RFC 7647

Clarifications for the Use of REFER with RFC 6665

📅 published September 2015, from ietf

RFC 7647: Clarifications for the Use of REFER with RFC 6665 💖

okay bestie, this is the title block 🎀 It's an IETF Standards Track doc (ISSN 2070-1721), dated September 2015. The authors are Robert Sparks at Oracle and Adam Roach (A.B. Roach) at Mozilla. It updates RFC 3515 🦋

Abstract 🌸

So the SIP REFER method leans on the SIP-Specific Event Notification framework, and that framework got a glow-up from RFC 6665. This doc is all about the fallout from those requirement changes. It also updates the REFER definition from RFC 3515 so everyone understands exactly what changed, no confusion allowed 💅

Status of This Memo ✨

This is an Internet Standards Track document. 🔥

It's a product of the Internet Engineering Task Force (IETF), so it represents the consensus of the IETF community. It got public review and was approved for publication by the Internet Engineering Steering Group (IESG). If you want more on Internet Standards, check Section 2 of RFC 5741.

Info about the current status, any errata, and how to send feedback lives at http://www.rfc-editor.org/info/rfc7647 🧚

Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved. 💜

This doc is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) as they stood on the publication date. Please actually read those, because they spell out your rights and restrictions. Code Components pulled out of this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions, and they come without warranty as described in the Simplified BSD License 🙈

Table of Contents 🍓

1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
2.  Conventions Used in This Document . . . . . . . . . . . . . .   2
3.  Use of GRUU Is Mandatory  . . . . . . . . . . . . . . . . . .   3
4.  Dialog Reuse Is Prohibited  . . . . . . . . . . . . . . . . .   3
5.  The 202 Response Code Is Deprecated . . . . . . . . . . . . .   4
6.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
  7.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
  7.2.  Informative References  . . . . . . . . . . . . . . . . .   5
Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   6
Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1. Introduction 🦋

Okay so the SIP REFER method is built on the SIP-Specific Event Notification framework, and that framework was revised by [RFC6665]. This doc breaks down what those requirement changes mean in RFC 6665 and updates [RFC3515] so the impact is crystal clear 💅

Here's the tea: 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 inside that dialog [RFC5057]. The "norefersub" extension from [RFC4488] asks to suppress this implicit subscription, but it can't prevent it from being created 😭

Some specialized environments (3GPP is one) use out-of-signaling agreements so in-dialog REFER requests using the RFC 4488 extension don't create a new subscription inside that dialog. In 3GPP, that behavior is based on capabilities advertised with media feature tags. But that mechanism doesn't stop extra dialog usages when you're talking to implementations that don't support it. The extensions in [RFC7614] give us a standardized way to avoid any additional dialog usage at all, and we love to see it 👑

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]. 🧠

3. Use of GRUU Is Mandatory 🔥

Section 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. Main character energy for GRUU 👑

A user agent (UA) accepting a REFER that creates a subscription MUST populate its Contact header field with a GRUU. No exceptions, bestie 💖

A UA that might possibly become a notifier (for example, 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 way, out-of-dialog REFER requests corresponding to any resulting INVITE dialogs actually arrive at this UA 🦋 Extensions can relax this requirement by defining a REFER request that cannot create an implicit subscription. That means the accepting UA doesn't become an RFC 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 GRUU use, plus the requirements in Section 4.5.2 of that document. It's giving strict, and honestly iconic 💅

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]. The exception is when the remote endpoint is an older RFC 3515 implementation that hasn't been updated to conform to RFC 6665 (you can tell because there's no GRUU in the remote target). So 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 it's 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. Plot twist 😭

A user agent that wants 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. User agents accepting REFER MUST be able to process that extension in requests they receive 🫶

If a user agent can be certain that no implicit subscription will be created by 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. A REFER like that gets its Contact header field populated with the dialog's local URI as specified in Section 12 of [RFC3261] 🌈

As described in Section 4.5.2 of [RFC6665], there are cases where a user agent may fall back to sharing existing dialogs for backwards-compatibility purposes. That only applies to a REFER when the peer hasn't provided a GRUU as its Contact in the existing dialog. In other words, when the peer is an RFC 3515 implementation that hasn't been updated to conform with RFC 6665 🍓

5. The 202 Response Code Is Deprecated 💀

Section 8.3.1 of [RFC6665] requires that elements not send a 202 response code to a subscribe request, and to 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 🔥 So, specifically, an element accepting a REFER request MUST NOT reply with a 202 response code, and it 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. Go read those, they matter 😭

7. References 📚

7.1. 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>.

[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>.

Acknowledgements 🫶

Christer Holmberg came up with the wording for the final paragraph of the introduction. Christer Holmberg and Ivo Sedlacek also gave super detailed comments during working group discussion of the document. Thank you, queens 👑

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.com
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