RTP Payload Format of Sun's CellB Video Encoding
RFC 2029

Document Type RFC - Proposed Standard (October 1996; No errata)
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Network Working Group                                      M. Speer
Request for Comment: 2029                                D. Hoffman
Category: Standards Track                    Sun Microsystems, Inc.
                                                       October 1996

            RTP Payload Format of Sun's CellB Video Encoding

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 packetization scheme for the CellB video
   encoding. The scheme proposed allows applications to transport CellB
   video flows over protocols used by RTP.  This document is meant for
   implementors of video applications that want to use RTP and CellB.

1. Introduction

   The Cell image compression algorithm is a variable bit-rate video
   coding scheme.  It provides "high" quality, low bit-rate image
   compression at low computational cost.   The bytestream that is
   produced by the Cell encoder consists of instructional codes and
   information about the compressed image.

   For futher information on Cell compression technology, refer to [1].
   Currently, there are two versions of the Cell compression technology:
   CellA and CellB.  CellA is primarily designed for the encoding of
   stored video intended for local display, and will not be discussed in
   this memo.

   CellB, derived from CellA, has been optimized for network-based video
   applications.  It is computationally symmetric in both encode and
   decode.  CellB utilizes a fixed colormap and vector quantization
   techniques in the YUV color space to achieve compression.

Speer & Hoffman             Standards Track                     [Page 1]
RFC 2029                   RTP Payload Format               October 1996

2. Network Packetization and Encapsulation

2.1 RTP Usage

   The RTP timestamp is in units of 90KHz. The same timestamp value is
   used for all packet payloads of a frame.  The RTP maker bit denotes
   the end of a frame.

2.2 CellB Header

   The packetization of the CellB bytestream is designed to make the
   resulting packet stream robust to packet loss.  To achieve this goal,
   an additional header is added to each RTP packet to uniquely identify
   the location of the first cell of the packet within the current
   frame.  In addition, the width and height of the frame in pixels is
   carried in each CellB packet header.  Although the size can only
   change between frames, it is carried in every packet to simplify the
   packet encoding.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Cell X Location         |      Cell Y Location          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Width of Image          |      Height of Image          |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                     Compressed CellB Data                     |
   |                             ....                              |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+

   All fields are 16-bit unsigned integers in network byte order, and
   are placed at the beginning of the payload for each RTP packet.  The
   Cell X and the Cell Y Location coordinates are expressed as cell
   coordinates, not pixel coordinates. Since cells represent 4x4 blocks
   of pixels, the X or Y dimension of the cell coordinates range in
   value from 0 through 1/4 of the of the same dimension in pixel
   coordinates.

2.3 Packetization Rules

   A packet can be of any size chosen by the implementor, up to a full
   frame.  All multi-byte codes must be completely contained within a
   packet.  In general, the implementor should avoid packet sizes that
   result in fragmentation by the network.

Speer & Hoffman             Standards Track                     [Page 2]
RFC 2029                   RTP Payload Format               October 1996

3. References

   1.      "Cell Image Compression Byte Stream Description,"
           ftp://playground.sun.com:/pub/multimedia/video/
           cellbytestream.ps.Z

   2.      Turletti, T., and C. Huitema, "RTP Payload Format
           for H.261 Video Streams", RFC 2032, October 1996.

   3.      Schulzrinne, H., Casner, S., Frederick, R., and
           V. Jacobson, "RTP: A Transport Protocol for Real-Time
           Applications", RFC 1889, January 1996.

   4.      Schulzrinne, H., "RTP Profile for Audio and Video
           Conferences with Minimal Control", RFC 1890,
           January 1996.

4 Authors' Addresses
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