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Internet Engineering Task Force (IETF)                         R. SparksRequest for Comments: 7647                                        OracleUpdates:3515                                                 A.B. RoachCategory: Standards Track                                        MozillaISSN: 2070-1721                                           September 2015Clarifications for the Use of REFER withRFC 6665Abstract   The SIP REFER method relies on the SIP-Specific Event Notification   framework.  That framework was revised byRFC 6665.  This document   highlights the implications of the requirement changes inRFC 6665,   and updates the definition of the REFER method described inRFC 3515   to clarify and disambiguate the impact of those changes.Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7647.Copyright Notice   Copyright (c) 2015 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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.Sparks & Roach               Standards Track                    [Page 1]

RFC 7647                  Refer Clarifications            September 2015Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Conventions Used in This Document . . . . . . . . . . . . . .23.  Use of GRUU Is Mandatory  . . . . . . . . . . . . . . . . . .34.  Dialog Reuse Is Prohibited  . . . . . . . . . . . . . . . . .35.  The 202 Response Code Is Deprecated . . . . . . . . . . . . .46.  Security Considerations . . . . . . . . . . . . . . . . . . .47.  References  . . . . . . . . . . . . . . . . . . . . . . . . .47.1.  Normative References  . . . . . . . . . . . . . . . . . .47.2.  Informative References  . . . . . . . . . . . . . . . . .5   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .6   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .61.  Introduction   The SIP REFER method relies on the SIP-Specific Event Notification   framework.  That framework was revised by [RFC6665].  This document   highlights the implications of the requirement changes inRFC 6665,   and updates [RFC3515] to clarify and disambiguate the impact of those   changes.   Accepting a REFER request (without invoking extensions) results in an   implicit SIP-Events subscription.  If that REFER was part of an   existing dialog, the implicit subscription creates a new, problematic   dialog usage within that dialog [RFC5057].  The "norefersub"   extension defined in [RFC4488] asks to suppress this implicit   subscription, but cannot prevent its creation.   There are implementations in some known specialized environments   (such as 3GPP) that use out-of-signaling agreements to ensure that   in-dialog REFER requests using theRFC 4488 extension do not create a   new subscription inside that dialog.  In the 3GPP environment, the   behavior is based on capabilities advertised using media feature   tags.  That mechanism does not, however, prevent additional dialog   usages when interoperating with implementations that do not support   the mechanism.  The extensions in [RFC7614] provide a standardized   mechanism that allows avoiding any additional dialog usage.2.  Conventions Used in This Document   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].Sparks & Roach               Standards Track                    [Page 2]

RFC 7647                  Refer Clarifications            September 20153.  Use of GRUU Is MandatorySection 4.5.1 of [RFC6665] makes GRUU [RFC5627] mandatory for   notifiers to implement and use as the local target in the   subscription created by the REFER request.   A user agent (UA) accepting a REFER that creates a subscription MUST   populate its Contact header field with a GRUU.   A UA that might possibly become a notifier (e.g., by accepting a   REFER request that creates a subscription) needs to include a GRUU in   the Contact header field of dialog-forming and target-refresh methods   (such as INVITE) [RFC7621].  This ensures that out-of-dialog REFER   requests corresponding to any resulting INVITE dialogs arrive at this   UA.  Extensions can relax this requirement by defining a REFER   request that cannot create an implicit subscription, thus not causing   the accepting UA to become anRFC 6665 notifier in the context of   this dialog.  [RFC7614] is an example of such an extension.4.  Dialog Reuse Is Prohibited   If a peer in an existing dialog has provided a GRUU as its Contact,   sending a REFER that might result in an additional dialog usage   within that dialog is prohibited.  This is a direct consequence of   [RFC6665] requiring the use of GRUU and the requirements inSection 4.5.2 of that document.   A user agent constructing a REFER request that could result in an   implicit subscription in a dialog MUST build it as an out-of-dialog   message as defined in [RFC3261], unless the remote endpoint is an   older implementation ofRFC 3515 that has not been updated to conform   toRFC 6665 (as determined by the absence of a GRUU in the remote   target).  Thus, the REFER request will have no tag parameter in its   To: header field.   Using the "norefersub" option tag [RFC4488] does not change this   requirement, even if used in a "Require" header field.  Even if the   recipient supports the "norefersub" mechanism, and accepts the   request with the option tag in the "Require" header field, it is   allowed to return a "Refer-Sub" header field with a value of "true"   in the response, and create an implicit subscription.   A user agent wishing to identify an existing dialog (such as for call   transfer as defined in [RFC5589]) MUST use the "Target-Dialog"   extension defined in [RFC4538] to do so, and user agents accepting   REFER MUST be able to process that extension in requests they   receive.Sparks & Roach               Standards Track                    [Page 3]

RFC 7647                  Refer Clarifications            September 2015   If a user agent can be certain that no implicit subscription will be   created as a result of sending a REFER request (such as by requiring   an extension that disallows any such subscription [RFC7614]), the   REFER request MAY be sent within an existing dialog (whether or not   the remote target is a GRUU).  Such a REFER will be constructed with   its Contact header field populated with the dialog's local URI as   specified inSection 12 of [RFC3261].   As described inSection 4.5.2 of [RFC6665], there are cases where a   user agent may fall back to sharing existing dialogs for backwards-   compatibility purposes.  This applies to a REFER only when the peer   has not provided a GRUU as its Contact in the existing dialog (i.e.,   when the peer is an implementation ofRFC 3515 that has not been   updated to conform withRFC 6665).5.  The 202 Response Code Is DeprecatedSection 8.3.1 of [RFC6665] requires that elements not send a 202   response code to a subscribe request, but use the 200 response code   instead.  Any 202 response codes received to a subscribe request are   treated as 200s.  These changes also apply to REFER.  Specifically,   an element accepting a REFER request MUST NOT reply with a 202   response code and MUST treat any 202 responses received as identical   to a 200 response.  Wherever [RFC3515] requires sending a 202   response code, a 200 response code MUST be sent instead.6.  Security Considerations   This document introduces no new security considerations directly.   The updated considerations in [RFC6665] apply to the implicit   subscription created by an accepted REFER request.7.  References7.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119,              DOI 10.17487/RFC2119, March 1997,              <http://www.rfc-editor.org/info/rfc2119>.   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,              A., Peterson, J., Sparks, R., Handley, M., and              E. Schooler, "SIP: Session Initiation Protocol",RFC 3261,              DOI 10.17487/RFC3261, June 2002,              <http://www.rfc-editor.org/info/rfc3261>.Sparks & Roach               Standards Track                    [Page 4]

RFC 7647                  Refer Clarifications            September 2015   [RFC3515]  Sparks, R., "The Session Initiation Protocol (SIP) Refer              Method",RFC 3515, DOI 10.17487/RFC3515, April 2003,              <http://www.rfc-editor.org/info/rfc3515>.   [RFC4538]  Rosenberg, J., "Request Authorization through Dialog              Identification in the Session Initiation Protocol (SIP)",RFC 4538, DOI 10.17487/RFC4538, June 2006,              <http://www.rfc-editor.org/info/rfc4538>.   [RFC5627]  Rosenberg, J., "Obtaining and Using Globally Routable User              Agent URIs (GRUUs) in the Session Initiation Protocol              (SIP)",RFC 5627, DOI 10.17487/RFC5627, October 2009,              <http://www.rfc-editor.org/info/rfc5627>.   [RFC6665]  Roach, A.B., "SIP-Specific Event Notification",RFC 6665,              DOI 10.17487/RFC6665, July 2012,              <http://www.rfc-editor.org/info/rfc6665>.   [RFC7621]  Roach, A.B., "A Clarification on the Use of Globally              Routable User Agent URIs (GRUUs) in the SIP Event              Notification Framework",RFC 7621, DOI 10.17487/RFC7621,              August 2015, <http://www.rfc-editor.org/info/rfc7621>.7.2.  Informative References   [RFC4488]  Levin, O., "Suppression of Session Initiation Protocol              (SIP) REFER Method Implicit Subscription",RFC 4488,              DOI 10.17487/RFC4488, May 2006,              <http://www.rfc-editor.org/info/rfc4488>.   [RFC5057]  Sparks, R., "Multiple Dialog Usages in the Session              Initiation Protocol",RFC 5057, DOI 10.17487/RFC5057,              November 2007, <http://www.rfc-editor.org/info/rfc5057>.   [RFC5589]  Sparks, R., Johnston, A., Ed., and D. Petrie, "Session              Initiation Protocol (SIP) Call Control - Transfer",BCP 149,RFC 5589, DOI 10.17487/RFC5589, June 2009,              <http://www.rfc-editor.org/info/rfc5589>.   [RFC7614]  Sparks, R., "Explicit Subscriptions for the REFER Method",RFC 7614, DOI 10.17487/RFC7614, August 2015,              <http://www.rfc-editor.org/info/rfc7614>.Sparks & Roach               Standards Track                    [Page 5]

RFC 7647                  Refer Clarifications            September 2015Acknowledgements   Christer Holmberg provided the formulation for the final paragraph of   the introduction.  Christer Holmberg and Ivo Sedlacek provided   detailed comments during working group discussion of the document.Authors' Addresses   Robert Sparks   Oracle   7460 Warren Parkway   Suite 300   Frisco, Texas  75034   United States   Email: rjsparks@nostrum.com   Adam Roach   Mozilla   Dallas, TX   United States   Phone: +1 650 903 0800 x863   Email: adam@nostrum.comSparks & Roach               Standards Track                    [Page 6]