rfc2035.txt

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Network Working Group                                            L. Berc
Request for Comments: 2035                 Digital Equipment Corporation
Category: Standards Track                                      W. Fenner
                                                              Xerox PARC
                                                            R. Frederick
                                                              Xerox PARC
                                                              S. McCanne
                                            Lawrence Berkeley Laboratory
                                                            October 1996


              RTP Payload Format for JPEG-compressed Video

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 the RTP payload format for JPEG video streams.
   The packet format is optimized for real-time video streams where
   codec parameters change rarely from frame to frame.

   This document is a product of the Audio-Video Transport working group
   within the Internet Engineering Task Force.  Comments are solicited
   and should be addressed to the working group's mailing list at rem-
   [e-mail] and/or the author(s).

1.  Introduction

   The Joint Photographic Experts Group (JPEG) standard [1,2,3] defines
   a family of compression algorithms for continuous-tone, still images.
   This still image compression standard can be applied to video by
   compressing each frame of video as an independent still image and
   transmitting them in series.  Video coded in this fashion is often
   called Motion-JPEG.

   We first give an overview of JPEG and then describe the specific
   subset of JPEG that is supported in RTP and the mechanism by which
   JPEG frames are carried as RTP payloads.

   The JPEG standard defines four modes of operation: the sequential DCT
   mode, the progressive DCT mode, the lossless mode, and the
   hierarchical mode.  Depending on the mode, the image is represented



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RFC 2035           RTP Payload Format for JPEG Video        October 1996


   in one or more passes.  Each pass (called a frame in the JPEG
   standard) is further broken down into one or more scans.  Within each
   scan, there are one to four components,which represent the three
   components of a color signal (e.g., "red, green, and blue", or a
   luminance signal and two chromanince signals).  These components can
   be encoded as separate scans or interleaved into a single scan.

   Each frame and scan is preceded with a header containing optional
   definitions for compression parameters like quantization tables and
   Huffman coding tables.  The headers and optional parameters are
   identified with "markers" and comprise a marker segment; each scan
   appears as an entropy-coded bit stream within two marker segments.
   Markers are aligned to byte boundaries and (in general) cannot appear
   in the entropy-coded segment, allowing scan boundaries to be
   determined without parsing the bit stream.

   Compressed data is represented in one of three formats: the
   interchange format, the abbreviated format, or the table-
   specification format.  The interchange format contains definitions
   for all the table used in the by the entropy-coded segments, while
   the abbreviated format might omit some assuming they were defined
   out-of-band or by a "previous" image.

   The JPEG standard does not define the meaning or format of the
   components that comprise the image.  Attributes like the color space
   and pixel aspect ratio must be specified out-of-band with respect to
   the JPEG bit stream.  The JPEG File Interchange Format (JFIF) [4] is
   a defacto standard that provides this extra information using an
   application marker segment (APP0).  Note that a JFIF file is simply a
   JPEG interchange format image along with the APP0 segment.  In the
   case of video, additional parameters must be defined out-of-band
   (e.g., frame rate, interlaced vs. non-interlaced, etc.).

   While the JPEG standard provides a rich set of algorithms for
   flexible compression, cost-effective hardware implementations of the
   full standard have not appeared.  Instead, most hardware JPEG video
   codecs implement only a subset of the sequential DCT mode of
   operation.  Typically, marker segments are interpreted in software
   (which "re-programs" the hardware) and the hardware is presented with
   a single, interleaved entropy-coded scan represented in the YUV color
   space.

2.  JPEG Over RTP

   To maximize interoperability among hardware-based codecs, we assume
   the sequential DCT operating mode [1,Annex F] and restrict the set of
   predefined RTP/JPEG "type codes" (defined below) to single-scan,
   interleaved images.  While this is more restrictive than even



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RFC 2035           RTP Payload Format for JPEG Video        October 1996


   baseline JPEG, many hardware implementation fall short of the
   baseline specification (e.g., most hardware cannot decode non-
   interleaved scans).

   In practice, most of the table-specification data rarely changes from
   frame to frame within a single video stream.  Therefore, RTP/JPEG
   data is represented in abbreviated format, with all of the tables
   omitted from the bit stream.  Each image begins immediately with the
   (single) entropy-coded scan.  The information that would otherwise be
   in both the frame and scan headers is represented entirely within a
   64-bit RTP/JPEG header (defined below) that lies between the RTP
   header and the JPEG scan and is present in every packet.

   While parameters like Huffman tables and color space are likely to
   remain fixed for the lifetime of the video stream, other parameters
   should be allowed to vary, notably the quantization tables and image
   size (e.g., to implement rate-adaptive transmission or allow a user
   to adjust the "quality level" or resolution manually).  Thus explicit
   fields in the RTP/JPEG header are allocated to represent this
   information.  Since only a small set of quantization tables are
   typically used, we encode the entire set of quantization tables in a
   small integer field.  The image width and height are encoded
   explicitly.

   Because JPEG frames are typically larger than the underlying
   network's maximum packet size, frames must often be fragmented into
   several packets.  One approach is to allow the network layer below
   RTP (e.g., IP) to perform the fragmentation.  However, this precludes
   rate-controlling the resulting packet stream or partial delivery in
   the presence of loss.  For example, IP will not deliver a fragmented
   datagram to the application if one or more fragments is lost, or IP
   might fragment an 8000 byte frame into a burst of 8 back-to-back
   packets.  Instead, RTP/JPEG defines a simple fragmentation and
   reassembly scheme at the RTP level.

3.  RTP/JPEG Packet Format

   The RTP timestamp is in units of 90000Hz.  The same timestamp must
   appear across all fragments of a single frame.  The RTP marker bit is
   set in the last packet of a frame.











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RFC 2035           RTP Payload Format for JPEG Video        October 1996


3.1.  JPEG header

   A special header is added to each packet that immediately follows the
   RTP header:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Type specific |              Fragment Offset                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Type     |       Q       |     Width     |     Height    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.1.1.  Type specific: 8 bits

   Interpretation depends on the value of the type field.

3.1.2.  Fragment Offset: 24 bits

   The Fragment Offset is the data offset in bytes of the current packet
   in the JPEG scan.

3.1.3.  Type: 8 bits

   The type field specifies the information that would otherwise be
   present in a JPEG abbreviated table-specification as well as the
   additional JFIF-style parameters not defined by JPEG.  Types 0-127
   are reserved as fixed, well-known mappings to be defined by this
   document and future revisions of this document.  Types 128-255 are
   free to be dynamically defined by a session setup protocol (which is
   beyond the scope of this document).

3.1.4.  Q: 8 bits

   The Q field defines the quantization tables for this frame using an
   algorithm that determined by the Type field (see below).

3.1.5.  Width: 8 bits

   This field encodes the width of the image in 8-pixel multiples (e.g.,
   a width of 40 denotes an image 320 pixels wide).

3.1.6.  Height: 8 bits

   This field encodes the height of the image in 8-pixel multiples
   (e.g., a height of 30 denotes an image 240 pixels tall).





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RFC 2035           RTP Payload Format for JPEG Video        October 1996


3.1.7.  Data

   The data following the RTP/JPEG header is an entropy-coded segment
   consisting of a single scan.  The scan header is not present and is
   inferred from the RTP/JPEG header.  The scan is terminated either
   implicitly (i.e., the point at which the image is fully parsed), or
   explicitly with an EOI marker.  The scan may be padded to arbitrary
   length with undefined bytes.  (Existing hardware codecs generate
   extra lines at the bottom of a video frame and removal of these lines
   would require a Huffman-decoding pass over the data.)

   As defined by JPEG, restart markers are the only type of marker that
   may appear embedded in the entropy-coded segment.  The "type code"
   determines whether a restart interval is defined, and therefore
   whether restart markers may be present. It also determines if the
   restart intervals will be aligned with RTP packets, allowing for
   partial decode of frames, thus increasing resiliance to packet drop.
   If restart markers are present, the 6-byte DRI segment (define
   restart interval marker [1, Sec. B.2.4.4] precedes the scan).

   JPEG markers appear explicitly on byte aligned boundaries beginning
   with an 0xFF.  A "stuffed" 0x00 byte follows any 0xFF byte generated
   by the entropy coder [1, Sec. B.1.1.5].

4.  Discussion

4.1.  The Type Field

   The Type field defines the abbreviated table-specification and
   additional JFIF-style parameters not defined by JPEG, since they are
   not present in the body of the transmitted JPEG data.  The Type field
   must remain constant for the duration of a session.

   Six type codes are currently defined.  They correspond to an
   abbreviated table-specification indicating the "Baseline DCT
   sequential" mode, 8-bit samples, square pixels, three components in
   the YUV color space, standard Huffman tables as defined in [1, Annex
   K.3], and a single interleaved scan with a scan component selector
   indicating components 0, 1, and 2 in that order.  The Y, U, and V
   color planes correspond to component numbers 0, 1, and 2,
   respectively.  Component 0 (i.e., the luminance plane) uses Huffman
   table number 0 and quantization table number 0 (defined below) and
   components 1 and 2 (i.e., the chrominance planes) use Huffman table
   number 1 and quantization table number 1 (defined below).

   Additionally, video is non-interlaced and unscaled (i.e., the aspect
   ratio is determined by the image width and height).  The frame rate
   is variable and explicit via the RTP timestamp.



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RFC 2035           RTP Payload Format for JPEG Video        October 1996


   Six RTP/JPEG types are currently defined that assume all of the
   above.  The odd types have different JPEG sampling factors from the
   even ones:

                        horizontal   vertical
           types   comp  samp. fact. samp. fact.
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |  0/2/4  |  0  |     2     |   1   |
          |  0/2/4  |  1  |     1     |   1   |
          |  0/2/4  |  2  |     1     |   1   |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |  1/3/5  |  0  |     2     |   2   |
          |  1/3/5  |  1  |     1     |   1   |
          |  1/3/5  |  2  |     1     |   1   |