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authorYuchung Cheng <ycheng@google.com>2012-05-02 13:30:03 +0000
committerDavid S. Miller <davem@davemloft.net>2012-05-02 20:56:10 -0400
commiteed530b6c67624db3f2cf477bac7c4d005d8f7ba (patch)
treec07096807ead2adb9d85e85d1a9cd1ada85755ac /include/net/tcp.h
parent1fbc340514fc3003514bd681b372e1f47ae6183f (diff)
tcp: early retransmit
This patch implements RFC 5827 early retransmit (ER) for TCP. It reduces DUPACK threshold (dupthresh) if outstanding packets are less than 4 to recover losses by fast recovery instead of timeout. While the algorithm is simple, small but frequent network reordering makes this feature dangerous: the connection repeatedly enter false recovery and degrade performance. Therefore we implement a mitigation suggested in the appendix of the RFC that delays entering fast recovery by a small interval, i.e., RTT/4. Currently ER is conservative and is disabled for the rest of the connection after the first reordering event. A large scale web server experiment on the performance impact of ER is summarized in section 6 of the paper "Proportional Rate Reduction for TCP”, IMC 2011. http://conferences.sigcomm.org/imc/2011/docs/p155.pdf Note that Linux has a similar feature called THIN_DUPACK. The differences are THIN_DUPACK do not mitigate reorderings and is only used after slow start. Currently ER is disabled if THIN_DUPACK is enabled. I would be happy to merge THIN_DUPACK feature with ER if people think it's a good idea. ER is enabled by sysctl_tcp_early_retrans: 0: Disables ER 1: Reduce dupthresh to packets_out - 1 when outstanding packets < 4. 2: (Default) reduce dupthresh like mode 1. In addition, delay entering fast recovery by RTT/4. Note: mode 2 is implemented in the third part of this patch series. Signed-off-by: Yuchung Cheng <ycheng@google.com> Acked-by: Neal Cardwell <ncardwell@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'include/net/tcp.h')
-rw-r--r--include/net/tcp.h15
1 files changed, 15 insertions, 0 deletions
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 0fb84de6da3..685437a16c9 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -252,6 +252,7 @@ extern int sysctl_tcp_max_ssthresh;
extern int sysctl_tcp_cookie_size;
extern int sysctl_tcp_thin_linear_timeouts;
extern int sysctl_tcp_thin_dupack;
+extern int sysctl_tcp_early_retrans;
extern atomic_long_t tcp_memory_allocated;
extern struct percpu_counter tcp_sockets_allocated;
@@ -797,6 +798,20 @@ static inline void tcp_enable_fack(struct tcp_sock *tp)
tp->rx_opt.sack_ok |= TCP_FACK_ENABLED;
}
+/* TCP early-retransmit (ER) is similar to but more conservative than
+ * the thin-dupack feature. Enable ER only if thin-dupack is disabled.
+ */
+static inline void tcp_enable_early_retrans(struct tcp_sock *tp)
+{
+ tp->do_early_retrans = sysctl_tcp_early_retrans &&
+ !sysctl_tcp_thin_dupack && sysctl_tcp_reordering == 3;
+}
+
+static inline void tcp_disable_early_retrans(struct tcp_sock *tp)
+{
+ tp->do_early_retrans = 0;
+}
+
static inline unsigned int tcp_left_out(const struct tcp_sock *tp)
{
return tp->sacked_out + tp->lost_out;