Skip to main content

5.1. RTP Header Usage

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.

Last updated on