Internet Engineering Task Force Audio-Video Transport Working Group INTERNET-DRAFT draft-ietf-avt-profile-02.txt H. Schulzrinne AT&T Bell Laboratories September 17, 1993 Expires: 10/01/93 Sample Profile for the Use of RTP for Audio and Video Conferences with Minimal Control Status of this Memo This document is an Internet Draft. 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. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a ``working draft'' or ``work in progress.'' Please check the I-D abstract listing contained in each Internet Draft directory to learn the current status of this or any other Internet Draft. Distribution of this document is unlimited. Abstract This note describes a profile for the use of the real-time transport protocol (RTP) 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 content index to encodings. 1 Introduction This profile defines aspects of RTP left unspecified in the RTP protocol definition (RFC TBD). This profile is intended for the use within audio and INTERNET-DRAFT draft-ietf-avt-profile-02.txt September 17, 1993 video conferences with minimal session control. In particular, no support for the negotiation of parameters or admission control is provided. Other profiles may make different choices for the items specified here. The profile specifies the use of RTP over unicast and multicast UDP as well as ST-II. For unicast UDP and ST-II, references to multicast addresses are to be ignored. The use of this profile is indicated by the use of a media-specific well-known port number. The profile may also be used with other port numbers. For example, the use of a particular session announcement tool could imply use of this profile. 2 Multiplexing and Demultiplexing Packets sharing the same multicast group address, the same destination port number and the same flow value belong to the same conference. Within a conference, a packet is mapped to a site (state) through its synchronization source identifier and network source port. Unless otherwise defined, flow 0 is used for audio and flow 1 for video. 3 FMT The content field within the FMT option describes the media encoding used. The four octets contain one of the encodings defined by the Internet Assigned Numbers Authority (IANA) or an encoding agreed upon by mutual consent of all conference participants. The names and the defined codings are defined in RFC TBD [media] and encoded in US-ASCII. Case is significant. If the name is shorter than four characters, it is padded with one or more space characters (ASCII 32 decimal). Experimental encodings should start with the letter 'X'. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |F| FMT | length |0|0| content | reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | name of encoding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | channels | sampling rate (Hz) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... encoding specific parameters ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: FMT for Audio For audio encodings, the index into the table of encodings is followed by H. Schulzrinne Expires 10/01/93 [Page 2] INTERNET-DRAFT draft-ietf-avt-profile-02.txt September 17, 1993 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |F| FMT | length |0|0| content | clock quality | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | name of video encoding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | version | encoding-specific parameters | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... encoding-specific parameters ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: FMT for Video a field containing a channel count and a sample rate field, measured in samples per second.(1) A channel count of zero is considered invalid. A packetization interval of 20 ms or a multiple thereof is suggested as it leads to integral sample counts for all common sampling rates. For video encodings, a one-octet numeric version identifier further describes the encoding. Unless otherwise defined, the version identifier has the value zero. 4 Standard Encodings Unless specified with the FMT option, the mapping between the content field in an RTP packet and encodings, sampling rates and channel counts is specified by Tables 1 and 2. Values of 31 and below cannot be redefined by FMT options. In other words, only values of 32 and above are valid in the content field within an FMT option. The receiver is expected to discard RTP packets containing media data with unknown content field values. Sites are expected to keep the mapping between content and encoding constant, so that lost packets containing FMT options do not lead the receiver to misinterpret media data. Additional standard encodings are defined by the Internet Assigned Numbers Authority. ------------------------------ 1. Fractional samples per second was considered excessive as the typical crystal accuraccy of 100 ppm translates into about one Hz or more of sampling rate inaccuracy. H. Schulzrinne Expires 10/01/93 [Page 3] INTERNET-DRAFT draft-ietf-avt-profile-02.txt September 17, 1993 index encoding sampling rate channels ________name______________(kHz)___________ 0 PCMU 8 1 1 1016 8 1 2 G721 8 1 3 GSM 8 1 4 G723 8 1 5 IDVI 8 1 ____10__L16________________44.1________2__ Table 1: Standard Audio Encodings _number__name_ 31 H261 30 Bolt 29 PicW 28 nv 27 CUSM 26 JPEG Table 2: Standard Video Encodings 5 Port Assignments and Miscellaneous ST-II SAP and UDP port 5005 is the default destination for multicast real-time data carried by RTP. A fixed port number is useful as it makes it less likely that a port number is chosen at random that is already in use at one or more hosts. Also, fixed port numbers allow traffic statistics and may simplify firewall implementations. A single fixed port number requires that hosts allow several processes to use a single UDP port with different multicast addresses. (The particular port number was chosen to lie in the range above 5000 to accomodate port number allocation practice within the Unix operating system, where port numbers below 1024 can only be used by privileged processes and port numbers between 1024 and 5000 are automatically assigned by the operating system.) Unicast connections may use the this or a set of mutually agreed-upon port numbers. RTCP messages should be sent periodically, with a period varying randomly between approximately 50% and 150% of the average. The average period should be chosen such that no more than 5% of the media bandwidth is consumed by RTCP messages from all sources, with a minimum of about 3 seconds. For example, for RTCP messages of around 100 bytes within a 64 kBytes/second audio conference, all sources combined should not exceed a H. Schulzrinne Expires 10/01/93 [Page 4] INTERNET-DRAFT draft-ietf-avt-profile-02.txt September 17, 1993 total message frequency of 4 messages per second. By scaling the message frequency with the (slowly increasing) number of observed participants, a new conference participant will quickly inform all other participants of its arrival. 6 Address of Author Henning Schulzrinne AT&T Bell Laboratories MH 2A244 600 Mountain Avenue Murray Hill, NJ 07974-0636 telephone: +1 908 582 2262 facsimile: +1 908 582 5809 electronic mail: hgs@research.att.com H. Schulzrinne Expires 10/01/93 [Page 5]