Internet Engineering Task Force R. Sparks Internet-Draft dynamicsoft Expires: August 3, 2001 February 2, 2001 SIP Call Control - Transfer draft-sip-cc-transfer-03 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 3, 2001. Copyright Notice Copyright (C) The Internet Society (2001). All Rights Reserved. Abstract This document defines a SIP extension within the Call Control Framework and demonstrates its use to provide Call Transfer capabilities. Sparks Expires August 3, 2001 [Page 1] Internet-Draft SIP Call Control - Transfer February 2001 Table of Contents 1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Changes from draft-sparks-sip-cc-transfer-02 . . . . . . . . 3 3. The REFER Method . . . . . . . . . . . . . . . . . . . . . . 3 3.1 The Refer-To Header . . . . . . . . . . . . . . . . . . . . 3 3.1.1 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.2 The Referred-By Header . . . . . . . . . . . . . . . . . . . 4 3.2.1 A PGP based signature-scheme . . . . . . . . . . . . . . . . 5 3.2.2 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.3 Header Field Support for the REFER Method . . . . . . . . . 6 3.4 Message Body Inclusion . . . . . . . . . . . . . . . . . . . 7 3.5 Responses within the REFER transaction . . . . . . . . . . . 7 3.6 Behavior of SIP User Agents . . . . . . . . . . . . . . . . 7 3.6.1 Accessing the referred-to resource . . . . . . . . . . . . . 7 3.6.2 Reporting on the results of the reference . . . . . . . . . 8 3.7 Behavior of SIP Registrars/Redirect Servers . . . . . . . . 8 3.8 Behavior of SIP Proxies . . . . . . . . . . . . . . . . . . 8 3.9 Prototypical REFER callflow . . . . . . . . . . . . . . . . 9 3.10 Security Considerations . . . . . . . . . . . . . . . . . . 9 4. Call Transfer . . . . . . . . . . . . . . . . . . . . . . . 9 4.1 Actors and Roles . . . . . . . . . . . . . . . . . . . . . . 9 4.2 Requirements . . . . . . . . . . . . . . . . . . . . . . . . 10 4.3 Using REFER to achieve Call Transfer . . . . . . . . . . . . 10 4.4 Unattended Transfer . . . . . . . . . . . . . . . . . . . . 11 4.4.1 Successful Unattended Transfer . . . . . . . . . . . . . . . 12 4.4.2 Failed Unattended Transfer . . . . . . . . . . . . . . . . . 13 4.5 Unattended Transfer with Consultation Hold . . . . . . . . . 13 4.5.1 Variation 1 : Exposes transfer target . . . . . . . . . . . 14 4.5.2 Variation 2 : Protects transfer target . . . . . . . . . . . 15 4.5.3 Consultation Hold in the presence of forking proxies . . . . 15 4.6 Attended Transfer . . . . . . . . . . . . . . . . . . . . . 16 4.7 Transfer with multiple parties . . . . . . . . . . . . . . . 16 5. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . 17 5.1 200 vs 202 . . . . . . . . . . . . . . . . . . . . . . . . . 17 5.2 Should REFER expire? . . . . . . . . . . . . . . . . . . . . 17 5.3 REFER is now dependent on sip-events . . . . . . . . . . . . 17 5.4 Registering the events with IANA . . . . . . . . . . . . . . 18 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 18 References . . . . . . . . . . . . . . . . . . . . . . . . . 18 Author's Address . . . . . . . . . . . . . . . . . . . . . . 18 Full Copyright Statement . . . . . . . . . . . . . . . . . . 19 Sparks Expires August 3, 2001 [Page 2] Internet-Draft SIP Call Control - Transfer February 2001 1. Overview This document defines a SIP[1] extension and details its use to provide Call Transfer capabilities. This is part of a family of Call Control extensions described in the Call Control Framework[2] document. The mechanisms discussed here are most closely related to traditional unattended and consultation hold transfers. Discussion of attended transfer (where all parties are briefly in a conference) is deferred until the conferencing features in this framework are addressed. This work has roots in draft-ietf-sip-cc-01[4] but some basic semantics are different. In particular, transfers are achieved through a new method that does not terminate the original signaling relationship. By disassociating transfers from the processing of BYE, these changes facilitate recovery of failed transfers and clarify state management in the participating entities. 2. Changes from draft-sparks-sip-cc-transfer-02 o Changed the REFER response to be immediate instead of waiting for the referred action to complete o Added use of NOTIFY to deliver the result of the referred action o Removed the claim of easy migration from BYE/ALSO 3. The REFER Method REFER is a SIP method as defined by RFC2543[1]. The REFER method indicates that the recipient should contact a third party using the contact information provided in the method. A success response indicates that the recipient was able to contact the third party. The REFER method follows the session's current signaling path. In particular, the Request-URI of the REFER method identifies the recipient. Unless stated otherwise, the protocol for emitting and responding to a REFER request are identical to those for a BYE request in [1]. The behavior of SIP entities not implementing the REFER (or any other unknown) method is explicitly defined in [1] and is not discussed further here. 3.1 The Refer-To Header Refer-To is a request-header as defined by [1]. It may only appear in a REFER request. Refer-To = ("Refer-To" | "r") ":" URL Sparks Expires August 3, 2001 [Page 3] Internet-Draft SIP Call Control - Transfer February 2001 A REFER method MUST contain exactly one Refer-To header. The Refer-To header MAY be encrypted as part of end-end encryption. The Contact header is an important part of the Route/Record-Route mechanism and is not available for this task. 3.1.1 Examples Refer-To: sip:alice@atlanta.com Refer-To: sip:bob@biloxi.com?Accept-Contact=sip:bobsdesk.biloxi.com?Ca ll-ID=55432@alicepc.atlanta.com Refer-To: sip:carol@cleveland.com;method=SUBSCRIBE Refer-To: http://www.ietf.org 3.2 The Referred-By Header Referred-By is a request-header as defined by [1]. It can appear in any request. It conveys the identity of the original REFERrer to the referred-to party, optionally proving the identity and that the REFERrer actually issued this reference. Referred-By = ("Referred-By" | "b") ":" referrer-url ";" ( referenced-url | ( referenced-url ";" ref-signature ) | ( ref-signature ";" referenced-url ) ) referrer-url = ( name-addr | addr-spec ) referenced-url = "ref" "=" "<" URL ">" ref-signature = signature-scheme *( ";" sig-scheme-params ) signature-scheme = "scheme" "=" token sig-scheme-parms = token "=" ( token | quoted-string ) The referrer-url contains the SIP URL of the party sending the REFER request. The referenced-url contains a copy of the URL placed in the Refer-To: header. Any occurrences of < or > in the referenced-url MUST be escaped. The ref-signature contains a signature over the concatenation of referrer-url and referenced-url. An example signature scheme is given in section 3.1.2. A REFER request MUST contain exactly one Referred-By header. (Open Issue: Should Refer Expire? (Section 5.2).) Sparks Expires August 3, 2001 [Page 4] Internet-Draft SIP Call Control - Transfer February 2001 The Referred-By header SHOULD be signed to help detection of REFERs from unauthorized third parties. A signed Referred-By header SHOULD include a Date header in the referrer-url to facilitate detection of replay attacks. A UA MAY reject a request containing an unsigned Referred-By header. A UA SHOULD verify the signature on any Referred-By header it receives. The Referred-By header MAY be encrypted as part of end-end encryption. 3.2.1 A PGP based signature-scheme One signature-scheme for Referred-By headers uses PGP as follows: signature-scheme = "scheme" "=" "pgp" sig-scheme-parms = pgp-version | signed-by | pgp-signature pgp-version, signed-by and pgp-signature are defined in section 15.1 of RFC2543, with the modification that the signature is computed across the concatenation of the referrer-url and the referenced-url. 3.2.2 Examples Referred-By: sip:alice@atlanta.com;ref= Referred-By: "Bob" ;ref=;scheme=pgp;pgp-version="5.0";signature="the signature" (Note that in the last example, the signature would be over the string "sip:bob@biloxi.comsip:alice@atlanta.com") Sparks Expires August 3, 2001 [Page 5] Internet-Draft SIP Call Control - Transfer February 2001 3.3 Header Field Support for the REFER Method This table adds a column to tables 4 and 5 in [1], describing header presence in a REFER method. See [1] for a key for the symbols used. A row for the Refer-To: and Referred-By request-header should be inferred, each mandatory for REFER. Refer-To is not applicable for all other methods. Referred-By is a general Request header. The enc and e-e columns in [1] apply to the REFER method unmodified. Header Where REFER Accept R - Accept-Encoding R - Accept-Language R o Allow R - Allow 405 m Authorization R o Call-ID gc m Contact R o Contact 1xx - Contact 2-6xx o Content-Encoding e - Content-Length e o Content-Type e - CSeq gc m Date g o Encryption g o Expires R o From gc m Hide R o Max-Forwards R o Organization g o Priority R - Proxy-Authenticate 407 o Proxy-Authorization R o Proxy-Require R o Require R o Retry-After R - Retry-After 404,480,486 o Retry-After 503 o Retry-After 600,603 o Response-Key R o Record-Route R o Record-Route 2xx o Route R o Server r o Subject R - Timestamp g o To gc(1) m Unsupported 420 o Sparks Expires August 3, 2001 [Page 6] Internet-Draft SIP Call Control - Transfer February 2001 User-Agent g o Via gc(2) m Warning r o WWW-Authenticate 401 o 3.4 Message Body Inclusion A REFER method may contain a body which SHOULD be processed according to its Content-Type. 3.5 Responses within the REFER transaction An agent responding to a REFER Method MUST return a 400 Bad Request if the request contained zero or more than one Refer-To headers. An agent responding to a REFER Method MUST return a 400 Bad Request if the request contained zero or more than one Referred-By headers. An agent (including proxies generating local responses) MAY return a 100 Trying or any appropriate 400-600 class response as prescribed by [1]. If the recipient's agent decides to contact the resource in the Refer-To header, a 200 OK response MUST be returned before the REFER transaction expires. (Open Issue: Should this be a 202 Accepted?) (Section 5.1) Editor's note - The previous version of this draft required the agent responding to REFER to wait until the referred action completed before sending a final response to the REFER. That final response reflected the success or failure of the referred action. This was infeasible due to the transaction timeout rules defined for non-INVITE requests in [1]. A REFER must always receive an immediate (within the lifetime of a non-INVITE transaction) final response. 3.6 Behavior of SIP User Agents 3.6.1 Accessing the referred-to resource A UA receiving a well-formed REFER request SHOULD request approval from the user to proceed (this request could be interactive or through configuration). Upon receiving approval from the user, the UA MUST contact the resource identified by the URL in the Refer-To: header. Note that if the URL is a SIP URL, it could contain header fields such as Call-Id that will be used to form the resulting request. If the URL is a SIP URL, the Referred-By header in the REFER request should be copied into the request sent to the referred-to resource. Sparks Expires August 3, 2001 [Page 7] Internet-Draft SIP Call Control - Transfer February 2001 3.6.2 Reporting on the results of the reference Once it is known whether the reference succeeded or failed, the UA receiving the REFER SHOULD notify the agent sending the refer using the NOTIFY mechanism defined in Event Notification in SIP[3] as if the the REFER had established a subscription. In particular: o The NOTIFY should reflect the To:, From:, and Call-ID headers from the REFER as if they had arrived in a SUBSCRIBE. o If the reference succeeded, the NOTIFY MUST contain Event: refersuccess o If the reference failed for any reason, or was not attempted after being accepted, the NOTIFY MUST contain Event: referfailure o The notifying UA SHOULD send exactly one NOTIFY with an event from the profile {refersuccess,referfailure}. If multiple notifications are sent, perhaps including events from other extension drafts, the UA MUST NOT send both refersuccess and referfailure events. o Analogous to the case for SUBSCRIBE described in [3], the agent that issued the REFER MUST be prepared to receive a NOTIFY before the REFER transaction completes. Open Issue: This makes REFER dependent on sip-events (Section 5.3) Open Issue: The events will need to be registered with IANA (Section 5.4) 3.7 Behavior of SIP Registrars/Redirect Servers Registrars and Redirect Servers SHOULD return a 603 to a REFER request, unless they are also playing some other SIP role. 3.8 Behavior of SIP Proxies SIP Proxies do not require modification to support the REFER method. Specifically, as required by [1], a proxy should process a REFER request the same way it processes an OPTIONS request. Sparks Expires August 3, 2001 [Page 8] Internet-Draft SIP Call Control - Transfer February 2001 3.9 Prototypical REFER callflow Agent A Agent B | | | REFER | |----------------------->| | 200 OK | |<-----------------------| | | | |-------> | | (whatever) | |<------ | | | NOTIFY | |<-----------------------| | 200 OK | |----------------------->| | | | | 3.10 Security Considerations The security requirements of [1] apply to the REFER method. This mechanism relies on providing contact information for the referred-to resource to the party being referred. Care should be taken to provide a suitably restricted URI if the referred to resource should be protected. Care should be taken when implementing the logic that determines whether or not to accept the REFER request. A UA not capable of accessing non-SIP URLs SHOULD NOT accept REFER requests to them. 4. Call Transfer 4.1 Actors and Roles There are three actors in a given transfer event, each playing one of the following roles: Transferee - the party being transferred to the Transfer Target. Transferor - the party initiating the transfer Transfer Target - the new party being introduced into a call with the Transferee. The following roles are used to describe transfer requirements and Sparks Expires August 3, 2001 [Page 9] Internet-Draft SIP Call Control - Transfer February 2001 scenarios: Originator - wishes to place a call to the Recipient. This actor is the source of the first INVITE in a session, to either a Facilitator or a Screener. Facilitator - receives a call or out-of-band request from the Originator, establishes a call to the Recipient through the Screener, and connects the Originator to the Recipient. Screener - receives a call ultimately intended for the Recipient and transfers the calling party to the Recipient if appropriate. Recipient - the party the Originator is ultimately connected to. 4.2 Requirements 1. Any party in a SIP session MUST be able to transfer any other party in that session at any point in that session. 2. The Transferor and the Transferee MUST NOT be removed from a session as part of a transfer transaction. At first glance, requirement 2 may seem to indicate that the user experience in a transfer must be significantly different from what a current PBX or Centrex user expects. As the call-flows in this document show, this is not the case. A client MAY preserve the current experience. In fact, without this requirement, some forms of the current experience (ringback on unattended transfer failure for instance) will be lost. 3. The Transferor MUST know whether or not the transfer was successful (this is significantly different from the requirements of draft-ietf-sip-cc-01). 4.3 Using REFER to achieve Call Transfer A REFER can be issued by the Transferor to cause the Transferee to issue an INVITE to the Transfer-Target. Note that a successful REFER transaction does not terminate the session between the Transferor and the Transferee. If those parties wish to terminate their session, they must do so with a subsequent BYE request. The media negotiated between the transferee and the transfer target is not affected by the media that had been negotiated between the transferor and the transferee. In particular, the INVITE issued by the Transferee will have the same SDP body it would have if he Transferee had initiated that INVITE on its own. Further, the Sparks Expires August 3, 2001 [Page 10] Internet-Draft SIP Call Control - Transfer February 2001 disposition of the media streams between the Transferor and the Transferee is not altered by the REFER method. Agents may alter a session's media through additional signaling. For example, they may make use of the SIP hold re-INVITE [1] or the conferencing extensions provided by this framework. 4.4 Unattended Transfer Unattended Transfer consists of the Transferor providing the Transfer Target's contact to the Transferee. The Transferee attempts to establish a session using that contact and reports the results of that attempt to the Transferor. The signaling relationship between the Transferor and Transferee is not terminated, so the call is recoverable if the Transfer Target cannot be reached. Note that the Transfer Target's contact information has been exposed to the Transferee. The provided contact can be used to make new calls in the future. The diagrams below show indicate the first line of each message. All messages in a particular diagram share the same Call-ID. In these diagrams, media is managed through reINVITE holds, but other mechanisms (mixing multiple media streams at the UA or using the conferencing extensions for example) are valid. Sparks Expires August 3, 2001 [Page 11] Internet-Draft SIP Call Control - Transfer February 2001 4.4.1 Successful Unattended Transfer Transferor Transferee Transfer | | Target | INVITE | | |<-------------------| | | 200 OK | | |------------------->| | | ACK | | |<-------------------| | | INVITE (hold) | | |------------------->| | | 200 OK | | |<-------------------| | | ACK | | |------------------->| | | REFER | | |------------------->| | | 200 OK | | |<-------------------| | | | INVITE | | |------------------->| | | 200 OK | | |<-------------------| | | ACK | | |------------------->| | NOTIFY (refersuccess) | |<-------------------| | | 200 OK | | |------------------->| | | BYE | | |------------------->| | | 200 OK | | |<-------------------| | | | BYE | | |<-------------------| | | 200 OK | | |------------------->| Sparks Expires August 3, 2001 [Page 12] Internet-Draft SIP Call Control - Transfer February 2001 4.4.2 Failed Unattended Transfer Transferor Transferee Transfer | | Target | | | | INVITE | | |<-------------------| | | 200 OK | | |------------------->| | | ACK | | |<-------------------| | | INVITE (hold) | | |------------------->| | | 200 OK | | |<-------------------| | | ACK | | |------------------->| | | REFER | | |------------------->| | | 200 OK | | |<-------------------| | | | INVITE | | |------------------->| | | 486 Busy Here | | |<-------------------| | | ACK | | |------------------->| | NOTIFY (referfailure) | |<-------------------| | | 200 OK | | |------------------->| | | INVITE (unhold) | | |------------------->| | | 200 OK | | |<-------------------| | | ACK | | |------------------->| | | BYE | | |------------------->| | | 200 OK | | |<-------------------| | 4.5 Unattended Transfer with Consultation Hold Transfer with Consultation Hold involves a session between the transferor and the transfer target before the transfer actually takes place. This is implemented with SIP Hold and Unattended Transfer as described above. Sparks Expires August 3, 2001 [Page 13] Internet-Draft SIP Call Control - Transfer February 2001 4.5.1 Variation 1 : Exposes transfer target The transferor places the transferee on hold, establishes a call with the transfer target to alert them to the impending transfer, terminates the connection with the transfer target, then proceeds with unattended transfer as above. This variation can be used to provide an experience similar to that expected by current PBX and Centrex users. To (hopefully) improve clarity, non-REFER transactions have been collapsed into one indicator with the arrow showing the direction of the request. Transferor Transferee Transfer | | Target | | | Call-ID:1 | INVITE/200 OK/ACK | | |<-------------------| | Call-ID:1 | INVITE (hold)/200 OK/ACK | |------------------->| | Call-ID:2 | INVITE/200 OK/ACK | | |---------------------------------------->| Call-ID:2 | BYE/200 OK | | |---------------------------------------->| Call-ID:1 | REFER | | |------------------->| | | 200 OK | | |<-------------------| | Call-ID:1 | | INVITE/200 OK/ACK | | |------------------->| Call-ID:1 | NOTIFY (refersuccess) | |<-------------------| | | 200 OK | | |------------------->| | Call-ID:1 | BYE/200 OK | | |------------------->| | Call-ID:1 | | BYE/200 OK | | |<-------------------| Sparks Expires August 3, 2001 [Page 14] Internet-Draft SIP Call Control - Transfer February 2001 4.5.2 Variation 2 : Protects transfer target The transferor places the transferee on hold, establishes a call with the transfer target and then reverses their roles, transferring the original transfer target to the original transferee. This has the advantage of hiding information about the original transfer target from the original transferee. On the other hand, the Transferee's experience is different that in current systems. The Transferee is effectively "called back" by the Transfer Target. Transferor Transferee Transfer | | Target | | | Call-ID:1 | INVITE/200 OK/ACK | | |<-------------------| | Call-ID:1 | INVITE (hold)/200 OK/ACK | |------------------->| | Call-ID:2 | INVITE/200 OK/ACK | | |---------------------------------------->| Call-ID:2 | INVITE (hold)/200 OK/ACK | |---------------------------------------->| Call-ID:2 | REFER | | |---------------------------------------->| | 200 Trying | | |<----------------------------------------| Call-ID:2 | | INVITE/200 OK/ACK | | |<-------------------| Call-ID:2 | NOTIFY (refersuccess) | |<----------------------------------------| | 200 OK | | |------------------->| | Call-ID:1 | BYE/200 OK | | |------------------->| | Call-ID:2 | BYE/200 OK | | |---------------------------------------->| Call-ID:2 | | BYE/200 OK | | |------------------->| 4.5.3 Consultation Hold in the presence of forking proxies It is worth noting that the examples given above abstract away any proxies that might be between the three parties. In 4.5.1 for example, the URL used to reach the Transfer Target may go through a forking proxy. There is no guarantee that the Transferee's and Transferor's invitations to the Transfer Target will reach the same endpoint. If the proxy forked in parallel, both invitations could cause multiple endpoints to ring. To increase the probability of the desired behavior of having the referred invite reach and ring only the same endpoint as the consultation invite, the Transferor SHOULD Sparks Expires August 3, 2001 [Page 15] Internet-Draft SIP Call Control - Transfer February 2001 issue the REFER request with the Refer-To: header containing the Contact the Transfer Target provided in its 200 OK to the Transferor's INVITE. If that REFER fails, the Transferor SHOULD issue another REFER with the Refer-To: header containing the URL it used to reach the Transfer Target, augmented with an Accept-Contact header containing the Contact the Transfer Target provided. 4.6 Attended Transfer In an attended transfer, the three actors participate in an ad-hoc conference as part of the event. Discussion of the implementation of attended transfer is thus deferred until the conferencing portion of the Call Control framework has been addressed. 4.7 Transfer with multiple parties In this example the Originator places call to the Facilitator who reaches the Recipient through the Screener. The Recipient's contact information is exposed to the Facilitator and the Originator. This example is provided for clarification of the semantics of the REFER method only and should not be used as the design of an implementation. Originator Facilitator Screener Recipient Call-ID | | | | 1 |INVITE/200 OK/ACK | |"Get Fred for me!" |----------->| | | "Right away!" 1 |INVITE (hold)/200 OK/ACK | | |<-----------| | | 2 | |INVITE/200 OK/ACK |"I have a call | |----------->| |from Mary for Fred" 2 | |INVITE (hold)/200 OK/ACK "Hold please" | |<-----------| | 3 | | |INVITE/200 OK/ACK | | |--------->|"You have a call | | | |from Mary" | | | | "Put her through" 3 | | |INVITE (hold)/200 OK/ACK | | |--------->| 2 | |REFER | | | |<-----------| | | |200 OK | | | |----------->| | 2 | |INVITE/200 OK/ACK | | |---------------------->|"This is Fred" 2 | |NOTIFY (refersuccess) | "Please hold for | |----------->| | Mary" | |200 OK | | | |<-----------| | Sparks Expires August 3, 2001 [Page 16] Internet-Draft SIP Call Control - Transfer February 2001 2 | |BYE/200 OK | | | |<-----------| | 3 | | |BYE/200 OK| | | |--------->| 2 | |INVITE (hold)/200 OK/ACK | |---------------------->| 1 |REFER | | | |<-----------| | | |200 OK | | | |----------->| | | 1 |INVITE/200 OK/ACK | | |----------------------------------->| "Hey Fred" 1 |NOTIFY (refersuccess) | | "Hello Mary" |----------->| | | |200 OK | | | |<-----------| | | 1 |BYE/200 OK | | | |<-----------| | | 2 | |BYE/200 OK | | | |---------------------->| 1 |BYE/200 OK | | | |<-----------------------------------| "See you later" 5. Open Issues 5.1 200 vs 202 Should an agent accepting a NOTIFY request return a 200 OK or a 202 Accepted? 5.2 Should REFER expire? Since REFER is effectively establishing a Subscription per [3], should the REFER request be required to contain an Expires: header? This would allow REFERer to specify how long he's willing to wait for the reference to complete. If the referred action exceeds that time, the agent processing the refer could return referfailure, stop trying, and free the state it needed for that processing. Without this or a similar mechanism, both agents involved in a REFER transaction could be forced to hold state indefinitely. 5.3 REFER is now dependent on sip-events By introducing NOTIFY, this work is prevented from moving to RFC until the sip-events draft moves to that level (that work is currently an individual submission). What needs to happen to our deliverable schedule to allow for completing the sip-events work? Sparks Expires August 3, 2001 [Page 17] Internet-Draft SIP Call Control - Transfer February 2001 5.4 Registering the events with IANA When we near the end of the process, the events we agree to use for this purpose (currently proposed as refersuccess and referfailure) will need to be registered with IANA per [3]. 6. Acknowledgments This draft is a collaborative product of the SIP working group. The editor thanks the following for their early contributions to this work: Ben Campbell, Chris Cunningham, Steve Donovan, Alan Johnston, Kevin Summers and Dean Willis. References [1] Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg, "SIP: Session Initiation Protocol", RFC 2543, March 1999. [2] Campbell, B., "Framework for SIP Call Control Extensions", draft-ietf-sip-cc-framework-00 (work in progress), March 2000. [3] Roach, A., "Event Notification in SIP", draft-roach-sip-subscibe-notify-02 (work in progress), November 2000. [4] Schulzrinne, H. and J. Rosenberg, "SIP Call Control Services", draft-ietf-sip-cc-01 (work in progress - expired), June 1999. Author's Address Robert J. Sparks dynamicsoft 5100 Tennyson Parkway Suite 1200 Plano, TX 75024 email: rsparks@dynamicsoft.com Sparks Expires August 3, 2001 [Page 18] Internet-Draft SIP Call Control - Transfer February 2001 Full Copyright Statement Copyright (C) The Internet Society (2001). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgement Funding for the RFC editor function is currently provided by the Internet Society. Sparks Expires August 3, 2001 [Page 19]