What is Typical RTP Packet size for H.264 video transmission

Question

I am encoding my video using JM reference software. The encoded file is divided into RTP packets. I can set a RTP packet size in bytes using configuration file. But i am unable to find what should be the RTP packet size for H.264 video used in video telephony? I know RTP packet size is variable but there should be some limit. Any help would be highly appreciated.

Answer 1

RTP Payload Format for H.264 Video

   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             |
  |                             ....                              |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    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.

     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.

---

H.264 also implements B-Frames. This type of frame fills in information between P-Frames. This means that the B-Frame will need to be held until the next P-frame arrives before the information can be used. B-Frames are not used in all modes of H.264. The encoder decides what type of frame is best suited. There are typically more P-frames than I-frames. Lab analysis has shown TelePresence I-frames to generally be 64K wide (50 packets @ 1316 bytes), while P- frames average 8K wide (9 packets at 900 bytes).

Hope this helps