Network Working Group R. Sparks
Request for Comments: 3515 dynamicsoft
Category: Standards Track April 2003

The Session Initiation Protocol (SIP) Refer Method

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

This document defines the REFER method. This Session Initiation
Protocol (SIP) extension requests that the recipient REFER to a
resource provided in the request. It provides a mechanism allowing
the party sending the REFER to be notified of the outcome of the
referenced request. This can be used to enable many applications,
including call transfer.

In addition to the REFER method, this document defines the the refer
event package and the Refer-To request header.

Table of Contents

1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. The REFER Method . . . . . . . . . . . . . . . . . . . . . . 3
2.1 The Refer-To Header Field . . . . . . . . . . . . . . . 3
2.2 Header Field Support for the REFER Method . . . . . . . 4
2.3 Message Body Inclusion. . . . . . . . . . . . . . . . . 5
2.4 Behavior of SIP User Agents . . . . . . . . . . . . . . 6
2.4.1 Forming a REFER request . . . . . . . . . . . . . 6
2.4.2 Processing a REFER request. . . . . . . . . . . . 6
2.4.3 Accessing the Referred-to Resource. . . . . . . . 6
2.4.4 Using SIP Events to Report the Results
of the Reference. . . . . . . . . . . . . . . . . 7
2.4.5 The Body of the NOTIFY. . . . . . . . . . . . . . 8
2.4.6 Multiple REFER Requests in a Dialog . . . . . . . 9
2.4.7 Using the Subscription-State Header
Field with Event Refer. . . . . . . . . . . . . . 9

2.5 Behavior of SIP Registrars/Redirect Servers . . . . . . 9
2.6 Behavior of SIP Proxies . . . . . . . . . . . . . . . . 10
3. Package Details: Event refer . . . . . . . . . . . . . . . . 10
3.1 Event Package Name. . . . . . . . . . . . . . . . . . . 10
3.2 Event Package Parameters. . . . . . . . . . . . . . . . 10
3.3 SUBSCRIBE Bodies. . . . . . . . . . . . . . . . . . . . 10
3.4 Subscription Duration . . . . . . . . . . . . . . . . . 10
3.5 NOTIFY Bodies . . . . . . . . . . . . . . . . . . . . . 11
3.6 Notifier processing of SUBSCRIBE requests . . . . . . . 11
3.7 Notifier Generation of NOTIFY Requests. . . . . . . . . 11
3.8 Subscriber Processing of NOTIFY Requests. . . . . . . . 11
3.9 Handling of Forked Requests . . . . . . . . . . . . . . 11
3.10 Rate of Notifications . . . . . . . . . . . . . . . . . 11
3.11 State Agents. . . . . . . . . . . . . . . . . . . . . . 11
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.1 Prototypical REFER callflow . . . . . . . . . . . . . . 12
4.2 Multiple REFERs in a dialog . . . . . . . . . . . . . . 14
5. Security Considerations . . . . . . . . . . . . . . . . . . 16
5.1 ConstrUCting a Refer-To URI . . . . . . . . . . . . . . 16
5.2 Authorization Considerations for REFER. . . . . . . . . 17
5.3 Considerations for the use of message/sipfrag . . . . . 18
5.3.1 Circumventing Privacy . . . . . . . . . . . . . . 18
5.3.2 Circumventing Confidentiality . . . . . . . . . . 19
5.3.3 Limiting the Breach . . . . . . . . . . . . . . . 19
5.3.4 Cut, Paste and Replay Considerations. . . . . . . 19
6. Historic Material . . . . . . . . . . . . . . . . . . . . . 20
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 20
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 21
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
9.1 Normative References. . . . . . . . . . . . . . . . . . 21
9.2 Informative References. . . . . . . . . . . . . . . . . 21
10. Intellectual Property Statement. . . . . . . . . . . . . . . 21
11. Author's Address . . . . . . . . . . . . . . . . . . . . . . 22
12. Full Copyright Statement . . . . . . . . . . . . . . . . . . 23

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. For instance, if Alice is in a call with Bob, and decides
Bob needs to talk to Carol, Alice can instruct her SIP user agent
(UA) to send a SIP REFER request to Bob's UA providing Carol's SIP
Contact information. Assuming Bob has given it permission, Bob's UA
will attempt to call Carol using that contact. Bob's UA will then
report whether it succeeded in reaching the contact to Alice's UA.

2. 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
contact a third party using the contact information provided in the
request.

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 implicitly establishes a subscription to the refer
event. Event subscriptions are defined in [2].

A REFER request MAY be placed outside the scope of a dialog created
with an INVITE. REFER creates a dialog, and MAY be Record-Routed,
hence MUST contain a single Contact header field value. REFERs
occurring inside an existing dialog MUST follow the Route/Record-
Route logic of that dialog.

2.1 The Refer-To Header Field

Refer-To is a request header field (request-header) as defined by
[1]. It only appears in a REFER request. It provides a URL to
reference.

Refer-To = ("Refer-To" / "r") HCOLON ( name-addr / addr-spec ) *
(SEMI generic-param)

The following should be interpreted as if it appeared in Table 3 of
RFC3261.

Header field where proxy ACK BYE CAN INV OPT REG
___________________________________________________________________
Refer-To R - - - - - -

The Refer-To header field MAY be encrypted as part of end-to-end
encryption.

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.example.com

Refer-To: <sip:bob@biloxi.example.net?Accept-Contact=sip:bobsdesk.
biloxi.example.net&Call-ID%3D55432%40alicepc.atlanta.example.com>

Refer-To: <sip:dave@denver.example.org?Replaces=12345%40192.168.118.3%3B
to-tag%3D12345%3Bfrom-tag%3D5FFE-3994>

Refer-To: <sip:carol@cleveland.example.orgmethod=SUBSCRIBE>

Refer-To: http://www.ietf.org

Long headers field values are line-wrapped here for clarity only.

2.2 Header Field Support for the REFER Method

This table adds a column to tables 2 and 3 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,
mandatory for REFER. Refer-To is not applicable for any other
methods. The proxy column in [1] applies 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 -
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

2.3 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.

2.4 Behavior of SIP User Agents

2.4.1 Forming a REFER request

REFER is a SIP request and is constructed as defined in [1]. A REFER
request MUST contain exactly one Refer-To header field value.

2.4.2 Processing a REFER request

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 as discussed in Section
2.4.3.

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 4xx-6xx 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.

If a REFER request is accepted (that is, a 2xx class response is
returned), the recipient MUST create a subscription and send
notifications of the status of the refer as described in Section
2.4.4.

2.4.3 Accessing the Referred-to Resource

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 INVITE (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].

2.4.4 Using SIP Events to Report the Results of the Reference

The NOTIFY mechanism defined in [2] MUST be used to inform the agent
sending the REFER of the status of the reference. The dialog
identifiers (To, From, and Call-ID) of each NOTIFY must match those
of the REFER as they would if the REFER had been a SUBSCRIBE request.

Each NOTIFY MUST contain an Event header field with a value of refer
and possibly an id parameter (see Section 2.4.6).

Each NOTIFY MUST contain a body of type "message/sipfrag" [3].

The creation of a subscription as defined by [2] always results in an
immediate NOTIFY. Analogous to the case for SUBSCRIBE described in
that document, the agent that issued the REFER MUST be prepared to
receive a NOTIFY before the REFER transaction completes.

The implicit subscription created by a REFER is the same as a
subscription created with a SUBSCRIBE request. The agent issuing the
REFER can terminate this subscription prematurely by unsubscribing
using the mechanisms described in [2]. Terminating a subscription,
either by explicitly unsubscribing or rejecting NOTIFY, is not an
indication that the referenced request should be withdrawn or
abandoned. In particular, an agent acting on a REFER request SHOULD
NOT issue a CANCEL to any referenced SIP requests because the agent
sending the REFER terminated its subscription to the refer event
before the referenced request completes.

The agent issuing the REFER may extend its subscription using the
subscription refresh mechanisms described in [2].

REFER is the only mechanism that can create a subscription to event
refer. If a SUBSCRIBE request for event refer is received for a
subscription that does not already exist, it MUST be rejected with a
403.

Notice that unlike SUBSCRIBE, the REFER transaction does not contain
a duration for the subscription in either the request or the
response. The lifetime of the state being subscribed to is
determined by the progress of the referenced request. The duration
of the subscription is chosen by the agent accepting the REFER and is
communicated to the agent sending the REFER in the subscription's
initial NOTIFY (using the Subscription-State expires header
parameter). Note that agents accepting REFER and not wishing to hold
subscription state can terminate the subscription with this initial
NOTIFY.

2.4.5 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. This body provides a complete statement of the status of the
referred action. The refer event package does not support state
deltas.

If a NOTIFY is generated when the subscription state is pending, its
body should consist only of a status line containing a response code
of 100.

A minimal, but complete, implementation can respond with a single
NOTIFY containing either the body:

SIP/2.0 100 Trying

if the subscription is pending, the body:

SIP/2.0 200 OK

if the reference was successful, the body:

SIP/2.0 503 Service Unavailable

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
(see Section 2.4.7).

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 5). 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.

2.4.6 Multiple REFER Requests in a Dialog

A REFER creates an implicit subscription sharing the dialog
identifiers 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. A SUBSCRIBE sent to refresh
or terminate this subscription MUST contain this id parameter.

2.4.7 Using the Subscription-State Header Field with Event Refer

Each NOTIFY must contain a Subscription-State header field as defined
in [2]. The final NOTIFY sent in response to a REFER MUST indicate
the subscription has been "terminated" with a reason of "noresource".
(The resource being subscribed to is the state of the referenced
request).

If a NOTIFY indicates a reason that indicates a re-subscribe is
appropriate according to [2], the agent sending the REFER is NOT
obligated to re-subscribe.

In the case where a REFER was accepted with a 202, but approval to
follow the reference was subsequently denied, the reason and retry-
after elements of the Subscription-State header field can be used to
indicate if and when the REFER can be re-attempted (as described for
SUBSCRIBE in [2]).

2.5 Behavior of SIP Registrars/Redirect Servers

A registrar that is unaware of the definition of the REFER method
will return a 501 response as defined in [1]. A registrar aware of
the definition of REFER SHOULD return a 405 response.

This specification places no requirements on redirect server behavior
beyond those specified in [1]. Thus, it is possible for REFER
requests to be redirected.

2.6 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. Package Details: Event refer

This document defines an event package as defined in [2].

3.1 Event Package Name

The name of this event package is "refer".

3.2 Event Package Parameters

This package uses the "id" parameter defined in [2]. Its use in
package is described in Section 2.4.6.

3.3 SUBSCRIBE Bodies

SUBSCRIBE bodies have no special meaning for this event package.

3.4 Subscription Duration

The duration of an implicit subscription created by a REFER request
is initially determined by the agent accepting the REFER and
communicated to the subscribing agent in the Subscription-State
header field's expire parameter in the first NOTIFY sent in the
subscription. Reasonable choices for this initial duration depend on
the type of request indicated in the Refer-To URI. The duration
SHOULD be chosen to be longer than the time the referenced request
will be given to complete. For example, if the Refer-To URI is a SIP
INVITE URI, the subscription interval should be longer than the
Expire value in the INVITE. Additional time MAY be included to
account for time needed to authorize the subscription. The
subscribing agent MAY extend the subscription by refreshing it, or
terminate it by unsubscribing. As described in Section 2.4.7, the
agent accepting the REFER will terminate the subscription when it
reports the final result of the reference, indicating that
termination in the Subscription-State header field.

3.5 NOTIFY Bodies

The bodies of NOTIFY requests for event refer are discussed in
Section 2.4.5.

3.6 Notifier processing of SUBSCRIBE requests

Notifier processing of SUBSCRIBE requests is discussed in Section
2.4.4.

3.7 Notifier Generation of NOTIFY Requests

Notifier generation of NOTIFY requests is discussed in Section 2.4.4.

3.8 Subscriber Processing of NOTIFY Requests

Subscriber processing of NOTIFY requests is discussed in Section
2.4.4.

3.9 Handling of Forked Requests

A REFER sent within the scope of an existing dialog will not fork. A
REFER sent outside the context of a dialog MAY fork, and if it is
accepted by multiple agents, MAY create multiple subscriptions.
These subscriptions are created and managed as per "Handling of
Forked Requests" in [2] as if the REFER had been a SUBSCRIBE. The
agent sending the REFER manages the state associated with each
subscription separately. It does NOT merge the state from the
separate subscriptions. The state is the status of the referenced
request at each of the accepting agents.

3.10 Rate of Notifications

An event refer NOTIFY might be generated each time new knowledge of
the status of a referenced requests becomes available. For instance,
if the REFER was to a SIP INVITE, NOTIFYs might be generated with
each provisional response and the final response to the INVITE.
Alternatively, the subscription might only result in two NOTIFY
requests, the immediate NOTIFY and the NOTIFY carrying the final
result of the reference. NOTIFYs to event refer SHOULD NOT be sent
more frequently than once per second.

3.11 State Agents

Separate state agents are not defined for event refer.

4. Examples

4.1 Prototypical REFER callflow

Agent A Agent B

F1 REFER
----------------------->
F2 202 Accepted
<-----------------------
F3 NOTIFY
<-----------------------
F4 200 OK
----------------------->


------->
(whatever)
<------

F5 NOTIFY
<-----------------------
F6 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@atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP agenta.atlanta.example.com;branch=z9hG4bK2293940223
To: <sip:b@atlanta.example.com>
From: <sip:a@atlanta.example.com>;tag=193402342
Call-ID: 898234234@agenta.atlanta.example.com
CSeq: 93809823 REFER
Max-Forwards: 70
Refer-To: (whatever URI)
Contact: sip:a@atlanta.example.com
Content-Length: 0

Message Two (F2)

SIP/2.0 202 Accepted
Via: SIP/2.0/UDP agenta.atlanta.example.com;branch=z9hG4bK2293940223
To: <sip:b@atlanta.example.com>;tag=4992881234
From: <sip:a@atlanta.example.com>;tag=193402342
Call-ID: 898234234@agenta.atlanta.example.com
CSeq: 93809823 REFER
Contact: sip:b@atlanta.example.com
Content-Length: 0

Message Three (F3)

NOTIFY sip:a@atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP agentb.atlanta.example.com;branch=z9hG4bK9922ef992-25
To: <sip:a@atlanta.example.com>;tag=193402342
From: <sip:b@atlanta.example.com>;tag=4992881234
Call-ID: 898234234@agenta.atlanta.example.com
CSeq: 1993402 NOTIFY
Max-Forwards: 70
Event: refer
Subscription-State: active;expires=(depends on Refer-To URI)
Contact: sip:b@atlanta.example.com
Content-Type: message/sipfrag;version=2.0
Content-Length: 20

SIP/2.0 100 Trying

Message Four (F4)

SIP/2.0 200 OK
Via: SIP/2.0/UDP agentb.atlanta.example.com;branch=z9hG4bK9922ef992-25
To: <sip:a@atlanta.example.com>;tag=193402342
From: <sip:b@atlanta.example.com>;tag=4992881234
Call-ID: 898234234@agenta.atlanta.example.com
CSeq: 1993402 NOTIFY
Contact: sip:a@atlanta.example.com
Content-Length: 0

Message Five (F5)

NOTIFY sip:a@atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP agentb.atlanta.example.com;branch=z9hG4bK9323394234
To: <sip:a@atlanta.example.com>;tag=193402342
From: <sip:b@atlanta.example.com>;tag=4992881234
Call-ID: 898234234@agenta.atlanta.example.com
CSeq: 1993403 NOTIFY
Max-Forwards: 70

Event: refer
Subscription-State: terminated;reason=noresource
Contact: sip:b@atlanta.example.com
Content-Type: message/sipfrag;version=2.0
Content-Length: 16

SIP/2.0 200 OK

Message Six (F6)

SIP/2.0 200 OK
Via: SIP/2.0/UDP agentb.atlanta.example.com;branch=z9hG4bK9323394234
To: <sip:a@atlanta.example.com>;tag=193402342
From: <sip:b@atlanta.example.com>;tag=4992881234
Call-ID: 898234234@agenta.atlanta.example.com
CSeq: 1993403 NOTIFY
Contact: sip:a@atlanta.example.com
Content-Length: 0

4.2 Multiple REFERs in a dialog

Message One above brings an implicit subscription dialog into
existence. Suppose Agent A issued a second REFER inside that dialog:

Agent A Agent B

F7 REFER
----------------------->
F8 202 Accepted
<-----------------------
F9 NOTIFY
<-----------------------
F10 200 OK
----------------------->
------->
(something different)
<------

F11 NOTIFY
<-----------------------
F12 200 OK
----------------------->



Message Seven (F7)

REFER sip:b@atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP agenta.atlanta.example.com;branch=z9hG4bK9390399231
To: <sip:b@atlanta.example.com>;tag=4992881234
From: <sip:a@atlanta.example.com>;tag=193402342
Call-ID: 898234234@agenta.atlanta.example.com
CSeq: 93809824 REFER
Max-Forwards: 70
Refer-To: (some different URI)
Contact: sip:a@atlanta.example.com
Content-Length: 0

Message Eight (F8)

SIP/2.0 202 Accepted
Via: SIP/2.0/UDP agenta.atlanta.example.com;branch=z9hG4bK9390399231
To: <sip:b@atlanta.example.com>;tag=4992881234
From: <sip:a@atlanta.example.com>;tag=193402342
Call-ID: 898234234@agenta.atlanta.example.com
CSeq: 93809824 REFER
Contact: sip:b@atlanta.example.com
Content-Length: 0

Message Nine (F9)

NOTIFY sip:a@atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP agentb.atlanta.example.com;branch=z9hG4bK9320394238995
To: <sip:a@atlanta.example.com>;tag=193402342
From: <sip:b@atlanta.example.com>;tag=4992881234
Call-ID: 898234234@agenta.atlanta.example.com
CSeq: 1993404 NOTIFY
Max-Forwards: 70
Event: refer;id=93809824
Subscription-State: active;expires=(depends on Refer-To URI)
Contact: sip:b@atlanta.example.com
Content-Type: message/sipfrag;version=2.0
Content-Length: 20

SIP/2.0 100 Trying

Message Ten (F10)

SIP/2.0 200 OK
Via: SIP/2.0/UDP agentb.atlanta.example.com;branch=z9hG4bK9320394238995
To: <sip:a@atlanta.example.com>;tag=193402342
From: <sip:b@atlanta.example.com>;tag=4992881234
Call-ID: 898234234@agenta.atlanta.example.com

CSeq: 1993404 NOTIFY
Contact: sip:a@atlanta.example.com
Content-Length: 0

Message Eleven (F11)

NOTIFY sip:a@atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP agentb.atlanta.example.com;branch=z9hG4bK2994a93eb-fe
To: <sip:a@atlanta.example.com>;tag=193402342
From: <sip:b@atlanta.example.com>;tag=4992881234
Call-ID: 898234234@agenta.atlanta.example.com
CSeq: 1993405 NOTIFY
Max-Forwards: 70
Event: refer;id=93809824
Subscription-State: terminated;reason=noresource
Contact: sip:b@atlanta.example.com
Content-Type: message/sipfrag;version=2.0
Content-Length: 16

SIP/2.0 200 OK

Message Twelve (F12)

SIP/2.0 200 OK
Via: SIP/2.0/UDP agentb.atlanta.example.com;branch=z9hG4bK2994a93eb-fe
To: <sip:a@atlanta.example.com>;tag=193402342
From: <sip:b@atlanta.example.com>;tag=4992881234
Call-ID: 898234234@agenta.atlanta.example.com
CSeq: 1993405 NOTIFY
Contact: sip:a@atlanta.example.com
Content-Length: 0

5. Security Considerations

The security considerations described in Section 26 of [1] apply to
the REFER transaction. In particular, the implementation
requirements and considerations in Section 26.3 address securing a
generic SIP transaction. Special consideration is warranted for the
authorization polices applied to REFER requests and for the use of
message/sipfrag to convey the results of the referenced request.

5.1 Constructing a Refer-To URI

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.

5.2 Authorization Considerations for REFER

As described in Section 2.4.2, an implementation can receive a REFER
requests with a Refer-To URI containing an arbitrary scheme. For
instance, a user could be referred to an online service such as a MUD
using a telnet URI. Customer service could refer a customer to an
order tracking web page using an HTTP URI. Section 2.4.2 allows a
user agent to reject a REFER request when it can not process the
referenced scheme. It also requires the user agent to obtain
authorization from its user before attempting to use the URI.
Generally, this could be achieved by prompting the user with the full
URI and a question such as "Do you wish to access this resource
(Y/N)". Of course, URIs can be arbitrarily long and are occasionally
constructed with malicious intent, so care should be taken to avoid
surprise even in the display of the URI itself (such as partial
display or crashing). Further, care should be taken to expose as
much information about the reference as possible to the user to
mitigate the risk of being misled into a dangerous decision. For
instance, the Refer-To header may contain a display name along with
the URI. Nothing ensures that any property implied by that display
name is shared by the URI. For instance, the display name may
contain "secure" or "president" and when the URI indicates
sip:agent59@telemarketing.example.com. Thus, prompting the user with
the display name alone is insufficient.

In some cases, the user can provide authorization for some REFER
requests ahead of time by providing policy to the user agent. This
is appropriate, for instance, for call transfer as discussed in [4].
Here, a properly authenticated REFER request within an existing SIP
dialog to a sip:, sips:, or tel: URI may be accepted through policy
without interactively obtaining the user's authorization. Similarly,
it may be appropriate to accept a properly authenticated REFER to an
HTTP URI if the referror is on an explicit list of approved
referrors. In the absence of such pre-provided authorization, the
user must interactively provide authorization to reference the
indicated resource.

To see the danger of a policy that blindly accepts and acts on an
HTTP URI, for example, consider a web server configured to accept
requests only from clients behind a small organization's firewall.
As it sits in this soft-creamy-middle environment where the small
organization trusts all its members and has little internal security,
the web server is frequently behind on maintenance, leaving it
vulnerable to attack through maliciously constructed URIs (resulting
perhaps in running arbitrary code provided in the URI). If a SIP UA
inside this firewall blindly accepted a reference to an arbitrary
HTTP URI, an attacker outside the firewall could compromise the web
server. On the other hand, if the UA's user has to take positive

action (such as responding to a prompt) before acting on this URI,
the risk is reduced to the same level as the user clicking on the URI
in a web-browser or email message.

The conclusion in the above paragraph generalizes to URIs with an
arbitrary scheme. An agent that takes automated action to access a
URI with a given scheme risks being used to indirectly attack another
host that is vulnerable to some security flaw related to that scheme.
This risk and the potential for harm to that other host is heightened
when the host and agent reside behind a common policy-enforcement
point such as a firewall. Furthermore, this agent increases its
exposure to denial of service attacks through resource exhaustion,
especially if each automated action involves opening a new
connection.

User agents should take care when handing an arbitrary URI to a
third-party service such as that provided by some modern operating
systems, particularly if the user agent is not aware of the scheme
and the possible ramifications using the protocols it indicates. The
opportunity for violating the principal of least surprise is very
high.

5.3 Considerations for the use of message/sipfrag

Using message/sipfrag bodies to return the progress and results of a
REFER request is extremely powerful. Careless use of that capability
can compromise confidentiality and privacy. Here are a couple of
simple, somewhat contrived, examples to demonstrate the potential for
harm.

5.3.1 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
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 URI 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.

5.3.2 Circumventing Confidentiality

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

5.3.3 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 2.4.5 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.

5.3.4 Cut, Paste and Replay Considerations

The mechanism defined in this specification is not directly
susceptible to abuse through copying the message/sipfrag bodies from
NOTIFY requests and inserting them, in whole or in part, in future
NOTIFY requests associated with the same or a different REFER. Under
this specification the agent replying to the REFER request is in
complete control of the content of the bodies of the NOTIFY it sends.
There is no mechanism defined here requiring this agent to faithfully
forward any information from the referenced party. Thus, saving a
body for later replay gives the agent no more ability to affect the
mechanism defined in this document at its peer than it has without
that body. Similarly, capture of a message/sipfrag body by
eavesdroppers will give them no more ability to affect this mechanism
than they would have without it.

Future extensions may place additional constraints on the agent
responding to REFER to allow using the message/sipfrag body part in a
NOTIFY to make statements like "I contacted the party you referred me
to, and here's cryptographic proof". These statements might be used

to affect the behavior of the receiving UA. This kind of extension
will need to define additional mechanism to protect itself from copy
based attacks.

6. 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.

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.

7. IANA Considerations

This document defines a new SIP method name (REFER), a new SIP header
field name with a compact form (Refer-To and r respectively), and an
event package (refer).

The following has been added to the method sub-registry under
http://www.iana.org/assignments/sip-parameters.

REFER [RFC3515]

The following information also has been be added to the header sub-
registry under http://www.iana.org/assignments/sip-parameters.

Header Name: Refer-To

Compact Form: r

Reference: RFC3515

This specification registers an event package, based on the
registration procedures 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: RFC3515

Person to Contact: Robert Sparks, rsparks@dynamicsoft.com

8. Acknowledgments

This document is a collaborative product of the SIP working group.

9. References

9.1 Normative References

[1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC3261, June 2002.

[2] Roach, A. B., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC3265, June 2002.

[3] Sparks, R., "Internet Media Type message/sipfrag", RFC3420,
November 2002.

9.2 Informative References

[4] Sparks, R. and A. Johnston, "Session Initiation Protocol Call
Control - Transfer", Work in Progress.

10. Intellectual Property Statement

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11. Author's Address

Robert J. Sparks
dynamicsoft
5100 Tennyson Parkway
Suite 1200
Plano, TX 75024

EMail: rsparks@dynamicsoft.com

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