2.11 TCP Setup and Teardown

3-way handshake(3步握手), simultaneous open(同步打开), and the TCP state machine(TCP状态机)

问题

  • 可靠通信通常受益于连接两端拥有的某种状态

  • 连接建立

    • 如何设置此状态?

  • 连接关闭

    • 如何清理(重用端口等)

Connection Setup

用到的东西： Connection Setup

wireshark

Filter : tcp.port == 80 and ip.addr == XXX.XXX.XXX.XXX

Connection Teardown

Connection Teardown

  • 两边都得发送 FIN

  • FIN 表示没有更多需要传递的数据

  • 关闭下的交换：

    • A->B

    • B->A

    • B->A

    • A->B

  • Can also have simultaneous(同步) close

  • Can A and B forget about closed socket after final message?

Problems with closed socket What if final ack is lost in the network? What if the same port pair is immediately reused for a new connection?

Cleaning Up safely

  • Problems with closed socket

    • 如果fin ack在网络中丢失了怎么办?

      • 如果第二步 closed 的时候 ack 丢失就直接关闭，对方永远不会知道连接是否已经关闭

    • 如果 the same port pair 立即被重新用于一个新连接，会发生什么情况?

      • corrupt the data because sequence number of spaces overlap(重叠)

  • 解决方法： “active” closer 走向 TIME WAIT

    • Active close is sending fin before receiving one.

    • Keep socket around for 2MSL (twice the"maximum segment lifetime")

  • 会对 severs 造成问题

    • OS 有很多 sockets 在 TIME WAIT,slow things down

    • 黑客: Can send RST and delete socket, set SO_LINGER socket option to time 0

    • OS 不会让你重新启动服务器，因为端口仍然在使用(So_REUSEADDR options让你重新绑定使用的端口号)

      完整的TCP FSM

      TCP

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