RFC 6184
RTP Payload Format for H.264 Video
5. RTP Payload Format
5.1. RTP Header Usage
The format of the RTP header is specified in RFC 3550 [5] and reprinted in Figure 1 for convenience. This payload format uses the fields of the header in a manner consistent with that specification. When one NAL unit is encapsulated per RTP packet, the RECOMMENDED RTP payload format is specified in Section 5.6. The RTP payload (and the settings for some RTP header bits) for aggregation packets and fragmentation units are specified in Sections 5.7.2 and 5.8, respectively. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |V=2|P|X| CC |M| PT | sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | timestamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | synchronization source (SSRC) identifier | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | contributing source (CSRC) identifiers | | .... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1. RTP header according to RFC 3550 The RTP header information to be set according to this RTP payload format is set as follows: Marker bit (M): 1 bit Set for the very last packet of the access unit indicated by the RTP timestamp, in line with the normal use of the M bit in video
formats, to allow an efficient playout buffer handling. For
aggregation packets (STAP and MTAP), the marker bit in the RTP
header MUST be set to the value that the marker bit of the last
NAL unit of the aggregation packet would have been if it were
transported in its own RTP packet. Decoders MAY use this bit as
an early indication of the last packet of an access unit but MUST
NOT rely on this property.
Informative note: Only one M bit is associated with an
aggregation packet carrying multiple NAL units. Thus, if a
gateway has re-packetized an aggregation packet into several
packets, it cannot reliably set the M bit of those packets.
Payload type (PT): 7 bits
The assignment of an RTP payload type for this new packet format
is outside the scope of this document and will not be specified
here. The assignment of a payload type has to be performed either
through the profile used or in a dynamic way.
Sequence number (SN): 16 bits
Set and used in accordance with RFC 3550. For the single NALU and
non-interleaved packetization mode, the sequence number is used to
determine decoding order for the NALU.
Timestamp: 32 bits
The RTP timestamp is set to the sampling timestamp of the content.
A 90 kHz clock rate MUST be used.
If the NAL unit has no timing properties of its own (e.g.,
parameter set and SEI NAL units), the RTP timestamp is set to the
RTP timestamp of the primary coded picture of the access unit in
which the NAL unit is included, according to Section 7.4.1.2 of
[1].
The setting of the RTP timestamp for MTAPs is defined in Section
5.7.2.
Receivers SHOULD ignore any picture timing SEI messages included
in access units that have only one display timestamp. Instead,
receivers SHOULD use the RTP timestamp for synchronizing the
display process.
If one access unit has more than one display timestamp carried in
a picture timing SEI message, then the information in the SEI
message SHOULD be treated as relative to the RTP timestamp, with
the earliest event occurring at the time given by the RTP
timestamp and subsequent events later, as given by the difference
in picture time values carried in the picture timing SEI message.
Let tSEI1, tSEI2, ..., tSEIn be the display timestamps carried in
the SEI message of an access unit, where tSEI1 is the earliest of
all such timestamps. Let tmadjst() be a function that adjusts the
SEI messages time scale to a 90-kHz time scale. Let TS be the RTP
timestamp. Then, the display time for the event associated with
tSEI1 is TS. The display time for the event with tSEIx, where x
is [2..n], is TS + tmadjst (tSEIx - tSEI1).
Informative note: Displaying coded frames as fields is needed
commonly in an operation known as 3:2 pulldown, in which film
content that consists of coded frames is displayed on a display
using interlaced scanning. The picture timing SEI message
enables carriage of multiple timestamps for the same coded
picture, and therefore the 3:2 pulldown process is perfectly
controlled. The picture timing SEI message mechanism is
necessary because only one timestamp per coded frame can be
conveyed in the RTP timestamp.
5.2. Payload Structures
The payload format defines three different basic payload structures.
A receiver can identify the payload structure by the first byte of
the RTP packet payload, which co-serves as the RTP payload header
and, in some cases, as the first byte of the payload. This byte is
always structured as a NAL unit header. The NAL unit type field
indicates which structure is present. The possible structures are as
follows.
Single NAL Unit Packet: Contains only a single NAL unit in the
payload. The NAL header type field is equal to the original NAL unit
type, i.e., in the range of 1 to 23, inclusive. Specified in Section
5.6.
Aggregation Packet: Packet type used to aggregate multiple NAL units
into a single RTP payload. This packet exists in four versions, the
Single-Time Aggregation Packet type A (STAP-A), the Single-Time
Aggregation Packet type B (STAP-B), Multi-Time Aggregation Packet
(MTAP) with 16-bit offset (MTAP16), and Multi-Time Aggregation Packet
(MTAP) with 24-bit offset (MTAP24). The NAL unit type numbers
assigned for STAP-A, STAP-B, MTAP16, and MTAP24 are 24, 25, 26, and
27, respectively. Specified in Section 5.7.
Fragmentation Unit: Used to fragment a single NAL unit over multiple
RTP packets. Exists with two versions, FU-A and FU-B, identified
with the NAL unit type numbers 28 and 29, respectively. Specified in
Section 5.8.
Informative note: This specification does not limit the size of
NAL units encapsulated in single NAL unit packets and
fragmentation units. The maximum size of a NAL unit encapsulated
in any aggregation packet is 65535 bytes.
Table 1 summarizes NAL unit types and the corresponding RTP packet
types when each of these NAL units is directly used as a packet
payload, and where the types are described in this memo.
Table 1. Summary of NAL unit types and the corresponding packet
types
NAL Unit Packet Packet Type Name Section
Type Type
-------------------------------------------------------------
0 reserved -
1-23 NAL unit Single NAL unit packet 5.6
24 STAP-A Single-time aggregation packet 5.7.1
25 STAP-B Single-time aggregation packet 5.7.1
26 MTAP16 Multi-time aggregation packet 5.7.2
27 MTAP24 Multi-time aggregation packet 5.7.2
28 FU-A Fragmentation unit 5.8
29 FU-B Fragmentation unit 5.8
30-31 reserved -
5.3. NAL Unit Header Usage
The structure and semantics of the NAL unit header were introduced in
Section 1.3. For convenience, the format of the NAL unit header is
reprinted below:
+---------------+
|0|1|2|3|4|5|6|7|
+-+-+-+-+-+-+-+-+
|F|NRI| Type |
+---------------+
This section specifies the semantics of F and NRI according to this
specification.
F: 1 bit
forbidden_zero_bit. A value of 0 indicates that the NAL unit
type octet and payload should not contain bit errors or other
syntax violations. A value of 1 indicates that the NAL unit
type octet and payload may contain bit errors or other syntax
violations.
MANEs SHOULD set the F bit to indicate detected bit errors in
the NAL unit. The H.264 specification requires that the F bit
be equal to 0. When the F bit is set, the decoder is advised
that bit errors or any other syntax violations may be present
in the payload or in the NAL unit type octet. The simplest
decoder reaction to a NAL unit in which the F bit is equal to 1
is to discard such a NAL unit and to conceal the lost data in
the discarded NAL unit.
NRI: 2 bits
nal_ref_idc. The semantics of value 00 and a non-zero value
remain unchanged from the H.264 specification. In other words,
a value of 00 indicates that the content of the NAL unit is not
used to reconstruct reference pictures for inter picture
prediction. Such NAL units can be discarded without risking
the integrity of the reference pictures. Values greater than
00 indicate that the decoding of the NAL unit is required to
maintain the integrity of the reference pictures.
In addition to the specification above, according to this RTP
payload specification, values of NRI indicate the relative
transport priority, as determined by the encoder. MANEs can
use this information to protect more important NAL units better
than they do less important NAL units. The highest transport
priority is 11, followed by 10, and then by 01; finally, 00 is
the lowest.
Informative note: Any non-zero value of NRI is handled
identically in H.264 decoders. Therefore, receivers need
not manipulate the value of NRI when passing NAL units to
the decoder.
An H.264 encoder MUST set the value of NRI according to the
H.264 specification (Subclause 7.4.1) when the value of
nal_unit_type is in the range of 1 to 12, inclusive. In
particular, the H.264 specification requires that the value of
NRI SHALL be equal to 0 for all NAL units having nal_unit_type
equal to 6, 9, 10, 11, or 12.
For NAL units having nal_unit_type equal to 7 or 8 (indicating
a sequence parameter set or a picture parameter set,
respectively), an H.264 encoder SHOULD set the value of NRI to
11 (in binary format). For coded slice NAL units of a primary
coded picture having nal_unit_type equal to 5 (indicating a
coded slice belonging to an IDR picture), an H.264 encoder
SHOULD set the value of NRI to 11 (in binary format).
For a mapping of the remaining nal_unit_types to NRI values,
the following example MAY be used and has been shown to be
efficient in a certain environment [14]. Other mappings MAY
also be desirable, depending on the application and the H.264
profile in use.
Informative note: Data partitioning is not available in
certain profiles, e.g., in the Main or Baseline profiles.
Consequently, the NAL unit types 2, 3, and 4 can occur only
if the video bitstream conforms to a profile in which data
partitioning is allowed and not in streams that conform to
the Main or Baseline profiles.
Table 2. Example of NRI values for coded slices and coded slice
data partitions of primary coded reference pictures
NAL Unit Type Content of NAL Unit NRI (binary)
----------------------------------------------------------------
1 non-IDR coded slice 10
2 Coded slice data partition A 10
3 Coded slice data partition B 01
4 Coded slice data partition C 01
Informative note: As mentioned before, the NRI value of non-
reference pictures is 00 as mandated by H.264.
An H.264 encoder SHOULD set the value of NRI for coded slice
and coded slice data partition NAL units of redundant coded
reference pictures equal to 01 (in binary format).
Definitions of the values for NRI for NAL unit types 24 to 29,
inclusive, are given in Sections 5.7 and 5.8 of this memo.
No recommendation for the value of NRI is given for NAL units
having nal_unit_type in the range of 13 to 23, inclusive,
because these values are reserved for ITU-T and ISO/IEC. No
recommendation for the value of NRI is given for NAL units
having nal_unit_type equal to 0 or in the range of 30 to 31,
inclusive, as the semantics of these values are not specified
in this memo.
5.4. Packetization Modes
This memo specifies three cases of packetization modes: o Single NAL unit mode o Non-interleaved mode o Interleaved mode The single NAL unit mode is targeted for conversational systems that comply with ITU-T Recommendation H.241 [3] (see Section 12.1). The non-interleaved mode is targeted for conversational systems that may not comply with ITU-T Recommendation H.241. In the non-interleaved mode, NAL units are transmitted in NAL unit decoding order. The interleaved mode is targeted for systems that do not require very low end-to-end latency. The interleaved mode allows transmission of NAL units out of NAL unit decoding order. The packetization mode in use MAY be signaled by the value of the OPTIONAL packetization-mode media type parameter. The used packetization mode governs which NAL unit types are allowed in RTP payloads. Table 3 summarizes the allowed packet payload types for each packetization mode. Packetization modes are explained in more detail in Section 6. Table 3. Summary of allowed NAL unit types for each packetization mode (yes = allowed, no = disallowed, ig = ignore) Payload Packet Single NAL Non-Interleaved Interleaved Type Type Unit Mode Mode Mode ------------------------------------------------------------- 0 reserved ig ig ig 1-23 NAL unit yes yes no 24 STAP-A no yes no 25 STAP-B no no yes 26 MTAP16 no no yes 27 MTAP24 no no yes 28 FU-A no yes yes 29 FU-B no no yes 30-31 reserved ig ig ig Some NAL unit or payload type values (indicated as reserved in Table 3) are reserved for future extensions. NAL units of those types SHOULD NOT be sent by a sender (direct as packet payloads, as aggregation units in aggregation packets, or as fragmented units in FU packets) and MUST be ignored by a receiver. For example, the payload types 1-23, with the associated packet type "NAL unit", are
allowed in "Single NAL Unit Mode" and in "Non-Interleaved Mode" but disallowed in "Interleaved Mode". However, NAL units of NAL unit types 1-23 can be used in "Interleaved Mode" as aggregation units in STAP-B, MTAP16, and MTAP24 packets as well as fragmented units in FU- A and FU-B packets. Similarly, NAL units of NAL unit types 1-23 can also be used in the "Non-Interleaved Mode" as aggregation units in STAP-A packets or fragmented units in FU-A packets, in addition to being directly used as packet payloads.5.5. Decoding Order Number (DON)
In the interleaved packetization mode, the transmission order of NAL units is allowed to differ from the decoding order of the NAL units. Decoding order number (DON) is a field in the payload structure or a derived variable that indicates the NAL unit decoding order. Rationale and examples of use cases for transmission out of decoding order and for the use of DON are given in Section 13. The coupling of transmission and decoding order is controlled by the OPTIONAL sprop-interleaving-depth media type parameter as follows. When the value of the OPTIONAL sprop-interleaving-depth media type parameter is equal to 0 (explicitly or per default), the transmission order of NAL units MUST conform to the NAL unit decoding order. When the value of the OPTIONAL sprop-interleaving-depth media type parameter is greater than 0: o the order of NAL units in an MTAP16 and an MTAP24 is not required to be the NAL unit decoding order, and o the order of NAL units generated by de-packetizing STAP-Bs, MTAPs, and FUs in two consecutive packets is not required to be the NAL unit decoding order. The RTP payload structures for a single NAL unit packet, an STAP-A, and an FU-A do not include DON. STAP-B and FU-B structures include DON, and the structure of MTAPs enables derivation of DON, as specified in Section 5.7.2. Informative note: When an FU-A occurs in interleaved mode, it always follows an FU-B, which sets its DON. Informative note: If a transmitter wants to encapsulate a single NAL unit per packet and transmit packets out of their decoding order, STAP-B packet type can be used. In the single NAL unit packetization mode, the transmission order of NAL units, determined by the RTP sequence number, MUST be the same as their NAL unit decoding order. In the non-interleaved packetization
mode, the transmission order of NAL units in single NAL unit packets, STAP-As, and FU-As MUST be the same as their NAL unit decoding order. The NAL units within an STAP MUST appear in the NAL unit decoding order. Thus, the decoding order is first provided through the implicit order within an STAP and then provided through the RTP sequence number for the order between STAPs, FUs, and single NAL unit packets. The signaling of the value of DON for NAL units carried in STAP-B, MTAP, and a series of fragmentation units starting with an FU-B is specified in Sections 5.7.1, 5.7.2, and 5.8, respectively. The DON value of the first NAL unit in transmission order MAY be set to any value. Values of DON are in the range of 0 to 65535, inclusive. After reaching the maximum value, the value of DON wraps around to 0. The decoding order of two NAL units contained in any STAP-B, MTAP, or a series of fragmentation units starting with an FU-B is determined as follows. Let DON(i) be the decoding order number of the NAL unit having index i in the transmission order. Function don_diff(m,n) is specified as follows: If DON(m) == DON(n), don_diff(m,n) = 0 If (DON(m) < DON(n) and DON(n) - DON(m) < 32768), don_diff(m,n) = DON(n) - DON(m) If (DON(m) > DON(n) and DON(m) - DON(n) >= 32768), don_diff(m,n) = 65536 - DON(m) + DON(n) If (DON(m) < DON(n) and DON(n) - DON(m) >= 32768), don_diff(m,n) = - (DON(m) + 65536 - DON(n)) If (DON(m) > DON(n) and DON(m) - DON(n) < 32768), don_diff(m,n) = - (DON(m) - DON(n)) A positive value of don_diff(m,n) indicates that the NAL unit having transmission order index n follows, in decoding order, the NAL unit having transmission order index m. When don_diff(m,n) is equal to 0, the NAL unit decoding order of the two NAL units can be in either order. A negative value of don_diff(m,n) indicates that the NAL unit having transmission order index n precedes, in decoding order, the NAL unit having transmission order index m. Values of DON-related fields (DON, DONB, and DOND; see Section 5.7) MUST be such that the decoding order determined by the values of DON, as specified above, conforms to the NAL unit decoding order.
If the order of two NAL units in NAL unit decoding order is switched and the new order does not conform to the NAL unit decoding order, the NAL units MUST NOT have the same value of DON. If the order of two consecutive NAL units in the NAL unit stream is switched and the new order still conforms to the NAL unit decoding order, the NAL units MAY have the same value of DON. For example, when arbitrary slice order is allowed by the video coding profile in use, all the coded slice NAL units of a coded picture are allowed to have the same value of DON. Consequently, NAL units having the same value of DON can be decoded in any order, and two NAL units having a different value of DON should be passed to the decoder in the order specified above. When two consecutive NAL units in the NAL unit decoding order have a different value of DON, the value of DON for the second NAL unit in decoding order SHOULD be the value of DON for the first, incremented by one. An example of the de-packetization process to recover the NAL unit decoding order is given in Section 7. Informative note: Receivers should not expect that the absolute difference of values of DON for two consecutive NAL units in the NAL unit decoding order will be equal to one, even in error-free transmission. An increment by one is not required, as at the time of associating values of DON to NAL units, it may not be known whether all NAL units are delivered to the receiver. For example, a gateway may not forward coded slice NAL units of non-reference pictures or SEI NAL units when there is a shortage of bitrate in the network to which the packets are forwarded. In another example, a live broadcast is interrupted by pre-encoded content, such as commercials, from time to time. The first intra picture of a pre-encoded clip is transmitted in advance to ensure that it is readily available in the receiver. When transmitting the first intra picture, the originator does not exactly know how many NAL units will be encoded before the first intra picture of the pre- encoded clip follows in decoding order. Thus, the values of DON for the NAL units of the first intra picture of the pre-encoded clip have to be estimated when they are transmitted, and gaps in values of DON may occur.5.6. Single NAL Unit Packet
The single NAL unit packet defined here MUST contain only one NAL unit of the types defined in [1]. This means that neither an aggregation packet nor a fragmentation unit can be used within a single NAL unit packet. A NAL unit stream composed by de-packetizing single NAL unit packets in RTP sequence number order MUST conform to the NAL unit decoding order. The structure of the single NAL unit packet is shown in Figure 2.
Informative note: The first byte of a NAL unit co-serves as the
RTP payload header.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F|NRI| Type | |
+-+-+-+-+-+-+-+-+ |
| |
| Bytes 2..n of a single NAL unit |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :...OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2. RTP payload format for single NAL unit packet
5.7. Aggregation Packets
Aggregation packets are the NAL unit aggregation scheme of this
payload specification. The scheme is introduced to reflect the
dramatically different MTU sizes of two key target networks: wireline
IP networks (with an MTU size that is often limited by the Ethernet
MTU size, roughly 1500 bytes) and IP-based or non-IP-based (e.g.,
ITU-T H.324/M) wireless communication systems with preferred
transmission unit sizes of 254 bytes or less. To prevent media
transcoding between the two worlds, and to avoid undesirable
packetization overhead, a NAL unit aggregation scheme is introduced.
Two types of aggregation packets are defined by this specification:
o Single-time aggregation packet (STAP): aggregates NAL units with
identical NALU-times. Two types of STAPs are defined, one without
DON (STAP-A) and another including DON (STAP-B).
o Multi-time aggregation packet (MTAP): aggregates NAL units with
potentially differing NALU-times. Two different MTAPs are
defined, differing in the length of the NAL unit timestamp offset.
Each NAL unit to be carried in an aggregation packet is encapsulated
in an aggregation unit. Please see below for the four different
aggregation units and their characteristics.
The structure of the RTP payload format for aggregation packets is presented in Figure 3. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |F|NRI| Type | | +-+-+-+-+-+-+-+-+ | | | | one or more aggregation units | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | :...OPTIONAL RTP padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3. RTP payload format for aggregation packets MTAPs and STAPs share the following packetization rules: o The RTP timestamp MUST be set to the earliest of the NALU-times of all the NAL units to be aggregated. o The type field of the NAL unit type octet MUST be set to the appropriate value, as indicated in Table 4. o The F bit MUST be cleared if all F bits of the aggregated NAL units are zero; otherwise, it MUST be set. o The value of NRI MUST be the maximum of all the NAL units carried in the aggregation packet. Table 4. Type field for STAPs and MTAPs Type Packet Timestamp offset DON-related fields field length (DON, DONB, DOND) (in bits) present -------------------------------------------------------- 24 STAP-A 0 no 25 STAP-B 0 yes 26 MTAP16 16 yes 27 MTAP24 24 yes The marker bit in the RTP header is set to the value that the marker bit of the last NAL unit of the aggregated packet would have if it were transported in its own RTP packet.
The payload of an aggregation packet consists of one or more aggregation units. See Sections 5.7.1 and 5.7.2 for the four different types of aggregation units. An aggregation packet can carry as many aggregation units as necessary; however, the total amount of data in an aggregation packet obviously MUST fit into an IP packet, and the size SHOULD be chosen so that the resulting IP packet is smaller than the MTU size. An aggregation packet MUST NOT contain fragmentation units, as specified in Section 5.8. Aggregation packets MUST NOT be nested; that is, an aggregation packet MUST NOT contain another aggregation packet.5.7.1. Single-Time Aggregation Packet (STAP)
A single-time aggregation packet (STAP) SHOULD be used whenever NAL units are aggregated that all share the same NALU-time. The payload of an STAP-A does not include DON and consists of at least one single-time aggregation unit, as presented in Figure 4. The payload of an STAP-B consists of a 16-bit unsigned decoding order number (DON) (in network byte order) followed by at least one single-time aggregation unit, as presented in Figure 5. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : | +-+-+-+-+-+-+-+-+ | | | | single-time aggregation units | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4. Payload format for STAP-A
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : decoding order number (DON) | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | single-time aggregation units | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5. Payload format for STAP-B The DON field specifies the value of DON for the first NAL unit in an STAP-B in transmission order. For each successive NAL unit in appearance order in an STAP-B, the value of DON is equal to (the value of DON of the previous NAL unit in the STAP-B + 1) % 65536, in which '%' stands for the modulo operation. A single-time aggregation unit consists of 16-bit unsigned size information (in network byte order) that indicates the size of the following NAL unit in bytes (excluding these two octets, but including the NAL unit type octet of the NAL unit), followed by the NAL unit itself, including its NAL unit type byte. A single-time aggregation unit is byte aligned within the RTP payload, but it may not be aligned on a 32-bit word boundary. Figure 6 presents the structure of the single-time aggregation unit. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : NAL unit size | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | NAL unit | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6. Structure for single-time aggregation unit
Figure 7 presents an example of an RTP packet that contains an STAP- A. The STAP contains two single-time aggregation units, labeled as 1 and 2 in the figure. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RTP Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |STAP-A NAL HDR | NALU 1 Size | NALU 1 HDR | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NALU 1 Data | : : + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | NALU 2 Size | NALU 2 HDR | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NALU 2 Data | : : | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | :...OPTIONAL RTP padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 7. An example of an RTP packet including an STAP-A containing two single-time aggregation units
Figure 8 presents an example of an RTP packet that contains an STAP- B. The STAP contains two single-time aggregation units, labeled as 1 and 2 in the figure. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RTP Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |STAP-B NAL HDR | DON | NALU 1 Size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NALU 1 Size | NALU 1 HDR | NALU 1 Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + : : + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | NALU 2 Size | NALU 2 HDR | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NALU 2 Data | : : | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | :...OPTIONAL RTP padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 8. An example of an RTP packet including an STAP-B containing two single-time aggregation units5.7.2. Multi-Time Aggregation Packets (MTAPs)
The NAL unit payload of MTAPs consists of a 16-bit unsigned decoding order number base (DONB) (in network byte order) and one or more multi-time aggregation units, as presented in Figure 9. DONB MUST contain the value of DON for the first NAL unit in the NAL unit decoding order among the NAL units of the MTAP. Informative note: The first NAL unit in the NAL unit decoding order is not necessarily the first NAL unit in the order in which the NAL units are encapsulated in an MTAP.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : decoding order number base | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | multi-time aggregation units | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 9. NAL unit payload format for MTAPs Two different multi-time aggregation units are defined in this specification. Both of them consist of 16 bits of unsigned size information of the following NAL unit (in network byte order), an 8-bit unsigned decoding order number difference (DOND), and n bits (in network byte order) of timestamp offset (TS offset) for this NAL unit, whereby n can be 16 or 24. The choice between the different MTAP types (MTAP16 and MTAP24) is application dependent: the larger the timestamp offset is, the higher the flexibility of the MTAP, but the overhead is also higher. The structure of the multi-time aggregation units for MTAP16 and MTAP24 are presented in Figures 10 and 11, respectively. The starting or ending position of an aggregation unit within a packet is not required to be on a 32-bit word boundary. The DON of the NAL unit contained in a multi-time aggregation unit is equal to (DONB + DOND) % 65536, in which % denotes the modulo operation. This memo does not specify how the NAL units within an MTAP are ordered, but, in most cases, NAL unit decoding order SHOULD be used. The timestamp offset field MUST be set to a value equal to the value of the following formula: if the NALU-time is larger than or equal to the RTP timestamp of the packet, then the timestamp offset equals (the NALU-time of the NAL unit - the RTP timestamp of the packet). If the NALU-time is smaller than the RTP timestamp of the packet, then the timestamp offset is equal to the NALU-time + (2^32 - the RTP timestamp of the packet).
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : NAL unit size | DOND | TS offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TS offset | | +-+-+-+-+-+-+-+-+ NAL unit | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 10. Multi-time aggregation unit for MTAP16 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : NAL unit size | DOND | TS offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TS offset | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | NAL unit | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 11. Multi-time aggregation unit for MTAP24 For the "earliest" multi-time aggregation unit in an MTAP, the timestamp offset MUST be zero. Hence, the RTP timestamp of the MTAP itself is identical to the earliest NALU-time. Informative note: The "earliest" multi-time aggregation unit is the one that would have the smallest extended RTP timestamp among all the aggregation units of an MTAP if the NAL units contained in the aggregation units were encapsulated in single NAL unit packets. An extended timestamp is a timestamp that has more than 32 bits and is capable of counting the wraparound of the timestamp field, thus enabling one to determine the smallest value if the timestamp wraps. Such an "earliest" aggregation unit may not be the first one in the order in which the aggregation units are encapsulated in an MTAP. The "earliest" NAL unit need not be the same as the first NAL unit in the NAL unit decoding order either. Figure 12 presents an example of an RTP packet that contains a multi- time aggregation packet of type MTAP16 that contains two multi-time aggregation units, labeled as 1 and 2 in the figure.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RTP Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |MTAP16 NAL HDR | decoding order number base | NALU 1 Size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NALU 1 Size | NALU 1 DOND | NALU 1 TS offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NALU 1 HDR | NALU 1 DATA | +-+-+-+-+-+-+-+-+ + : : + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | NALU 2 SIZE | NALU 2 DOND | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NALU 2 TS offset | NALU 2 HDR | NALU 2 DATA | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | : : | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | :...OPTIONAL RTP padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 12. An RTP packet including a multi-time aggregation packet of type MTAP16 containing two multi-time aggregation units
Figure 13 presents an example of an RTP packet that contains a multi- time aggregation packet of type MTAP24 that contains two multi-time aggregation units, labeled as 1 and 2 in the figure. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RTP Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |MTAP24 NAL HDR | decoding order number base | NALU 1 Size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NALU 1 Size | NALU 1 DOND | NALU 1 TS offs | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |NALU 1 TS offs | NALU 1 HDR | NALU 1 DATA | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + : : + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | NALU 2 SIZE | NALU 2 DOND | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NALU 2 TS offset | NALU 2 HDR | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NALU 2 DATA | : : | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | :...OPTIONAL RTP padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 13. An RTP packet including a multi-time aggregation packet of type MTAP24 containing two multi-time aggregation units5.8. Fragmentation Units (FUs)
This payload type allows fragmenting a NAL unit into several RTP packets. Doing so on the application layer instead of relying on lower-layer fragmentation (e.g., by IP) has the following advantages: o The payload format is capable of transporting NAL units bigger than 64 kbytes over an IPv4 network that may be present in pre- recorded video, particularly in High-Definition formats (there is a limit of the number of slices per picture, which results in a limit of NAL units per picture, which may result in big NAL units). o The fragmentation mechanism allows fragmenting a single NAL unit and applying generic forward error correction as described in Section 12.5.
Fragmentation is defined only for a single NAL unit and not for any aggregation packets. A fragment of a NAL unit consists of an integer number of consecutive octets of that NAL unit. Each octet of the NAL unit MUST be part of exactly one fragment of that NAL unit. Fragments of the same NAL unit MUST be sent in consecutive order with ascending RTP sequence numbers (with no other RTP packets within the same RTP packet stream being sent between the first and last fragment). Similarly, a NAL unit MUST be reassembled in RTP sequence number order. When a NAL unit is fragmented and conveyed within fragmentation units (FUs), it is referred to as a fragmented NAL unit. STAPs and MTAPs MUST NOT be fragmented. FUs MUST NOT be nested; that is, an FU MUST NOT contain another FU. The RTP timestamp of an RTP packet carrying an FU is set to the NALU- time of the fragmented NAL unit. Figure 14 presents the RTP payload format for FU-As. An FU-A consists of a fragmentation unit indicator of one octet, a fragmentation unit header of one octet, and a fragmentation unit payload. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | FU indicator | FU header | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | FU payload | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | :...OPTIONAL RTP padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 14. RTP payload format for FU-A Figure 15 presents the RTP payload format for FU-Bs. An FU-B consists of a fragmentation unit indicator of one octet, a fragmentation unit header of one octet, a decoding order number (DON) (in network byte order), and a fragmentation unit payload. In other words, the structure of FU-B is the same as the structure of FU-A, except for the additional DON field.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | FU indicator | FU header | DON | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| | | | FU payload | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | :...OPTIONAL RTP padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 15. RTP payload format for FU-B NAL unit type FU-B MUST be used in the interleaved packetization mode for the first fragmentation unit of a fragmented NAL unit. NAL unit type FU-B MUST NOT be used in any other case. In other words, in the interleaved packetization mode, each NALU that is fragmented has an FU-B as the first fragment, followed by one or more FU-A fragments. The FU indicator octet has the following format: +---------------+ |0|1|2|3|4|5|6|7| +-+-+-+-+-+-+-+-+ |F|NRI| Type | +---------------+ Values equal to 28 and 29 in the type field of the FU indicator octet identify an FU-A and an FU-B, respectively. The use of the F bit is described in Section 5.3. The value of the NRI field MUST be set according to the value of the NRI field in the fragmented NAL unit. The FU header has the following format: +---------------+ |0|1|2|3|4|5|6|7| +-+-+-+-+-+-+-+-+ |S|E|R| Type | +---------------+ S: 1 bit When set to one, the Start bit indicates the start of a fragmented NAL unit. When the following FU payload is not the start of a fragmented NAL unit payload, the Start bit is set to zero.
E: 1 bit
When set to one, the End bit indicates the end of a fragmented
NAL unit, i.e., the last byte of the payload is also the last
byte of the fragmented NAL unit. When the following FU
payload is not the last fragment of a fragmented NAL unit, the
End bit is set to zero.
R: 1 bit
The Reserved bit MUST be equal to 0 and MUST be ignored by the
receiver.
Type: 5 bits
The NAL unit payload type as defined in Table 7-1 of [1].
The value of DON in FU-Bs is selected as described in Section 5.5.
Informative note: The DON field in FU-Bs allows gateways to
fragment NAL units to FU-Bs without organizing the incoming NAL
units to the NAL unit decoding order.
A fragmented NAL unit MUST NOT be transmitted in one FU; that is, the
Start bit and End bit MUST NOT both be set to one in the same FU
header.
The FU payload consists of fragments of the payload of the fragmented
NAL unit so that if the fragmentation unit payloads of consecutive
FUs are sequentially concatenated, the payload of the fragmented NAL
unit can be reconstructed. The NAL unit type octet of the fragmented
NAL unit is not included as such in the fragmentation unit payload,
but rather the information of the NAL unit type octet of the
fragmented NAL unit is conveyed in the F and NRI fields of the FU
indicator octet of the fragmentation unit and in the type field of
the FU header. An FU payload MAY have any number of octets and MAY
be empty.
Informative note: Empty FUs are allowed to reduce the latency of a
certain class of senders in nearly lossless environments. These
senders can be characterized in that they packetize NALU fragments
before the NALU is completely generated and, hence, before the
NALU size is known. If zero-length NALU fragments were not
allowed, the sender would have to generate at least one bit of
data of the following fragment before the current fragment could
be sent. Due to the characteristics of H.264, where sometimes
several macroblocks occupy zero bits, this is undesirable and can
add delay. However, the (potential) use of zero-length NALU
fragments should be carefully weighed against the increased risk
of the loss of at least a part of the NALU because of the
additional packets employed for its transmission.
If a fragmentation unit is lost, the receiver SHOULD discard all following fragmentation units in transmission order corresponding to the same fragmented NAL unit. A receiver in an endpoint or in a MANE MAY aggregate the first n-1 fragments of a NAL unit to an (incomplete) NAL unit, even if fragment n of that NAL unit is not received. In this case, the forbidden_zero_bit of the NAL unit MUST be set to one to indicate a syntax violation.6. Packetization Rules
The packetization modes are introduced in Section 5.2. The packetization rules common to more than one of the packetization modes are specified in Section 6.1. The packetization rules for the single NAL unit mode, the non-interleaved mode, and the interleaved mode are specified in Sections 6.2, 6.3, and 6.4, respectively.6.1. Common Packetization Rules
All senders MUST enforce the following packetization rules, regardless of the packetization mode in use: o Coded slice NAL units or coded slice data partition NAL units belonging to the same coded picture (and thus sharing the same RTP timestamp value) MAY be sent in any order; however, for delay- critical systems, they SHOULD be sent in their original decoding order to minimize the delay. Note that the decoding order is the order of the NAL units in the bitstream. o Parameter sets are handled in accordance with the rules and recommendations given in Section 8.4. o MANEs MUST NOT duplicate any NAL unit except for sequence or picture parameter set NAL units, as neither this memo nor the H.264 specification provides means to identify duplicated NAL units. Sequence and picture parameter set NAL units MAY be duplicated to make their correct reception more probable, but any such duplication MUST NOT affect the contents of any active sequence or picture parameter set. Duplication SHOULD be performed on the application layer and not by duplicating RTP packets (with identical sequence numbers).
Senders using the non-interleaved mode and the interleaved mode MUST
enforce the following packetization rule:
o In an RTP translator, MANEs MAY convert single NAL unit packets
into one aggregation packet, convert an aggregation packet into
several single NAL unit packets, or mix both concepts. The RTP
translator SHOULD take into account at least the following
parameters: path MTU size, unequal protection mechanisms (e.g.,
through packet-based FEC according to RFC 5109 [18], especially
for sequence and picture parameter set NAL units and coded slice
data partition A NAL units), bearable latency of the system, and
buffering capabilities of the receiver.
Informative note: An RTP translator is required to handle RTP
Control Protocol (RTCP) as per RFC 3550.
6.2. Single NAL Unit Mode
This mode is in use when the value of the OPTIONAL packetization-mode
media type parameter is equal to 0 or the packetization-mode is not
present. All receivers MUST support this mode. It is primarily
intended for low-delay applications that are compatible with systems
using ITU-T Recommendation H.241 [3] (see Section 12.1). Only single
NAL unit packets MAY be used in this mode. STAPs, MTAPs, and FUs
MUST NOT be used. The transmission order of single NAL unit packets
MUST comply with the NAL unit decoding order.
6.3. Non-Interleaved Mode
This mode is in use when the value of the OPTIONAL packetization-mode
media type parameter is equal to 1. This mode SHOULD be supported.
It is primarily intended for low-delay applications. Only single NAL
unit packets, STAP-As, and FU-As MAY be used in this mode. STAP-Bs,
MTAPs, and FU-Bs MUST NOT be used. The transmission order of NAL
units MUST comply with the NAL unit decoding order.
6.4. Interleaved Mode
This mode is in use when the value of the OPTIONAL packetization-mode
media type parameter is equal to 2. Some receivers MAY support this
mode. STAP-Bs, MTAPs, FU-As, and FU-Bs MAY be used. STAP-As and
single NAL unit packets MUST NOT be used. The transmission order of
packets and NAL units is constrained as specified in Section 5.5.
7. De-Packetization Process
The de-packetization process is implementation dependent. Therefore, the following description should be seen as an example of a suitable implementation. Other schemes may also be used as long as the output for the same input is the same as the process described below. The same output means that the resulting NAL units and their order are identical. Optimizations relative to the described algorithms are likely possible. Section 7.1 presents the de-packetization process for the single NAL unit and non-interleaved packetization modes, whereas Section 7.2 describes the process for the interleaved mode. Section 7.3 includes additional de-packetization guidelines for intelligent receivers. All normal RTP mechanisms related to buffer management apply. In particular, duplicated or outdated RTP packets (as indicated by the RTP sequence number and the RTP timestamp) are removed. To determine the exact time for decoding, factors such as a possible intentional delay to allow for proper inter-stream synchronization must be factored in.7.1. Single NAL Unit and Non-Interleaved Mode
The receiver includes a receiver buffer to compensate for transmission delay jitter. The receiver stores incoming packets in reception order into the receiver buffer. Packets are de-packetized in RTP sequence number order. If a de-packetized packet is a single NAL unit packet, the NAL unit contained in the packet is passed directly to the decoder. If a de-packetized packet is an STAP-A, the NAL units contained in the packet are passed to the decoder in the order in which they are encapsulated in the packet. For all the FU-A packets containing fragments of a single NAL unit, the de-packetized fragments are concatenated in their sending order to recover the NAL unit, which is then passed to the decoder. Informative note: If the decoder supports arbitrary slice order, coded slices of a picture can be passed to the decoder in any order, regardless of their reception and transmission order.7.2. Interleaved Mode
The general concept behind these de-packetization rules is to reorder NAL units from transmission order to the NAL unit decoding order. The receiver includes a receiver buffer, which is used to compensate for transmission delay jitter and to reorder NAL units from transmission order to the NAL unit decoding order. In this section, the receiver operation is described under the assumption that there
is no transmission delay jitter. To differentiate the receiver buffer from a practical receiver buffer that is also used for compensation of transmission delay jitter, the receiver buffer is hereafter called the de-interleaving buffer in this section. Receivers SHOULD also prepare for transmission delay jitter, i.e., either reserve separate buffers for transmission delay jitter buffering and de-interleaving buffering or use a receiver buffer for both transmission delay jitter and de-interleaving. Moreover, receivers SHOULD take transmission delay jitter into account in the buffering operation, e.g., by additional initial buffering before starting of decoding and playback. This section is organized as follows: Subsection 7.2.1 presents how to calculate the size of the de-interleaving buffer. Subsection 7.2.2 specifies the receiver process on how to organize received NAL units to the NAL unit decoding order.7.2.1. Size of the De-Interleaving Buffer
In either Offer/Answer or declarative Session Description Protocol (SDP) usage, the sprop-deint-buf-req media type parameter signals the requirement for the de-interleaving buffer size. Therefore, it is RECOMMENDED to set the de-interleaving buffer size, in terms of number of bytes, equal to or greater than the value of the sprop- deint-buf-req media type parameter. When the SDP Offer/Answer model or any other capability exchange procedure is used in session setup, the properties of the received stream SHOULD be such that the receiver capabilities are not exceeded. In the SDP Offer/Answer model, the receiver can indicate its capabilities to allocate a de-interleaving buffer with the deint- buf-cap media type parameter. See Section 8.1 for further information on the deint-buf-cap and sprop-deint-buf-req media type parameters and Section 8.2.2 for further information on their use in the SDP Offer/Answer model.7.2.2. De-Interleaving Process
There are two buffering states in the receiver: initial buffering and buffering while playing. Initial buffering occurs when the RTP session is initialized. After initial buffering, decoding and playback are started, and the buffering-while-playing mode is used. Regardless of the buffering state, the receiver stores incoming NAL units, in reception order, in the de-interleaving buffer as follows. NAL units of aggregation packets are stored in the de-interleaving buffer individually. The value of DON is calculated and stored for each NAL unit.
The receiver operation is described below with the help of the following functions and constants: o Function AbsDON is specified in Section 8.1. o Function don_diff is specified in Section 5.5. o Constant N is the value of the OPTIONAL sprop-interleaving-depth media type parameter (see Section 8.1) incremented by 1. Initial buffering lasts until one of the following conditions is fulfilled: o There are N or more VCL NAL units in the de-interleaving buffer. o If sprop-max-don-diff is present, don_diff(m,n) is greater than the value of sprop-max-don-diff, in which n corresponds to the NAL unit having the greatest value of AbsDON among the received NAL units and m corresponds to the NAL unit having the smallest value of AbsDON among the received NAL units. o Initial buffering has lasted for the duration equal to or greater than the value of the OPTIONAL sprop-init-buf-time media type parameter. The NAL units to be removed from the de-interleaving buffer are determined as follows: o If the de-interleaving buffer contains at least N VCL NAL units, NAL units are removed from the de-interleaving buffer and passed to the decoder in the order specified below until the buffer contains N-1 VCL NAL units. o If sprop-max-don-diff is present, all NAL units m for which don_diff(m,n) is greater than sprop-max-don-diff are removed from the de-interleaving buffer and passed to the decoder in the order specified below. Herein, n corresponds to the NAL unit having the greatest value of AbsDON among the NAL units in the de- interleaving buffer.
The order in which NAL units are passed to the decoder is specified
as follows:
o Let PDON be a variable that is initialized to 0 at the beginning
of the RTP session.
o For each NAL unit associated with a value of DON, a DON distance
is calculated as follows. If the value of DON of the NAL unit is
larger than the value of PDON, the DON distance is equal to DON -
PDON. Otherwise, the DON distance is equal to 65535 - PDON + DON
+ 1.
o NAL units are delivered to the decoder in ascending order of DON
distance. If several NAL units share the same value of DON
distance, they can be passed to the decoder in any order.
o When a desired number of NAL units have been passed to the
decoder, the value of PDON is set to the value of DON for the last
NAL unit passed to the decoder.
7.3. Additional De-Packetization Guidelines
The following additional de-packetization rules may be used to
implement an operational H.264 de-packetizer:
o Intelligent RTP receivers (e.g., in gateways) may identify lost
coded slice data partitions A (DPAs). If a lost DPA is detected,
after taking into account possible retransmission and FEC, a
gateway may decide not to send the corresponding coded slice data
partitions B and C, as their information is meaningless for H.264
decoders. In this way, a MANE can reduce network load by
discarding useless packets without parsing a complex bitstream.
o Intelligent RTP receivers (e.g., in gateways) may identify lost
FUs. If a lost FU is found, a gateway may decide not to send the
following FUs of the same fragmented NAL unit, as their
information is meaningless for H.264 decoders. In this way, a
MANE can reduce network load by discarding useless packets without
parsing a complex bitstream.
o Intelligent receivers having to discard packets or NALUs should
first discard all packets/NALUs in which the value of the NRI
field of the NAL unit type octet is equal to 0. This will
minimize the impact on user experience and keep the reference
pictures intact. If more packets have to be discarded, then
packets with a numerically lower NRI value should be discarded
before packets with a numerically higher NRI value. However,
discarding any packets with an NRI bigger than 0 very likely leads
to decoder drift and SHOULD be avoided.
(page 39 continued on part 3)
