From 3e59cb0ddfd2c59991f38e89352ad8a3c71b2374 Mon Sep 17 00:00:00 2001 From: Yuchung Cheng Date: Fri, 17 May 2013 13:45:05 +0000 Subject: tcp: remove bad timeout logic in fast recovery tcp_timeout_skb() was intended to trigger fast recovery on timeout, unfortunately in reality it often causes spurious retransmission storms during fast recovery. The particular sign is a fast retransmit over the highest sacked sequence (SND.FACK). Currently the RTO timer re-arming (as in RFC6298) offers a nice cushion to avoid spurious timeout: when SND.UNA advances the sender re-arms RTO and extends the timeout by icsk_rto. The sender does not offset the time elapsed since the packet at SND.UNA was sent. But if the next (DUP)ACK arrives later than ~RTTVAR and triggers tcp_fastretrans_alert(), then tcp_timeout_skb() will mark any packet sent before the icsk_rto interval lost, including one that's above the highest sacked sequence. Most likely a large part of scorebard will be marked. If most packets are not lost then the subsequent DUPACKs with new SACK blocks will cause the sender to continue to retransmit packets beyond SND.FACK spuriously. Even if only one packet is lost the sender may falsely retransmit almost the entire window. The situation becomes common in the world of bufferbloat: the RTT continues to grow as the queue builds up but RTTVAR remains small and close to the minimum 200ms. If a data packet is lost and the DUPACK triggered by the next data packet is slightly delayed, then a spurious retransmission storm forms. As the original comment on tcp_timeout_skb() suggests: the usefulness of this feature is questionable. It also wastes cycles walking the sack scoreboard and is actually harmful because of false recovery. It's time to remove this. Signed-off-by: Yuchung Cheng Acked-by: Eric Dumazet Acked-by: Neal Cardwell Acked-by: Nandita Dukkipati Signed-off-by: David S. Miller --- include/linux/tcp.h | 1 - 1 file changed, 1 deletion(-) (limited to 'include/linux/tcp.h') diff --git a/include/linux/tcp.h b/include/linux/tcp.h index 5adbc33d1ab..472120b4fac 100644 --- a/include/linux/tcp.h +++ b/include/linux/tcp.h @@ -246,7 +246,6 @@ struct tcp_sock { /* from STCP, retrans queue hinting */ struct sk_buff* lost_skb_hint; - struct sk_buff *scoreboard_skb_hint; struct sk_buff *retransmit_skb_hint; struct sk_buff_head out_of_order_queue; /* Out of order segments go here */ -- cgit v1.2.3-70-g09d2