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# Transport-Wide Congestion Control
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This RTP header extension is an extended version of the extension defined in
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<https://tools.ietf.org/html/draft-holmer-rmcat-transport-wide-cc-extensions-01>
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**Name:** "Transport-wide congenstion control 02"
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**Formal name:**
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<http://www.webrtc.org/experiments/rtp-hdrext/transport-wide-cc-02>
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**Status:** This extension is defined here to allow for experimentation. Once
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experience has shown that it is useful, we intend to make a proposal based on
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it for standardization in the IETF.
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The original extension defines a transport-wide sequence number that is used in
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feedback packets for congestion control. The original implementation sends these
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feedback packets at a periodic interval. The extended version presented here has
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two changes compared to the original version:
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* Feedback is sent only on request by the sender, therefore, the extension has
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two optional bytes that signals that a feedback packet is requested.
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* The sender determines if timing information should be included or not in the
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feedback packet. The original version always include timing information.
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Contact <kron@google.com> or <sprang@google.com> for more info.
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## RTP header extension format
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### Data layout overview
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Data layout of transport-wide sequence number
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1-byte header + 2 bytes of data:
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0 1 2
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| ID | L=1 |transport-wide sequence number |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Data layout of transport-wide sequence number and optional feedback request
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1-byte header + 4 bytes of data:
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0 1 2 3
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| ID | L=3 |transport-wide sequence number |T| seq count |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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|seq count cont.|
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+-+-+-+-+-+-+-+-+
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### Data layout details
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The data is written in the following order,
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* transport-wide sequence number (16-bit unsigned integer)
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* feedback request (optional) (16-bit unsigned integer)<br>
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If the extension contains two extra bytes for feedback request, this means
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that a feedback packet should be generated and sent immediately. The feedback
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request consists of a one-bit field giving the flag value T and a 15-bit
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field giving the sequence count as an unsigned number.
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- If the bit T is set the feedback packet must contain timing information.
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- seq count specifies how many packets of history that should be included in
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the feedback packet. If seq count is zero no feedback should be be
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generated, which is equivalent of sending the two-byte extension above.
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This is added as an option to allow for a fixed packet header size.
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