Network Working Group                Audio-Video Transport Working Group
Request for Comments: 1890                                H. Schulzrinne
Category: Standards Track                                      GMD Fokus
                                                            January 1996


    RTP Profile for Audio and Video Conferences with Minimal Control

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.

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.






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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.




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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.




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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.





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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.  Audio

4.1 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    S



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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.





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