In the above command a value of 0 is specified, which sets the number of requests allowed down a single socket to unlimited.
上面的命令中指定了值0，将允许通过单个套接字发送的请求数量设置为无限大。
tcp_usrreq () now calls soisconnecting (), which sets the state of the socket on the client host as SYN_SENT.
tcp_usrreq () 现在调用 soisconnecting ()，它可以将客户端主机上的套接字的状态设置为 SYN_SENT。
Lists allow the same socket to appear more than once in each list, but doing so doesn't make sense: conceptually, what is passed are sets of sockets.
列表中可以允许同一个socket多次出现，但是这样做一点意义都没有：从概念上讲，传入的应该是由socket组成的set。
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