tech-invite   World Map     

IETF     RFCs     Groups     SIP     ABNFs    |    3GPP     Specs     Glossaries     Architecture     IMS     UICC    |    search     info

RFC 6184

 
 
 

RTP Payload Format for H.264 Video

Part 3 of 4, p. 39 to 67
Prev RFC Part       Next RFC Part

 


prevText      Top      Up      ToC       Page 39 
8.  Payload Format Parameters

   This section specifies the parameters that MAY be used to select
   optional features of the payload format and certain features of the
   bitstream.  The parameters are specified here as part of the media
   subtype registration for the ITU-T H.264 | ISO/IEC 14496-10 codec.  A
   mapping of the parameters into the Session Description Protocol (SDP)
   [6] is also provided for applications that use SDP.  Equivalent
   parameters could be defined elsewhere for use with control protocols
   that do not use SDP.

   Some parameters provide a receiver with the properties of the stream
   that will be sent.  The names of all these parameters start with
   "sprop" for stream properties.  Some of these "sprop" parameters are
   limited by other payload or codec configuration parameters.  For
   example, the sprop-parameter-sets parameter is constrained by the
   profile-level-id parameter.

8.1.  Media Type Registration

   The media subtype for the ITU-T H.264 | ISO/IEC 14496-10 codec has
   been allocated from the IETF tree.

   Media Type name:     video

   Media subtype name:  H264

   Required parameters: none

   OPTIONAL parameters:

      profile-level-id:
         A base16 [7] (hexadecimal) representation of the following
         three bytes in the sequence parameter set NAL unit is specified
         in [1]: 1) profile_idc, 2) a byte herein referred to as
         profile-iop, composed of the values of constraint_set0_flag,
         constraint_set1_flag, constraint_set2_flag,
         constraint_set3_flag, constraint_set4_flag,
         constraint_set5_flag, and reserved_zero_2bits in bit-
         significance order, starting from the most-significant bit, and
         3) level_idc.  Note that reserved_zero_2bits is required to be
         equal to 0 in [1], but other values for it may be specified in
         the future by ITU-T or ISO/IEC.

Top      Up      ToC       Page 40 
         The profile-level-id parameter indicates the default sub-
         profile (i.e., the subset of coding tools that may have been
         used to generate the stream or that the receiver supports) and
         the default level of the stream or the receiver supports.

         The default sub-profile is indicated collectively by the
         profile_idc byte and some fields in the profile-iop byte.
         Depending on the values of the fields in the profile-iop byte,
         the default sub-profile may be the set of coding tools
         supported by one profile, or a common subset of coding tools of
         multiple profiles, as specified in Section 7.4.2.1.1 of [1].
         The default level is indicated by the level_idc byte, and, when
         profile_idc is equal to 66, 77, or 88 (the Baseline, Main, or
         Extended profile) and level_idc is equal to 11, additionally by
         bit 4 (constraint_set3_flag) of the profile-iop byte.  When
         profile_idc is equal to 66, 77, or 88 (the Baseline, Main, or
         Extended profile), level_idc is equal to 11, and bit 4
         (constraint_set3_flag) of the profile-iop byte is equal to 1,
         the default level is Level 1b.

         Table 5 lists all profiles defined in Annex A of [1] and, for
         each of the profiles, the possible combinations of profile_idc
         and profile-iop that represent the same sub-profile.

Top      Up      ToC       Page 41 
            Table 5.  Combinations of profile_idc and profile-iop
            representing the same sub-profile corresponding to the full
            set of coding tools supported by one profile.  In the
            following, x may be either 0 or 1, while the profile names
            are indicated as follows.  CB: Constrained Baseline profile,
            B: Baseline profile, M: Main profile, E: Extended profile,
            H: High profile, H10: High 10 profile, H42: High 4:2:2
            profile, H44: High 4:4:4 Predictive profile, H10I: High 10
            Intra profile, H42I: High 4:2:2 Intra profile, H44I: High
            4:4:4 Intra profile, and C44I: CAVLC 4:4:4 Intra profile.

              Profile     profile_idc        profile-iop
                          (hexadecimal)      (binary)

              CB          42 (B)             x1xx0000
                 same as: 4D (M)             1xxx0000
                 same as: 58 (E)             11xx0000
              B           42 (B)             x0xx0000
                 same as: 58 (E)             10xx0000
              M           4D (M)             0x0x0000
              E           58                 00xx0000
              H           64                 00000000
              H10         6E                 00000000
              H42         7A                 00000000
              H44         F4                 00000000
              H10I        6E                 00010000
              H42I        7A                 00010000
              H44I        F4                 00010000
              C44I        2C                 00010000

         For example, in the table above, profile_idc equal to 58
         (Extended) with profile-iop equal to 11xx0000 indicates the
         same sub-profile corresponding to profile_idc equal to 42
         (Baseline) with profile-iop equal to x1xx0000.  Note that other
         combinations of profile_idc and profile-iop (not listed in
         Table 5) may represent a sub-profile equivalent to the common
         subset of coding tools for more than one profile.  Note also
         that a decoder conforming to a certain profile may be able to
         decode bitstreams conforming to other profiles.

         If the profile-level-id parameter is used to indicate
         properties of a NAL unit stream, it indicates that, to decode
         the stream, the minimum subset of coding tools a decoder has to
         support is the default sub-profile, and the lowest level the
         decoder has to support is the default level.

Top      Up      ToC       Page 42 
         If the profile-level-id parameter is used for capability
         exchange or session setup, it indicates the subset of coding
         tools, which is equal to the default sub-profile, that the
         codec supports for both receiving and sending.  If max-recv-
         level is not present, the default level from profile-level-id
         indicates the highest level the codec wishes to support.  If
         max-recv-level is present, it indicates the highest level the
         codec supports for receiving.  For either receiving or sending,
         all levels that are lower than the highest level supported MUST
         also be supported.

            Informative note: Capability exchange and session setup
            procedures should provide means to list the capabilities for
            each supported sub-profile separately.  For example, the
            one-of-N codec selection procedure of the SDP Offer/Answer
            model can be used (Section 10.2 of [8]).  The one-of-N codec
            selection procedure may also be used to provide different
            combinations of profile_idc and profile-iop that represent
            the same sub-profile.  When there are many different
            combinations of profile_idc and profile-iop that represent
            the same sub-profile, using the one-of-N codec selection
            procedure may result in a fairly large SDP message.
            Therefore, a receiver should understand the different
            equivalent combinations of profile_idc and profile-iop that
            represent the same sub-profile and be ready to accept an
            offer using any of the equivalent combinations.

         If no profile-level-id is present, the Baseline profile,
         without additional constraints at Level 1, MUST be inferred.

      max-recv-level:
         This parameter MAY be used to indicate the highest level a
         receiver supports when the highest level is higher than the
         default level (the level indicated by profile-level-id).  The
         value of max-recv-level is a base16 (hexadecimal)
         representation of the two bytes after the syntax element
         profile_idc in the sequence parameter set NAL unit specified in
         [1]: profile-iop (as defined above) and level_idc.  If the
         level_idc byte of max-recv-level is equal to 11 and bit 4 of
         the profile-iop byte of max-recv-level is equal to 1 or if the
         level_idc byte of max-recv-level is equal to 9 and bit 4 of the
         profile-iop byte of max-recv-level is equal to 0, the highest
         level the receiver supports is Level 1b.  Otherwise, the
         highest level the receiver supports is equal to the level_idc
         byte of max-recv-level divided by 10.

         max-recv-level MUST NOT be present if the highest level the
         receiver supports is not higher than the default level.

Top      Up      ToC       Page 43 
      max-mbps, max-smbps, max-fs, max-cpb, max-dpb, and max-br:
         These parameters MAY be used to signal the capabilities of a
         receiver implementation.  These parameters MUST NOT be used for
         any other purpose.  The highest level conveyed in the value of
         the profile-level-id parameter or the max-recv-level parameter
         MUST be such that the receiver is fully capable of supporting.
         max-mbps, max-smbps, max-fs, max-cpb, max-dpb, and max-br MAY
         be used to indicate capabilities of the receiver that extend
         the required capabilities of the signaled highest level, as
         specified below.

         When more than one parameter from the set (max-mbps, max-smbps,
         max-fs, max-cpb, max-dpb, max-br) is present, the receiver MUST
         support all signaled capabilities simultaneously.  For example,
         if both max-mbps and max-br are present, the signaled highest
         level with the extension of both the frame rate and bitrate is
         supported.  That is, the receiver is able to decode NAL unit
         streams in which the macroblock processing rate is up to max-
         mbps (inclusive), the bitrate is up to max-br (inclusive), the
         coded picture buffer size is derived as specified in the
         semantics of the max-br parameter below, and the other
         properties comply with the highest level specified in the value
         of the profile-level-id parameter or the max-recv-level
         parameter.

         If a receiver can support all the properties of Level A, the
         highest level specified in the value of the profile-level-id
         parameter or the max-recv-level parameter MUST be Level A
         (i.e., MUST NOT be lower than Level A).  In other words, a
         receiver MUST NOT signal values of max-mbps, max-fs, max-cpb,
         max-dpb, and max-br that taken together meet the requirements
         of a higher level compared to the highest level specified in
         the value of the profile-level-id parameter or the max-recv-
         level parameter.

            Informative note: When the OPTIONAL media type parameters
            are used to signal the properties of a NAL unit stream, max-
            mbps, max-smbps, max-fs, max-cpb, max-dpb, and max-br are
            not present, and the value of profile-level-id must always
            be such that the NAL unit stream complies fully with the
            specified profile and level.

      max-mbps: The value of max-mbps is an integer indicating the
         maximum macroblock processing rate in units of macroblocks per
         second.  The max-mbps parameter signals that the receiver is
         capable of decoding video at a higher rate than is required by
         the signaled highest level conveyed in the value of the
         profile-level-id parameter or the max-recv-level parameter.

Top      Up      ToC       Page 44 
         When max-mbps is signaled, the receiver MUST be able to decode
         NAL unit streams that conform to the signaled highest level,
         with the exception that the MaxMBPS value in Table A-1 of [1]
         for the signaled highest level is replaced with the value of
         max-mbps.  The value of max-mbps MUST be greater than or equal
         to the value of MaxMBPS given in Table A-1 of [1] for the
         highest level.  Senders MAY use this knowledge to send pictures
         of a given size at a higher picture rate than is indicated in
         the signaled highest level.

      max-smbps: The value of max-smbps is an integer indicating the
         maximum static macroblock processing rate in units of static
         macroblocks per second, under the hypothetical assumption that
         all macroblocks are static macroblocks.  When max-smbps is
         signaled, the MaxMBPS value in Table A-1 of [1] should be
         replaced with the result of the following computation:

         o  If the parameter max-mbps is signaled, set a variable
            MaxMacroblocksPerSecond to the value of max-mbps.
            Otherwise, set MaxMacroblocksPerSecond equal to the value of
            MaxMBPS in Table A-1 [1] for the signaled highest level
            conveyed in the value of the profile-level-id parameter or
            the max-recv-level parameter.

         o  Set a variable P_non-static to the proportion of non-static
            macroblocks in picture n.

         o  Set a variable P_static to the proportion of static
            macroblocks in picture n.

         o  The value of MaxMBPS in Table A-1 of [1] should be
            considered by the encoder to be equal to:

            MaxMacroblocksPerSecond * max-smbps / (P_non-static *
            max-smbps + P_static * MaxMacroblocksPerSecond)

         The encoder should recompute this value for each picture.  The
         value of max-smbps MUST be greater than or equal to the value
         of MaxMBPS given explicitly as the value of the max-mbps
         parameter or implicitly in Table A-1 of [1] for the signaled
         highest level.  Senders MAY use this knowledge to send pictures
         of a given size at a higher picture rate than is indicated in
         the signaled highest level.

      max-fs: The value of max-fs is an integer indicating the maximum
         frame size in units of macroblocks.  The max-fs parameter
         signals that the receiver is capable of decoding larger picture
         sizes than are required by the signaled highest level conveyed

Top      Up      ToC       Page 45 
         in the value of the profile-level-id parameter or the max-recv-
         level parameter.  When max-fs is signaled, the receiver MUST be
         able to decode NAL unit streams that conform to the signaled
         highest level, with the exception that the MaxFS value in Table
         A-1 of [1] for the signaled highest level is replaced with the
         value of max-fs.  The value of max-fs MUST be greater than or
         equal to the value of MaxFS given in Table A-1 of [1] for the
         highest level.  Senders MAY use this knowledge to send larger
         pictures at a proportionally lower frame rate than is indicated
         in the signaled highest level.

      max-cpb: The value of max-cpb is an integer indicating the maximum
         coded picture buffer size in units of 1000 bits for the VCL HRD
         parameters and in units of 1200 bits for the NAL HRD
         parameters.  Note that this parameter does not use units of
         cpbBrVclFactor and cpbBrNALFactor (see Table A-1 of [1]).  The
         max-cpb parameter signals that the receiver has more memory
         than the minimum amount of coded picture buffer memory required
         by the signaled highest level conveyed in the value of the
         profile-level-id parameter or the max-recv-level parameter.
         When max-cpb is signaled, the receiver MUST be able to decode
         NAL unit streams that conform to the signaled highest level,
         with the exception that the MaxCPB value in Table A-1 of [1]
         for the signaled highest level is replaced with the value of
         max-cpb (after taking cpbBrVclFactor and cpbBrNALFactor into
         consideration when needed).  The value of max-cpb (after taking
         cpbBrVclFactor and cpbBrNALFactor into consideration when
         needed) MUST be greater than or equal to the value of MaxCPB
         given in Table A-1 of [1] for the highest level.  Senders MAY
         use this knowledge to construct coded video streams with
         greater variation of bitrate than can be achieved with the
         MaxCPB value in Table A-1 of [1].

            Informative note: The coded picture buffer is used in the
            hypothetical reference decoder (Annex C of H.264).  The use
            of the hypothetical reference decoder is recommended in
            H.264 encoders to verify that the produced bitstream
            conforms to the standard and to control the output bitrate.
            Thus, the coded picture buffer is conceptually independent
            of any other potential buffers in the receiver, including
            de-interleaving and de-jitter buffers.  The coded picture
            buffer need not be implemented in decoders as specified in
            Annex C of H.264, but rather standard-compliant decoders can
            have any buffering arrangements provided that they can
            decode standard-compliant bitstreams.  Thus, in practice,
            the input buffer for a video decoder can be integrated with
            de-interleaving and de-jitter buffers of the receiver.

Top      Up      ToC       Page 46 
      max-dpb: The value of max-dpb is an integer indicating the maximum
         decoded picture buffer size in units of 8/3 macroblocks.  The
         max-dpb parameter signals that the receiver has more memory
         than the minimum amount of decoded picture buffer memory
         required by the signaled highest level conveyed in the value of
         the profile-level-id parameter or the max-recv-level parameter.
         When max-dpb is signaled, the receiver MUST be able to decode
         NAL unit streams that conform to the signaled highest level,
         with the exception that the MaxDpbMbs value in Table A-1 of [1]
         for the signaled highest level is replaced with the value of
         max-dpb * 3 / 8.  Consequently, a receiver that signals max-dpb
         MUST be capable of storing the following number of decoded
         frames, complementary field pairs, and non-paired fields in its
         decoded picture buffer:

            Min(max-dpb * 3 / 8 / ( PicWidthInMbs * FrameHeightInMbs),
            16)

         Wherein PicWidthInMbs and FrameHeightInMbs are defined in [1].

         The value of max-dpb MUST be greater than or equal to the value
         of MaxDpbMbs * 3 / 8, wherein the value of MaxDpbMbs is given
         in Table A-1 of [1] for the highest level.  Senders MAY use
         this knowledge to construct coded video streams with improved
         compression.

            Informative note: This parameter was added primarily to
            complement a similar codepoint in the ITU-T Recommendation
            H.245, so as to facilitate signaling gateway designs.  The
            decoded picture buffer stores reconstructed samples.  There
            is no relationship between the size of the decoded picture
            buffer and the buffers used in RTP, especially
            de-interleaving and de-jitter buffers.

            Informative note: In RFC 3984, which this document
            obsoletes, the unit of this parameter was 1024 bytes.  The
            unit has been changed to 8/3 macroblocks in this document.
            The reason for this change was due to the changes from the
            2003 version of the H.264 specification referenced by RFC
            3984 to the 2010 version of the H.264 specification
            referenced by this document, particularly the changes to
            Table A-1 in the H.264 specification due to addition of
            color formats and bit depths not supported earlier.  The
            changed semantics of this parameter keeps backward
            compatibility to RFC 3984 and supports all profiles defined
            in the 2010 version of the H.264 specification.

Top      Up      ToC       Page 47 
      max-br: The value of max-br is an integer indicating the maximum
         video bitrate in units of 1000 bits per second for the VCL HRD
         parameters and in units of 1200 bits per second for the NAL HRD
         parameters.  Note that this parameter does not use units of
         cpbBrVclFactor and cpbBrNALFactor (see Table A-1 of [1]).

         The max-br parameter signals that the video decoder of the
         receiver is capable of decoding video at a higher bitrate than
         is required by the signaled highest level conveyed in the value
         of the profile-level-id parameter or the max-recv-level
         parameter.

         When max-br is signaled, the video codec of the receiver MUST
         be able to decode NAL unit streams that conform to the signaled
         highest level, with the following exceptions in the limits
         specified by the highest level:

         o  The value of max-br (after taking cpbBrVclFactor and
            cpbBrNALFactor into consideration when needed) replaces the
            MaxBR value in Table A-1 of [1] for the highest level.

         o  When the max-cpb parameter is not present, the result of the
            following formula replaces the value of MaxCPB in Table A-1
            of [1]: (MaxCPB of the signaled level) * max-br / (MaxBR of
            the signaled highest level).

         For example, if a receiver signals capability for Main profile
         Level 1.2 with max-br equal to 1550, this indicates a maximum
         video bitrate of 1550 kbits/sec for VCL HRD parameters, a
         maximum video bitrate of 1860 kbits/sec for NAL HRD parameters,
         and a CPB size of 4036458 bits (1550000 / 384000 * 1000 *
         1000).

         The value of max-br (after taking cpbBrVclFactor and
         cpbBrNALFactor into consideration when needed) MUST be greater
         than or equal to the value MaxBR given in Table A-1 of [1] for
         the signaled highest level.

         Senders MAY use this knowledge to send higher bitrate video as
         allowed in the level definition of Annex A of H.264 to achieve
         improved video quality.

            Informative note: This parameter was added primarily to
            complement a similar codepoint in the ITU-T Recommendation
            H.245, so as to facilitate signaling gateway designs.  The
            assumption that the network is capable of handling such
            bitrates at any given time cannot be made from the value of

Top      Up      ToC       Page 48 
            this parameter.  In particular, no conclusion can be drawn
            that the signaled bitrate is possible under congestion
            control constraints.

      redundant-pic-cap:
         This parameter signals the capabilities of a receiver
         implementation.  When equal to 0, the parameter indicates that
         the receiver makes no attempt to use redundant coded pictures
         to correct incorrectly decoded primary coded pictures.  When
         equal to 0, the receiver is not capable of using redundant
         slices; therefore, a sender SHOULD avoid sending redundant
         slices to save bandwidth.  When equal to 1, the receiver is
         capable of decoding any such redundant slice that covers a
         corrupted area in a primary decoded picture (at least partly),
         and therefore a sender MAY send redundant slices.  When the
         parameter is not present, a value of 0 MUST be used for
         redundant-pic-cap.  When present, the value of redundant-pic-
         cap MUST be either 0 or 1.

         When the profile-level-id parameter is present in the same
         signaling as the redundant-pic-cap parameter and the profile
         indicated in profile-level-id is such that it disallows the use
         of redundant coded pictures (e.g., Main profile), the value of
         redundant-pic-cap MUST be equal to 0.  When a receiver
         indicates redundant-pic-cap equal to 0, the received stream
         SHOULD NOT contain redundant coded pictures.

            Informative note: Even if redundant-pic-cap is equal to 0,
            the decoder is able to ignore redundant codec pictures
            provided that the decoder supports a profile (Baseline,
            Extended) in which redundant coded pictures are allowed.

            Informative note: Even if redundant-pic-cap is equal to 1,
            the receiver may also choose other error concealment
            strategies to replace or complement decoding of redundant
            slices.

      sprop-parameter-sets:
         This parameter MAY be used to convey any sequence and picture
         parameter set NAL units (herein referred to as the initial
         parameter set NAL units) that can be placed in the NAL unit
         stream to precede any other NAL units in decoding order.  The
         parameter MUST NOT be used to indicate codec capability in any
         capability exchange procedure.  The value of the parameter is a
         comma-separated (',') list of base64 [7] representations of
         parameter set NAL units as specified in Sections 7.3.2.1 and

Top      Up      ToC       Page 49 
         7.3.2.2 of [1].  Note that the number of bytes in a parameter
         set NAL unit is typically less than 10, but a picture parameter
         set NAL unit can contain several hundred bytes.

            Informative note: When several payload types are offered in
            the SDP Offer/Answer model, each with its own sprop-
            parameter-sets parameter, the receiver cannot assume that
            those parameter sets do not use conflicting storage
            locations (i.e., identical values of parameter set
            identifiers).  Therefore, a receiver should buffer all
            sprop-parameter-sets and make them available to the decoder
            instance that decodes a certain payload type.

         The sprop-parameter-sets parameter MUST only contain parameter
         sets that are conforming to the profile-level-id, i.e., the
         subset of coding tools indicated by any of the parameter sets
         MUST be equal to the default sub-profile, and the level
         indicated by any of the parameter sets MUST be equal to the
         default level.

      sprop-level-parameter-sets:
         This parameter MAY be used to convey any sequence and picture
         parameter set NAL units (herein referred to as the initial
         parameter set NAL units) that can be placed in the NAL unit
         stream to precede any other NAL units in decoding order and
         that are associated with one or more levels different than the
         default level.  The parameter MUST NOT be used to indicate
         codec capability in any capability exchange procedure.

         The sprop-level-parameter-sets parameter contains parameter
         sets for one or more levels that are different than the default
         level.  All parameter sets associated with one level are
         clustered and prefixed with a three-byte field that has the
         same syntax as profile-level-id.  This enables the receiver to
         install the parameter sets for one level and discard the rest.
         The three-byte field is named PLId, and all parameter sets
         associated with one level are named PSL, which has the same
         syntax as sprop-parameter-sets.  Parameter sets for each level
         are represented in the form of PLId:PSL, i.e., PLId followed by
         a colon (':') and the base64 [7] representation of the initial
         parameter set NAL units for the level.  Each pair of PLId:PSLs
         is also separated by a colon.  Note that a PSL can contain
         multiple parameter sets for that level, separated with commas
         (',').

         The subset of coding tools indicated by each PLId field MUST be
         equal to the default sub-profile, and the level indicated by
         each PLId field MUST be different than the default level.  All

Top      Up      ToC       Page 50 
         sequence parameter sets contained in each PSL MUST have the
         three bytes from profile_idc to level_idc, inclusive, equal to
         the preceding PLId.

            Informative note: This parameter allows for efficient level
            downgrade or upgrade in SDP Offer/Answer and out-of-band
            transport of parameter sets simultaneously.

      use-level-src-parameter-sets:
         This parameter MAY be used to indicate a receiver capability.
         The value MAY be equal to either 0 or 1.  When the parameter is
         not present, the value MUST be inferred to be equal to 0.  The
         value 0 indicates that the receiver does not understand the
         sprop-level-parameter-sets parameter, does not understand the
         "fmtp" source attribute as specified in Section 6.3 of [9],
         will ignore sprop-level-parameter-sets when present, and will
         ignore sprop-parameter-sets when conveyed using the "fmtp"
         source attribute.  The value 1 indicates that the receiver
         understands the sprop-level-parameter-sets parameter,
         understands the "fmtp" source attribute as specified in Section
         6.3 of [9], and is capable of using parameter sets contained in
         the sprop-level-parameter-sets or contained in the sprop-
         parameter-sets that is conveyed using the "fmtp" source
         attribute.

            Informative note: An RFC 3984 receiver does not understand
            sprop-level-parameter-sets, use-level-src-parameter-sets, or
            the "fmtp" source attribute as specified in Section 6.3 of
            [9].  Therefore, during SDP Offer/Answer, an RFC 3984
            receiver as the answerer will simply ignore sprop-level-
            parameter-sets when present in an offer and sprop-parameter-
            sets conveyed using the "fmtp" source attribute, as
            specified in Section 6.3 of [9].  Assume that the offered
            payload type was accepted at a level lower than the default
            level.  If the offered payload type included sprop-level-
            parameter-sets or included sprop-parameter-sets conveyed
            using the "fmtp" source attribute and if the offerer sees
            that the answerer has not included use-level-src-parameter-
            sets equal to 1 in the answer, the offerer knows that
            in-band transport of parameter sets is needed.

      in-band-parameter-sets:
         This parameter MAY be used to indicate a receiver capability.
         The value MAY be equal to either 0 or 1.  The value 1 indicates
         that the receiver discards out-of-band parameter sets in sprop-
         parameter-sets and sprop-level-parameter-sets; therefore, the
         sender MUST transmit all parameter sets in-band.  The value 0
         indicates that the receiver utilizes out-of-band parameter sets

Top      Up      ToC       Page 51 
         included in sprop-parameter-sets and/or sprop-level-parameter-
         sets.  However, in this case, the sender MAY still choose to
         send parameter sets in-band.  When in-band-parameter-sets is
         equal to 1, use-level-src-parameter-sets MUST NOT be present or
         MUST be equal to 0.  When the parameter is not present, this
         receiver capability is not specified, and therefore the sender
         MAY send out-of-band parameter sets only, it MAY send in-band-
         parameter-sets only, or it MAY send both.

      level-asymmetry-allowed:
         This parameter MAY be used in SDP Offer/Answer to indicate
         whether level asymmetry, i.e., sending media encoded at a
         different level in the offerer-to-answerer direction than the
         level in the answerer-to-offerer direction, is allowed.  The
         value MAY be equal to either 0 or 1.  When the parameter is not
         present, the value MUST be inferred to be equal to 0.  The
         value 1 in both the offer and the answer indicates that level
         asymmetry is allowed.  The value of 0 in either the offer or
         the answer indicates that level asymmetry is not allowed.

         If level-asymmetry-allowed is equal to 0 (or not present) in
         either the offer or the answer, level asymmetry is not allowed.
         In this case, the level to use in the direction from the
         offerer to the answerer MUST be the same as the level to use in
         the opposite direction.

      packetization-mode:
         This parameter signals the properties of an RTP payload type or
         the capabilities of a receiver implementation.  Only a single
         configuration point can be indicated; thus, when capabilities
         to support more than one packetization-mode are declared,
         multiple configuration points (RTP payload types) must be used.

         When the value of packetization-mode is equal to 0 or
         packetization-mode is not present, the single NAL mode MUST be
         used.  This mode is in use in standards using ITU-T
         Recommendation H.241 [3] (see Section 12.1).  When the value of
         packetization-mode is equal to 1, the non-interleaved mode MUST
         be used.  When the value of packetization-mode is equal to 2,
         the interleaved mode MUST be used.  The value of packetization-
         mode MUST be an integer in the range of 0 to 2, inclusive.

      sprop-interleaving-depth:
         This parameter MUST NOT be present when packetization-mode is
         not present or the value of packetization-mode is equal to 0 or
         1.  This parameter MUST be present when the value of
         packetization-mode is equal to 2.

Top      Up      ToC       Page 52 
         This parameter signals the properties of an RTP packet stream.
         It specifies the maximum number of VCL NAL units that precede
         any VCL NAL unit in the RTP packet stream in transmission order
         and that follow the VCL NAL unit in decoding order.
         Consequently, it is guaranteed that receivers can reconstruct
         NAL unit decoding order when the buffer size for NAL unit
         decoding order recovery is at least the value of sprop-
         interleaving-depth + 1 in terms of VCL NAL units.

         The value of sprop-interleaving-depth MUST be an integer in the
         range of 0 to 32767, inclusive.

      sprop-deint-buf-req:
         This parameter MUST NOT be present when packetization-mode is
         not present or the value of packetization-mode is equal to 0 or
         1.  It MUST be present when the value of packetization-mode is
         equal to 2.

         sprop-deint-buf-req signals the required size of the
         de-interleaving buffer for the RTP packet stream.  The value of
         the parameter MUST be greater than or equal to the maximum
         buffer occupancy (in units of bytes) required in such a
         de-interleaving buffer that is specified in Section 7.2.  It is
         guaranteed that receivers can perform the de-interleaving of
         interleaved NAL units into NAL unit decoding order, when the
         de-interleaving buffer size is at least the value of sprop-
         deint-buf-req in terms of bytes.

         The value of sprop-deint-buf-req MUST be an integer in the
         range of 0 to 4294967295, inclusive.

            Informative note: sprop-deint-buf-req indicates the required
            size of the de-interleaving buffer only.  When network
            jitter can occur, an appropriately sized jitter buffer has
            to be provisioned for as well.

      deint-buf-cap:
         This parameter signals the capabilities of a receiver
         implementation and indicates the amount of de-interleaving
         buffer space in units of bytes that the receiver has available
         for reconstructing the NAL unit decoding order.  A receiver is
         able to handle any stream for which the value of the sprop-
         deint-buf-req parameter is smaller than or equal to this
         parameter.

         If the parameter is not present, then a value of 0 MUST be used
         for deint-buf-cap.  The value of deint-buf-cap MUST be an
         integer in the range of 0 to 4294967295, inclusive.

Top      Up      ToC       Page 53 
            Informative note: deint-buf-cap indicates the maximum
            possible size of the de-interleaving buffer of the receiver
            only.  When network jitter can occur, an appropriately sized
            jitter buffer has to be provisioned for as well.

      sprop-init-buf-time:
         This parameter MAY be used to signal the properties of an RTP
         packet stream.  The parameter MUST NOT be present if the value
         of packetization-mode is equal to 0 or 1.

         The parameter signals the initial buffering time that a
         receiver MUST wait before starting decoding to recover the NAL
         unit decoding order from the transmission order.  The parameter
         is the maximum value of (decoding time of the NAL unit -
         transmission time of a NAL unit), assuming reliable and
         instantaneous transmission, the same timeline for transmission
         and decoding, and commencement of decoding when the first
         packet arrives.

         An example of specifying the value of sprop-init-buf-time
         follows.  A NAL unit stream is sent in the following
         interleaved order, in which the value corresponds to the
         decoding time and the transmission order is from left to right:

               0  2  1  3  5  4  6  8  7 ...

         Assuming a steady transmission rate of NAL units, the
         transmission times are:

               0  1  2  3  4  5  6  7  8 ...

         Subtracting the decoding time from the transmission time
         column-wise results in the following series:

               0 -1  1  0 -1  1  0 -1  1 ...

         Thus, in terms of intervals of NAL unit transmission times, the
         value of sprop-init-buf-time in this example is 1.  The
         parameter is coded as a non-negative base10 integer
         representation in clock ticks of a 90-kHz clock.  If the
         parameter is not present, then no initial buffering time value
         is defined.  Otherwise, the value of sprop-init-buf-time MUST
         be an integer in the range of 0 to 4294967295, inclusive.

Top      Up      ToC       Page 54 
         In addition to the signaled sprop-init-buf-time, receivers
         SHOULD take into account the transmission delay jitter
         buffering, including buffering for the delay jitter caused by
         mixers, translators, gateways, proxies, traffic-shapers, and
         other network elements.

      sprop-max-don-diff:
         This parameter MAY be used to signal the properties of an RTP
         packet stream.  It MUST NOT be used to signal transmitter,
         receiver, or codec capabilities.  The parameter MUST NOT be
         present if the value of packetization-mode is equal to 0 or 1.
         sprop-max-don-diff is an integer in the range of 0 to 32767,
         inclusive.  If sprop-max-don-diff is not present, the value of
         the parameter is unspecified.  sprop-max-don-diff is calculated
         as follows:

            sprop-max-don-diff = max{AbsDON(i) - AbsDON(j)},
            for any i and any j>i,

         where i and j indicate the index of the NAL unit in the
         transmission order and AbsDON denotes a decoding order number
         of the NAL unit that does not wrap around to 0 after 65535.  In
         other words, AbsDON is calculated as follows: let m and n be
         consecutive NAL units in transmission order.  For the very
         first NAL unit in transmission order (whose index is 0),
         AbsDON(0) = DON(0).  For other NAL units, AbsDON is calculated
         as follows:

            If DON(m) == DON(n), AbsDON(n) = AbsDON(m)

            If (DON(m) < DON(n) and DON(n) - DON(m) < 32768),
              AbsDON(n) = AbsDON(m) + DON(n) - DON(m)

            If (DON(m) > DON(n) and DON(m) - DON(n) >= 32768),
              AbsDON(n) = AbsDON(m) + 65536 - DON(m) + DON(n)

            If (DON(m) < DON(n) and DON(n) - DON(m) >= 32768),
              AbsDON(n) = AbsDON(m) - (DON(m) + 65536 - DON(n))

            If (DON(m) > DON(n) and DON(m) - DON(n) < 32768),
              AbsDON(n) = AbsDON(m) - (DON(m) - DON(n))

         where DON(i) is the decoding order number of the NAL unit
         having index i in the transmission order.  The decoding order
         number is specified in Section 5.5.

Top      Up      ToC       Page 55 
            Informative note: Receivers may use sprop-max-don-diff to
            trigger which NAL units in the receiver buffer can be passed
            to the decoder.

      max-rcmd-nalu-size:
         This parameter MAY be used to signal the capabilities of a
         receiver.  The parameter MUST NOT be used for any other
         purposes.  The value of the parameter indicates the largest
         NALU size in bytes that the receiver can handle efficiently.
         The parameter value is a recommendation, not a strict upper
         boundary.  The sender MAY create larger NALUs but must be aware
         that the handling of these may come at a higher cost than NALUs
         conforming to the limitation.

         The value of max-rcmd-nalu-size MUST be an integer in the range
         of 0 to 4294967295, inclusive.  If this parameter is not
         specified, no known limitation to the NALU size exists.
         Senders still have to consider the MTU size available between
         the sender and the receiver and SHOULD run MTU discovery for
         this purpose.

         This parameter is motivated by, for example, an IP to H.223
         video telephony gateway, where NALUs smaller than the H.223
         transport data unit will be more efficient.  A gateway may
         terminate IP; thus, MTU discovery will normally not work beyond
         the gateway.

            Informative note: Setting this parameter to a lower than
            necessary value may have a negative impact.

      sar-understood:
         This parameter MAY be used to indicate a receiver capability
         and nothing else.  The parameter indicates the maximum value of
         aspect_ratio_idc (specified in [1]) smaller than 255 that the
         receiver understands.  Table E-1 of [1] specifies
         aspect_ratio_idc equal to 0 as "unspecified"; 1 to 16,
         inclusive, as specific Sample Aspect Ratios (SARs); 17 to 254,
         inclusive, as "reserved"; and 255 as the Extended SAR, for
         which SAR width and SAR height are explicitly signaled.
         Therefore, a receiver with a decoder according to [1]
         understands aspect_ratio_idc in the range of 1 to 16,
         inclusive, and aspect_ratio_idc equal to 255, in the sense that
         the receiver knows exactly what the SAR is.  For such a
         receiver, the value of sar-understood is 16.  In the future, if
         Table E-1 of [1] is extended, e.g., such that the SAR for
         aspect_ratio_idc equal to 17 is specified, then for a receiver
         with a decoder that understands the extension, the value of

Top      Up      ToC       Page 56 
         sar-understood is 17.  For a receiver with a decoder according
         to the 2003 version of [1], the value of sar-understood is 13,
         as the minimum reserved aspect_ratio_idc therein is 14.

         When sar-understood is not present, the value MUST be inferred
         to be equal to 13.

      sar-supported:
         This parameter MAY be used to indicate a receiver capability
         and nothing else.  The value of this parameter is an integer in
         the range of 1 to sar-understood, inclusive, equal to 255.  The
         value of sar-supported equal to N smaller than 255 indicates
         that the receiver supports all the SARs corresponding to H.264
         aspect_ratio_idc values (see Table E-1 of [1]) in the range
         from 1 to N, inclusive, without geometric distortion.  The
         value of sar-supported equal to 255 indicates that the receiver
         supports all sample aspect ratios that are expressible using
         two 16-bit integer values as the numerator and denominator,
         i.e., those that are expressible using the H.264
         aspect_ratio_idc value of 255 (Extended_SAR, see Table E-1 of
         [1]), without geometric distortion.

         H.264-compliant encoders SHOULD NOT send an aspect_ratio_idc
         equal to 0 or an aspect_ratio_idc larger than sar-understood
         and smaller than 255.  H.264-compliant encoders SHOULD send an
         aspect_ratio_idc that the receiver is able to display without
         geometrical distortion.  However, H.264-compliant encoders MAY
         choose to send pictures using any SAR.

         Note that the actual sample aspect ratio or extended sample
         aspect ratio, when present, of the stream is conveyed in the
         Video Usability Information (VUI) part of the sequence
         parameter set.

      Encoding considerations:
         This type is only defined for transfer via RTP (RFC 3550).

      Security considerations:
         See Section 9 of RFC 6184.

      Public specification:
         Please refer to RFC 6184 and its Section 17.

      Additional information:
         None

      File extensions:  none

Top      Up      ToC       Page 57 
      Macintosh file type code:  none

      Object identifier or OID:  none

      Person & email address to contact for further information:
         Ye-Kui Wang, yekui.wang@huawei.com

      Intended usage:  COMMON

      Author:
         Ye-Kui Wang, yekui.wang@huawei.com

      Change controller:
         IETF Audio/Video Transport working group delegated from the
         IESG.

8.2.  SDP Parameters

   The receiver MUST ignore any parameter unspecified in this memo.

8.2.1.  Mapping of Payload Type Parameters to SDP

   The media type video/H264 string is mapped to fields in the Session
   Description Protocol (SDP) [6] as follows:

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

   o  The encoding name in the "a=rtpmap" line of SDP MUST be H264 (the
      media subtype).

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

   o  The OPTIONAL parameters profile-level-id, max-recv-level, max-
      mbps, max-smbps, max-fs, max-cpb, max-dpb, max-br, redundant-pic-
      cap, use-level-src-parameter-sets, in-band-parameter-sets, level-
      asymmetry-allowed, packetization-mode, sprop-interleaving-depth,
      sprop-deint-buf-req, deint-buf-cap, sprop-init-buf-time, sprop-
      max-don-diff, max-rcmd-nalu-size, sar-understood, and sar-
      supported, when present, MUST be included in the "a=fmtp" line of
      SDP.  These parameters are expressed as a media type string, in
      the form of a semicolon-separated list of parameter=value pairs.

   o  The OPTIONAL parameters sprop-parameter-sets and sprop-level-
      parameter-sets, when present, MUST be included in the "a=fmtp"
      line of SDP or conveyed using the "fmtp" source attribute as
      specified in Section 6.3 of [9].  For a particular media format
      (i.e., RTP payload type), a sprop-parameter-sets or sprop-level-
      parameter-sets MUST NOT be both included in the "a=fmtp" line of

Top      Up      ToC       Page 58 
      SDP and conveyed using the "fmtp" source attribute.  When included
      in the "a=fmtp" line of SDP, these parameters are expressed as a
      media type string, in the form of a semicolon-separated list of
      parameter=value pairs.  When conveyed using the "fmtp" source
      attribute, these parameters are only associated with the given
      source and payload type as parts of the "fmtp" source attribute.

         Informative note: Conveyance of sprop-parameter-sets and sprop-
         level-parameter-sets using the "fmtp" source attribute allows
         for out-of-band transport of parameter sets in topologies like
         Topo-Video-switch-MCU [29].

   An example of media representation in SDP is as follows (Baseline
   profile, Level 3.0, some of the constraints of the Main profile may
   not be obeyed):

      m=video 49170 RTP/AVP 98
      a=rtpmap:98 H264/90000
      a=fmtp:98 profile-level-id=42A01E;
                packetization-mode=1;
                sprop-parameter-sets=<parameter sets data>

8.2.2.  Usage with the SDP Offer/Answer Model

   When H.264 is offered over RTP using SDP in an Offer/Answer model [8]
   for negotiation for unicast usage, the following limitations and
   rules apply:

   o  The parameters identifying a media format configuration for H.264
      are profile-level-id and packetization-mode.  These media format
      configuration parameters (except for the level part of profile-
      level-id) MUST be used symmetrically; that is, the answerer MUST
      either maintain all configuration parameters or remove the media
      format (payload type) completely if one or more of the parameter
      values are not supported.  Note that the level part of profile-
      level-id includes level_idc, and, for indication of Level 1b when
      profile_idc is equal to 66, 77, or 88, bit 4
      (constraint_set3_flag) of profile-iop.  The level part of profile-
      level-id is changeable.

         Informative note: The requirement for symmetric use does not
         apply for the level part of profile-level-id and does not apply
         for the other stream properties and capability parameters.

         Informative note: In H.264 [1], all the levels except for Level
         1b are equal to the value of level_idc divided by 10.  Level 1b
         is a level higher than Level 1.0 but lower than Level 1.1 and
         is signaled in an ad hoc manner, because the level was

Top      Up      ToC       Page 59 
         specified after Level 1.0 and Level 1.1.  For the Baseline,
         Main, and Extended profiles (with profile_idc equal to 66, 77,
         and 88, respectively), Level 1b is indicated by level_idc equal
         to 11 (i.e., same as Level 1.1) and constraint_set3_flag equal
         to 1.  For other profiles, Level 1b is indicated by level_idc
         equal to 9 (but note that Level 1b for these profiles are still
         higher than Level 1, which has level_idc equal to 10 and lower
         than Level 1.1).  In SDP Offer/Answer, an answer to an offer
         may indicate a level equal to or lower than the level indicated
         in the offer.  Due to the ad hoc indication of Level 1b,
         offerers and answerers must check the value of bit 4
         (constraint_set3_flag) of the middle octet of the parameter
         profile-level-id, when profile_idc is equal to 66, 77, or 88
         and level_idc is equal to 11.

      To simplify the handling and matching of these configurations, the
      same RTP payload type number used in the offer SHOULD also be used
      in the answer, as specified in [8].  An answer MUST NOT contain
      the payload type number used in the offer unless the configuration
      is exactly the same as in the offer.

         Informative note: When an offerer receives an answer, it has to
         compare payload types not declared in the offer based on the
         media type (i.e., video/H264) and the above media configuration
         parameters with any payload types it has already declared.
         This will enable it to determine whether the configuration in
         question is new or if it is equivalent to configuration already
         offered, since a different payload type number may be used in
         the answer.

   o  When present, the parameter max-recv-level declares the highest
      level supported for receiving.  In case max-recv-level is not
      present, the highest level supported for receiving is equal to the
      default level indicated by the level part of profile-level-id.
      When present, max-recv-level MUST be higher than the default
      level.

   o  The parameter level-asymmetry-allowed indicates whether level
      asymmetry is allowed.

      If level-asymmetry-allowed is equal to 0 (or not present) in
      either the offer or the answer, level asymmetry is not allowed.
      In this case, the level to use in the direction from the offerer
      to the answerer MUST be the same as the level to use in the
      opposite direction, and the common level to use is equal to the
      lower value of the default level in the offer and the default
      level in the answer.

Top      Up      ToC       Page 60 
      Otherwise, level-asymmetry-allowed equals 1 in both the offer and
      the answer, and level asymmetry is allowed.  In this case, the
      level to use in the offerer-to-answerer direction MUST be equal to
      the highest level the answerer supports for receiving, and the
      level to use in the answerer-to-offerer direction MUST be equal to
      the highest level the offerer supports for receiving.

      When level asymmetry is not allowed, level upgrade is not allowed,
      i.e., the default level in the answer MUST be equal to or lower
      than the default level in the offer.

   o  The parameters sprop-deint-buf-req, sprop-interleaving-depth,
      sprop-max-don-diff, and sprop-init-buf-time describe the
      properties of the RTP packet stream that the offerer or answerer
      is sending for the media format configuration.  This differs from
      the normal usage of the Offer/Answer parameters: normally such
      parameters declare the properties of the stream that the offerer
      or the answerer is able to receive.  When dealing with H.264, the
      offerer assumes that the answerer will be able to receive media
      encoded using the configuration being offered.

         Informative note: The above parameters apply for any stream
         sent by a declaring entity with the same configuration; i.e.,
         they are dependent on their source.  Rather than being bound to
         the payload type, the values may have to be applied to another
         payload type when being sent, as they apply for the
         configuration.

   o  The capability parameters max-mbps, max-smbps, max-fs, max-cpb,
      max-dpb, max-br, redundant-pic-cap, max-rcmd-nalu-size, sar-
      understood, and sar-supported MAY be used to declare further
      capabilities of the offerer or answerer for receiving.  These
      parameters MUST NOT be present when the direction attribute is
      "sendonly" and when the parameters describe the limitations of
      what the offerer or answerer accepts for receiving streams.

   o  An offerer has to include the size of the de-interleaving buffer,
      sprop-deint-buf-req, in the offer for an interleaved H.264 stream.
      To enable the offerer and answerer to inform each other about
      their capabilities for de-interleaving buffering in receiving
      streams, both parties are RECOMMENDED to include deint-buf-cap.
      For interleaved streams, it is also RECOMMENDED to consider
      offering multiple payload types with different buffering
      requirements when the capabilities of the receiver are unknown.

   o  The sprop-parameter-sets or sprop-level-parameter-sets parameter,
      when present (included in the "a=fmtp" line of SDP or conveyed
      using the "fmtp" source attribute as specified in Section 6.3 of

Top      Up      ToC       Page 61 
      [9]), is used for out-of-band transport of parameter sets.
      However, when out-of-band transport of parameter sets is used,
      parameter sets MAY still be additionally transported in-band.

      The answerer MAY use either out-of-band or in-band transport of
      parameter sets for the stream it is sending, regardless of whether
      out-of-band parameter sets transport has been used in the offerer-
      to-answerer direction.  Parameter sets included in an answer are
      independent of those parameter sets included in the offer, as they
      are used for decoding two different video streams, one from the
      answerer to the offerer and the other in the opposite direction.

      The following rules apply to transport of parameter sets in the
      offerer-to-answerer direction.

         o  An offer MAY include either or both of sprop-parameter-sets
            and sprop-level-parameter-sets.  If neither sprop-parameter-
            sets nor sprop-level-parameter-sets is present in the offer,
            then only in-band transport of parameter sets is used.

         o  If the answer includes in-band-parameter-sets equal to 1,
            then the offerer MUST transmit parameter sets in-band.
            Otherwise, the following applies.

               o  If the level to use in the offerer-to-answerer
                  direction is equal to the default level in the offer,
                  the following applies.

                     When there is a sprop-parameter-sets included in
                     the "a=fmtp" line in the offer, the answerer MUST
                     be prepared to use the parameter sets included in
                     the sprop-parameter-sets for decoding the incoming
                     NAL unit stream.

                     When there is a sprop-parameter-sets conveyed using
                     the "fmtp" source attribute in the offer, the
                     following applies.  If the answer includes use-
                     level-src-parameter-sets equal to 1 or the "fmtp"
                     source attribute, the answerer MUST be prepared to
                     use the parameter sets included in the sprop-
                     parameter-sets for decoding the incoming NAL unit
                     stream;  otherwise, the offerer MUST transmit
                     parameter sets in-band.

                     When sprop-parameter-sets is not present in the
                     offer, the offerer MUST transmit parameter sets in-
                     band.

Top      Up      ToC       Page 62 
                     The answerer MUST ignore sprop-level-parameter-
                     sets, when present (either included in the "a=fmtp"
                     line or conveyed using the "fmtp" source attribute)
                     in the offer.

               o  Otherwise, the level to use in the offerer-to-answerer
                  direction is not equal to the default level in the
                  offer, and the following applies.

                     The answerer MUST ignore sprop-parameter-sets, when
                     present (either included in the "a=fmtp" line or
                     conveyed using the "fmtp" source attribute) in the
                     offer.

                     When neither use-level-src-parameter-sets is equal
                     to 1 nor the "fmtp" source attribute is present in
                     the answer, the answerer MUST ignore sprop-level-
                     parameter-sets, when present in the offer, and the
                     offerer MUST transmit parameter sets in-band.

                     When either use-level-src-parameter-sets is equal
                     to 1 or the "fmtp" source attribute is present in
                     the answer, the answerer MUST be prepared to use
                     the parameter sets that are included in sprop-
                     level-parameter-sets for the accepted level (i.e.,
                     the default level in the answer), when present in
                     the offer, for decoding the incoming NAL unit
                     stream, and ignore all other parameter sets
                     included in sprop-level-parameter-sets.

                     When no parameter sets for the level to use in the
                     offerer-to-answerer direction are present in sprop-
                     level-parameter-sets in the offer, the offerer MUST
                     transmit parameter sets in-band.

      The following rules apply to the transport of parameter sets in
      the answerer-to-offerer direction.

         o  An answer MAY include either sprop-parameter-sets or sprop-
            level-parameter-sets but MUST NOT include both.  If neither
            sprop-parameter-sets nor sprop-level-parameter-sets is
            present in the answer, then only in-band transport of
            parameter sets is used.

         o  If the offer includes in-band-parameter-sets equal to 1, the
            answerer MUST NOT include sprop-parameter-sets or sprop-
            level-parameter-sets in the answer and MUST transmit
            parameter sets in-band.  Otherwise, the following applies.

Top      Up      ToC       Page 63 
               o  If the level to use in the answerer-to-offerer
                  direction is equal to the default level in the answer,
                  the following applies.

                     When there is a sprop-parameter-sets included in
                     the "a=fmtp" line in the answer, the offerer MUST
                     be prepared to use the parameter sets included in
                     the sprop-parameter-sets for decoding the incoming
                     NAL unit stream.

                     When there is a sprop-parameter-sets conveyed using
                     the "fmtp" source attribute in the answer, the
                     following applies.  If the offer includes use-
                     level-src-parameter-sets equal to 1 or the "fmtp"
                     source attribute, the offerer MUST be prepared to
                     use the parameter sets included in the sprop-
                     parameter-sets for decoding the incoming NAL unit
                     stream;  otherwise, the answerer MUST transmit
                     parameter sets in-band.

                     When sprop-parameter-sets is not present in the
                     answer, the answerer MUST transmit parameter sets
                     in-band.

                     The offerer MUST ignore sprop-level-parameter-sets,
                     when present (either included in the "a=fmtp" line
                     or conveyed using the "fmtp" source attribute) in
                     the answer.

               o  Otherwise, the level to use in the answerer-to-offerer
                  direction is not equal to the default level in the
                  answer, and the following applies.

                     The offerer MUST ignore sprop-parameter-sets when
                     present (either included in the "a=fmtp" line of
                     SDP or conveyed using the "fmtp" source attribute)
                     in the answer.

                     When neither use-level-src-parameter-sets is equal
                     to 1 nor the "fmtp" source attribute is present in
                     the offer, the offerer MUST ignore sprop-level-
                     parameter-sets, when present, and the answerer MUST
                     transmit parameter sets in-band.

                     When either use-level-src-parameter-sets is equal
                     to 1 or the "fmtp" source attribute is present in
                     the offer, the offerer MUST be prepared to use the
                     parameter sets that are included in sprop-level-

Top      Up      ToC       Page 64 
                     parameter-sets for the level to use in the
                     answerer-to-offerer direction, when present in the
                     answer, for decoding the incoming NAL unit stream,
                     and ignore all other parameter sets included in
                     sprop-level-parameter-sets in the answer.

                     When no parameter sets for the level to use in the
                     answerer-to-offerer direction are present in sprop-
                     level-parameter-sets in the answer, the answerer
                     MUST transmit parameter sets in-band.

      When sprop-parameter-sets or sprop-level-parameter-sets is
      conveyed using the "fmtp" source attribute as specified in Section
      6.3 of [9], the receiver of the parameters MUST store the
      parameter sets included in the sprop-parameter-sets or sprop-
      level-parameter-sets for the accepted level and associate them
      with the source given as a part of the "fmtp" source attribute.
      Parameter sets associated with one source MUST only be used to
      decode NAL units conveyed in RTP packets from the same source.
      When this mechanism is in use, SSRC collision detection and
      resolution MUST be performed as specified in [9].

         Informative note: Conveyance of sprop-parameter-sets and sprop-
         level-parameter-sets using the "fmtp" source attribute may be
         used in topologies like Topo-Video-switch-MCU [29] to enable
         out-of-band transport of parameter sets.

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

   o  The media format configuration is identified by "profile-level-
      id", including the level part, and packetization-mode.  These
      media format configuration parameters (including the level part of
      profile-level-id) MUST be used symmetrically; that is, the
      answerer MUST either maintain all configuration parameters or
      remove the media format (payload type) completely.  Note that this
      implies that the level part of profile-level-id for Offer/Answer
      in multicast is not changeable.

      To simplify the handling and matching of these configurations, the
      same RTP payload type number used in the offer SHOULD also be used
      in the answer, as specified in [8].  An answer MUST NOT contain a
      payload type number used in the offer unless the configuration is
      the same as in the offer.

   o  Parameter sets received MUST be associated with the originating
      source and MUST only be used in decoding the incoming NAL unit
      stream from the same source.

Top      Up      ToC       Page 65 
   o  The rules for other parameters are the same as above for unicast
      as long as the above rules are obeyed.

   Table 6 lists the interpretation of all the media type parameters
   that MUST be used for the different direction attributes.

       Table 6.  Interpretation of parameters for different direction
                 attributes

                                              sendonly --+
                                           recvonly --+  |
                                        sendrecv --+  |  |
                                                   |  |  |
                profile-level-id                   C  C  P
                max-recv-level                     R  R  -
                packetization-mode                 C  C  P
                sprop-deint-buf-req                P  -  P
                sprop-interleaving-depth           P  -  P
                sprop-max-don-diff                 P  -  P
                sprop-init-buf-time                P  -  P
                max-mbps                           R  R  -
                max-smbps                          R  R  -
                max-fs                             R  R  -
                max-cpb                            R  R  -
                max-dpb                            R  R  -
                max-br                             R  R  -
                redundant-pic-cap                  R  R  -
                deint-buf-cap                      R  R  -
                max-rcmd-nalu-size                 R  R  -
                sar-understood                     R  R  -
                sar-supported                      R  R  -
                in-band-parameter-sets             R  R  -
                use-level-src-parameter-sets       R  R  -
                level-asymmetry-allowed            O  -  -
                sprop-parameter-sets               S  -  S
                sprop-level-parameter-sets         S  -  S

             Legend:

             C: configuration for sending and receiving streams
             O: offer/answer mode
             P: properties of the stream to be sent
             R: receiver capabilities
             S: out-of-band parameter sets
             -: not usable (when present, SHOULD be ignored)

Top      Up      ToC       Page 66 
   Parameters used for declaring receiver capabilities are in general
   downgradable; that is, they express the upper limit for a sender's
   possible behavior.  Thus, a sender MAY select to set its encoder
   using only lower/less or equal values of these parameters.

   Parameters declaring a configuration point are not changeable, with
   the exception of the level part of the profile-level-id parameter for
   unicast usage.

   When a sender's capabilities are declared and non-downgradable
   parameters are used in this declaration, these parameters express a
   configuration that is acceptable for the sender to receive streams.
   In order to achieve high interoperability levels, it is often
   advisable to offer multiple alternative configurations, e.g., for the
   packetization mode.  It is impossible to offer multiple
   configurations in a single payload type.  Thus, when multiple
   configuration offers are made, each offer requires its own RTP
   payload type associated with the offer.

   A receiver SHOULD understand all media type parameters, even if it
   only supports a subset of the payload format's functionality.  This
   ensures that a receiver is capable of understanding when an offer to
   receive media can be downgraded to what is supported by the receiver
   of the offer.

   An answerer MAY extend the offer with additional media format
   configurations.  However, to enable their usage, in most cases, a
   second offer is required from the offerer to provide the stream
   property parameters that the media sender will use.  This also has
   the effect that the offerer has to be able to receive this media
   format configuration, not only to send it.

   If an offerer wishes to have non-symmetric capabilities between
   sending and receiving, the offerer can allow asymmetric levels via
   level-asymmetry-allowed being equal to 1.  Alternatively, the offerer
   could offer different RTP sessions, i.e., different media lines
   declared as "recvonly" and "sendonly", respectively.  This may have
   further implications on the system and may require additional
   external semantics to associate the two media lines.

8.2.3.  Usage in Declarative Session Descriptions

   When H.264 over RTP is offered with SDP in a declarative style, as in
   Real Time Streaming Protocol (RTSP) [27] or Session Announcement
   Protocol (SAP) [28], the following considerations are necessary.

Top      Up      ToC       Page 67 
   o  All parameters capable of indicating both stream properties and
      receiver capabilities are used to indicate only stream properties.
      For example, in this case, the parameter profile-level-id declares
      only the values used by the stream, not the capabilities for
      receiving streams.  The result of this is that the following
      interpretation of the parameters MUST be used:

      Declaring actual configuration or stream properties:

         - profile-level-id
         - packetization-mode
         - sprop-interleaving-depth
         - sprop-deint-buf-req
         - sprop-max-don-diff
         - sprop-init-buf-time

      Out-of-band transporting of parameter sets:

         - sprop-parameter-sets
         - sprop-level-parameter-sets

      Not usable (when present, they SHOULD be ignored):

         - max-mbps
         - max-smbps
         - max-fs
         - max-cpb
         - max-dpb
         - max-br
         - max-recv-level
         - redundant-pic-cap
         - max-rcmd-nalu-size
         - deint-buf-cap
         - sar-understood
         - sar-supported
         - in-band-parameter-sets
         - level-asymmetry-allowed
         - use-level-src-parameter-sets

   o  A receiver of the SDP is required to support all parameters and
      values of the parameters provided; otherwise, the receiver MUST
      reject (RTSP) or not participate in (SAP) the session.  It falls
      on the creator of the session to use values that are expected to
      be supported by the receiving application.


Next RFC Part