TCP 之 TCP_NEW_SYN_RECV状态

概述

以前的TCP请求控制块没有独立的状态，而是依赖于他们的父控制块的状态，也就是TCP_LISTEN状态，现在要把请求控制块加入到全局的ehash中，所以需要一个状态，而TCP_SYN_RECV状态被fast open sokets使用了，所以新加了一个TCP_NEW_SYN_RECV状态；

以下截取在kernel git，地址：https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/log/?qt=grep&q=TCP_NEW_SYN_RECV

TCP_SYN_RECV state is currently used by fast open sockets.

Initial TCP requests (the pseudo sockets created when a SYN is received)
are not yet associated to a state. They are attached to their parent,
and the parent is in TCP_LISTEN state.

This commit adds TCP_NEW_SYN_RECV state, so that we can convert
TCP stack to a different schem gradually.

------------------------
We need to identify request sock when they'll be visible in
global ehash table.

ireq_state is an alias to req.__req_common.skc_state.

Its value is set to TCP_NEW_SYN_RECV

代码分析

以下代码以服务器端处于LISTEN状态，等待接收syn为前提；

接收SYN

在服务器接收了syn之后，会调用tcp_conn_request来处理连接请求，其中调用inet_reqsk_alloc来创建请求控制块，可见请求控制块的ireq_state被初始化为TCP_NEW_SYN_RECV；

struct request_sock *inet_reqsk_alloc(const struct request_sock_ops *ops,
                      struct sock *sk_listener,
                      bool attach_listener)
{
    struct request_sock *req = reqsk_alloc(ops, sk_listener,
                           attach_listener);

    if (req) {
        struct inet_request_sock *ireq = inet_rsk(req);

        kmemcheck_annotate_bitfield(ireq, flags);
        ireq->opt = NULL;
#if IS_ENABLED(CONFIG_IPV6)
        ireq->pktopts = NULL;
#endif
        atomic64_set(&ireq->ir_cookie, 0);
        ireq->ireq_state = TCP_NEW_SYN_RECV;
        write_pnet(&ireq->ireq_net, sock_net(sk_listener));
        ireq->ireq_family = sk_listener->sk_family;
    }

    return req;
}

该部分详细分析请移步：<TCP被动打开 之 第一次握手-接收SYN>；

接收ACK

tcp_v4_rcv函数中会对TCP_NEW_SYN_RECV进行处理，如果连接检查成功，则需要新建控制块来处理连接，这个新建控制块的状态将会使用TCP_SYN_RECV状态；

该部分详细分析请移步：<TCP被动打开 之 第三次握手-接收ACK>；