将sysctl tcp_retries1设置为3时，TCP数据包被重传7次-为什么？

Ubuntu 12.04

我试图更好地理解，当目标未收到确认时，TCP将尝试重传数据包多少次。阅读了tcp手册页后，似乎很明显这是由sysctl tcp_retries1控制的：

tcp_retries1 (integer; default: 3)
           The number of times TCP will attempt to retransmit a  packet  on
           an  established connection normally, without the extra effort of
           getting the network layers involved.  Once we exceed this number
           of retransmits, we first have the network layer update the route
           if possible before each new retransmit.  The default is the  RFC
           specified minimum of 3.

我的系统设置为默认值3：

# cat /proc/sys/net/ipv4/tcp_retries1 
3

为了测试这一点，我通过ssh从系统A（172.16.249.138）连接到系统B（172.16.249.137），并在控制台上启动了一个简单的打印循环。然后，在发生此通信时，我突然断开了B与网络的连接。

在另一个终端中，我在系统A上运行“ tcpdump host 172.16.249.137”。以下是输出中的相关行（为清楚起见添加了行号）。

00: ...
01: 13:29:46.994715 IP 172.16.249.138.50489 > 172.16.249.137.ssh: Flags [.], ack 5989441, win 80, options [nop,nop,TS val 1957286 ecr 4294962520], length 0
02: 13:29:46.995084 IP 172.16.249.138.50489 > 172.16.249.137.ssh: Flags [.], ack 5989441, win 186, options [nop,nop,TS val 1957286 ecr 4294962520], length 0    
03: 13:29:47.040360 IP 172.16.249.138.50489 > 172.16.249.137.ssh: Flags [P.], seq 29136:29184, ack 5989441, win 186, options [nop,nop,TS val 1957298 ecr 4294962520], length 48
04: 13:29:47.086552 IP 172.16.249.138.50489 > 172.16.249.137.ssh: Flags [.], ack 5989441, win 376, options [nop,nop,TS val 1957309 ecr 4294962520], length 0
05: 13:29:47.680608 IP 172.16.249.138.50489 > 172.16.249.137.ssh: Flags [P.], seq 29136:29184, ack 5989441, win 376, options [nop,nop,TS val 1957458 ecr 4294962520], length 48
06: 13:29:48.963721 IP 172.16.249.138.50489 > 172.16.249.137.ssh: Flags [P.], seq 29136:29184, ack 5989441, win 376, options [nop,nop,TS val 1957779 ecr 4294962520], length 48
07: 13:29:51.528564 IP 172.16.249.138.50489 > 172.16.249.137.ssh: Flags [P.], seq 29136:29184, ack 5989441, win 376, options [nop,nop,TS val 1958420 ecr 4294962520], length 48
08: 13:29:56.664384 IP 172.16.249.138.50489 > 172.16.249.137.ssh: Flags [P.], seq 29136:29184, ack 5989441, win 376, options [nop,nop,TS val 1959704 ecr 4294962520], length 48
09: 13:30:06.936480 IP 172.16.249.138.50489 > 172.16.249.137.ssh: Flags [P.], seq 29136:29184, ack 5989441, win 376, options [nop,nop,TS val 1962272 ecr 4294962520], length 48
10: 13:30:27.480381 IP 172.16.249.138.50489 > 172.16.249.137.ssh: Flags [P.], seq 29136:29184, ack 5989441, win 376, options [nop,nop,TS val 1967408 ecr 4294962520], length 48
11: 13:31:08.504033 IP 172.16.249.138.50489 > 172.16.249.137.ssh: Flags [P.], seq 29136:29184, ack 5989441, win 376, options [nop,nop,TS val 1977664 ecr 4294962520], length 48
12: 13:31:13.512437 ARP, Request who-has 172.16.249.137 tell 172.16.249.138, length 28
13: 13:31:14.512336 ARP, Request who-has 172.16.249.137 tell 172.16.249.138, length 28
14: 13:31:15.512241 ARP, Request who-has 172.16.249.137 tell 172.16.249.138, length 28

如果我正确地解释了这个（可能不是），那么系统B永远不会确认行3的数据包。然后，A每次增加其重传计时器（每次将其大约加倍），都重试发送此数据包7次（第5-11行）。时间）。

为什么将数据包重传7次而不是3次？

注意：我在注意到一些pcap文件后执行了此正式测试，这些文件通过HTTP连接进行了6-7次重传，因此重传的次数似乎并不特定于SSH。

我相信您通过杀死.137服务器上的连接创建了一个孤立套接字。因此，使用的内核参数将是tcp_orphan_retries-通用linux默认值为7。

您可以在此处获得有关所创建条件和结果的描述：http : //www.linuxinsight.com/proc_sys_net_ipv4_tcp_orphan_retries.html