标签：open() 时延 初始 errno cli nis lse switch nal
一、TCP概述
简介：
传输控制协议（TCP）是一种面向连接的、可靠的、基于字节流的传输层通信协议，由IETF的RFC 793定义。在简化的计算机网络OSI模型中，它完成第四层传输层所指定的功能。用户数据报协议（UDP）是同一层内另一个重要的传输协议。
应用层向TCP层发送用于网间传输的、用8位字节表示的数据流，然后TCP把数据流分割成适当长度的报文段（通常受该计算机连接的网络的数据链路层的最大传输单元（MTU）的限制）。之后TCP把结果包传给IP层，由它来透过网络将包传送给接收端实体的TCP层。TCP为了保证不发生丢包，就给每个包一个序号，同时序号也保证了传送到接收端实体的包的按序接收。然后接收端实体对已成功收到的包发回一个相应的确认信息（ACK）；如果发送端实体在合理的往返时延（RTT）内未收到确认，那么对应的数据包就被假设为已丢失并进行重传。TCP用一个校验和函数来检验数据是否有错误，在发送和接收时都要计算校验和。
报文格式：
序号：Seq序号，占32位，用来标识从TCP源端向目的端发送的字节流，发起方发送数据时对此进行标记。
（2）确认序号：Ack序号，占32位，只有ACK标志位为1时，确认序号字段才有效，Ack=Seq+1。
（3）标志位：共6个，即URG、ACK、PSH、RST、SYN、FIN等，
运作方式：
TCP协议的运行可划分为三个阶段：连接创建(connection establishment)、数据传送（data transfer）和连接终止（connection termination）。操作系统将TCP连接抽象为套接字表示的本地端点（local end-point），作为编程接口给程序使用。在TCP连接的生命期内，本地端点要经历一系列的状态改变。
二、TCP的三次握手
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。
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。
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。
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。
系统调用是调用socket相关系统调用的入口函数
1.connect
当客户端调用系统调用connect连接服务端的时候，经过socket系统调用入口函数的分发，最终调用的是__sys_connect()函数.
SYSCALL_DEFINE3(connect, int, fd, struct sockaddr __user *, uservaddr, int, addrlen) { struct socket *sock; struct sockaddr_storage address; int err, fput_needed; //根据fd，找到对应的socket。11 sock = sockfd_lookup_light(fd, &err, &fput_needed); if (!sock) goto out; 16 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; // 调用sock的连接函数27 err = sock->ops->connect(sock, (struct sockaddr *)&address, addrlen, sock->file->f_flags); out_put: fput_light(sock->file, fput_needed); out: return err; }
sockfd_lookup_light()找到具体的socket实例。其中调用了__inet_stream_connect函数。
int __inet_stream_connect(struct socket *sock, struct sockaddr *uaddr, int addr_len, int flags, int is_sendmsg) { struct sock *sk = sock->sk; int err; long timeo; /* * uaddr can be NULL and addr_len can be 0 if: * sk is a TCP fastopen active socket and * TCP_FASTOPEN_CONNECT sockopt is set and * we already have a valid cookie for this socket. * In this case, user can call write() after connect(). * write() will invoke tcp_sendmsg_fastopen() which calls * __inet_stream_connect(). */ if (uaddr) { 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: err = -EISCONN; goto out; case SS_CONNECTING: if (inet_sk(sk)->defer_connect) err = is_sendmsg ? -EINPROGRESS : -EISCONN; else err = -EALREADY; /* Fall out of switch with err, set for this state */ break; case SS_UNCONNECTED: //没有建立连接 err = -EISCONN; if (sk->sk_state != TCP_CLOSE) goto out; if (BPF_CGROUP_PRE_CONNECT_ENABLED(sk)) { err = sk->sk_prot->pre_connect(sk, uaddr, addr_len); if (err) goto out; } err = sk->sk_prot->connect(sk, uaddr, addr_len); //这一行发送一个报文，这是TCP连接的第一步 if (err < 0) goto out; sock->state = SS_CONNECTING; if (!err && inet_sk(sk)->defer_connect) goto out; /* Just entered SS_CONNECTING state; the only * difference is that return value in non-blocking * case is EINPROGRESS, rather than EALREADY. */ err = -EINPROGRESS; break; } timeo = sock_sndtimeo(sk, flags & O_NONBLOCK); if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) { //客户端发出请求连接后，等待服务端的响应。 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)) //等待第二次连接 goto out; err = sock_intr_errno(timeo); 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. */ sock->state = SS_CONNECTED; //连接建立成功 err = 0; out: return err; sock_error: err = sock_error(sk) ? : -ECONNABORTED; sock->state = SS_UNCONNECTED; if (sk->sk_prot->disconnect(sk, flags)) sock->state = SS_DISCONNECTING; goto out; }
2.accept
调用accept系统调用时，，具体调用的是__sys_accept4函数。
int __sys_accept4(int fd, struct sockaddr __user *upeer_sockaddr, int __user *upeer_addrlen, int flags) { struct socket *sock, *newsock; struct file *newfile; int err, len, newfd, fput_needed; struct sockaddr_storage address; if (flags & ~(SOCK_CLOEXEC | SOCK_NONBLOCK)) return -EINVAL; if (SOCK_NONBLOCK != O_NONBLOCK && (flags & SOCK_NONBLOCK)) flags = (flags & ~SOCK_NONBLOCK) | O_NONBLOCK; sock = sockfd_lookup_light(fd, &err, &fput_needed); //查找服务器端监听的socket if (!sock) goto out; err = -ENFILE; newsock = sock_alloc(); //分配一个socket if (!newsock) goto out_put; newsock->type = sock->type; newsock->ops = sock->ops; /* * We don‘t need try_module_get here, as the listening socket (sock) * has the protocol module (sock->ops->owner) held. */ __module_get(newsock->ops->owner); newfd = get_unused_fd_flags(flags); if (unlikely(newfd < 0)) { err = newfd; sock_release(newsock); goto out_put; } newfile = sock_alloc_file(newsock, flags, sock->sk->sk_prot_creator->name); if (IS_ERR(newfile)) { err = PTR_ERR(newfile); put_unused_fd(newfd); goto out_put; } err = security_socket_accept(sock, newsock); if (err) goto out_fd; err = sock->ops->accept(sock, newsock, sock->file->f_flags, false); //调用socket上对应的accept if (err < 0) goto out_fd; if (upeer_sockaddr) { len = newsock->ops->getname(newsock, (struct sockaddr *)&address, 2); if (len < 0) { err = -ECONNABORTED; goto out_fd; } err = move_addr_to_user(&address, len, upeer_sockaddr, upeer_addrlen); if (err < 0) goto out_fd; } /* File flags are not inherited via accept() unlike another OSes. */ fd_install(newfd, newfile); err = newfd; out_put: fput_light(sock->file, fput_needed); out: return err; out_fd: fput(newfile); put_unused_fd(newfd); goto out_put; }
分配一个新的socket对象，这个socket的新对象当有新连接进来时，用于与客户端进行连接，然后调用inet_accept():
int inet_accept(struct socket *sock, struct socket *newsock, int flags, bool kern) { struct sock *sk1 = sock->sk; int err = -EINVAL; struct sock *sk2 = sk1->sk_prot->accept(sk1, flags, &err, kern); //调用inet_csk_accept if (!sk2) goto do_err; lock_sock(sk2); sock_rps_record_flow(sk2); WARN_ON(!((1 << sk2->sk_state) & (TCPF_ESTABLISHED | TCPF_SYN_RECV | TCPF_CLOSE_WAIT | TCPF_CLOSE))); sock_graft(sk2, newsock); newsock->state = SS_CONNECTED; //建立连接 err = 0; release_sock(sk2); do_err: return err; }
上面这个函数用于监听连接。
struct sock *inet_csk_accept(struct sock *sk, int flags, int *err, bool kern) { struct inet_connection_sock *icsk = inet_csk(sk); struct request_sock_queue *queue = &icsk->icsk_accept_queue; struct request_sock *req; struct sock *newsk; int error; lock_sock(sk); /* We need to make sure that this socket is listening, * and that it has something pending. */ error = -EINVAL; if (sk->sk_state != TCP_LISTEN) goto out_err; /* Find already established connection */ if (reqsk_queue_empty(queue)) { long timeo = sock_rcvtimeo(sk, flags & O_NONBLOCK); /* If this is a non blocking socket don‘t sleep */ error = -EAGAIN; if (!timeo) goto out_err; error = inet_csk_wait_for_connect(sk, timeo); //等待连接 if (error) goto out_err; } req = reqsk_queue_remove(queue, sk); newsk = req->sk; if (sk->sk_protocol == IPPROTO_TCP && tcp_rsk(req)->tfo_listener) { spin_lock_bh(&queue->fastopenq.lock); if (tcp_rsk(req)->tfo_listener) { /* We are still waiting for the final ACK from 3WHS * so can‘t free req now. Instead, we set req->sk to * NULL to signify that the child socket is taken * so reqsk_fastopen_remove() will free the req * when 3WHS finishes (or is aborted). */ req->sk = NULL; req = NULL; } spin_unlock_bh(&queue->fastopenq.lock); }
此函数等待客户端的连接，作用于第二次握手的过程。
综上：服务器端先初始化Socket，然后与端口绑定(bind)，对端口进行监听(listen)，调用accept阻塞，等待客户端连接。连接成功，这时客户端与服务器端的连接就建立了。
标签：open() 时延 初始 errno cli nis lse switch nal
原文地址：https://www.cnblogs.com/ft-97/p/12104292.html
