tcp: improve PRR loss recovery

This patch improves TCP PRR loss recovery behavior for a corner
case. Previously during PRR conservation-bound mode, it strictly
sends the amount equals to the amount newly acked or s/acked.

The patch changes s.t. PRR may send additional amount that was banked
previously (e.g. application-limited) in the conservation-bound
mode, similar to the slow-start mode. This unifies and simplifies the
algorithm further and may improve the recovery latency. This change
still follow the general packet conservation design principle and
always keep inflight/cwnd below the slow start threshold set
by the congestion control module.

PRR is described in RFC 6937. We'll include this change in the
latest revision rfc6937-bis as well.

Reported-by: Neal Cardwell <>
Signed-off-by: Yuchung Cheng <>
Signed-off-by: Neal Cardwell <>
Signed-off-by: Eric Dumazet <>
Signed-off-by: Jakub Kicinski <>
1 file changed