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The features "call completion on busy subscriber" and "call completion on no reply" allow the calling party of a failed call to be notified when the called party becomes available to receive a call. This document describes an architecture for implementing these features in the Session Initiation Protocol: "Call completion" implementations at the caller's and callee's endpoints cooperate to place the caller's request for call completion into a queue at the callee's endpoint, and, when a caller's request is ready to be serviced, re-attempt the original, failed call.
1.
Introduction
2.
Requirements terminology
3.
Terminology
4.
Overview
4.1.
General
4.2.
Differences from SS
5.
Detailed Description of the Call-Completion Mechanism
5.1.
Caller's Call-Completion Agent
5.2.
Callee's Call-Completion Monitor
5.3.
The Original Call Is Made
5.4.
Call-Completion Is Activated
5.5.
The Call-Completion Request Is Queued
5.6.
Call-Completion Is Invoked
5.7.
The caller is busy on receipt of the CC recall
5.8.
Data Provided in the Call-Completion Event Package
6.
Examples
7.
Call Completion Event Package
7.1.
Event Package Name
7.2.
Event Package Parameters
7.3.
SUBSCRIBE Bodies
7.4.
Subscribe Duration
7.5.
NOTIFY Bodies
7.6.
Subscriber Generation of SUBSCRIBE Requests
7.7.
Notifier Processing of SUBSCRIBE Requests
7.8.
Notifier Generation of NOTIFY Requests
7.9.
Subscriber Processing of NOTIFY Requests
7.10.
Handling of Forked Requests
7.11.
Rate of Notifications
7.12.
State Agents
8.
Call-completion information format
8.1.
call-completion-state
8.2.
service-retention
8.3.
cc-URI
9.
Security Considerations
10.
IANA Considerations
11.
Acknowledgments
12.
Normative References
Appendix A.
Example Caller's Agent
Appendix B.
Example Callee's Monitor
§
Authors' Addresses
§
Intellectual Property and Copyright Statements
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The call-completion architecture is an end-to-end design driven by the interactions between two types of agents, a "caller's agent" which operates on behalf of the original caller, and a "callee's monitor", which operates on behalf of the original callee. The two agents are associated with the two UAs, though they may be provided as services by the endpoints' home proxies or other network elements. Though it is expected that a UA that "implements call completion" will have both types of agents so that it can participate in call completion as both caller and callee, the two agents are independent of each other.
In order to allow flexibility and innovation, most of the interaction between the caller's agent and the caller-user(s) and the caller's UA(s) is out of the scope of this document.
Similarly, most of the interaction between the callee's monitor and the callee-user(s) and the callee's UA(s) is out of the scope of this document, as is also the policy by which the callee's monitor arbitrates between multiple call-completion requests.
As a proof-of-concept, simple agents and monitors can be devised that interact with users and UAs entirely through standard SIP mechanisms [RFC3265] (Roach, A., “Session Initiation Protocol (SIP)-Specific Event Notification,” June 2002.), [RFC4235] (Rosenberg, J., Schulzrinne, H., and R. Mahy, “An INVITE-Initiated Dialog Event Package for the Session Initiation Protocol (SIP),” November 2005.) and [RFC3515] (Sparks, R., “The Session Initiation Protocol (SIP) Refer Method,” April 2003.), as described in the Appendixes.
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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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).
This document uses terms from [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.).
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For the purpose of this service, we provide the following terminologies:
CC, or call completion: a service which allows a call which failed to reach a desired destination user to be automatically retried at a later time when the destination appears to be available. Martin: we should avoid the expression 'retry' because it's not a trying, but a notification when it's possible to recall. Therefore for me the description from the abstract seems to be more appropriate: to be notified when the called party becomes available to receive a call.
CC indicator: the data in responses to the INVITE of the original call which indicate that CC is available for this call.
CC request: (1) the indication by the caller to the caller's agent that the caller desires CC for a failed original call, (2) the indication transmitted from the caller's agent to the callee's monitor of the desire for CC processing, (3) the entry in the callee's monitor queue representing the original call and the caller's request for CC processing. Martin: At least for 1) and 2) there may be a need for a differentiation in future. Perhaps we should use differnet terms: 1) request 2) subscription 3) entry ? Other suggestions?
CCBS, or Completion of Calls to Busy Subscriber: CC service when the initial failure was that the destination UA was busy.
CCNR, or Completion of Calls on No Reply: a CC service when the initial failure was that the destination UA was not answered.
CCBS/CCNR service duration timer, or CC service duration timer: maximum time a CC request may remain active within the network.
CC call: a call from the caller to the callee, triggered by the CC service when it determines that the callee is available.
CC recall: (1) the action of the callee's monitor selecting a particular CC request as one that should initiate a CC call, (2) the indication from the caller's agent to the caller that it is now possible to initiate a CC call, (3) the indication by the caller to initiate a CC call. Martin: IMO 3) belongs more to the CC call.
CC recall events: Event notifications of event package "call-completion", sent by the callee's monitor to the caller's agent to inform it of the status of its CC request.
Call-completion queue: a buffer at the callee' monitor which stores incoming calls which have failed or may have failed. Note: This buffer may or may not be organized as a queue. The use of the term "queue" is by analogy with SS7 usage.
Caller, calling user, originator, or call-completion user: the initiator of the original call and the CC request. The user on whose behalf the call-completion call is made.
Callee, called user, destination, or call-completion target: a destination of the original call, and a target of the call-completion call.
Callee's monitor, or monitor: a component which implements the call-completion queue for destination user(s)/UA(s), and performs the associated tasks, including sending CC recall events, analogous to the destination local exchange's role in SS7 CC.
Caller's agent, or agent: a component which makes CC requests and responds to CC recall events on behalf of originating user(s)/UA(s), analogous to the originating local exchange's role in SS7 CC.
Original call: the initial call which failed to reach a desired destination.
Suspended CC request: a CC request which is temporarily not to be selected for CC recall.
Retain option: a characteristic of the call-completion service; if supported, call-completion calls which again encounter a busy callee will not be queued again, but the position of the caller's entry in the queue is retained.
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The call-completion architecture augments each caller's UA (or UAC) which wishes to be able to use the call-completion features with with a "call-completion agent" (also written as "CC agent", "agent", or "caller's agent"). It augments each callee's UA (or UAS) which wishes to be able to be the target of the call-completion features with a "call-completion monitor" (also written as "CC monitor", "monitor", or "callee's monitor"). The agent and monitor functions can be integrated into the respective UAs, be independent end-systems, or be provided by a centralized application server.
In order to request call-completion, the caller's agent subscribes to the call-completion event package (as defined in chapter 6) of the callee's monitor. This subscription is used to coordinate with the monitor (and indirectly with other caller's agents and other callee's monitors) to implement the call-completion features.
When the caller's UA makes a call to a callee that fails (e.g., because the callee was busy or the callee did not answer), and the caller wishes to use CC to contact the callee later, the caller instructs the caller's agent to activate the CC feature.
The caller's agent sends a SUBSCRIBE request for the call-completion event package to the original destination URI of the call. This SUBSCRIBE reaches the callee's monitor. The callee's monitor uses the existence of the subscription to know that the caller is interested in using the CC feature in regard to the specified original call. The monitor keeps a list or queue of failed calls to the callee, and of the caller's agent's subscriptions, which indicate the callers that are waiting to use the CC features.
When the callee's monitor judges that the callee and/or callee's UA is available for call-completion, the callee's monitor selects (usually) one request to be the next caller to execute call-completion to the callee. The callee's monitor sends a call-completion event update to the selected caller's agent's subscription, telling it to begin execution of call-completion.
When the caller's agent receives this update, it calls the caller's UA or otherwise tests whether the caller is available to take advantage of call-completion. If the caller is available, the agent directs the caller's UA to make again the call to the callee. This call is identified as a call-completion call so it can be given precedence in reaching the callee's UA.
If the caller is not available on the receipt of the ready for recall notification, the CC agent suspends the CC request at the CC monitor. The CC agent resumes the CC request once the caller becomes available for CC again. On the receipt of the suspension from the top CC agent in the queue, the CC monitor shall perform the callee monitoring for the next not suspended CC agent in the queue. On the receipt of the resume from the previously suspended top CC agent in the queue the CC monitor shall perform the callee monitoring for this top CC agent.
When the call completion call fails there are two possible options: the CC feature has to be activated again, or CC remains activated and the original CC request retains its position in the queue, possibly with the possibility to update the subscription.
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SIP call completion differs in some ways from the CCBS and CCNR features of SS7 (which is used in the PSTN). For ease of understanding, we enumerate some of the differences here.
SIP call completion does not fundamentally distinguish "call completion on no reply" (CCNR) from "call completion on busy subscriber" (CCBS), because the network does not need to make a distinction, and given the potential complexity of SIP routing, agents in the network may not be able to. However, SIP call completion operations MAY carry an 'm' parameter to label call completion actions as to the original cause of the failure. Currently, the only defined values are "BS" and "NR". Other values may be defined in the future, and SIP call completion operations should succeed even if the 'm' parameter is omitted. However, in these latter cases, interoperation with SS7 is likely to be impaired.
Due to the complex forking situations that are possible in SIP, a call may "fail" from the point of view of the user and yet have a "success" response from SIP's point of view. (This can happen even in simple situations: e.g., a call to a busy user that fails over to his voicemail receives a SIP success response, even though the caller may consider it "busy subscriber".) Thus, the calling user must be able to invoke call completion even when the original call appeared to succeed. To support this, the caller's agent (and to a lesser degree the callee's monitor) must record successful calls as well as unsuccessful calls.
In SIP, only the caller's UA or service and the callee's UA or service needs specifically to support call completion in order that call completion work successfully between the UAs. Intermediate SIP systems (proxies or B2BUAs) do not need specifically to implement call completion; they only need to be transparent to the usual range of SIP messages.
JRE: How does "caller's UA or service" relate to "caller's agent", and how does "callee's UA or service" relate to "callee's monitor"?
Dale: The caller's UA or the caller's service implements the caller's agent. Similar for the callee. What I was trying to emphasize is that while the caller's and callee's "end" of the service path must explicitly implement CC, any transit networks in between do not.
MH: Is it possible to express this with something like 'instances on the originating resp. terminating side', in difference to 'intermediate instances'? Otherwise I think we have to explain the term 'service' more deeply.
Due to flexibility needed to support legacy systems that are not optimized to support call completion, there are a larger number of situations in SIP where call completion services are offered but eventually cannot be successfully executed.
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Note: Many of the actions of call completion can be taken with the actor possessing greater or lesser information about the particular situation, depending on the specific messages involved. This complexity is due to a number of factors: the complexities of SIP forking, interoperation with other networks (specifically SS7), the availability and use of optional SIP features to assist CC, and any future enhancements of CC (either in SIP or SS7). Thus, in some of the following descriptions, several alternative actions are presented based on the information that the actor possesses. CC implementations MUST properly choose between the described alternatives, and MAY augment those alternatives based on any additional information they possess.
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The call-completion architecture augments each caller's UA (or UAC) which wishes to be able to use the call-completion features with a "call-completion agent".
An agent may be an integrated part of the UA, a separate end-system, integrated with a proxy serving the UA, or a function of an application server that provides these services to many UAs.
An agent may service more than one UA as a collective group if it is common that a caller or population of users will be shared between the UAs, and especially if the UAs share an AOR.
The caller's agent must be capable of performing a number of functions relative to the UA(s). The method by which it does so is outside the scope of this document, but an example method is described in appendix A.
The agent monitors calls made from the UA(s) in order to determine their destinations and (potentially) their final response statuses, and the Call-Info headers of final responses and any HERFP provisional responses.
The callers using the UA(s) can indicate to the agent when they wish to avail themselves of CC for a recently-made call which failed to reach their chosen destination.
The agent monitors the status of the UA(s) to determine when they are available to be used for a CC call.
The agent can communicate to the UA(s) that CC recall is in progress and to inquire if the relevant calling user is available for the CC call.
The agent can order the UA(s) at which the relevant calling user is available to generate a CC call to the callee.
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The call-completion architecture augments each callee's UA (or UAS) which wishes to be able to be the target of the call-completion features with a "call-completion monitor".
A monitor may be an integrated part of the UA, a separate end-system, integrated with a proxy serving the UA, or a function of an application server that provides these services to many UAs.
A monitor may service more than one UA as a collective group if it is common that a callee or population of users will be shared between the UAs, and especially if the UAs share an AOR.
The callee's monitor must be capable of performing a number of functions relative to the UA(s). The method by which it does so is outside the scope of this document, but an example method is described in appendix B.
The monitor monitors calls made to the UA(s) in order to determine their callers and (potentially) their final response statuses.
The monitor may supply the callee's UAS(s) with Call-Info header values for final responses and any HERFP provisional responses.
The monitor receives SUBSCRIBEs for the call-completion event package directed to the URIs serviced by the UA(s) and any URIs that the monitor provides for use in Call-Info headers.
The callees using the UA(s) may be able to indicate to the monitor when they wish to receive CC calls.
The monitor has a method of monitoring the status of the UA(s) and/or their users to determine when they are "available" for a CC call, that is, in a suitable state to receive a CC call. In a system with rich presence information, the presence information may directly provide this status. In a more restricted system, this determination MAY depend on the mode of the CC call in question, which is provided by the 'm' parameter. E.g., a UA is considered available for CCBS ("m=BS") when it is not busy, but a UA is considered available for CCNR ("m=CR") when it becomes not busy after being busy with an established call.
The monitor maintains information about the set of INVITEs that have been received by the UA(s) that may not have been considered successful by the calling user. In practice, the monitor may remove knowledge about an incoming dialog from its set if its CC policy establishes that the dialog is no longer eligible for CC requests.
The CC monitor MAY provide a URI to which the CC agent can subscribe for call completion. When applicable, the CC URI SHALL be sent to the CC agent in the Call Info header of an appropriate response message to the initial INVITE, according to the following scheme:
Call-Info:monitor-URI;purpose=call-completion;m=XX Note that this particular format for the information in a response has not been fixed yet.
The 'm' parameter defines the "mode" of call completion and can currently have the values 'BS' for CCBS and 'NR' for CCNR. It is possible that other values will be defined in future. It is also allowed to omit the 'm' parameter entirely. Implementations MUST accept CC operations in which the 'm' parameter is missing or has an unknown value, and perform them as well as is possible in their environment (which is likely to be with degraded service, especially in interoperation with SS7).
Issue: How does the CC monitor inform the CC agent if CC retention is supported or not? MH: service retention parameter
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The caller's UA sends an INVITE to a request URI. One or more forks of this request reach one or more of the callee's UAs. By hypothesis, none of the callee's UAs returns a success response, as otherwise, call completion services would not be needed for this call. However, the caller's INVITE might succeed at some other UA that the calling user considers insufficient to satisfy his needs. E.g., a call that is not answered by the callee user may connect to the callee user's voicemail server. Eventually, the INVITE fails, or the resulting dialog(s) are terminated.
The caller's agent MUST record the From URI and MAY record the final request status that the caller's UA received and the contents of Call-Info headers of final responses and any HERFP provisional responses. The callee's monitor MUST record the From URI and MAY record the final request status(es) returned by the callee's UA(s).
Note that the caller's UA may not receive any response from any of the callee's UA(s), as the final response returned to the caller's UA may have been from a fork that reached a UA that was not the callee's.
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The calling user indicates to the caller's agent that he wishes to invoke call-completion services on the recent call. Note that from the SIP point of view, the INVITE may be successful, but from the user's point of view, the call may be unsuccessful. E.g., the call may have connected to the callee's voicemail, which would return a 200 status to the INVITE but from the caller's point of view is "no reply".
Question: At this point, it seems that the best choice is that the caller's agent need not determine what type of CC is being requested (CCNR vs. CCBS), as (1) it cannot determine this from the INVITE final response, (2) it would be a burden to make the calling user to specify it, and (3) the callee's monitor can determine this from the responses returned by the callee's UAs. MH: Dependent if ‘m’ parameter is included in the response.
The caller's agent subscribes to the call-completion event package using the request URI of the original call. This SUBSCRIBE should be routed in much the same way as the original INVITE, but ultimately being routed not to the callee's UAs but to the callee's monitor. The Event header of the subscribe specifies the call-completion event package with a parameter call_id={Call-Id of the original call}.
Question: Should the specification of the original call be done in the SUBSCRIBE body rather than in an event-param?
The SUBSCRIBE should have headers to optimize its routing. In particular, it SHOULD contain "Request-Disposition: parallel, no-cancel", and an Accept-Contact header to eliminate callee UAs that are not acceptable to the calling user.
The callee's monitor(s) that receive the SUBSCRIBE establish subscriptions. These subscriptions represent the caller's agent's request for call-completion services. The callee's monitor must be prepared to receive multiple forks of a single SUBSCRIBE, and should respond 482 (Merged Request) to all but one fork. The callee's monitor must be prepared to receive SUBSCRIBEs regarding original calls that it has no knowledge of, and should respond 404 (Not Found) to such SUBSCRIBEs. The monitor may apply additional restrictions as to which caller's agents may subscribe.
The caller's agent must be prepared to receive multiple responses to the SUBSCRIBE and to have multiple subscriptions established. The agent must also be prepared to have the SUBSCRIBE fail, in which case, CC cannot be invoked for this original call.
The call-completion event package returns various information to the caller's agent, but the vital datum is that it contains an indication whether the callee's monitor has chosen the caller's agent to perform the next CC call to the callee. This datum is initially false.
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The continuation of the caller's agent's subscription indicates that the caller's agent is prepared to initiate the CC call when it is selected by the callee's monitor. If the caller's agent becomes unwilling to initiate the CC call (e.g., because the calling user has deactivated CC or because the caller's UA becomes busy), the caller's agent must terminate or suspend the subscription(s). (Currently, no method of suspending a subscription is defined.) If the caller's agent later becomes willing again to initiate CC for the original call, it may resume the suspended subscription(s) or initiate new one(s).
If the callee's monitor becomes aware that, according to its policy, the original call referenced by a subscription will never be selected for call-completion, it should terminate the subscription. (And respond to any attempt to start a new subscription for that original call with 404.)
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The callee's monitor has a policy regarding when and how it selects CC requests to be activated. This policy may take into account the type of the requests (CCNR vs. CCBS), the state of the callee's UA(s), the order in which the original calls arrived, and any previous CC attempts for the same original call. Usually the callee's monitor will choose only one CC request for activation at a time, but if the callee's UA(s) can support multiple calls, it may choose more than one.
The callee's monitor changes the "call completion active" datum for the chosen caller's agent from false to true. This triggers a notification for the agent's subscription.
The agent receives the notification with the CC active datum set to true. It then terminates or suspends all other CC subscriptions for this original call, and all CC subscriptions for all other original calls, in order to prevent any other CC requests from this caller from being activated. The agent then determines whether the calling user is available for the CC call, usually by calling the caller's UA(s).
If the calling user is not available, the caller's agent indicates this to the callee's monitor by terminating the CC subscription.
If the calling user is available, the caller's agent causes the caller's UA to initiate a call to the request URI (which is expected to be routed to the callee's UA(s)).
Question: Should the callee's monitor supply a URI which should be used in the CC call? This seems like it would be more reliable, as the monitor is probably "for" a particular callee URI, and it has no information about the destinations of any other forks of the original call.
Question: The CC must be marked in some way as a CC call in order for the callee's monitor to know that the CC activation is being acted upon by the caller's agent. And the marking must include the original Call-Id to allow correlation with the original call. Possibilities for a marking are a special URI-parameter on the request URI or a special header.
The callee's UA(s) and any associated proxies may give the CC call precedence over non-CC calls.
The callee's monitor supervises the receiving of the CC call. If the CC call does not arrive at the callee's UA(s) promptly, the monitor will withdraw CC activation from the caller's agent by changing the value of its CC active datum to false. Similarly, if the CC call fails, the monitor will withdraw CC activation. Depending on its policy, the same original call may be selected again for CC activation at a later time. If the CC call succeeds, the monitor will also withdraw CC activation, but the original call will never again be selected for CC activation (and in practice, can be deleted from the monitor's records).
Question: Is that last statement true? Can a call appear to succeed from the monitor's point of view but fail from the calling user's point of view?
Once the CC call has failed, or if it has succeeded, once the CC call has been terminated, the callee's monitor's policy may select another CC request for activation.
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If the caller is found to be busy previous to or on receipt of the CC recall, then the CC request shall be suspended until the caller becomes not busy again. The CC agent shall send a PUBLISH request informing about the PIDF state 'closed' to the CC monitor, with the URI of the caller in the From header and the To header. The Request-URI of this PUBLISH request contains the monitor URI as received in the Call-Info header of the 486 response.
If a queue entry is suspended, it is stepped over during CC processing at the CC monitor.
When the caller is no longer busy, then the CC request shall be resumed. The CC agent shall send a PUBLISH request informing about the PIDF state 'open' to the CC monitor, with the URI of the caller in the From header and the To header. The Request-URI of this PUBLISH request contains the monitor URI as received in the Call-Info header of the 486 response. When a CC request becomes resumed, then, if the callee becomes not busy again and there is no entry in the CC queue which is currently being processed, the CC monitor shall process the destination B queue as described above.
If the processing of a CC request results in suspending that CC request, the monitor shall stop the recall timer and attempt to process the next CC request in the queue.
In case of the CC agent had sent several CC suspension requests to different CC monitors and the caller becomes not busy again, the CC agent shall send a CC resumption request to each CC monitor for which there is a suspended CC request.
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Question: What format should the event package data be presented in? This draft proposes a simple attribute-value format. We might also consider yet another XML format.
The only necessary information to be provided by the call-completion event package is the CC activation datum, whose value is false (meaning that this CC request has not been chosen for activation) or true (meaning that it has).
Question: If we decide to let the callee's monitor provide the request URI for the CC call, that request URI should probably be a mandatory datum as well.
The event package may provide information about the callee's monitor's policy. In particular, the PSTN CC feature gives an indication of the "service retention" attribute, which indicates whether the CC request can be continued to a later time if the call-completion call fails due to the callee's UA(s) being busy.
If the callee has a caller-queuing facility, we want to treat the call-completion queue as part of the queuing facility, and include in the event package information regarding the state of the queue, such as number of callers ahead of this caller and expected wait time. In that case, this data should probably not trigger a notification every time it changes, but rather at suitable time increments.
Need to revise
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A basic flow, with only the most significant messages shown, is this:
Caller Callee
sip:123@a.com sip:456@b.com
| |
| INVITE sip:456@b.com |
|------------------------->|
| |
| 487 |
| Call-Info: <sip:456@b.com;monitor>;purpose=call-completion;m=NR
|<-------------------------|
| |
| SUBSCRIBE sip:456@b.com;monitor;id=xxxx;m=NR
| Request-Disposition: parallel, no-cancel
|------------------------->|
| |
| 200 |
|<-------------------------|
| |
| SUBSCRIBE sip:456@b.com;id=xxxx;m=NR
| Request-Disposition: parallel, no-cancel
|------------------------->|
| |
| 482 |
|<-------------------------|
| |
| NOTIFY sip:123@a.com |
|<-------------------------|
| |
| INVITE sip:456@b.com;id=xxxx;m=NR
|------------------------->|
| |
The original call is an ordinary INVITE. It fails due to no-response (ring-no-answer). In this case, the callee's governing proxy generates a 487 response because the proxy canceled the INVITE to the UA when it rang too long without an answer. The response carries a Call-Info header with "purpose=call-completion". The Call-Info header positively indicates that CC is available for this failed fork of the call. The "m=NR" parameter indicates that it failed due to no-response, which is useful for PSTN interworking and assessing presence information in the callee's monitor.
The URI in the Call-Info header is the where the caller's agent should subscribe for call-completion processing. Ideally, it is a globally-routable URI for the callee's monitor. In practice, it may be the callee's AOR, and the SUBSCRIBE will be routed to the callee's monitor because it specifies "Event: call-completion".
CC activation is done by sending a SUBSCRIBE to any known monitor URIs. These can be provided by the Call-Info header in the response to the INVITE. These can also be provided by any HERFP provisional responses that are received. (More about that below.) Additionally, the caller's agent needs to include the original request-URI in its set of monitor URIs, because the call may have forked to additional callees whose responses the caller has not seen. (A SUBSCRIBE to the request-URI alone is used in cases where the caller's agent has not received or cannot remember any monitor URI.)
The caller's agent adds to the URIs the 'id' and (if possible) 'm' parameters.
In this case, the caller's agent forks the SUBSCRIBE to two destinations, with appropriate Request-Disposition. The first SUBSCRIBE is to the URI from Call-Info. The second SUBSCRIBE is to the original request-URI, and reaches the same callee's monitor. Because it has the same Call-Id as the SUBSCRIBE that has already reached the monitor, the monitor rejects it with a 482, thus avoiding redundant subscriptions.
Eventually, this caller is selected for CC, and is informed of this via a NOTIFY. This NOTIFY carries a URI to which the CC completion INVITE should be sent. In practice, this may be the AOR of the callee.
The caller generates a new INVITE to the URI specified in the NOTIFY, or if there was no such URI or if the caller's agent cannot remember it, it may use the original request-URI. The caller adds the 'id' and (if possible) 'm' parameters, to specify CC processing.
Complication: The caller's agent cannot remember the URIs returned in the Call-Info header and the call-completion NOTIFY.
The caller's agent uses the request-URI of the original INVITE. This may not provide ideal routing, but in simple cases it is likely to reach the desired callee/callee's monitor.
Complication: Multiple callees are contacted in a deep forking structure.
The caller's agent attempts to reach these by including a SUBSCRIBE to the original request-URI.
Complication: Depending on the request-URI to reach all callee's monitors is not reliable in deep forking structures.
The best solution I know of is to use the proposed 130 response code (draft-mahy-sipping-herfp-fix) to allow callee UAs or their governing proxies to directly carry callee's monitor URIs back to the caller. The body of the 130 is a sipfrag which is the failure response of the original call at that callee. In this application, most of the apparatus described in the referenced draft is ignored, the body sipfrag is only examined for a Call-Info header with purpose=call-completion. If it is found, the URI is added to the list of URIs to be contacted for CC activation.
(While the proposed 130 response code and the proposed 199 response code (draft-holmberg-sipping-199) are generated in nearly identical circumstances, they have different purposes and only the 130 carries the final response as its body. If both purposes are to be served, the two proposals should be merged.)
Fortunately, the 130 response proposal has good upward-compatibility -- only the callee UA/proxy and the caller UA need to understand its semantics, and if a caller UA that does not understand 130 receives one, it will just discard it.
An example call flow is:
Caller Proxy Callee Callee
sip:123@a.com b.com sip:456@b.com sip:789@b.com
| | | |
| INVITE sip:abc@b.com | |
|------------------>| | |
| | INVITE sip:456@b.com |
| |------------------>| |
| | | |
| | 487 | |
| | Call-Info: <sip:456@b.com;monitor> |
| | ;purpose=call-completion;m=NR
| |<------------------| |
| | | |
| | 130 carrying: | |
| | 487 | |
| | Call-Info: <sip:456@b.com;monitor> |
| | ;purpose=call-completion;m=NR
|<--------------------------------------| |
| | | |
| | INVITE sip:789@b.com |
| |-------------------------------------->|
| | | |
| | 487 | |
| | Call-Info: <sip:789@b.com;monitor> |
| | ;purpose=call-completion;m=NR
| |<--------------------------------------|
| | | |
| | 130 carrying: | |
| | 487 | |
| | Call-Info: <sip:789@b.com;monitor> |
| | ;purpose=call-completion;m=NR
|<----------------------------------------------------------|
| | | |
| 487 | | |
| Call-Info: <sip:789@b.com;monitor>;purpose=call-completion;m=NR
|<------------------| | |
| | | |
| SUBSCRIBE sip:456@b.com;monitor;id=xxxx;m=NR |
| Request-Disposition: parallel, no-cancel |
|-------------------------------------->| |
| | | |
| 200 | | |
|<--------------------------------------| |
| | | |
| SUBSCRIBE sip:789@b.com;monitor;id=xxxx;m=NR |
| Request-Disposition: parallel, no-cancel |
|---------------------------------------------------------->|
| | | |
| 200 | | |
|<----------------------------------------------------------|
| | | |
| SUBSCRIBE sip:abc@b.com;id=xxxx;m=NR | |
| Request-Disposition: parallel, no-cancel |
|------------------>| | |
| | | |
| | SUBSCRIBE sip:456@b.com;id=xxxx;m=NR |
| | Request-Disposition: parallel, no-cancel
| |------------------>| |
| | | |
| | 482 | |
| |<------------------| |
| | | |
| | SUBSCRIBE sip:789@b.com;id=xxxx;m=NR |
| | Request-Disposition: parallel, no-cancel
| |-------------------------------------->|
| | | |
| | 482 | |
| |<--------------------------------------|
| | | |
| 482 | | |
|<------------------| | |
| | | |
| NOTIFY sip:123@a.com | |
|<--------------------------------------| |
| | | |
| INVITE sip:456@b.com;id=xxxx;m=NR | |
|-------------------------------------->| |
| | | |
| TOC |
This section specifies the call-completion event package, in accordance with section 4.4 of [RFC3265].
| TOC |
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.
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No package-specific Event header parameters are defined for this event package.
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[RFC3265] requires package definitions to define the usage, if any, of bodies in SUBSCRIBE requests. A SUBSCRIBE request for a call-completion package MAY contain a body. This body defines a filter to be applied to the subscription. Filter documents are not specified in this document.
The SUBSCRIBE request MAY contain an Accept header field. If no such header field is present, it has a default value of "application/call-completion". If the header field is present, it MUST include "application/call-completion".
A SUBSCRIBE request for a call-completion package MAY contain a body. This body defines a filter to be applied to the subscription. Filter documents are not specified in this document, and may be the subject of future standardization activity.
A SUBSCRIBE request requests call-completion information regarding calls recently made from the same originator to the destination UA(s) serviced by the notifier. Calls are defined to be "from the same originator" if the URI-part of the From header value in the INVITE is the same as the URI-part of the From header value in the SUBSCRIBE. (The other parts of the From header values are not considered, as they do not have canonical representations or a defined equality comparison.)
John: Is this intended to be deleted from the final document? If so, fine. If not, I am not sure this captures the true reason for leaving out the tag, say.
Dale: I had not considered that the tag must obviously be left out. I was more concerned with the display-name, of which there are many formats, and there are no clear rules for comparison or canonical representation.
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[RFC3265] requires package definitions to define a default value for subscription durations, and to discuss reasonable choices for durations when they are explicitly specified.
If a SUBSCRIBE does not explicitly request a duration, the default requested duration is 3600 seconds, as that is the highest timer value recommended for the call completion services in ETSI and ITU-T. It is RECOMMENDED that subscribers request, and that notifiers grant, a subscription time of at least 3600 seconds.
If a notifier can determine that, according to its policy, after certain duration the requested subscription cannot proceed to "ready" state, it SHOULD reduce the granted subscription time to that duration. If a notifier can determine that, according to its policy, the requested subscription cannot proceed to "ready" state, it should refuse the subscription. For example, in many cases when resuming a subscription the granted duration will be less than 3600 seconds.
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[RFC3265] requires package definitions to describe the allowed set if body types in NOTIFY requests, and to specify the default value to be used when there is no Accept header field in the SUBSCRIBE request A NOTIFY for a call-completion package MUST contain a body that describes the call-completion states.
As described in [RFC3265], the NOTIFY message will contain bodies that describe the state of the subscribed resource. This body is in a format listed in the Accept header field of the SUBSCRIBE, or in a package-specific default format if the Accept header field was 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.
| TOC |
Subscribers MUST generate SUBSCRIBE requests when they want to subscribe to the call-completion event package at the terminating side in order to receive call-completion notifications. The generation of SUBSCRIBE requests MAY imply the usage of call-completion service specific timers. An example of such an implementation can be found in 3GPP TS 24.642.
Issue: timers have to be specified, a timer section has to be added.
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Upon receiving a subscription refresh, the notifier MUST set the "expires" parameter of the Subscription-State header to the current remaining duration of the subscription regardless of the value received in the Expires header (if present) of the subscription refresh.
If a subscription is not successful because the call-completion queue has reached the maximum allowed number of entries (short term denial), the notifier MUST send a 480 Temporarily Unavailable response to the subscriber. If a subscription is not successful because an error has occurred that prevents and will continue to prevent the call-completion service (long term denial), the notifier MUST send a 403 Forbidden response to the subscriber.
[Need to insert text about handling if there is no call to be completed.]
A notifier MAY receive multiple forks of the same SUBSCRIBE. (Multiple forks are, as always, identified by having the same Call-Id.) In such a case, the notifier SHOULD reject all but one of the SUBSCRIBEs with a 482 Merged Request response (unless some other failure response applies).
The call-completion information can be sensitive. Therefore, all subscriptions SHOULD be handled with consideration of the issues discussed in Section 9.
| TOC |
Notifiers MUST generate NOTIFY requests when a call-completion service condition occurs at the terminating side that needs to be sent towards the originating side. A NOTIFY that is sent with non-zero expiration MUST contain the "call-completion-state" parameter. The parameter's value MUST be "queued" if the call-completion request represented by the subscription is not at this time selected by the monitor for CC recall, and the parameter's value MUST be "ready" if the request is at this time selected by the monitor for CC recall.
A NOTIFY sent with a zero expiration (e.g., as a confirmation of a request to unsubscribe) MAY contain the "call-completion-state" parameter.
When the callee's monitor selects the request for CC recall, the notifier MUST send a NOTIFY to the subscription of the selected request. This NOTIFY MUST contain the "call-completion-state" parameter set to "ready".
When the callee's monitor withdraws selection of the request for CC recall (e.g., because the agent has not initiated the CC recall INVITE promptly, or because the agent has suspended the request from being considered for CC recall), the notifier MUST send a NOTIFY to the subscription of the selected request. This NOTIFY MUST contain the "call-completion-state" parameter set to "queued".
If the call-completion subscription was successful and the retention option is supported at the callee, the NOTIFY MUST contain the "retention-option" parameter.
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The subscriber processing of NOTIFY requests MAY trigger additional CC service procedures, as described in this document and possibly in other documents (e.g., 3GPP TS 24.642).
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Forked requests are expected to be common for the call-completion event type. The subscriber MUST be prepared to process NOTIFY requests from multiple notifiers and to coordinate its processing of the information obtained from them in accordance with the procedures in this document.
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The call completion service typically involves a single notification per notifier and per subscription but MAY involve several notifications separated by a call completion call that failed due to a busy call completion target. Typically, notifications will be separated by at least tens of seconds. Notifiers SHOULD NOT generate more than two notifications in any ten-second interval.
John: A common situation might be a successful subscription to CCBS, resulting in an initial NOTIFY "queued", and then the callee becomes free a couple seconds later, resulting a second NOTIFY "ready". Similarly following resume. It does not seem reasonable to suppress or delay that second NOTIFY. I think the recommendation is too severe. Perhaps NOTIFYs in response to SUBSCRIBEs need to be excluded from this.
Dale: I'm quite willing to modify this prescription. I only included it because RFC 3265 says that there must be a prescription; I believe that if a monitor generate NOTIFYs only in response to relevant events, its NOTIFY rate will be low enough.
However each of in the two cases you mention, there are only two NOTIFYs, so the prescription as stated cannot be violation, since a violation requires 3 NOTIFYs.
| TOC |
State agents have no defined role in the handling of the call-completion package.
| TOC |
The following syntax specification uses the Augmented Backus-Naur Form (ABNF) as described in RFC 2234. The formal syntax for the application/call-completion MIME type is described below. In general, the call-completion body is to be interpreted in the same way as SIP headers: (1) the names of the lines are case-insensitive, (2) the lines can be continued over line boundaries if the succeeding lines start with horizontal white space, (3) lines with unknown names are to be ignored, and (4) names starting with "X-" are reserved for non-standardized uses. Two lines with the same name MUST NOT be present, except when specifically permitted.
call-completion = *(cc-header CRLF)
cc-header = cc-state / cc-service-retention / cc-URI / extension-header
The above rules whose names start with "cc-" are described below. Other rules are described in RFC 3261.
| TOC |
The cc-state line indicates the call-completion status of a particular user with an entry in a call-completion queue. It MUST be present unless the expiration time indicated in the NOTIFY is zero.
cc-state = "state" HCOLON ( "queued" / "ready" )
The value "queued" indicates that a user's entry in the call-completion queue is not selected for CC recall. The value "ready" indicates that a user's entry in the call-completion queue has been selected for CC recall.
| TOC |
The service-retention line indicates the support of the retain option. The retain option, if supported at the callee, will maintain the entry in the call-completion queue, if a call-completion call has failed due to destination busy condition. If present, this parameter indicates that the retain option is supported, otherwise it is not supported. This parameter MAY be present in NOTIFY requests.
cc-service-retention = "service-retention" HCOLON "true"
| TOC |
The cc-URI line provides a URI (possibly in the form of a name-addr) which the agent SHOULD use as the request-URI of the CC recall INVITE. It SHOULD NOT be provided and MUST be ignored unless the cc-state value is "ready". The URI SHOULD be globally routable. The syntax provides for generic-params in the value, but this document defines no such parameters. Parameters that are not understood by the subscriber MUST be ignored.
cc-URI = "URI" HCOLON (name-addr / addr-spec) *(SEMI generic-param)
| TOC |
The use of the CC facility allows the caller's agent to determine some status information regarding the callee. The information is confined to a busy/not-busy indication, and is to a considerable degree protected by the necessity of presenting the Call-Id of a recent call to the callee in order to obtain information.
The CC facility may enhance the effectiveness of SPIT by the following technique: The caller makes calls to a group of targets. The caller then requests CC for the calls that do not connect to the targets. The CC calls resulting are probably more likely to reach the targets than original calls to a further group of targets.
| TOC |
This specification registers an event package, based on the registration procedures defined in . The followings is the information required for such a registration:
Package Name: call-completion
Package or Template-Package: This is a package.
Published Document: RFC XXXX(Note for RFC Editor: Please fill in XXXX with the RFC number of this specification).
Person to Contact: Martin Huelsemann, martin.huelsemann@telekom.de
| TOC |
Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA).
| TOC |
| [RFC2119] | Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML). |
| [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 (TXT). |
| [RFC3265] | Roach, A., “Session Initiation Protocol (SIP)-Specific Event Notification,” RFC 3265, June 2002 (TXT). |
| [RFC3515] | Sparks, R., “The Session Initiation Protocol (SIP) Refer Method,” RFC 3515, April 2003 (TXT). |
| [RFC4235] | Rosenberg, J., Schulzrinne, H., and R. Mahy, “An INVITE-Initiated Dialog Event Package for the Session Initiation Protocol (SIP),” RFC 4235, November 2005 (TXT). |
| TOC |
This section outlines how an autonomous caller's agent can operate using only standard SIP features to interact with the caller's UA. This example is suitable only as a learning aid, as its performance is poor.
The agent monitors calls made from the UA(s) by subscribing to the dialog event package of the UA(s). Since this does not give the agent access to the Call-Info headers of responses, the agent is limited to using the original request URI for making CC requests.
The UA(s) or their proxy routes calls made with either of two special dial sequences to the agent, which interprets the INVITEs as indications to make a CC request with BS or NR mode for the latest call made by the UA.
The agent monitors the status of the UA(s) for availability to be used for a CC call by examining the dialog events.
The agent indicates to the UA(s) that CC recall is in progress by making call to the UA(s). If the UA is answered, the agent assumes that the caller is available and plays pre-recorded audio to indicate that CC recall is in progress.
After playing the pre-recorded audio, the caller's agent uses REFER to order the UA to make the CC call to the callee.
| TOC |
This section outlines how an autonomous callee's monitor can operate using only standard SIP features to interact with the callee's UA. This example is suitable only as a learning aid, as its performance is poor.
The monitor monitors calls made to the UA(s) by subscribing to the UA(s) dialog events. This enables it to determine their Call-Id's and their final response statuses.
The monitor does not supply Call-Info header values. The proxy for the UA(s) routes to the monitor any SUBSCRIBEs for the call-completion event package directed to the URIs serviced by the UA(s).
The callees cannot indicate to the monitor when they wish to receive CC calls.
The monitor monitors the status of the UA(s) to determine when they are in a suitable state to receive a CC call by watching the busy/not-busy status of the UA(s): a UA is available for CCBS when it is not busy, but a UA is available for CCNR when it becomes not busy after being busy with an established call.
| TOC |
| Dale R. Worley | |
| Pingtel Corp. | |
| 10 North Ave. | |
| Burlington, MA, 01803 | |
| US | |
| Phone: | +1 781 229 0533 x173 |
| Email: | dworley@pingtel.com |
| URI: | URI: http://www.pingtel.com |
| Martin Huelsemann | |
| Deutsche Telekom | |
| Heinrich-Hertz-Strasse 3-7 | |
| Darmstadt, 64307 | |
| Germany | |
| Phone: | +4961516282765 |
| Email: | martin.huelsemann@telekom.de |
| Denis Alexeitsev | |
| Deutsche Telekom | |
| Heinrich-Hertz-Strasse 3-7 | |
| Darmstadt, 64307 | |
| Germany | |
| Phone: | +4961516282773 |
| Email: | d.alexeitsev@telekom.de |
| TOC |
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