From eed530b6c67624db3f2cf477bac7c4d005d8f7ba Mon Sep 17 00:00:00 2001 From: Yuchung Cheng Date: Wed, 2 May 2012 13:30:03 +0000 Subject: [PATCH] tcp: early retransmit MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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 Acked-by: Neal Cardwell Signed-off-by: David S. Miller --- Documentation/networking/ip-sysctl.txt | 14 ++++++++++++++ include/linux/tcp.h | 1 + include/net/tcp.h | 15 +++++++++++++++ net/ipv4/sysctl_net_ipv4.c | 10 ++++++++++ net/ipv4/tcp.c | 3 +++ net/ipv4/tcp_input.c | 15 +++++++++++++++ net/ipv4/tcp_minisocks.c | 1 + 7 files changed, 59 insertions(+) diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt index 9b569a2d9c60..34916e792d9d 100644 --- a/Documentation/networking/ip-sysctl.txt +++ b/Documentation/networking/ip-sysctl.txt @@ -190,6 +190,20 @@ tcp_cookie_size - INTEGER tcp_dsack - BOOLEAN Allows TCP to send "duplicate" SACKs. +tcp_early_retrans - INTEGER + Enable Early Retransmit (ER), per RFC 5827. ER lowers the threshold + for triggering fast retransmit when the amount of outstanding data is + small and when no previously unsent data can be transmitted (such + that limited transmit could be used). + Possible values: + 0 disables ER + 1 enables ER + 2 enables ER but delays fast recovery and fast retransmit + by a fourth of RTT. This mitigates connection falsely + recovers when network has a small degree of reordering + (less than 3 packets). + Default: 2 + tcp_ecn - INTEGER Enable Explicit Congestion Notification (ECN) in TCP. ECN is only used when both ends of the TCP flow support it. It is useful to diff --git a/include/linux/tcp.h b/include/linux/tcp.h index 278af9ea42d4..7859b416d46e 100644 --- a/include/linux/tcp.h +++ b/include/linux/tcp.h @@ -372,6 +372,7 @@ struct tcp_sock { repair : 1, unused : 1; u8 repair_queue; + u8 do_early_retrans:1;/* Enable RFC5827 early-retransmit */ /* RTT measurement */ u32 srtt; /* smoothed round trip time << 3 */ diff --git a/include/net/tcp.h b/include/net/tcp.h index 0fb84de6da36..685437a16c97 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; diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c index 33417f84e07f..ef32956ed655 100644 --- a/net/ipv4/sysctl_net_ipv4.c +++ b/net/ipv4/sysctl_net_ipv4.c @@ -27,6 +27,7 @@ #include static int zero; +static int two = 2; static int tcp_retr1_max = 255; static int ip_local_port_range_min[] = { 1, 1 }; static int ip_local_port_range_max[] = { 65535, 65535 }; @@ -676,6 +677,15 @@ static struct ctl_table ipv4_table[] = { .mode = 0644, .proc_handler = proc_dointvec }, + { + .procname = "tcp_early_retrans", + .data = &sysctl_tcp_early_retrans, + .maxlen = sizeof(int), + .mode = 0644, + .proc_handler = proc_dointvec_minmax, + .extra1 = &zero, + .extra2 = &two, + }, { .procname = "udp_mem", .data = &sysctl_udp_mem, diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c index 9670af341931..6802c89bc44d 100644 --- a/net/ipv4/tcp.c +++ b/net/ipv4/tcp.c @@ -395,6 +395,7 @@ void tcp_init_sock(struct sock *sk) tp->mss_cache = TCP_MSS_DEFAULT; tp->reordering = sysctl_tcp_reordering; + tcp_enable_early_retrans(tp); icsk->icsk_ca_ops = &tcp_init_congestion_ops; sk->sk_state = TCP_CLOSE; @@ -2495,6 +2496,8 @@ static int do_tcp_setsockopt(struct sock *sk, int level, err = -EINVAL; else tp->thin_dupack = val; + if (tp->thin_dupack) + tcp_disable_early_retrans(tp); break; case TCP_REPAIR: diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index be8e09d2c6b1..e042cabb695e 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -99,6 +99,7 @@ int sysctl_tcp_thin_dupack __read_mostly; int sysctl_tcp_moderate_rcvbuf __read_mostly = 1; int sysctl_tcp_abc __read_mostly; +int sysctl_tcp_early_retrans __read_mostly = 2; #define FLAG_DATA 0x01 /* Incoming frame contained data. */ #define FLAG_WIN_UPDATE 0x02 /* Incoming ACK was a window update. */ @@ -906,6 +907,7 @@ static void tcp_init_metrics(struct sock *sk) if (dst_metric(dst, RTAX_REORDERING) && tp->reordering != dst_metric(dst, RTAX_REORDERING)) { tcp_disable_fack(tp); + tcp_disable_early_retrans(tp); tp->reordering = dst_metric(dst, RTAX_REORDERING); } @@ -988,6 +990,9 @@ static void tcp_update_reordering(struct sock *sk, const int metric, #endif tcp_disable_fack(tp); } + + if (metric > 0) + tcp_disable_early_retrans(tp); } /* This must be called before lost_out is incremented */ @@ -2492,6 +2497,16 @@ static int tcp_time_to_recover(struct sock *sk) tcp_is_sack(tp) && !tcp_send_head(sk)) return 1; + /* Trick#6: TCP early retransmit, per RFC5827. To avoid spurious + * retransmissions due to small network reorderings, we implement + * Mitigation A.3 in the RFC and delay the retransmission for a short + * interval if appropriate. + */ + if (tp->do_early_retrans && !tp->retrans_out && tp->sacked_out && + (tp->packets_out == (tp->sacked_out + 1) && tp->packets_out < 4) && + !tcp_may_send_now(sk)) + return 1; + return 0; } diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c index 3cabafb5cdd1..6f6a91832826 100644 --- a/net/ipv4/tcp_minisocks.c +++ b/net/ipv4/tcp_minisocks.c @@ -482,6 +482,7 @@ struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, newtp->sacked_out = 0; newtp->fackets_out = 0; newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH; + tcp_enable_early_retrans(newtp); /* So many TCP implementations out there (incorrectly) count the * initial SYN frame in their delayed-ACK and congestion control -- 2.20.1