Request for Comments: 4488
Category: Standards Track
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 © The Internet Society (2006).
The 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.
Table of Contents
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
The REFER specification  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.
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 RFC 2119 .
To simplify discussions of the REFER method and its extensions, the three terms below are being used throughout the document:
- REFER-Issuer: the UA issuing the REFER request
- REFER-Recipient: the UA receiving the REFER request
- REFER-Target: the UA designated in the Refer-To Uniform Resource Identifier (URI)
The REFER specification mandates that every REFER creates an implicit subscription between the REFER-Issuer and the REFER-Recipient. This subscription results in at least one NOTIFY being sent from the REFER-Recipient to the REFER-Issuer. The REFER-Recipient may choose to cancel the implicit subscription with this NOTIFY. The REFER- Issuer may choose to cancel this implicit subscription with an explicit SUBSCRIBE (Expires: 0) after receipt of the initial NOTIFY.
One purpose of requiring the implicit subscription and initial NOTIFY is to allow for the situation where the REFER request gets forked and the REFER-Issuer needs a way to see the multiple dialogs that may be established as a result of the forked REFER. This is the same approach used to handle forking of SUBSCRIBE  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.
This document defines a new SIP header field: "Refer-Sub". This header field is meaningful and MAY be used with a REFER request and the corresponding 2XX response only. This header field set to "false" specifies that a REFER-Issuer requests that the REFER- Recipient doesn't establish an implicit subscription and the resultant dialog. Note that when using this extension, the REFER remains a target refresh request (as in the default case -- when the extension is not used).
This document adds the following entry to Table 2 of . 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 18.104.22.168 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.
5. Preventing Forking of REFER Requests
The REFER specification allows for the possibility of forking a REFER request that is sent outside of an existing dialog. In addition, a proxy may fork an unknown method type. Should forking occur, the sender of the REFER with "Refer-Sub" will not be aware as only a single 2xx response will be forwarded by the forking proxy. As a result, the responsibility is on the issuer of the REFER with "Refer- Sub" to ensure that no forking will result.
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.
An example of REFER that suppresses the implicit subscription is shown below. Note that the conventions used in the SIP Torture Test Messages  document are reused, specifically the <allOneLine> tag.
REFER sip:firstname.lastname@example.org SIP/2.0 Via: SIP/2.0/TCP issuer.example.com;branch=z9hG4bK-a-1 From: <sip:email@example.com>;tag=1a <allOneLine> To: sip:firstname.lastname@example.org;opaque=urn:uuid:f8 1d4fae-7dec-11d0-a765-00a0c91e6bf6;grid=99a </allOneLine> Call-ID: email@example.com CSeq: 234234 REFER Max-Forwards: 70 Refer-To: <sip:firstname.lastname@example.org;method=INVITE> Refer-Sub: false Supported: norefersub Contact: sip:email@example.com Content-Length: 0
7. IANA Considerations
This document registers a new SIP header field "Refer-Sub". This header field is only meaningful for the REFER request defined in RFC 3515  and the corresponding response. The following information has been added to the SIP Header field sub-registry in the SIP Parameters Registry:
- Header Name: Refer-Sub
- Compact Form: None
- 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:
- Name: norefersub
- Description: This option tag specifies a User Agent ability of accepting a REFER request without establishing an implicit subscription (compared to the default case defined in RFC 3515 ).
8. Security Considerations
The purpose of this SIP extension is to modify the expected behavior of the REFER-Recipient. The change in behavior is for the REFER- Recipient not to establish a dialog and not to send NOTIFY messages back to the REFER-Issuer. As such, a malicious inclusion of a "Refer-Sub" header field set to "false" reduces the processing and state requirements on the recipient. As a result, its use in a denial-of-service attack seems limited.
On the other hand, by inserting a "Refer-Sub" header field set to "false", a man-in-the-middle (MitM) can potentially exploit the mechanism for easier (than an interception) suppression of the notifications from the REFER-Receiver without the REFER-Issuer noticing it. Also, by removing a "Refer-Sub" header field set to "false", a MitM can cause the REFER-Receiver to generate notifications over the implicit dialog that otherwise had been suppressed by the REFER-Issuer.
To protect against these kinds of MitM attacks, integrity protection should be used. For example, the REFER-Issuer could use S/MIME as discussed in RFC 3261  to protect against these kinds of attacks.
The SIP community would like to thank Sriram Parameswar for his ideas, originally presented in "Suppressing Refer Implicit Subscription" (October 2002), which served as the basis for this specification.
10.1. Normative References
 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.
10.2. Informative References
 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.
Orit Levin Microsoft Corporation One Microsoft Way Redmond, WA 98052 USA Phone: 425-722-2225 EMail: firstname.lastname@example.org
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