Network Working Group O. Levin Request for Comments: 4488 Microsoft Corporation Category: Standards Track May 2006 Suppression of Session Initiation Protocol (SIP) REFER Method Implicit Subscription Status of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2006).
AbstractThe Session Initiation Protocol (SIP) REFER extension as defined in RFC 3515 automatically establishes a typically short-lived event subscription used to notify the party sending a REFER request about the receiver's status in executing the transaction requested by the REFER. These notifications are not needed in all cases. This specification provides a way to prevent the automatic establishment of an event subscription and subsequent notifications using a new SIP extension header field that may be included in a REFER request. 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 2 3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 2 4. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 3 5. Preventing Forking of REFER Requests . . . . . . . . . . . . . 4 6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5 8. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 10.1. Normative References . . . . . . . . . . . . . . . . . . 7 10.2. Informative References . . . . . . . . . . . . . . . . . 7
3] specifies that every REFER creates an implicit subscription between the REFER-Issuer and the REFER- Recipient. This document defines a new SIP header field: "Refer-Sub" meaningful within a REFER transaction only. This header field, when set to "false", specifies that a REFER-Issuer requests that the REFER- Recipient doesn't establish an implicit subscription and the resultant dialog. This document defines a new option tag: "norefersub". This tag, when included in the Supported header field, indicates that a User Agent (UA) is capable of accepting a REFER request without creating an implicit subscription when acting as a REFER-Recipient. RFC 2119 . To simplify discussions of the REFER method and its extensions, the three terms below are being used throughout the document: o REFER-Issuer: the UA issuing the REFER request o REFER-Recipient: the UA receiving the REFER request o REFER-Target: the UA designated in the Refer-To Uniform Resource Identifier (URI) 4] requests. Where the
REFER-Issuer explicitly specifies that forking not occur, the requirement that an implicit subscription be established is unnecessary. Another purpose of the NOTIFY is to inform the REFER-Issuer of the progress of the SIP transaction that results from the REFER at the REFER-Recipient. In the case where the REFER-Issuer is already aware of the progress of the requested operation, such as when the REFER- Issuer has an explicit subscription to the dialog event package at the REFER-Recipient, the implicit subscription and resultant NOTIFY traffic related to the REFER can create an unnecessary network overhead. 2]. The additions to this table are also provided for extension methods at the time of publication of this document. This is provided as a courtesy to the reader and is not normative in any way: Header field where proxy ACK BYE CAN INV OPT REG MSG Refer-Sub R, 2xx - - - - - - - Header field where SUB NOT REF INF UPD PRA PUB Refer-Sub R, 2xx - - o - - - - The Refer-Sub header field MAY be encrypted as part of end-to-end encryption. The syntax of the header field follows the BNF defined below: Refer-Sub = "Refer-Sub" HCOLON refer-sub-value *(SEMI exten) refer-sub-value = "true" / "false" exten = generic-param where the syntax of generic-param is defined in .
The "Refer-Sub" header field set to "false" MAY be used by the REFER- Issuer only when the REFER-Issuer can be certain that the REFER request will not be forked. If the REFER-Recipient supports the extension and is willing to process the REFER transaction without establishing an implicit subscription, it MUST insert the "Refer-Sub" header field set to "false" in the 2xx response to the REFER-Issuer. In this case, no implicit subscription is created. Consequently, no new dialog is created if this REFER was issued outside any existing dialog. If the REFER-Issuer inserts the "Refer-Sub" header field set to "false", but the REFER-Recipient doesn't grant the suggestion (i.e., either does not include the "Refer-Sub" header field or includes the "Refer-Sub" header field set to "true" in the 2xx response), an implicit subscription is created as in the default case. This document also defines a new option tag, "norefersub". This tag, when included in the Supported header field, specifies that a User Agent (UA) is capable of accepting a REFER request without creating an implicit subscription when acting as a REFER-Recipient. The REFER-Issuer can know the capabilities of the REFER-Recipient from the presence of the option tags in the Supported header field of the dialog initiating request or response. Another way of learning the capabilities would be by using presence, such as defined in . However, if the capabilities of the REFER-Recipient are not known, using the "norefersub" tag with the Require header field is NOT RECOMMENDED. This is due to the fact that in the event the REFER- Recipient doesn't support the extension, in order to fall back to the normal REFER, the REFER-Issuer will need to issue a new REFER transaction thus resulting in additional round-trips. As described in Section 126.96.36.199 in , a REFER-Recipient will reject a REFER request containing a Require: norefersub header field with a 420 (Bad Extension) response unless it supports this extension. Note that Require: norefersub can be present with a Refer-Sub: false header field.
If a REFER request to a given Request-URI might fork, the REFER- Issuer SHOULD NOT include the "Refer-Sub" header field. The REFER- Issuer SHOULD use standardized mechanisms for ensuring the REFER request does not fork. In absence of any other mechanism, the Request-URI of the REFER request SHOULD have Globally Routable User Agent URI (GRUU) properties according to the definitions of  as those properties ensure the request will not fork. 7] document are reused, specifically the <allOneLine> tag. REFER sip:email@example.com SIP/2.0 Via: SIP/2.0/TCP issuer.example.com;branch=z9hG4bK-a-1 From: <sip:firstname.lastname@example.org>;tag=1a <allOneLine> To: sip:email@example.com;opaque=urn:uuid:f8 1d4fae-7dec-11d0-a765-00a0c91e6bf6;grid=99a </allOneLine> Call-ID: firstname.lastname@example.org CSeq: 234234 REFER Max-Forwards: 70 Refer-To: <sip:email@example.com;method=INVITE> Refer-Sub: false Supported: norefersub Contact: sip:firstname.lastname@example.org Content-Length: 0 RFC 3515  and the corresponding response. The following information has been added to the SIP Header field sub-registry in the SIP Parameters Registry: o Header Name: Refer-Sub o Compact Form: None o Reference: RFC 4488 This document also registers a new SIP option tag, "norefersub", adding it to the SIP Option Tags sub-registry in the SIP Parameters Registry. The required information for this registration, as specified in RFC 3261 , is:
 Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  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.  Sparks, R., "The Session Initiation Protocol (SIP) Refer Method", RFC 3515, April 2003.  Roach, A.B., "Session Initiation Protocol (SIP)-Specific Event Notification", RFC 3265, June 2002.  Rosenberg, J., "Obtaining and Using Globally Routable User Agent (UA) URIs (GRUU) in the Session Initiation Protocol (SIP)", Work in Progress, October 2005.  Lonnfors, M. and K. Kiss, "Session Initiation Protocol (SIP) User Agent Capability Extension to Presence Information Data Format (PIDF)", Work in Progress, January 2006.  Sparks, R., Ed., Hawrylyshen, A., Johnston, A., Rosenberg, J., and H. Schulzrinne, "Session Initiation Protocol (SIP) Torture Test Messages", RFC 4475, May 2006.
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