Internet DRAFT - draft-hanes-dispatch-fax-capability
draft-hanes-dispatch-fax-capability
DISPATCH D. Hanes
Internet-Draft G. Salgueiro
Intended status: Standards Track Cisco Systems
Expires: August 19, 2013 K. Fleming
Digium, Inc.
February 15, 2013
Indicating Fax over IP Capability
in the Session Initiation Protocol (SIP)
draft-hanes-dispatch-fax-capability-08
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 initiation. Consequently, fax
calls can be routed to SIP user agents that are not fax capable. A
'fax' media feature tag implemented in conjunction with caller
preferences 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.
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This Internet-Draft will expire on August 19, 2013.
Copyright Notice
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Usage of the sip.fax Parameter . . . . . . . . . . . . . . . . 4
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
Fax communications in the Session Initiation Protocol (SIP) [RFC3261]
are handled in a "voice first" manner. Indications that a user
desires to use a fax transport protocol, such as ITU-T T.38 [T38], to
send a fax are not known when the initial INVITE message is sent.
The call is set up as a voice call first and then only after it is
connected, does a switchover to the T.38 [T38] protocol occur. This
is problematic in that fax calls can be routed inadvertently to SIP
user agents (UAs) 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 [RFC3841]
can be used. Feature preferences are a part of RFC 3841 [RFC3841]
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 are routed to 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 [RFC3841] 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 Session Description Protocol (SDP) portion of the
SIP [RFC3261] communication. However, fax is somewhat unique in that
the ultimate intention of the call is not accurately signaled in the
initial SDP exchange. Specifically, indications of T.38 [T38] or any
other fax transport protocol in the call are not known when the call
is initiated by an INVITE message. Fax calls are always considered
voice calls until after they are connected. This results in the
possibility 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 has registered two
SIP UAs. The first SIP UA is not fax capable but the second one
supports the T.38 [T38] fax protocol. Currently, SIP servers are
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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.
To resolve this issue of calls not arriving at a UA supporting fax,
this document defines a new media feature tag specific to fax per RFC
3840 [RFC3840]. Caller preferences as defined in RFC3841 [RFC3841]
can then be used for registering UAs that support fax and routing fax
calls to these UAs. Thus, Alice can express up front that she
prefers a T.38 [T38] 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. Usage of the sip.fax Parameter
The sip.fax media feature tag is a new string parameter, defined in
this document, that allows a call to indicate a fax preference. A
receiving UA includes the "sip.fax" media feature tag in the Contact
header field of REGISTER messages to indicate that it is fax-capable,
and a SIP Registrar includes this tag in the Contact header field of
its 200 OK response to confirm the registration of this preference,
all as per RFC 3840 [RFC3840].
A calling UA SHOULD include the "sip.fax" media feature tag in the
Accept-Contact header of an INVITE request in order to express its
desire for a call to be routed to a fax capable UA. Otherwise,
without this tag, fax call determination is not possible until after
the call is connected. If a calling UA does so, and the SIP network
elements that process the call (including the called UA(s)) implement
RFC 3840 and RFC 3841 procedures, then the call will be
preferentially routed to UAs that have advertised their support for
this feature (by including it in the Contact header of their REGISTER
requests, as documented above).
It is possible for the calling UA to utilize additional procedures in
RFC 3840 and RFC 3841 to express a requirement (instead of a
preference) that its call be delivered to fax-capable UAs. However,
the calling UA SHOULD NOT require the "sip.fax" media type. Doing so
could result in call failure for a number of reasons, not only
because there may not be any receiving UAs registered that have
advertised their support for this feature, but also because one or
more SIP network elements that process the call may not support RFC
3840 and RFC 3841 processing. A calling UA that wishes to express
this requirement should be prepared to relax it to a preference if it
receives a failure response indicating that the requirement mechanism
itself is not supported by the called UA(s), their proxies, or other
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SIP network elements.
When calls do connect through the use of "sip.fax" either as a
preference or a requirement, then UAs should follow standard fax
negotiation procedures documented in ITU-T T.38 [T38] for T.38 fax
calls and ITU-T G.711 [G711] and ITU-T V.152 [V152] sections 6 and
6.1 for fax passthrough calls. Subsequently, the "sip.fax" feature
tag has two allowed values: "t38" and "passthrough". The "t38" value
indicates that the impending call will utilize the ITU-T T.38 [T38]
protocol for the fax transmission. The "passthrough" value indicates
that the ITU-T G.711 [G711] codec will be used to transport the fax
call.
5. Example
Bob registers with the fax media feature tag. The message flow is
shown in Figure 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:5060;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@pc33.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:5060;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:bob@biloxi.example.com SIP/2.0
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74b43
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>
Accept-Contact: *;+sip.fax="t38"
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Contact: <sip:alice@client.atlanta.example.com;transport=tcp>
Content-Type: application/sdp
Content-Length: 151
6. Security Considerations
The security considerations related to the use of media feature tags
from Section 11.1 of RFC 3840 [RFC3840] apply.
7. 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].
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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
[T38] fax protocol ("t38") or the passthrough method of fax
transmission using the ITU-T G.711 [G711] audio codec
("passthrough").
Values appropriate for use with this feature tag: Token with an
equality relationship. Values are:
t38: The device supports the image/t38 media type [RFC3326] and
implements ITU-T T.38 [T38] for transporting the ITU-T T.30
[T30] and ITU-T T.4 [T4] fax data over IP.
passthrough: The device supports the audio/pcmu and audio/pcma
media types [RFC4856] for transporting ITU-T T.30 [T30] and
ITU-T T.4 [T4] fax data using the ITU-T G.711 [G711] audio
codec. Additional implementation recommendations are in ITU-T
V.152 [V152] Sections 6 and 6.1.
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 Fax over IP (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: The security considerations related to the
use of media feature tags from Section 11.1 of RFC 3840 [RFC3840]
apply.
[[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.]]
8. Acknowledgements
This document is a result of the unique cooperation between the SIP
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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.
This memo has benefited from the discussion and review of the
DISPATCH working group, especially the detailed and thoughtful
comments and corrections of Dan Wing, Paul Kyzivat, Christer
Holmberg, Charles Eckel, Hadriel Kaplan, Tom Yu, Dale Worley, Adrian
Farrel and Pete Resnick.
The authors also thank Gonzalo Camarillo for his review and AD
sponsorship of this draft and DISPATCH WG chair, Mary Barnes, for her
review and support.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[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.
[T38] International Telecommunication Union, "Procedures for
real-time Group 3 facsimile communication over IP
Networks", ITU-T Recommendation T.38, October 2010.
9.2. Informative References
[G711] International Telephone and Telegraph Consultative
Committee, "Pulse Code Modulation (PCM) of Voice
Frequencies", CCITT Recommendation G.711, 1972.
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[RFC2506] Holtman, K., Mutz, A., and T. Hardie, "Media Feature Tag
Registration Procedure", BCP 31, RFC 2506, March 1999.
[RFC3326] Schulzrinne, H., Oran, D., and G. Camarillo, "The Reason
Header Field for the Session Initiation Protocol (SIP)",
RFC 3326, December 2002.
[RFC4856] Casner, S., "Media Type Registration of Payload Formats in
the RTP Profile for Audio and Video Conferences",
RFC 4856, February 2007.
[T30] International Telecommunication Union, "Procedures for
document facsimile transmission in the general switched
telephone network", ITU-T Recommendation T.30,
September 2005.
[T4] International Telecommunication Union, "Standardization of
Group 3 facsimile terminals for document transmission",
ITU-T Recommendation T.4, July 2003.
[V152] International Telecommunication Union, "Procedures for
supporting voice-band data over IP networks", ITU-
T Recommendation V.152, September 2010.
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
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Kevin P. Fleming
Digium, Inc.
445 Jan Davis Drive NW
Huntsville, AL 35806
US
Email: kevin@kpfleming.us
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