rfc2435.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

   quantization table mapping is dynamic and can change on every frame.
   Decoders MUST NOT depend on any previous version of the tables, and
   need to reload these tables on every frame.  Packets MUST NOT contain
   Q = 255 and Length = 0.

3.1.9.  JPEG Payload

   The data following the RTP/JPEG headers 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



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   explicitly with an EOI marker.  The scan may be padded to arbitrary
   length with undefined bytes.  (Some 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.)

   The type code determines whether restart markers are present.  If a
   type supports restart markers, the packet MUST contain a non-zero
   Restart Interval value in a Restart Marker Header and restart markers
   MUST appear on byte aligned boundaries beginning with an 0xFF between
   MCUs at that interval.  Additional 0xFF bytes MAY appear between
   restart intervals.  This can be used in the packetization process to
   align data to something like a word boundary for more efficient
   copying.  Restart markers MUST NOT appear anywhere else in the JPEG
   payload.  Types which do not support restart makers MUST NOT contain
   restart markers anywhere in the JPEG payload. All packets MUST
   contain a "stuffed" 0x00 byte following any true 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.

   Three ranges of the type field are currently defined. Types 0-63 are
   reserved as fixed, well-known mappings to be defined by this document
   and future revisions of this document. Types 64-127 are the same as
   types 0-63, except that restart markers are present in the JPEG data
   and a Restart Marker header appears immediately following the main
   JPEG header. Types 128-255 are free to be dynamically defined by a
   session setup protocol (which is beyond the scope of this document).

   Of the first group of fixed mappings, types 0 and 1 are currently
   defined, along with the corresponding types 64 and 65 that indicate
   the presence of restart markers.  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 1, 2, and 3 in that order.  The Y, U, and V color planes
   correspond to component numbers 1, 2, and 3, respectively.  Component
   1 (i.e., the luminance plane) uses Huffman table number 0 and
   quantization table number 0 (defined below) and components 2 and 3
   (i.e., the chrominance planes) use Huffman table number 1 and
   quantization table number 1 (defined below).




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   Type numbers 2-5 are reserved and SHOULD NOT be used.  Applications
   based on previous versions of this document (RFC 2035) should be
   updated to indicate the presence of restart markers with type 64 or
   65 and the Restart Marker header.

   The two RTP/JPEG types currently defined are described below:

                            horizontal   vertical   Quantization
           types  component samp. fact. samp. fact. table number
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |       |  1 (Y)  |     2     |     1     |     0     |
         | 0, 64 |  2 (U)  |     1     |     1     |     1     |
         |       |  3 (V)  |     1     |     1     |     1     |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |       |  1 (Y)  |     2     |     2     |     0     |
         | 1, 65 |  2 (U)  |     1     |     1     |     1     |
         |       |  3 (V)  |     1     |     1     |     1     |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   These sampling factors indicate that the chrominance components of
   type 0 video is downsampled horizontally by 2 (often called 4:2:2)
   while the chrominance components of type 1 video are downsampled both
   horizontally and vertically by 2 (often called 4:2:0).

   Types 0 and 1 can be used to carry both progressively scanned and
   interlaced image data.  This is encoded using the Type-specific field
   in the main JPEG header.  The following values are defined:

      0 : Image is progressively scanned.  On a computer monitor, it can
          be displayed as-is at the specified width and height.

      1 : Image is an odd field of an interlaced video signal.  The
          height specified in the main JPEG header is half of the height
          of the entire displayed image.  This field should be de-
          interlaced with the even field following it such that lines
          from each of the images alternate.  Corresponding lines from
          the even field should appear just above those same lines from
          the odd field.

      2 : Image is an even field of an interlaced video signal.

      3 : Image is a single field from an interlaced video signal, but
          it should be displayed full frame as if it were received as
          both the odd & even fields of the frame.  On a computer
          monitor, each line in the image should be displayed twice,
          doubling the height of the image.





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   Appendix B contains C source code for transforming the RTP/JPEG
   header parameters into the JPEG frame and scan headers that are
   absent from the data payload.

4.2.  The Q Field

   For JPEG types 0 and 1 (and their corresponding types 64 and 65), Q
   values between 1 and 99 inclusive are defined as follows.  Other
   values less than 128 are reserved.  Additional types are encouraged
   to use this definition if applicable.

   Both type 0 and type 1 JPEG require two quantization tables.  These
   tables are calculated as follows.  For 1 <= Q <= 99, the Independent
   JPEG Group's formula [5] is used to produce a scale factor S as:

           S = 5000 / Q          for  1 <= Q <= 50
             = 200 - 2 * Q       for 51 <= Q <= 99

   This value is then used to scale Tables K.1 and K.2 from [1]
   (saturating each value to 8 bits) to give quantization table numbers
   0 and 1, respectively.  C source code is provided in Appendix A to
   compute these tables.

   For Q values 128-255, dynamically defined quantization tables are
   used.  These tables may be specified either in-band or out of band by
   something like a session setup protocol, but the Quantization Table
   header MUST be present in the first packet of every frame. When the
   tables are specified out of band, they may be omitted from the packet
   by setting the Length field in this header to 0.

   When the quantization tables are sent in-band, they need not be sent
   with every frame.  Like the out of band case, frames which do not
   contain tables will have a Quantization Table header with a Length
   field of 0.  While this does decrease the overhead of including the
   tables, new receivers will be unable to properly decode frames from
   the time they start up until they receive the tables.

4.3.  Fragmentation and Reassembly

   Since JPEG frames can be large, they must often be fragmented.
   Frames SHOULD be fragmented into packets in a manner avoiding
   fragmentation at a lower level.  If support for partial frame
   decoding is desired, frames SHOULD be fragmented such that each
   packet contains an integral number of restart intervals (see below).

   Each packet that makes up a single frame MUST have the same
   timestamp, and the RTP marker bit MUST be set on the last packet in a
   frame.  The fragment offset field of each packet is set to the byte



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   offset of its payload data within the original frame.  Packets making
   up a frame SHOULD be sent sequentially and the fragments they contain
   MUST NOT overlap one another.

   An entire frame can be identified as a sequence of packets beginning
   with a packet having a zero fragment offset and ending with a packet
   having the RTP marker bit set.  Missing packets can be detected
   either with RTP sequence numbers or with the fragment offset and
   lengths of each packet.  Reassembly could be carried out without the
   offset field (i.e., using only the RTP marker bit and sequence
   numbers), but an efficient single-copy implementation would not
   otherwise be possible in the presence of misordered packets.
   Moreover, if the last packet of the previous frame (containing the
   marker bit) were dropped, then a receiver could not always detect
   that the current frame is entirely intact.

4.4.  Restart Markers

   Restart markers indicate a point in the JPEG stream at which the
   Huffman decoder and DC predictors are reset, allowing partial
   decoding starting at that point.  To fully take advantage of this,
   however, a decoder must know which MCUs of a frame a particular
   restart interval encodes.  While the original JPEG specification does
   provide a small sequence number field in the restart markers for this
   purpose, it is not large enough to properly cope with the loss of an
   entire packet's worth of data at a typical network MTU size.  The
   RTP/JPEG Restart Marker header contains the additional information
   needed to accomplish this.

   The size of restart intervals SHOULD be chosen to always allow an
   integral number of restart intervals to fit within a single packet.
   This will guarantee that packets can be decoded independently from
   one another.  If a restart interval ends up being larger than a
   packet, the F and L bits in the Restart Marker header can be used to
   fragment it, but the resulting set of packets must all be received by
   a decoder for that restart interval to be decoded properly.

   Once a decoder has received either a single packet with both the F
   and L bits set on or a contiguous sequence of packets (based on the
   RTP sequence number) which begin with an F bit and end with an L bit,
   it can begin decoding.  The position of the MCU at the beginning of
   the data can be determined by multiplying the Restart Count value by
   the Restart Interval value.  A packet (or group of packets as
   identified by the F and L bits) may contain any number of consecutive
   restart intervals.

   To accommodate encoders which generate frames with restart markers in
   them but cannot fragment the data in this manner, the Restart Count



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   field may be set to 0x3FFF with the F and L bits both set to 1.  This
   indicates to decoders that the entire frame must be reassembled
   before decoding it.

5.  Security Considerations

   RTP packets using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [6], and any appropriate RTP profile (for example [7]).
   This implies that confidentiality of the media streams is achieved by
   encryption. Because the data compression used with this payload
   format is applied end-to-end, encryption may be performed after
   compression so there is no conflict between the two operations.

   A potential denial-of-service threat exists for data encodings using
   compression techniques that have non-uniform receiver-end
   computational load.  The attacker can inject pathological datagrams
   into the stream which are complex to decode and cause the receiver to
   be overloaded.  However, this encoding does not exhibit any
   significant non-uniformity.

   Another potential denial-of-service threat exists around the
   fragmentation mechanism presented here.  Receivers should be prepared
   to limit the total amount of data associated with assembling received
   frames so as to avoid resource exhaustion.

   As with any IP-based protocol, in some circumstances a receiver may
   be overloaded simply by the receipt of too many packets, either
   desired or undesired.  Network-layer authentication may be used to
   discard packets from undesired sources, but the processing cost of
   the authentication itself may be too high.  In a multicast
   environment, pruning of specific sources will be implemented in a
   future version of IGMP [8] and in multicast routing protocols to
   allow a receiver to select which sources are allowed to reach it.

   A security review of this payload format found no additional
   considerations beyond those in the RTP specification.














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6.  Authors' Addresses

   Lance M. Berc
   Systems Research Center
   Digital Equipment Corporation
   130 Lytton Ave
   Palo Alto CA 94301

   Phone: [phone]
   EMail: [e-mail]


   William C. Fenner
   Xerox PARC
   3333 Coyote Hill Road
   Palo Alto, CA 94304

   Phone: [phone]
   EMail: [e-mail]


   Ron Frederick
   Xerox PARC
   3333 Coyote Hill Road
   Palo Alto, CA 94304

   Phone: [phone]
   EMail: [e-mail]


   Steven McCanne
   University of California at Berkeley
   Electrical Engineering and Computer Science
   633 Soda Hall
   Berkeley, CA 94720

   Phone: [phone]
   EMail: [e-mail]


   Paul Stewart
   Xerox PARC
   3333 Coyote Hill Road
   Palo Alto, CA 94304

   Phone: [phone]
   EMail: [e-mail]




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7.  References


   [1]  ISO DIS 10918-1. Digital Compression and Coding of Continuous-
        tone Still Images (JPEG), CCITT Recommendation T.81.

   [2]  William B. Pennebaker, Joan L. Mitchell, JPEG: Still Image Data
        Compression Standard, Van Nostrand Reinhold, 1993.

   [3]  Gregory K. Wallace, The JPEG Sill Picture Compression Standard,
        Communications of the ACM, April 1991, Vol 34, No. 1, pp. 31-44.

   [4]  The JPEG File Interchange Format.  Maintained by C-Cube
        Microsystems, Inc., and available in
        ftp://ftp.uu.net/graphics/jpeg/jfif.ps.gz.

   [5]  Tom Lane et. al., The Independent JPEG Group software JPEG
        codec.  Source code available in
        ftp://ftp.uu.net/graphics/jpeg/jpegsrc.v6a.tar.gz.

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