tcp 重传超时次数

数据被重发以后若还是收不到应答，则进行再次发送。此时等待确认应答时间会以2倍、4倍的指数函数延长。
此外，数据也不会被无限、反复的重发。达到一定的重发次数之后，如果仍然没有任何确认应答返回，就会判断为网络或者对端主机发生了异常，强制关闭连接。

linux的设置

最小重传时间是200ms
最大重传时间是120s
重传次数为15

TCP retransmits an unacknowledged packet up to tcp_retries2 sysctl setting times (defaults to 15) using an exponential backoff timeout for which each retransmission timeout is between TCP_RTO_MIN (200 ms) and TCP_RTO_MAX (120 seconds). Once the 15th retry expires (by default), the TCP stack will notify the layers above (ie. app) of a broken connection.

The value of TCP_RTO_MIN and TCP_RTO_MAX is hardcoded in the Linux kernel and defined by the following constants:

#define TCP_RTO_MAX ((unsigned)(120*HZ))
#define TCP_RTO_MIN ((unsigned)(HZ/5))

Linux 2.6+ uses HZ of 1000ms, so TCP_RTO_MIN is ~200 ms and TCP_RTO_MAX is ~120 seconds. Given a default value of tcp_retries set to 15, it means that it takes 924.6 seconds before a broken network link is notified to the upper layer (ie. application), since the connection is detected as broken when the last (15th) retry expires.

image.png

From the Linux kernel doc
tcp_retries2 - INTEGER
This value influences the timeout of an alive TCP connection,
when RTO retransmissions remain unacknowledged.
Given a value of N, a hypothetical TCP connection following
exponential backoff with an initial RTO of TCP_RTO_MIN would
retransmit N times before killing the connection at the (N+1)th RTO.

The default value of 15 yields a hypothetical timeout of 924.6
seconds and is a lower bound for the effective timeout.
TCP will effectively time out at the first RTO which exceeds the
hypothetical timeout.

RFC 1122 recommends at least 100 seconds for the timeout,
which corresponds to a value of at least 8.

参考地址
https://pracucci.com/linux-tcp-rto-min-max-and-tcp-retries2.html