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rfc:rfc4175

Network Working Group L. Gharai Request for Comments: 4175 USC/ISI Category: Standards Track C. Perkins

                                                 University of Glasgow
                                                        September 2005
             RTP Payload Format for Uncompressed 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.

Copyright Notice

 Copyright (C) The Internet Society (2005).

Abstract

 This memo specifies a packetization scheme for encapsulating
 uncompressed video into a payload format for the Real-time Transport
 Protocol, RTP.  It supports a range of standard- and high-definition
 video formats, including common television formats such as ITU
 BT.601, and standards from the Society of Motion Picture and
 Television Engineers (SMPTE), such as SMPTE 274M and SMPTE 296M.  The
 format is designed to be applicable and extensible to new video
 formats as they are developed.

1. Introduction

 This memo defines a scheme to packetize uncompressed, studio-quality
 video streams for transport using RTP [RTP].  It supports a range of
 standard and high-definition video formats, including ITU-R BT.601
 [601], SMPTE 274M [274] and SMPTE 296M [296].
 Formats for uncompressed standard definition television are defined
 by ITU Recommendation BT.601 [601] along with bit-serial and parallel
 interfaces in Recommendation BT.656 [656].  These formats allow both
 625-line and 525-line operation, with 720 samples per digital active
 line, 4:2:2 color sub-sampling, and 8- or 10-bit digital
 representation.

Gharai & Perkins Standards Track [Page 1] RFC 4175 RTP Payload Format for Uncompressed Video September 2005

 The representation of uncompressed high-definition television is
 specified in SMPTE standards 274M [274] and 296M [296].  SMPTE 274M
 defines a family of scanning systems with an image format of
 1920x1080 pixels with progressive and interlaced scanning, while
 SMPTE 296M defines systems with an image size of 1280x720 pixels and
 progressive scanning.  In progressive scanning, scan lines are
 displayed in sequence from top to bottom of a full frame.  In
 interlaced scanning, a frame is divided into its odd and even scan
 lines (called fields) and the two fields are displayed in succession.
 SMPTE 274M and 296M define images with aspect ratios of 16:9, and
 define the digital representation for RGB and YCbCr components.  In
 the case of YCbCr components, the Cb and Cr components are
 horizontally sub-sampled by a factor of two (4:2:2 color encoding).
 Although these formats differ in their details, they are structurally
 very similar.  This memo specifies a payload format to encapsulate
 these and other similar video formats for transport within RTP.

2. Conventions Used in This Document

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [2119].

3. Payload Design

 Each scan line of digital video is packetized into one or more RTP
 packets.  If the data for a complete scan line exceeds the network
 MTU, the scan line SHOULD be fragmented into multiple RTP packets,
 each smaller than the MTU.  A single RTP packet MAY contain data for
 more than one scan line.  Only the active samples are included in the
 RTP payload: inactive samples and the contents of horizontal and
 vertical blanking SHOULD NOT be transported.  In instances where
 ancillary data is being transmitted, the sender and receiver can
 disambiguate between ancillary and video data via scan line numbers.
 That is, the ancillary data will use scan line numbers that are not
 within the scope of the video frame.
 Scan line numbers are included in the RTP payload header, along with
 a field identifier for interlaced video.
    For SMPTE 296M format video, valid scan line numbers are from 26
    through 745, inclusive.  For progressive scan SMPTE 274M format
    video, valid scan lines are from scan line 42 through 1121,
    inclusive.  For interlaced scan SMPTE 274M format video, valid
    scan line numbers for field one (F=0) are from 21 to 560 and valid
    scan line numbers for the second field (F=1) are from 584 to 1123.
    For ITU-R BT.601 format video, the blanking intervals defined in

Gharai & Perkins Standards Track [Page 2] RFC 4175 RTP Payload Format for Uncompressed Video September 2005

    BT.656 are used: for 625 line video, lines 24 to 310 of field one
    (F=0) and 337 to 623 of the second field (F=1) are valid; for 525
    line video, lines 21 to 263 of the first field, and 284 to 525 of
    the second field are valid.  Other formats (e.g., [372]) may
    define different ranges of active lines.
 The payload header contains a 16-bit extension to the standard 16-bit
 RTP sequence number, thereby extending the sequence number to 32 bits
 and enabling the payload format to accommodate high data rates
 without ambiguity.  This is necessary as the 16-bit RTP sequence
 number will roll over very quickly for high data rates.  For example,
 for a 1-Gbps video stream with packet sizes of at least 1000 octets,
 the standard RTP packet will roll over in 0.5 seconds, which can be a
 problem for detecting loss and out-of-order packets particularly in
 instances where the round-trip time is greater than half a second.
 The extended 32-bit number allows for a longer wrap-around time of
 approximately nine hours.
 Each scan line comprises an integer number of pixels.  Each pixel is
 represented by a number of samples.  Samples may be coded as 8-, 10-,
 12-, or 16-bit values.  A sample may represent a color component or a
 luminance component of the video.  Color samples may be shared
 between adjacent pixels.  The sharing of color samples between
 adjacent pixels is known as color sub-sampling.  This is typically
 done in the YCbCr color space for the purpose of reducing the size of
 the image data.
 Pixels that share sample values MUST be transported together as a
 "pixel group".  If 10-bit or 12-bit samples are used, each pixel may
 also comprise a non-integer number of octets.  In this case, several
 pixels MUST be combined into an octet-aligned pixel group for
 transmission.  These restrictions simplify the operation of receivers
 by ensuring that the complete payload is octet aligned, and that
 samples relating to a single pixel are not fragmented across multiple
 packets [ALF].
 For example, in YCbCr video with 4:1:1 color sub-sampling, each group
 of 4 adjacent pixels comprises 6 samples, Y1 Y2 Y3 Y4 Cr Cb, with the
 Cr and Cb values being shared between all 4 pixels.  If samples are
 8-bit values, the result is a group of 4 pixels comprising 6 octets.
 If, however, samples are 10-bit values, the resulting 60-bit group is
 not octet aligned.  To be both octet aligned and appropriately
 framed, two groups of 4 adjacent pixels must be collected, thereby
 becoming octet aligned on a 15-octet boundary.  This length is
 referred to as the pixel group size ("pgroup").

Gharai & Perkins Standards Track [Page 3] RFC 4175 RTP Payload Format for Uncompressed Video September 2005

 Formally, the "pgroup" parameter is the size in octets of the
 smallest grouping of pixels such that 1) the grouping comprises an
 integer number of octets; and 2) if color sub-sampling is used,
 samples are only shared within the grouping.  When packetizing
 digital active line content, video data MUST NOT be fragmented within
 a pgroup.
 Video content is almost always associated with additional information
 such as audio tracks, time code, etc.  In professional digital video
 applications, this data is commonly embedded in non-active portions
 of the video stream (horizontal and vertical blanking periods) so
 that precise and robust synchronization is maintained.  This payload
 format requires that applications using such synchronized ancillary
 data SHOULD deliver it in separate RTP sessions that operate
 concurrently with the video session.  The normal RTP mechanisms
 SHOULD be used to synchronize the media.

4. RTP Packetization

 The standard RTP header is followed by a 2-octet payload header that
 extends the RTP Sequence Number, and by a 6-octet payload header for
 each line (or partial line) of video included.  One or more lines, or
 partial lines, of video data follow.  This format makes the payload
 header 32-bit aligned in the common case, where one scan line (or
 fragment) of video is included in each RTP packet.
 For example, if two lines of video are encapsulated, the payload
 format will be as shown in Figure 1.

Gharai & Perkins Standards Track [Page 4] RFC 4175 RTP Payload Format for Uncompressed Video September 2005

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | V |P|X|   CC  |M|    PT       |       Sequence Number         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           Time Stamp                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                             SSRC                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Extended Sequence Number    |            Length             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |F|          Line No            |C|           Offset            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            Length             |F|          Line No            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |C|           Offset            |                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               .
    .                                                               .
    .                 Two (partial) lines of video data             .
    .                                                               .
    +---------------------------------------------------------------+
   Figure 1: RTP Payload Format showing two (partial) lines of video

4.1. The RTP Header

 The fields of the fixed RTP header have their usual meaning, with the
 following additional notes:
 Payload Type (PT): 7 bits
   A dynamically allocated payload type field that designates the
   payload as uncompressed video.
 Timestamp: 32 bits
   For progressive scan video, the timestamp denotes the sampling
   instant of the frame to which the RTP packet belongs.  Packets MUST
   NOT include data from multiple frames, and all packets belonging to
   the same frame MUST have the same timestamp.
   For interlaced video, the timestamp denotes the sampling instant of
   the field to which the RTP packet belongs.  Packets MUST NOT
   include data from multiple fields, and all packets belonging to the
   same field MUST have the same timestamp.  Use of field timestamps,
   rather than a frame timestamp and field indicator bit, is needed to
   support reverse 3-2 pulldown.

Gharai & Perkins Standards Track [Page 5] RFC 4175 RTP Payload Format for Uncompressed Video September 2005

   A 90-kHz timestamp SHOULD be used in both cases.  If the sampling
   instant does not correspond to an integer value of the clock (as
   may be the case when interleaving), the value SHALL be truncated to
   the next lowest integer, with no ambiguity.
 Marker bit (M): 1 bit
   If progressive scan video is being transmitted, the marker bit
   denotes the end of a video frame.  If interlaced video is being
   transmitted, it denotes the end of the field.  The marker bit MUST
   be set to 1 for the last packet of the video frame/field.  It MUST
   be set to 0 for other packets.
 Sequence Number: 16 bits
   The low-order bits for RTP sequence number.  The standard 16-bit
   sequence number is augmented with another 16 bits in the payload
   header in order avoid problems due to wrap-around when operating at
   high rate rates.

4.2. Payload Header

 Extended Sequence Number: 16 bits
   The high order bits of the extended 32-bit sequence number, in
   network byte order.
 Length: 16 bits
   Number of octets of data included from this scan line, in network
   byte order.  This MUST be a multiple of the pgroup value.
 Line No.: 15 bits
   Scan line number of encapsulated data, in network byte order.
   Successive RTP packets MAY contains parts of the same scan line
   (with an incremented RTP sequence number, but the same timestamp),
   if it is necessary to fragment a line.
 Offset: 15 bits
   Offset of the first pixel of the payload data within the scan line.
   If YCbCr format data is being transported, this is the pixel offset
   of the luminance sample; if RGB format data is being transported,
   it is the pixel offset of the red sample; if BGR format data is
   being transported, it is the pixel offset of the blue sample.  The

Gharai & Perkins Standards Track [Page 6] RFC 4175 RTP Payload Format for Uncompressed Video September 2005

   value is in network byte order.  The offset has a value of zero if
   the first sample in the payload corresponds to the start of the
   line, and increments by one for each pixel.
 Field Identification (F): 1 bit
   Identifies which field the scan line belongs to, for interlaced
   data.  F=0 identifies the first field and F=1 the second field.
   For progressive scan data (e.g., SMPTE 296M format video), F MUST
   always be set to zero.
 Continuation (C): 1 bit
   Determines if an additional scan line header follows the current
   scan line header in the RTP packet.  Set to 1 if an additional
   header follows, implying that the RTP packet is carrying data for
   more than one scan line.  Set to 0 otherwise.  Several scan lines
   MAY be included in a single packet, up to the path MTU limit.  The
   only way to determine the number of scan lines included per packet
   is to parse the payload headers.

4.3. Payload Data

 Depending on the video format, each RTP packet can include either a
 single complete scan line, a single fragment of a scan line, or one
 (or more) complete scan lines and scan line fragments.  The length of
 each scan line or scan line fragment MUST be an integer multiple of
 the pgroup size in octets.  Scan lines SHOULD be fragmented so that
 the resulting RTP packet is smaller than the path MTU.
 It is possible that the scan line length is not evenly divisible by
 the number of pixels in a pgroup, so the final pixel data of a scan
 line does not align to either an octet or a pgroup boundary.
 Nonetheless, the payload MUST contain a whole number of pgroups; the
 sender MUST fill the remaining bits of the final pgroup with zero and
 the receiver MUST ignore the fill data. (In effect, the trailing edge
 of the image is black-filled to a pgroup boundary.)
 For RGB format video, samples are packed in order Red-Green-Blue.
 For BGR format video, samples are packed in order Blue-Green-Red.
 For both formats, if 8-bit samples are used, the pgroup is 3 octets.
 If 10-bit samples are used, samples from 4 adjacent pixels form 15-
 octet pgroups.  If 12-bit samples are used, samples from 2 adjacent
 pixels form 9-octet pgroups.  If 16-bit samples are used, each pixel
 forms a separate 6-octet pgroup.

Gharai & Perkins Standards Track [Page 7] RFC 4175 RTP Payload Format for Uncompressed Video September 2005

 For RGBA format video, samples are packed in order Red-Green-Blue-
 Alpha.  For BGRA format video, samples are packed in order Blue-
 Green-Red-Alpha.  For 8-, 10-, 12-, or 16-bit samples, each pixel
 forms its own pgroup, with octet sizes of 4, 5, 6, and 8,
 respectively.
 If the video is in YCbCr format, the packing of samples into the
 payload depends on the color sub-sampling used.
 For YCbCr 4:4:4 format video, samples are packed in order Cb-Y-Cr for
 both interlaced and progressive frames.  If 8-bit samples are used,
 the pgroup is 3 octets.  If 10-bit samples are used, samples from 4
 adjacent pixels form 15-octet pgroups.  If 12-bit samples are used,
 samples from 2 adjacent pixels form 9-octet pgroups.  If 16-bit
 samples are used, each pixel forms a separate 6-octet pgroup.
 For YCbCr 4:2:2 format video, the Cb and Cr components are
 horizontally sub-sampled by a factor of two (each Cb and Cr sample
 corresponds to two Y components).  Samples are packed in order Cb0-
 Y0-Cr0-Y1 for both interlaced and progressive scan lines.  For 8-,
 10-, 12-, or 16-bit samples, the pgroup is formed from two adjacent
 pixels (4, 5, 6, or 8 octets, respectively).
 For YCbCr 4:1:1 format video, the Cb and Cr components are
 horizontally sub-sampled by a factor of four (each Cb and Cr sample
 corresponds to four Y components).  Samples are packed in order Cb0-
 Y0-Y1-Cr0-Y2-Y3 for both interlaced and progressive scan lines.  For
 8-, 10-, 12-, or 16-bit samples, the pgroup is formed from four
 adjacent pixels (6, 15, 9, or 12 octets, respectively).
 For YCbCr 4:2:0 video, the Cb and Cr components are sub-sampled by a
 factor of two both horizontally and vertically.  Therefore,
 chrominance samples are shared between certain adjacent lines.
 Figure 2 shows the composition of luminance and chrominance samples
 for a 6x6 pixel grid of 4:2:0 YCbCr video.  The pixel group is a
 group of four pixels arranged in a 2x2 matrix.  The octet size of the
 pgroup for progressive scan 4:2:0 video with samples sizes of 8, 10,
 12, and 16 bits is 6, 15, 9, and 12 octets, respectively.  For
 interlaced 4:2:0 video, the corresponding pgroups are 4, 5, 6, and 8
 octets.

Gharai & Perkins Standards Track [Page 8] RFC 4175 RTP Payload Format for Uncompressed Video September 2005

     line 0:  Y00   Y01   Y02   Y03   Y04   Y05
              Cb00 Cr00   Cb01 Cr01   Cb02 Cr02
     line 1:  Y10   Y11   Y12   Y13   Y14   Y15
     line 2:  Y20   Y21   Y22   Y23   Y24   Y25
              Cb10 Cr10   Cb11 Cr11   Cb12 Cr12
     line 3:  Y30   Y31   Y32   Y33   Y34   Y35
     line 4:  Y40   Y41   Y42   Y43   Y44   Y45
              Cb20 Cr20   Cb21 Cr21   Cb22 Cr22
     line 5:  Y50   Y51   Y52   Y53   Y54   Y55
   Figure 2: Chrominance/luminance composition in 4:2:0 YCbCr video
 When packetizing progressive scan 4:2:0 YCbCr video, samples from two
 consecutive scan lines are included in each packet.  The scan line
 number in the payload header is set to that of the first scan line of
 the pair:
   line 0/1:
   Y00-Y01-Y10-Y11-Cb00-Cr00 Y02-Y03-Y12-Y13-Cb01-Cr01
                                         Y04-Y05-Y14-Y15-Cb02-Cr02
   line 2/3:
   Y20-Y21-Y30-Y31-Cb10-Cr10 Y22-Y23-Y32-Y33-Cb11-Cr11
                                         Y24-Y25-Y34-Y35-Cb12-Cr12
   line 4/5:
   Y40-Y41-Y50-Y51-Cb20-Cr20 Y42-Y43-Y52-Y53-Cb21-Cr21
                                         Y44-Y45-Y54-Y55-Cb22-Cr22
   Figure 3: Packetization of progressive 4:2:0 YCbCr video
 For interlaced transport, chrominance samples are transported with
 every other line.  The first set of chrominance samples may be
 transported with either the first line of field 0, or the first line
 of field 1.  Figure 4 illustrates the transport of chrominance
 samples starting with the first line of field 0 (signaled by the
 "top-field-first" MIME parameter).

Gharai & Perkins Standards Track [Page 9] RFC 4175 RTP Payload Format for Uncompressed Video September 2005

   field 0:
      line 0: Y00-Y01-Cb00-Cr00 Y02-Y03-Cb01-Cr01 Y04-Y05-Cb02-Cr02
      line 2: Y20-Y21 Y22-Y23 Y24-Y25
      line 4: Y40-Y41-Cb20-Cr20 Y42-Y43-Cb21-Cr21 Y44-Y45-Cb22-Cr22
   field 1:
      line 1: Y10-Y11 Y12-Y13 Y14-Y15
      line 3: Y30-Y31-Cb10-Cr10 Y32-Y33-Cb11 Cr11 Y34-Y35-Cb12-Cr12
      line 5: Y50-Y51 Y52-Y53 Y54-Y55
   Figure 4: Packetization of interlaced 4:2:0 YCbCr video with
             top-field-first.
 Chrominance values may be sampled with different offsets relative to
 luminance values.  For instance, in Figure 2, chrominance values are
 sampled at the same distance from neighboring luminance samples.  It
 is also possible for a chrominance sample to be co-sited with a
 luminance sample, as in Figure 5:
     line 0:  Y00-C   Y01   Y02-C   Y03   Y04-C   Y05
     line 1:  Y10     Y11   Y12     Y13   Y14     Y15
     line 2:  Y20-C   Y21   Y22-C   Y23   Y24-C   Y25
     line 3:  Y30     Y31   Y32     Y33   Y34     Y35
     line 4:  Y40-C   Y41   Y42-C   Y43   Y44-C   Y45
     line 5:  Y50     Y51   Y52     Y53   Y54     Y55
   Figure 5: Co-sited video sampling in 4:2:0 YCbCr video where C
             designates a CbCr pair
 In general, chrominance values may be placed between luminance
 samples or co-sited.  Positions can be designated by an integer
 numbering system starting from left to right and top to bottom.  The
 position matrices shown in Figures 6, 7, and 8 apply for 4:2:0,
 4:2:2, and 4:1:1 video, respectively:
     line N:    Y[0] [1] Y[2]   Y[0] [1] Y[2]
                 [3] [4] Y[5]    [3] [4]  [5]
     line N+1:  Y[6] [7] Y[8]   Y[6] [7] Y[8]
   Figure 6: Chrominance position matrix for 4:2:0 YCbCr video

Gharai & Perkins Standards Track [Page 10] RFC 4175 RTP Payload Format for Uncompressed Video September 2005

     line N:    Y[0] [1] Y[2] [3]  Y[0] [1] Y[2] [3]
     line N+1:  Y[0] [1] Y[2] [3]  Y[0] [1] Y[2] [3]
   Figure 7: Chrominance position matrix for 4:2:2 YCbCr video
     line N:    Y[0] [1] Y[2] [3] Y[4] [5] Y[6]
     line N+1:  Y[0] [1] Y[2] [3] Y[4] [5] Y[6]
   Figure 8: Chrominance position matrix for 4:1:1 YCbCr video
 Although these positions do not affect the packetization order of
 chrominance and luminance samples, the information is needed for
 interpolation prior to display and therefore should be signaled to
 the receiver.

5. RTCP Considerations

 RTCP SHOULD be used as specified in RFC 3550 [RTP].  It is to be
 noted that the sender's octet count in SR packets and the cumulative
 number of packets lost will wrap around quickly for high data rate
 streams.  This means that these two fields may not accurately
 represent octet count and number of packets lost since the beginning
 of transmission, as defined in RFC 3550.  Therefore, for network
 monitoring purposes, other means of keeping track of these variables
 SHOULD be used.

6. IANA Considerations

 The IANA has registered one new MIME subtype along with an associated
 RTP Payload Format, and has created two sub-parameter registries, as
 described in the following.

6.1. MIME type registration

 MIME media type name: video
 MIME subtype name: raw
 Required parameters:
   rate: The RTP timestamp clock rate.  Applications using this
   payload format SHOULD use a value of 90000.
   sampling: Determines the color (sub-)sampling mode of the video
   stream.  Currently defined values are RGB, RGBA, BGR, BGRA,
   YCbCr-4:4:4, YCbCr-4:2:2, YCbCr-4:2:0, and YCbCr-4:1:1.  New values
   may be registered as described in section 6.2 of RFC 4175.

Gharai & Perkins Standards Track [Page 11] RFC 4175 RTP Payload Format for Uncompressed Video September 2005

   width: Determines the number of pixels per line.  This is an
   integer between 1 and 32767.
   height: Determines the number of lines per frame.  This is an
   integer between 1 and 32767.
   depth: Determines the number of bits per sample.  This is an
   integer with typical values including 8, 10, 12, and 16.
   colorimetry: This parameter defines the set of colorimetric
   specifications and other transfer characteristics for the video
   source, by reference to an external specification.  Valid values
   and their specification are:
        BT601-5      ITU Recommendation BT.601-5 [601]
        BT709-2      ITU Recommendation BT.709-2 [709]
        SMPTE240M    SMPTE standard 240M [240]
   New values may be registered as described in section 6.2 of RFC
   4175.
 Optional parameters:
   Interlace: If this OPTIONAL parameter is present, it indicates that
   the video stream is interlaced.  If absent, progressive scan is
   implied.
   Top-field-first: If this OPTIONAL parameter is present, it
   indicates that chrominance samples are packetized starting with the
   first line of field 0.  Its absence implies that chrominance
   samples are packetized starting with the first line of field 1.
   chroma-position: This OPTIONAL parameter defines the position of
   chrominance samples relative to luminance samples.  It is either a
   single integer or a comma separated pair of integers.  Integer
   values range from 0 to 8, as specified in Figures 6-8 of RFC 4175.
   A single integer implies that Cb and Cr are co-sited.  A comma
   separated pair of integers designates the locations of Cb and Cr
   samples, respectively.  In its absence, a single value of zero is
   assumed for color-subsampled video (chroma-position=0).
   gamma: An OPTIONAL floating point gamma correction value.

Gharai & Perkins Standards Track [Page 12] RFC 4175 RTP Payload Format for Uncompressed Video September 2005

 Encoding considerations:
   Uncompressed video is only transmitted over RTP as specified in RFC
   4175.  No file format media type has been defined to go with this
   transmission media type at this time.
 Security considerations: See section 9 of RFC 4175.
 Interoperability considerations: NONE.
 Published specification: RFC 4175.
 Applications which use this media type: Video communication.
 Additional information: None
 Person & email address to contact for further information:
   Ladan Gharai <ladan@isi.edu>
   IETF Audio/Video Transport working group.
 Intended usage: COMMON
 Author: Ladan Gharai <ladan@isi.edu>
 Change controller: IETF AVT Working Group
       delegated from the IESG

6.2. Parameter Registration

 New values of the "sampling" parameter MAY be registered with the
 IANA provided they reference an RFC or other permanent and readily
 available specification (the Specification Required policy of RFC
 2434 [2434]).  A new registration MUST define the packing order of
 samples and a valid combinations of color and sub-sampling modes.
 New values of the "colorimetry" parameter MAY be registered with the
 IANA provided they reference an RFC or other permanent and readily
 available specification if colorimetric parameters and other
 applicable transfer characteristics (the Specification Required
 policy of RFC 2434 [2434]).

7. Mapping MIME Parameters into SDP

 The information carried in the MIME media type specification has a
 specific mapping to fields in the Session Description Protocol (SDP)
 [SDP], which is commonly used to describe RTP sessions.  When SDP is
 used to specify sessions transporting uncompressed video, the mapping
 is as follows:

Gharai & Perkins Standards Track [Page 13] RFC 4175 RTP Payload Format for Uncompressed Video September 2005

  1. The MIME type ("video") goes in SDP "m=" as the media name.
  1. The MIME subtype (payload format name) goes in SDP "a=rtpmap" as

the encoding name.

  1. Remaining parameters go in the SDP "a=fmtp" attribute by copying

them directly from the MIME media type string as a semicolon-

    separated list of parameter=value pairs.
 A sample SDP mapping for uncompressed video is as follows:
   m=video 30000 RTP/AVP 112
   a=rtpmap:112 raw/90000
   a=fmtp:112 sampling=YCbCr-4:2:2; width=1280; height=720; depth=10;
                            colorimetry=BT.709-2; chroma-position=1
 In this example, a dynamic payload type 112 is used for uncompressed
 video.  The RTP sampling clock is 90 kHz.  Note that the "a=fmtp:"
 line has been wrapped to fit this page, and will be a single long
 line in the SDP file.

8. Security Considerations

 RTP packets using the payload format defined in this specification
 are subject to the security considerations discussed in the RTP
 specification [RTP] and any appropriate RTP profile.  This implies
 that confidentiality of the media streams is achieved by encryption.
 This payload type does not exhibit any significant non-uniformity in
 the receiver side computational complexity for packet processing to
 cause a potential denial-of-service threat.
 It is important to note that uncompressed video can have immense
 bandwidth requirements (up to 270 Mbps for standard-definition video,
 and approximately 1 Gbps for high-definition video).  This is
 sufficient to cause potential for denial-of-service if transmitted
 onto most currently available Internet paths.
 Accordingly, if best-effort service is being used, users of this
 payload format MUST monitor packet loss to ensure that the packet
 loss rate is within acceptable parameters.  Packet loss is considered
 acceptable if a TCP flow across the same network path, and
 experiencing the same network conditions, would achieve an average
 throughput, measured on a reasonable timescale, that is not less than
 the RTP flow is achieving.  This condition can be satisfied by
 implementing congestion control mechanisms to adapt the transmission

Gharai & Perkins Standards Track [Page 14] RFC 4175 RTP Payload Format for Uncompressed Video September 2005

 rate (or the number of layers subscribed for a layered multicast
 session), or by arranging for a receiver to leave the session if the
 loss rate is unacceptably high.
 This payload format may also be used in networks that provide
 quality-of-service guarantees.  If enhanced service is being used,
 receivers SHOULD monitor packet loss to ensure that the service that
 was requested is actually being delivered.  If it is not, then they
 SHOULD assume that they are receiving best-effort service and behave
 accordingly.

9. Relation to RFC 2431

 In comparison with RFC 2431, this memo specifies support for a wider
 variety of uncompressed video, in terms of frame size, color sub-
 sampling and sample sizes.  Although [BT656] can transport up to 4096
 scan lines and 2048 pixels per line, our payload type can support up
 to 32768 scan lines and pixels per line.  Also, RFC 2431 only address
 4:2:2 YCbCr data, while this memo covers YCbCr, RGB, RGBA, BGR, BGRA,
 and most common color sub-sampling schemes.  Given the variety of
 video types that we cover, this memo also assumes out-of-band
 signaling for sample size and data types (RFC 2431 uses in band
 signaling).

10. Relation to RFC 3497

 RFC 3497 [292RTP] specifies a RTP payload format for encapsulating
 SMPTE 292M video.  The SMPTE 292M standard defines a bit-serial
 digital interface for local area High-Definition Television (HDTV)
 transport.  As a transport medium, SMPTE 292M utilizes 10-bit words
 and a fixed 1.485 Gbps (and 1.485/1.001 Gbps) data rate.  SMPTE 292M
 is typically used in the broadcast industry for the transport of
 other video formats such as SMPTE 260M, SMPTE 295M, SMPTE 274M, and
 SMPTE 296M.
 RFC 3497 defines a circuit emulation for the transport of SMPTE 292M
 over RTP.  It is very specific to SMPTE 292 and has been designed to
 be interoperable with existing broadcast equipment with a constant
 rate of 1.485 Gbps.
 This memo defines a flexible native packetization scheme that can
 packetize any uncompressed video, at varying data rates.  In
 addition, unlike RFC 3497, this memo only transports active video
 pixels (i.e., horizontal and vertical blanking are not transported).

Gharai & Perkins Standards Track [Page 15] RFC 4175 RTP Payload Format for Uncompressed Video September 2005

11. Acknowledgements

 The authors are grateful to Philippe Gentric, Chuck Harrison, Stephan
 Wenger, and Dave Singer for their feedback.
 This memo is based upon work supported by the U.S. National Science
 Foundation (NSF) under Grant No. 0230738.  Any opinions, findings,
 and conclusions or recommendations expressed in this material are
 those of the authors and do not necessarily reflect the views of NSF.

Normative References

 [RTP]    Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
          "RTP: A Transport Protocol for Real-Time Applications", STD
          64, RFC 3550, July 2003.
 [2119]   Bradner, S., "Key words for use in RFCs to Indicate
          Requirement Levels", BCP 14, RFC 2119, March 1997.
 [2434]   Narten, T. and H. Alvestrand, "Guidelines for Writing an
          IANA Considerations Section in RFCs", BCP 26, RFC 2434,
          October 1998.
 [601]    International Telecommunication Union, "Studio encoding
          parameters of digital television for standard 4:3 and wide
          screen 16:9 aspect ratios", Recommendation BT.601, October
          1995.
 [709]    International Telecommunication Union, "Parameter Values for
          HDTV Standards for Production and International Programme
          Exchange", Recommendation BT.709-2
 [240]    Society of Motion Picture and Television Engineers,
          "Television - Signal Parameters - 1125-Line High-Definition
          Production", SMPTE 240M-1999.

Informative References

 [274]    Society of Motion Picture and Television Engineers,
          "1920x1080 Scanning and Analog and Parallel Digital
          Interfaces for Multiple Picture Rates", SMPTE 274M-1998.
 [296]    Society of Motion Picture and Television Engineers,
          "1280x720 Scanning, Analog and Digital Representation and
          Analog Interfaces", SMPTE 296M-1998.

Gharai & Perkins Standards Track [Page 16] RFC 4175 RTP Payload Format for Uncompressed Video September 2005

 [372]    Society of Motion Picture and Television Engineers, "Dual
          Link 292M Interface for 1920 x 1080 Picture Raster", SMPTE
          372M-2002.
 [ALF]    Clark, D. D., and Tennenhouse, D. L., "Architectural
          Considerations for a New Generation of Protocols", In
          Proceedings of SIGCOMM '90 (Philadelphia, PA, Sept. 1990),
          ACM.
 [SDP]    Handley, M. and V. Jacobson, "SDP: Session Description
          Protocol", RFC 2327, April 1998.
 [BT656]  Tynan, D., "RTP Payload Format for BT.656 Video Encoding",
          RFC 2431, October 1998.
 [292RTP] Gharai, L., Perkins, C., Goncher, G., and A. Mankin, "RTP
          Payload Format for Society of Motion Picture and Television
          Engineers (SMPTE) 292M Video", RFC 3497, March 2003.
 [656]    International Telecommunication Union, "Interfaces for
          Digital Component Video Signals in 525-line and 625-line
          Television Systems Operating at the 4:2:2 Level of
          Recommendation ITU-R BT.601 (Part A)", Recommendation
          BT.656, April 1998.

Authors' Addresses

 Ladan Gharai
 USC Information Sciences Institute
 3811 N. Fairfax Drive, #200
 Arlington, VA 22203
 USA
 EMail: ladan@isi.edu
 Colin Perkins
 University of Glasgow
 Department of Computing Science
 17 Lilybank Gardens
 Glasgow G12 8QQ
 United Kingdom
 EMail: csp@csperkins.org

Gharai & Perkins Standards Track [Page 17] RFC 4175 RTP Payload Format for Uncompressed Video September 2005

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Gharai & Perkins Standards Track [Page 18]

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