draft-herlein-speex-rtp-profile-03

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 RFC 2119 .

Speex is based on the CELP encoding technique with support for either narrowband (nominal 8kHz), wideband (nominal 16kHz) or ultra-wideband (nominal 32kHz), and (non-optimal) rates up to 48 kHz sampling also available. The main characteristics can be summarized as follows: Free software/open-source Integration of wideband and narrowband in the same bit-stream Wide range of bit-rates available Dynamic bit-rate switching and variable bit-rate (VBR) Voice Activity Detection (VAD, integrated with VBR) Variable complexity

For RTP based transportation of Speex encoded audio the standard RTP header [2] is followed by one or more payload data blocks. An optional padding terminator may also be used.

The RTP header begins with an octet of fields (V, P, X, and CC) to support specialized RTP uses (see and for details). For Speex the following values are used. Version (V): 2 bits This field identifies the version of RTP. The version used by this specification is two . Padding (P): 1 bit If the padding bit is set, the packet contains one or more additional padding octets at the end which are not part of the payload. P is set if the total packet size is less than the MTU. Extension (X): 1 bit If the extension, X, bit is set, the fixed header MUST be followed by exactly one header extension, with a format defined in Section 5.3.1. of . CSRC count (CC): 4 bits The CSRC count contains the number of CSRC identifiers. Marker (M): 1 bit The M bit indicates if the packet contains comfort noise. This field is used in conjunction with the cng SDP attribute and is detailed further in section 5 below. In normal usage this bit is set if the packet contains comfort noise. Payload Type (PT): 7 bits An RTP profile for a class of applications is expected to assign a payload type for this format, or a dynamically allocated payload type SHOULD be chosen which designates the payload as Speex. Sequence number: 16 bits The sequence number increments by one for each RTP data packet sent, and may be used by the receiver to detect packet loss and to restore packet sequence. This field is detailed further in . Timestamp: 32 bits A timestamp representing the sampling time of the first sample of the first Speex packet in the RTP packet. The clock frequency MUST be set to the sample rate of the encoded audio data. Speex uses 20 msec frames and a variable sampling rate clock. The RTP timestamp MUST be in units of 1/X of a second where X is the sample rate used. Speex uses a nominal 8kHz sampling rate for narrowband use, a nominal 16kHz sampling rate for wideband use, and a nominal 32kHz sampling rate for ultra-wideband use. SSRC/CSRC identifiers: These two fields, 32 bits each with one SSRC field and a maximum of 16 CSRC fields, are as defined in .

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 , 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 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. It is RECOMMENDED that values of 8000, 16000 and 32000 be used for normal internet telephony applications, though the sample rate is supported at rates as low as 6000 Hz and as high as 48 kHz. 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.

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.

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 are able to detect the the bitrate from the payload and are responsible for detecting the 20 msec boundaries between each frame.

Full definition of the MIME type for Speex will be part of the Ogg Vorbis MIME type definition application . MIME media type name: audio MIME subtype: speex Optional parameters: Required parameters: to be included in the Ogg MIME specification. Encoding considerations: Security Considerations: See Section 6 of RFC 3047. Interoperability considerations: none Published specification: Applications which use this media type: Additional information: none Person & email address to contact for further information: Greg Herlein <gherlein@herlein.com> Jean-Marc Valin <jean-marc.valin@hermes.usherb.ca> Intended usage: COMMON Author/Change controller: Author: Greg Herlein <gherlein@herlein.com> Change controller: Greg Herlein <gherlein@herlein.com> Change controller: IETF AVT Working Group This transport type signifies that the content is to be interpreted according to this document if the contents are transmitted over RTP. Should this transport type appear over a lossless streaming protocol such as TCP, the content encapsulation should be interpreted as an Ogg Stream in accordance with , with the exception that the content of the Ogg Stream may be assumed to be Speex audio and Speex audio only.

When conveying information by SDP , 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 . 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). 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. penh: use of perceptual enhancement. 1 indicates to the decoder that perceptual enhancement is recommended, 0 indicates that it is not. Defaults to on (1). 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 3. The offerer may suggest to the remote decoder to activate its perceptual enhancement filter like this: m=audio 8088 RTP/AVP 97 a=rtmap:97 speex/8000 a=fmtp:97 penh=1 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;penh=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. In the example below the ptime value is set to 40, indicating that 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.

Application is underway to make Speex a standard ITU codec. However, until that is finalized, Speex MAY be used in H.323 by using a non-standard codec block definition in the H.245 codec capability negotiations.

For Speex use in H.245 based systems, the fields in the NonStandardMessage should be: t35CountryCode = Hex: B5 t35Extension = Hex: 00 manufacturerCode = Hex: 0026 [Length of the Binary Sequence (8 bit number)] [Binary Sequence consisting of an ASCII string, no NULL terminator] The binary sequence is an ascii string merely for ease of use. The string is not null terminated. The format of this string is speex [optional variables] The optional variables are identical to those used for the SDP a=fmtp strings discussed in section 5 above. The string is built to be all on one line, each key-value pair separated by a semi-colon. The optional variables MAY be omitted, which causes the default values to be assumed. They are: ebw=narrow;mode=3;vbr=off;cng=off;ptime=20;sr=8000;penh=no; The fifth octet of the block is the length of the binary sequence. NOTE: this method can result in the advertising of a large number of Speex 'codecs' based on the number of variables possible. For most VoIP applications, use of the default binary sequence of 'speex' is RECOMMENDED to be used in addition to all other options. This maximizes the chances that two H.323 based applications that support Speex can find a mutual codec.

Dynamic payload type codes MUST be negotiated 'out-of-band' for the assignment of a dynamic payload type from the range of 96-127. H.323 applications MUST use the H.245 H2250LogicalChannelParameters encoding to accomplish this.

RTP packets using the payload format defined in this specification are subject to the security considerations discussed in the RTP specification , 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.

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 <brian@streamcomm.com> 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.