Definition of Events for Modem, Fax, and Text Telephony Signals
RFC 4734

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Network Working Group                                     H. Schulzrinne
Request for Comments: 4734                                   Columbia U.
Obsoletes: 2833                                                T. Taylor
Updates: 4733                                                     Nortel
Category: Standards Track                                  December 2006

    Definition of Events for Modem, Fax, and Text Telephony Signals

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2006).

Abstract

   This memo updates RFC 4733 to add event codes for modem, fax, and
   text telephony signals when carried in the telephony event RTP
   payload.  It supersedes the assignment of event codes for this
   purpose in RFC 2833, and therefore obsoletes that part of RFC 2833.

Schulzrinne & Taylor        Standards Track                     [Page 1]
RFC 4734         Modem, Fax, and Text Telephony Events     December 2006

Table of Contents

   1. Introduction ....................................................3
      1.1. Terminology ................................................3
      1.2. Overview ...................................................3
   2. Definitions of Events for Control of Data, Fax, and Text
      Telephony Sessions ..............................................5
      2.1. V.8 bis Events .............................................5
           2.1.1. Handling of Congestion ..............................9
      2.2. V.21 Events ...............................................10
           2.2.1. Handling of Congestion .............................11
      2.3. V.8 Events ................................................12
           2.3.1. Handling of Congestion .............................15
      2.4. V.25 Events ...............................................15
           2.4.1. Handling of Congestion .............................17
      2.5. V.32/V.32bis Events .......................................18
           2.5.1. Handling of Congestion .............................19
      2.6. T.30 Events ...............................................19
           2.6.1. Handling of Congestion .............................23
      2.7. Events for Text Telephony .................................23
           2.7.1. Signal Format Indicators for Text Telephony ........23
           2.7.2. Use of Events with V.18 Modems .....................27
      2.8. A Generic Indicator .......................................28
   3. Strategies for Handling Fax and Modem Signals ..................29
   4. Example of V.8 Negotiation .....................................30
      4.1. Simultaneous Transmission of Events and
           Retransmitted Events Using RFC 2198 Redundancy ............35
      4.2. Simultaneous Transmission of Events and Voice-Band
           Data Using RFC 2198 Redundancy ............................37
   5. Security Considerations ........................................39
   6. IANA Considerations ............................................40
   7. Acknowledgements ...............................................42
   8. References .....................................................43
      8.1. Normative References ......................................43
      8.2. Informative References ....................................44

Schulzrinne & Taylor        Standards Track                     [Page 2]
RFC 4734         Modem, Fax, and Text Telephony Events     December 2006

1.  Introduction

1.1.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [1].

   In addition to those defined for specific events, this document uses
   the following abbreviations:

   Fax    facsimile

   HDLC   High-level Data Link Control

   PSTN   Public Switched (circuit) Telephone Network

1.2.  Overview

   This document extends the set of telephony events defined within the
   framework of RFC 4733 [5] to include the control events and tones
   that can appear on a subscriber line serving a fax machine, a modem,
   or a text telephony device.  The events are organized into several
   groups, corresponding to the ITU-T Recommendation in which they are
   defined.  Their purpose is to support negotiation, start-up and
   takedown of fax, modem, or text telephony sessions and transitions
   between operating modes.  The actual fax, modem, and text payload is
   typically carried by other payload types (e.g., V.150.1 [32] modem
   relay, voice-band data as formalized in ITU-T Rec. V.152 [33],