Internet Engineering Task Force (IETF) D. Hanes
Request for Comments: 6913 G. Salgueiro
Category: Standards Track Cisco Systems
ISSN: 2070-1721 K. Fleming
Digium, Inc.
March 2013
Indicating Fax over IP Capability
in the Session Initiation Protocol (SIP)
Abstract
This document defines and registers with IANA the new "fax" media
feature tag for use with the Session Initiation Protocol (SIP).
Currently, fax calls are indistinguishable from voice calls 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 is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6913.
Hanes, et al. Standards Track [Page 1]RFC 6913 Fax Media Feature Tag March 2013
Copyright Notice
Copyright (c) 2013 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
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 . . . . . . . . . . . . . . . . . . . . . . . . . 2
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 . . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . . 8
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 UAs, 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 UAs.
Hanes, et al. Standards Track [Page 2]RFC 6913 Fax Media Feature Tag March 2013
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 RFC 2119 [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 UAs that are 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
unable to know at the time that 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 that supports
fax, this document defines a new media feature tag specific to fax,
per RFC 3840 [RFC3840]. Caller preferences, as defined in RFC 3841
[RFC3841], can then be used for registering UAs that support fax and
for 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.
Hanes, et al. Standards Track [Page 3]
RFC 6913 Fax Media Feature Tag March 2013
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 includes the "sip.fax" tag
and the SIP network elements that process the call (including the
called UAs) implement the procedures of RFC 3840 and RFC 3841, 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
defined 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 the
processing defined in RFC 3840 and RFC 3841. 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 UAs,
their proxies, or other SIP network elements.
When calls do connect through the use of "sip.fax" either as a
preference or a requirement, 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.
Hanes, et al. Standards Track [Page 4]
RFC 6913 Fax Media Feature Tag March 2013
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
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.
Hanes, et al. Standards Track [Page 5]RFC 6913 Fax Media Feature Tag March 2013
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].
Media feature tag name: sip.fax
ASN.1 Identifier: 1.3.6.1.8.4.25
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.
Hanes, et al. Standards Track [Page 6]RFC 6913 Fax Media Feature Tag March 2013
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: RFC 6913
Security Considerations: The security considerations related to the
use of media feature tags from Section 11.1 of RFC 3840 [RFC3840]
apply.
8. 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 who 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.
Hanes, et al. Standards Track [Page 7]RFC 6913 Fax Media Feature Tag March 2013
[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.
[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.
Hanes, et al. Standards Track [Page 8]RFC 6913 Fax Media Feature Tag March 2013
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
Hanes, et al. Standards Track [Page 9]
