Internet Engineering Task Force J. Elwell
Internet Draft Siemens
R. Jesske
Deutsche Telekom
J. McMillen
Avaya Inc.
draft-elwell-sipping-redirection-reason-01.txt
Expires: April 2005 October 2004
SIP Reason header extension for indicating redirection reasons
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Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This examines the need for signalling additional information
concerning the reason for redirection in SIP and proposes an
extension to the Reason header.
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Table of Contents
1 Introduction....................................................3
2 Overview of proposed solution...................................4
3 Extension to the Reason header..................................4
4 Examples........................................................5
4.1 Redirection with reason "Forward Immediate"...................5
4.2 Call Forwarding Unconditional.................................7
5 IANA considerations.............................................9
6 Security Considerations.........................................9
7 Author's Addresses..............................................9
8 Normative References...........................................10
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1 Introduction
Central to SIP [2] is the concept of redirecting or retargeting a
request by a proxy, whereby the Request-URI in the original request
is replaced before forwarding the request on the next hop. Sometimes
this is due to normal rerouting behaviour of the proxy (e.g.,
resolving an address-of-record URI to a contact URI). At other times
it is due to more application-related reasons, e.g., where a user has
made arrangements for calls to that user under certain conditions to
be forwarded to a different destination. Also retargeting can be
performed as a result of a 3xx response from a redirect server.
Different 3xx response codes reflect different reasons for rejecting
the request. A 3xx response can be accompanied by a Reason header [1]
giving more information.
The History-Info header [3] provides a means for conveying
information about a retarget to the final destination UAS and also
back to the UAC. In addition to providing the retargeted-from and
retargeted-to URIs for each recorded retarget, this header also
conveys a reason by means of the Reason header. The Reason header
accompanies the retargeted-from URI and reflects the reason why
attempts to reach that target failed, normally in the form of the SIP
response code concerned.
However, there is nothing in either a 3xx response or the History-
Info header, including the Reason header, to indicate an explicit
reason for the redirection request or the retarget respectively. At
present the reason is implicit in the reason for failure of the
request to the original target. Sometimes this might give an accurate
picture of what is happening, but not always. Consider the following
cases:
1. A device acts as a redirect server because it is busy. None of
the 3xx response codes can reflect that the reason for retargeting
to the URI given in the Contact header of the 3xx response is
because the existing target is busy.
2. A device acts as a redirect server because it alerts the user
but fails to get a reply within a certain time. None of the 3xx
response codes can reflect that the reason for retargeting to the
URI given in the Contact header of the 3xx response is because the
existing target failed to answer.
3. A proxy is scripted to retarget requests without first
attempting to forward them to the original target. Retargeting may
be unconditional or based on certain conditions such as date, time,
the source of the request or caller preferences. Because it does
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this without forwarding the request to the original target, no SIP
response code is applicable.
4. A proxy is scripted to perform hunting or distribution of calls
among a number of different targets. When forwarding a request to a
target selected from a list of candidate targets, the reason for
retargeting is because of hunting or distribution, rather than
because of any failure of the existing target.
5. In the hunting or distribution scenario above, forwarding a
request to one target from the list of candidate targets may fail
for a particular reason (e.g., busy), leading to selection of
another target from the list. However, the reason for retargeting
is because of hunting or distribution, not specifically because the
previous target had a certain condition.
With these considerations in mind, there is a need to provide a
solution for conveying a more precise reason for redirection in a 3xx
response or a History-Info header.
In addition, in some circumstances a proxy can generate a 181 "Call
Is Being Forwarded" response. In this case no History-Info header
would be included, since the response does not come from a UAS. At
present a 181 response does not convey a precise reason for
forwarding. Provision of a reason for forwarding might be of use to
the UAC, which otherwise would need to wait for a later response
containing a History-Info header.
2 Overview of proposed solution
The proposed solution is to enhance the Reason header with new
values. This can be done by defining a new "protocol" value and then
defining specific new reason values under the umbrella of the new
"protocol" value.
Candidate reasons include forward unavailable, forward busy, forward
no reply, forward immediate, deflection, hunting, mobile not
reachable.
Note that selection of the new target may depend on several other
conditions (e.g., relating to date, time, the source of the request
or caller preferences), but the reasons suggested above should be
sufficient to convey the main circumstance leading to the retarget.
3 Extension to the Reason header
This document defines the following new protocol value for the
protocol field of the Reason header field in [1]:
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Redirection: The cause parameter contains a reason for redirection
of a request.
This document defines the following redirection cause codes:
Value Default Text Description
1 Normal redirection The call has been retargeted for
normal routing reasons
2 Forward unavailable The call has been retargeted
because the called user is
unavailable (no registered contact).
3 Forward busy The call has been retargeted
because the called user is busy.
4 Forward no reply The call has been retargeted
because the called user has been
alerted but has failed to reply.
5 Forward immediate The call has been retargeted
immediately without determining
whether the called user is
unavailable or busy and without
alerting the user.
6 Deflection The call has been retargeted as a
result of a request by the called
user in response to alerting.
7 Hunting The call has been retargeted to an
individual member of the hunt group
at which it was previously targeted.
8 Mobile not reachable The call has been retargeted
because the called mobile user is
not reachable
Example syntax is as follows:
Reason: redirection;cause=3 ;text="Forward busy"
4 Examples
4.1 Redirection with reason "Forward Immediate"
Alice Proxy Bob Carol
| | | |
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| INVITE F1 | | |
|--------------->| INVITE F2 | |
| |------------->| |
|(100 Trying) F3 | | |
|<---------------| 302 Moved Temporarily F4 |
| |<-------------| |
| | ACK F5 | |
| |------------->| |
| | | INVITE F6 |
| |--------------------------------->|
| | | 180 Ringing F7 |
| |<---------------------------------|
| 180 Ringing F8 | | |
|<---------------| | 200 OK F9 |
| |<---------------------------------|
| 200 OK F10 | | |
|<---------------| | |
| ACK F11 | | |
|--------------->| | ACK F12 |
| |--------------------------------->|
Assuming the entity sending the INVITE supports the History-Info
header, the INVITE would look like this:
F1 (INVITE) Alice -> Proxy
INVITE sip:Bob@example.com; SIP/2.0
From: <sip:Alice@example.com>;tag=2
To: <sip:Bob@example.com >
Call-ID: 12345600@example.com
CSeq: 1 INVITE
History-Info: <sip:Bob@example.com>;index=1
...
The call is then redirected to a contact URI <sip:carol@example.com>
in a 302 response. The response would be as follows:
F3 (302 Moved Temporarily) Bob -> Proxy
SIP/2.0 302 Moved temporarily
From: <sip:Alice @example.com>;tag=2
To: <sip:Bob@example.com>;tag=3
Call-ID: 12345600@example.com
CSeq: 1 INVITE
Contact: <sip:Carol@example.com>
Reason: Redirection;cause=5;text="Forward immediate"
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The call would be retargeted to the contact URI. The first History-
Info header would be augmented with the two reasons for retargeting
(SIP 302 and redirection 5)). A second History-Info header would be
added with the new retargeted-to Request-URI:
F6 (INVITE) ยก Proxy -> Carol
INVITE sip:Carol@example.com SIP/2.0
From: <sip:Alice@example.com>;tag=2
To: <sip:Bob@example.com>
Call-ID: 12345600@example.com
CSeq: 1 INVITE
History-Info: <sip:Bob@example.com?Reason: SIP;cause=302;
text="Moved temporarily"?Reason: Redirection; cause=5;text="Forward
immediate">;index=1, <sip:Carol@example.com>;index=2
The "index 1" entry indicates that the call to Bob was retargeted
because of SIP response code 302 and redirection reason CFI.
The "index 2" entry indicates that the call to Carol has not yet been
further retargeted.
4.2 Call Forwarding Unconditional
This example is taken from [4], augmented to show the use of History-
Info and Reason headers.
Alice Proxy Gateway
| | |
| INVITE F1 | |
|--------------->| |
|(100 Trying) F2 | |
|<---------------| |
| (181 Call Is Being Forwarded) F3
|<---------------| INVITE F4 |
| |------------->|
| |180 Ringing F5|
| 180 Ringing F6 |<-------------|
|<---------------| 200 OK F7 |
| 200 OK F8 |<-------------|
|<---------------| |
| ACK F9 | |
|--------------->| ACK F10 |
| |------------->|
| Both way RTP Established |
|<=============================>|
| BYE F11 | |
|--------------->| BYE F12 |
| |------------->|
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| | 200 OK F13 |
| 200 OK F14 |<-------------|
|<---------------| |
| | |
Bob wants all calls forwarded to a destination in the PSTN (which is
just another URI to the proxy server). Alice calls Bob. The proxy
server rewrites the Request URI, and forwards the INVITE to a
Gateway. Details of messaging behind the Gateway are not shown.
Message Details
F3 (181 Call is Being Forwarded) Proxy -> Alice
SIP/2.0 181 Call is Being Forwarded
Via: SIP/2.0/TLS
client.atlanta.example.com:5061;branch=z9hG4bK74bf9
;received=192.0.2.103
From: Alice <sips:alice@atlanta.example.com>;tag=1234567
To: Bob <sips:bob@biloxi.example.com>
Call-ID: 12345600@atlanta.example.com
CSeq: 1 INVITE
Content-Length: 0
Reason: Redirection;cause=5;text="Forward immediate"
F4 INVITE Proxy -> Gateway
INVITE sips:+19727293660@gw1.example.com;user=phone SIP/2.0
Via: SIP/2.0/TLS ss1.example.com:5061;branch=z9hG4bK83749.1
Via: SIP/2.0/TLS
client.atlanta.example.com:5061;branch=z9hG4bK74bf9
;received=192.0.2.103
Record-Route: <sips:ss1.example.com;lr>
Max-Forwards: 69
From: Alice <sips:alice@atlanta.example.com>;tag=1234567
To: Bob <sips:bob@biloxi.example.com>
Call-ID: 12345600@atlanta.example.com
CSeq: 1 INVITE
Contact: <sips:alice@client.atlanta.example.com>
History-Info: <sips:bob@biloxi.example.com?Reason: Redirection;
cause=5;text="Forward immediate">;index=1,
<sips:+19727293660@gw1.example.com;user=phone>;index=2
Content-Type: application/sdp
Content-Length: ...
v=0
o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
s=Session SDP
c=IN IP4 client.atlanta.example.com
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t=3034423619 0
m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000
5 IANA considerations
This document defines one new value for the SIP Reason header [1]
protocol namespace. The new value is "Redirection" and indicates the
use of cause value defined in this document.
This document also creates an IANA registry for cause values that
populate the cause field of the Reason header when protocol value
"Redirection" is used and corresponding default values that populate
the text field. The current cause and text values in this new
registry are as follows:
Cause value Default text value Reference
----------- ------------------------------
1 Normal redirection This document
2 Forward unavailable This document
3 Forward busy This document
4 Forward no reply This document
5 Forward immediate This document
6 Deflection This document
7 Hunting This document
8 Mobile not reachable This document
New values for this registry can only be defined by means of a
published standard.
6 Security Considerations
The security considerations of [1] apply. When the Reason header
field is embedded within a History-Info header field, the security
considerations of [3] apply.
Unauthorised insertion, deletion of modification of the Reason header
field can provide misleading information to users and applications.
Eavesdropping on this header field can reveal information about a
user. Securing of SIP connections by TLS can combat this problem.
A SIP entity that can provide a redirection reason in a Reason header
field SHOULD be able to suppress this in accordance with privacy
requirements of the user concerned.
7 Author's Addresses
John Elwell
Siemens Communications
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Technology Drive
Beeston
Nottingham, UK, NG9 1LA
email: john.elwell@siemens.com
Roland Jesske
Deutsch Telekom
Am Kavalleriesand 3
Germany-64295 Darmstadt
email: r.jesske@t-com.net
Joanne McMillen
Avaya Inc.
1300 W. 120th Ave.
Westminster, CO 80234-2726
email: joanne@avaya.com
8 Normative References
[1] H. Schulzrinne, D. Oran, G. Camarillo, "The Reason Header for the
Session Initiation Protocol (SIP)", RFC 3326.
[2] J. Rosenberg, H. Schulzrinne, et al., "SIP: Session initiation
protocol", RFC 3261.
[3] M. Barnes, "An Extension to the Session Initiation Protocol for
Request History Information", draft-ietf-sipping-history-info-03
(work in progress).
[4] A. Johnston et alia, "Session Initiation Protocol Service
Examples", draft-ietf-sipping-service-examples-07 (work in progress).
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