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Versions: 00 01 02 03 04 05 06 RFC 3515
Network Working Group R. Sparks
Internet-Draft dynamicsoft
Expires: January 16, 2002 July 18, 2001
The Refer Method
draft-ietf-sip-refer-00.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2001). 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-sparks-sip-cc-transfer-04 . . . . . . . 3
3. The REFER Method . . . . . . . . . . . . . . . . . . . . . 3
3.1 The Refer-To Header . . . . . . . . . . . . . . . . . . . 3
3.1.1 Examples . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2 The Referred-By Header . . . . . . . . . . . . . . . . . . 4
3.2.1 A PGP based signature-scheme . . . . . . . . . . . . . . . 5
3.2.2 Examples . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.3 Header Field Support for the REFER Method . . . . . . . . 5
3.4 Message Body Inclusion . . . . . . . . . . . . . . . . . . 7
3.5 Responses within the REFER transaction . . . . . . . . . . 7
3.6 Behavior of SIP User Agents . . . . . . . . . . . . . . . 7
3.6.1 Accessing the referred-to resource . . . . . . . . . . . . 7
3.6.2 Reporting on the results of the reference . . . . . . . . 7
3.6.2.1 Using NOTIFY . . . . . . . . . . . . . . . . . . . . . . . 7
3.6.2.2 The body of the NOTIFY . . . . . . . . . . . . . . . . . . 8
3.7 Behavior of SIP Registrars/Redirect Servers . . . . . . . 9
3.8 Behavior of SIP Proxies . . . . . . . . . . . . . . . . . 9
3.9 Prototypical REFER callflow . . . . . . . . . . . . . . . 9
3.10 Security Considerations . . . . . . . . . . . . . . . . . 11
3.10.1 Circumventing privacy . . . . . . . . . . . . . . . . . . 11
3.10.2 Circumventing security . . . . . . . . . . . . . . . . . . 11
3.10.3 Limiting the breach . . . . . . . . . . . . . . . . . . . 12
4. Historic Material . . . . . . . . . . . . . . . . . . . . 12
5. Open Issues . . . . . . . . . . . . . . . . . . . . . . . 12
5.1 REFER is now dependent on sip-events . . . . . . . . . . . 12
5.2 Registering the refer event with IANA . . . . . . . . . . 13
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . 13
References . . . . . . . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . 13
Full Copyright Statement . . . . . . . . . . . . . . . . . 14
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1. Overview
This document defines the REFER method. This SIP 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.
Editor's note: Per working group consensus, draft-ietf-sip-cc-
transfer-04 was split into two drafts. This draft specifies the
REFER method itself. Its companion (name to be determined) details
the use of REFER to achieve call transfer.
2. Changes from draft-sparks-sip-cc-transfer-04
o Split the document
o Simplified the expression of the Referred-By grammar
o Resolved inconsistency in Referred-By signature normative text and
example
3. The REFER Method
REFER is a SIP method as defined by RFC2543 [1]. The REFER method
indicates that the recipient (identified by the Request-URI) should
contact a third party using the contact information provided in the
method. A success response indicates that the recipient was able to
contact the third party.
Unless stated otherwise, the protocol for emitting and responding to
a REFER request are identical to those for a BYE request in [1]. The
behavior of SIP entities not implementing the REFER (or any other
unknown) method is explicitly defined in [1].
A REFER request MAY be placed outside the scope of a call-leg created
with an INVITE. REFER MAY be Record-Routed, hence MUST contain a
single Contact header. REFERs occurring inside an existing call-leg
MUST follow the Route/Record-Route logic of that call-leg. REFERs
occurring outside an existing call-leg effectively create a new call-
leg following the behavior of SUBSCRIBE specified [2].
3.1 The Refer-To Header
Refer-To is a request-header as defined by [1]. It may only appear
in a REFER request. It provides a URL to reference.
Refer-To = ("Refer-To" | "r") ":" URL
A REFER method MUST contain exactly one Refer-To header.
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The Refer-To header MAY be encrypted as part of end-end encryption.
The Contact header is an important part of the Route/Record-Route
mechanism and is not available to be used to indicate the target of the
reference.
3.1.1 Examples
Refer-To: sip:alice@atlanta.com
Refer-To: sip:bob@biloxi.com?Accept-Contact=sip:bobsdesk.
biloxi.com&Call-ID=55432@alicepc.atlanta.com
Refer-To: <sip:dave@denver.com?Replaces:12345@192.168.118.3;
to-tag=12345;from-tag=5FFE-3994>
Refer-To: sip:carol@cleveland.com;method=SUBSCRIBE
Refer-To: http://www.ietf.org
Long headers are line-wrapped here for clarity only.
3.2 The Referred-By Header
Referred-By is a request-header as defined by [1]. It can appear in
any request. It conveys the identity of the original REFERrer to the
referred-to party, optionally proving the identity and that the
REFERrer actually issued this reference.
Referred-By = ("Referred-By" | "b") ":"
referrer-url [ referenced-url ]
referrer-url = ( name-addr | addr-spec )
referenced-url = ";" "ref" "=" "<" URL ">" [ ref-signature ]
ref-signature = ";" signature-scheme *( sig-scheme-params )
signature-scheme = "scheme" "=" token
sig-scheme-parms = ";" token "=" ( token | quoted-string )
The referrer-url contains the SIP URL of the party sending the REFER
request. The referenced-url contains a copy of the URL placed in the
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Refer-To: header. Any occurrences of < or > in the referenced-url
MUST be escaped. The ref-signature contains a signature over the
concatenation of addr-spec portion of the referrer-url and the entire
referenced-url. An example signature scheme is given in section
Section 3.2.1.
A REFER request MUST contain exactly one Referred-By header.
The Referred-By header SHOULD be signed to help detection of REFERs
from unauthorized third parties. A signed Referred-By header SHOULD
include a Date header in the referrer-url to facilitate detection of
replay attacks.
A UA MAY reject a request containing an unsigned Referred-By header.
A UA SHOULD verify the signature on any Referred-By header it
receives.
The Referred-By header MAY be encrypted as part of end-end
encryption.
3.2.1 A PGP based signature-scheme
One signature-scheme for Referred-By headers uses PGP as follows:
signature-scheme = "scheme" "=" "pgp"
sig-scheme-parms = pgp-version | signed-by | pgp-signature
pgp-version, signed-by and pgp-signature are defined in section 15.1
of RFC2543, with the modification that the signature is computed
across the concatenation of the addr-spec portion of the referrer-url
and the entire referenced-url.
3.2.2 Examples
Referred-By: sip:alice@atlanta.com;ref=<http:www.ietf.org>
Referred-By: "Bob" <sip:bob@biloxi.com>;
ref=<sip:alice@atlanta.com>;scheme=pgp;
pgp-version="5.0";signature="the signature"
(Note that in the last example, the signature would be over the
string "sip:bob@biloxi.comsip:alice@atlanta.com")
3.3 Header Field Support for the REFER Method
This table adds a column to tables 4 and 5 in [1], describing header
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presence in a REFER method. See [1] for a key for the symbols used.
A row for the Refer-To: and Referred-By request-header should be
inferred, each mandatory for REFER. Refer-To is not applicable for
any other methods. Referred-By is a general Request header. The enc
and e-e columns in [1] apply to the REFER method unmodified.
Header Where REFER
Accept R -
Accept-Encoding R -
Accept-Language R o
Allow R -
Allow 405 m
Authorization R o
Call-ID gc m
Contact R m
Contact 1xx -
Contact 2-6xx o
Content-Encoding e -
Content-Length e o
Content-Type e -
CSeq gc m
Date g o
Encryption g o
Expires R o
From gc m
Hide R o
Max-Forwards R o
Organization g o
Priority R -
Proxy-Authenticate 407 o
Proxy-Authorization R o
Proxy-Require R o
Require R o
Retry-After R -
Retry-After 404,480,486 o
Retry-After 503 o
Retry-After 600,603 o
Response-Key R o
Record-Route R o
Record-Route 2xx o
Route R o
Server r o
Subject R -
Timestamp g o
To gc(1) m
Unsupported 420 o
User-Agent g o
Via gc(2) m
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Warning r o
WWW-Authenticate 401 o
3.4 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.
3.5 Responses within the REFER transaction
An agent responding to a REFER Method MUST return a 400 Bad Request
if the request contained zero or more than one Refer-To headers. An
agent responding to a REFER Method MUST return a 400 Bad Request if
the request contained zero or more than one Referred-By headers. An
agent (including proxies generating local responses) MAY return a 100
Trying or any appropriate 400-600 class response as prescribed by
[1]. If the recipient's agent decides to contact the resource in the
Refer-To header, a 202 Accepted response MUST be returned before the
REFER transaction expires.
3.6 Behavior of SIP User Agents
3.6.1 Accessing the referred-to resource
A UA receiving a well-formed REFER request SHOULD request approval
from the user to proceed (this request could be interactive or
through configuration). Upon receiving approval from the user, the
UA MUST contact the resource identified by the URL in the Refer-To:
header. Note that if the URL is a SIP URL, it could contain header
fields such as Call-Id that may be used to form the resulting
request. If the URL is a SIP URL, the Referred-By header in the
REFER request should be copied into the request sent to the referred-
to resource.
The resource identified by the Refer-To: URL is contacted using the
normal mechanisms for that URL type. For example, if the URL is a
SIP INVITE URL, the UA would issue a new INVITE using all of the
normal rules for sending an INVITE defined in [1].
3.6.2 Reporting on the results of the reference
3.6.2.1 Using NOTIFY
Once it is known whether the reference succeeded or failed, the UA
receiving the REFER SHOULD notify the agent sending the refer using
the NOTIFY mechanism defined in Event Notification in SIP [2] as if
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the REFER had established a subscription. In particular:
o Each NOTIFY should reflect the To:, From:, and Call-ID headers
from the REFER as if they had arrived in a SUBSCRIBE.
o Each NOTIFY MUST contain an event header of Event: refer
o Each NOTIFY MUST contain a body of type "application/sip". The
contents of this body are detailed in Section 3.6.2.2
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.
3.6.2.2 The body of the NOTIFY
Each NOTIFY MUST contain a body of type "application/sip". This body
MUST begin with a SIP Response Status-Line as defined in [1]. The
response class in this status line indicates the success of the
referred action. The body MAY contain other SIP headers 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 or the body:
SIP/2.0 503 Service Unavailable
if the reference failed.
An implementation MAY include more of a SIP message in that body to
convey more information. Warning headers received in responses to
the referred action are good candidates. In fact, if the reference
was to a SIP URL, 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 3.10).
Implementers must carefully consider what they choose to include.
Note that if the reference was to a non-SIP URL, 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
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contain the application/sip body of "SIP/2.0 200 OK"
3.7 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.
3.8 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.
3.9 Prototypical REFER callflow
Agent A Agent B
| |
| REFER |
|----------------------->|
| 202 Accepted |
|<-----------------------|
| |
| |------->
| | (whatever)
| |<------
| |
| NOTIFY |
|<-----------------------|
| 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
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REFER sip:b@agentland SIP/2.0
Via: SIP/2.0/UDP agenta.agentland
To: <sip:b@agentland>
From: <sip:a@agentland>;tag=193402342
Call-ID: 898234234@agenta.agentland
CSeq: 93809823 REFER
Refer-To: (whatever URL)
Referred-By: <sip:a@agentland>;ref=<whatever URL>;
scheme=pgp;pgp-version="5.0";signature="signature goes here"
Contact: sip:a@agentland
Content-Length: 0
Message Two
SIP/2.0 202 Accepted
Via: SIP/2.0/UDP agenta.agentland
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
NOTIFY sip:a@agentland SIP/2.0
Via: SIP/2.0/UDP agentb.agentland
To: <sip:a@agentland>;tag=193402342
From: <sip:b@agentland>;tag=4992881234
Call-ID: 898234234@agenta.agentland
CSeq: 1993402 NOTIFY
Event: refer
Contact: sip:b@agentland
Content-Type: application/sip
Content-Length: 16
SIP/2.0 200 OK
Message Four
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SIP/2.0 200 OK
Via: SIP/2.0/UDP agentb.agentland
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
3.10 Security Considerations
The security requirements 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.
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 URLs SHOULD NOT accept REFER requests to them.
Using application/sip 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.
3.10.1 Circumventing privacy
Suppose Alice has a user-agent that accepts REFER requests to SIP
INVITE URLs, 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
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.
3.10.2 Circumventing security
Suppose Alice, with the same user agent as above, is working at a
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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 URL that he suspects might
have something to do with it. He sends a REFER to ALICE with the
mysterious URL and as Alice connects to the FOO, Mallory gets NOTIFYs
with bodies containing
Server: FOO/v0.9.7
3.10.3 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 3.6.2.2 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.
4. 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 now 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.
Earlier 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.
5. Open Issues
5.1 REFER is now dependent on sip-events
This work is prevented from moving to RFC until the sip-events draft
moves to that level.
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5.2 Registering the refer event with IANA
When we near the end of the process, the refer event will need to be
registered with IANA per [2].
6. Acknowledgments
This draft is a collaborative product of the SIP working group.
References
[1] Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,
"SIP: Session Initiation Protocol", RFC 2543, March 1999.
[2] Roach, A., "SIP-Specific Event Notification", draft-sip-events-
00 (work in progress), July 2001.
Author's Address
Robert J. Sparks
dynamicsoft
5100 Tennyson Parkway
Suite 1200
Plano, TX 75024
EMail: rsparks@dynamicsoft.com
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