Indicating Fax over IP Capability in the Session Initiation Protocol (SIP)
draft-hanes-dispatch-fax-capability-01
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | David Hanes , Gonzalo Salgueiro , Kevin P. Fleming | ||
| Last updated | 2012-08-31 | ||
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draft-hanes-dispatch-fax-capability-01
DISPATCH D. Hanes
Internet-Draft G. Salgueiro
Intended status: Standards Track Cisco Systems
Expires: March 4, 2013 K. Fleming
Digium, Inc.
August 31, 2012
Indicating Fax over IP Capability
in the Session Initiation Protocol (SIP)
draft-hanes-dispatch-fax-capability-01
Abstract
This document defines and registers with IANA the new 'fax' media
feature tag for use with SIP. Currently, fax calls are
indistinguishable from voice at call setup. Consequently, fax calls
can be routed to SIP user agents that are not fax capable. A 'fax'
media feature tag implemented in conjunction with RFC 3841 allows for
more accurate fax call routing.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 4, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7
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1. Introduction
Fax communications in the Session Initiation Protocol (SIP) are
handled in a "voice first" manner. Indications that a user desires
to use a fax transport protocol, such as ITU-T T.38, to send a fax
are not known during the call setup. The call is set up as a voice
call first and then only after it is connected, does a switchover to
the T.38 protocol occur. This is problematic in that fax calls can
be routed inadvertently to SIP user agents that are not fax capable.
To ensure that fax calls are routed to fax capable SIP user agents,
an implementation of caller preferences defined in RFC 3841 is
necessary. Feature preferences are a part of RFC 3841 that would
allow UAs to express their preference for receiving fax
communications. Subsequently SIP servers take these preferences into
account to increase the likelihood that fax calls land at fax capable
SIP user agents.
This document defines the 'fax' media feature tag for use in the SIP
tree as per Section 12.1 of RFC 3840 [RFC3840]. This feature tag
will be applied per RFC 3841 as a feature preference for fax capable
UAs.
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 [RFC2119].
3. Motivation
In the majority of circumstances, it is preferred that capabilities
be handled in the SDP portion of the SIP communication. However, fax
is somewhat unique in that the ultimate intention of the call is not
accurately signaled in the initial SDP exchange. Fax is one of the
few situations where a media feature tag indicating a capability is
highly predictive of the ultimate communication request that will be
made in the near future but is not indicated by the current SDP.
Specifically, indications of T.38 or any other fax transport protocol
in the call are not known when the call is setup. Fax calls are
always considered voice calls until after they are connected. This
results in increased chances of fax calls being received by SIP user
agents not capable of handling fax transmissions.
For example, Alice wants to send a fax to Bob. Bob registers two SIP
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UAs. The first SIP UA is not fax capable but the second one supports
the T.38 fax protocol. Currently, SIP servers are unable to know
when the call starts that Alice prefers a fax capable SIP UA to
handle her call. Additionally, the SIP servers are also not aware of
which of Bob's SIP UAs are fax capable.
An implementation of RFC3841 changes this scenario and feature
preferences are used to resolve this issue. With RFC 3841, Alice can
express up front that she prefers a T.38 fax capable SIP UA for this
call. At the same time, Bob's SIP UAs have expressed their fax
capabilities as well during registration. Now when Alice places a
fax call to Bob, the call is appropriately routed to Bob's fax
capable SIP UA.
4. Example
Bob registers with the fax media feature tag. The message flow is
shown in FFigure 1:
SIP Registrar Bob's SIP UA
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| |
| REGISTER F1 |
|<------------------------------|
| |
| 200 OK F2 |
|------------------------------>|
| |
Figure 1: Fax Media Feature Tag SIP Registration Example
F1 REGISTER Bob -> Registrar
REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/TCP bob-TP@example.com;branch=z9hG4bK309475a2
From: <sip:bob-tp@example.com>;tag=a6c85cf
To: <sip:bob-tp@pexample.com>
Call-ID: a84b4c76e66710
Max-Forwards: 70
CSeq: 116 REGISTER
Contact: <sip:bob-tp@example.com;transport=tcp>;+sip.fax="t38"
Expires: 3600
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The registrar responds with a 200 OK:
F2 200 OK Registrar -> Bob
SIP/2.0 200 OK
From: <sip:bob-tp@example.com>;tag=a6c85cf
To: <sip:bob-tp@example.com>;tag=1263390604
Contact: <sip:bob-tp@example.com;transport=tcp>;+sip.fax="t38"
Expires: 120
Call-ID: a84b4c76e66710
Via: SIP/2.0/TCP bob-TP@example.com;branch=z9hG4bK309475a2
CSeq: 116 REGISTER
Expires: 3600
Callers desiring to express a preference for fax will include the
sip.fax media feature tag in the Accept-Contact header of their
INVITE.
INVITE sip:UserY@example.com SIP/2.0
From: sip:UserX@operator.com
To: sip:UserY@example.com
Accept-Contact: +sip.fax="t38"
Content-Type: application/sdp
5. Security Considerations
The security considerations related to the use of media feature tags
from Section 11.1 of RFC 3840 [RFC3840] apply.
6. IANA Considerations
This specification adds a new media feature tag to the SIP Media
Feature Tag Registration Tree per the procedures defined in RFC 2506
[RFC2506] and RFC 3840 [RFC3840].
Media feature tag name: sip.fax
ASN.1 Identifier: 1.3.6.1.8.4.{PH}
Summary of the media feature indicated by this tag: This feature tag
indicates whether a communications device supports the ITU-T T.38
fax protocol ("t38") or the passthrough method of fax transmission
using an audio codec ("passthrough").
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Values appropriate for use with this feature tag: Token with an
equality relationship. Typical values include:
t38: The device supports the ITU-T T.38 standard for fax
communication.
passthrough: The device supports the transmission of fax using an
audio codec.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation mechanisms: This
feature tag is most useful in a communications application for the
early identification of a FoIP call.
Examples of typical use: Ensuring a fax call is routed to a fax
capable SIP UA.
Related standards or documents: RFCXXXX
Security Considerations: Security considerations for this media
feature tag are discussed in Section 5 of this document.
[[NOTE TO RFC EDITOR: Please change {PH} above to the correct
identifier for this entry in the IANA registry for
iso.org.dod.internet.features.sip-tree (1.3.6.1.8.4)]]
[[NOTE TO RFC EDITOR: Please change XXXX to the number assigned to
this specification, and remove this paragraph on publication.]]
7. Acknowledgements
This document is a result of the unique cooperation between the SIP
Forum and the i3 Forum who embarked on a groundbreaking international
test program for FoIP to improve the interoperability and reliability
of fax communications over IP networks, especially tandem networks.
The authors would like to acknowledge the effort and dedication of
all the members of the Fax-over-IP (FoIP) Task Group in the SIP Forum
and the communications carriers of the I3 Forum that contributed to
this global effort.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
"Indicating User Agent Capabilities in the Session
Initiation Protocol (SIP)", RFC 3840, August 2004.
[RFC3841] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
Preferences for the Session Initiation Protocol (SIP)",
RFC 3841, August 2004.
8.2. Informative References
[RFC2506] Holtman, K., Mutz, A., and T. Hardie, "Media Feature Tag
Registration Procedure", BCP 31, RFC 2506, March 1999.
[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.
Authors' Addresses
David Hanes
Cisco Systems
7200-10 Kit Creek Road
Research Triangle Park, NC 27709
US
Email: dhanes@cisco.com
Gonzalo Salgueiro
Cisco Systems
7200-12 Kit Creek Road
Research Triangle Park, NC 27709
US
Email: gsalguei@cisco.com
Kevin P. Fleming
Digium, Inc.
445 Jan Davis Drive NW
Huntsville, AL 35806
US
Email: kevin@kpfleming.us
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