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Versions: (draft-klemets-avt-rtp-vc1) 00 01 02 03 04 05 06 RFC 4425

Internet Engineering Task Force
Internet Draft                                               A. Klemets
Document: draft-ietf-avt-rtp-vc1-05.txt                       Microsoft
Expires: July 2006                                         January 2006


               RTP Payload Format for Video Codec 1 (VC-1)

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

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Copyright Notice

      Copyright (C) The Internet Society (2006).

Abstract

   This memo specifies an RTP payload format for encapsulating Video
   Codec 1 (VC-1) compressed bit streams, as defined by the Society of
   Motion Picture and Television Engineers (SMPTE) standard, SMPTE 421M.
   SMPTE is the main standardizing body in the motion imaging industry
   and the SMPTE 421M standard defines a compressed video bit stream
   format and decoding process for television.


Table of Contents

   1. Introduction..................................................2
      1.1 Conventions used in this document.........................3
   2. Definitions and abbreviations.................................3


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   3. Overview of VC-1 .............................................5
      3.1 VC-1 bit stream layering model............................5
      3.2 Bit-stream Data Units in Advanced profile.................6
      3.3 Decoder initialization parameters.........................6
      3.4 Ordering of frames........................................8
   4. Encapsulation of VC-1 format bit streams in RTP ..............8
      4.1 Access Units .............................................8
      4.2 Fragmentation of VC-1 frames ............................10
      4.3 Time stamp considerations................................10
      4.4 Random Access Points ....................................12
      4.5 Removal of HRD parameters................................12
      4.6 Repeating the Sequence Layer header .....................13
      4.7 Signaling of media type parameters.......................13
      4.8 The "mode=1" media type parameter........................14
      4.9 The "mode=3" media type parameter........................14
   5. RTP Payload Format syntax....................................15
      5.1 RTP header usage.........................................15
      5.2 AU header syntax.........................................16
      5.3 AU Control field syntax..................................17
   6. RTP Payload format parameters................................18
      6.1 Media type Registration..................................18
      6.2 Mapping of media type parameters to SDP..................26
      6.3 Usage with the SDP Offer/Answer Model....................26
      6.4 Usage in Declarative Session Descriptions................28
   7. Security Considerations......................................29
   8. Congestion Control...........................................30
   9. IANA Considerations..........................................31
   10. References..................................................31
      10.1 Normative references....................................31
      10.2 Informative references..................................31

1. Introduction

   This memo specifies an RTP payload format for the video coding
   standard Video Codec 1, also known as VC-1.  The specification for
   the VC-1 bit stream format and decoding process is published by the
   Society of Motion Picture and Television Engineers (SMPTE) as SMPTE
   421M [1].

   VC-1 has a broad applicability, being suitable for low bit rate
   Internet streaming applications to HDTV broadcast and Digital Cinema
   applications with nearly lossless coding.  The overall performance of
   VC-1 is such that bit rate savings of more than 50% are reported [9],
   when compared against MPEG-2.  See [9] for further details about how
   VC-1 compares against other codecs, such as MPEG-4 and H.264/AVC.
   (In [9], VC-1 is referred to by its earlier name, VC-9.)

   VC-1 is widely used for downloading and streaming of movies on the
   Internet, in the form of Windows Media Video 9 (WMV-9) [9], because


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   the WMV-9 codec is compliant with the VC-1 standard.  VC-1 has also
   recently been adopted as a mandatory compression format for the high-
   definition DVD formats HD DVD and Blu-ray.

   SMPTE 421M defines the VC-1 bit stream syntax and specifies
   constraints that must be met by VC-1 conformant bit streams.  SMPTE
   421M also specifies the complete process required to decode the bit
   stream.  However, it does not specify the VC-1 compression algorithm,
   thus allowing for different ways to implement a VC-1 encoder.

   The VC-1 bit stream syntax has three profiles. Each profile has
   specific bit stream syntax elements and algorithms associated with
   it.  Depending on the application in which VC-1 is used, some
   profiles may be more suitable than others.  For example, Simple
   profile is designed for low bit rate Internet streaming and for
   playback on devices that can only handle low complexity decoding.
   Advanced profile is designed for broadcast applications, such as
   digital TV, HD DVD or HDTV.  Advanced profile is the only VC-1
   profile that supports interlaced video frames and non-square pixels.

   Section 2 defines the abbreviations used in this document.  Section 3
   provides a more detailed overview of VC-1.  Sections 4 and 5 define
   the RTP payload format for VC-1, and section 6 defines the media type
   and SDP parameters for VC-1.  See section 7 for security
   considerations, and section 8 for congestion control requirements.

1.1 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 BCP 14, RFC 2119 [2].

2. Definitions and abbreviations

   This document uses the definitions in SMPTE 421M [1].  For
   convenience, the following terms from SMPTE 421M are restated here:

   B-picture: A picture that is coded using motion compensated
   prediction from past and/or future reference fields or frames.  A B-
   picture cannot be used for predicting any other picture.

   BI-picture: A B-picture that is coded using information only from
   itself.  A BI-picture cannot be used for predicting any other
   picture.

   Bit-stream data unit (BDU): A unit of the compressed data which may
   be parsed (i.e., syntax decoded) independently of other information
   at the same hierarchical level.  A BDU can be, for example, a
   sequence layer header, an entry-point header, a frame, or a slice.


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   Encapsulated BDU (EBDU): A BDU which has been encapsulated using the
   encapsulation mechanism described in Annex E of SMPTE 421M [1], to
   prevent emulation of the start code prefix in the bit stream.

   Entry-point: A point in the bit stream that offers random access.

   frame: A frame contains lines of spatial information of a video
   signal.  For progressive video, these lines contain samples starting
   from one time instant and continuing through successive lines to the
   bottom of the frame.  For interlaced video, a frame consists of two
   fields, a top field and a bottom field.  One of these fields will
   commence one field period later than the other.

   interlace: The property of frames where alternating lines of the
   frame represent different instances in time.  In an interlaced frame,
   one of the fields is meant to be displayed first.

   I-picture: A picture coded using information only from itself.

   level: A defined set of constraints on the values which may be taken
   by the parameters (such as bit rate and buffer size) within a
   particular profile.  A profile may contain one or more levels.

   P-picture: A picture that is coded using motion compensated
   prediction from past reference fields or frames.

   picture: For progressive video, a picture is identical to a frame,
   while for interlaced video, a picture may refer to a frame, or the
   top field or the bottom field of the frame depending on the context.

   profile: A defined subset of the syntax of VC-1, with a specific set
   of coding tools, algorithms, and syntax associated with it.  There
   are three VC-1 profiles: Simple, Main and Advanced.

   progressive: The property of frames where all the samples of the
   frame represent the same instance in time.

   random access: A random access point in the bit stream is defined by
   the following guarantee: If decoding begins at this point, all frames
   needed for display after this point will have no decoding dependency
   on any data preceding this point, and are also present in the
   decoding sequence after this point.  A random access point is also
   called an entry-point.

   sequence: A coded representation of a series of one or more pictures.
   In VC-1 Advanced profile, a sequence consists of a series of one or
   more entry-point segments, where each entry-point segment consists of
   a series of one or more pictures, and where the first picture in each


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   entry-point segment provides random access.  In VC-1 Simple and Main
   profiles, the first picture in each sequence is an I-picture.

   slice: A consecutive series of macroblock rows in a picture, which
   are encoded as a single unit.

   start codes (SC): 32-bit codes embedded in that coded bit stream that
   are unique, and identify the beginning of a BDU.  Start codes consist
   of a unique three-byte Start Code Prefix (SCP), and a one-byte Start
   Code Suffix (SCS).

3. Overview of VC-1

   The VC-1 bit stream syntax consists of three profiles: Simple, Main,
   and Advanced.  Simple profile is designed for low bit rates and for
   low complexity applications, such as playback of media on personal
   digital assistants.  The maximum bit rate supported by Simple profile
   is 384 kbps.  Main profile is targets high bit rate applications,
   such as streaming and TV over IP.  Main profile supports B-pictures,
   which provide improved compression efficiency at the cost of higher
   complexity.

   Certain features that can be used to achieve high compression
   efficiency, such as non-square pixels and support for interlaced
   pictures, are only included in Advanced profile.  The maximum bit
   rate supported by the Advanced profile is 135 Mbps, making it
   suitable for nearly lossless encoding of HDTV signals.
   Only Advanced profile supports carrying user-data (meta-data) in-band
   with the compressed bit stream.  The user-data can be used for closed
   captioning support, for example.

   Of the three profiles, only Advanced profile allows codec
   configuration parameters, such as the picture aspect ratio, to be
   changed through in-band signaling in the compressed bit stream.

   For each of the profiles, a certain number of "levels" have been
   defined.  Unlike a "profile", which implies a certain set of features
   or syntax elements, a "level" is a set of constraints on the values
   of parameters in a profile, such as the bit rate or buffer size.  VC-
   1 Simple profile has two levels, Main profile has three, and Advanced
   profile has five levels.  See Annex D of SMPTE 421M [1] for a
   detailed list of the profiles and levels.

3.1 VC-1 bit stream layering model

   The VC-1 bit stream is defined as a hierarchy of layers.  This is
   conceptually similar to the notion of a protocol stack of networking
   protocols.  The outermost layer is called the sequence layer.  The
   other layers are entry-point, picture, slice, macroblock and block.


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   In Simple and Main profiles, a sequence in the sequence layer
   consists of a series of one or more coded pictures.  In Advanced
   profile, a sequence consists of one or more entry-point segments,
   where each entry-point segment consists of a series of one or more
   pictures, and where the first picture in each entry-point segment
   provides random access.  A picture is decomposed into macroblocks.  A
   slice comprises one or more contiguous rows of macroblocks.

   The entry-point and slice layers are only present in Advanced
   profile.  In Advanced profile, the start of each entry-point layer
   segment indicates a random access point.  In Simple and Main profiles
   each I-picture is a random access point.

   Each picture can be coded as an I-picture, P-picture, skipped
   picture, BI-picture, or as a B-picture.  These terms are defined in
   section 2 of this document and in section 4.12 of SMPTE 421M [1].

3.2 Bit-stream Data Units in Advanced profile

   In Advanced profile, each picture and slice is considered a Bit-
   stream Data Unit (BDU).  A BDU is always byte-aligned and is defined
   as a unit that can be parsed (i.e., syntax decoded) independently of
   other information in the same layer.

   The beginning of a BDU is signaled by an identifier called Start Code
   (SC).  Sequence layer headers and entry-point headers are also BDUs
   and thus can be easily identified by their Start Codes.  See Annex E
   of SMPTE 421M [1] for a complete list of Start Codes.  Blocks and
   macroblocks are not BDUs and thus do not have a Start Code and are
   not necessarily byte-aligned.

   The Start Code consists of four bytes.  The first three bytes are
   0x00, 0x00 and 0x01.  The fourth byte is called the Start Code Suffix
   (SCS) and it is used to indicate the type of BDU that follows the
   Start Code.  For example, the SCS of a sequence layer header (0x0F)
   is different from the SCS of an entry-point header (0x0E).  The Start
   Code is always byte-aligned and is transmitted in network byte order.

   To prevent accidental emulation of the Start Code in the coded bit
   stream, SMPTE 421M defines an encapsulation mechanism that uses byte
   stuffing.  A BDU which has been encapsulated by this mechanism is
   referred to as an Encapsulated BDU, or EBDU.

3.3 Decoder initialization parameters

   In VC-1 Advanced profile, the sequence layer header contains
   parameters that are necessary to initialize the VC-1 decoder.



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   The parameters apply to all entry-point segments until the next
   occurrence of a sequence layer header in the coded bit stream.

   The parameters in the sequence layer header include the Advanced
   profile level, the maximum dimensions of the coded pictures, the
   aspect ratio, interlace information, the frame rate and up to 31
   leaky bucket parameter sets for the Hypothetical Reference Decoder
   (HRD).

   Section 6.1 of SMPTE 421M [1] provides the formal specification of
   the sequence layer header.

   A sequence layer header is not defined for VC-1 Simple and Main
   profiles.  For these profiles, decoder initialization parameters MUST
   be conveyed out-of-band.  The decoder initialization parameters for
   Simple and Main profiles include the maximum dimensions of the coded
   picture, and a leaky bucket parameter set for the HRD.  Section 4.7
   specifies how the parameters are conveyed by this RTP payload format.

   Each leaky bucket parameter set for the HRD specifies a peak
   transmission bit rate and a decoder buffer capacity.  The coded bit
   stream is restricted by these parameters.  The HRD model does not
   mandate buffering by the decoder.  Its purpose is to limit the
   encoder's bit rate fluctuations according to a basic buffering model,
   so that the resources necessary to decode the bit stream are
   predictable.  The HRD has a constant-delay mode and a variable-delay
   mode.  The constant-delay mode is appropriate for broadcast and
   streaming applications, while the variable-delay mode is designed for
   video conferencing applications.

   Annex C of SMPTE 421M [1] specifies the usage of the hypothetical
   reference decoder for VC-1 bit streams.  A general description of the
   theory of the HRD can be found in [10].

   For Simple and Main profiles, the current buffer fullness value for
   the HRD leaky bucket is signaled using the BF syntax element in the
   picture header of I-pictures and BI-pictures.

   For Advanced profile, the entry-point header specifies current buffer
   fullness values for the leaky buckets in the HRD.  The entry-point
   header also specifies coding control parameters that are in effect
   until the occurrence of the next entry-point header in the bit
   stream.  The concept of an entry-point layer applies only to VC-1
   Advanced profile.  See Section 6.2 of SMPTE 421M [1] for the formal
   specification of the entry-point header.






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3.4 Ordering of frames

   Frames are transmitted in the same order in which they are captured,
   except if B-pictures or BI-pictures are present in the coded bit
   stream.  A BI-picture is a special kind of B-picture, and in the
   remainder of this section the terms B-picture and B-frame also apply
   to BI-pictures and BI-frames, respectively.

   When B-pictures are present in the coded bit stream, the frames are
   transmitted such that the frames that the B-pictures depend on are
   transmitted first.  This is referred to as the coded order of the
   frames.

   The rules for how a decoder converts frames from the coded order to
   the display order are stated in section 5.4 of SMPTE 421M [1].  In
   short, if B-pictures may be present in the coded bit stream, a
   hypothetical decoder implementation needs to buffer one additional
   decoded frame.  When an I-frame or a P-frame is received, the frame
   can be decoded immediately but it is not displayed until the next I-
   or P-frame is received.  However, B-frames are displayed immediately.

   Figure 1 illustrates the timing relationship between the capture of
   frames, their coded order, and the display order of the decoded
   frames, when B-pictures are present in the coded bit stream.  The
   figure shows that the display of frame P4 is delayed until frame P7
   is received, while frames B2 and B3 are displayed immediately.


   Capture:        |I0  P1  B2  B3  P4  B5  B6  P7  B8  B9  ...
                   |
   Coded order:    |        I0  P1  P4  B2  B3  P7  B5  B6  ...
                   |
   Display order:  |            I0  P1  B2  B3  P4  B5  B6  ...
                   |
                   |+---+---+---+---+---+---+---+---+---+--> time
                    0   1   2   3   4   5   6   7   8   9

   Figure 1.  Frame reordering when B-pictures are present.

   If B-pictures are not present, the coded order and the display order
   are identical, and frames can then be displayed without additional
   delay shown in Figure 1.

4. Encapsulation of VC-1 format bit streams in RTP

4.1 Access Units

   Each RTP packet contains an integral number of application data units
   (ADUs).  For VC-1 format bit streams, an ADU is equivalent to one


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   Access Unit (AU).  An Access Unit is defined as the AU header
   (defined in section 5.2) followed by a variable length payload, with
   the rules and constraints described in sections 4.1 and 4.2.  Figure
   2 shows the layout of an RTP packet with multiple AUs.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+- .. +-+-+-+-+
   | RTP     | AU(1) | AU(2) |     | AU(n) |
   | Header  |       |       |     |       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+- .. +-+-+-+-+

   Figure 2.  RTP packet structure.

   Each Access Unit MUST start with the AU header defined in section
   5.2.  The AU payload MUST contain data belonging to exactly one VC-1
   frame.  This means that data from different VC-1 frames will always
   be in different AUs, however, it possible for a single VC-1 frame to
   be fragmented across multiple AUs (see section 4.2.)

   In the case of interlaced video, a VC-1 frame consists of two fields
   that may be coded as separate pictures.  The two pictures still
   belong to the same VC-1 frame.

   The following rules apply to the contents of each AU payload when VC-
   1 Advanced profile is used:

   - The AU payload MUST contain VC-1 bit stream data in EBDU format
     (i.e., the bit stream must use the byte-stuffing encapsulation
     mode defined in Annex E of SMPTE 421M [1].)

   - The AU payload MAY contain multiple EBDUs, e.g., a sequence layer
     header, an entry-point header, a frame (picture) header, a field
     header, and multiple slices and the associated user-data.
     (However, all slices and their corresponding macroblocks MUST
     belong to the same video frame.)

   - The AU payload MUST start at an EBDU boundary, except when the AU
     payload contains a fragmented frame, in which case the rules in
     section 4.2 apply.

   When VC-1 Simple or Main profiles are used, the AU payload MUST start
   at the beginning of a frame, except when the AU payload contains a
   fragmented frame.  Section 4.2 describes how to handle fragmented
   frames.

   Access Units MUST be byte-aligned.  If the data in an AU (EBDUs in
   the case of Advanced profile and frame in the case of Simple and
   Main) does not end at an octet boundary, up to 7 zero-valued padding
   bits MUST be added to achieve octet-alignment.



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4.2 Fragmentation of VC-1 frames

   Each AU payload SHOULD contain a complete VC-1 frame.  However, if
   this would cause the RTP packet to exceed the MTU size, the frame
   SHOULD be fragmented into multiple AUs to avoid IP-level
   fragmentation.  When an AU contains a fragmented frame, this MUST be
   indicated by setting the FRAG field in the AU header as defined in
   section 5.3.

   AU payloads that do not contain a fragmented frame, or that contain
   the first fragment of a frame, MUST start at an EBDU boundary if
   Advanced profile is used.  In this case, for Simple and Main
   profiles, the AU payload MUST start at the beginning of a frame.

   If Advanced profile is used, AU payloads that contain a fragment of a
   frame other than the first fragment, SHOULD start at an EBDU
   boundary, such as at the start of a slice.

   However, slices are only defined for Advanced profile, and are not
   always used.  Blocks and macroblocks are not BDUs (have no Start
   Code) and are not byte-aligned.  Therefore, it may not always be
   possible to continue a fragmented frame at an EBDU boundary.  One can
   determine if an AU payload starts at an EBDU boundary by inspecting
   the first three bytes of the AU payload.  The AU payload starts at an
   EBDU boundary if the first three bytes are identical to the Start
   Code Prefix (i.e., 0x00, 0x00, 0x01.)

   In the case of Simple and Main profiles, since the blocks and
   macroblocks are not byte-aligned, the fragmentation boundary may be
   chosen arbitrarily.

   If an RTP packet contains an AU with the last fragment of a frame,
   additional AUs SHOULD NOT be included in the RTP packet.

   If the PTS Delta field in the AU header is present, each fragment of
   a frame MUST have the same presentation time.  If the DTS Delta field
   in the AU header is present, each fragment of a frame MUST have the
   same decode time.

4.3 Time stamp considerations

   VC-1 video frames MUST be transmitted in the coded order.  Coded
   order implies that no frames are dependent on subsequent frames, as
   discussed in section 3.4.  When a video frame consists of a single
   picture, the presentation time of the frame is identical to the
   presentation time of the picture.  When the VC-1 interlace coding
   mode is used, frames may contain two pictures, one for each field.
   In that case, the presentation time of a frame is the presentation
   time of the field that is displayed first.


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   The RTP timestamp field MUST be set to the presentation time of the
   video frame contained in the first AU in the RTP packet.  The
   presentation time can be used as the timestamp field in the RTP
   header because it differs from the sampling instant of the frame only
   by an arbitrary constant offset.

   If the video frame in an AU has a presentation time that differs from
   the RTP timestamp field, then the presentation time MUST be specified
   using the PTS Delta field in the AU header.  Since the RTP timestamp
   field must be identical to the presentation time of the first video
   frame, this can only happen if an RTP packet contains multiple AUs.
   The syntax of the PTS Delta field is defined in section 5.2.

   The decode time of a VC-1 frame is always monotonically increasing
   when the video frames are transmitted in the coded order.  If neither
   B- nor BI-pictures are present in the coded bit stream, then the
   decode time of a frame SHALL be equal to the presentation time of the
   frame.  A BI-picture is a special kind of B-picture, and in the
   remainder of this section the terms B-picture and B-frame also apply
   to BI-pictures and BI-frames, respectively.

   If B-pictures may be present in the coded bit stream, then the decode
   times of frames are determined as follows:

   - Non-B frames:  The decode time SHALL be equal to the presentation
     time of the previous non-B frame in the coded order.

   - B-frames:  The decode time SHALL be equal to the presentation time
     of the B-frame.

   As an example, consider Figure 1 in section 3.4.  The decode time of
   non-B frame P4 is 4 time units, which is equal to the presentation
   time of the previous non-B frame in the coded order, which is P1.  On
   the other hand, the decode time of B-frame B2 is 5 time units, which
   is identical to its presentation time.

   If the decode time of a video frame differs from its presentation
   time, then the decode time MUST be specified using the DTS Delta
   field in the AU header.  The syntax of the DTS Delta field is defined
   in section 5.2.

   Knowing if the stream will contain B-pictures may help the receiver
   allocate resources more efficiently and can reduce delay, as an
   absence of B-pictures in the stream implies that no reordering
   of frames will be needed between the decoding process and the display
   of the decoded frames.  This may be important for interactive
   applications.



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   The receiver SHALL assume that the coded bit stream may contain B-
   pictures in the following cases:

   - Advanced profile: If the value of the "bpic" media type parameter
     defined in section 6.1 is 1, or if the "bpic" parameter is not
     specified.

   - Main profile: If the MAXBFRAMES field in STRUCT_C decoder
     initialization parameter has a non-zero value.  STRUCT_C is
     conveyed in the "config" media type parameter, which is defined in
     section 6.1.

   Simple profile does not use B-pictures.

4.4 Random Access Points

   The entry-point header contains information that is needed by the
   decoder to decode the frames in that entry-point segment.  This means
   that in the event of lost RTP packets the decoder may be unable to
   decode frames until the next entry-point header is received.

   The first frame after an entry-point header is a random access points
   into the coded bit stream.  Simple and Main profiles do not have
   entry-point headers, so for those profiles each I-picture is a random
   access point.

   To allow the RTP receiver to detect that an RTP packet which was lost
   contained a random access point, this RTP payload format defines a
   field called "RA Count".  This field is present in every AU, and its
   value is incremented (modulo 256) for every random access point.  For
   additional details, see the definition of "RA Count" in section 5.2.

   To make it easy to determine if an AU contains a random access point,
   this RTP payload format also defines a bit called the "RA" flag in
   the AU Control field.  This bit is set to 1 only on those AU's that
   contain a random access point.  The RA bit is defined in section 5.3.

4.5 Removal of HRD parameters

   The sequence layer header of Advanced profile may include up to 31
   leaky bucket parameter sets for the Hypothetical Reference Decoder
   (HRD).  Each leaky bucket parameter set specifies a possible peak
   transmission bit rate (HRD_RATE) and a decoder buffer capacity
   (HRD_BUFFER).  (See section 3.3 for additional discussion about the
   HRD.)

   If the actual peak transmission rate is known by the RTP sender, the
   RTP sender MAY remove all leaky bucket parameter sets except for the
   one corresponding to the actual peak transmission rate.


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   For each leaky bucket parameter set in the sequence layer header,
   there is also parameter in the entry-point header that specifies the
   initial fullness (HRD_FULL) of the leaky bucket.

   If the RTP sender has removed any leaky bucket parameter sets from
   the sequence layer header, then for any removed leaky bucket
   parameter set, it MUST also remove the corresponding HRD_FULL
   parameter in the entry-point header.

   Removing leaky bucket parameter sets, as described above, may
   significantly reduce the size of the sequence layer headers and the
   entry-point headers.

4.6 Repeating the Sequence Layer header

   To improve robustness against loss of RTP packets, it is RECOMMENDED
   that if the sequence layer header changes, it should be repeated
   frequently in the bit stream.  In this is case, it is RECOMMENDED
   that the number of leaky bucket parameters in the sequence layer
   header and the entry point headers be reduced to one, as described in
   section 4.5.  This will help reduce the overhead caused by repeating
   the sequence layer header.

   Any data in the VC-1 bit stream, including repeated copies of the
   sequence header itself, must be accounted for when computing the
   leaky bucket parameter for the HRD.  (See section 3.3 for a
   discussion about the HRD.)

   If the value of TFCNTRFLAG in the sequence layer header is 1, each
   picture header contains a frame counter field (TFCNTR).  Each time
   the sequence layer header is inserted in the bit stream, the value of
   this counter MUST be reset.

   To allow the RTP receiver to detect that an RTP packet which was lost
   contained a new sequence layer header, the AU Control field defines a
   bit called the "SL" flag.  This bit is toggled when a sequence layer
   header is transmitted, but only if that header is different from the
   most recently transmitted sequence layer header.  The SL bit is
   defined in section 5.3.

4.7 Signaling of media type parameters

   When this RTP payload format is used with SDP, the decoder
   initialization parameters described in section 3.3 MUST be signaled
   in SDP using the media type parameters specified in section 6.1.
   Section 6.2 specifies how to map the media type parameters to SDP
   [5], and section 6.3 defines rules specific to the SDP Offer/Answer



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   model, and section 6.4 defines rules for when SDP is used in a
   declarative style.

   When Simple or Main profiles are used, it is not possible to change
   the decoder initialization parameters through the coded bit stream.
   Any changes to the decoder initialization parameters would have to be
   done through out-of-band means, e.g., by a SIP [14] re-invite or
   similar means that convey an updated session description.

   When Advanced profile is used, the decoder initialization parameters
   MAY be changed by inserting a new sequence layer header or an entry-
   point header in the coded bit stream.

   The sequence layer header specifies the VC-1 level, the maximum size
   of the coded pictures and optionally also the maximum frame rate.
   The media type parameters "level", "width", "height" and "framerate"
   specify upper limits for these parameters.  Thus, the sequence layer
   header MAY specify values that are lower than the values of the media
   type parameters "level", "width", "height" or "framerate", but the
   sequence layer header MUST NOT exceed the values of any of these
   media type parameters.

4.8 The "mode=1" media type parameter

   In certain applications using Advanced profile, the sequence layer
   header never changes.  This MAY be signaled with the media type
   parameter "mode=1". (The "mode" parameter is defined in section 6.1.)
   The "mode=1" parameter serves as a "hint" to the RTP receiver that
   all sequence layer headers in the bit stream will be identical.  If
   "mode=1" is signaled and a sequence layer header is present in the
   coded bit stream, then it MUST be identical to the sequence layer
   header specified by the "config" media type parameter.

   Since the sequence layer header never changes in "mode=1", the RTP
   sender MAY remove it from the bit stream.  Note, however, that if the
   value of TFCNTRFLAG in the sequence layer header is 1, each picture
   header contains a frame counter field (TFCNTR).  This field is reset
   each time the sequence layer header occurs in the bit stream.  If the
   RTP sender chooses to remove the sequence layer header, then it MUST
   ensure that the resulting bit stream is still compliant with the VC-1
   specification (e.g., by adjusting the TFCNTR field, if necessary.)

4.9 The "mode=3" media type parameter

   In certain applications using Advanced profile, both the sequence
   layer header and the entry-point header never change.  This MAY be
   signaled with the media type parameter "mode=3".  The same rules
   apply to "mode=3" as for "mode=1", described in section 4.8.
   Additionally, if "mode=3" is signaled, then the RTP sender MAY


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   "compress" the coded bit stream by not including sequence layer
   headers and entry-point headers in the RTP packets.

   The RTP receiver MUST "decompress" the coded bit stream by re-
   inserting the entry-point headers prior to delivering the coded bit
   stream to the VC-1 decoder.  The sequence layer header does not need
   to be decompressed by the receiver, since it never changes.

   If "mode=3" is signaled and the RTP receiver receives a complete AU
   or the first fragment of an AU, and the RA bit is set to 1 but the AU
   does not begin with an entry-point header, then this indicates that
   entry-point header has been "compressed".  In that case, the RTP
   receiver MUST insert an entry-point header at the beginning of the
   AU.  When inserting the entry-point header, the RTP receiver MUST use
   the one that was specified by the "config" media type parameter.

5. RTP Payload Format syntax

5.1 RTP header usage

   The format of the RTP header is specified in RFC 3550 [3] and is
   reprinted in Figure 3 for convenience.

      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=2|P|X|  CC   |M|     PT      |       sequence number         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           timestamp                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           synchronization source (SSRC) identifier            |
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
     |            contributing source (CSRC) identifiers             |
     |                             ....                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 3.  RTP header according to RFC 3550

   The fields of the fixed RTP header have their usual meaning, which is
   defined in RFC 3550 and by the RTP profile in use, with the following
   additional notes:

   Marker bit (M): 1 bit
           This bit is set to 1 if the RTP packet contains an Access
           Unit containing a complete VC-1 frame, or the last fragment
           of a VC-1 frame.





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   Payload type (PT): 7 bits
           This document does not assign an RTP payload type for this
           RTP payload format. The assignment of a payload type has to
           be performed either through the RTP profile used or in a
           dynamic way.

   Sequence Number: 16 bits
           The RTP receiver can use the sequence number field to recover
           the coded order of the VC-1 frames.  (A typical VC-1 decoder
           will require the VC-1 frames to be delivered in coded order.)
           When VC-1 frames have been fragmented across RTP packets, the
           RTP receiver can use the sequence number field to ensure that
           no fragment is missing.

   Timestamp: 32 bits
           The RTP timestamp is set to the presentation time of the VC-1
           frame in the first Access Unit.
           A clock rate of 90 kHz MUST be used.

5.2 AU header syntax

   The Access Unit header consists of a one-byte AU Control field, the
   RA Count field and 3 optional fields.  All fields MUST be written in
   network byte order.  The structure of the AU header is illustrated in
   Figure 4.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |AU     | RA    |  AUP  | PTS   | DTS   |
   |Control| Count |  Len  | Delta | Delta |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 4.  Structure of AU header.

   AU Control: 8 bits
           The usage of the AU Control field is defined in section 5.3.

   RA Count: 8 bits
           Random Access Point Counter.  This field is a binary modulo
           256 counter.  The value of this field MUST be incremented by
           1 each time an AU is transmitted where the RA bit in the AU
           Control field is set to 1.  The initial value of this field
           is undefined and MAY be chosen randomly.

   AUP Len: 16 bits
           Access Unit Payload Length.  Specifies the size, in bytes, of
           the payload of the Access Unit.  The field does not include
           the size of the AU header itself.  The field MUST be included
           in each AU header in an RTP packet, except for the last AU
           header in the packet.  If this field is not included, the


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           payload of the Access Unit SHALL be assumed to extend to the
           end of the RTP payload.

   PTS Delta: 32 bits
           Presentation time delta.  Specifies the presentation time of
           the frame as a 2's complement offset (delta) from the
           timestamp field in the RTP header of this RTP packet.  The
           PTS Delta field MUST use the same clock rate as the timestamp
           field in the RTP header.
           This field SHOULD NOT be included in the first AU header in
           the RTP packet, because the RTP timestamp field specifies the
           presentation time of the frame in the first AU.  If this
           field is not included, the presentation time of the frame
           SHALL be assumed to be specified by the timestamp field in
           the RTP header.

   DTS Delta: 32 bits
           Decode time delta.  Specifies the decode time of the frame as
           a 2's complement offset (delta) between the presentation time
           and the decode time.  Note that if the presentation time is
           larger than the decode time, this results in a value for the
           DTS Delta field that is greater than zero.  The DTS Delta
           field MUST use the same clock rate as the timestamp field in
           the RTP header.  If this field is not included, the decode
           time of the frame SHALL be assumed to be identical to the
           presentation time of the frame.

5.3 AU Control field syntax

   The structure of the 8-bit AU Control field is shown in Figure 5.

     0    1    2    3    4    5    6    7
   +----+----+----+----+----+----+----+----+
   |  FRAG   | RA | SL | LP | PT | DT | R  |
   +----+----+----+----+----+----+----+----+

   Figure 5.  Syntax of AU Control field.

   FRAG: 2 bits
           Fragmentation Information.  This field indicates if the AU
           payload contains a complete frame or a fragment of a frame.
           It MUST be set as follows:
           0: The AU payload contains a fragment of a frame other than
           the first or last fragment.
           1: The AU payload contains the first fragment of a frame.
           2: The AU payload contains the last fragment of a frame.
           3: The AU payload contains a complete frame (not fragmented.)




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   RA: 1 bit
           Random Access Point indicator.  This bit MUST be set to 1 if
           the AU contains a frame that is a random access point.  In
           the case of Simple and Main profiles, any I-picture is a
           random access point.
           In the case of Advanced profile, the first frame after an
           entry-point header is a random access point.
           If entry-point headers are not transmitted at every random
           access point, this MUST be indicated using the media type
           parameter "mode=3".

   SL: 1 bit
           Sequence Layer Counter.  This bit MUST be toggled, i.e.,
           changed from 0 to 1 or from 1 to 0, if the AU contains a
           sequence layer header and if it is different from the most
           recently transmitted sequence layer header.  Otherwise, the
           value of this bit must be identical to the value of the SL
           bit in the previous AU.
           The initial value of this bit is undefined and MAY be chosen
           randomly.
           The bit MUST be 0 for Simple and Main profile bit streams or
           if the sequence layer header never changes.

   LP: 1 bit
           Length Present.  This bit MUST be set to 1 if the AU header
           includes the AUP Len field.

   PT: 1 bit
           PTS Delta Present.  This bit MUST be set to 1 if the AU
           header includes the PTS Delta field.

   DT: 1 bit
           DTS Delta Present.  This bit MUST be set to 1 if the AU
           header includes the DTS Delta field.

   R: 1 bit
           Reserved.  This bit MUST be set to 0 and MUST be ignored by
           receivers.

6. RTP Payload format parameters

6.1 Media type Registration

   This registration uses the template defined in RFC 4288 [7] and
   follows RFC 3555 [8].

   Type name:  video

   Subtype name:  vc1


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   Required parameters:

         profile:
           The value is an integer identifying the VC-1 profile.  The
           following values are defined:
           0: Simple profile.
           1: Main profile.
           3: Advanced profile.

           If the profile parameter is used to indicate properties of a
           coded bit stream, it indicates the VC-1 profile that a
           decoder has to support when it decodes the bit stream.

           If the profile parameter is used for capability exchange or
           in a session setup procedure, it indicates the VC-1 profile
           that the codec supports.

         level:
           The value is an integer specifying the level of the VC-1
           profile.
           For Advanced profile, valid values are 0 to 4, which
           correspond to levels L0 to L4, respectively.  For Simple and
           Main profiles, the following values are defined:
           1: Low Level
           2: Medium Level
           3: High Level (only valid for Main profile)

           If the level parameter is used to indicate properties of a
           coded bit stream, it indicates the highest level of the VC-1
           profile that a decoder has to support when it decodes the bit
           stream.  Note that support for a level implies support for
           all numerically lower levels of the given profile.

           If the level parameter is used for capability exchange or in
           a session setup procedure, it indicates the highest level of
           the VC-1 profile that the codec supports.  See section 6.3 of
           RFC XXXX for specific rules for how this parameter is used
           with the SDP Offer/Answer model.

   Optional parameters:

         config:
           The value is a base16 [6] (hexadecimal) representation of an
           octet string that expresses the decoder initialization
           parameters.  Decoder initialization parameters are mapped
           onto the base16 octet string in an MSB-first basis.  The
           first bit of the decoder initialization parameters MUST be
           located at the MSB of the first octet.  If the decoder


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           initialization parameters are not multiple of 8 bits, in the
           last octet up to 7 zero-valued padding bits MUST be added to
           achieve octet alignment.

           For Simple and Main profiles, the decoder initialization
           parameters are STRUCT_C, as defined in Annex J of SMPTE 421M
           [1].

           For Advanced profile, the decoder initialization parameters
           are a sequence layer header directly followed by an entry-
           point header.  The two headers MUST be in EBDU format,
           meaning that they must include their Start Codes and must use
           the encapsulation method defined in Annex E of SMPTE 421M
           [1].

         width:
           The value is an integer greater than zero, specifying the
           maximum horizontal size of the coded picture, in luma samples
           (pixels in the luma picture.)

           For Simple and Main profiles, the value SHALL be identical to
           the actual horizontal size of the coded picture.
           For Advanced profile, the value SHALL be greater than, or
           equal to, the largest horizontal size of the coded picture.

           If this parameter is not specified, it defaults to the
           maximum horizontal size allowed by the specified profile and
           level.

         height:
           The value is an integer greater than zero, specifying the
           maximum vertical size of the coded picture in luma samples
           (pixels in the luma picture.)

           For Simple and Main profiles, the value SHALL be identical to
           the actual vertical size of the coded picture.
           For Advanced profile, the value SHALL be greater than, or
           equal to, the largest vertical size of the coded picture.

           If this parameter is not specified, it defaults to the
           maximum vertical size allowed by the specified profile and
           level.

         bitrate:
           The value is an integer greater than zero, specifying the
           peak transmission rate of the coded bit stream in bits per
           second.  The number does not include the overhead caused by
           RTP encapsulation, i.e., it does not include the AU headers,
           or any of the RTP, UDP or IP headers.


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           If this parameter is not specified, it defaults to the
           maximum bit rate allowed by the specified profile and level.
           (See the values for "RMax" in Annex D of SMPTE 421M [1].)

         buffer:
           The value is an integer specifying the leaky bucket size, B,
           in milliseconds, required to contain a stream transmitted at
           the transmission rate specified by the bitrate parameter.
           This parameter is defined in the hypothetical reference
           decoder model for VC-1, in Annex C of SMPTE 421M [1].

           Note that this parameter relates to the codec bit stream
           only, and does not account for any buffering time that may be
           required to compensate for jitter in the network.

           If this parameter is not specified, it defaults to the
           maximum buffer size allowed by the specified profile and
           level.  (See the values for "BMax" and "RMax" in Annex D of
           SMPTE 421M [1].)

         framerate:
           The value is an integer greater than zero, specifying the
           maximum number of frames per second in the coded bit stream,
           multiplied by 1000 and rounded to the nearest integer value.
           For example, 30000/1001 (approximately 29.97) frames per
           second is represented as 29970.

           This parameter can be used to control resource allocation at
           the receiver.  For example, a receiver may choose to perform
           additional post-processing on decoded frames only if the
           frame rate is expected to be low.  The parameter MUST NOT be
           used for pacing of the rendering process, since the actual
           frame rate may differ from the specified value.

           If the parameter is not specified, it defaults to the maximum
           frame rate allowed by the specified profile and level.

         bpic:
           This parameter signals that B- and BI-pictures may be present
           when Advanced profile is used.  If this parameter is present,
           and B- or BI-pictures may be present in the coded bit stream,
           this parameter MUST be equal to 1.
           A value of 0 indicates that B- and BI-pictures SHALL NOT be
           present in the coded bit stream, even if the sequence layer
           header changes.  It is RECOMMENDED to include this parameter,
           with a value of 0, if neither B- nor BI-pictures are included
           in the coded bit stream.



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           This parameter MUST NOT be used with Simple and Main
           profiles. (For Main profile, the presence of B- and BI-
           pictures is indicated by the MAXBFRAMES field in STRUCT_C
           decoder initialization parameter.)

           For Advanced profile, if this parameter is not specified, a
           value of 1 SHALL be assumed.

         mode:
           The value is an integer specifying the use of the sequence
           layer header and the entry-point header.  This parameter is
           only defined for Advanced profile.  The following values are
           defined:
           0: Both the sequence layer header and the entry-point header
           may change, and changed headers will be included in the RTP
           packets.
           1: The sequence layer header specified in the config
           parameter never changes.  The rules in section 4.8 of RFC
           XXXX MUST be followed.
           3: The sequence layer header and the entry-point header
           specified in the config parameter never change.  The rules in
           section 4.9 of RFC XXXX MUST be followed.

           If the mode parameter is not specified, a value of 0 SHALL be
           assumed.  The mode parameter SHOULD be specified if modes 1
           or 3 apply to the VC-1 bit stream.

         max-width, max-height, max-bitrate, max-buffer, max-framerate:
           These parameters are defined for use in a capability exchange
           procedure.  The parameters do not signal properties of the
           coded bit stream, but rather upper limits or preferred values
           for the "width", "height", "bitrate", "buffer" and
           "framerate" parameters.  Section 6.3 of RFC XXXX provides
           specific rules for these parameters are used with the SDP
           Offer/Answer model.

           Receivers that signal support for a given profile and level
           MUST support the maximum values for these parameters for that
           profile and level.  For example, a receiver that indicates
           support for Main profile, Low level, must support a width of
           352 pixels and height of 288 pixels, even if this requires
           scaling the image to fit the resolution of a smaller display
           device.

           A receiver MAY use any of the max-width, max-height, max-
           bitrate, max-buffer and max-framerate parameters to indicate
           preferred capabilities.  For example, a receiver may choose
           to specify values for max-width and max-height that match the



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           resolution of its display device, since a bit stream encoded
           using those parameters would not need to be rescaled.

           If any of the max-width, max-height, max-bitrate, max-buffer
           and max-framerate parameters signal a capability that is less
           than the required capabilities of the signaled profile and
           level, then the parameter SHALL be interpreted as a preferred
           value for that capability.

           Any of the parameters MAY also be used to signal capabilities
           that exceed the required capabilities of the signaled profile
           and level.  In that case, the parameter SHALL be interpreted
           as the maximum value that can be supported for that
           capability.

           When more than one parameter from the set (max-width, max-
           height, max-bitrate, max-buffer and max-framerate) is
           present, all signaled capabilities MUST be supported
           simultaneously.

           A sender or receiver MUST NOT use these parameters to signal
           capabilities that meet the requirements of a higher level of
           the VC-1 profile than the one specified in the "level"
           parameter, if the sender or receiver can support all the
           properties of the higher level, except if specifying a higher
           level is not allowed due to other restrictions.  (As an
           example of such a restriction, in the SDP Offer/Answer model,
           the value of the level parameter that can be used in an
           Answer is limited by what was specified in the Offer.)

         max-width:
           The value is an integer greater than zero, specifying a
           horizontal size for the coded picture, in luma samples
           (pixels in the luma picture.)  If the value is less than the
           maximum horizontal size allowed by the profile and level,
           then the value specifies the preferred horizontal size.
           Otherwise, it specifies the maximum horizontal size that is
           supported.

           If this parameter is not specified, it defaults to the
           maximum horizontal size allowed by the specified profile and
           level.

         max-height:
           The value is an integer greater than zero, specifying a
           vertical size for the coded picture, in luma samples (pixels
           in the luma picture.)  If the value is less than the maximum
           vertical size allowed by the profile and level, then the



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           value specifies the preferred vertical size.  Otherwise, it
           specifies the maximum vertical size that is supported.

           If this parameter is not specified, it defaults to the
           maximum vertical size allowed by the specified profile and
           level.

         max-bitrate:
           The value is an integer greater than zero, specifying a peak
           transmission rate for the coded bit stream in bits per
           second.  The number does not include the overhead caused by
           RTP encapsulation, i.e., it does not include the AU headers,
           or any of the RTP, UDP or IP headers.

           If the value is less than the maximum bit rate allowed by the
           profile and level, then the value specifies the preferred bit
           rate.  Otherwise, it specifies the maximum bit rate that is
           supported.

           If this parameter is not specified, it defaults to the
           maximum bit rate allowed by the specified profile and level.
           (See the values for "RMax" in Annex D of SMPTE 421M [1].)

         max-buffer:
           The value is an integer specifying a leaky bucket size, B, in
           milliseconds, required to contain a stream transmitted at the
           transmission rate specified by the max-bitrate parameter.
           This parameter is defined in the hypothetical reference
           decoder model for VC-1, in Annex C of SMPTE 421M [1].

           Note that this parameter relates to the codec bit stream
           only, and does not account for any buffering time that may be
           required to compensate for jitter in the network.

           If the value is less than the maximum leaky bucket size
           allowed by the max-bitrate parameter and the profile and
           level, then the value specifies the preferred leaky bucket
           size.  Otherwise, it specifies the maximum leaky bucket size
           that is supported for the bit rate specified by the max-
           bitrate parameter.

           If this parameter is not specified, it defaults to the
           maximum buffer size allowed by the specified profile and
           level.  (See the values for "BMax" and "RMax" in Annex D of
           SMPTE 421M [1].)

         max-framerate:
           The value is an integer greater than zero, specifying a
           number of frames per second for the coded bit stream.  The


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           value is the frame rate multiplied by 1000 and rounded to the
           nearest integer value.  For example, 30000/1001
           (approximately 29.97) frames per second is represented as
           29970.

           If the value is less than the maximum frame rate allowed by
           the profile and level, then the value specifies the preferred
           frame rate.  Otherwise, it specifies the maximum frame rate
           that is supported.

           If the parameter is not specified, it defaults to the maximum
           frame rate allowed by the specified profile and level.

   Encoding considerations:
           This media type is framed and contains binary data.

   Security considerations:
           See Section 7 of RFC XXXX.

   Interoperability considerations:
           None.

   Published specification:
           RFC XXXX.

   Applications which use this media type:
           Multimedia streaming and conferencing tools.

   Additional Information:
           None.

   Person & email address to contact for further information:
           Anders Klemets <anderskl@microsoft.com>
           IETF AVT working group.

   Intended Usage:
           COMMON

   Restrictions on usage:
           This media type depends on RTP framing, and hence is only
           defined for transfer via RTP [3].

   Authors:
           Anders Klemets

   Change controller:
           IETF Audio/Video Transport Working Group delegated from the
           IESG.



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6.2 Mapping of media type parameters to SDP

   The information carried in the media type specification has a
   specific mapping to fields in the Session Description Protocol (SDP)
   [4].  If SDP is used to specify sessions using this payload format,
   the mapping is done as follows:

   o The media name in the "m=" line of SDP MUST be video (the type
     name).

   o The encoding name in the "a=rtpmap" line of SDP MUST be vc1 (the
     subtype name).

   o The clock rate in the "a=rtpmap" line MUST be 90000.

   o The REQUIRED parameters "profile" and "level" MUST be included in
     the "a=fmtp" line of SDP.
     These parameters are expressed in the form of a semicolon
     separated list of parameter=value pairs.

   o The OPTIONAL parameters "config", "width", "height", "bitrate",
     "buffer", "framerate", "bpic", "mode", "max-width", "max-height",
     "max-bitrate", "max-buffer" and "max-framerate", when present,
     MUST be included in the "a=fmtp" line of SDP.
     These parameters are expressed in the form of a semicolon
     separated list of parameter=value pairs:

         a=fmtp:<dynamic payload type> <parameter
         name>=<value>[,<value>][; <parameter name>=<value>]

   o Any unknown parameters to the device that uses the SDP MUST be
     ignored.  For example, parameters defined in later specifications
     MAY be copied into the SDP and MUST be ignored by receivers that
     do not understand them.

6.3 Usage with the SDP Offer/Answer Model

   When VC-1 is offered over RTP using SDP in an Offer/Answer model [5]
   for negotiation for unicast usage, the following rules and
   limitations apply:

   o The "profile" parameter MUST be used symmetrically, i.e., the
     answerer MUST either maintain the parameter or remove the media
     format (payload type) completely if the offered VC-1 profile is
     not supported.

   o The "level" parameter specifies the highest level of the VC-1
     profile supported by the codec.



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     The answerer MUST NOT specify a numerically higher level in the
     answer than what was specified in the offer. The answerer MAY
     specify a level that is lower than what was specified in the
     offer, i.e., the level parameter can be "downgraded".

     If the offer specifies the sendrecv or sendonly direction
     attribute, and the answer downgrades the level parameter, this may
     require a new offer to specify an updated "config" parameter.  If
     the "config" parameter cannot be used with the level specified in
     the answer, then the offerer MUST initiate another Offer/Answer
     round, or not use media format (payload type).

   o The parameters "config", "bpic", "width", "height", "framerate",
     "bitrate", "buffer" and "mode", describe the properties of the VC-
     1 bit stream that the offerer or answerer is sending for this
     media format configuration.

     In the case of unicast usage and when the direction attribute in
     the offer or answer is recvonly, the interpretation of these
     parameters is undefined and they MUST NOT be used.

   o The parameters "config", "width", "height", "bitrate" and "buffer"
     MUST be specified when the direction attribute is sendrecv or
     sendonly.

   o The parameters "max-width", "max-height", "max-framerate", "max-
     bitrate" and "max-buffer" MAY be specified in an offer or an
     answer, and their interpretation is as follows:

     When the direction attribute is sendonly, the parameters describe
     the limits of the VC-1 bit stream that the sender is capable of
     producing for the given profile and level, and for any lower level
     of the same profile.

     When the direction attribute is recvonly or sendrecv, the
     parameters describe properties of the receiver implementation.  If
     the value of a property is less than what is allowed by the level
     of the VC-1 profile, then it SHALL be interpreted as a preferred
     value and the sender's VC-1 bit stream SHOULD NOT exceed it.  If
     the value of a property is greater than what is allowed by the
     level of the VC-1 profile, then it SHALL be interpreted as the
     upper limit of the value that the receiver accepts for the given
     profile and level, and for any lower level of the same profile.

     For example, if a recvonly or sendrecv offer specifies
     "profile=0;level=1;max-bitrate=48000", then 48 kbps is merely a
     suggested bit rate, because all receiver implementations of Simple
     profile, Low level, are required to support bit rates of up to 96
     kbps.  Assuming that the offer is accepted, the answerer should


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     specify "bitrate=48000" in the answer, but any value up to 96000
     is allowed.  But if the offer specifies "max-bitrate=200000", this
     means that the receiver implementation supports a maximum of 200
     kbps for the given profile and level (or lower level.)  In this
     case, the answerer is allowed to answer with a bitrate parameter
     of up to 200000.

   o If an offerer wishes to have non-symmetrical capabilities between
     sending and receiving, e.g., use different levels in each
     direction, then the offerer has to offer different RTP sessions.
     This can be done by specifiying different media lines declared as
     "recvonly" and "sendonly", respectively.

   For streams being delivered over multicast, the following rules apply
   in addition:

   o The "level" parameter specifies the highest level of the VC-1
     profile used by the participants in the multicast session.  The
     value of this parameter MUST NOT be changed by the answerer.
     Thus, a payload type can either be accepted unaltered or removed.

   o The parameters "config", "bpic", "width", "height", "framerate",
     "bitrate", "buffer" and "mode", specify properties of the VC-1 bit
     stream that will be sent, and/or received, on the multicast
     session.  The parameters MAY be specified even if the direction
     attribute is recvonly.

     The values of these parameters MUST NOT be changed by the
     answerer.  Thus, a payload type can either be accepted unaltered
     or removed.

   o The values of the parameters "max-width", "max-height", "max-
     framerate", "max-bitrate" and "max-buffer" MUST be supported by
     the answerer for all streams declared as sendrecv or recvonly.
     Otherwise, one of the following actions MUST be performed: the
     media format is removed, or the session rejected.

6.4 Usage in Declarative Session Descriptions

   When VC-1 is offered over RTP using SDP in a declarative style, as in
   RTSP [12] or SAP [13], the following rules and limitations apply.

   o The parameters "profile" and "level" indicate only the properties
     of the coded bit stream.  They do not imply a limit on capabilties
     supported by the sender.

   o The parameters "config", "width", "height", "bitrate" and "buffer"
     MUST be specified.



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   o The parameters "max-width", "max-height", "max-framerate", "max-
     bitrate" and "max-buffer" MUST NOT be used.

   An example of media representation in SDP is as follows (Simple
   profile, Medium level):

   m=video 49170 RTP/AVP 98
   a=rtpmap:98 vc1/90000
   a=fmtp:98 profile=0;level=2;width=352;height=288;framerate=15000;
   bitrate=384000;buffer=2000;config=4e291800

7. Security Considerations

   RTP packets using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [4], and in any appropriate RTP profile.  This implies
   that confidentiality of the media streams is achieved by encryption;
   for example, through the application of SRTP [11].

   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 RTP packets
   into the stream that are complex to decode and that cause the
   receiver to be overloaded.  VC-1 is particularly vulnerable to such
   attacks, because it is possible for an attacker to generate RTP
   packets containing frames that affect the decoding process of many
   future frames.  Therefore, the usage of data origin authentication
   and data integrity protection of at least the RTP packet is
   RECOMMENDED; for example, with SRTP [11].

   Note that the appropriate mechanism to ensure confidentiality and
   integrity of RTP packets and their payloads is very dependent on the
   application and on the transport and signaling protocols employed.
   Thus, although SRTP is given as an example above, other possible
   choices exist.

   VC-1 bit streams can carry user-data, such as closed captioning
   information and content meta-data.  The VC-1 specification does not
   define how to interpret user-data.  Identifiers for user-data are
   required to be registered with SMPTE.  It is conceivable for types of
   user-data to be defined to include programmatic content, such as
   scripts or commands that would be executed by the receiver.
   Depending on the type of user-data, it might be possible for a sender
   to generate user-data in a non-compliant manner to crash the receiver
   or make it temporarily unavailable.  Senders that transport VC-1 bit
   streams SHOULD ensure that the user-data is compliant with the
   specification registered with SMPTE (see Annex F of [1].)  Receivers
   SHOULD prevent malfunction in case of non-compliant user-data.



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   It is important to note that VC-1 streams can have very high
   bandwidth requirements (up to 135 Mbps for high-definition video.)
   This is sufficient to cause potential for denial-of-service if
   transmitted onto many Internet paths.  Therefore, users of this
   payload format MUST comply with the congestion control requirements
   described in section 8.

8. Congestion Control

   Congestion control for RTP SHALL be used in accordance with RFC 3550
   [3], and with any applicable RTP profile; e.g., RFC 3551 [15].

   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 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.

   The bit rate adaptation necessary for obeying the congestion control
   principle is easily achievable when real-time encoding is used.  When
   pre-encoded content is being transmitted, bandwidth adaptation
   requires one or more of the following:

   - The availability of more than one coded representation of the same
     content at different bit rates.  The switching between the
     different representations can normally be performed in the same
     RTP session, by switching streams at random access point
     boundaries.

   - The existence of non-reference frames (e.g., B-frames) in the bit
     stream.  Non-reference frames can be discarded by the transmitter
     prior to encapsulation in RTP.  If the frames contain the TFCNTR
     (Temporal Reference Frame Counter) syntax element, it will require
     updating the TFCNTR fields of other frames to ensure that the
     field remains continuous.  Because TFCNTR counts the frames in the
     display order, which is different from the order in which they are
     transmitted (the coded order), it will require the transmitter to
     "look ahead", or buffer, of some number of frames.

   Only when non-downgradable parameters (such as the VC-1 "profile"
   parameter) are required to be changed does it become necessary to
   terminate and re-start the media stream.  This may be accomplished by
   using a different RTP payload type.


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   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. IANA Considerations

   IANA is requested to register the media type "video/vc1" and the
   associated RTP payload format, as specified in section 6.1 of this
   document, in the Media Types registry and in the RTP Payload Format
   MIME types registry.

10. References

10.1 Normative references

   [1] Society of Motion Picture and Television Engineers, "VC-1
       Compressed Video Bitstream Format and Decoding Process", SMPTE
       421M.
   [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.
   [3] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
       "RTP: A Transport Protocol for Real-Time Applications", STD 64,
       RFC 3550, July 2003.
   [4] Handley, M. and V. Jacobson, "SDP: Session Description Protocol",
       RFC 2327, April 1998.
   [5] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
       Session Description Protocol (SDP)", RFC 3264, June 2002.
   [6] Josefsson, S., Ed., "The Base16, Base32, and Base64 Data
       Encodings", RFC 3548, July 2003.
   [7] Freed, N. and Klensin, J., "Media Type Specifications and
       Registration Procedures", BCP 13, RFC 4288, December 2005.
   [8] Casner, S. and P. Hoschka, "MIME Type Registration of RTP Payload
       Formats", RFC 3555, July 2003.

10.2 Informative references

   [9] Srinivasan, S., Hsu, P., Holcomb, T., Mukerjee, K., Regunathan,
       S.L., Lin, B., Liang, J., Lee, M., and J. Ribas-Corbera, "Windows
       Media Video 9: overview and applications", Signal Processing:
       Image Communication, Volume 19, Issue 9, October 2004.
   [10]Ribas-Corbera, J., Chou, P.A., and S.L. Regunathan, "A
       generalized hypothetical reference decoder for H.264/AVC", IEEE
       Transactions on Circuits and Systems for Video Technology, August
       2003.



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   [11]Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
       Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
       3711, March 2004.
   [12]Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
       Protocol (RTSP)", RFC 2326, April 1998.
   [13]Handley, M., Perkins, C., and E. Whelan, "Session Announcement
       Protocol", RFC 2974, October 2000.
   [14]Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,
       "SIP: Session Initiation Protocol", RFC 2543, March 1999.
   [15]Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
       Conferences with Minimal Control", STD 65, RFC 3551, July 2003.

Author's Addresses

   Anders Klemets
   Microsoft Corp.
   1 Microsoft Way
   Redmond, WA 98052
   USA
   Email: anderskl@microsoft.com

Acknowledgements

   Thanks to Regis Crinon, Miska Hannuksela, Colin Perkins, Shankar
   Regunathan, Gary Sullivan, Stephan Wenger and Magnus Westerlund for
   providing detailed feedback on this document.

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   ietf-ipr@ietf.org.

Full Copyright Statement

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   This document is subject to the rights, licenses and restrictions
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