Network Working Group                Audio-Video Transport Working GroupRequest for Comments: 1890                                H. SchulzrinneCategory: Standards Track                                      GMD Fokus                                                            January 1996    RTP Profile for Audio and Video Conferences with Minimal ControlStatus 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.Abstract   This memo describes a profile for the use of the real-time transport   protocol (RTP), version 2, and the associated control protocol, RTCP,   within audio and video multiparticipant conferences with minimal   control. It provides interpretations of generic fields within the RTP   specification suitable for audio and video conferences.  In   particular, this document defines a set of default mappings from   payload type numbers to encodings.   The document also describes how audio and video data may be carried   within RTP. It defines a set of standard encodings and their names   when used within RTP. However, the encoding definitions are   independent of the particular transport mechanism used. The   descriptions provide pointers to reference implementations and the   detailed standards. This document is meant as an aid for implementors   of audio, video and other real-time multimedia applications.1.  Introduction   This profile defines aspects of RTP left unspecified in the RTP   Version 2 protocol definition (RFC 1889). This profile is intended   for the use within audio and video conferences with minimal session   control. In particular, no support for the negotiation of parameters   or membership control is provided. The profile is expected to be   useful in sessions where no negotiation or membership control are   used (e.g., using the static payload types and the membership   indications provided by RTCP), but this profile may also be useful in   conjunction with a higher-level control protocol.Schulzrinne                 Standards Track                     [Page 1]RFC 1890                       AV Profile                   January 1996   Use of this profile occurs by use of the appropriate applications;   there is no explicit indication by port number, protocol identifier   or the like.   Other profiles may make different choices for the items specified   here.2.  RTP and RTCP Packet Forms and Protocol Behavior   The section "RTP Profiles and Payload Format Specification"   enumerates a number of items that can be specified or modified in a   profile. This section addresses these items. Generally, this profile   follows the default and/or recommended aspects of the RTP   specification.   RTP data header: The standard format of the fixed RTP data header is        used (one marker bit).   Payload types: Static payload types are defined in Section 6.   RTP data header additions: No additional fixed fields are appended to        the RTP data header.   RTP data header extensions: No RTP header extensions are defined, but        applications operating under this profile may use such        extensions. Thus, applications should not assume that the RTP        header X bit is always zero and should be prepared to ignore the        header extension. If a header extension is defined in the        future, that definition must specify the contents of the first        16 bits in such a way that multiple different extensions can be        identified.   RTCP packet types: No additional RTCP packet types are defined by        this profile specification.   RTCP report interval: The suggested constants are to be used for the        RTCP report interval calculation.   SR/RR extension: No extension section is defined for the RTCP SR or        RR packet.   SDES use: Applications may use any of the SDES items described.        While CNAME information is sent every reporting interval, other        items should be sent only every fifth reporting interval.   Security: The RTP default security services are also the default        under this profile.Schulzrinne                 Standards Track                     [Page 2]RFC 1890                       AV Profile                   January 1996   String-to-key mapping:  A user-provided string ("pass phrase") is        hashed with the MD5 algorithm to a 16-octet digest. An n-bit key        is extracted from the digest by taking the first n bits from the        digest. If several keys are needed with a total length of 128        bits or less (as for triple DES), they are extracted in order        from that digest. The octet ordering is specified in RFC 1423,        Section 2.2. (Note that some DES implementations require that        the 56-bit key be expanded into 8 octets by inserting an odd        parity bit in the most significant bit of the octet to go with        each 7 bits of the key.)   It is suggested that pass phrases are restricted to ASCII letters,   digits, the hyphen, and white space to reduce the the chance of   transcription errors when conveying keys by phone, fax, telex or   email.   The pass phrase may be preceded by a specification of the encryption   algorithm. Any characters up to the first slash (ASCII 0x2f) are   taken as the name of the encryption algorithm. The encryption format   specifiers should be drawn from RFC 1423 or any additional   identifiers registered with IANA. If no slash is present, DES-CBC is   assumed as default. The encryption algorithm specifier is case   sensitive.   The pass phrase typed by the user is transformed to a canonical form   before applying the hash algorithm. For that purpose, we define   return, tab, or vertical tab as well as all characters contained in   the Unicode space characters table. The transformation consists of   the following steps: (1) convert the input string to the ISO 10646   character set, using the UTF-8 encoding as specified in Annex P to   ISO/IEC 10646-1:1993 (ASCII characters require no mapping, but ISO   8859-1 characters do); (2) remove leading and trailing white space   characters; (3) replace one or more contiguous white space characters   by a single space (ASCII or UTF-8 0x20); (4) convert all letters to   lower case and replace sequences of characters and non-spacing   accents with a single character, where possible. A minimum length of   16 key characters (after applying the transformation) should be   enforced by the application, while applications must allow up to 256   characters of input.   Underlying protocol: The profile specifies the use of RTP over        unicast and multicast UDP. (This does not preclude the use of        these definitions when RTP is carried by other lower-layer        protocols.)   Transport mapping: The standard mapping of RTP and RTCP to        transport-level addresses is used.Schulzrinne                 Standards Track                     [Page 3]RFC 1890                       AV Profile                   January 1996   Encapsulation: No encapsulation of RTP packets is specified.3.  Registering Payload Types   This profile defines a set of standard encodings and their payload   types when used within RTP. Other encodings and their payload types   are to be registered with the Internet Assigned Numbers Authority   (IANA). When registering a new encoding/payload type, the following   information should be provided:        o name and description of encoding, in particular the RTP         timestamp clock rate; the names defined here are 3 or 4         characters long to allow a compact representation if needed;        o indication of who has change control over the encoding (for         example, ISO, CCITT/ITU, other international standardization         bodies, a consortium or a particular company or group of         companies);        o any operating parameters or profiles;        o a reference to a further description, if available, for         example (in order of preference) an RFC, a published paper, a         patent filing, a technical report, documented source code or a         computer manual;        o for proprietary encodings, contact information (postal and         email address);        o the payload type value for this profile, if necessary (see         below).   Note that not all encodings to be used by RTP need to be assigned a   static payload type. Non-RTP means beyond the scope of this memo   (such as directory services or invitation protocols) may be used to   establish a dynamic mapping between a payload type drawn from the   range 96-127 and an encoding. For implementor convenience, this   profile contains descriptions of encodings which do not currently   have a static payload type assigned to them.   The available payload type space is relatively small. Thus, new   static payload types are assigned only if the following conditions   are met:        o The encoding is of interest to the Internet community at         large.Schulzrinne                 Standards Track                     [Page 4]RFC 1890                       AV Profile                   January 1996        o It offers benefits compared to existing encodings and/or is         required for interoperation with existing, widely deployed         conferencing or multimedia systems.        o The description is sufficient to build a decoder.4.  Audio4.1 Encoding-Independent Recommendations   For applications which send no packets during silence, the first   packet of a talkspurt (first packet after a silence period) is   distinguished by setting the marker bit in the RTP  data header.   Applications without silence suppression set the bit to zero.   The RTP clock rate used for generating the RTP timestamp is   independent of the number of channels and the encoding; it equals the   number of sampling periods per second.  For N-channel encodings, each   sampling period (say, 1/8000 of a second) generates N samples. (This   terminology is standard, but somewhat confusing, as the total number   of samples generated per second is then the sampling rate times the   channel count.)   If multiple audio channels are used, channels are numbered left-to-   right, starting at one. In RTP audio packets, information from   lower-numbered channels precedes that from higher-numbered channels.   For more than two channels, the convention followed by the AIFF-C   audio interchange format should be followed [1], using the following   notation:   l    left   r    right   c    center   S    surround   F    front   R    rear   channels    description                 channel                               1     2     3     4     5     6   ___________________________________________________________   2           stereo          l     r   3                           l     r     c   4           quadrophonic    Fl    Fr    Rl    Rr   4                           l     c     r     S   5                           Fl    Fr    Fc    Sl    Sr   6                           l     lc    c     r     rc    SSchulzrinne                 Standards Track                     [Page 5]RFC 1890                       AV Profile                   January 1996   Samples for all channels belonging to a single sampling instant must   be within the same packet. The interleaving of samples from different   channels depends on the encoding. General guidelines are given in   Section 4.2 and 4.3.   The sampling frequency should be drawn from the set: 8000, 11025,   16000, 22050, 24000, 32000, 44100 and 48000 Hz. (The Apple Macintosh   computers have native sample rates of 22254.54 and 11127.27, which   can be converted to 22050 and 11025 with acceptable quality by   dropping 4 or 2 samples in a 20 ms frame.) However, most audio   encodings are defined for a more restricted set of sampling   frequencies. Receivers should be prepared to accept multi-channel   audio, but may choose to only play a single channel.   The following recommendations are default operating parameters.   Applications should be prepared to handle other values. The ranges   given are meant to give guidance to application writers, allowing a   set of applications conforming to these guidelines to interoperate   without additional negotiation. These guidelines are not intended to   restrict operating parameters for applications that can negotiate a   set of interoperable parameters, e.g., through a conference control   protocol.   For packetized audio, the default packetization interval should have   a duration of 20 ms, unless otherwise noted when describing the   encoding. The packetization interval determines the minimum end-to-   end delay; longer packets introduce less header overhead but higher   delay and make packet loss more noticeable. For non-interactive   applications such as lectures or links with severe bandwidth   constraints, a higher packetization delay may be appropriate. A   receiver should accept packets representing between 0 and 200 ms of   audio data. This restriction allows reasonable buffer sizing for the   receiver.4.2 Guidelines for Sample-Based Audio Encodings   In sample-based encodings, each audio sample is represented by a   fixed number of bits. Within the compressed audio data, codes for   individual samples may span octet boundaries. An RTP audio packet may   contain any number of audio samples, subject to the constraint that   the number of bits per sample times the number of samples per packet   yields an integral octet count. Fractional encodings produce less   than one octet per sample.   The duration of an audio packet is determined by the number of   samples in the packet.Schulzrinne                 Standards Track                     [Page 6]