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Indicating Fax over IP Capability in the Session Initiation Protocol (SIP)
RFC 6913

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",

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