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Linux TCP 参数设置

查询tcp相关内核参数

sysctl -a|grep ipv4|grep -i --color tcp

[root@localhost ~]# sysctl -a|grep ipv4|grep -i --color tcp
sysctl: reading key "net.ipv6.conf.all.stable_secret"
sysctl: reading key "net.ipv6.conf.default.stable_secret"
sysctl: reading key "net.ipv6.conf.eth0.stable_secret"
sysctl: reading key "net.ipv6.conf.lo.stable_secret"
net.ipv4.tcp_abort_on_overflow = 0
net.ipv4.tcp_adv_win_scale = 1
net.ipv4.tcp_allowed_congestion_control = reno cubic
net.ipv4.tcp_app_win = 31
net.ipv4.tcp_autocorking = 1
net.ipv4.tcp_available_congestion_control = reno cubic
net.ipv4.tcp_available_ulp =
net.ipv4.tcp_base_mss = 1024
net.ipv4.tcp_challenge_ack_limit = 1000
net.ipv4.tcp_comp_sack_delay_ns = 1000000
net.ipv4.tcp_comp_sack_nr = 44
net.ipv4.tcp_congestion_control = cubic
net.ipv4.tcp_dsack = 1
net.ipv4.tcp_early_demux = 1
net.ipv4.tcp_early_retrans = 3
net.ipv4.tcp_ecn = 2
net.ipv4.tcp_ecn_fallback = 1
net.ipv4.tcp_fack = 0
net.ipv4.tcp_fastopen = 1
net.ipv4.tcp_fastopen_blackhole_timeout_sec = 3600
net.ipv4.tcp_fastopen_key = 00000000-00000000-00000000-00000000
net.ipv4.tcp_fin_timeout = 60
net.ipv4.tcp_frto = 2
net.ipv4.tcp_fwmark_accept = 0
net.ipv4.tcp_invalid_ratelimit = 500
net.ipv4.tcp_keepalive_intvl = 75
net.ipv4.tcp_keepalive_probes = 9
net.ipv4.tcp_keepalive_time = 7200
net.ipv4.tcp_l3mdev_accept = 0
net.ipv4.tcp_limit_output_bytes = 262144
net.ipv4.tcp_low_latency = 0
net.ipv4.tcp_max_orphans = 65536
net.ipv4.tcp_max_reordering = 300
net.ipv4.tcp_max_syn_backlog = 512
net.ipv4.tcp_max_tw_buckets = 65536
net.ipv4.tcp_mem = 10908        14547   21816
net.ipv4.tcp_min_rtt_wlen = 300
net.ipv4.tcp_min_snd_mss = 48
net.ipv4.tcp_min_tso_segs = 2
net.ipv4.tcp_moderate_rcvbuf = 1
net.ipv4.tcp_mtu_probing = 0
net.ipv4.tcp_no_metrics_save = 0
net.ipv4.tcp_notsent_lowat = 4294967295
net.ipv4.tcp_orphan_retries = 0
net.ipv4.tcp_pacing_ca_ratio = 120
net.ipv4.tcp_pacing_ss_ratio = 200
net.ipv4.tcp_probe_interval = 600
net.ipv4.tcp_probe_threshold = 8
net.ipv4.tcp_recovery = 1
net.ipv4.tcp_reordering = 3
net.ipv4.tcp_retrans_collapse = 1
net.ipv4.tcp_retries1 = 3
net.ipv4.tcp_retries2 = 15
net.ipv4.tcp_rfc1337 = 0
net.ipv4.tcp_rmem = 4096        87380   6291456
net.ipv4.tcp_sack = 1
net.ipv4.tcp_slow_start_after_idle = 1
net.ipv4.tcp_stdurg = 0
net.ipv4.tcp_syn_retries = 6
net.ipv4.tcp_synack_retries = 5
net.ipv4.tcp_syncookies = 1
net.ipv4.tcp_thin_linear_timeouts = 0
net.ipv4.tcp_timestamps = 1
net.ipv4.tcp_tso_win_divisor = 3
net.ipv4.tcp_tw_reuse = 2
net.ipv4.tcp_window_scaling = 1
net.ipv4.tcp_wmem = 4096        16384   4194304
net.ipv4.tcp_workaround_signed_windows = 0

#sysctl -a | grep keepalive
net.ipv4.tcp_keepalive_intvl = 75
net.ipv4.tcp_keepalive_probes = 9
net.ipv4.tcp_keepalive_time = 7200
sysctl: reading key "net.ipv6.conf.all.stable_secret"
sysctl: reading key "net.ipv6.conf.default.stable_secret"
sysctl: reading key "net.ipv6.conf.eth0.stable_secret"
sysctl: reading key "net.ipv6.conf.lo.stable_secret"

在TCP链接建立之后，如果应用程序或者上层协议一直不发送数据，或者隔很长时间才发送一次数据，当链接很久没有数据报文传输时如何去确定对方还在线，到底是掉线了还是确实没有数据传输，链接还需不需要保持，这种情况在TCP协议设计中是需要考虑到的。

TCP协议通过keepalive这种巧妙的方式去解决这个问题，其原理就是TCP内嵌的一个心跳包。当超过一段时间之后，TCP自动发送一个数据为空的报文给对方，如果对方回应了这个报文，说明对方还在线，链接可以继续保持，如果对方没有报文返回，并且重试了多次之后则认为链接丢失，没有必要保持链接。

tcp_keepalive_time
一个连接需要TCP开始发送keepalive探测数据包之前的空闲时间，以秒为单位。

tcp_keepalive_probes
发送TCP keepalive探测数据包的最大数量，默认是9.如果发送9个keepalive探测包后对端仍然没有响应，就关掉这个连接。

tcp_keepalive_intvl
发送两个TCP keepalive探测数据包的间隔时间，默认是75秒。

下面是Linux默认值，可以看出在默认参数下，若tcp对端不通知就掉线，需要经过7200+9*75=7875秒=131.25分钟=2.1875小时才能确认对方掉线。

$ sysctl -a | grep keepalive
net.ipv4.tcp_keepalive_time = 7200
net.ipv4.tcp_keepalive_probes = 9
net.ipv4.tcp_keepalive_intvl = 75

1. client 端仍然存在，网络连接状况良好。此时 client 端会返回一个 ACK 。server 端接收到 ACK 后重置计时器(复位存活定时器)，在 2 小时后再发送探测。如果 2 小时内连接上有数据传输，那么在该时间基础上向后推延 2 个小时。

2. 客户端异常关闭，或是网络断开。在这两种情况下， client 端都不会响应。服务器没有收到对其发出探测的响应，并且在一定时间后重复发送 keep-alive packet ，并且重复发送一定次数。重复发送次数后没有收到响应，则会进入TIME_WAIT状态，此状态下等待2*MSL时间后连接关闭。MSL，即Maximum Segment Lifetime，是一个数据分片（报文）在网络中能够生存的最长时间，在RFC 793中定义MSL通常为2分钟。

在Linux系统中查看MSL值，默认为1分钟，如下图：

bash-4.1$ cat /proc/sys/net/ipv4/tcp_fin_timeout
60

# sysctl -a | grep tcp_tw
net.ipv4.tcp_tw_reuse = 2
sysctl: reading key "net.ipv6.conf.all.stable_secret"
sysctl: reading key "net.ipv6.conf.default.stable_secret"
sysctl: reading key "net.ipv6.conf.eth0.stable_secret"
sysctl: reading key "net.ipv6.conf.lo.stable_secret"

参考

Linux 下的 TCP参数优化
参考URL: https://www.91kuzhan.com/article-16468-1.html
专栏文章 linux 下 TCP 参数优化详解 (1)
参考URL: https://testerhome.com/topics/25431