TCP 四次握手 [英] TCP four-way handshake

问题描述

四路握手连接终止可以减少到三路甚至两路一.三向握手连接建立是否有可能扩展到四向?

Four-way handshake connection termination can be reduced to three-way and even two way one. Is it possible the three-way handshake connection establishment would be extended to four-way?

SYN=>
<=ACK
<=SYN
ACK=>

推荐答案

鉴于 SYN 和 ACK 的语义，应该可以在不同的数据包中发送 SYN+ACK 并且延迟握手.例如.客户端发送 SYN，服务器回复 ACK 以确认客户端希望建立新连接，但它尚未授予该愿望.稍后服务器发送 SYN 并从客户端获取匹配的 ACK 并建立连接.但我怀疑是否有人以这种方式建立连接，而且可能是某些操作系统会在其上发出吱吱声.

Given the semantics of SYN and ACK it should be possible to send SYN+ACK in different packets and those delay the handshake. E.g. client sends a SYN, server replies with an ACK to acknowledge the wish of the client for a new connection, but it does not grant the wish yet. Later the server sends a SYN and gets the matching ACK back from the client and the connection is established. But I doubt that anybody does connection establishment this way and it might be, that some OS will croak on it.

但是，四次握手还有另一种情况，但是数据包的顺序不同.如果双方尝试同时与另一方建立连接，则可能会发生这种情况，例如两者都向对等方发送 SYN，并返回 ACK.它在 RFC 793 (TCP) 第 3.4 节中进行了描述.但我怀疑您永远不会看到这样的握手，因为它不适合一端等待连接而另一端进行连接的典型客户端-服务器-场景.

But, there is another scenario for a four-way-handshake, however with a different ordering of the packets. It could happen, if both side try to establish a connection to the other side at the same time, e.g. both send a SYN to the peer, and get an ACK back. It is described in the RFC 793 (TCP) section 3.4. But I doubt you will ever see such a handshake, because it does not fit into the typical client-server-scenario where one end is waiting for connects and the other end does the connect.

你想象的握手存在，它被称为分裂握手".请参阅 http://hackmageddon.com/2011/04/17/tcp-split-handshake-attack-explained/ .正如我所料，它并未得到普遍支持.

the handshake you envision exists and it is called "split handshake". See http://hackmageddon.com/2011/04/17/tcp-split-handshake-attack-explained/ . And like I expected, it is not universally supported.

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