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5.3. NAL Unit Header Usage

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 TypeContent of NAL UnitNRI (binary)
1non-IDR coded slice10
2Coded slice data partition A10
3Coded slice data partition B01
4Coded slice data partition C01

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.

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