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DISPATCH                                                        D. Hanes
Internet-Draft                                              G. Salgueiro
Intended status: Standards Track                           Cisco Systems
Expires: April 11, 2013                                       K. Fleming
                                                            Digium, Inc.
                                                         October 8, 2012

                   Indicating Fax over IP Capability
                in the Session Initiation Protocol (SIP)


   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 [RFC3841]
   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 April 11, 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  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8

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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[T38], 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[T38] 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 [RFC3841]
   is necessary.  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 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 [RFC3841] as a feature preference for
   fax capable UAs.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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[T38] 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.

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   For example, Alice wants to send a fax to Bob. Bob registers 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 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 [RFC3841] changes this scenario and
   feature preferences are used to resolve this issue.  With RFC 3841
   [RFC3841], 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.  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"

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       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;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 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:{PH}

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   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

   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 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 (]]

   [[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

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   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, and Dale Worley.

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.

   [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.

8.2.  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.

   [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.

   [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",

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              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

   Email: dhanes@cisco.com

   Gonzalo Salgueiro
   Cisco Systems
   7200-12 Kit Creek Road
   Research Triangle Park, NC  27709

   Email: gsalguei@cisco.com

   Kevin P. Fleming
   Digium, Inc.
   445 Jan Davis Drive NW
   Huntsville, AL  35806

   Email: kevin@kpfleming.us

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