To reduce latency when delivering messages on channel with low traffic
volume and with packet loss, where retransmissions are not driven by
fast retransmit but by T3-RTX timer, set the I-SACK bit (RFC7053) when
the congestion window is low.
Note that RFC7053 doesn't have to be negotiated, as is explained in
https://www.rfc-editor.org/rfc/rfc7053.html#section-6, and if the
receiver doesn't support it, it will delay SACKs as is done today.
When T3-RTX fires, the congestion window will be set to one MTU and any
future sent message will only send one MTU's worth of data before waiting
for the receiver's SACK until more data is sent. Delayed SACK, which is
normally used, could delay the next packet from being sent unecessarily
long. Setting I-SACK when the congestion window is small will make the
receiver always send a SACK for every received packet with a DATA chunk
in it. By not setting it always, it will not affect the packet rate when
the channel is fully utilized.
This modification improves latency in the aforementioned situations
without significantly affecting bandwidth. While this change increases
SACK frequency in specific scenarios, the impact on overall network load
is expected to be minimal.
Bug: webrtc:396080535
Change-Id: If4a5aa960969f1385c9ea59baa7e2d52caf25626
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/377140
Commit-Queue: Victor Boivie <boivie@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#43897}
f6902aff95