RTP Payload Format for the Speex Codec
RFC 5574

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Network Working Group                                         G. Herlein
Request for Comments: 5574                                   Independent
Category: Standards Track                                       J. Valin
                                                     Xiph.Org Foundation
                                                            A. Heggestad
                                                             Creytiv.com
                                                              A. Moizard
                                                                 Antisip
                                                               June 2009

                 RTP Payload Format for the Speex Codec

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) 2009 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 in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   Speex is an open-source voice codec suitable for use in VoIP (Voice
   over IP) type applications.  This document describes the payload
   format for Speex-generated bit streams within an RTP packet.  Also
   included here are the necessary details for the use of Speex with the
   Session Description Protocol (SDP).

Herlein, et al.             Standards Track                     [Page 1]
RFC 5574                         Speex                         June 2009

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................3
   3. RTP Usage for Speex .............................................3
      3.1. RTP Speex Header Fields ....................................3
      3.2. RTP Payload Format for Speex ...............................4
      3.3. Speex Payload ..............................................4
      3.4. Example Speex Packet .......................................5
      3.5. Multiple Speex Frames in an RTP Packet .....................5
   4. IANA Considerations .............................................6
      4.1. Media Type Registration ....................................6
           4.1.1. Registration of Media Type Audio/Speex ..............6
   5. SDP Usage of Speex ..............................................8
      5.1. Example Supporting All Modes, Prefer Mode 4 ...............10
      5.2. Example Supporting Only Modes 3 and 5 .....................10
      5.3. Example with Variable Bit-Rate and Comfort Noise ..........10
      5.4. Example with Voice Activity Detection .....................11
      5.5. Example with Multiple Sampling Rates ......................11
      5.6. Example with Ptime and Multiple Speex Frames ..............11
      5.7. Example with Complete Offer/Answer Exchange ...............12
   6. Implementation Guidelines ......................................12
   7. Security Considerations ........................................12
   8. Acknowledgments ................................................13
   9. References .....................................................13
      9.1. Normative References ......................................13
      9.2. Informative References ....................................13

Herlein, et al.             Standards Track                     [Page 2]
RFC 5574                         Speex                         June 2009

1.  Introduction

   Speex is based on the Code Excited Linear Prediction [CELP] encoding
   technique with support for either narrowband (nominal 8 kHz),
   wideband (nominal 16 kHz), or ultra-wideband (nominal 32 kHz).  The
   main characteristics can be summarized as follows:

   o  Free software/open-source

   o  Integration of wideband and narrowband in the same bit-stream

   o  Wide range of bit-rates available

   o  Dynamic bit-rate switching and variable bit-rate (VBR)

   o  Voice Activity Detection (VAD, integrated with VBR)

   o  Variable complexity

   The Speex codec supports a wide range of bit-rates from 2.15 kbit/s
   to 44 kbit/s.  In some cases however, it may not be possible for an
   implementation to include support for all rates (e.g., because of
   bandwidth or RAM or CPU constraints).  In those cases, to be
   compliant with this specification, implementations MUST support at
   least narrowband (8 kHz) encoding and decoding at 8 kbit/s bit-rate