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authorYuchung Cheng <ycheng@google.com>2013-05-17 13:45:05 +0000
committerDavid S. Miller <davem@davemloft.net>2013-05-19 23:51:17 -0700
commit3e59cb0ddfd2c59991f38e89352ad8a3c71b2374 (patch)
tree8bac5e7632178f7ff879d8c9e0ca8fc15a1f7299 /include/net/tcp.h
parent3cc7587b30032b7c4dd9610a55a77519e84da7db (diff)
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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 <ycheng@google.com> Acked-by: Eric Dumazet <edumazet@google.com> Acked-by: Neal Cardwell <ncardwell@google.com> Acked-by: Nandita Dukkipati <nanditad@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'include/net/tcp.h')
-rw-r--r--include/net/tcp.h1
1 files changed, 0 insertions, 1 deletions
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 5bba80f..e1c3723 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1193,7 +1193,6 @@ static inline void tcp_mib_init(struct net *net)
static inline void tcp_clear_retrans_hints_partial(struct tcp_sock *tp)
{
tp->lost_skb_hint = NULL;
- tp->scoreboard_skb_hint = NULL;
}
static inline void tcp_clear_all_retrans_hints(struct tcp_sock *tp)
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