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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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 19, 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



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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.  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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