RFC 6184
RTP Payload Format for H.264 Video
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.
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.
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.
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.
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.
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
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.
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.
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
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
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
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
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.
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.
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.
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.
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
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
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
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
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.
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
[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.
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.
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-
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.
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)
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.
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 page on part 4)
