AVT G. Herlein Internet-Draft Intended status: Standards Track J. Valin Expires: October 24, 2007 University of Sherbrooke A. Heggestad April 22, 2007 RTP Payload Format for the Speex Codec draft-ietf-avt-rtp-speex-01 (non-final) Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on October 24, 2007. Copyright Notice Copyright (C) The Internet Society (2007). Herlein, et al. Expires October 24, 2007 [Page 1] Internet-Draft Speex April 2007 Abstract Speex is an open-source voice codec suitable for use in Voice over IP (VoIP) 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. Expires October 24, 2007 [Page 2] Internet-Draft Speex April 2007 Editors Note All references to RFC XXXX are to be replaced by references to the RFC number of this memo, when published. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. RTP usage for Speex . . . . . . . . . . . . . . . . . . . . . 6 3.1. RTP Speex Header Fields . . . . . . . . . . . . . . . . . 6 3.2. RTP payload format for Speex . . . . . . . . . . . . . . . 6 3.3. Speex payload . . . . . . . . . . . . . . . . . . . . . . 6 3.4. Example Speex packet . . . . . . . . . . . . . . . . . . . 7 3.5. Multiple Speex frames in a RTP packet . . . . . . . . . . 7 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 4.1. Media Type Registration . . . . . . . . . . . . . . . . . 9 4.1.1. Registration of media type audio/speex . . . . . . . . 9 5. SDP usage of Speex . . . . . . . . . . . . . . . . . . . . . . 11 6. Security Considerations . . . . . . . . . . . . . . . . . . . 14 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16 8.1. Normative References . . . . . . . . . . . . . . . . . . . 16 8.2. Informative References . . . . . . . . . . . . . . . . . . 16 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17 Intellectual Property and Copyright Statements . . . . . . . . . . 18 Herlein, et al. Expires October 24, 2007 [Page 3] Internet-Draft Speex April 2007 1. Introduction Speex is based on the CELP [CELP] encoding technique with support for either narrowband (nominal 8kHz), wideband (nominal 16kHz) or ultra- wideband (nominal 32kHz). 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 To be compliant with this specification, implementations MUST support 8 kHz sampling rate (narrowband)" and SHOULD support 8 kbps bitrate. The sampling rate MUST be 8, 16 or 32 kHz. Herlein, et al. Expires October 24, 2007 [Page 4] Internet-Draft Speex April 2007 2. Terminology 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 [RFC2119] and indicate requirement levels for compliant RTP implementations. Herlein, et al. Expires October 24, 2007 [Page 5] Internet-Draft Speex April 2007 3. RTP usage for Speex 3.1. RTP Speex Header Fields The RTP header is defined in the RTP specification [RFC3550]. This section defines how fields in the RTP header are used. Payload Type (PT): The assignment of an RTP payload type for this packet format is outside the scope of this document; it is specified by the RTP profile under which this payload format is used, or signaled dynamically out-of-band (e.g., using SDP). Marker (M) bit: The M bit is set to one to indicate that the RTP packet payload contains at least one complete frame Extension (X) bit: Defined by the RTP profile used. Timestamp: A 32-bit word that corresponds to the sampling instant for the first frame in the RTP packet. 3.2. RTP payload format for Speex The RTP payload for Speex has the format shown in Figure 1. No additional header fields specific to this payload format are required. For RTP based transportation of Speex encoded audio the standard RTP header [RFC3550] is followed by one or more payload data blocks. An optional padding terminator may also be used. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RTP Header | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | one or more frames of Speex .... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | one or more frames of Speex .... | padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: RTP payload for Speex 3.3. Speex payload For the purposes of packetizing the bit stream in RTP, it is only necessary to consider the sequence of bits as output by the Speex encoder [speexenc], and present the same sequence to the decoder. The payload format described here maintains this sequence. A typical Speex frame, encoded at the maximum bitrate, is approx. 110 Herlein, et al. Expires October 24, 2007 [Page 6] Internet-Draft Speex April 2007 octets and the total number of Speex frames SHOULD be kept less than the path MTU to prevent fragmentation. Speex frames MUST NOT be fragmented across multiple RTP packets, An RTP packet MAY contain Speex frames of the same bit rate or of varying bit rates, since the bit-rate for a frame is conveyed in band with the signal. The encoding and decoding algorithm can change the bit rate at any 20 msec frame boundary, with the bit rate change notification provided in-band with the bit stream. Each frame contains both "mode" (narrowband, wideband or ultra-wideband) and "sub-mode" (bit-rate) information in the bit stream. No out-of-band notification is required for the decoder to process changes in the bit rate sent by the encoder. Sampling rate values of 8000, 16000 or 32000 Hz MUST be used. Any other sampling rates MUST NOT be used. The RTP payload MUST be padded to provide an integer number of octets as the payload length. These padding bits are LSB aligned in network octet order and consist of a 0 followed by all ones (until the end of the octet). This padding is only required for the last frame in the packet, and only to ensure the packet contents ends on an octet boundary. 3.4. Example Speex packet In the example below we have a single Speex frame with 5 bits of padding to ensure the packet size falls on an octet boundary. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RTP Header | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | ..speex data.. | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ..speex data.. |0 1 1 1 1| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3.5. Multiple Speex frames in a RTP packet Below is an example of two Speex frames contained within one RTP packet. The Speex frame length in this example fall on an octet boundary so there is no padding. Speex codecs [speexenc] are able to detect the bitrate from the Herlein, et al. Expires October 24, 2007 [Page 7] Internet-Draft Speex April 2007 payload and are responsible for detecting the 20 msec boundaries between each frame. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RTP Header | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | ..speex frame 1.. | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ..speex frame 1.. | ..speex frame 2.. | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ..speex frame 2.. | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Herlein, et al. Expires October 24, 2007 [Page 8] Internet-Draft Speex April 2007 4. IANA Considerations This document defines the Speex media type. 4.1. Media Type Registration This section describes the media types and names associated with this payload format. The section registers the media types, as per RFC4288 [RFC4288] 4.1.1. Registration of media type audio/speex Media type name: audio Media subtype name: speex Required parameters: None Optional parameters: ptime: see RFC 4566. SHOULD be a multiple of 20 msec. maxptime: see RFC 4566. SHOULD be a multiple of 20 msec. Encoding considerations: This media type is framed and binary, see section 4.8 in [RFC4288]. Security considerations: See Section 6 Interoperability considerations: None. Published specification: RFC XXXX [This RFC]. Applications which use this media type: Audio streaming and conferencing applications. Additional information: none Person and email address to contact for further information : Herlein, et al. Expires October 24, 2007 [Page 9] Internet-Draft Speex April 2007 Alfred E. Heggestad: aeh@db.org Intended usage: COMMON Restrictions on usage: This media type depends on RTP framing, and hence is only defined for transfer via RTP [RFC3550]. Transport within other framing protocols is not defined at this time. Author: Alfred E. Heggestad Change controller: IETF Audio/Video Transport working group delegated from the IESG. Herlein, et al. Expires October 24, 2007 [Page 10] Internet-Draft Speex April 2007 5. SDP usage of Speex When conveying information by SDP [RFC4566], the encoding name MUST be set to "speex". An example of the media representation in SDP for offering a single channel of Speex at 8000 samples per second might be: m=audio 8088 RTP/AVP 97 a=rtpmap:97 speex/8000 Note that the RTP payload type code of 97 is defined in this media definition to be 'mapped' to the speex codec at an 8kHz sampling frequency using the 'a=rtpmap' line. Any number from 96 to 127 could have been chosen (the allowed range for dynamic types). The value of the sampling frequency is typically 8000 for narrow band operation, 16000 for wide band operation, and 32000 for ultra-wide band operation. If for some reason the offerer has bandwidth limitations, the client may use the "b=" header, as explained in SDP [RFC4566]. The following example illustrates the case where the offerer cannot receive more than 10 kbit/s. m=audio 8088 RTP/AVP 97 b=AS:10 a=rtmap:97 speex/8000 In this case, if the remote part agrees, it should configure its Speex encoder so that it does not use modes that produce more than 10 kbit/s. Note that the "b=" constraint also applies on all payload types that may be proposed in the media line ("m="). An other way to make recommendations to the remote Speex encoder is to use its specific parameters via the a=fmtp: directive. The following parameters are defined for use in this way: ptime: duration of each packet in milliseconds. sr: actual sample rate in Hz. ebw: encoding bandwidth - either 'narrow' or 'wide' or 'ultra' (corresponds to nominal 8000, 16000, and 32000 Hz sampling rates). Herlein, et al. Expires October 24, 2007 [Page 11] Internet-Draft Speex April 2007 vbr: variable bit rate - either 'on' 'off' or 'vad' (defaults to off). If on, variable bit rate is enabled. If off, disabled. If set to 'vad' then constant bit rate is used but silence will be encoded with special short frames to indicate a lack of voice for that period. cng: comfort noise generation - either 'on' or 'off'. If off then silence frames will be silent; if 'on' then those frames will be filled with comfort noise. mode: Speex encoding mode. Can be {1,2,3,4,5,6,any} defaults to 3 in narrowband, 6 in wide and ultra-wide. Examples: m=audio 8008 RTP/AVP 97 a=rtpmap:97 speex/8000 a=fmtp:97 mode=4 This examples illustrate an offerer that wishes to receive a Speex stream at 8000Hz, but only using speex mode 4. Several Speex specific parameters can be given in a single a=fmtp line provided that they are separated by a semi-colon: a=fmtp:97 mode=any;mode=1 The offerer may indicate that it wishes to send variable bit rate frames with comfort noise: m=audio 8088 RTP/AVP 97 a=rtmap:97 speex/8000 a=fmtp:97 vbr=on;cng=on The "ptime" attribute is used to denote the packetization interval (ie, how many milliseconds of audio is encoded in a single RTP packet). Since Speex uses 20 msec frames, ptime values of multiples of 20 denote multiple Speex frames per packet. Values of ptime which are not multiples of 20 MUST be ignored and clients MUST use the default value of 20 instead. Implementations SHOULD support ptime of 20 msec (i.e. one frame per packet) In the example below the ptime value is set to 40, indicating that Herlein, et al. Expires October 24, 2007 [Page 12] Internet-Draft Speex April 2007 there are 2 frames in each packet. m=audio 8008 RTP/AVP 97 a=rtpmap:97 speex/8000 a=ptime:40 Note that the ptime parameter applies to all payloads listed in the media line and is not used as part of an a=fmtp directive. Values of ptime not multiple of 20 msec are meaningless, so the receiver of such ptime values MUST ignore them. If during the life of an RTP session the ptime value changes, when there are multiple Speex frames for example, the SDP value must also reflect the new value. Care must be taken when setting the value of ptime so that the RTP packet size does not exceed the path MTU. Herlein, et al. Expires October 24, 2007 [Page 13] Internet-Draft Speex April 2007 6. Security Considerations RTP packets using the payload format defined in this specification are subject to the security considerations discussed in the RTP specification [RFC3550], and any appropriate RTP profile. This implies that confidentiality of the media streams is achieved by encryption. Because the data compression used with this payload format is applied end-to-end, encryption may be performed after compression so there is no conflict between the two operations. A potential denial-of-service threat exists for data encodings using compression techniques that have non-uniform receiver-end computational load. The attacker can inject pathological datagrams into the stream which are complex to decode and cause the receiver to be overloaded. However, this encoding does not exhibit any significant non-uniformity. As with any IP-based protocol, in some circumstances a receiver may be overloaded simply by the receipt of too many packets, either desired or undesired. Network-layer authentication may be used to discard packets from undesired sources, but the processing cost of the authentication itself may be too high. Herlein, et al. Expires October 24, 2007 [Page 14] Internet-Draft Speex April 2007 7. Acknowledgements The authors would like to thank Equivalence Pty Ltd of Australia for their assistance in attempting to standardize the use of Speex in H.323 applications, and for implementing Speex in their open source OpenH323 stack. The authors would also like to thank Brian C. Wiles of StreamComm for his assistance in developing the proposed standard for Speex use in H.323 applications. The authors would also like to thank the following members of the Speex and AVT communities for their input: Ross Finlayson, Federico Montesino Pouzols, Henning Schulzrinne, Magnus Westerlund. Thanks to former authors of this document; Simon Morlat, Roger Hardiman, Phil Kerr Herlein, et al. Expires October 24, 2007 [Page 15] Internet-Draft Speex April 2007 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. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. 8.2. Informative References [CELP] "CELP, U.S. Federal Standard 1016.", National Technical Information Service (NTIS) website http://www.ntis.gov/. [RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and Registration Procedures", BCP 13, RFC 4288, December 2005. [speexenc] Valin, J., "Speexenc/speexdec, reference command-line encoder/decoder", Speex website http://www.speex.org/. Herlein, et al. Expires October 24, 2007 [Page 16] Internet-Draft Speex April 2007 Authors' Addresses Greg Herlein 2034 Filbert Street San Francisco, California 94123 United States Email: gherlein@herlein.com Jean-Marc Valin University of Sherbrooke Department of Electrical and Computer Engineering University of Sherbrooke 2500 blvd Universite Sherbrooke, Quebec J1K 2R1 Canada Email: jean-marc.valin@usherbrooke.ca Alfred E. Heggestad Biskop J. Nilssonsgt. 20a Oslo 0659 Norway Email: aeh@db.org Herlein, et al. Expires October 24, 2007 [Page 17] Internet-Draft Speex April 2007 Full Copyright Statement Copyright (C) The Internet Society (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Herlein, et al. Expires October 24, 2007 [Page 18]