This RTP header extension is an extended version of the extension defined in https://tools.ietf.org/html/draft-holmer-rmcat-transport-wide-cc-extensions-01

Name: “Transport-wide congenstion control 02”

Formal name: http://www.webrtc.org/experiments/rtp-hdrext/transport-wide-cc-02

Status: This extension is defined here to allow for experimentation. Once experience has shown that it is useful, we intend to make a proposal based on it for standardization in the IETF.

The original extension defines a transport-wide sequence number that is used in feedback packets for congestion control. The original implementation sends these feedback packets at a periodic interval. The extended version presented here has two changes compared to the original version:

• Feedback is sent only on request by the sender, therefore, the extension has two optional bytes that signals that a feedback packet is requested.
• The sender determines if timing information should be included or not in the feedback packet. The original version always include timing information.

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RTP header extension format

Data layout overview

Data layout of transport-wide sequence number 1-byte header + 2 bytes of data:

  0                   1                   2
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  ID   | L=1   |transport-wide sequence number |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Data layout of transport-wide sequence number and optional feedback request 1-byte header + 4 bytes of data:

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  ID   | L=3   |transport-wide sequence number |T|  seq count  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |seq count cont.|
 +-+-+-+-+-+-+-+-+

Data layout details

The data is written in the following order,

• transport-wide sequence number (16-bit unsigned integer)
• feedback request (optional) (16-bit unsigned integer)
  If the extension contains two extra bytes for feedback request, this means that a feedback packet should be generated and sent immediately. The feedback request consists of a one-bit field giving the flag value T and a 15-bit field giving the sequence count as an unsigned number.
  • If the bit T is set the feedback packet must contain timing information.
  • seq count specifies how many packets of history that should be included in the feedback packet. If seq count is zero no feedback should be be generated, which is equivalent of sending the two-byte extension above. This is added as an option to allow for a fixed packet header size.