bliss D. Worley Internet-Draft Avaya Inc. Intended status: Standards Track M. Huelsemann Expires: November 7, 2011 R. Jesske D. Alexeitsev Deutsche Telekom May 06, 2011 Call Completion for Session Initiation Protocol (SIP) draft-ietf-bliss-call-completion-10 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 queuing call- completion requests, this document references the usage of the the dialog event package (RFC 4235) that is described as 'automatic redial' in the SIP Service Examples (RFC 5359). For the realization of a more comprehensive solution with queuing 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. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any Worley, et al. Expires November 7, 2011 [Page 1] Internet-Draft Call Completion May 2011 time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on November 7, 2011. Copyright Notice Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. 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. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Worley, et al. Expires November 7, 2011 [Page 2] Internet-Draft Call Completion May 2011 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Requirements terminology . . . . . . . . . . . . . . . . . . . 5 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4.1. Call-Completion architecture . . . . . . . . . . . . . . . 7 4.2. Call-Completion procedures . . . . . . . . . . . . . . . . 9 4.3. Automatic redial as a fallback . . . . . . . . . . . . . . 11 4.4. Differences from SS7 . . . . . . . . . . . . . . . . . . . 11 5. Call-Completion Queue model . . . . . . . . . . . . . . . . . 12 6. Caller's Call-Completion Agent behavior . . . . . . . . . . . 13 6.1. Receiving the CC possible indication . . . . . . . . . . . 13 6.2. Subscribing to CC . . . . . . . . . . . . . . . . . . . . 13 6.3. Receiving a CC recall notification . . . . . . . . . . . . 14 6.4. Initiating a CC call . . . . . . . . . . . . . . . . . . . 14 6.5. Suspending CC . . . . . . . . . . . . . . . . . . . . . . 14 6.6. Resuming CC . . . . . . . . . . . . . . . . . . . . . . . 14 7. Callee's Call-Completion Monitor behavior . . . . . . . . . . 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 . . . . . . . . . . . . . . . . 18 7.6. Receiving a CC resumption . . . . . . . . . . . . . . . . 18 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 . . . . . . . 23 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 status . . . . . . . . . . . . . . . . . . 25 10.2. Call Completion service-retention indication . . . . . . . 25 10.3. Call Completion 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 . . . . 27 12.3. SIP/SIPS URI parameter 'm' . . . . . . . . . . . . . . . . 27 Worley, et al. Expires November 7, 2011 [Page 3] Internet-Draft Call Completion May 2011 12.4. 'purpose=call-completion' header parameter for Call-Info . . . . . . . . . . . . . . . . . . . . . . . . 27 12.5. 'm' header parameter for Call-Info . . . . . . . . . . . . 28 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28 14. Normative References . . . . . . . . . . . . . . . . . . . . . 28 Appendix A. Example Caller's Agent . . . . . . . . . . . . . . . 29 Appendix B. Example Callee's Monitor . . . . . . . . . . . . . . 29 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30 Worley, et al. Expires November 7, 2011 [Page 4] Internet-Draft Call Completion May 2011 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. This draft defines the following CC features: Call Completion on Busy Subscriber (CCBS): The callee is busy. The caller is recalled after the callee is not busy any longer. Call Completion on No Reply (CCNR): The callee does not answer the call. The caller is recalled after the callee has completed a new call. Call Completion on Not Logged-in (CCNL): The callee is not registered. The caller is recalled after the callee has registered again. 2. Requirements terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. This document uses terms from [RFC3261]. 3. Terminology For the purpose of this service, we provide the following terminology: 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 November 7, 2011 [Page 5] Internet-Draft Call Completion May 2011 original call which indicate that CC is available for this call. CC indicator: an indiction 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 the caller's request for CC processing, that is, the caller's call- completion subscription. 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 for initiation of a CC call, resulting in an indication from the caller's agent to the caller that it is now possible to initiate a CC call. CC recall events: event notifications of event package "call- completion", sent by the callee's monitor to the caller's agent to inform it of the status of its CC request. CC recall timer: maximum time the callee's monitor will wait for the caller's response to a CC recall CC queue: a buffer at the callee's monitor which stores incoming calls which are target for call completion. Note: This buffer may or may not be organized as a queue. The use of the term "queue" is by analogy with SS7 usage. Caller, calling user, originator, or CC user: the initiator of the original call and the CC request. The user on whose behalf the CC Worley, et al. Expires November 7, 2011 [Page 6] Internet-Draft Call Completion May 2011 call is made. Callee, called user, destination, or CC target: a destination of the original call, and a target of the CC call. 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. CCE: call-completion entity Failed call: a call which does not reach a desired callee, from the caller's point of view. Note that a failed call may be successful from the SIP point of view; e.g., if the call reached the callee's voicemail, but the caller desired to speak to the callee in person, the INVITE receives a 200 response, but the caller considers the call to have failed. Original call: the initial call which failed to reach a desired destination. Retain option: a characteristic of the CC service; if supported, CC calls which again encounter a busy callee will not be queued again, but the position of the caller's entry in the queue is retained. Note that SIP CC always operates with the retain option active; a failed CC call does not cause the CC request to lose its position in the queue. Suspended CC request: a CC request which is temporarily not to be selected for CC recall. 4. Solution 4.1. Call-Completion architecture The call-completion architecture augments each caller's UA (or UAC) which wishes to 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 Worley, et al. Expires November 7, 2011 [Page 7] Internet-Draft Call Completion May 2011 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 centralized application servers. The two functions, though they are associated with the two UAs, also may be provided as services by the endpoints' home proxies or other network elements. Though it is expected that a UA that implements call completion will have both types of functions so that it can participate in call completion as both caller and callee, the two functions are independent of each other. 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 header fields 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 field 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. Worley, et al. Expires November 7, 2011 [Page 8] Internet-Draft Call Completion May 2011 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 the callers estimated to not have been successful. 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. 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 can depend on the mode of the CC call in question, which is provided by the 'm' parameter. E.g., a UA is considered available for CCBS ("m=BS") when it is not busy, but a UA is considered available for CCNR ("m=NR") when it becomes not busy after being busy with an established call. The monitor maintains information about the set of INVITEs 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 activations. 4.2. Call-Completion procedures The caller's UA sends an INVITE to a request URI. One or more forks of this request reach one or more of the callee's UAs. If the call- completion feature is available, the callee's monitor (note there can be a monitor for each of the callee's UAs) inserts a Call-Info header field with its URI and with "purpose=call-completion" in appropriate non-100 provisional or final response messages to the initial INVITE and forwards them to the caller. On receipt of a non-100 provisional or a final response with the indication that the call-completion feature is available, the calling user can invoke the feature. The 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 have been successful, but from the user's point of view, the call may have been unsuccessful. E.g., the call may have connected to the callee's voicemail, which would return a 200 status to the INVITE but from the caller's point of view is "no reply". Worley, et al. Expires November 7, 2011 [Page 9] Internet-Draft Call Completion May 2011 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 with other caller's agents and other callee's monitors) to implement the call-completion features. The caller's agent sends a SUBSCRIBE request for the call-completion event package to the original destination URI of the call and to all known monitor URIs (which are provided by Call-Info header fields in provisional and final responses to the INVITE). This SUBSCRIBE reaches the callee's monitor. The callee's monitor uses the existence of the subscription to know that the caller is interested in using the CC feature in regard to the specified callee. The monitor keeps a list or queue 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 by adding a specific SIP URI parameter, 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. Worley, et al. Expires November 7, 2011 [Page 10] Internet-Draft Call Completion May 2011 4.3. Automatic redial as a fallback Automatic redial is a simple end-to-end design. An automatic redial scenario is described in [RFC5359], section 2.17. This solution is based on the usage of the dialog event package. When the callee is busy when the call arrives, the caller subscribes to the callee's call state. The callee's UA sends a notification when the callee's call state changes. This means the caller is also 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 sucessful recalls. There is no interaction between call completion and automatic redial, as there is a difference in the behavior of the callee's monitor and the caller when using the dialog event package for receiving dialog information or for aggregating a call completion state. 4.4. Differences from SS7 SIP call completion differs in some ways from the CCBS and CCNR features of SS7 (which is used in the PSTN). For ease of understanding, we enumerate some of the differences here. Due to the complex forking situations that are possible in SIP, a call may "fail" from the point of view of the user and yet have a "success" response from SIP's point of view. (This can happen even in simple situations: e.g., a call to a busy user that fails over to his voicemail receives a SIP success response, even though the caller may consider it "busy subscriber".) Thus, the calling user must be able to invoke call completion even when the original call appeared to succeed. To support this, the caller's agent must record successful calls as well as unsuccessful calls. In SIP, only the caller's UA or service system on the originating 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 Worley, et al. Expires November 7, 2011 [Page 11] Internet-Draft Call Completion May 2011 situations in SIP where call completion services are offered but eventually cannot be successfully executed. 5. Call-Completion Queue model The callee's monitor manages CC for a single URI. This URI is likely to be a published AOR, or more likely "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 call-completion entities (called "CCEs") which represent CC requests, or equivalently, the existing incoming call-completion subscriptions. This set is also called a queue, because a queue data structure often aids in implementing the monitor's policies for selecting CCEs for CC recall. Each CCE has an availability state, which is either "available" (for recall) or "not available" (for recall). It is not visible via subscriptions. Each CCE has a recall state which is visible via subscriptions. The recall state is either "queued" or "ready". Each CCE carries the From URI of the SUBSCRIBE request that caused its creation. CC subscriptions arrive at the monitor by being addressed to the managed URI, or URIs the monitor returns in Call-Info header fields. The request URI of the SUBSCRIBE request determines the queue to which the resulting CCE is added. The resulting subscription reports the status of the queue. The base event data is the status of all the CCEs in the queue, but the data returned by each subscription is filtered to report only the status of that subscription's CCE. (Further standardization may define means for obtaining more comprehensive information about a queue.) When a CCE is created, it is given the availability state "available" and recall state "queued". The monitor may receive PIDF bodies [RFC3863] 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 CCE is identified by the request-URI (if it was taken from a call-completion event notification and identifies the CCE) or the From URI of the request (matching the From URI recorded in the CCE). Receipt of a PUBLISH with 'status' of 'open' sets the availability state of the CCE to 'available'; 'status' of 'closed' sets the availability state of the CCE to 'not-available'. Worley, et al. Expires November 7, 2011 [Page 12] Internet-Draft Call Completion May 2011 The monitor MUST select for recall only CC requests whose CCE's have availability state 'available', and for which the callee appears to be available considering the 'm' value of the CCE. Within that constraint, the monitor's selections are determined by its policy. Often, a monitor will choose the acceptable CCE that has been in the queue the longest. When the monitor has selected a CCE for recall, it changes the CCE's recall state from 'queued' to 'ready', which triggers a notification on the CCE's subscription. If a selected subscriber then suspends its request by sending a PUBLISH with status 'closed', the CCE becomes not-available, and the monitor changes the CCE's recall state to 'queued'. This may cause another CCE (e.g., that has been in the queue for less time) to be selected for recall. 6. Caller's Call-Completion Agent behavior 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 header fields of provisional and final responses. 6.2. Subscribing to CC For CC activation the agent MUST send a SUBSCRIBE to all known monitor URIs. A monitor URI can be provided by the Call-Info header field in provisional and final responses to the INVITE sent back by the callee's CC monitor(s). Additionally, the caller's agent SHOULD include the original request-URI in its set of monitor URIs, if it is unclear if the call has forked to additional callees whose responses the caller has not seen. A SUBSCRIBE to the original request-URI alone is used in cases where the caller's agent has not received or 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 'm' parameter in the Call-Info header field, if a Call-Info header field was received. To minimize redundant subscriptions, these SUBSCRIBEs SHOULD have the same Call-Id, that is, be presented as forks of the same transaction, if the caller's agent is capable of doing so. The SUBSCRIBE SHOULD have header fields to optimize its routing. In particular, it SHOULD contain "Request-Disposition: parallel, no- cancel", and an Accept-Contact header field to eliminate callee UAs that are not acceptable to the calling user. Worley, et al. Expires November 7, 2011 [Page 13] Internet-Draft Call Completion May 2011 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 or suspends the subscription(s). 6.3. Receiving a CC recall 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 it SHOULD suspend all CC subscriptions for all other original calls, in order to prevent any other CC requests from this caller from being activated. The agent confirms that the calling user would be able to initiate a CC call, e.g. by calling the caller's UA(s). 6.4. Initiating a CC call If the calling user is available for the CC call and willing to initiate the CC call, the caller's agent causes the caller's UA to generate a new INVITE towards the callee. The caller MAY add a 'm' parameter, in order to specify his preferences in CC processing and to prioritize the CC call. The INVITE SHOULD be addressed to the URI specified in the NOTIFY in the cc-URI, or if not available it SHOULD use the URI returned in the Call-Info header field, or if not available it MAY use the request-URI of the original INVITE, if this URI was recorded. 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 not available for the CC recall, or if determined by the CC agent's suspension policy, the CC request SHALL be suspended by the CC agent until the caller becomes not busy again, or if the conditions relevant to the CC agent's suspension policy have changed. 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 field, 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, or if the conditions relevant to the CC agent's suspension policy have changed, then the CC request Worley, et al. Expires November 7, 2011 [Page 14] Internet-Draft Call Completion May 2011 SHALL be 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. In the case where the CC agent has sent several CC suspension requests to different CC monitors and the caller becomes not busy again, as determined by the CC agent's resumption policy the CC agent MAY send a CC resumption request to each CC monitor for which there is a suspended CC request. Note that the CC agent's suspension policy may allow suspensions that are manually initiated and thus should not be automatically resumed 7. Callee's Call-Completion Monitor behavior 7.1. Sending the CC possible indication The callee's monitor MUST record the From URI and MAY record the final request status(es) returned by the callee's UA(s). If the callee's monitor wants to enable the caller to make use of the CC service, it inserts a Call-Info header field with "purpose=call- completion" in an appropriate response message to the initial INVITE and forwards it to the caller. The Call-Info header field positively indicates that CC is available for this failed fork of the call. The callee's monitor SHOULD insert a URI in the Call-Info header field where the caller's agent should subscribe for call-completion processing. Ideally, it is a globally-routable URI for the callee's monitor. In practice, it may be the callee's AOR, and the SUBSCRIBE will be routed to the callee's monitor only because it specifies "Event: call-completion". When applicable, the Call-Info header field MUST be set up according to the following scheme: Call-Info:monitor-URI;purpose=call-completion;m=XX The 'm' parameter defines the "mode" of call completion. The "m=NR" parameter indicates that it failed due to no-response, the "m=BS" parameter indicates that it failed due to busy subscriber, and the "m=NL" parameter indicates that it failed due to not registered subscriber. The 'm' parameter is useful for PSTN interworking and assessing presence information in the callee's monitor. It is possible that other values will be defined in future. It is also allowed to omit the 'm' parameter entirely. Implementations MUST accept CC operations in which the 'm' parameter is missing or has an Worley, et al. Expires November 7, 2011 [Page 15] Internet-Draft Call Completion May 2011 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). 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 header fields. The SUBSCRIBEs SHALL be processed in accordance with the procedures defined in [RFC3265]. The callee's monitor(s) that receive the SUBSCRIBE establish subscriptions. These subscriptions represent the caller's agent's request for call-completion services. The callee's monitor MUST be prepared to receive multiple forks of a single SUBSCRIBE, and SHOULD respond 482 (Merged Request) to all but one fork. 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 continuation of the caller's agent's subscription indicates to the callee's monitor that the caller's agent is prepared to initiate the CC call if it is selected for the 'ready' state. If the callee's monitor becomes aware that, according to its policy, a subscription is unlikely to be selected for call-completion, it SHOULD terminate the subscription. 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 is an indication about the cc-state, which in the beginning is 'queued'. The notification SHOULD also contain a URI which can be used for suspension requests. Ideally, it is a globally-routable URI for the callee's monitor. In practice, it may be the callee's AOR, and the SUBSCRIBE will be routed to the callee's monitor only because it specifies "Event: call-completion". The call-completion event package provides limited information about the callee's monitor's policy. In particular, like in the PSTN, the "'cc-service-retention" datum gives an indication of the "service Worley, et al. Expires November 7, 2011 [Page 16] Internet-Draft Call Completion May 2011 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'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 CC requests arrived, the length of time the CC requests have been active, 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. Usually the callee's monitor will choose the oldest active request. When the callee's monitor changes the state datum for the chosen subscription from "queued" to "ready", the monitor MUST send a notification for the agent's subscription with the CC-state set to 'ready'. The notification SHOULD also contain in the cc-URI a URI which should be used in the CC call. In practice, this may be the AOR of the callee. The monitor SHALL start a recall timer. It is RECOMMENDED to use a default value between 10 and 20 seconds, which corresponds to the recommendation for the call completion services in ETSI and ITU-T. 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. On arrival of the CC call the recall timer SHALL be stopped. If the CC call does not arrive at the callee's UA(s) before expiry of the recall timer, the monitor SHOULD withdraw the CC activation from the caller's agent by changing the value of its state datum to "queued". 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 terminate the CC activation. Once the CC call has been terminated, successfully or unsuccessfully, the callee's monitor's policy MAY select another CC request for activation according to the monitor's policy. Worley, et al. Expires November 7, 2011 [Page 17] Internet-Draft Call Completion May 2011 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 SHALL ensure that the request is set to a 'queued' state, and then the monitor SHALL attempt to process another CC request in the queue according to the monitor's policy. 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 in subclause 7.3 above. 8. Examples A basic flow, with only the most significant messages shown, is this: Worley, et al. Expires November 7, 2011 [Page 18] Internet-Draft Call Completion May 2011 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| |------------------------->| | | | NOTIFY sip:123@y.a.com | [CC terminated] | Expires = 0 | | | Worley, et al. Expires November 7, 2011 [Page 19] Internet-Draft Call Completion May 2011 The original call is an ordinary INVITE. It fails due to no-response (ring-no-answer). In this case, the callee's governing proxy generates a 487 response because the proxy canceled the INVITE to the UA when it rang too long without an answer. The 487 response carries a Call-Info header field with "purpose=call-completion". The Call- Info header field positively indicates that CC is available for this failed fork of the call. The "m=NR" parameter indicates that it failed due to no-response, which is useful for PSTN interworking and assessing presence information in the callee's monitor. The URI in the Call-Info header field () 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 field 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 November 7, 2011 [Page 20] Internet-Draft Call Completion May 2011 Finally the subscription for the CC request is terminated by the monitor. 9. Call Completion Event Package This section specifies the call-completion event package, in accordance with section 4.4 of [RFC3265]. The call-completion event package has the media type "application/call-completion". Note that if the callee has a caller-queuing facility, the callee's monitor may want to treat the call-completion queue as part of the queuing facility, and include in the event package information regarding the state of the queue. How this information is conveyed is left for further standardization. 9.1. Event Package Name The SIP Events specification requires package definitions to specify the name of their package or template-package. The name of this package is "call-completion". This value appears in the Event and Allow-events header fields. 9.2. Event Package Parameters No package-specific Event header field parameters are defined for this event package. 9.3. SUBSCRIBE Bodies [RFC3265] requires package definitions to define the usage, if any, of bodies in SUBSCRIBE requests. The SUBSCRIBE request MAY contain an Accept header field. If no such header field is present, it has a default value of "application/ call-completion". If the header field is present, it MUST include "application/call-completion". A SUBSCRIBE request for a call-completion package MAY contain a body. This body defines a filter to be applied to the subscription. Filter documents are not specified in this document, and may be the subject of future standardization activity. A SUBSCRIBE request requests call-completion information regarding calls recently made from the same 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 field value in the INVITE is the same as the URI-part of the From header field value in Worley, et al. Expires November 7, 2011 [Page 21] Internet-Draft Call Completion May 2011 the SUBSCRIBE. 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. Worley, et al. Expires November 7, 2011 [Page 22] Internet-Draft Call Completion May 2011 9.6. Subscriber Generation of SUBSCRIBE Requests Subscribers MUST generate SUBSCRIBE requests when they want to subscribe to the call-completion event package at the terminating side in order to receive call-completion notifications. The generation of SUBSCRIBE requests MAY imply the usage of a call- completion service specific timer as described in section 9.4. 9.7. Notifier Processing of SUBSCRIBE Requests Upon receiving a subscription refresh, the notifier MUST set the "expires" parameter of the Subscription-State header field to the current remaining duration of the subscription regardless of the value received in the Expires header field (if present) of the subscription refresh. If a subscription is not successful because the call-completion queue has reached the maximum allowed number of entries (short term denial), the notifier MUST send a 480 Temporarily Unavailable response to the subscriber. If a subscription is not successful because an error has occurred that prevents and will continue to prevent the call-completion service (long term denial), the notifier MUST send a 403 Forbidden response to the subscriber. A notifier MAY receive multiple forks of the same SUBSCRIBE. (Multiple forks are, as always, identified by having the same Call-Id.) In such a case, the notifier SHOULD reject all but one of the SUBSCRIBEs with a 482 Merged Request response unless some other failure response applies. The call-completion information can be sensitive. Therefore, all subscriptions SHOULD be handled with consideration of the issues discussed in section 11. 9.8. Notifier Generation of NOTIFY Requests Notifiers MUST generate NOTIFY requests when a call-completion service condition occurs at the terminating side that needs to be sent towards the originating side. A NOTIFY that is sent with non- zero expiration MUST contain the "cc-state" parameter. The parameter's value MUST be "queued" if the call-completion request represented by the subscription is not at this time selected by the 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 November 7, 2011 [Page 23] Internet-Draft Call Completion May 2011 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 avoid useless recalls, 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 November 7, 2011 [Page 24] Internet-Draft Call Completion May 2011 Form (ABNF) as described in [RFC5234]. The formal syntax for the application/call-completion MIME type is described below. In general, the call-completion body is to be interpreted in the same way as SIP headers: (1) the names of the lines are case-insensitive, (2) the lines can be continued over line boundaries if the succeeding lines start with horizontal white space, (3) lines with unknown names are to be ignored, and (4) names starting with "X-" are reserved for non-standardized uses. Two lines with the same name MUST NOT be present, except when specifically permitted. call-completion = 1*(cc-header CRLF) cc-header = cc-state / cc-service-retention / cc-URI / extension- header The above rules whose names start with "cc-" are described below. Other rules are described in [RFC3261]. 10.1. Call Completion status The cc-state line indicates the 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 subscriber's entry in the call- completion queue is not selected for CC recall. The value "ready" indicates that a user's entry in the call-completion queue has been selected for CC recall. 10.2. Call Completion service-retention indication The service-retention line indicates the support of the retain option. The retain option, if supported at the callee, will maintain the entry in the 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. Call Completion URI The cc-URI line provides a URI (possibly in the form of a name-addr) which the agent SHOULD use as the request-URI of the CC recall INVITE. 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 November 7, 2011 [Page 25] Internet-Draft Call Completion May 2011 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 in legitimate use, will be subscribed to stereotyped ways that limit the disclosure of status information: 1. When a subscriber is selected for CC, a call should arrive promptly for the callee, or the subscription should be terminated. This expectation may be violated by a race condition between selection of the subscription for CC and the caller becoming unavailable, but it should be rare that a single subscription will exhibit the race more than once. 2. Subscriptions should not remain suspended for longer than the expected duration of a call (a call by the caller to a third party). 3. Subscriptions should be initiated only shortly after failed incoming calls. 4. Most of the time, a callee should have no queued subscriptions. Violations of these expectations should be detected by the callee's monitor and reported as possible attempts at privacy violation. The CC facility may enhance the effectiveness of SPIT by the following technique: The caller makes calls to a group of 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 Worley, et al. Expires November 7, 2011 [Page 26] Internet-Draft Call Completion May 2011 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. 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 field parameter of the Call-Info header field. This modifies the registry Header Field Parameters and Parameter Values by adding this RFC as a Reference to the line for Header Field "Call- Info" and Parameter Name "purpose": Header Field Parameter Name Predefined Values Reference Worley, et al. Expires November 7, 2011 [Page 27] Internet-Draft Call Completion May 2011 --------------------- --------------------------- ------------------------- --------- Call-Info purpose Yes [ RFC3261][RFC5367][this RFC] 12.5. 'm' header parameter for Call-Info This RFC adds a new header field parameter 'm' of the Call-Info header field. 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] 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. [RFC3863] Sugano , H., Fujimoto, S., Klyne , G., Bateman, A., Carr, W., and J. Peterson , "Presence Information Data Format (PIDF)", August 2004. [RFC4235] Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE- Initiated Dialog Event Package for the Session Initiation Protocol (SIP)", RFC 4235, November 2005. Worley, et al. Expires November 7, 2011 [Page 28] Internet-Draft Call Completion May 2011 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. [RFC5359] Johnston, A., Sparks, R., Cunningham, C., Donovan, S., and K. Summers, "Session Initiation Protocol Service Examples", BCP 144, RFC 5359, October 2008. Appendix A. Example Caller's Agent This section outlines how an autonomous caller's agent can operate using only standard SIP features to interact with the caller's UA. This example is suitable only as a learning aid, as its performance is poor. The agent monitors calls made from the UA(s) by subscribing to the dialog event package of the UA(s). The UA(s) or their proxy routes calls made with either of two special dial sequences to the agent, which interprets the INVITEs as indications to make a CC request with BS or NR or NL mode for the latest call made by the UA. The agent monitors the status of the UA(s) for availability to be used for a CC call by examining the dialog events. The agent indicates to the UA(s) that CC recall is in progress by making call to the UA(s). If the UA is answered, the agent assumes that the caller is available and plays pre-recorded audio to indicate that CC recall is in progress. After playing the pre-recorded audio, the caller's agent uses REFER to order the UA to make the CC call to the callee. Appendix B. Example Callee's Monitor This section outlines how an autonomous callee's monitor can operate using only standard SIP features to interact with the callee's UA. This example is suitable only as a learning aid, as its performance is poor. The 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 Worley, et al. Expires November 7, 2011 [Page 29] Internet-Draft Call Completion May 2011 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 Avaya Inc. 600 Technology Park Dr. Billerica, MA, 01821 US Phone: +1 978 671 3465 Email: dworley@avaya.com URI: http://www.avaya.com Martin Huelsemann Deutsche Telekom Heinrich-Hertz-Strasse 3-7 Darmstadt, 64307 Germany Phone: +4961516282765 Email: martin.huelsemann@telekom.de URI: http://www.telekom.de Roland Jesske Deutsche Telekom Heinrich-Hertz-Strasse 3-7 Darmstadt, 64307 Germany Phone: +4961516282766 Email: r.jesske@telekom.de URI: http://www.telekom.de Worley, et al. Expires November 7, 2011 [Page 30] Internet-Draft Call Completion May 2011 Denis Alexeitsev Deutsche Telekom Heinrich-Hertz-Strasse 3-7 Darmstadt 64307 Germany Phone: +49-6151-628 2773 Email: d.alexeitsev@telekom.de URI: http://www.telekom.de Worley, et al. Expires November 7, 2011 [Page 31]