什么计时器可以防止TCP窗口更新数据包丢失

Question

下面是我在连接末尾看到的数据包序列（需要更多数据）：

No.     Time            Source                Destination           Protocol Length Info
 122160 21:24:37.421824 192.168.15.121        72.21.81.253          TCP      60     41200 > http [ACK] Seq=462 Ack=2984241 Win=5152 Len=0
 122162 21:24:37.445528 72.21.81.253          192.168.15.121        TCP      1514   [TCP segment of a reassembled PDU]
 122163 21:24:37.445796 72.21.81.253          192.168.15.121        TCP      1514   [TCP segment of a reassembled PDU]
 122164 21:24:37.446087 72.21.81.253          192.168.15.121        TCP      1514   [TCP segment of a reassembled PDU]
 122171 21:24:37.481802 192.168.15.121        72.21.81.253          TCP      60     41200 > http [ACK] Seq=462 Ack=2988621 Win=784 Len=0
 122184 21:24:37.744838 72.21.81.253          192.168.15.121        TCP      838    [TCP Window Full] [TCP segment of a reassembled PDU]
 122185 21:24:37.745048 192.168.15.121        72.21.81.253          TCP      60     [TCP ZeroWindow] 41200 > http [ACK] Seq=462 Ack=2989405 Win=0 Len=0
 122190 21:24:38.014841 72.21.81.253          192.168.15.121        TCP      60     [TCP Keep-Alive] http > 41200 [ACK] Seq=2989404 Ack=462 Win=15872 Len=0
 122191 21:24:38.014993 192.168.15.121        72.21.81.253          TCP      60     [TCP ZeroWindow] 41200 > http [ACK] Seq=462 Ack=2989405 Win=0 Len=0
 122232 21:24:38.534437 72.21.81.253          192.168.15.121        TCP      60     [TCP Keep-Alive] http > 41200 [ACK] Seq=2989404 Ack=462 Win=15872 Len=0
 122233 21:24:38.534599 192.168.15.121        72.21.81.253          TCP      60     [TCP ZeroWindow] 41200 > http [ACK] Seq=462 Ack=2989405 Win=0 Len=0
 122314 21:24:39.564525 72.21.81.253          192.168.15.121        TCP      60     [TCP Keep-Alive] http > 41200 [ACK] Seq=2989404 Ack=462 Win=15872 Len=0
 122315 21:24:39.564680 192.168.15.121        72.21.81.253          TCP      60     [TCP ZeroWindow] 41200 > http [ACK] Seq=462 Ack=2989405 Win=0 Len=0
 122361 21:24:43.403052 192.168.15.121        72.21.81.253          TCP      60     [TCP Window Update] 41200 > http [ACK] Seq=462 Ack=2989405 Win=119904 Len=0
 122892 21:25:45.161896 192.168.15.121        72.21.81.253          TCP      60     41200 > http [FIN, ACK] Seq=462 Ack=2989405 Win=186720 Len=0
 122902 21:25:45.373289 192.168.15.121        72.21.81.253          TCP      60     41200 > http [FIN, ACK] Seq=462 Ack=2989405 Win=186720 Len=0
 122927 21:25:45.813267 192.168.15.121        72.21.81.253          TCP      60     41200 > http [FIN, ACK] Seq=462 Ack=2989405 Win=186720 Len=0
 122936 21:25:46.693275 192.168.15.121        72.21.81.253          TCP      60     41200 > http [FIN, ACK] Seq=462 Ack=2989405 Win=186720 Len=0
 122956 21:25:48.453337 192.168.15.121        72.21.81.253          TCP      60     41200 > http [FIN, ACK] Seq=462 Ack=2989405 Win=186720 Len=0
 123009 21:25:51.983392 192.168.15.121        72.21.81.253          TCP      60     41200 > http [FIN, ACK] Seq=462 Ack=2989405 Win=186720 Len=0
 123061 21:25:59.033566 192.168.15.121        72.21.81.253          TCP      60     41200 > http [FIN, ACK] Seq=462 Ack=2989405 Win=186720 Len=0
 123262 21:26:13.153852 192.168.15.121        72.21.81.253          TCP      60     41200 > http [FIN, ACK] Seq=462 Ack=2989405 Win=186720 Len=0
 123460 21:26:41.394469 192.168.15.121        72.21.81.253          TCP      60     41200 > http [FIN, ACK] Seq=462 Ack=2989405 Win=186720 Len=0

接收者在21:24:43重新打开窗户，再也没有听到发送者的声音。一分钟后，接收器超时连接（关闭由应用程序启动），并发送一系列未确认的FIN-ACK。

看起来与发送方的通信就这样丢失了（捕获是在接收方的网络上进行的）。如果不是，那么是否应该一直期待对Fin-ACK的确认，即使在一段足够长的时间使对等体忘记了连接之后也是如此？

Answer 1

正如您注意到的，一旦接收窗口中的空间可用，接收方就会发送一个新的ACK并更新窗口大小。

为了处理ACK丢失的情况，发送方保留一个“持久定时器”，该定时器偶尔会重新发送一个数据包（“窗口探测”），以便测试水并查看是否确实存在未报告的接收空间。

持久定时器的值不是显式指定的，而是计算的往返时间的函数以及一些指数回退。RFC1122第4.2.2.14节提供了完整的详细信息，请访问:http://tools.ietf.org/html/RFC1122#page-92