . . . . . . 26
9.3. SUBSCRIBE bodies . . . . . . . . . . . . . . . . . . . . . 26
9.4. Subscribe duration . . . . . . . . . . . . . . . . . . . . 27
9.5. NOTIFY bodies . . . . . . . . . . . . . . . . . . . . . . 27
9.6. Subscriber generation of SUBSCRIBE requests . . . . . . . 28
9.7. Notifier processing of SUBSCRIBE requests . . . . . . . . 28
9.8. Notifier generation of NOTIFY requests . . . . . . . . . . 28
9.9. Subscriber processing of NOTIFY requests . . . . . . . . . 29
9.10. Handling of forked requests . . . . . . . . . . . . . . . 29
9.11. Rate of notifications . . . . . . . . . . . . . . . . . . 29
9.12. State agents . . . . . . . . . . . . . . . . . . . . . . . 30
10. Call-completion information format . . . . . . . . . . . . . . 30
10.1. Call-completion status . . . . . . . . . . . . . . . . . . 30
10.2. Call-completion service-retention indication . . . . . . . 30
10.3. Call-completion URI . . . . . . . . . . . . . . . . . . . 31
11. Security considerations . . . . . . . . . . . . . . . . . . . 31
12. IANA considerations . . . . . . . . . . . . . . . . . . . . . 32
12.1. SIP event package registration for call-completion . . . . 33
12.2. MIME registration for application/call-completion . . . . 33
12.3. SIP/SIPS URI parameter 'm' . . . . . . . . . . . . . . . . 34
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12.4. Call-Completion purpose parameter value . . . . . . . . . 34
12.5. 'm' header parameter for Call-Info . . . . . . . . . . . . 35
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 35
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
14.1. Normative References . . . . . . . . . . . . . . . . . . . 35
14.2. Informative References . . . . . . . . . . . . . . . . . . 36
Appendix A. Example Caller's Agent . . . . . . . . . . . . . . . 37
Appendix B. Example Callee's Monitor . . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38
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1. Introduction
The call completion (CC) feature allows the caller of a failed call
to have the call completed without having to make a new call attempt
while guessing when the callee becomes available. When the caller
requests the use of the CC feature, the callee will be monitored for
its availability. When the callee becomes available the callee will
be given a certain timeframe for initiating a call. If the callee
does not initiate a new call within this timeframe, then the caller
will be recalled. When the caller accepts the CC recall then a CC
call to the callee will automatically start. If several callers have
requested the CC feature on the same callee, they will be recalled in
a predefined order, which is usually the order in which they have
requested the CC feature.
This draft defines the following CC features:
Call Completion on Busy Subscriber (CCBS): The callee is busy. The
caller is recalled after the callee is not busy any longer.
Call Completion on No Reply (CCNR): The callee does not answer the
call. The caller is recalled after the callee has completed a new
call.
Call Completion on Not Logged-in (CCNL): The callee is not
registered. The caller is recalled after the callee has registered
again.
2. Requirements terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This document uses terms from [RFC3261].
3. Terminology
For the purpose of this service, we provide the following
terminology:
Callee: a destination of the original call, and a target of the CC
call.
Caller: The initiator of the original call and the CC request. The
user on whose behalf the CC call is made.
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Callee's monitor: a logical component which implements the call-
completion queue for destination user(s)/UA(s), and performs the
associated tasks, including sending CC recall events, analogous to
the destination local exchange's role in SS7 CC.
Caller's agent: a logical component which makes CC requests and
responds to CC recall events on behalf of originating user(s)/UA(s),
analogous to the originating local exchange's role in SS7 CC.
CC, or call completion: a service which allows a caller who failed to
reach a desired callee to be notified when the callee becomes
available to receive a call.
CC activation: the indication by the caller to the caller's agent
that the caller desires CC for a failed original call; this implies
an indication transmitted from the caller's agent to the callee's
monitor of the desire for CC processing.
CCBS, or Call Completion on Busy Subscriber: a CC service when the
initial failure was that the destination UA was busy.
CCNR, or Call Completion on No Reply: a CC service when the initial
failure was that the destination UA did not answer.
CCNL, or Call Completion on Not Logged-in: a CC service when the
initial failure was that the destination UA was not registered.
CC call: a call from the caller to the callee, triggered by the CC
service when it has determined that the callee is available.
CC indicator: an indication in the CC call INVITE used to prioritize
the call at the destination.
CC possible indication: the data in responses to the INVITE of the
original call which indicate that CC is available for the call.
CC recall: the action of the callee's monitor selecting a particular
CC request for initiation of a CC call, resulting in an indication
from the caller's agent to the caller that it is now possible to
initiate a CC call.
CC recall events: event notifications of event package "call-
completion", sent by the callee's monitor to the caller's agent to
inform it of the status of its CC request.
CC recall timer: maximum time the callee's monitor will wait for the
caller's response to a CC recall.
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CC request: the entry in the callee's monitor queue representing the
caller's request for CC processing, that is, the caller's call-
completion subscription.
CC service duration timer: maximum time a CC request may remain
active within the network.
CC queue: a buffer at the callee's monitor which stores incoming
calls which are target for call completion. Note: This buffer may or
may not be organized as a queue. The use of the term "queue" is by
analogy with SS7 usage.
CCE, or call-completion entity: the representation of a CC request,
or equivalently, an existing call-completion subscription within a
callee's monitor's queue
Failed call: a call which does not reach a desired callee, from the
caller's point of view. Note that a failed call may be successful
from the SIP point of view; e.g., if the call reached the callee's
voicemail, but the caller desired to speak to the callee in person,
the INVITE receives a 200 response, but the caller considers the call
to have failed.
Notifier: the user agent that generates NOTIFY requests for the
purpose of notifying subscribers of the callee's availibility; for
the CC service this is task of the callee's monitor.
Original call: the initial call which failed to reach a desired
destination.
Retain option: a characteristic of the CC service; if supported, CC
calls which again encounter a busy callee will not be queued again,
but the position of the caller's entry in the queue is retained.
Note that SIP CC always operates with the retain option active; a
failed CC call does not cause the CC request to lose its position in
the queue.
Subscriber: the user agent that receives NOTIFY requests with
information of the callee's availibility; for the CC service this is
task of the caller's agent.
Suspended CC request: a CC request which is temporarily not to be
selected for CC recall.
4. Solution
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4.1. Call-completion architecture
The call-completion architecture augments each caller's UA (or UAC)
wishing to use the call-completion features with a "call-completion
agent" (also written as "caller's agent").
It augments each callee's UA (or UAS) wishing to be the target of the
call-completion features with a "call-completion monitor" (also
written as "callee's monitor").
The caller's agent and callee's monitor functions can be integrated
into the respective UAs, be independent end-systems, or be provided
by centralized application servers. The two functions, though
associated with the two UAs (caller and callee), also may be provided
as services by the endpoints' home proxies or by other network
elements. Though it is expected that a UA that implements call
completion will have both functions so that it can participate in
call completion as both caller and callee, the two functions are
independent of each other.
A caller's agent may service more than one UA as a collective group
if a caller or population of users will be shared between the UAs,
and especially if the UAs share an AOR.
The caller's agent monitors calls made from the caller's UA(s) in
order to determine their destinations and (potentially) their final
response statuses, and the Call-Info header fields of provisional and
final responses for to invoke the call completion feature.
A callee's monitor may service more than one UA as a collective group
if a callee or population of users will be shared between the UAs,
and especially if the UAs share an AOR. The callee's monitor may
supply the callee's UAS(s) with Call-Info header field values for
provisional and final responses.
The callee's monitor also instantiates a presence server used to
monitor caller's availability for CC recall.
The callees using the UA(s) may be able to indicate to the callee's
monitor when they wish to receive CC calls.
In order to allow flexibility and innovation, most of the interaction
between the caller's agent, the caller-user(s) and the caller's UA(s)
is out of the scope of this document. Similarly, most of the
interaction between the callee's monitor, the callee(s) and the
callee's UA(s) is out of the scope of this document, as is also the
policy by which the callee's monitor arbitrates between multiple
call-completion requests.
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The caller's agent must be capable of performing a number of
functions relative to the UA(s). The method by which it does so is
outside the scope of this document, but an example method is
described in Appendix A. The callee's monitor must be capable of
performing a number of functions relative to the UA(s). The method
by which it does so is outside the scope of this document, but an
example method is described in Appendix B.
As a proof of concept, simple caller's agents and callee's monitors
can be devised that interact with users and UAs entirely through
standard SIP mechanisms [RFC6665], [RFC4235] and [RFC3515], as
described in the Appendixes.
The callers using the UA(s) can indicate to the caller's agent when
they wish to avail themselves of CC for a recently-made call which
the callers determined to unsuccessful. The caller's agent monitors
the status of the caller's UA(s) to determine when they are available
to be used for a CC recall. The caller's agent can communicate to
the caller's UA(s) that a CC recall is in progress and to inquire if
the relevant caller is available for the CC recall.
The callee's monitor may utilize several methods to monitor the
status of the callee's UA(s) and/or their users for availability to
receive a CC call. This can be achieved through monitoring calls
made to the callee's UA(s) to determine their caller's and their
final response' statuses. And in a system with rich presence
information, the presence information may directly provide this
status. In a more restricted system, this determination can depend
on the mode of the CC call in question, which is provided by the URI
'm' parameter. E.g., a UA is considered available for CCBS ("m=BS")
when it is not busy, but a UA is considered available for CCNR
("m=NR") when it becomes not busy after being busy with an
established call.
The callee's monitor maintains information about the set of INVITEs
received by the callee's UA(s) considered unsuccessful by the caller.
In practice, the callee's monitor may remove knowledge about an
incoming dialog from its set if local policy at the callee's monitor
establishes that the dialog is no longer eligible for CC activations.
4.2. Call-completion procedures
The caller's UA sends an INVITE to a request URI. One or more forks
of this request reach one or more of the callee's UAs. If the call-
completion feature is available, the callee's monitor (note there can
be a monitor for each of the callee's UAs) inserts a Call-Info header
field with its URI and with "purpose=call-completion" in appropriate
non-100 provisional or final response to the initial INVITE and
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forwards them to the caller. The provisional response SHOULD be sent
reliably, if the INVITE contained a Supported header field with the
option tag 100rel. On receipt of a non-100 provisional or a final
response with the indication that the call-completion feature is
available, the calling user can invoke the CC feature.
The caller indicates to the caller's agent that he wishes to invoke
call-completion services on the recent call. Note that from the SIP
point of view, the INVITE may have been successful, but from the
user's point of view, the call may have been unsuccessful. E.g., the
call may have connected to the callee's voicemail, which would return
a 200 status to the INVITE but from the caller's point of view is "no
reply".
In order to receive information necessary for the caller to complete
the call at the callee, the caller's agent subscribes to the call-
completion event package at the callee's monitor.
The possibility of the caller to complete the call at the callee is
also known as the call-completion state (cc-state) of the caller.
The cc-states comprehend the values 'queued' and 'ready' (for call-
completion).
In order to receive information from all destinations where the
callee will be reachable, the caller's agent sends a SUBSCRIBE
request for the call-completion event package to the original
destination URI of the call and to all known callee's monitor URIs
(which are provided by Call-Info header fields in provisional and
final responses to the INVITE). The callee's monitor uses the
subscription as an indication that caller is interested in using the
CC feature with regard to the specified callee. The callee's monitor
keeps a list or queue of the caller's agent's subscriptions,
representing the requests from the caller's agent to the callee's
monitors for call-completion services. These subscriptions are
created, refreshed, and terminated according to the procedures of
[RFC6665].
Upon receiving a SUBSCRIBE request from the caller's agent, the
callee's monitor instantiates a presence state for the caller's UA
which can be modified by the caller's UA to indicate its availability
for CC call. The status at the presence upon instantiation is
"open".
When the callee's monitor determines the callee and/or callee's UA
available for a CC call, it selects a caller to execute the CC call
and sends a call-completion event update (''cc-state: ready'') via a
NOTIFY request to the selected caller's agent's subscription, telling
it to begin the CC call to the callee's UA.
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When the caller's agent receives this update, it initiates a CC
recall by calling the caller's UA, and then starts the CC call to the
callee's UA, using 3rd party call control procedures in accordance
with [RFC3725]. The caller's agent can also check by other means
whether the caller is available to initiate the CC call to the
callee's UA. If the caller is available, the caller's agent directs
the caller's UA to initiate the CC call to the callee's UA.
The caller's agent marks the CC call as such by adding a specific SIP
URI parameter to the Request-URI, so it can be given precedence by
the callee's monitor in reaching the callee's UA.
If the caller is not available on the receipt of the "ready for
recall" notification, the caller's agent suspends the CC request at
the callee's monitor by sending a PUBLISH request containing presence
information to the callee's monitor's presence server, informing
about the presence status 'closed'. Once the caller becomes
available for a CC call again, the caller's agent resumes the CC
request by sending another PUBLISH request to the callee's monitor,
informing about the presence status 'open'.
On the receipt of the suspension request, the callee's monitor
performs the monitoring for the next non-suspended CC request in the
queue. On the receipt of the resume from the previously suspended
caller's agent that was at the top of the queue, the callee's monitor
performs the callee monitoring for this caller's agent.
When the CC call fails there are two possible options: the CC feature
has to be activated again by caller's agent subscribing to callee's
monitor, or CC remains activated and the original CC request retains
its position in the queue if retain option is supported.
The retain option (see section 3) determines the callee's monitor's
behavior when a CC call fails. If the retain option is supported, CC
remains activated and the original CC request retains its position in
the queue. Otherwise the CC feature is deactivated, and the caller's
agent would have to subscribe again to reactivate it.
A monitor that supports the retain option provides the cc-service-
retention header in its call-completion events. A caller's agent
that also supports the retain option uses the presence of this header
to know not to generate a new CC request after a failed CC call.
Monitors not supporting the retain option do not provide the cc-
service-retention header. A failed CC call causes the CC request to
be deleted from the queue, and these monitors will terminate the
corresponding caller's agent's subscription to inform that agent that
its CC request is no longer in the queue. A caller's agent that does
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not support the retain option can also terminate its subscription
when a CC call fails, so it is possible that both the caller's agent
and the monitor may be signalling the termination of the subscription
concurrently. This is a normal SIP Events [ [RFC6665]] scenario.
After the subscription is terminated, the caller's agent may create a
new subscription (as described in section 6.2) to reactivate the CC
feature for the original call.
4.3. Automatic redial as a fallback
Automatic redial is a simple end-to-end design. An automatic redial
scenario is described in [RFC5359], section 2.17. This solution is
based on the usage of the dialog event package. When the callee is
busy when the call arrives, the caller subscribes to the callee's
call state. The callee's UA sends a notification when the callee's
call state changes. This means the caller is also notified when the
callee's call state changes to 'terminated'. The caller is alerted,
then the caller's UA starts a call establishment to the callee again.
If several callers have subscribed to a busy callee's call state,
they will be notified at the same time that the call state has
changed to 'terminated'. The problem of this solution is, that it
might happen that several recalls are started at the same time. This
means it is a heuristic approach with no guarantee of success.
There is no interaction between call completion and automatic redial,
as there is a difference in the behavior of the callee's monitor and
the caller when using the dialog event package for receiving dialog
information or for aggregating a call completion state.
4.4. Differences from SS7
SIP call completion differs in some ways from the CCBS and CCNR
features of SS7 (which is used in the PSTN). For ease of
understanding, we enumerate some of the differences here.
As there is no equivalent to the forking mechnism in SS7, in the PSTN
calls can be clearly differentiated between successful or
unsuccessful. Due to the complex forking situations that are
possible in SIP, a call may "fail" from the point of view of the user
and yet have a "success" response from SIP's point of view. (This
can happen even in simple situations: e.g., a call to a busy user
that fails over to his voicemail receives a SIP success response,
even though the caller may consider it "busy subscriber".) Thus, the
caller must be able to invoke call completion even when the original
call appeared to succeed. To support this, the caller's agent must
record successful calls as well as unsuccessful calls.
In SIP, only the caller's UA or service system on the originating
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side and the callee's UA or service system on the terminating side
need to support call completion for call completion to work
successfully between the UAs. Intermediate SIP systems (proxies or
B2BUAs) do not need to implement call completion; they only need to
be transparent to the usual range of SIP messages. In the PSTN also
intermediate nodes like media gateway controllers have to implement
the call completion service.
5. Call-completion queue model
The callee's monitor manages CC for a single URI. This URI is likely
to be a published AOR, or more likely "non-voicemail AOR", but it may
be as narrowly scoped as a single UA's contact URI. The callee's
monitor manages a dynamic set of call-completion entities (called
"CCEs") which represent CC requests, or equivalently, the existing
incoming call-completion subscriptions. This set is also called a
queue, because a queue data structure often aids in implementing the
callee's monitor's policies for selecting CCEs for CC recall.
Each CCE has an availability state, determined through caller's
presence status at the callee's monitor. A presence status of
''open'' represents CCE's availability state of 'available' and a
presence status of "closed" represents CCE's availability state of
'unavailable'.
Each CCE has a recall state which is visible via subscriptions. The
recall state is either "queued" or "ready".
Each CCE carries the From URI of the SUBSCRIBE request that caused
its creation.
CC subscriptions arrive at the callee's monitor by addressing the
URIs the callee's monitor returns in Call-Info header fields. The
request URI of the SUBSCRIBE request determines the queue to which
the resulting CCE is added. The resulting subscription reports the
status of the queue. The base event data is the status of all the
CCEs in the queue, but the data returned by each subscription is
filtered to report only the status of that subscription's CCE.
(Further standardization may define means for obtaining more
comprehensive information about a queue.)
When a CCE is created, it is given the availability state "available"
and recall state "queued".
When the callee's monitor receives PIDF bodies [RFC3863] via PUBLISH
requests [ [RFC3903]], these PUBLISH requests are expected to be sent
by subscribers to indirectly suspend and resume their CC requests by
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modifying its CCE availability state. A CCE is identified by the
request-URI (if it was taken from a call-completion event
notification which identifies the CCE) or the From URI of the request
(matching the From URI recorded in the CCE). Receipt of a PUBLISH
with 'status' of 'open' sets the availability state of the CCE to
'available' (resume); 'status' of 'closed' sets the availability
state of the CCE to 'not-available' (suspend).
A CC request is eligible for recall only when its CCE's availability
state is "available" and the "m" value of the CCE also indicates an
available state. The callee's monitor MUST NOT select for recall any
CC requests that fail to meet those criteria. Within that
constraint, the callee's monitor's selections are determined by its
local policy. Often, a callee's monitor will choose the acceptable
CCE that has been in the queue the longest. When the callee's
monitor has selected a CCE for recall, it changes the CCE's recall
state from 'queued' to 'ready', which triggers a notification on the
CCE's subscription.
If a selected subscriber then suspends its request by sending a
PUBLISH with the presence status 'closed', the CCE becomes not-
available, and the callee's monitor changes the CCE's recall state to
'queued'. This may cause another CCE (e.g., that has been in the
queue for less time) to be selected for recall.
The caller's presence status at the callee's monitor is terminated
when the caller completes its CC call or when the caller's agent's
subscription at the callee's monitor is terminated.
6. Caller's agent behavior
6.1. Receiving the CC possible indication
The caller's agent MUST record the From URI and SHOULD record the
final request status that the caller's UA received along with the
contents of Call-Info header fields of provisional and final
responses.
Note that receiving a CC possible indication also depends on the
aggregation of final responses by proxies, in case of 4xx responses
some 4xx responses are more likely to be sent to the caller.
6.2. Subscribing to CC
For CC activation the caller's agent MUST send a SUBSCRIBE to all
known callee's monitor URIs. A callee's monitor URI may be provided
in the Call-Info header field in provisional and final responses to
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the INVITE sent back by the callee's monitor(s). Additionally, the
caller's agent SHOULD include the original request-URI that it sent
the original INVITE to, in its set of callee's monitor URIs, when it
is unclear if the call has forked to additional callees whose
responses the caller has not seen. A SUBSCRIBE to the original
request-URI alone is used in cases where the caller's agent has not
received or does not remember any callee's monitor URI. The caller's
agent SHOULD add an 'm' parameter to these URIs in order to indicate
the desired call-completion proceeding at the callee's monitor. The
'm' parameter SHOULD have the value of the 'm' parameter received in
the Call-Info header field of the responses to the original INVITE.
To minimize redundant subscriptions, these SUBSCRIBEs SHOULD be
presented as forks of the same transaction as defined by section
8.2.2.2 of [RFC3261], if the caller's agent is capable of doing so.
The agent MUST NOT maintain more than one CC request for a single
caller and directed to a single original destination URI. If a
caller requests CC a second time for the same destination URI, the
agent MUST consolidate the new request with the existing CC request
by either reusing the existing CC subscriptions or terminating and
then recreating them. For this purpose, equality of callers is
determined by comparing caller's AORs and equality of destination
URIs is determined by comparing them per [RFC3261] section 19.1.4.
When generating these SUBSCRIBEs, the From URI MUST be the caller's
AOR. The To URI SHOULD be the destination URI of the original call
(if the agent knows that and can insert it into the To header), and
otherwise MUST be the request-URI of the SUBSCRIBE.
The SUBSCRIBE SHOULD have header fields to optimize its routing. In
particular, it SHOULD contain "Request-Disposition: parallel", and an
Accept-Contact header field to eliminate callee UAs that are not
acceptable to the caller.
The caller's agent MUST be prepared to receive multiple responses for
multiple forks of the SUBSCRIBE and to have multiple subscriptions
established. The caller's agent must also be prepared to have the
SUBSCRIBE fail, in which case, CC cannot be invoked for this original
call.
If the caller's agent no longer wants to initiate the CC call (e.g.,
because the caller has deactivated CC), the caller's agent terminates
the subscription in accordance with [RFC6665] or suspends the
subscription(s) as specified in subclause 6.5.
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6.3. Receiving a CC recall notification
When receiving a NOTIFY with the cc-state set to 'ready', the
caller's agent SHOULD suspend all other subscriptions to CC, by
following the step in section 6.5, in order to prevent any other CC
requests from this caller to receive CC recalls. The caller's agent
starts the CC recall to the caller by confirming that the caller
would be able to initiate a CC call, e.g. by calling the caller's
UA(s).
6.4. Initiating a CC call
If the caller is available for the CC call and willing to initiate
the CC call, the caller's agent causes the caller's UA to generate a
new INVITE towards the callee. The caller's UA MAY add a 'm' URI
parameter with the value of the 'm' parameter received in Call-Info
header in the response to original INVITE, in order to specify his
preferences in CC processing and to prioritize the CC call. The
INVITE SHOULD be addressed to the URI specified in the cc-URI of the
NOTIFY, or if not available it SHOULD use the URI in the Call-Info
header field of the response to the original INVITE, or if not
available it MAY use the request-URI of the original INVITE, if this
URI was recorded. Note that the latter choice may not provide ideal
routing, but in simple cases it is likely to reach the desired
callee/callee's monitor.
6.5. Suspending CC
If the caller is not available for the CC recall, the CC request
SHALL be suspended by the caller's agent until the caller becomes
available again, or if the conditions relevant to the caller's
agent's local policy for suspensions have changed. To suspend the CC
request, the caller's agent SHALL publish the caller's presence state
by sending a PUBLISH request to each callee's monitor where the
presence server for CC resides in accordance with the procedures
described in [RFC3903] , giving the PIDF state 'closed' for the
caller's identity as presentity. The PUBLISH request SHOULD contain
an Expires header field with a value that corresponds to the current
value of the remaining CC subscription duration.
Each PUBLISH SHOULD be sent to the CC URI as received in the NOTIFY,
or within the corresponding SUBSCRIBE dialog, or if that is not
possible, to the corresponding callee's monitor URI received in the
Call-Info header field of the NOTIFY, or if one is not available, the
Contact address of the subscription.
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6.6. Resuming CC
When the caller is no longer busy, or if the conditions relevant to
the caller's agent's suspension policy have changed, then the CC
request SHALL be resumed by the caller's agent. To resume a CC
request, the caller's agent SHALL publish the Caller's presence state
by sending a PUBLISH request to each callee's monitor a PUBLISH
request in accordance with the procedures described in [RFC3903] ,
informing about the PIDF state 'open' but otherwise be constructed as
same as the suspend PUBLISH request. These PUBLISH requests are sent
to presence server that are instantiated at a CC monitor.
In the case where the caller's agent has sent several CC suspension
requests to different callee's monitors and the caller becomes
available again, as determined by the caller's agent's local policy
about resumption the caller's agent MAY send a PUBLISH to resume a CC
request to each callee's monitor for which there is a suspended CC
request. Note that the caller's agent's policy about resumption may
prescribe a manual resumption and thus a suspended CC request should
not be automatically resumed.
7. Callee's monitor behavior
7.1. Sending the CC possible indication
The callee's monitor MUST record the From URI and MAY record the
final request status(es) returned by the callee's UA(s).
If the callee's monitor wants to enable the caller to make use of the
CC service, it MUST insert a Call-Info header field with
"purpose=call-completion" in the final response message (e.g. in a
486 to enable call-completion due to busy subscriber) and at least
one non-100 provisional response message (e.g. in a 180 due to no
response) to the initial INVITE when forwarding it to the caller.
The non-100 provisional response message SHOULD be sent reliably if
the INVITE contained a Supported header field with the option tag
100rel. The Call-Info header field values defined in this
specification positively indicates that CC is available for the
failed fork of the call.
The callee's monitor SHOULD insert a URI in the Call-Info header
field where the caller's agent should subscribe for call-completion.
Ideally, it is a globally-routable URI [RFC5627] for the callee's
monitor. In practice, it may be the callee's AOR, and the SUBSCRIBE
will be routed to the callee's monitor only because it specifies
"Event: call-completion".
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In order to enable call-completion, the Call-Info header field MUST
be set up according to the following scheme:
Call-Info:monitor-URI;purpose=call-completion;m=XX
The 'm' parameter defines the "mode" of call completion. The "m=NR"
parameter indicates that it failed due to lack of response, the
"m=BS" parameter indicates that it failed due to busy subscriber, and
the "m=NL" parameter indicates that it failed due to non registered
subscriber (no devices are registered for the AoR contacted). The
'm' parameter is useful for PSTN interworking and assessing presence
information in the callee's monitor. It is possible that other
values will be defined in future. It is also allowed to omit the 'm'
parameter entirely. Implementations MUST accept CC operations in
which the 'm' parameter is missing or has an unknown value, and
execute them at its best in their environment (which is likely to be
a degraded service, especially when interoperating with SS7).
7.2. Receiving a CC subscription
The callee's monitor MUST be prepared to receive SUBSCRIBEs for the
call-completion event package directed to the URIs of UA(s) that it
is servicing and any URIs that the callee's monitor provides in Call-
Info header fields. The SUBSCRIBEs MUST be processed in accordance
with the procedures defined in [RFC6665].
The callee's monitor(s) that receive the SUBSCRIBE establish
subscriptions. These subscriptions represent the caller's agent's
request for call-completion services.
If the monitor receives two or more SUBSCRIBEs that have the same
Call-Id header field value and the monitor considers the request-URIs
of the received SUBSCRIBEs to request the status of the same set of
UAs, then they are redundant forks of one SUBSCRIBE request, and the
monitor SHOULD reject all but one of the requests with 482 (Merged
Request) responses.
The monitor MAY determine that an incoming CC SUBSCRIBE is a
duplicate of an existing CC subscription if: (1) the Call-Id header
field values are different, (2) the From URIs (i.e., the caller's
AORs) are the same (per [RFC3261] section 19.1.4), (3) the To URIs
(which should be the request-URI of the original call) have the same
user and hostpart components, and (4) the monitor considers the
request-URIs of the received SUBSCRIBEs to request the status of the
same set of UAs.
If the monitor determines that a new subscription is a duplicate of
an existing subscription, it MAY terminate the existing subscription
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in accordance with the procedures defined in [RFC6665]. In any case,
it MUST establish the new subscription.
The callee's monitor may apply restrictions as to which caller's
agents may subscribe.
The continuation of the caller's agent's subscription indicates to
the callee's monitor that the caller's agent is prepared to initiate
the CC call if it is selected for the 'ready' state. If the callee's
monitor becomes aware of a subscription which cannot be selected for
a CC recall, it SHOULD terminate the subscription in accordance with
[RFC6665].
7.3. Sending a CC notification
The call-completion event package returns various information to the
caller's agent, but the vital datum it contains is the cc-state of
the caller's agent's CC request in the CC queue, which in the
beginning is 'queued'. When the cc-state of the agent's request
changes, the callee's monitor MUST send a NOTIFY for a call-
completion event to the caller's agent. The notification SHOULD also
contain a URI which can be used for suspension requests. Ideally, it
is a globally-routable URI [RFC5627] for the callee's monitor. In
practice, it may be the callee's AOR, and the SUBSCRIBE will be
routed to the callee's monitor only because it specifies "Event:
call-completion".
The call-completion event package provides limited information about
the callee's monitor's policy. In particular, like in the PSTN, the
"'cc-service-retention" datum gives an indication of the "service
retention" attribute, which indicates whether the CC request can be
continued to a later time if the CC call fails due to the callee's
UA(s) being busy. If the callee's monitor supports the service-
retention option, the callee's monitor SHOULD include the cc-service-
retention parameter.
The callee's monitor has a policy regarding when and how it selects
CC requests for the recall. This policy may take into account the
type of the requests (e. g. CCNR vs. CCBS), the state of the
callee's UA(s), the order in which the CC requests arrived, the
length of time the CC requests have been active, and any previous
attempts of CC activations for the same original call. Usually the
callee's monitor will choose only one CC request for the recall at a
time, but if the callee's UA(s) can support multiple calls, it may
choose more than one. Usually the callee's monitor will choose the
oldest active request.
When the callee's monitor changes the state datum for the chosen
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subscription from "queued" to "ready", the callee's monitor MUST send
a NOTIFY for the caller's agent's subscription with the cc-state set
to 'ready' (recall notification). The NOTIFY SHOULD also contain in
the cc-URI a URI to be used in the CC call. In practice, this may be
the AOR of the callee.
Upon sending the recall notification the callee's monitor MUST start
a recall timer. It is RECOMMENDED to use a value between 10 and 20
seconds, which corresponds to the recommendation for the call
completion services in ETSI [ETS300.356-18] and ITU-T [ITU-T.Q.733].
7.4. Receiving a CC call
The callee's UA(s) and the callee's monitor may give the CC call
precedence over non-CC calls by evaluating the presence of the 'm'
URI parameter and the From header of the INVITE request. The
callee's monitor supervises the receiving of the CC call. Upon
arrival of the CC call the recall timer MUST be stopped. If the CC
call does not arrive at the callee's UA(s) before the expiry of the
recall timer, the callee's monitor SHOULD stop processing the recall
and change the value of the cc-state datum to "queued" if it supports
the retain option and terminate the subscription along with the queue
if it doesn't support the retain option. Similarly, if the CC call
is not accepted, the callee's monitor will stop the CC recall
processing. Depending on its policy, the same original call may be
selected again for a CC recall at a later time. If the CC call
succeeds, the callee's monitor MUST terminate the relevant
subscription in accordance with [RFC6665], and MUST remove any
associated presence event state used for suspend and resume for the
caller of the CC call.
Once the CC call has been terminated, successfully or unsuccessfully,
the callee's monitor's policy MAY select another CC request for a
recall according to the callee's monitor's policy. Note that
according to the callee's monitor's policy several recalls may be
processed at the same time.
7.5. Receiving a CC suspension
The monitor may receive PUBLISH requests to suspend CC requests from
caller's agent as described in section 6.5. The PUBLISH requests may
be received via the URI it manages, any URI that it inserts into a
Call-Info header, any contact URI it uses as a notifier for "call-
completion" events, or any URI it returns as the "URI" line of the
call-completion event packages.
The receipt of the PUBLISH request initiates a presence event state
for the caller's identity at the presence server functionality of the
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callee's monitor as specified in [RFC3903] , together with a logical
presence server if this has not been done before for another call.
Note: The presence server may initiate a presence event state for the
caller's identity at the receipt of SUBSCRIBE request as well,
dependent on the implementation.
The monitor SHOULD identify the addressed CCE by the request-URI of
the PUBLISH request, or if that is not possible, by the From URI.
If the processing of a CC request results in suspending that CC
request by receiving a PUBLISH request from caller's agent as
described in section 6.5, the callee's monitor MUST stop the recall
timer and MUST ensure that the request is set to a 'queued' state,
and then the callee's monitor MUST attempt to process another CC
request in the queue according to the callee's monitor's local
policy.
7.6. Receiving a CC resumption
When a CC request becomes resumed by receiving a PUBLISH request from
caller's agent as described in section 6.6, the presence event state
for the caller's identity at the presence server functionality of the
CC monitor MUST be modified as described in [RFC3903]. If the callee
is not busy and there is no entry in the CC queue which is currently
being processed, the callee's monitor MUST process the queue as
described in section 7.3 above.
8. Examples
A basic call flow, with only the most significant messages of a call-
completion activation and invocation shown, is as follows (please
note this is an example and there may be variations in the failure
responses):
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Caller Callee
sip:123@a.com sip:456@b.com
| |
| INVITE sip:456@b.com | [original call]
| From: sip:123@a.com |
|------------------------->|
| |
| 487 |
| Call-Info:<sip:456@z.b.com>;purpose=call-completion;m=NR
|<-------------------------|
| |
| SUBSCRIBE sip:456@z.b.com;m=NR [initial SUBSCRIBE]
| From: sip:123@a.com |
| Contact: sip:123@y.a.com |
| Request-Disposition: parallel
| Call-Id: abcd-efgh |
| Event: call-completion |
|------------------------->|
| |
| 200 |
|<-------------------------|
| |
| NOTIFY sip:123@y.a.com | [initial NOTIFY]
| Body: status: queued |
|<-------------------------|
| |
| SUBSCRIBE sip:456@b.com;m=NR [another init. SUB.]
| From: sip:foo@example.com|
| Request-Disposition: parallel
| Call-Id: abcd-efgh |
| Event: call-completion |
|------------------------->|
| |
| 482 | [duplicate SUB. rej.]
|<-------------------------|
| |
| NOTIFY sip:123@y.a.com | [CC invoked]
| Body: status: ready |
| URI: sip:recall@z.b.com
|<-------------------------|
| |
| INVITE sip:recall@z.b.com;m=NR [CC call]
| From: sip:foo@example.com|
|------------------------->|
| |
| NOTIFY sip:123@y.a.com | [CC terminated]
| Expires = 0 |
|<-------------------------|
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The original call is an ordinary INVITE. It fails due to no-response
(ring-no-answer). In this case, the callee's governing proxy
generates a 487 response because the proxy canceled the INVITE to the
UA when it rang too long without an answer. The 487 response carries
a Call-Info header field with "purpose=call-completion". The Call-
Info header field positively indicates that CC is available for this
failed fork of the call. The "m=NR" parameter indicates that it
failed due to no-response, which is useful for PSTN interworking and
assessing presence information in the callee's monitor.
The URI in the Call-Info header field (<sip:456@z.b.com>) is where
the caller's agent should subscribe for call-completion processing.
Ideally, it is a globally-routable URI for the callee's monitor. In
practice, it may be the callee's AOR, and the SUBSCRIBE will be
routed to the callee's monitor only because it specifies "Event:
call-completion".
CC is activated by sending a SUBSCRIBE to all known callee's monitor
URIs. These can be provided by the Call-Info header field in the
response to the INVITE.
Additionally, the caller's agent needs to include the original
request-URI in its set of callee's monitor URIs, because the call may
have forked to additional callees whose responses the caller has not
seen. (A SUBSCRIBE to the request-URI alone is used in cases where
the caller's agent has not received or cannot remember any callee's
monitor URI.)
The caller's agent adds to these URIs an 'm' parameter (if possible).
In this case, the caller's agent forks the SUBSCRIBE to two
destinations as defined by section 8.2.2.2 of [RFC3261], with
appropriate Request-Disposition. The first SUBSCRIBE is to the URI
from Call-Info.
The second SUBSCRIBE is to the original request-URI, and reaches the
same callee's monitor. Because it has the same Call-Id as the
SUBSCRIBE that has already reached the callee's monitor, the callee's
monitor rejects it with a 482, thus avoiding redundant subscriptions.
The initial NOTIFY for the successful SUBSCRIBE has "state: queued"
in its body. Eventually, this caller is selected for CC, and is
informed of this via a NOTIFY containing "state: ready". This NOTIFY
carries a URI to which the INVITE for CC call should be sent. In
practice, this may be the AOR of the callee.
The caller generates a new INVITE to the URI specified in the NOTIFY,
or if there was no such URI or if the caller's agent cannot remember
it, it may use the original request-URI. The caller adds the 'm'
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parameters (if possible), to specify CC processing.
Finally the subscription for the CC request is terminated by the
callee's monitor.
Another flow, with only the most significant messages of call-
completion suspension and resumption shown, is as follows:
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Caller Callee
sip:123@a.com sip:456@b.com
| |
| NOTIFY sip:123@y.a.com | [CC notification, caller not
| Body: status: ready | available for CC recall]
| URI: sip:recall@z.b.com
|<-------------------------|
| |
| 200 |
|------------------------->|
| |
| PUBLISH sip:456@z.b.com | [non-availibility for recall
| From: sip:123@a.com | is published]
| Contact: sip:123@y.a.com |
| Event: presence
| Content-Type: 'app/pidf' |
| Body: status=closed
|------------------------->|
| |
| 200 |
|<-------------------------|
| |
| | [caller becomes available
| | again]
| |
| PUBLISH sip:456@z.b.com | [availibility for recall
| From: sip:123@a.com | is published]
| Contact: sip:123@y.a.com |
| Event: presence
| Content-Type: 'app/pidf' |
| Body: status=open
|------------------------->|
| |
| 200 |
|<-------------------------|
| |
The caller is selected for CC, and is informed of this via a NOTIFY
request containing "state: ready". At this time, the caller is not
available for the CC recall.
For updating his presence event state at the presence server
functionality at the callee, the caller generates a PUBLISH request
to the CC URI as received in the NOTIFY, or within the corresponding
SUBSCRIBE dialog, or if that is not possible, to the corresponding
callee's monitor URI received in the Call-Info header field of the
NOTIFY, or if one is not available, the Contact address of the
subscription, informing about the PIDF state 'closed' .
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When the caller is again available for the CC recall, he caller
updates his presence event state at the presence server functionality
at the callee by generating a PUBLISH request informing about the
PIDF state 'open', but otherwise constructed as same as the suspend
PUBLISH request.
9. Call-completion event package
This section specifies the call-completion event package, in
accordance with section 4.4 of [RFC6665]. The call-completion event
package has the media type "application/call-completion".
Note that if the callee has a caller-queuing facility, the callee's
monitor may want to treat the call-completion queue as part of the
queuing facility, and include in the event package information
regarding the state of the queue. How this information is conveyed
is left for further standardization.
9.1. Event package name
The SIP Events specification requires package definitions to specify
the name of their package or template-package. The name of this
package is "call-completion". This value appears in the Event and
Allow-events header fields.
9.2. Event package parameters
No package-specific Event header field parameters are defined for
this event package.
9.3. SUBSCRIBE bodies
[RFC6665] requires package definitions to define the usage, if any,
of bodies in SUBSCRIBE requests.
The SUBSCRIBE request MAY contain an Accept header field. If no such
header field is present, it has a default value of "application/
call-completion". If the header field is present, it MUST include
"application/call-completion".
A SUBSCRIBE request for a call-completion package MAY contain a body.
This body defines a filter to be applied to the subscription. Filter
documents are not specified in this document, and may be the subject
of future standardization activity.
A SUBSCRIBE request requests call-completion information regarding
calls recently made from the same caller to the callee UA(s) serviced
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by the notifier. Calls are defined to be "from the same caller" if
the URI-part of the From header field value in the INVITE is the same
as the URI-part of the From header field value in the SUBSCRIBE.
9.4. Subscribe duration
[RFC6665] requires package definitions to define a default value for
subscription durations, and to discuss reasonable choices for
durations when they are explicitly specified.
If a SUBSCRIBE does not explicitly request a duration, the default
requested duration is 3600 seconds, as that is the highest service
duration timer value recommended for the call completion services in
ETSI [ETS300.356-18] and ITU-T [ITU-T.Q.733]. As because of the
subscription duration no explicit timer is needed, and the
subscription duration can be seen as an equivalent to the SS7 service
duration timer, this specification refers to the subscription
duration also as the service duration timer. It is RECOMMENDED that
subscribers request, and that notifiers grant, a subscription time of
at least 3600 seconds.
If a notifier can determine that, according to its policy, after a
certain duration the requested subscription can not any more proceed
to "ready" state, it SHOULD reduce the granted subscription time to
that duration. If a notifier can determine that, according to its
policy, the requested subscription can never proceed to "ready"
state, it should refuse the subscription.
9.5. NOTIFY bodies
[RFC6665] requires package definitions to describe the allowed set of
body types in NOTIFY requests, and to specify the default value to be
used when there is no Accept header field in the SUBSCRIBE request.
A NOTIFY for a call-completion package MUST contain a body that
describes the call-completion states.
As described in [RFC6665], the NOTIFY message will contain bodies
that describe the state of the subscribed resource. This body is in
a format listed in the Accept header field of the SUBSCRIBE, or in a
package-specific default format if the Accept header field was
omitted from the SUBSCRIBE.
In this event package, the body of the notification contains a call-
completion document. All subscribers and notifiers MUST support the
"application/call-completion" data format described in section 10.
The SUBSCRIBE request MAY contain an Accept header field. If no such
header field is present, it has a default value of "application/
call-completion". If the header field is present, it MUST include
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"application/call-completion". Of course, the notifications
generated by the server MUST be in one of the formats specified in
the Accept header field in the SUBSCRIBE request.
9.6. Subscriber generation of SUBSCRIBE requests
Subscribers MUST generate SUBSCRIBE requests when they want to
subscribe to the call-completion event package at the terminating
side in order to receive call-completion notifications. The
generation of SUBSCRIBE requests can imply the usage of a call-
completion service specific timer as described in section 9.4.
9.7. Notifier processing of SUBSCRIBE requests
Upon receiving a subscription refresh, the notifier MUST set the
"expires" parameter of the Subscription-State header field to a value
not higher than the current remaining duration of the subscription
regardless of the value received in the Expires header field (if
present) of the subscription refresh.
If a subscription is not successful because the call-completion queue
has reached the maximum allowed number of entries (short term
denial), the notifier MUST send a 480 Temporarily Unavailable
response to the subscriber, possibly with a Retry-after header field
in accordance with the notifier's policy. If a subscription is not
successful because a condition has occurred that prevents and will
continue to prevent the call-completion service (long term denial),
the notifier MUST send a 403 Forbidden response to the subscriber.
A notifier MAY receive multiple forks of the same SUBSCRIBE, as
defined by section 8.2.2.2 of [RFC3261]. In such a case, the
notifier MUST reject all but one of the SUBSCRIBEs with a 482 Merged
Request response unless some other failure response applies.
The call-completion information can be sensitive. Therefore, all
subscriptions SHOULD be handled with consideration of the security
considerations discussed in section 11, in particular for verifying
the identity of the subscriber.
9.8. Notifier generation of NOTIFY requests
Notifiers MUST generate NOTIFY requests when the CC request's state
changes to 'queued' or to 'ready (for call-completion)'. A NOTIFY
that is sent with non-zero expiration MUST contain the "cc-state"
parameter. The parameter's value MUST be "queued" if the call-
completion request represented by the subscription is not at this
time selected by the callee's monitor for CC recall, and the
parameter's value MUST be "ready" if the request is at this time
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selected by the callee's monitor for CC recall.
A NOTIFY sent with a zero expiration (e.g., as a confirmation of a
request to unsubscribe) MAY contain the "cc-state" parameter.
When the callee's monitor withdraws the selection of the request for
the CC recall (e.g., because the caller's agent has not initiated the
CC recall INVITE before the CC recall timer expires, or because the
agent has suspended the request from being considered for CC recall),
the notifier MUST send a NOTIFY to the subscription of the selected
request. This NOTIFY MUST contain the "cc-state" parameter set to
"queued".
If the call-completion subscription was successful and the retain
option is supported at the callee, the NOTIFY MUST contain the "cc-
service-retention" parameter.
9.9. Subscriber processing of NOTIFY requests
When receiving a NOTIFY requests with the cc-state set to 'ready',
subscribers SHOULD suspend all other CC subscriptions for the
original call at other notifiers. The receipt of a NOTIFY request
with the cc-state set to 'ready' by the subscriber will also cause an
interaction with the instances at the subscribers side hat are
responsible for starting the CC recall.
9.10. Handling of forked requests
Forked requests are expected to be common for the call-completion
event type. The subscriber MUST be prepared to process NOTIFY
requests from multiple notifiers and to coordinate its processing of
the information obtained from them in accordance with the procedures
in this document.
9.11. Rate of notifications
The call completion service typically involves a single notification
per notifier and per subscription that notifies about the change to
'ready (for call-completion)', but MAY involve several notifications
about the change to the 'ready' state, separated by a call completion
call that failed due to a busy callee . Typically, notifications
will be separated by at least tens of seconds. Notifiers SHOULD NOT
generate more than three notifications for one subscription in any
ten-second interval. Since it is important to avoid useless recalls,
a notifier SHOULD send state changes to "queued" from "ready"
promptly. Thus, a notifier SHOULD NOT send a state change to "ready"
as the third notification in a ten-second interval, as that would
make it impossible to promptly send a further state change to
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"queued".
9.12. State agents
State agents have no defined role in the handling of the call-
completion package.
10. Call-completion information format
The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) as described in [RFC5234]. The formal syntax for the
application/call-completion MIME type is described below. In
general, the call-completion body is to be interpreted in the same
way as SIP headers: (1) the names of the lines are case-insensitive,
(2) the lines can be continued over line boundaries if the succeeding
lines start with horizontal white space, and (3) lines with unknown
names are to be ignored. The header lines defined in this document
can occur at most once in any given call-completion document.
call-completion = 1*(cc-header CRLF)
cc-header = cc-state / cc-service-retention / cc-URI / extension-
header
The above rules whose names start with "cc-" are described below.
Other rules are described in [RFC3261].
10.1. Call-completion status
The cc-state line indicates the CC status of a particular user with
an entry in a CC queue. It MUST be present unless the expiration
time indicated in the NOTIFY is zero.
cc-state = "cc-state" HCOLON ( "queued" / "ready" )
The value "queued" indicates that a subscriber's entry in the call-
completion queue is not selected for CC recall. The value "ready"
indicates that a user's entry in the call-completion queue has been
selected for CC recall.
10.2. Call-completion service-retention indication
The service-retention line indicates the support of the retain
option. The retain option, if supported at the callee, will maintain
the entry in the CC queue, if a CC call has failed due to callee busy
condition. If present, this parameter indicates that the retain
option is supported, otherwise it is not supported. This parameter
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MAY be present in NOTIFY requests.
cc-service-retention = "cc-service-retention" HCOLON "true"
10.3. Call-completion URI
The cc-URI line provides a URI (possibly in the form of a name-addr)
which the agent SHOULD use as the request-URI of the CC recall INVITE
and the suspend/resume PUBLISH. It SHOULD be provided in all
NOTIFYs. The URI SHOULD be globally routable and SHOULD uniquely
identify the CCE in question. The syntax provides for generic-params
in the value, but this document defines no such parameters.
Parameters that are not understood by the subscriber MUST be retained
with the URI.
cc-URI = "cc-URI" HCOLON addr-spec
11. Security considerations
The CC facility allows the caller's agent to determine some status
information regarding the callee. This information intrinsically
diminishes the privacy of the callee; in order to protect
sufficiently the privacy of the callee, the overall amount of
disclosure must be limited, and the amount of disclosure to any
single caller must be limited.
Of course, if a caller is not permitted to call the callee, that
caller should not be allowed to establish a CC subscription. Callers
that are particularly sensitive about their privacy may reject all CC
subscriptions. But in the ordinary case, the optimal protection is
to permit any caller to subscribe, but prevent any caller from
subscribing for too long, or too often, or in a pattern that does not
reveal to the callee (through CC calls) that the subscriptions are
taking place.
In legitimate use, CC event subscriptions will be made in stereotyped
ways that limit the disclosure of status information:
1. When a subscriber is selected for CC, a call should arrive
promptly for the callee, or the subscription should be
terminated. This expectation may be violated by a race condition
between selection of the subscription for CC and the caller
becoming unavailable, but it should be rare that a single
subscription will exhibit the race condition more than once.
2. Subscriptions should not remain suspended for longer than the
expected duration of a call (a call by the caller to a third
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party).
3. Subscriptions should be initiated only shortly after failed
incoming calls.
4. Most of the time, a callee should have no queued subscriptions.
Violations of these expectations should be detected by the callee's
monitor and reported as possible attempts at privacy violation.
The CC facility may enhance the effectiveness of Spam over Internet
Telephony (SPIT) by the following technique: The caller makes calls
to a group of callees. The caller then requests CC for the calls
that do not connect to the callees. The CC calls resulting are
probably more likely to reach the callees than original calls to a
further group of targets.
In order to prevent Denial of Service (DoS) attacks and manipulations
of the call-completion queue by suspending other call-completion
entries than the own, a mechanism to correlate the identity of the
original caller and the generator of the SUBSCRIBE and PUBLISH
request is needed. The RECOMMENDED mechanism to authenticate the
identity of the originator of call-completion relevant requests is
the SIP Identity mechanism [RFC4474] . Alternatively, CC agents and
monitors within an administrative domain or federation of domains MAY
use the mechanism described in [RFC3325] to authenticate their
identity with a P-Asserted-Identity header field.
Furthermore, the use of presence server functionality to suspend or
resume SHOULD be limited to a caller which has an active queue in the
callee's monitor. This can be achieved first by monitoring and
logging incoming call to the callee and the destination where CC
indication was sent, then to ensure subscription to the call
completion package is permitted only within a short timeframe after
the initial call failed and to only accept PUBLISH request to the
presence server functionality if there is an active queue for the
caller in question.
Note that regarding the authentication/authorization/billing logic
subject to operator policy CC calls or subscriptions do not differ
from other basic calls or event subscriptions.
12. IANA considerations
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12.1. SIP event package registration for call-completion
This specification registers an event package, based on the
registration procedures defined in [RFC6665]. The followings is the
information required for such a registration:
Package Name: call-completion
Is this registration for a Template-Package: No.
Published Document: RFC XXXX (Note for RFC Editor: Please fill in
XXXX with the RFC number of this specification).
Person and e-mail to contact for further information: Martin
Huelsemann, martin.huelsemann@telekom.de
12.2. MIME registration for application/call-completion
MIME media type name: application
MIME subtype name: call-completion
Required parameters: none.
Optional parameters: none.
Encoding considerations: Consists of lines of UTF-8-encoded
characters, ended with CR-LF
Security considerations: There are no security considerations
internal to the media type. Its typical usage involves the security
considerations described in RFC XXXX
(Note for RFC Editor: Please fill in XXXX with the RFC number of this
specification).
Interoperability considerations: See RFC XXXX (Note for RFC Editor:
Please fill in XXXX with the RFC number of this specification).
Published specification: RFC XXXX (Note for RFC Editor: Please fill
in XXXX with the RFC number of this specification)
Applications that use this media type: the implementations of the
call-completion features of the Session Initiation Protocol
Additional information:
Magic number(s): none
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File extension(s): not expected to be stored in files
Macintosh file type code(s): not expected to be stored in files
Person & email address to contact for further information: Martin
Huelsemann, martin.huelsemann@telekom.de
Intended usage: LIMITED USE
Restrictions on usage: none
Author/Change controller: the IETF
12.3. SIP/SIPS URI parameter 'm'
This specification defines one new SIP/SIPS URI parameter 'm' as per
the registry created by [RFC3969]. It is used to identify that an
INVITE request is a CC call, or to further identify that a SUBSCRIBE
request is for the call-completion event package. The parameter may
have a value that describes the type of the call-completion
operation, as described in this specification.
Name of the Parameter: m
Predefined Values : yes
RFC Reference : [RFC XXXX]
(Note for RFC Editor: Please fill in XXXX with the RFC number of this
specification)
12.4. Call-Completion purpose parameter value
This specification adds a new predefined value "call-completion" for
the "purpose" header field parameter of the Call-Info header field.
This modifies the registry header field parameters and parameter
values by adding this RFC as a reference to the line for header field
"Call-Info" and parameter name "purpose":
Header Field: Call-Info
Parameter Name: purpose
Predefined Values: yes
Reference: [RFC3261].[RFC5367][[RFC XXXX]]
(Note for RFC Editor: Please fill in XXXX with the RFC number of this
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specification)
12.5. 'm' header parameter for Call-Info
This specification extends [RFC3261] to add a new header field
parameter 'm' to the Call-Info header field. This adds a row to the
registry header field parameters and parameter values:
Header Field: Call-Info
Parameter Name: m
Predefined Values: yes
Reference: [RFC XXXX]
This RFC predefines the values 'BS', 'NR' and 'NL' .
(Note for RFC Editor: Please fill in XXXX with the RFC number of this
specification)
13. Acknowledgements
Thanks to Paul Kyzivat, John Elwell, Keith Drage, Andrew Hutton,
Thomas Stach, Dennis Luebbers and Christer Holmberg who provided
helpful comments, feedback and suggestions.
14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3515] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[RFC3863] Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr,
W., and J. Peterson, "Presence Information Data Format
(PIDF)", RFC 3863, August 2004.
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[RFC3903] Niemi, A., "Session Initiation Protocol (SIP) Extension
for Event State Publication", RFC 3903, October 2004.
[RFC3969] Camarillo, G., "The Internet Assigned Number Authority
(IANA) Uniform Resource Identifier (URI) Parameter
Registry for the Session Initiation Protocol (SIP)",
BCP 99, RFC 3969, December 2004.
[RFC4235] Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE-
Initiated Dialog Event Package for the Session Initiation
Protocol (SIP)", RFC 4235, November 2005.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5367] Camarillo, G., Roach, A., and O. Levin, "Subscriptions to
Request-Contained Resource Lists in the Session Initiation
Protocol (SIP)", RFC 5367, October 2008.
[RFC5627] Rosenberg, J., "Obtaining and Using Globally Routable User
Agent URIs (GRUUs) in the Session Initiation Protocol
(SIP)", RFC 5627, October 2009.
[RFC6665] Roach, A., "SIP-Specific Event Notification", RFC 6665,
July 2012.
14.2. Informative References
[ETS300.356-18]
"Completion of Calls to Busy Subscriber (CCBS)
supplementary service", February 1995.
[ITU-T.Q.733]
"DESCRIPTION FOR CALL COMPLETION SUPPLEMENTARY SERVICES
USING SS No. 7", February 1995.
[RFC3325] Jennings, C., Peterson, J., and M. Watson, "Private
Extensions to the Session Initiation Protocol (SIP) for
Asserted Identity within Trusted Networks", RFC 3325,
November 2002.
[RFC3725] Rosenberg, J., Peterson, J., Schulzrinne, H., and G.
Camarillo, "Best Current Practices for Third Party Call
Control (3pcc) in the Session Initiation Protocol (SIP)",
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BCP 85, RFC 3725, April 2004.
[RFC5359] Johnston, A., Sparks, R., Cunningham, C., Donovan, S., and
K. Summers, "Session Initiation Protocol Service
Examples", BCP 144, RFC 5359, October 2008.
Appendix A. Example Caller's Agent
This section outlines how an autonomous caller's agent can operate
using only standard SIP features to interact with the caller's UA.
This example is suitable only as a learning aid, as its performance
is poor.
The agent monitors calls made from the UA(s) by subscribing to the
dialog event package of the UA(s).
The UA(s) or their proxy routes calls made with either of two special
dial sequences to the agent, which interprets the INVITEs as
indications to make a CC request with BS or NR or NL mode for the
latest call made by the UA.
The agent monitors the status of the UA(s) for availability to be
used for a CC call by examining the dialog events.
The agent indicates to the UA(s) that CC recall is in progress by
making call to the UA(s). If the UA is answered, the agent assumes
that the caller is available and plays pre-recorded audio to indicate
that CC recall is in progress.
After playing the pre-recorded audio, the caller's agent uses REFER
to order the UA to make the CC call to the callee.
Appendix B. Example Callee's Monitor
This section outlines how an autonomous callee's monitor can operate
using only standard SIP features to interact with the callee's UA.
This example is suitable only as a learning aid, as its performance
is poor.
The callee's monitor monitors calls made to the UA(s) by subscribing
to the UA(s) dialog events. This enables it to determine their Call-
Id's and their final response statuses.
The proxy for the UA(s) routes to the callee's monitor any SUBSCRIBEs
for the call-completion event package directed to the URIs serviced
by the UA(s).
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The callee's monitor monitors the status of the UA(s) to determine
when they are in a suitable state to receive a CC call by watching
the busy/not-busy status of the UA(s): e.g. a UA is available for
CCBS when it is not busy, but a UA is available for CCNR when it
becomes not busy after being busy with an established call.
Authors' Addresses
Dale R. Worley
Ariadne Internet Services, Inc.
738 Main St.
Waltham, MA, 02451
US
Phone: +1 781 647 9199
Email: worley@ariadne.com
URI: http://
Martin Huelsemann
Deutsche Telekom
Heinrich-Hertz-Strasse 3-7
Darmstadt, 64307
Germany
Phone: +4961515812765
Email: martin.huelsemann@telekom.de
URI: http://www.telekom.de
Roland Jesske
Deutsche Telekom
Heinrich-Hertz-Strasse 3-7
Darmstadt, 64307
Germany
Phone: +4961515812766
Email: r.jesske@telekom.de
URI: http://www.telekom.de
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Denis Alexeitsev
TeleFLASH
Mainzer Landstrasse 47
Frankfurt 60329
Germany
Phone: +49-69-257-378-230
Email: alexeitsev@teleflash.com
URI: http://www.teleflash.com
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