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tcp: don't mark recently sent packets lost on RTO
An RTO event indicates the head has not been acked for a long time after its last (re)transmission. But the other packets are not necessarily lost if they have been only sent recently (for example due to application limit). This patch would prohibit marking packets sent within an RTT to be lost on RTO event, using similar logic in TCP RACK detection. Normally the head (SND.UNA) would be marked lost since RTO should fire strictly after the head was sent. An exception is when the most recent RACK RTT measurement is larger than the (previous) RTO. To address this exception the head is always marked lost. Congestion control interaction: since we may not mark every packet lost, the congestion window may be more than 1 (inflight plus 1). But only one packet will be retransmitted after RTO, since tcp_retransmit_timer() calls tcp_retransmit_skb(...,segs=1). The connection still performs slow start from one packet (with Cubic congestion control). This commit was tested in an A/B test with Google web servers, and showed a reduction of 2% in (spurious) retransmits post timeout (SlowStartRetrans), and correspondingly reduced DSACKs (DSACKIgnoredOld) by 7%. Signed-off-by: Yuchung Cheng <[email protected]> Signed-off-by: Neal Cardwell <[email protected]> Reviewed-by: Eric Dumazet <[email protected]> Reviewed-by: Soheil Hassas Yeganeh <[email protected]> Reviewed-by: Priyaranjan Jha <[email protected]> Signed-off-by: David S. Miller <[email protected]>
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