bliss D. Worley Internet-Draft Nortel Networks Corp. Intended status: Standards Track M. Huelsemann Expires: April 29, 2010 R. Jesske D. Alexeitsev Deutsche Telekom October 26, 2009 Call Completion for Session Initiation Protocol (SIP) draft-ietf-bliss-call-completion-05 Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. 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The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 29, 2010. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the Worley, et al. Expires April 29, 2010 [Page 1] Internet-Draft Call Completion October 2009 document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract The call completion features allow the calling user of a failed call to be notified when the called user becomes available to receive a call. For the realization of a basic solution without queueing call- completion requests, this document references the usage of the the dialog event package [RFC4235] as described as 'automatic redial' in [RFC5359]. For the realization of a more comprehensive solution with queueing call-completion requests, this document introduces an architecture for implementing these features in the Session Initiation Protocol: "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 the original, failed call. The deployment of a certain SIP call-completion solution is also dependent on the needed level of interoperability with existing call- completion solutions in other networks. Worley, et al. Expires April 29, 2010 [Page 2] Internet-Draft Call Completion October 2009 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Requirements terminology . . . . . . . . . . . . . . . . . . . 5 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4.1. Call-Completion architecture . . . . . . . . . . . . . . . 7 4.2. Call-Completion procedures . . . . . . . . . . . . . . . . 9 4.3. Automatic redial as a fallback . . . . . . . . . . . . . . 10 4.4. Differences from SS7 . . . . . . . . . . . . . . . . . . . 11 5. Call-Completion Queue model . . . . . . . . . . . . . . . . . 12 6. Caller's Call-Completion Agent behaviour . . . . . . . . . . . 13 6.1. Receiving the CC possible indication . . . . . . . . . . . 13 6.2. Subscribing to CC . . . . . . . . . . . . . . . . . . . . 13 6.3. Receiving a CC notification . . . . . . . . . . . . . . . 14 6.4. Initiating a CC call . . . . . . . . . . . . . . . . . . . 14 6.5. Suspending CC . . . . . . . . . . . . . . . . . . . . . . 14 6.6. Resuming CC . . . . . . . . . . . . . . . . . . . . . . . 14 7. Callee's Call-Completion Monitor behaviour . . . . . . . . . . 15 7.1. Sending the CC possible indication . . . . . . . . . . . . 15 7.2. Receiving a CC subscription . . . . . . . . . . . . . . . 16 7.3. Sending a CC notification . . . . . . . . . . . . . . . . 16 7.4. Receiving a CC call . . . . . . . . . . . . . . . . . . . 17 7.5. Receiving a CC suspension . . . . . . . . . . . . . . . . 17 7.6. Receiving a CC resumption . . . . . . . . . . . . . . . . 17 8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 9. Call Completion Event Package . . . . . . . . . . . . . . . . 21 9.1. Event Package Name . . . . . . . . . . . . . . . . . . . . 21 9.2. Event Package Parameters . . . . . . . . . . . . . . . . . 21 9.3. SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . . . 21 9.4. Subscribe Duration . . . . . . . . . . . . . . . . . . . . 22 9.5. NOTIFY Bodies . . . . . . . . . . . . . . . . . . . . . . 22 9.6. Subscriber Generation of SUBSCRIBE Requests . . . . . . . 22 9.7. Notifier Processing of SUBSCRIBE Requests . . . . . . . . 23 9.8. Notifier Generation of NOTIFY Requests . . . . . . . . . . 23 9.9. Subscriber Processing of NOTIFY Requests . . . . . . . . . 24 9.10. Handling of Forked Requests . . . . . . . . . . . . . . . 24 9.11. Rate of Notifications . . . . . . . . . . . . . . . . . . 24 9.12. State Agents . . . . . . . . . . . . . . . . . . . . . . . 24 10. Call-completion information format . . . . . . . . . . . . . . 24 10.1. call-completion-state . . . . . . . . . . . . . . . . . . 25 10.2. service-retention . . . . . . . . . . . . . . . . . . . . 25 10.3. cc-URI . . . . . . . . . . . . . . . . . . . . . . . . . . 25 11. Security Considerations . . . . . . . . . . . . . . . . . . . 26 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 12.1. SIP Event Package Registration for call-completion . . . . 26 12.2. MIME Registration for application/call-completion . . . . 26 12.3. SIP/SIPS URI parameter 'm' . . . . . . . . . . . . . . . . 27 Worley, et al. Expires April 29, 2010 [Page 3] Internet-Draft Call Completion October 2009 12.4. 'purpose=call-completion' header parameter for Call-Info . . . . . . . . . . . . . . . . . . . . . . . . 27 12.5. 'm' header parameter for Call-Info . . . . . . . . . . . . 27 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28 14. Normative References . . . . . . . . . . . . . . . . . . . . . 28 Appendix A. Example Caller's Agent . . . . . . . . . . . . . . . 28 Appendix B. Example Callee's Monitor . . . . . . . . . . . . . . 29 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29 Worley, et al. Expires April 29, 2010 [Page 4] Internet-Draft Call Completion October 2009 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 when the callee becomes available. When the caller requests the CC feature, the callee will be monitored for becoming available. When the callee becomes available he will be given a certain time for initiating a call himself. If the callee does not initiate a new call within this time, then the caller will be recalled. When the caller accepts the CC recall then a CC call to the callee will be automatically started. 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. So far the following modes of the CC feature have been specified: 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: CC, or call completion: a service which allows a caller who failed to reach a desired destination user to be notified when the called party becomes available to receive a call. CC possible indication: the data in responses to the INVITE of the Worley, et al. Expires April 29, 2010 [Page 5] Internet-Draft Call Completion October 2009 original call which indicate that CC is available for this call. CC indicator: a iniction in the CC call INVITE used to prioritize the call at the destination. 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. CC request: the entry in the callee's monitor queue representing the original call and the caller's request for CC processing. CCBS, or Completion of Calls to Busy Subscriber: a 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. CCNL, or Completion of Call on Not Logged-in: a CC service when the initial failure was that the destination UA was not registered. CCBS/CCNR/CCNL 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 has determined that the callee is available. CC recall: the action of the callee's monitor selecting a particular CC request as one that should initiate 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 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 CC user: the initiator of the original call and the CC request. The user on whose behalf the CC call is made. Callee, called user, destination, or CC target: a destination of the original call, and a target of the CC call. Worley, et al. Expires April 29, 2010 [Page 6] Internet-Draft Call Completion October 2009 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. 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. Failed call: a call which does not reach a desired callee, from the caller's point of view. Note that a failed call may be successful from the SIP point of view; e.g., if the call reached the callee's voicemail, but the caller desired to speak to the callee in person, the INVITE receives a 200 response, but the caller considers the call to have failed. Original call: the initial call which failed to reach a desired destination. Suspended CC request: a CC request which is temporarily not to be selected for CC recall. 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 4. Solution 4.1. Call-Completion architecture 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" (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. The two functions, though they are associated with the two UAs, also may be provided as services by the endpoints' home proxies or other network elements. Though it is expected that a UA that "implements call completion" will have both types of agents so that it can participate in call completion as both caller and Worley, et al. Expires April 29, 2010 [Page 7] Internet-Draft Call Completion October 2009 callee, the two agents are independent of each other. 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 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 provisional and final responses. 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 monitor may supply the callee's UAS(s) with Call-Info header values for provisional and final responses. The callees using the UA(s) may be able to indicate to the monitor when they wish to receive CC calls. 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. 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 agents and monitors can be devised that interact with users and UAs entirely through standard SIP mechanisms [RFC3265], [RFC4235] and [RFC3515], as described in the Appendixes. 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 recall. The agent can communicate to the UA(s) that a CC recall is in progress and to inquire if the relevant calling user is available for the CC recall. The agent can order the UA(s) at which the relevant calling user is available to generate a CC call to the callee. Worley, et al. Expires April 29, 2010 [Page 8] Internet-Draft Call Completion October 2009 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. This can be achieved by monitoring calls made to the UA(s) in order to determine their callers and (potentially) their final response statuses. In a system with rich presence information, the presence information may directly provide this status. In a more restricted system, this determination 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=NR") 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. 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. 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. Eventually, the INVITE fails, or the resulting dialog(s) are terminated. If the call-completion feature is available, the callee's monitor inserts a Call-Info header with "purpose=call-completion" in an appropriate non-100 provisional or final response message to the initial INVITE and forward them to the caller. 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". In order to request call-completion, the caller's agent subscribes to the call-completion event package of the callee's monitor. This subscription is used to coordinate with the monitor (and indirectly Worley, et al. Expires April 29, 2010 [Page 9] Internet-Draft Call Completion October 2009 with other caller's agents and other callee's monitors) to implement the call-completion features. The caller's agent sends a SUBSCRIBE request for the call-completion event package to all known monitor URIs and 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 (CC recall). 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 (CC call). This call is marked as a CC call so it can be given precedence by the monitor in reaching the callee's UA. If the caller is not available on the receipt of the "ready for recall" notification, the CC agent suspends the CC request at the CC monitor. The CC agent resumes the CC request once the caller becomes available for CC again. On the receipt of the suspension from the CC agent at the top of 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 CC agent that was at the top of the queue the CC monitor shall perform the callee monitoring for this 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 (retain option), optionally with the possibility to update the subscription. 4.3. Automatic redial as a fallback Automatic redial is a simple end-to-end design. An automatic redial scenario is described in [RFC5359], section 2.17. This solution is based on the usage of the dialog event package. When the callee is Worley, et al. Expires April 29, 2010 [Page 10] Internet-Draft Call Completion October 2009 busy when the call arrives, the caller subscribes to the callee's call state. The callee's UA sends a notification when the callee's call state changes. This means the caller is also nofified when the callee's call state changes to 'terminated'. The caller is alerted, then the caller's UA starts a call establishment to the callee again. If several callers have subscribed to a busy callee's call state, they will be notified at the same time that the call state has changed to 'terminated'. The problem of this solution is, that it might occur that several CC recalls are started at the same time. This means it is a heuristic approach with no guarantee in the order of sucessfull recalls. There is no interaction between call completion and automatic redial, as there is a difference in the behaviour of the callee's monitor and the caller when using the dialog event package for receiving dialog information or for aggregating a call completion state. 4.4. Differences from SS7 SIP call completion differs in some ways from the CCBS and CCNR features of SS7 (which is used in the PSTN). For ease of understanding, we enumerate some of the differences here. Due to the complex forking situations that are possible in SIP, a call may "fail" from the point of view of the user and yet have a "success" response from SIP's point of view. (This can happen even in simple situations: e.g., a call to a busy user that fails over to his voicemail receives a SIP success response, even though the caller may consider it "busy subscriber".) Thus, the 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 system on the originating side and the callee's UA or service system on the terminating side need 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. 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. Worley, et al. Expires April 29, 2010 [Page 11] Internet-Draft Call Completion October 2009 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 "an AOR without its voicemail", but it may be as narrowly scoped as a single UA's contact URI. The monitor manages a dynamic set of entities which represent calls eligible for completion ("CECEs" for short). When an INVITE reaches the URI but does not result in a confirmed dialog, a CECE is created which carries the From-URI of the INVITE. (If a CECE already exists carrying the From-URI, no additional CECE is created, however its lifetime timer is reset to 0.) A CECE is removed from the set after its lifetime timer exceeds a particular time limit. A subset of the CECEs are organized into a queue. Each CECE has an availability state, which is either "available for recall" or "not available for recall". This availability state is only significant for CECEs in the queue. It is not visible via subscriptions. Each CECE has a recall state which is visible via subscriptions. The recall state is either "queued" or "ready". This recall state is only significant for CECEs in the queue. CC subscriptions arrive at the the monitor by being addressed to the managed URI. The CECEs being subscribed to are identified by the Request URI. When a CECE is destroyed, any subscriptions to its state are terminated. The monitor maintains "call-completion" subscriptions to all of the CECEs. (And it knows the identities of all the CECEs, since it creates them.) When a CECE becomes the target of a subscription, the CECE is added to the queue (if it is not already there), and is given the availability state "available" and recall state "queued". When a CECE loses its last subscription, the CECE is removed from the queue (though it remains in the set of CECEs). The monitor may receive PIDF bodies (RFC 3863), via PUBLISH requests directed at its URI. These PUBLISH requests are expected to be sent by subscribers to suspend and resume their CC requests. A PIDF body contains an 'entity' attribute of the 'presence' element which is the From-URI of the original call in question, which therefore identifies a CECE. Receipt of a PUBLISH with 'status' of 'open' sets the availability state of the CECE to 'available'; 'status' of 'closed' sets the availability state of the CECE to 'not-available'. The monitor uses the following algorithm to select callers for recall: When the callee is available for recall, and there is at least one CECE in the queue with availability state 'available', then Worley, et al. Expires April 29, 2010 [Page 12] Internet-Draft Call Completion October 2009 the monitor selects from these CECEs the one that has resided in the queue the longest. The CECE's recall state is changed from 'queued' to 'ready', which triggers a notification on the CECE's subscription. When the monitor discovers that an CC INVITE has arrived containing a From-URI of a CECE, it deletes the CECE from the set. However if that INVITE fails, it may cause the creation of another CECE with the same From-URI, or if the retain option is supported, the CECE is retained in the set. If the monitor discovers after a time interval that no INVITE arrives containing the From-URI of the CECE, it deletes the CECE. This may cause another caller to be selected for recall. If the subscriber suspends its request by sending a PUBLISH with status 'closed', the CECE becomes not-available, and the monitor changes the CECE's recall state to 'queued'. This may cause another CECE (that has been in the queue for less time) to be selected for recall. 6. Caller's Call-Completion Agent behaviour 6.1. Receiving the CC possible indication 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 provisional and final responses. 6.2. Subscribing to CC For CC activation the agent MUST send a SUBSCRIBE to all known monitor URIs. These can be provided by the Call-Info header in provisional and final responses to the INVITE. Additionally, the caller's agent MUST 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 MAY add an 'm' parameter to these URIs. The 'm' parameter SHOULD have the value of the the 'm' parameter in the Call-Info header, if a Call-Info header was received. 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. Worley, et al. Expires April 29, 2010 [Page 13] Internet-Draft Call Completion October 2009 If the caller's agent becomes unwilling to initiate the CC call (e.g., because the calling user has deactivated CC), the caller's agent terminates the subscription(s). 6.3. Receiving a CC notification When receiving a CC notification with the cc-state set to 'ready', the caller's agent SHOULD terminate or suspend all other CC subscriptions (at other monitors) 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). 6.4. Initiating a CC call If the calling user is available, the caller's agent causes the caller's UA to generate a new INVITE to the URI specified in the NOTIFY. The caller MAY add the 'm' parameters (if possible), to specify CC processing and prioritize the CC call. If the caller's agent cannot remember the URIs returned in the Call-Info header and the call-completion NOTIFY or if there was no such URI, it SHOULD use 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. 6.5. Suspending CC If the caller is found to be busy previous to or on receipt of the CC recall, then the CC request is suspended by the CC agent until the caller becomes not busy again. To suspend the CC request, the CC agent SHALL send a PUBLISH request to each CC monitor, giving the PIDF state 'closed' for the caller's identity as presentity. Each PUBLISH SHOULD be sent to the URI as received in the NOTIFY, or within the corresponding SUBSCRIBE dialog, or if that is not possible, to the corresponding monitor URI as received in the Call- Info header, or if one is not available, the Contact address of the subscription. If a queue entry is suspended, it is stepped over during CC processing at the CC monitor. 6.6. Resuming CC When the caller is no longer busy, then the CC request is resumed by the CC agent. To resume a CC request, the CC agent SHALL send to each CC monitor a PUBLISH request informing about the PIDF state 'open' but otherwise constructed as the suspend PUBLISH request. Each PUBLISH SHOULD be sent to the URI as received in the NOTIFY, or Worley, et al. Expires April 29, 2010 [Page 14] Internet-Draft Call Completion October 2009 within the corresponding SUBSCRIBE dialog, or if that is not possible, to the corresponding monitor URI as received in the Call- Info header, or if one is not available, the Contact address of the subscription. In the case where the CC agent has 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. 7. Callee's Call-Completion Monitor behaviour 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). The callee's monitor MAY insert a Call-Info header with "purpose=call-completion" in an appropriate response message to the initial INVITE and forward them to the caller. The Call-Info header positively indicates that CC is available for this failed fork of the call. The callee's monitor SHOULD insert a URI in the Call-Info header where the caller's agent should subscribe for call-completion processing. Ideally, it is a globally-routable URI for the callee's monitor. In practice, it may be the callee's AOR, and the SUBSCRIBE will be routed to the callee's monitor only because it specifies "Event: call-completion". When applicable, the Call-Info header MUST be set up according to the following scheme: Call-Info:monitor-URI;purpose=call-completion;m=XX The 'm' parameter defines the "mode" of call completion. The "m=NR" parameter indicates that it failed due to no-response, the "m=BS" parameter indicates that it failed due to busy subscriber, and the "m=NL" parameter indicates that it failed due to not registered subscriber. The 'm' parameter is useful for PSTN interworking and assessing presence information in the callee's monitor. It is possible that other values will be defined in future. It is also allowed to omit the 'm' parameter entirely. Implementations MUST accept CC operations in which the 'm' parameter is missing or has an unknown value, and perform them as well as is possible in their environment (which is likely to be with degraded service, especially in interoperation with SS7). Worley, et al. Expires April 29, 2010 [Page 15] Internet-Draft Call Completion October 2009 7.2. Receiving a CC subscription The monitor MUST be prepared to receive SUBSCRIBEs for the call- completion event package directed to the URIs of UA(s) serviced by the monitor and any URIs that the monitor provides for use in Call- Info headers. 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 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 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 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. 7.3. Sending a CC notification When the cc-state of the agent's request changes, the monitor MUST send a NOTIFY for a call-completion event to the agent. The call- completion event package returns various information to the caller's agent, but the vital datum is that it contains an indication about the cc-state, which in the beginning is 'queued'. The notification MAY also contain a URI which should be used for suspension requestst. Ideally, it is a globally-routable URI for the callee's monitor. In practice, it may be the callee's AOR, and the SUBSCRIBE will be routed to the callee's monitor only because it specifies "Event: call-completion". The call-completion event package provides 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 call-completion call fails due to Worley, et al. Expires April 29, 2010 [Page 16] Internet-Draft Call Completion October 2009 the callee's UA(s) being busy. If the callee's monitor supports the service-retention option, the monitor SHOULD include the cc-service- retention parameter. 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 (e. g. 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. When the callee's monitor changes the "call completion active" datum for the chosen caller's agent from false to true, the monitor MUST send a notification for the agent's subscription with the CC-state set to 'ready'. The notification SHOULD also contain also a URI which should be used in the CC call. In practice, this may be the AOR of the callee. 7.4. Receiving a CC call The callee's UA(s) and any associated proxies may give the CC call precedence over non-CC calls. The callee's monitor supervises the receiving of the CC call. If the CC call does not arrive at the callee's UA(s) promptly, the monitor should withdraw the 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 should withdraw CC activation. 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. 7.5. Receiving a CC suspension 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. 7.6. Receiving a CC resumption When a CC request becomes resumed, then, if the callee is not busy and there is no entry in the CC queue which is currently being processed, the CC monitor SHALL process the queue as described above. Worley, et al. Expires April 29, 2010 [Page 17] Internet-Draft Call Completion October 2009 8. Examples A basic flow, with only the most significant messages shown, is this: Worley, et al. Expires April 29, 2010 [Page 18] Internet-Draft Call Completion October 2009 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:;purpose=call-completion;m=NR |<-------------------------| | | | SUBSCRIBE sip:456@z.b.com;m=NR [initial SUBSCRIBE] | From: sip:123@a.com | | Contact: sip:123@y.a.com | | Request-Disposition: parallel, no-cancel | Call-Id: abcd-efgh | | Event: call-completion | |------------------------->| | | | 200 | |<-------------------------| | | | NOTIFY sip:123@y.a.com | [initial NOTIFY] | Body: status: queued | |<-------------------------| | | | SUBSCRIBE sip:456@b.com;m=NR [another init. SUB.] | From: sip:foo@example.com| | Request-Disposition: parallel, no-cancel | Call-Id: abcd-efgh | | Event: call-completion | |------------------------->| | | | 482 | [duplicate SUB. rejected] |<-------------------------| | | | 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 recall] | From: sip:foo@example.com| |------------------------->| | | The original call is an ordinary INVITE. It fails due to no-response (ring-no-answer). In this case, the callee's governing proxy Worley, et al. Expires April 29, 2010 [Page 19] Internet-Draft Call Completion October 2009 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 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 only because it specifies "Event: call-completion". CC activation is done by sending a SUBSCRIBE to all known monitor URIs. These can be provided by the Call-Info header in the response to the INVITE. 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 these URIs an 'm' parameter (if possible). In this case, the caller's agent forks the SUBSCRIBE to two destinations, with appropriate Request-Disposition. The first SUBSCRIBE is to the URI from Call-Info. The second SUBSCRIBE is to the original request-URI, and reaches the same callee's monitor. Because it has the same Call-Id as the SUBSCRIBE that has already reached the monitor, the 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 CC 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 'm' parameters (if possible), to specify CC processing. Worley, et al. Expires April 29, 2010 [Page 20] Internet-Draft Call Completion October 2009 9. Call Completion Event Package This section specifies the call-completion event package, in accordance with section 4.4 of [RFC3265]. The call-completion event package has the media type "application/call-completion". Note that if the callee has a caller-queuing facility, the callee's monitor may want to treat the call-completion queue as part of the queuing facility, and include in the event package information regarding the state of the queue. How this information is conveyed is left for further standardization. 9.1. Event Package Name The SIP Events specification requires package definitions to specify the name of their package or template-package. The name of this package is "call-completion". This value appears in the Event and Allow-events header fields. 9.2. Event Package Parameters No package-specific Event header parameters are defined for this event package. 9.3. SUBSCRIBE Bodies [RFC3265] requires package definitions to define the usage, if any, of bodies in SUBSCRIBE requests. The SUBSCRIBE request MAY contain an Accept header field. If no such header field is present, it has a default value of "application/ call-completion". If the header field is present, it MUST include "application/call-completion". A SUBSCRIBE request for a call-completion package MAY contain a body. This body defines a filter to be applied to the subscription. Filter documents are not specified in this document, and may be the subject of future standardization activity. A SUBSCRIBE request requests call-completion information regarding calls recently made from the same 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. Worley, et al. Expires April 29, 2010 [Page 21] Internet-Draft Call Completion October 2009 9.4. Subscribe Duration [RFC3265] requires package definitions to define a default value for subscription durations, and to discuss reasonable choices for durations when they are explicitly specified. If a SUBSCRIBE does not explicitly request a duration, the default requested duration is 3600 seconds, as that is the highest service duration timer value recommended for the call completion services in ETSI 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. 9.5. NOTIFY Bodies [RFC3265] requires package definitions to describe the allowed set of body types in NOTIFY requests, and to specify the default value to be used when there is no Accept header field in the SUBSCRIBE request. A NOTIFY for a call-completion package MUST contain a body that describes the call-completion states. As described in [RFC3265], the NOTIFY message will contain bodies that describe the state of the subscribed resource. This body is in a format listed in the Accept header field of the SUBSCRIBE, or in a package-specific default format if the Accept header field was omitted from the SUBSCRIBE. In this event package, the body of the notification contains a call- completion document. All subscribers and notifiers MUST support the "application/call-completion" data format described in section 8. The SUBSCRIBE request MAY contain an Accept header field. If no such header field is present, it has a default value of "application/ call-completion". If the header field is present, it MUST include "application/call-completion". Of course, the notifications generated by the server MUST be in one of the formats specified in the Accept header field in the SUBSCRIBE request. 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 Worley, et al. Expires April 29, 2010 [Page 22] Internet-Draft Call Completion October 2009 side in order to receive call-completion notifications. The generation of SUBSCRIBE requests MAY imply the usage of a call- completion service specific timer as described in section 9.4. 9.7. Notifier Processing of SUBSCRIBE Requests Upon receiving a subscription refresh, the notifier MUST set the "expires" parameter of the Subscription-State header 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. If it is not possible to correlate a SUBSCRIBE request with a queue entry then the notifier SHOULD send a 481 Call/Transaction Does Not Exist response. A notifier MAY receive multiple forks of the same SUBSCRIBE. (Multiple forks are, as always, identified by having the same Call-Id.) In such a case, the notifier SHOULD reject all but one of the SUBSCRIBEs with a 482 Merged Request response unless some other failure response applies. The call-completion information can be sensitive. Therefore, all subscriptions SHOULD be handled with consideration of the issues discussed in Section 11. 9.8. Notifier Generation of NOTIFY Requests Notifiers MUST generate NOTIFY requests when a call-completion service condition occurs at the terminating side that needs to be sent towards the originating side. A NOTIFY that is sent with non- zero expiration MUST contain the "cc-state" parameter. The parameter's value MUST be "queued" if the call-completion request represented by the subscription is not at this time selected by the 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 "cc-state" parameter. Worley, et al. Expires April 29, 2010 [Page 23] Internet-Draft Call Completion October 2009 When the callee's monitor withdraws selection of the request for CC recall (e.g., because the agent has not initiated the CC recall INVITE promptly, or because the agent has suspended the request from being considered for CC recall), the notifier MUST send a NOTIFY to the subscription of the selected request. This NOTIFY MUST contain the "cc-state" parameter set to "queued". If the call-completion subscription was successful and the retention option is supported at the callee, the NOTIFY MUST contain the "cc- service-retention" parameter. 9.9. Subscriber Processing of NOTIFY Requests The subscriber processing of NOTIFY requests MAY trigger additional CC service procedures, as described in this document and possibly in other documents. 9.10. Handling of Forked Requests Forked requests are expected to be common for the call-completion event type. The subscriber MUST be prepared to process NOTIFY requests from multiple notifiers and to coordinate its processing of the information obtained from them in accordance with the procedures in this document. 9.11. Rate of Notifications The call completion service typically involves a single notification per notifier and per subscription but MAY involve several notifications separated by a call completion call that failed due to a busy call completion target. 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 to avoid useless re-calls, a notifier SHOULD send state changes to "queued" promptly. Thus, a notifier SHOULD NOT send a state change to "ready" as the third notification in a ten-second interval, as that would make it impossible to promptly send a further state change to "queued". 9.12. State Agents State agents have no defined role in the handling of the call- completion package. 10. Call-completion information format The following syntax specification uses the Augmented Backus-Naur Worley, et al. Expires April 29, 2010 [Page 24] Internet-Draft Call Completion October 2009 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. 10.1. call-completion-state The cc-state line indicates the call-completion status of a particular user with an entry in a call-completion queue. It MUST be present unless the expiration time indicated in the NOTIFY is zero. cc-state = "cc-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. 10.2. service-retention The service-retention line indicates the support of the retain option. The retain option, if supported at the callee, will maintain the entry in the call-completion queue, if a call-completion call has failed due to destination busy condition. If present, this parameter indicates that the retain option is supported, otherwise it is not supported. This parameter MAY be present in NOTIFY requests. cc-service-retention = "cc-service-retention" HCOLON "true" 10.3. cc-URI The cc-URI line provides a URI (possibly in the form of a name-addr) which the agent SHOULD use as the request-URI of the CC recall INVITE. It SHOULD NOT be provided and MUST be ignored unless the cc- state value is "ready". The URI SHOULD be globally routable. The Worley, et al. Expires April 29, 2010 [Page 25] Internet-Draft Call Completion October 2009 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 = "cc-URI" HCOLON (name-addr / addr-spec) *(SEMI generic- param) 11. Security Considerations The use of the CC facility allows the caller's agent to determine some status information regarding the callee. The information is confined to a busy/not-busy indication, and 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. 12. IANA Considerations 12.1. SIP Event Package Registration for call-completion 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 12.2. MIME Registration for application/call-completion MIME media type name: application MIME subtype name: call-completion Required parameters: none. Worley, et al. Expires April 29, 2010 [Page 26] Internet-Draft Call Completion October 2009 Optional parameters: none. to be defined 12.3. SIP/SIPS URI parameter 'm' This RFC adds a URI parameter 'm'. 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 RFC. This adds a row to the registry SIP/SIPS URI Parameters: Parameter Name Predefined Values Reference ---------------- ------------------- -------------- m Yes [this RFC] 12.4. 'purpose=call-completion' header parameter for Call-Info This RFC adds a new predefined value "call-completion" for the "purpose" header parameter of the Call-Info header. 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 Parameter Name Predefined Values Reference ------------- --------------- ----------------- --------- Call-Info purpose Yes [RFC3261] [RFC5367] [this RFC] 12.5. 'm' header parameter for Call-Info This RFC adds a new header parameter 'm' of the Call-Info header. This adds a row to the registry Header Field Parameters and Parameter Values: Header Field Parameter Name Predefined Values Reference ------------- --------------- ----------------- ---------- Call-Info m Yes [this RFC] Worley, et al. Expires April 29, 2010 [Page 27] Internet-Draft Call Completion October 2009 13. Acknowledgements Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). 14. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. [RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific Event Notification", RFC 3265, June 2002. [RFC3515] Sparks, R., "The Session Initiation Protocol (SIP) Refer Method", RFC 3515, April 2003. [RFC4235] Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE- Initiated Dialog Event Package for the Session Initiation Protocol (SIP)", RFC 4235, November 2005. [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. Worley, et al. Expires April 29, 2010 [Page 28] Internet-Draft Call Completion October 2009 The agent indicates to the UA(s) that CC recall is in progress by making call to the UA(s). If the UA is answered, the agent assumes that the caller is available and plays pre-recorded audio to indicate that CC recall is in progress. After playing the pre-recorded audio, the caller's agent uses REFER to order the UA to make the CC call to the callee. Appendix B. Example Callee's Monitor This section outlines how an autonomous callee's monitor can operate using only standard SIP features to interact with the callee's UA. This example is suitable only as a learning aid, as its performance is poor. The monitor monitors calls made to the UA(s) by subscribing to the UA(s) dialog events. This enables it to determine their Call-Id's and their final response statuses. The 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 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 Nortel Networks Corp. 600 Technology Park Dr. Billerica, MA, 01821 US Phone: +1 781 229 0533 x173 Email: dworley@nortel.com URI: URI: http://www.nortel.com Worley, et al. Expires April 29, 2010 [Page 29] Internet-Draft Call Completion October 2009 Martin Huelsemann Deutsche Telekom Heinrich-Hertz-Strasse 3-7 Darmstadt, 64307 Germany Phone: +4961516282765 Email: martin.huelsemann@telekom.de Roland Jesske Deutsche Telekom Heinrich-Hertz-Strasse 3-7 Darmstadt, 64307 Germany Phone: +4961516282766 Email: r.jesske@telekom.de Denis Alexeitsev Deutsche Telekom Friedrich-Ebert-Allee Bonn 53113 Germany Phone: +49-228-18112010 Email: d.alexeitsev@telekom.de Worley, et al. Expires April 29, 2010 [Page 30]