Network Working Group R. Sparks
Internet-Draft dynamicsoft
Expires: October 25, 2002 April 26, 2002
The Refer Method
draft-sparks-sip-refer-split-00
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document defines the REFER method. This SIP extension requests
that the recipient REFER to a resource provided in the request. This
can be used to enable many applications, including Call Transfer.
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Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Changes from draft-ietf-sip-refer-02 . . . . . . . . . . . . 3
3. The REFER Method . . . . . . . . . . . . . . . . . . . . . . 3
4. Header Field Support for the REFER Method . . . . . . . . . 4
5. Message Body Inclusion . . . . . . . . . . . . . . . . . . . 6
6. Behavior of SIP User Agents . . . . . . . . . . . . . . . . 6
6.1 Accessing the referred-to resource . . . . . . . . . . . . . 6
6.2 UA Responses within the REFER transaction . . . . . . . . . 6
6.3 Reporting on the results of the reference . . . . . . . . . 7
6.3.1 Using NOTIFY . . . . . . . . . . . . . . . . . . . . . . . . 7
6.3.2 The body of the NOTIFY . . . . . . . . . . . . . . . . . . . 7
6.3.3 Multiple REFER requests in a dialog . . . . . . . . . . . . 8
6.4 Behavior of SIP Registrars/Redirect Servers . . . . . . . . 8
6.5 Behavior of SIP Proxies . . . . . . . . . . . . . . . . . . 9
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1 Prototypical REFER callflow . . . . . . . . . . . . . . . . 9
7.2 Multiple REFERs in a dialog . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . 12
8.1 Circumventing privacy . . . . . . . . . . . . . . . . . . . 12
8.2 Circumventing security . . . . . . . . . . . . . . . . . . . 13
8.3 Limiting the breach . . . . . . . . . . . . . . . . . . . . 13
9. Historic Material . . . . . . . . . . . . . . . . . . . . . 13
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . 14
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 14
References . . . . . . . . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . 15
Full Copyright Statement . . . . . . . . . . . . . . . . . . 16
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1. Overview
This document defines the REFER method. This SIP [1] extension
requests that the recipient REFER to a resource provided in the
request. This can be used to enable many applications, including
Call Transfer.
2. Changes from draft-ietf-sip-refer-02
This section will be removed before the draft is fowarded to the IESG
o Removed Referred-By to a separate draft.
o Repaired syntax errors (particularly escaping) in example URIs
o Changed syntax definitions to use RFC2234 ABNF
o Incorporated other last call feedback
o Replaced the header tables to match RFC3261
o Changed syntax of Refer-To to allow name-addr (not backwards
compatible)
o Redo IANA section to align with Events/Presence
o Replaced the cseq= Event header parameter with the sip-events id=
parameter when handling multiple subscriptions within a dialog
3. The REFER Method
REFER is a SIP method as defined by RFC3261 [1]. The REFER method
indicates that the recipient (identified by the Request-URI) should
be interpreted as if it appeared in Table 3 of RFC 3261.
Header field where proxy ACK BYE CAN INV OPT REG
___________________________________________________________________
Refer-To R - - - - - -
Refer-To = ("Refer-To" / "r") HCOLON ( name-addr / addr-spec )
A REFER method MUST contain exactly one Refer-To header field value.
The Refer-To header field MAY be encrypted as part of end-end
encryption.
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The Contact header field is an important part of the Route/Record-Route
mechanism and is not available to be used to indicate the target of the
reference.
Examples
Refer-To: sip:alice@atlanta.com
Refer-To: sip:bob@biloxi.com?Accept-Contact%3Dsip:bobsdesk.
biloxi.com&%3BCall-ID%3D55432%40alicepc.atlanta.com
Refer-To: sip:dave@denver.com?Replaces%3D12345%40192.168.118.3%3B
to-tag%3D12345%3Bfrom-tag%3D5FFE-3994
Refer-To: sip:carol@cleveland.com;method=SUBSCRIBE
Refer-To: http://www.ietf.org
Long headers field values are line-wrapped here for clarity only.
4. Header Field Support for the REFER Method
This table adds a column to tables 4 and 5 in [1], describing header
field presence in a REFER method. See [1] for a key for the symbols
used. A row for the Refer-To: request-header should be inferred,
each mandatory for REFER. Refer-To is not applicable for any other
methods. The enc and e-e columns in [1] apply to the REFER method
unmodified.
Header Where REFER
Accept R o
Accept 2xx -
Accept 415 c
Accept-Encoding R o
Accept-Encoding 2xx -
Accept-Encoding 415 c
Accept-Language R o
Accept-Language 2xx -
Accept-Language 415 c
Alert-Info -
Allow Rr o
Allow 405 m
Authentication-Info 2xx o
Authorization R o
Call-ID c m
Call-Info -
Contact R m
Contact 1xx -
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Contact 2xx m
Contact 3-6xx o
Content-Disposition o
Content-Encoding o
Content-Language o
Content-Length o
Content-Type *
CSeq c m
Date o
Error-Info 3-6xx o
Expires R o
From c m
In-Reply-To -
Max-Forwards R m
Min-Expires -
MIME-Version o
Organization o
Priority R -
Proxy-Authenticate 401 o
Proxy-Authenticate 407 m
Proxy-Authorization R o
Proxy-Require R o
Record-Route R o
Record-Route 2xx,18x o
Reply-To -
Require c
Retry-After 404,413,480,486 o
Retry-After 500,503 o
Retry-After 600,603 o
Route R c
Server r o
Subject R -
Supported R,2xx o
Timestamp o
To c(1) m
Unsupported 420 o
User-Agent o
Via c(2) m
Warning r o
WWW-Authenticate 401 m
WWW-Authenticate 407 o
Table 1: Header Field Support
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5. Message Body Inclusion
A REFER method MAY contain a body. This specification assigns no
meaning to such a body. A receiving agent may choose to process the
body according to its Content-Type.
6. Behavior of SIP User Agents
6.1 Accessing the referred-to resource
A UA accepting a well-formed REFER request SHOULD request approval
from the user to proceed (this request could be satisfied with an
interactive query or through accessing configured policy). If
approval is granted, the UA MUST contact the resource identified by
the URI in the Refer-To: header field value. Note that if the URI is
a SIP URI, it could contain header fields such as Call-Id that may be
used to form the resulting request.
The resource identified by the Refer-To: URI is contacted using the
normal mechanisms for that URI type. For example, if the URI is a
SIP URI indicating an INVITE should be generated (using a
method=INVITE URI parameter for example), the UA would issue a new
INVITE using all of the normal rules for sending an INVITE defined in
[1].
6.2 UA Responses within the REFER transaction
If the approval sought above for a well formed REFER request is
immediately denied, the UA MAY decline the request.
An agent responding to a REFER Method MUST return a 400 Bad Request
if the request contained zero or more than one Refer-To header field
values.
An agent (including proxies generating local responses) MAY return a
100 Trying or any appropriate 400-600 class response as prescribed by
[1].
Care should be taken when implementing the logic that determines
whether or not to accept the REFER request. A UA not capable of
accessing non-SIP URIs SHOULD NOT accept REFER requests to them.
If no final response has been generated according to the rules above,
the UA MUST return a 202 Accepted response before the REFER
transaction expires.
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6.3 Reporting on the results of the reference
6.3.1 Using NOTIFY
If a REFER request is accepted (with a 202 Accepted response), the UA
receiving the REFER SHOULD notify the agent sending the REFER of the
status of the reference. This is done using the NOTIFY mechanism
defined in [2] as if the REFER had established a subscription. In
particular:
o Each NOTIFY should reflect the To:, From:, and Call-ID header
fields from the REFER as if they had arrived in a SUBSCRIBE.
o Each NOTIFY MUST contain an event header field of Event: refer
o Each NOTIFY MUST contain a body of type "message/sipfrag" [3].
o Analogous to the case for SUBSCRIBE described in [2], the agent
that issued the REFER MUST be prepared to receive a NOTIFY before
the REFER transaction completes.
6.3.2 The body of the NOTIFY
Each NOTIFY MUST contain a body of type "message/sipfrag" [3]. The
body of a NOTIFY MUST begin with a SIP Response Status-Line as
defined in [1]. The response class in this status line indicates the
status of the referred action. The body MAY contain other SIP header
fields to provide information about the outcome of the referenced
action.
A minimal, but complete, implementation can respond with a single
NOTIFY containing either the body:
SIP/2.0 200 OK
if the reference was successful, the body:
SIP/2.0 503 Service Unavailable
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if the reference failed, or the body:
SIP/2.0 603 Declined
if the REFER request was accepted before approval to follow the
reference could be obtained and that approval was subsequently
denied.
An implementation MAY include more of a SIP message in that body to
convey more information. Warning header field values received in
responses to the referred action are good candidates. In fact, if
the reference was to a SIP URI, the entire response to the referenced
action could be returned (perhaps to assist with debugging).
However, doing so could have grave security repercussions (see
Section 8). Implementers must carefully consider what they choose to
include.
Note that if the reference was to a non-SIP URI, status in any
NOTIFYs to the referrer must still be in the form of SIP Response
Status-Lines. The minimal implementation discussed above is
sufficient to provide a basic indication of success or failure. For
example, if a client receives a REFER to a HTTP URL, and is
successful in accessing the resource, its NOTIFY to the referrer can
contain the message/sipfrag body of "SIP/2.0 200 OK". If the
notifier wishes to return additional non-SIP protocol specific
information about the status of the request, it may place it in the
body of the sipfrag message.
6.3.3 Multiple REFER requests in a dialog
A REFER creates an implicit subscription sharing the dialog
identifers in the REFER request. If more than one REFER is issued in
the same dialog (a second attempt at transferring a call for
example), the dialog identifiers do not provide enough information to
associate the resulting NOTIFYs with the proper REFER.
Thus, for the second and subsequent REFER requests a UA receives in a
given dialog, it MUST include an id parameter[2] in the Event header
field of each NOTIFY containing the sequence number (the number from
the CSeq header field value) of the REFER this NOTIFY is associated
with. This id parameter MAY be included in NOTIFYs to the first
REFER a UA receives in a given dialog.
6.4 Behavior of SIP Registrars/Redirect Servers
Registrars and Redirect Servers SHOULD return a 603 to a REFER
request, unless they are also playing some other SIP role.
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6.5 Behavior of SIP Proxies
SIP Proxies do not require modification to support the REFER method.
Specifically, as required by [1], a proxy should process a REFER
request the same way it processes an OPTIONS request.
7. Examples
7.1 Prototypical REFER callflow
Agent A Agent B
| |
| F1 REFER |
|----------------------->|
| F2 202 Accepted |
|<-----------------------|
| |
| |------->
| | (whatever)
| |<------
| |
| F3 NOTIFY |
|<-----------------------|
| F4 200 OK |
|----------------------->|
| |
| |
Here are examples of what the four messages between Agent A and Agent
B might look like if the reference to (whatever) that Agent B makes
is successful. The details of this flow indicate this particular
REFER occurs outside a session (there is no To: tag in the REFER
request). If the REFER occurs inside a session, there would be a
non-empty To: tag in the request.
Message One (F1)
REFER sip:b@agentland SIP/2.0
Via: SIP/2.0/UDP agenta.agentland;branch=2293940223
To: <sip:b@agentland>
From: <sip:a@agentland>;tag=193402342
Call-ID: 898234234@agenta.agentland
CSeq: 93809823 REFER
Max-Forwards: 70
Refer-To: (whatever URI)
Contact: sip:a@agentland
Content-Length: 0
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Message Two (F2)
SIP/2.0 202 Accepted
Via: SIP/2.0/UDP agenta.agentland;branch=2293940223
To: <sip:b@agentland>;tag=4992881234
From: <sip:a@agentland>;tag=193402342
Call-ID: 898234234@agenta.agentland
CSeq: 93809823 REFER
Contact: sip:b@agentland
Content-Length: 0
Message Three (F3)
NOTIFY sip:a@agentland SIP/2.0
Via: SIP/2.0/UDP agentb.agentland;branch=9922ef992-25
To: <sip:a@agentland>;tag=193402342
From: <sip:b@agentland>;tag=4992881234
Call-ID: 898234234@agenta.agentland
CSeq: 1993402 NOTIFY
Max-Forwards: 70
Event: refer
Contact: sip:b@agentland
Content-Type: message/sipfrag;version=2.0
Content-Length: 16
SIP/2.0 200 OK
Message Four (F4)
SIP/2.0 200 OK
Via: SIP/2.0/UDP agentb.agentland;branch=9922ef992-25
To: <sip:a@agentland>;tag=193402342
From: <sip:b@agentland>;tag=4992881234
Call-ID: 898234234@agenta.agentland
CSeq: 1993402 NOTIFY
Contact: sip:a@agentland
Content-Length: 0
7.2 Multiple REFERs in a dialog
Message One above brings an implicit subscription dialog into
existance. Suppose Agent A issued a second REFER inside that dialog:
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Agent A Agent B
| |
| F5 REFER |
|----------------------->|
| F6 202 Accepted |
|<-----------------------|
| |
| |------->
| | (something different)
| |<------
| |
| F7 NOTIFY |
|<-----------------------|
| F8 200 OK |
|----------------------->|
| |
| |
Message Five (F5)
REFER sip:b@agentland SIP/2.0
Via: SIP/2.0/UDP agenta.agentland;branch=9390399231
To: <sip:b@agentland>;tag=4992881234
From: <sip:a@agentland>;tag=193402342
Call-ID: 898234234@agenta.agentland
CSeq: 93809824 REFER
Max-Forwards: 70
Refer-To: (some different URI)
Contact: sip:a@agentland
Content-Length: 0
Message Six (F6)
SIP/2.0 202 Accepted
Via: SIP/2.0/UDP agenta.agentland;branch=9390399231
To: <sip:b@agentland>;tag=4992881234
From: <sip:a@agentland>;tag=193402342
Call-ID: 898234234@agenta.agentland
CSeq: 93809824 REFER
Contact: sip:b@agentland
Content-Length: 0
Message Seven (F7)
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NOTIFY sip:a@agentland SIP/2.0
Via: SIP/2.0/UDP agentb.agentland;branch=2994a93eb-fe
To: <sip:a@agentland>;tag=193402342
From: <sip:b@agentland>;tag=4992881234
Call-ID: 898234234@agenta.agentland
CSeq: 1993403 NOTIFY
Max-Forwards: 70
Event: refer;id=93809824
Contact: sip:b@agentland
Content-Type: message/sipfrag;version=2.0
Content-Length: 16
SIP/2.0 200 OK
Message Eight (F8)
SIP/2.0 200 OK
Via: SIP/2.0/UDP agentb.agentland;branch=2994a93eb-fe
To: <sip:a@agentland>;tag=193402342
From: <sip:b@agentland>;tag=4992881234
Call-ID: 898234234@agenta.agentland
CSeq: 1993403 NOTIFY
Contact: sip:a@agentland
Content-Length: 0
8. Security Considerations
The security considerations of [1] apply to the REFER method.
This mechanism relies on providing contact information for the
referred-to resource to the party being referred. Care should be
taken to provide a suitably restricted URI if the referred to
resource should be protected.
Using message/sipfrag bodies to return the progress and results of a
REFER request is extremely powerful. Careless use of that capability
will compromise security and privacy. Here are a couple of simple,
somewhat contrived, examples to demonstrate the potential for harm.
8.1 Circumventing privacy
Suppose Alice has a user-agent that accepts REFER requests to SIP
INVITE URIs, and NOTIFYs the referrer of the progress of the INVITE
by copying each response to the INVITE into the body of a NOTIFY.
Suppose further that Carol has a reason to avoid Mallory and has
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configured her system at her proxy to only accept calls from a
certain set of people she trusts (including Alice), so that Mallory
doesn't learn when she's around, or what user agent she's actually
using.
Mallory can send a REFER to Alice, with a Refer-To: indicating Carol.
If Alice can reach Carol, the 200 OK Carol sends gets returned to
Mallory in a NOTIFY, letting him know not only that Carol is around,
but also the IP address of the agent she's using.
8.2 Circumventing security
Suppose Alice, with the same user agent as above, is working at a
company that is working on the greatest SIP device ever invented -
the SIP FOO. The company has been working for months building the
device and the marketing materials, carefully keeping the idea, even
the name of the idea secret (since a FOO is one of those things that
anybody could do if they'd just had the idea first). FOO is up and
running, and anyone at the company can use it, but it's not available
outside the company firewall.
Mallory has heard rumor that Alice's company is onto something big,
and has even managed to get his hands on a URI that he suspects might
have something to do with it. He sends a REFER to ALICE with the
mysterious URI and as Alice connects to the FOO, Mallory gets NOTIFYs
with bodies containing
Server: FOO/v0.9.7
8.3 Limiting the breach
For each of these cases, and in general, returning a carefully
selected subset of the information available about the progress of
the reference through the NOTIFYs mitigates risk. The minimal
implementation described in Section 6.3.2 exposes the least
information about what the agent operating on the REFER request has
done, and is least likely to be a useful tool for malicious users.
9. Historic Material
This method was initially motivated by the call-transfer application.
Starting as TRANSFER, and later generalizing to REFER, this method
improved on the BYE/Also concept of the expired draft-ietf-sip-cc-01
by disassociating transfers from the processing of BYE. These
changes facilitate recovery of failed transfers and clarify state
management in the participating entities.
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Early versions of this work required the agent responding to REFER to
wait until the referred action completed before sending a final
response to the REFER. That final response reflected the success or
failure of the referred action. This was infeasible due to the
transaction timeout rules defined for non-INVITE requests in [1]. A
REFER must always receive an immediate (within the lifetime of a non-
INVITE transaction) final response.
10. IANA Considerations
(Note to RFC Editor: Please fill in all occurances of XXXX in this
section with the RFC number of this specification).
This document defines a new SIP method name (REFER), a new SIP header
name with a compact form (Refer-To and r respectively), and an event
package (refer).
SIP Method names are not currently registered with IANA.
The following information should be added to the header sub-registry
under http://www.iana.org/assignmemts/sip-parameters.
Header Name: Refer-To
Compact Form: r
Reference: RFC XXXX
This specification registers an event package, based on the
registration proceedures defined in [2]. The following is the
information required for such a registration:
Package Name: refer
Package or Package-Template: This is a package.
Published Specification: RFC XXXX
Person to Contact: Robert Sparks, rsparks@dynamicsoft.com
11. Acknowledgments
This draft is a collaborative product of the SIP working group.
References
[1] PlaceHolder, A., "Placeholder", RFC 3261, placeholder 2002.
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[2] PlaceHolder, A., "Placeholder", RFC 3265, placeholder 2002.
[3] PlaceHolder, A., "Placeholder", draft-sparks-sip-mimetypes (work
in progress), placeholder 2002.
Author's Address
Robert J. Sparks
dynamicsoft
5100 Tennyson Parkway
Suite 1200
Plano, TX 75024
EMail: rsparks@dynamicsoft.com
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