RFC 5368
Referring to Multiple Resources in the Session Initiation Protocol (SIP)
RFC 5368: Referring to Multiple Resources in the Session Initiation Protocol (SIP) 🎀
okay bestie, let's get into RFC 5368! 🦋 it's titled Referring to Multiple Resources in the Session Initiation Protocol (SIP), it came out from the Network Working Group in October 2008, and it's a Standards Track document. the authors are G. Camarillo (Ericsson), A. Niemi (Nokia), M. Isomaki (Nokia), M. Garcia-Martin (Ericsson), and H. Khartabil (Ericsson Australia) 👑 the title block doesn't list anything it obsoletes or updates, so there's nothing like that to track ✨
Status of This Memo 📜
this doc is an Internet standards track protocol for the Internet community, and it's asking for discussion and suggestions for improvements 💬 for the current standardization state and status of this protocol, check the "Internet Official Protocol Standards" (STD 1) 🔍 and distribution of this memo is unlimited 🌸
Abstract 💖
so this doc adds extensions to the SIP REFER method so it can refer to multiple resources in one single request 🔥 the extensions are pointers to Uniform Resource Identifier (URI) lists in the Refer-To header field, plus the "multiple-refer" SIP option-tag ✨
Table of Contents 🧚
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Overview of Operation . . . . . . . . . . . . . . . . . . . . 3
4. The multiple-refer SIP Option-Tag . . . . . . . . . . . . . . 4
5. Suppressing REFER's Implicit Subscription . . . . . . . . . . 4
6. URI-List Format . . . . . . . . . . . . . . . . . . . . . . . 5
7. Behavior of SIP REFER-Issuers . . . . . . . . . . . . . . . . 6
8. Behavior of REFER-Recipients . . . . . . . . . . . . . . . . . 6
9. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
10. Security Considerations . . . . . . . . . . . . . . . . . . . 10
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
12.1. Normative References . . . . . . . . . . . . . . . . . . 10
12.2. Informative References . . . . . . . . . . . . . . . . . 111. Introduction ✨
okay so RFC 3261 (SIP) [RFC3261] got extended by RFC 3515 [RFC3515] with a REFER method 💅 it lets a user agent (UA) ask a second UA to send a SIP request to a third party 🫶 picture this: Alice is on a call with Bob and decides Bob needs to talk to Carol 👀 Alice's SIP UA sends a REFER request to Bob's UA with Carol's SIP Contact information. if Bob gives it permission, Bob's UA tries to call Carol using that contact, meaning it sends an INVITE request to that contact 📞
a bunch of apps need the second UA to start transactions toward a whole set of destinations 🍓 like, the moderator of a conference might want the conference server to send BYE requests to a group of participants 😭 or that same moderator might want the conference server to INVITE a set of new participants 🎉
so we define an extension to the REFER method so REFER requests can refer other user agents (like conference servers) to multiple destinations 🦋 on top of that, this mechanism uses the suppression of the REFER method implicit subscription specified in RFC 4488 [RFC4488] 💜
2. Terminology 📚
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 BCP 14, RFC 2119 [RFC2119] and indicate requirement levels for compliant implementations. 🔑
this doc reuses these terms from RFC 3261 [RFC3261]:
- User Agent (UA)
- User Agent Client (UAC)
- User Agent Server (UAS)
and it defines these new terms 🌈
- REFER-Issuer: a user agent issuing a REFER request.
- REFER-Recipient: an entity that receives a REFER request and forwards a SIP request to a number of REFER-Targets. the REFER-Recipient is typically a network entity, like a URI-list server, that acts as a UAS for REFER requests and as a UAC for other SIP requests 🧚
- REFER-Target: a UA of the intended final recipient of a SIP request generated by the REFER-Recipient 🎯
3. Overview of Operation 🌸
this doc is an application of URI-list services [RFC5363], and it lets a URI-list service receive a SIP REFER request that has a list of targets in it 📋 the URI-list service invokes the requested SIP method toward each target on the list 💫 this kind of URI-list service is called a REFER-Recipient all throughout this doc 👑
this doc defines an extension to the SIP REFER method from RFC 3515 [RFC3515] 🎀 it lets a SIP UAC put a URI list, as specified in RFC 4826 [RFC4826], of REFER-Targets into a REFER request and send it to a REFER-Recipient 💌 the REFER-Recipient creates a new SIP request for each entry in the URI list and sends it to each REFER-Recipient 🤔
the URI list with the targets gets used together with RFC 5364 [RFC5364] so the sender can indicate the role (e.g., 'to', 'cc', or anonymous) the REFER-Target plays in the signalling 🫶
we represent multiple targets of a REFER request with a URI list as specified in RFC 4826 [RFC4826] 📝 a REFER-Issuer that wants to refer a REFER-Recipient to a set of destinations creates a SIP REFER request 🦋 the Refer-To header contains a pointer to a URI list, which is included in a body part, and there's an option-tag in the Require header field: "multiple-refer" 💖 that option-tag means the requirement to support the functionality in this spec ✨
when the REFER-Recipient gets such a request, it creates a new request per REFER-Target and sends them, one to each REFER-Target 🚀
this doc does not give REFER-Issuers any mechanism to find out the results of a REFER request with multiple REFER-Targets 😭 it also doesn't support the implicit subscription mechanism that's part of the SIP REFER method 🙈 how REFER-Issuers get kept informed about the results of a REFER is service specific 🧠 for example, a REFER-Issuer sending a REFER request to invite a set of participants to a conference can find out which participants got successfully brought into the conference by subscribing to the conference state event package specified in RFC 4575 [RFC4575] 🎊
4. The multiple-refer SIP Option-Tag 🏷️
we define a new SIP option-tag for the Require and Supported header fields: multiple-refer 💅
a user agent that includes the multiple-refer option-tag in a Supported header field is showing it complies with this spec ✅
a user agent generating a REFER with a pointer to a URI list in its Refer-To header field MUST include the multiple-refer option-tag in the Require header field of the REFER 🔒
5. Suppressing REFER's Implicit Subscription 🙅♀️
REFER requests with a single REFER-Target implicitly establish a subscription to the refer event 📬 the REFER-Issuer gets informed about the result of the transaction toward the REFER-Target through this implicit subscription 💌 as RFC 3515 [RFC3515] describes, NOTIFY requests sent because of an implicit subscription created by a REFER request contain a body of type message/sipfrag, RFC 3420 [RFC3420], which describes the status of the transaction the REFER-Recipient started 🌈
now, when a REFER-Issuer generates a REFER with multiple REFER-targets, the REFER-Issuer is usually already subscribed to other event packages that can give the info about the results of the transactions toward the REFER-Targets 🧩 like, a moderator telling a conference server to send a BYE request to a set of participants is usually subscribed to the conference state event package for the conference 🎪 notifications to that event package keep the moderator and the rest of the subscribers updated on the current list of conference participants 👑
most apps using the multiple REFER technology described in this memo don't need its implicit subscription 🙌 so a SIP REFER-Issuer generating a REFER request with multiple REFER-Targets SHOULD include the norefersub option-tag in a Require header field, and SHOULD include a Refer-Sub header field set to false to say that no notifications about the requests should be sent to the REFER-Issuer 🫧 the REFER-Recipient SHOULD honor that suggestion and also include a Refer-Sub header field set to false in the 200 (OK) response 💫 the norefersub SIP option-tag and the Refer-Sub header field are specified in RFC 4488 [RFC4488] 📖
RFC 4488 [RFC4488] says that one condition for the REFER-Issuer to include a Refer-Sub header is that the REFER-Issuer is sure the REFER request will not fork. 👀
at the time of writing, there's no extension that lets you report the status of several transactions over the implicit subscription tied to a REFER dialog 😤 that's the reason this doc recommends using the norefersub option-tag 💜 if a future extension defines that, REFER-Issuers using it could skip the norefersub option-tag and use the new extension instead ✨
6. URI-List Format 🎀
Okay so, RFC 5363 [RFC5363] says that every URI-list service has to spell out a default format for its 'recipient-list' bodies 💅. That's just the rule for this whole family of services.
For REFER-Issuers and REFER-Recipients, the default format is RFC 4826 [RFC4826] extended with RFC 5364 [RFC5364] ✨. REFER-Recipients that handle 'recipient-list' bodies MUST support both of these formats, and both REFER-Issuers and REFER-Recipients MAY support other formats too 🦋.
RFC 5364 [RFC5364] lets each URI get tagged with a 'copyControl' attribute set to "to", "cc", or "bcc" 💌. That tells you what role the target plays when it receives the referred SIP request. Bestie, though, a 'copyControl' attribute doesn't always make sense. It can go on INVITE requests, but it's nonsense on mid-dialog requests like BYE 🙈.
URIs can also get the 'anonymize' attribute (also from RFC 5364 [RFC5364]). That stops the REFER-Recipient from showing the target URI in a URI list 🫶.
RFC 5364 [RFC5364] also defines a 'recipient-list-history' body, which holds the list of targets 📜. For conference services, the default format for that body is again RFC 4826 [RFC4826] extended with RFC 5364 [RFC5364]. REFER-Recipients that support this specification MUST support both of these formats, while REFER-Targets MAY support them. Both REFER-Recipients and REFER-Targets MAY support other formats as well 🌸.
Now, RFC 4826 [RFC4826] has extra features like hierarchical lists and entries that reference the XML Configuration Access Protocol (XCAP) root URI. The multiple REFER service in this document doesn't need any of that 😭. Lowkey extra, but it's fine.
Figure 1 shows a flat list that follows the resource list document 🌈.
<?xml version="1.0" encoding="UTF-8"?>
<resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists"
xmlns:cp="urn:ietf:params:xml:ns:copycontrol">
<list>
<entry uri="sip:bill@example.com" cp:copyControl="to" />
<entry uri="sip:joe@example.org" cp:copyControl="cc" />
<entry uri="sip:ted@example.net" cp:copyControl="bcc" />
</list>
</resource-lists>
Figure 1: URI list7. Behavior of SIP REFER-Issuers 🧚
As we covered in Sections 4 and 5, a SIP REFER-Issuer that makes a REFER request with multiple REFER-Targets puts "multiple-refer" and "norefersub" in the Require header field 💖. If it makes sense, it also sets a Refer-Sub header field to "false" ✨.
The REFER-Issuer then packs the set of REFER-Targets into a recipient-list body whose disposition type is 'recipient-list', as defined in RFC 5363 [RFC5363] 📦. Section 6 has more on that URI-list body.
The Refer-To header field of a REFER request with multiple REFER-Targets MUST contain a pointer. That's a Content-ID Uniform Resource Locator (URL) as per RFC 2392 [RFC2392], and it has to point at the body part carrying the URI list 🎯. The REFER-Issuer SHOULD NOT include any particular URI more than once in the URI list 🙅♀️.
RFC 4826 [RFC4826] has hierarchical lists and entries by reference relative to the XCAP root URI. Neither of those is needed here 💅. So when the default resource list document is in use, SIP REFER-Issuers generating REFER requests with multiple REFER-Targets SHOULD use flat lists (no hierarchical lists) and SHOULD NOT use <entry-ref> elements 🌷.
8. Behavior of REFER-Recipients 👑
The REFER-Recipient follows the rules in Section 2.4.2 of RFC 3515 [RFC3515] to figure out the status code of the response to the REFER 📮.
The REFER-Recipient SHOULD not create an implicit subscription, and it SHOULD add a Refer-Sub header field set to "false" in the 200 OK response 🙌.
The incoming REFER request usually has a URI-list document, or a reference to one, with the actual list of targets 📃. If that URI list includes resources tagged with 'copyControl' set to "to" or "cc", and the request is appropriate for the service (for example, it's not received mid-dialog), the REFER-Recipient SHOULD include a URI list in each of the outgoing requests 💫. That list SHOULD be formatted according to RFC 4826 [RFC4826] and RFC 5364 [RFC5364]. The REFER-Recipient MUST follow the procedures in RFC 4826 [RFC4826] for handling the 'anonymize', 'count', and 'copyControl' attributes 🧾.
Section 4 of RFC 5363 [RFC5363] talks about what to do when duplicated URIs show up in a URI list 👯. To avoid duplicated requests, REFER-Recipients MUST take those actions from RFC 5363 [RFC5363] into account so they don't send a duplicate request to the same target 🚫.
If the REFER-Recipient includes a URI list in an outgoing request, it MUST include a Content-Disposition header field, specified in RFC 2183 [RFC2183], with the value set to 'recipient-list-history'. It also needs a 'handling' parameter, specified in RFC 3204 [RFC3204], set to "optional" 🏷️.
The multiple REFER service doesn't use hierarchical lists or lists with entries that reference the XCAP root URI 🙃. So a REFER-Recipient that gets a URI list with more information than Section 6 describes MAY discard all of the extra information 🗑️.
Last thing: the REFER-Recipient follows the rules in RFC 3515 [RFC3515] to generate the requests towards the REFER-Targets. It acts as if it got a regular REFER (no URI list) for each URI in the URI list 🔁.
9. Example 🎬
Figure 2 shows an example flow where a REFER-Issuer sends a multiple-REFER request to the focus of a conference, which acts as the REFER-Recipient 🎀. The REFER-Recipient generates a BYE request per REFER-Target 👋. Details on using a REFER request to remove participants from a conference are in RFC 4579 [RFC4579] 📚.
+--------+ +---------+ +--------+ +--------+ +--------+
| REFER | | REFER | | REFER | | REFER | | REFER |
| issuer | |recipient| |target 1| |target 2| |target 3|
| | | | | | | | | |
| Carol | | (focus) | | Bill | | Joe | | Ted |
+--------+ +---------+ +--------+ +--------+ +--------+
| 1. REFER | | | |
| ---------------->| | | |
| 2. 202 Accepted | | | |
|<---------------- | 3. BYE | | |
| | ----------->| | |
| | 4. BYE | | |
| | ----------------------->| |
| | 5. BYE | | |
| | ----------------------------------->|
| | 6. 200 OK | | |
| |<----------- | | |
| | 7. 200 OK | | |
| |<----------------------- | |
| | 8. 200 OK | | |
| |<----------------------------------- |
| | | | |
| | | | |
| | | | |
Figure 2: Example flow of a REFER request containing
multiple REFER-TargetsThe REFER request (1) has a Refer-To header field that points to the message body, which holds a list of the URIs of the REFER-Targets 📝. In this example, the URI list doesn't include the 'copyControl' attribute extension 🙂. The REFER's Require header field carries the "multiple-refer" and "norefersub" option-tags 🏃♀️. The Request-URI is set to a Globally Routable User Agent URI (GRUU) [SIP-GRUU], which guarantees the REFER request won't fork 🛡️. The Refer-Sub header field is set to "false" to ask for the implicit subscription to be suppressed 🤫. Figure 3 shows what this REFER request looks like. The resource list document holds the list of REFER-Target URIs, along with the method of the SIP request that the REFER-Recipient generates 💜.
REFER sip:conf-123@example.com;gruu;opaque=hha9s8d-999a SIP/2.0
Via: SIP/2.0/TCP client.chicago.example.com
;branch=z9hG4bKhjhs8ass83
Max-Forwards: 70
To: "Conference 123" <sip:conf-123@example.com>
From: Carol <sip:carol@chicago.example.com>;tag=32331
Call-ID: d432fa84b4c76e66710
CSeq: 2 REFER
Contact: <sip:carol@client.chicago.example.com>
Refer-To: <cid:cn35t8jf02@example.com>
Refer-Sub: false
Require: multiple-refer, norefersub
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY
Allow-Events: dialog
Accept: application/sdp, message/sipfrag
Content-Type: application/resource-lists+xml
Content-Disposition: recipient-list
Content-Length: 362
Content-ID: <cn35t8jf02@example.com>
<?xml version="1.0" encoding="UTF-8"?>
<resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<list>
<entry uri="sip:bill@example.com?method=BYE" />
<entry uri="sip:joe@example.org?method=BYE" />
<entry uri="sip:ted@example.net?method=BYE" />
</list>
</resource-lists>
Figure 3: REFER request with multiple REFER-TargetsFigure 4 shows the BYE request (3) that the REFER-Recipient sends to the first REFER-Target 👣.
BYE sip:bill@example.com SIP/2.0
Via: SIP/2.0/TCP conference.example.com
;branch=z9hG4bKhjhs8assmm
Max-Forwards: 70
From: "Conference 123" <sip:conf-123@example.com>;tag=88734
To: <sip:bill@example.com>;tag=29872
Call-ID: d432fa84b4c34098s812
CSeq: 34 BYE
Content-Length: 0
Figure 4: BYE request10. Security Considerations 🔐
Heads up, this part is serious bestie 😤. RFC 5363 [RFC5363] covers the issues tied to SIP URI-list services. A REFER-Recipient that accepts REFER requests with multiple REFER-targets is acting as a URI-list service, so implementations of that server MUST follow the security-related rules in RFC 5363 [RFC5363] 🛑. Those rules include opt-in lists and mandatory authentication and authorization of clients 🔒.
Real talk: these rules aren't optional extras. Opt-in lists plus mandatory auth and authz are the baseline, or your server is wide open 🚨.
REFER-Recipients SHOULD only accept REFER requests within the context of an application the REFER-Recipient understands (for example, a conferencing application) 🧠. That means REFER-Recipients MUST NOT accept REFER requests for methods they do not understand 🙅♀️. The idea behind these two rules is that REFER-Recipients should not be used as dumb servers whose only job is to fan out random messages they don't understand 📣. Iconic behaviour, honestly 💅✨.
11. IANA Considerations ✨
okay so this section is about registration paperwork 📋 and honestly it's kinda iconic. This document defines a new SIP option-tag called "multiple-refer" 🎀 and it's been registered in the SIP Parameters registry, so it's officially on the list now 💅
The "Option Tags" section of the SIP Parameter Registry got a brand new row, and it looks like this:
+-----------------+-------------------------------------+-----------+
| Name | Description | Reference |
+-----------------+-------------------------------------+-----------+
| multiple-refer | This option tag indicates support | [RFC5368] |
| | for REFER requests that contain a | |
| | resource list document describing | |
| | multiple REFER targets. | |
+-----------------+-------------------------------------+-----------+
Table 1: Registration of the 'multiple-refer' option-tag in SIPso basically the tag name is multiple-refer, it says it means support for REFER requests carrying a resource list document with multiple REFER targets 🦋, and it points back to [RFC5368] 🌸
12. References
bestie, this is the reading list 📚 Normative references are the ones you genuinely need to know to implement this stuff, and informative ones are more like good background 🧚 Every single one is listed below with all its details, no cap 💜
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2183] Troost, R., Dorner, S., and K. Moore, "Communicating
Presentation Information in Internet Messages: The
Content-Disposition Header Field", RFC 2183, August 1997.
[RFC2392] Levinson, E., "Content-ID and Message-ID Uniform Resource
Locators", RFC 2392, August 1998.
[RFC3204] Zimmerer, E., Peterson, J., Vemuri, A., Ong, L., Audet,
F., Watson, M., and M. Zonoun, "MIME media types for ISUP
and QSIG Objects", RFC 3204, December 2001.
[RFC3261] 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.
[RFC3420] Sparks, R., "Internet Media Type message/sipfrag",
RFC 3420, November 2002.
[RFC3515] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[RFC4488] Levin, O., "Suppression of Session Initiation Protocol
(SIP) REFER Method Implicit Subscription", RFC 4488,
May 2006.
[RFC4826] Rosenberg, J., "Extensible Markup Language (XML) Formats
for Representing Resource Lists", RFC 4826, May 2007.
[RFC5363] Camarillo, G. and A.B. Roach, "Framework and Security
Considerations for Session Initiation Protocol (SIP) URI-
List Services", RFC 5363, October 2008.
[RFC5364] Garcia-Martin, M. and G. Camarillo, "Extensible Markup
Language (XML) Format Extension for Representing Copy
Control Attributes in Resource Lists", RFC 5364,
October 2008.12.2. Informative References
[RFC4575] Rosenberg, J., Schulzrinne, H., and O. Levin, "A Session
Initiation Protocol (SIP) Event Package for Conference
State", RFC 4575, August 2006.
[RFC4579] Johnston, A. and O. Levin, "Session Initiation Protocol
(SIP) Call Control - Conferencing for User Agents",
BCP 119, RFC 4579, August 2006.
[SIP-GRUU] Rosenberg, J., "Obtaining and Using Globally Routable
User Agent (UA) URIs (GRUU) in the Session Initiation
Protocol (SIP)", Work in Progress, October 2007.Authors' Addresses ✨
meet the crew 🫶 Here's where you can find each author:
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
EMail: Gonzalo.Camarillo@ericsson.com
Aki Niemi
Nokia
P.O. Box 321
NOKIA GROUP, FIN 00045
Finland
EMail: Aki.Niemi@nokia.com
Markus Isomaki
Nokia
P.O. Box 100
NOKIA GROUP, FIN 00045
Finland
EMail: markus.isomaki@nokia.com
Miguel A. Garcia-Martin
Ericsson
Via de los Poblados 13
Madrid 28033
Spain
EMail: miguel.a.garcia@ericsson.com
Hisham Khartabil
Ericsson Australia
EMail: hisham.khartabil@gmail.comFull Copyright Statement
the legal part 🙈 but it matters, so here it is word for word:
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