G.729.1 RTP Payload Format Update: Discontinuous Transmission (DTX) Support
RFC 5459

Network Working Group                                         A. Sollaud
Request for Comments: 5459                                France Telecom
Updates: 4749                                               January 2009
Category: Standards Track

                   G.729.1 RTP Payload Format Update:
                Discontinuous Transmission (DTX) Support

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.

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Abstract

   This document updates the Real-time Transport Protocol (RTP) payload
   format to be used for the International Telecommunication Union
   (ITU-T) Recommendation G.729.1 audio codec.  It adds Discontinuous
   Transmission (DTX) support to the RFC 4749 specification, in a
   backward-compatible way.  An updated media type registration is
   included for this payload format.

Sollaud                     Standards Track                     [Page 1]
RFC 5459               G.729.1 DTX Support in RTP           January 2009

Table of Contents

   1. Introduction ....................................................2
   2. Background ......................................................2
   3. RTP Header Usage ................................................3
   4. Payload Format ..................................................3
   5. Payload Format Parameters .......................................4
      5.1. Media Type Registration Update .............................4
      5.2. Mapping to SDP Parameters ..................................5
           5.2.1. DTX Offer-Answer Model Considerations ...............5
           5.2.2. DTX Declarative SDP Considerations ..................6
   6. Congestion Control ..............................................6
   7. Security Considerations .........................................6
   8. IANA Considerations .............................................6
   9. References ......................................................6
      9.1. Normative References .......................................6
      9.2. Informative References .....................................7

1.  Introduction

   The International Telecommunication Union (ITU-T) Recommendation
   G.729.1 [ITU-G.729.1] is a scalable and wideband extension of the
   Recommendation G.729 [ITU-G.729] audio codec.  [RFC4749] specifies
   the payload format for packetization of G.729.1-encoded audio signals
   into the Real-time Transport Protocol (RTP) [RFC3550].

   Annex C of G.729.1 [ITU-G.729.1-C] adds Discontinuous Transmission
   (DTX) support to G.729.1.  This document updates the RTP payload
   format to allow usage of this Annex.

   Only changes or additions to [RFC4749] will be described in the
   following sections.

   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].

2.  Background

   G.729.1 supports Discontinuous Transmission (DTX), a.k.a. "silence
   suppression".  It means that the coder includes a Voice Activity
   Detection (VAD) algorithm to determine if an audio frame contains
   silence or actual audio.  During silence periods, the coder may
   significantly decrease the transmitted bit rate by sending a small
   frame called a Silence Insertion Descriptor (SID) and then stop
   transmission.  The receiver's decoder will generate comfort noise
   (CNG) according to the parameters contained in the SID.  This DTX/CNG
   scheme is specified in [ITU-G.729.1-C].

Sollaud                     Standards Track                     [Page 2]
RFC 5459               G.729.1 DTX Support in RTP           January 2009

   The G.729.1 SID has an embedded structure.  The core SID is the same
   as the legacy G.729 SID [ITU-G.729-B].  The first enhancement layer
   adds some parameters for narrowband comfort noise, while a second