1. 概述

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TCP的连接建立过程通常称为三次握手(three-way handshake)，即客户端与服务端彼此交换用于建立连接的初始SYN segment与ACK。

连接建立过程中，任一端出现接收不到对端ACK时都将导致初始SYN segment重传，Linux内核为该情况设置了重传间隔与重传次数：

1. 重传间隔：若当前为第N次重传，则与上一次发送或重传的间隔时间为2的(N -1)次方秒，但最长间隔不超过120秒；内核的TCP_TIMEOUT_INIT/TCP_RTO_MIN/TCP_RTO_MAX宏分别定义了间隔时间的初始值/最小值/最大值
2. 重传次数：客户端由tcp_syn_retries指定，对应的proc文件为/proc/sys/net/ipv4/tcp_syn_retries，sysctl参数为net.ipv4.tcp_syn_retries；服务端由tcp_synack_retries指定，对应的proc文件为/proc/sys/net/ipv4/tcp_synack_retries，sysctl参数为net.ipv4.tcp_synack_retries

2. 示例

操作系统：CentOS Linux release 7.5.1804 (Core)

内核版本：3.10.0-862.2.3.el7.x86_64

服务端IP：22.99.22.111

客户端IP：22.99.22.101

示例程序详见 Linux TCP那些事儿之被动套接字（一）

服务端启动程序：

# ./tcp_server -b 1024 &

客户端启动程序：

# ./tcp_client -i 22.99.22.111 -p 8888 -n 1 &

服务端与客户端分别启动tcpdump观察连接情况：

# tcpdump -i eth0 -nn -S host 22.99.22.111 and host 22.99.22.101 and tcp

2.1 - 正常连接

服务端与客户端的tcpdump输出：

21:40:49.989951 IP 22.99.22.101.38118 > 22.99.22.111.8888: Flags [S], seq 231965702, win 29200, options [mss 1460,sackOK,TS val 11012209 ecr 0,nop,wscale 7], length 0 21:40:49.989998 IP 22.99.22.111.8888 > 22.99.22.101.38118: Flags [S.], seq 513997240, ack 231965703, win 28960, options [mss 1460,sackOK,TS val 14396437 ecr 11012209,nop,wscale 7], length 0 21:40:49.990223 IP 22.99.22.101.38118 > 22.99.22.111.8888: Flags [.], ack 513997241, win 229, options [nop,nop,TS val 11012210 ecr 14396437], length 0

21:40:49.991145 IP 22.99.22.101.38118 > 22.99.22.111.8888: Flags [S], seq 231965702, win 29200, options [mss 1460,sackOK,TS val 11012209 ecr 0,nop,wscale 7], length 0 21:40:49.991499 IP 22.99.22.111.8888 > 22.99.22.101.38118: Flags [S.], seq 513997240, ack 231965703, win 28960, options [mss 1460,sackOK,TS val 14396437 ecr 11012209,nop,wscale 7], length 0 21:40:49.991541 IP 22.99.22.101.38118 > 22.99.22.111.8888: Flags [.], ack 513997241, win 229, options [nop,nop,TS val 11012210 ecr 14396437], length 0

查看连接状态，两端均显示连接已建立

# ss -atn | grep 8888 LISTEN     1      128          *:8888                     *:* ESTAB      0      0      22.99.22.111:8888               22.99.22.101:38118

# ss -atn | grep 8888 ESTAB      0      0      22.99.22.101:38118              22.99.22.111:8888

2.2 - 客户端超时

在服务端添加防火墙规则，丢弃向客户端发送的syn+ack

# iptables -I OUTPUT -p tcp --destination 22.99.22.101 --tcp-flags ALL ACK,SYN -j DROP # # iptables -L Chain INPUT (policy ACCEPT) target     prot opt source               destination  Chain FORWARD (policy ACCEPT) target     prot opt source               destination  Chain OUTPUT (policy ACCEPT) target     prot opt source               destination DROP       tcp  --  anywhere             22.99.22.101         tcp flags:FIN,SYN,RST,PSH,ACK,URG/SYN,ACK

客户端设置重传次数，默认为6，修改为4

# cat /proc/sys/net/ipv4/tcp_syn_retries 6 # echo 4 > /proc/sys/net/ipv4/tcp_syn_retries # cat /proc/sys/net/ipv4/tcp_syn_retries 4

再次启动服务端与客户端程序，两端的tcpdump输出均显示客户端发生了4次初始SYN的重传，每次间隔分别为1s/2s/4s/8s，总间隔为15s

21:49:29.425637 IP 22.99.22.101.38120 > 22.99.22.111.8888: Flags [S], seq 1034503734, win 29200, options [mss 1460,sackOK,TS val 11531645 ecr 0,nop,wscale 7], length 0 21:49:30.428747 IP 22.99.22.101.38120 > 22.99.22.111.8888: Flags [S], seq 1034503734, win 29200, options [mss 1460,sackOK,TS val 11532648 ecr 0,nop,wscale 7], length 0 21:49:32.433200 IP 22.99.22.101.38120 > 22.99.22.111.8888: Flags [S], seq 1034503734, win 29200, options [mss 1460,sackOK,TS val 11534652 ecr 0,nop,wscale 7], length 0 21:49:36.437044 IP 22.99.22.101.38120 > 22.99.22.111.8888: Flags [S], seq 1034503734, win 29200, options [mss 1460,sackOK,TS val 11538656 ecr 0,nop,wscale 7], length 0 21:49:44.454342 IP 22.99.22.101.38120 > 22.99.22.111.8888: Flags [S], seq 1034503734, win 29200, options [mss 1460,sackOK,TS val 11546673 ecr 0,nop,wscale 7], length 0

21:49:29.426515 IP 22.99.22.101.38120 > 22.99.22.111.8888: Flags [S], seq 1034503734, win 29200, options [mss 1460,sackOK,TS val 11531645 ecr 0,nop,wscale 7], length 0 21:49:30.429581 IP 22.99.22.101.38120 > 22.99.22.111.8888: Flags [S], seq 1034503734, win 29200, options [mss 1460,sackOK,TS val 11532648 ecr 0,nop,wscale 7], length 0 21:49:32.434015 IP 22.99.22.101.38120 > 22.99.22.111.8888: Flags [S], seq 1034503734, win 29200, options [mss 1460,sackOK,TS val 11534652 ecr 0,nop,wscale 7], length 0 21:49:36.437559 IP 22.99.22.101.38120 > 22.99.22.111.8888: Flags [S], seq 1034503734, win 29200, options [mss 1460,sackOK,TS val 11538656 ecr 0,nop,wscale 7], length 0 21:49:44.454826 IP 22.99.22.101.38120 > 22.99.22.111.8888: Flags [S], seq 1034503734, win 29200, options [mss 1460,sackOK,TS val 11546673 ecr 0,nop,wscale 7], length 0

查看连接状态，服务端的syn+ack由于被自身drop而一直处于LISTEN状态，客户端在重传期间处于SYN-SENT状态，且超过重传次数后因连接超时而返回

# ss -atn | grep 8888 LISTEN     0      128          *:8888                     *:*

# ss -atn | grep 8888 SYN-SENT   0      1      22.99.22.101:38120              22.99.22.111:8888 # connect() failed: Connection timed out

2.3 - 服务端超时

在客户端添加防火墙规则，丢弃向服务端发送的ack

# iptables -I OUTPUT -p tcp --destination 22.99.22.111 --dport 8888 --tcp-flags ALL ACK -j DROP # # iptables -L Chain INPUT (policy ACCEPT) target     prot opt source               destination  Chain FORWARD (policy ACCEPT) target     prot opt source               destination  Chain OUTPUT (policy ACCEPT) target     prot opt source               destination DROP       tcp  --  anywhere             22.99.22.111         tcp dpt:ddi-tcp-1 flags:FIN,SYN,RST,PSH,ACK,URG/ACK

客户端设置重传次数，默认为5，修改为3

# sysctl net.ipv4.tcp_synack_retries net.ipv4.tcp_synack_retries = 5 # [root@localhost code]# sysctl -w net.ipv4.tcp_synack_retries=3 net.ipv4.tcp_synack_retries = 3 # # sysctl net.ipv4.tcp_synack_retries net.ipv4.tcp_synack_retries = 3

再次启动服务端与客户端程序，两端的tcpdump输出均显示服务端发生了3次初始SYN的重传，每次间隔分别为1s/2s/4s，总间隔为7s

22:03:56.982987 IP 22.99.22.101.38122 > 22.99.22.111.8888: Flags [S], seq 985103784, win 29200, options [mss 1460,sackOK,TS val 12399201 ecr 0,nop,wscale 7], length 0 22:03:56.983034 IP 22.99.22.111.8888 > 22.99.22.101.38122: Flags [S.], seq 703914964, ack 985103785, win 28960, options [mss 1460,sackOK,TS val 15783430 ecr 12399201,nop,wscale 7], length 0 22:03:57.985014 IP 22.99.22.111.8888 > 22.99.22.101.38122: Flags [S.], seq 703914964, ack 985103785, win 28960, options [mss 1460,sackOK,TS val 15784432 ecr 12399201,nop,wscale 7], length 0 22:03:59.993905 IP 22.99.22.111.8888 > 22.99.22.101.38122: Flags [S.], seq 703914964, ack 985103785, win 28960, options [mss 1460,sackOK,TS val 15786441 ecr 12399201,nop,wscale 7], length 0 22:04:04.008496 IP 22.99.22.111.8888 > 22.99.22.101.38122: Flags [S.], seq 703914964, ack 985103785, win 28960, options [mss 1460,sackOK,TS val 15790456 ecr 12399201,nop,wscale 7], length 0

22:03:56.983313 IP 22.99.22.101.38122 > 22.99.22.111.8888: Flags [S], seq 985103784, win 29200, options [mss 1460,sackOK,TS val 12399201 ecr 0,nop,wscale 7], length 0 22:03:56.983746 IP 22.99.22.111.8888 > 22.99.22.101.38122: Flags [S.], seq 703914964, ack 985103785, win 28960, options [mss 1460,sackOK,TS val 15783430 ecr 12399201,nop,wscale 7], length 0 22:03:57.985870 IP 22.99.22.111.8888 > 22.99.22.101.38122: Flags [S.], seq 703914964, ack 985103785, win 28960, options [mss 1460,sackOK,TS val 15784432 ecr 12399201,nop,wscale 7], length 0 22:03:59.994857 IP 22.99.22.111.8888 > 22.99.22.101.38122: Flags [S.], seq 703914964, ack 985103785, win 28960, options [mss 1460,sackOK,TS val 15786441 ecr 12399201,nop,wscale 7], length 0 22:04:04.009283 IP 22.99.22.111.8888 > 22.99.22.101.38122: Flags [S.], seq 703914964, ack 985103785, win 28960, options [mss 1460,sackOK,TS val 15790456 ecr 12399201,nop,wscale 7], length 0

查看连接状态，服务端的初始SYN被客户端丢弃而在重传期间一直处于SYN-RECV状态，客户端已接收到服务端对初始SYN的ACK确认而处于ESTAB状态，导致半开(half-open)连接

# ss -atn | grep 8888 LISTEN     0      128          *:8888                     *:* SYN-RECV   0      0      22.99.22.111%if355612354:8888               22.99.22.101:38122

# ss -atn | grep 8888 ESTAB      0      0      22.99.22.101:38122              22.99.22.111:8888