TCP是一种面向连接、可靠、基于字节流的传输协议，位于TCP/IP模型的传输层。
- 面向连接：不同于UDP，TCP协议需要通信双方确定彼此已经建立连接后才可以进行数据传输；
- 可靠：连接建立的双方在进行通信时，TCP保证了不会存在数据丢失，或是数据丢失后存在拯救丢失的措施；
- 字节流：实际传输中，不论是何种数据，TCP都按照字节的方式传输，而非以数据包为单位。
1.TCP建立连接的三次握手
TCP是一个面向连接的协议，无论哪一方向另一方发送数据之前，都必须先在双方之间建立一条连接，所谓三次握手（Three-Way Handshake）即建立TCP连接，就是指建立一个TCP连接时，需要客户端和服务端总共发送3个包以确认连接的建立。在socket编程中，这一过程由客户端执行connect来触发，整个流程如下图所示：
（1）第一次握手：Client将标志位SYN置为1，随机产生一个值seq=J，并将该数据包发送给Server，Client进入SYN_SENT状态，等待Server确认。
（2）第二次握手：Server收到数据包后由标志位SYN=1知道Client请求建立连接，Server将标志位SYN和ACK都置为1，ack=J+1，随机产生一个值seq=K，并将该数据包发送给Client以确认连接请求，Server进入SYN_RCVD状态。
（3）第三次握手：Client收到确认后，检查ack是否为J+1，ACK是否为1，如果正确则将标志位ACK置为1，ack=K+1，并将该数据包发送给Server，Server检查ack是否为K+1，ACK是否为1，如果正确则连接建立成功，Client和Server进入ESTABLISHED状态，完成三次握手，随后Client与Server之间可以开始传输数据了。
简单来说，就是：
1、建立连接时，客户端发送SYN包（SYN=i）到服务器，并进入到SYN-SEND状态，等待服务器确认；
2、服务器收到SYN包，必须确认客户的SYN（ack=i+1）,同时自己也发送一个SYN包（SYN=k）,即SYN+ACK包，此时服务器进入SYN-RECV状态；
3、客户端收到服务器的SYN+ACK包，向服务器发送确认报ACK（ack=k+1）,此包发送完毕，客户端和服务器进入ESTABLISHED状态，完成三次握手，客户端与服务器开始传送数据。
TCP状态转换图：
CLOSED：起始点，在超时或者连接关闭时候进入此状态，这并不是一个真正的状态，而是这个状态图的假想起点和终点。
LISTEN：服务器端等待连接的状态。服务器经过socket，bind，listen函数之后进入此状态，开始监听客户端发过来的连接请求。此称为应用程序被动打开（等到客户端连接请求）。
SYN_SENT：第一次握手发生阶段，客户端发起连接。客户端调用connect，发送SYN给服务器端，然后进入SYN_SENT状态，等待服务器端确认（三次握手中的第二个报文）。如果服务器端不能连接，则直接进入CLOSED状态。
SYN_RCVD：第二次握手发生阶段，跟3对应，这里是服务器端接收到了客户端的SYN，此时服务器由LISTEN进入SYN_RCVD状态，同时服务器端回应一个ACK，然后再发送一个SYN即SYN+ACK给客户端。状态图中还描绘了这样一种情况，当客户端在发送SYN的同时也收到服务器端的SYN请求，即两个同时发起连接请求，那么客户端就会从SYN_SENT转换到SYN_REVD状态。
ESTABLISHED：第三次握手发生阶段，客户端接收到服务器端的ACK包（ACK，SYN）之后，也会发送一个ACK确认包，客户端进入ESTABLISHED状态，表明客户端这边已经准备好，但TCP需要两端都准备好才可以进行数据传输。服务器端收到客户端的ACK之后会从SYN_RCVD状态转移到ESTABLISHED状态，表明服务器端也准备好进行数据传输了。这样客户端和服务器端都是ESTABLISHED状态，就可以进行后面的数据传输了。所以ESTABLISHED也可以说是一个数据传送状态。
2.内核源码分析
客户端调用connect主动发起连接：
/* * Attempt to connect to a socket with the server address. The address * is in user space so we verify it is OK and move it to kernel space. * * For 1003.1g we need to add clean support for a bind to AF_UNSPEC to * break bindings * * NOTE: 1003.1g draft 6.3 is broken with respect to AX.25/NetROM and * other SEQPACKET protocols that take time to connect() as it doesn't * include the -EINPROGRESS status for such sockets. */ int __sys_connect(int fd, struct sockaddr __user *uservaddr, int addrlen) { struct socket *sock; struct sockaddr_storage address; int err, fput_needed; sock = sockfd_lookup_light(fd, &err, &fput_needed); if (!sock) goto out; //将地址对象从用户空间拷贝到内核空间 err = move_addr_to_kernel(uservaddr, addrlen, &address); if (err < 0) goto out_put; err = security_socket_connect(sock, (struct sockaddr *)&address, addrlen); if (err) goto out_put; err = sock->ops->connect(sock, (struct sockaddr *)&address, addrlen, sock->file->f_flags); out_put: fput_light(sock->file, fput_needed); out: return err; } SYSCALL_DEFINE3(connect, int, fd, struct sockaddr __user *, uservaddr, int, addrlen) { return __sys_connect(fd, uservaddr, addrlen); }
该函数根据文件描述符找到指定的socket对象，将地址信息从用户空间拷贝到内核空间，调用指定类型套接字的connect函数。
对应流式套接字的connect函数是inet_stream_connect：
int inet_stream_connect(struct socket *sock, struct sockaddr *uaddr, int addr_len, int flags) { int err; lock_sock(sock->sk); err = __inet_stream_connect(sock, uaddr, addr_len, flags); release_sock(sock->sk); return err; } /* * Connect to a remote host. There is regrettably still a little * TCP 'magic' in here. */ //1. 检查socket地址长度和使用的协议族。 //2. 检查socket的状态，必须是SS_UNCONNECTED或SS_CONNECTING。 //3. 调用tcp_v4_connect()来发送SYN包。 //4. 等待后续握手的完成： int __inet_stream_connect(struct socket *sock, struct sockaddr *uaddr, int addr_len, int flags) { struct sock *sk = sock->sk; int err; long timeo; if (addr_len < sizeof(uaddr->sa_family)) return -EINVAL; if (uaddr->sa_family == AF_UNSPEC) { err = sk->sk_prot->disconnect(sk, flags); sock->state = err ? SS_DISCONNECTING : SS_UNCONNECTED; goto out; } switch (sock->state) { default: err = -EINVAL; goto out; case SS_CONNECTED: //已经是连接状态 err = -EISCONN; goto out; case SS_CONNECTING: //正在连接 err = -EALREADY; /* Fall out of switch with err, set for this state */ break; case SS_UNCONNECTED: err = -EISCONN; if (sk->sk_state != TCP_CLOSE) goto out; //对于流式套接字,sock->ops为 inet_stream_ops --> inet_stream_connect --> tcp_prot --> tcp_v4_connect //对于数据报套接字,sock->ops为 inet_dgram_ops --> inet_dgram_connect --> udp_prot --> ip4_datagram_connect err = sk->sk_prot->connect(sk, uaddr, addr_len); if (err < 0) goto out; sock->state = SS_CONNECTING; /* Just entered SS_CONNECTING state; the only * difference is that return value in non-blocking * case is EINPROGRESS, rather than EALREADY. */ err = -EINPROGRESS; break; } //获取阻塞时间timeo。如果socket是非阻塞的，则timeo是0 //connect()的超时时间为sk->sk_sndtimeo，在sock_init_data()中初始化为MAX_SCHEDULE_TIMEOUT,表示无限等待，可以通过SO_SNDTIMEO选项来修改 timeo = sock_sndtimeo(sk, flags & O_NONBLOCK); if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) { int writebias = (sk->sk_protocol == IPPROTO_TCP) && tcp_sk(sk)->fastopen_req && tcp_sk(sk)->fastopen_req->data ? 1 : 0; /* Error code is set above */ if (!timeo || !inet_wait_for_connect(sk, timeo, writebias)) goto out; err = sock_intr_errno(timeo); //进程收到信号，如果err为-ERESTARTSYS，接下来库函数会重新调用connect() if (signal_pending(current)) goto out; } /* Connection was closed by RST, timeout, ICMP error * or another process disconnected us. */ if (sk->sk_state == TCP_CLOSE) goto sock_error; /* sk->sk_err may be not zero now, if RECVERR was ordered by user * and error was received after socket entered established state. * Hence, it is handled normally after connect() return successfully. */ //更新socket状态为连接已建立 sock->state = SS_CONNECTED; //清除错误码 err = 0; out: return err; sock_error: err = sock_error(sk) ? : -ECONNABORTED; sock->state = SS_UNCONNECTED; //如果使用的是TCP，则sk_prot为tcp_prot，disconnect为tcp_disconnect() if (sk->sk_prot->disconnect(sk, flags)) //如果失败 sock->state = SS_DISCONNECTING; goto out; }
该函数首先检查socket地址长度和使用的协议族，检查socket的状态，必须是SS_UNCONNECTED或SS_CONNECTING，调用实现协议的connect函数，对于流式套接字，实现协议是tcp，调用的是tcp_v4_connect()，对于阻塞调用，等待后续握手的完成；对于非阻塞调用，则直接返回 -EINPROGRESS。
tcp_v4_connect的源代码：
/* This will initiate an outgoing connection. */ int tcp_v4_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len) { struct sockaddr_in *usin = (struct sockaddr_in *)uaddr; struct inet_sock *inet = inet_sk(sk); struct tcp_sock *tp = tcp_sk(sk); __be16 orig_sport, orig_dport; __be32 daddr, nexthop; struct flowi4 *fl4; struct rtable *rt; int err; struct ip_options_rcu *inet_opt; if (addr_len < sizeof(struct sockaddr_in)) return -EINVAL; if (usin->sin_family != AF_INET) return -EAFNOSUPPORT; nexthop = daddr = usin->sin_addr.s_addr; inet_opt = rcu_dereference_protected(inet->inet_opt, lockdep_sock_is_held(sk)); //将下一跳地址和目的地址的临时变量都暂时设为用户提交的地址。 if (inet_opt && inet_opt->opt.srr) { if (!daddr) return -EINVAL; nexthop = inet_opt->opt.faddr; } //源端口 orig_sport = inet->inet_sport; //目的端口 orig_dport = usin->sin_port; fl4 = &inet->cork.fl.u.ip4; //如果使用了来源地址路由，选择一个合适的下一跳地址。 rt = ip_route_connect(fl4, nexthop, inet->inet_saddr, RT_CONN_FLAGS(sk), sk->sk_bound_dev_if, IPPROTO_TCP, orig_sport, orig_dport, sk); if (IS_ERR(rt)) { err = PTR_ERR(rt); if (err == -ENETUNREACH) IP_INC_STATS(sock_net(sk), IPSTATS_MIB_OUTNOROUTES); return err; } if (rt->rt_flags & (RTCF_MULTICAST | RTCF_BROADCAST)) { ip_rt_put(rt); return -ENETUNREACH; } //进行路由查找，并校验返回的路由的类型，TCP是不被允许使用多播和广播的 if (!inet_opt || !inet_opt->opt.srr) daddr = fl4->daddr; //更新目的地址临时变量——使用路由查找后返回的值 if (!inet->inet_saddr) inet->inet_saddr = fl4->saddr; sk_rcv_saddr_set(sk, inet->inet_saddr); //如果还没有设置源地址，和本地发送地址，则使用路由中返回的值 if (tp->rx_opt.ts_recent_stamp && inet->inet_daddr != daddr) { /* Reset inherited state */ tp->rx_opt.ts_recent = 0; tp->rx_opt.ts_recent_stamp = 0; if (likely(!tp->repair)) tp->write_seq = 0; } if (tcp_death_row.sysctl_tw_recycle && !tp->rx_opt.ts_recent_stamp && fl4->daddr == daddr) tcp_fetch_timewait_stamp(sk, &rt->dst); //保存目的地址及端口 inet->inet_dport = usin->sin_port; sk_daddr_set(sk, daddr); inet_csk(sk)->icsk_ext_hdr_len = 0; if (inet_opt) inet_csk(sk)->icsk_ext_hdr_len = inet_opt->opt.optlen; //设置最小允许的mss值 536 tp->rx_opt.mss_clamp = TCP_MSS_DEFAULT; /* Socket identity is still unknown (sport may be zero). * However we set state to SYN-SENT and not releasing socket * lock select source port, enter ourselves into the hash tables and * complete initialization after this. */ //套接字状态被置为 TCP_SYN_SENT， tcp_set_state(sk, TCP_SYN_SENT); err = inet_hash_connect(&tcp_death_row, sk); if (err) goto failure; sk_set_txhash(sk); //动态选择一个本地端口，并加入 hash 表，与bind(2)选择端口类似 rt = ip_route_newports(fl4, rt, orig_sport, orig_dport, inet->inet_sport, inet->inet_dport, sk); if (IS_ERR(rt)) { err = PTR_ERR(rt); rt = NULL; goto failure; } /* OK, now commit destination to socket. */ sk->sk_gso_type = SKB_GSO_TCPV4; sk_setup_caps(sk, &rt->dst); if (!tp->write_seq && likely(!tp->repair)) tp->write_seq = secure_tcp_sequence_number(inet->inet_saddr, inet->inet_daddr, inet->inet_sport, usin->sin_port); inet->inet_id = tp->write_seq ^ jiffies; //函数用来根据 sk 中的信息，构建一个完成的 syn 报文，并将它发送出去。 err = tcp_connect(sk); rt = NULL; if (err) goto failure; return 0; failure: /* * This unhashes the socket and releases the local port, * if necessary. */ tcp_set_state(sk, TCP_CLOSE); ip_rt_put(rt); sk->sk_route_caps = 0; inet->inet_dport = 0; return err; }
该函数完成了路由查找，得到下一跳地址，并更新socket对象的下一跳地址，将socket对象的状态设置为TCP_SYN_SENT，如果没设置序号初值，则选定一个随机初值， 调用函数tcp_connect完成报文构建和发送。
tcp_connect的源码：
/* Build a SYN and send it off. */ int tcp_connect(struct sock *sk) { struct tcp_sock *tp = tcp_sk(sk); struct sk_buff *buff; int err; //初始化传输控制块中与连接相关的成员 tcp_connect_init(sk); if (unlikely(tp->repair)) { tcp_finish_connect(sk, NULL); return 0; } //分配skbuff --> 为SYN段分配报文并进行初始化 buff = sk_stream_alloc_skb(sk, 0, sk->sk_allocation, true); if (unlikely(!buff)) return -ENOBUFS; //构建syn报文 //在函数tcp_v4_connect中write_seq已经被初始化随机值 tcp_init_nondata_skb(buff, tp->write_seq++, TCPHDR_SYN); tp->retrans_stamp = tcp_time_stamp; //将报文添加到发送队列上 tcp_connect_queue_skb(sk, buff); //显式拥塞通告 ---> //路由器在出现拥塞时通知TCP。当TCP段传递时，路由器使用IP首部中的2位来记录拥塞，当TCP段到达后， //接收方知道报文段是否在某个位置经历过拥塞。然而，需要了解拥塞发生情况的是发送方，而非接收方。因 //此，接收方使用下一个ACK通知发送方有拥塞发生，然后，发送方做出响应，缩小自己的拥塞窗口。 tcp_ecn_send_syn(sk, buff); /* Send off SYN; include data in Fast Open. */ err = tp->fastopen_req ? tcp_send_syn_data(sk, buff) : //构造tcp头和ip头并发送 tcp_transmit_skb(sk, buff, 1, sk->sk_allocation); if (err == -ECONNREFUSED) return err; /* We change tp->snd_nxt after the tcp_transmit_skb() call * in order to make this packet get counted in tcpOutSegs. */ tp->snd_nxt = tp->write_seq; tp->pushed_seq = tp->write_seq; TCP_INC_STATS(sock_net(sk), TCP_MIB_ACTIVEOPENS); /* Timer for repeating the SYN until an answer. */ //启动重传定时器 inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, inet_csk(sk)->icsk_rto, TCP_RTO_MAX); return 0; }
该函数完成：
1 初始化套接字跟连接相关的字段
2 申请sk_buff空间
3 将sk_buff初始化为syn报文，实质是操作tcp_skb_cb，在初始化TCP头的时候会用到
4 调用tcp_connect_queue_skb()函数将报文sk_buff添加到发送队列sk->sk_write_queue
5 调用tcp_transmit_skb()函数构造tcp头，然后交给网络层。
6 初始化重传定时器
tcp_connect_queue_skb()函数的原理主要是移动sk_buff的data指针，然后填充TCP头。再然后将报文交给网络层，将报文发出。
这样，三次握手中的第一次握手在客户端的层面完成，报文到达服务端，由服务端处理完毕后，第一次握手完成，客户端socket状态变为TCP_SYN_SENT。
tcp_v4_rcv()源码：
/* * From tcp_input.c */ int tcp_v4_rcv(struct sk_buff *skb) { struct net *net = dev_net(skb->dev); int sdif = inet_sdif(skb); const struct iphdr *iph; const struct tcphdr *th; bool refcounted; struct sock *sk; int ret; if (skb->pkt_type != PACKET_HOST) goto discard_it; /* Count it even if it's bad */ __TCP_INC_STATS(net, TCP_MIB_INSEGS); if (!pskb_may_pull(skb, sizeof(struct tcphdr))) goto discard_it; th = (const struct tcphdr *)skb->data; if (unlikely(th->doff < sizeof(struct tcphdr) / 4)) goto bad_packet; if (!pskb_may_pull(skb, th->doff * 4)) goto discard_it; /* An explanation is required here, I think. * Packet length and doff are validated by header prediction, * provided case of th->doff==0 is eliminated. * So, we defer the checks. */ if (skb_checksum_init(skb, IPPROTO_TCP, inet_compute_pseudo)) goto csum_error; th = (const struct tcphdr *)skb->data; iph = ip_hdr(skb); lookup: sk = __inet_lookup_skb(&tcp_hashinfo, skb, __tcp_hdrlen(th), th->source, th->dest, sdif, &refcounted); if (!sk) goto no_tcp_socket; process: if (sk->sk_state == TCP_TIME_WAIT) goto do_time_wait; if (sk->sk_state == TCP_NEW_SYN_RECV) { struct request_sock *req = inet_reqsk(sk); bool req_stolen = false; struct sock *nsk; sk = req->rsk_listener; if (unlikely(tcp_v4_inbound_md5_hash(sk, skb))) { sk_drops_add(sk, skb); reqsk_put(req); goto discard_it; } if (tcp_checksum_complete(skb)) { reqsk_put(req); goto csum_error; } if (unlikely(sk->sk_state != TCP_LISTEN)) { inet_csk_reqsk_queue_drop_and_put(sk, req); goto lookup; } /* We own a reference on the listener, increase it again * as we might lose it too soon. */ sock_hold(sk); refcounted = true; nsk = NULL; if (!tcp_filter(sk, skb)) { th = (const struct tcphdr *)skb->data; iph = ip_hdr(skb); tcp_v4_fill_cb(skb, iph, th); nsk = tcp_check_req(sk, skb, req, false, &req_stolen); } if (!nsk) { reqsk_put(req); if (req_stolen) { /* Another cpu got exclusive access to req * and created a full blown socket. * Try to feed this packet to this socket * instead of discarding it. */ tcp_v4_restore_cb(skb); sock_put(sk); goto lookup; } goto discard_and_relse; } if (nsk == sk) { reqsk_put(req); tcp_v4_restore_cb(skb); } else if (tcp_child_process(sk, nsk, skb)) { tcp_v4_send_reset(nsk, skb); goto discard_and_relse; } else { sock_put(sk); return 0; } } if (unlikely(iph->ttl < inet_sk(sk)->min_ttl)) { __NET_INC_STATS(net, LINUX_MIB_TCPMINTTLDROP); goto discard_and_relse; } if (!xfrm4_policy_check(sk, XFRM_POLICY_IN, skb)) goto discard_and_relse; if (tcp_v4_inbound_md5_hash(sk, skb)) goto discard_and_relse; nf_reset(skb); if (tcp_filter(sk, skb)) goto discard_and_relse; th = (const struct tcphdr *)skb->data; iph = ip_hdr(skb); tcp_v4_fill_cb(skb, iph, th); skb->dev = NULL; if (sk->sk_state == TCP_LISTEN) { ret = tcp_v4_do_rcv(sk, skb); goto put_and_return; } sk_incoming_cpu_update(sk); bh_lock_sock_nested(sk); tcp_segs_in(tcp_sk(sk), skb); ret = 0; if (!sock_owned_by_user(sk)) { ret = tcp_v4_do_rcv(sk, skb); } else if (tcp_add_backlog(sk, skb)) { goto discard_and_relse; } bh_unlock_sock(sk); put_and_return: if (refcounted) sock_put(sk); return ret; no_tcp_socket: if (!xfrm4_policy_check(NULL, XFRM_POLICY_IN, skb)) goto discard_it; tcp_v4_fill_cb(skb, iph, th); if (tcp_checksum_complete(skb)) { csum_error: __TCP_INC_STATS(net, TCP_MIB_CSUMERRORS); bad_packet: __TCP_INC_STATS(net, TCP_MIB_INERRS); } else { tcp_v4_send_reset(NULL, skb); } discard_it: /* Discard frame. */ kfree_skb(skb); return 0; discard_and_relse: sk_drops_add(sk, skb); if (refcounted) sock_put(sk); goto discard_it; do_time_wait: if (!xfrm4_policy_check(NULL, XFRM_POLICY_IN, skb)) { inet_twsk_put(inet_twsk(sk)); goto discard_it; } tcp_v4_fill_cb(skb, iph, th); if (tcp_checksum_complete(skb)) { inet_twsk_put(inet_twsk(sk)); goto csum_error; } switch (tcp_timewait_state_process(inet_twsk(sk), skb, th)) { case TCP_TW_SYN: { struct sock *sk2 = inet_lookup_listener(dev_net(skb->dev), &tcp_hashinfo, skb, __tcp_hdrlen(th), iph->saddr, th->source, iph->daddr, th->dest, inet_iif(skb), sdif); if (sk2) { inet_twsk_deschedule_put(inet_twsk(sk)); sk = sk2; tcp_v4_restore_cb(skb); refcounted = false; goto process; } } /* to ACK */ /* fall through */ case TCP_TW_ACK: tcp_v4_timewait_ack(sk, skb); break; case TCP_TW_RST: tcp_v4_send_reset(sk, skb); inet_twsk_deschedule_put(inet_twsk(sk)); goto discard_it; case TCP_TW_SUCCESS:; } goto discard_it; }
该函数主要工作就是根据tcp头部信息查到处理报文的socket对象，然后检查socket状态做不同处理，我们这里是监听状态TCP_LISTEN，直接调用函数tcp_v4_do_rcv():
/* The socket must have it's spinlock held when we get * here, unless it is a TCP_LISTEN socket. * * We have a potential double-lock case here, so even when * doing backlog processing we use the BH locking scheme. * This is because we cannot sleep with the original spinlock * held. */ int tcp_v4_do_rcv(struct sock *sk, struct sk_buff *skb) { struct sock *rsk; if (sk->sk_state == TCP_ESTABLISHED) { /* Fast path */ struct dst_entry *dst = sk->sk_rx_dst; sock_rps_save_rxhash(sk, skb); sk_mark_napi_id(sk, skb); if (dst) { if (inet_sk(sk)->rx_dst_ifindex != skb->skb_iif || !dst->ops->check(dst, 0)) { dst_release(dst); sk->sk_rx_dst = NULL; } } tcp_rcv_established(sk, skb); return 0; } if (tcp_checksum_complete(skb)) goto csum_err; if (sk->sk_state == TCP_LISTEN) { struct sock *nsk = tcp_v4_cookie_check(sk, skb); if (!nsk) goto discard; if (nsk != sk) { if (tcp_child_process(sk, nsk, skb)) { rsk = nsk; goto reset; } return 0; } } else sock_rps_save_rxhash(sk, skb); if (tcp_rcv_state_process(sk, skb)) { rsk = sk; goto reset; } return 0; reset: tcp_v4_send_reset(rsk, skb); discard: kfree_skb(skb); /* Be careful here. If this function gets more complicated and * gcc suffers from register pressure on the x86, sk (in %ebx) * might be destroyed here. This current version compiles correctly, * but you have been warned. */ return 0; csum_err: TCP_INC_STATS(sock_net(sk), TCP_MIB_CSUMERRORS); TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS); goto discard; } EXPORT_SYMBOL(tcp_v4_do_rcv);
首先函数检查当前是否处于半连接状态，并调用tcp_v4_hnd_req检查报文的状态字段，再针对报文类型调用不同函数进行处理，若是SYN报文，则调用tcp_rcv_state_process函数，进入到下一阶段，客户端收到服务端的SYN+ACK，并发送ACK。
接着就是调用tcp_rcv_state_process()：
/* * This function implements the receiving procedure of RFC 793 for * all states except ESTABLISHED and TIME_WAIT. * It's called from both tcp_v4_rcv and tcp_v6_rcv and should be * address independent. */ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb) { struct tcp_sock *tp = tcp_sk(sk); struct inet_connection_sock *icsk = inet_csk(sk); const struct tcphdr *th = tcp_hdr(skb); struct request_sock *req; int queued = 0; bool acceptable; switch (sk->sk_state) { case TCP_CLOSE: goto discard; case TCP_LISTEN: if (th->ack) return 1; if (th->rst) goto discard; if (th->syn) { if (th->fin) goto discard; /* It is possible that we process SYN packets from backlog, * so we need to make sure to disable BH and RCU right there. */ rcu_read_lock(); local_bh_disable(); acceptable = icsk->icsk_af_ops->conn_request(sk, skb) >= 0; local_bh_enable(); rcu_read_unlock(); if (!acceptable) return 1; consume_skb(skb); return 0; } goto discard; case TCP_SYN_SENT: tp->rx_opt.saw_tstamp = 0; tcp_mstamp_refresh(tp); queued = tcp_rcv_synsent_state_process(sk, skb, th); if (queued >= 0) return queued; /* Do step6 onward by hand. */ tcp_urg(sk, skb, th); __kfree_skb(skb); tcp_data_snd_check(sk); return 0; } tcp_mstamp_refresh(tp); tp->rx_opt.saw_tstamp = 0; req = tp->fastopen_rsk; if (req) { bool req_stolen; WARN_ON_ONCE(sk->sk_state != TCP_SYN_RECV && sk->sk_state != TCP_FIN_WAIT1); if (!tcp_check_req(sk, skb, req, true, &req_stolen)) goto discard; } if (!th->ack && !th->rst && !th->syn) goto discard; if (!tcp_validate_incoming(sk, skb, th, 0)) return 0; /* step 5: check the ACK field */ acceptable = tcp_ack(sk, skb, FLAG_SLOWPATH | FLAG_UPDATE_TS_RECENT | FLAG_NO_CHALLENGE_ACK) > 0; if (!acceptable) { if (sk->sk_state == TCP_SYN_RECV) return 1; /* send one RST */ tcp_send_challenge_ack(sk, skb); goto discard; } switch (sk->sk_state) { case TCP_SYN_RECV: tp->delivered++; /* SYN-ACK delivery isn't tracked in tcp_ack */ if (!tp->srtt_us) tcp_synack_rtt_meas(sk, req); /* Once we leave TCP_SYN_RECV, we no longer need req * so release it. */ if (req) { inet_csk(sk)->icsk_retransmits = 0; reqsk_fastopen_remove(sk, req, false); /* Re-arm the timer because data may have been sent out. * This is similar to the regular data transmission case * when new data has just been ack'ed. * * (TFO) - we could try to be more aggressive and * retransmitting any data sooner based on when they * are sent out. */ tcp_rearm_rto(sk); } else { tcp_init_transfer(sk, BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB); tp->copied_seq = tp->rcv_nxt; } smp_mb(); tcp_set_state(sk, TCP_ESTABLISHED); sk->sk_state_change(sk); /* Note, that this wakeup is only for marginal crossed SYN case. * Passively open sockets are not waked up, because * sk->sk_sleep == NULL and sk->sk_socket == NULL. */ if (sk->sk_socket) sk_wake_async(sk, SOCK_WAKE_IO, POLL_OUT); tp->snd_una = TCP_SKB_CB(skb)->ack_seq; tp->snd_wnd = ntohs(th->window) << tp->rx_opt.snd_wscale; tcp_init_wl(tp, TCP_SKB_CB(skb)->seq); if (tp->rx_opt.tstamp_ok) tp->advmss -= TCPOLEN_TSTAMP_ALIGNED; if (!inet_csk(sk)->icsk_ca_ops->cong_control) tcp_update_pacing_rate(sk); /* Prevent spurious tcp_cwnd_restart() on first data packet */ tp->lsndtime = tcp_jiffies32; tcp_initialize_rcv_mss(sk); tcp_fast_path_on(tp); break; case TCP_FIN_WAIT1: { int tmo; /* If we enter the TCP_FIN_WAIT1 state and we are a * Fast Open socket and this is the first acceptable * ACK we have received, this would have acknowledged * our SYNACK so stop the SYNACK timer. */ if (req) { /* We no longer need the request sock. */ reqsk_fastopen_remove(sk, req, false); tcp_rearm_rto(sk); } if (tp->snd_una != tp->write_seq) break; tcp_set_state(sk, TCP_FIN_WAIT2); sk->sk_shutdown |= SEND_SHUTDOWN; sk_dst_confirm(sk); if (!sock_flag(sk, SOCK_DEAD)) { /* Wake up lingering close() */ sk->sk_state_change(sk); break; } if (tp->linger2 < 0) { tcp_done(sk); NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTONDATA); return 1; } if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq && after(TCP_SKB_CB(skb)->end_seq - th->fin, tp->rcv_nxt)) { /* Receive out of order FIN after close() */ if (tp->syn_fastopen && th->fin) tcp_fastopen_active_disable(sk); tcp_done(sk); NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTONDATA); return 1; } tmo = tcp_fin_time(sk); if (tmo > TCP_TIMEWAIT_LEN) { inet_csk_reset_keepalive_timer(sk, tmo - TCP_TIMEWAIT_LEN); } else if (th->fin || sock_owned_by_user(sk)) { /* Bad case. We could lose such FIN otherwise. * It is not a big problem, but it looks confusing * and not so rare event. We still can lose it now, * if it spins in bh_lock_sock(), but it is really * marginal case. */ inet_csk_reset_keepalive_timer(sk, tmo); } else { tcp_time_wait(sk, TCP_FIN_WAIT2, tmo); goto discard; } break; } case TCP_CLOSING: if (tp->snd_una == tp->write_seq) { tcp_time_wait(sk, TCP_TIME_WAIT, 0); goto discard; } break; case TCP_LAST_ACK: if (tp->snd_una == tp->write_seq) { tcp_update_metrics(sk); tcp_done(sk); goto discard; } break; } /* step 6: check the URG bit */ tcp_urg(sk, skb, th); /* step 7: process the segment text */ switch (sk->sk_state) { case TCP_CLOSE_WAIT: case TCP_CLOSING: case TCP_LAST_ACK: if (!before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) break; /* fall through */ case TCP_FIN_WAIT1: case TCP_FIN_WAIT2: /* RFC 793 says to queue data in these states, * RFC 1122 says we MUST send a reset. * BSD 4.4 also does reset. */ if (sk->sk_shutdown & RCV_SHUTDOWN) { if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq && after(TCP_SKB_CB(skb)->end_seq - th->fin, tp->rcv_nxt)) { NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTONDATA); tcp_reset(sk); return 1; } } /* Fall through */ case TCP_ESTABLISHED: tcp_data_queue(sk, skb); queued = 1; break; } /* tcp_data could move socket to TIME-WAIT */ if (sk->sk_state != TCP_CLOSE) { tcp_data_snd_check(sk); tcp_ack_snd_check(sk); } if (!queued) { discard: tcp_drop(sk, skb); } return 0; } EXPORT_SYMBOL(tcp_rcv_state_process);