2012-10-23 12:21:34 +00:00
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From patchwork Mon Aug 6 21:04:43 2012
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Content-Type: text/plain; charset="utf-8"
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MIME-Version: 1.0
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Content-Transfer-Encoding: 7bit
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Subject: [net-next] tcp: ecn: dont delay ACKS after CE
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Date: Mon, 06 Aug 2012 11:04:43 -0000
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From: Eric Dumazet <eric.dumazet@gmail.com>
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X-Patchwork-Id: 175453
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Message-Id: <1344287083.26674.83.camel@edumazet-glaptop>
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To: David Miller <davem@davemloft.net>
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Cc: netdev <netdev@vger.kernel.org>,
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Neal Cardwell <ncardwell@google.com>
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From: Eric Dumazet <edumazet@google.com>
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While playing with CoDel and ECN marking, I discovered a
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non optimal behavior of receiver of CE (Congestion Encountered)
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segments.
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In pathological cases, sender has reduced its cwnd to low values,
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and receiver delays its ACK (by 40 ms).
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While RFC 3168 6.1.3 (The TCP Receiver) doesn't explicitly recommend
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to send immediate ACKS, we believe its better to not delay ACKS, because
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a CE segment should give same signal than a dropped segment, and its
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quite important to reduce RTT to give ECE/CWR signals as fast as
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possible.
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Note we already call tcp_enter_quickack_mode() from TCP_ECN_check_ce()
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if we receive a retransmit, for the same reason.
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Signed-off-by: Eric Dumazet <edumazet@google.com>
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Cc: Neal Cardwell <ncardwell@google.com>
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Acked-by: Neal Cardwell <ncardwell@google.com>
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---
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net/ipv4/tcp_input.c | 6 +++++-
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1 file changed, 5 insertions(+), 1 deletion(-)
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--
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More majordomo info at http://vger.kernel.org/majordomo-info.html
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--- a/net/ipv4/tcp_input.c
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+++ b/net/ipv4/tcp_input.c
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2012-10-24 13:05:22 +00:00
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@@ -237,7 +237,11 @@ static inline void TCP_ECN_check_ce(stru
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2012-10-23 12:21:34 +00:00
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tcp_enter_quickack_mode((struct sock *)tp);
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break;
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case INET_ECN_CE:
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- tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
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+ if (!(tp->ecn_flags & TCP_ECN_DEMAND_CWR)) {
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+ /* Better not delay acks, sender can have a very low cwnd */
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+ tcp_enter_quickack_mode((struct sock *)tp);
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+ tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
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+ }
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/* fallinto */
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default:
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tp->ecn_flags |= TCP_ECN_SEEN;
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