connect及bind、listen、accept背后的三次握手

一、基础知识

 

TCP通过称为“主动确认重传”(PAR)的方式提供可靠的通信传输层的协议数据单元(PDU)称为段。使用PAR的设备重新发送数据单元,直到它收到确认为止。如果接收端接收的数据单元已损坏(使用用于错误检测的传输层的校验和功能检查数据),则接收端将丢弃该段。因此,发送方必须重新发送未收到确认的数据单元。通过上述机制,可以实现在发送方(客户端)和接收方(服务器)之间交换三个段,以建立可靠的TCP连接。这一机制是这样工作的

    • 步骤1(SYN):第一步,客户端要与服务器建立连接,因此它发送一个带有SYN(同步序列号)的段,该段通知服务器客户端可能开始通信以及以段以什么序列号开始通信。
    • 步骤2(SYN + ACK):服务器通过设置SYN-ACK信号位来响应客户端请求。Acknowledgement(ACK)表示收到的段的响应,SYN表示服务器端的段从哪个序列号开始通信。
    • 步骤3(ACK):在最后一步中,客户端确认服务器的响应,并且双方都建立了可靠的连接,它们将开始实际的数据传输

步骤1、2建立一个方向的连接参数(序列号),并确认该参数。步骤2、3为另一个方向建立连接参数(序列号),并确认该参数。利用这些,建立了全双工通信。

注–在客户端和服务器之间建立连接时,会随机选择初始序列号。

 

二、实验过程

SYSCALL_DEFINE2(socketcall, int, call, unsigned long __user *, args)
{
    unsigned long a[AUDITSC_ARGS];
    unsigned long a0, a1;
    int err;
    unsigned int len;
 
    if (call < 1 || call > SYS_SENDMMSG)
        return -EINVAL;
    call = array_index_nospec(call, SYS_SENDMMSG + 1);
 
    len = nargs[call];
    if (len > sizeof(a))
        return -EINVAL;
 
    /* copy_from_user should be SMP safe. */
    if (copy_from_user(a, args, len))
        return -EFAULT;
 
    err = audit_socketcall(nargs[call] / sizeof(unsigned long), a);
    if (err)
        return err;
 
    a0 = a[0];
    a1 = a[1];
 
    switch (call) {
    case SYS_SOCKET:                                                      #call=1
        err = __sys_socket(a0, a1, a[2]);
        break;
    case SYS_BIND:                                                        #call=2
        err = __sys_bind(a0, (struct sockaddr __user *)a1, a[2]);
        break;
    case SYS_CONNECT:                                                     #call=3
        err = __sys_connect(a0, (struct sockaddr __user *)a1, a[2]);
        break;
    case SYS_LISTEN:                                                      #call=4
        err = __sys_listen(a0, a1);
        break;
    case SYS_ACCEPT:                                                      #call=5
        err = __sys_accept4(a0, (struct sockaddr __user *)a1,
                    (int __user *)a[2], 0);
        break;
    case SYS_GETSOCKNAME:                                                 #call=6
        err =
            __sys_getsockname(a0, (struct sockaddr __user *)a1,
                      (int __user *)a[2]);
        break;
    case SYS_GETPEERNAME:                                                 #call=7
        err =
            __sys_getpeername(a0, (struct sockaddr __user *)a1,
                      (int __user *)a[2]);
        break;
    case SYS_SOCKETPAIR:                                                  #call=8
        err = __sys_socketpair(a0, a1, a[2], (int __user *)a[3]);
        break;
    case SYS_SEND:                                                        #call=9
        err = __sys_sendto(a0, (void __user *)a1, a[2], a[3],
                   NULL, 0);
        break;
    case SYS_SENDTO:                                                      #call=10
        err = __sys_sendto(a0, (void __user *)a1, a[2], a[3],
                   (struct sockaddr __user *)a[4], a[5]);
        break;
    case SYS_RECV:                                                        #call=11
        err = __sys_recvfrom(a0, (void __user *)a1, a[2], a[3],
                     NULL, NULL);
        break;
    case SYS_RECVFROM:                                                    #call=12
        err = __sys_recvfrom(a0, (void __user *)a1, a[2], a[3],
                     (struct sockaddr __user *)a[4],
                     (int __user *)a[5]);
        break;
    case SYS_SHUTDOWN:                                                    #call=13
        err = __sys_shutdown(a0, a1);
        break;
    case SYS_SETSOCKOPT:                                                  #call=14
        err = __sys_setsockopt(a0, a1, a[2], (char __user *)a[3],
                       a[4]);
        break;
    case SYS_GETSOCKOPT:                                                  #call=15
        err =
            __sys_getsockopt(a0, a1, a[2], (char __user *)a[3],
                     (int __user *)a[4]);
        break;
    case SYS_SENDMSG:                                                     #call=16
        err = __sys_sendmsg(a0, (struct user_msghdr __user *)a1,
                    a[2], true);
        break;
    case SYS_SENDMMSG:                                                    #call=17
        err = __sys_sendmmsg(a0, (struct mmsghdr __user *)a1, a[2],
                     a[3], true);
        break;
    case SYS_RECVMSG:                                                     #call=18
        err = __sys_recvmsg(a0, (struct user_msghdr __user *)a1,
                    a[2], true);
        break;
    case SYS_RECVMMSG:                                                    #call=19
        if (IS_ENABLED(CONFIG_64BIT) || !IS_ENABLED(CONFIG_64BIT_TIME))
            err = __sys_recvmmsg(a0, (struct mmsghdr __user *)a1,
                         a[2], a[3],
                         (struct __kernel_timespec __user *)a[4],
                         NULL);
        else
            err = __sys_recvmmsg(a0, (struct mmsghdr __user *)a1,
                         a[2], a[3], NULL,
                         (struct old_timespec32 __user *)a[4]);
        break;
    case SYS_ACCEPT4:                                                      #call=20
        err = __sys_accept4(a0, (struct sockaddr __user *)a1,
                    (int __user *)a[2], a[3]);
        break;
    default:
        err = -EINVAL;
        break;
    }
    return err;

   在上次实验中,我们发现了sys_socketcall根据传入call的数值决定调用的函数,又根据gdb信息不难定位到__sys_socket,__sys_connect,__sys_listen,__sys_accept4四个函数,因此打开gdb,连接并打断点,如图所示。

 

  也就是说在这些函数的调用后,我们实现了TCP通信,那么我们接下来就依次看看具体的源代码来看看TCP的三次握手是怎么实现的。

  (1)首先是__sys_socket,代码如下

int __sys_socket(int family, int type, int protocol)
{
        int retval;
        struct socket *sock;
        int flags;

        /* Check the SOCK_* constants for consistency.  */
        BUILD_BUG_ON(SOCK_CLOEXEC != O_CLOEXEC);
        BUILD_BUG_ON((SOCK_MAX | SOCK_TYPE_MASK) != SOCK_TYPE_MASK);
        BUILD_BUG_ON(SOCK_CLOEXEC & SOCK_TYPE_MASK);
        BUILD_BUG_ON(SOCK_NONBLOCK & SOCK_TYPE_MASK);

        flags = type & ~SOCK_TYPE_MASK;
        if (flags & ~(SOCK_CLOEXEC | SOCK_NONBLOCK))
                return -EINVAL;
        type &= SOCK_TYPE_MASK;

        if (SOCK_NONBLOCK != O_NONBLOCK && (flags & SOCK_NONBLOCK))
                flags = (flags & ~SOCK_NONBLOCK) | O_NONBLOCK;

        retval = sock_create(family, type, protocol, &sock);
        if (retval < 0)
                return retval;
        return sock_map_fd(sock, flags & (O_CLOEXEC | O_NONBLOCK));
}  

  在这个函数中,主要是调用了sock_create和sock_map_fd函数,找到这两个函数,源码如下:

int sock_create(int family, int type, int protocol, struct socket **res)
{
        return __sock_create(current->nsproxy->net_ns, family, type, protocol, res, 0);
}

  调用了__sock_create函数,源码如下:

int __sock_create(struct net *net, int family, int type, int protocol,
                         struct socket **res, int kern)
{
        int err;
        struct socket *sock;
        const struct net_proto_family *pf;

        /*
         *      Check protocol is in range
         */
        if (family < 0 || family >= NPROTO)
                return -EAFNOSUPPORT;
        if (type < 0 || type >= SOCK_MAX)
                return -EINVAL;

        /* Compatibility.

           This uglymoron is moved from INET layer to here to avoid
           deadlock in module load.
         */
        if (family == PF_INET && type == SOCK_PACKET) {
                pr_info_once("%s uses obsolete (PF_INET,SOCK_PACKET)\n",
                             current->comm);
                family = PF_PACKET;
        }

        err = security_socket_create(family, type, protocol, kern);
        if (err)
                return err;

        /*
         *      Allocate the socket and allow the family to set things up. if
         *      the protocol is 0, the family is instructed to select an appropriate
         *      default.
         */
        sock = sock_alloc();
        if (!sock) {
                net_warn_ratelimited("socket: no more sockets\n");
                return -ENFILE; /* Not exactly a match, but its the
                                   closest posix thing */
        }

        sock->type = type;

#ifdef CONFIG_MODULES
        /* Attempt to load a protocol module if the find failed.
         *
         * 12/09/1996 Marcin: But! this makes REALLY only sense, if the user
         * requested real, full-featured networking support upon configuration.
 * Otherwise module support will break!
         */
        if (rcu_access_pointer(net_families[family]) == NULL)
                request_module("net-pf-%d", family);
#endif

        rcu_read_lock();
        pf = rcu_dereference(net_families[family]);
        err = -EAFNOSUPPORT;
        if (!pf)
                goto out_release;

        /*
         * We will call the ->create function, that possibly is in a loadable
         * module, so we have to bump that loadable module refcnt first.
         */
        if (!try_module_get(pf->owner))
                goto out_release;

        /* Now protected by module ref count */
        rcu_read_unlock();

        err = pf->create(net, sock, protocol, kern);
        if (err < 0)
                goto out_module_put;

        /*
         * Now to bump the refcnt of the [loadable] module that owns this
         * socket at sock_release time we decrement its refcnt.
         */
        if (!try_module_get(sock->ops->owner))
                goto out_module_busy;

        /*
         * Now that we're done with the ->create function, the [loadable]
         * module can have its refcnt decremented
         */
        module_put(pf->owner);
        err = security_socket_post_create(sock, family, type, protocol, kern);
        if (err)
                goto out_sock_release;
        *res = sock;

        return 0;

out_module_busy:
        err = -EAFNOSUPPORT;
out_module_put:
        sock->ops = NULL;
        module_put(pf->owner);
out_sock_release:
        sock_release(sock);
        return err;
out_release:
        rcu_read_unlock();
        goto out_sock_release;
}
struct socket *sock_alloc(void)
{
        struct inode *inode;
        struct socket *sock;

        inode = new_inode_pseudo(sock_mnt->mnt_sb);
        if (!inode)
                return NULL;

        sock = SOCKET_I(inode);

        inode->i_ino = get_next_ino();
        inode->i_mode = S_IFSOCK | S_IRWXUGO;
        inode->i_uid = current_fsuid();
        inode->i_gid = current_fsgid();
        inode->i_op = &sockfs_inode_ops;

        return sock;
}
EXPORT_SYMBOL(sock_alloc);

  可以看到,调用了sock_alloc函数初始化socket的相关信息。

  sock_map_fd()主要用于对socket的*file指针初始化,经过sock_map_fd()操作后,socket就通过其*file指针与VFS管理的文件进行了关联,便可以进行文件的各种操作,如read、write、lseek、ioctl等. 

static int sock_map_fd(struct socket *sock, int flags)
{
        struct file *newfile;
        int fd = get_unused_fd_flags(flags);
        if (unlikely(fd < 0)) {
                sock_release(sock);
                return fd;
        }

        newfile = sock_alloc_file(sock, flags, NULL);
        if (likely(!IS_ERR(newfile))) {
                fd_install(fd, newfile);
                return fd;
        }

        put_unused_fd(fd);
        return PTR_ERR(newfile);
}

 

(2)其次是__sys_connect,源码如下

int __sys_connect(int fd, struct sockaddr __user *uservaddr, int addrlen)
{
        struct socket *sock;
        struct sockaddr_storage address;
        int err, fput_needed;
    //获得socket
        sock = sockfd_lookup_light(fd, &err, &fput_needed);
        if (!sock)
                goto out;
    //将地址移动到内核空间 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);
      //对于流式套接字,sock->ops为 inet_stream_ops -->inet_stream_connect
        //对于数据报套接字,sock->ops为 inet_dgram_ops --> inet_dgram_connect
out_put: fput_light(sock->file, fput_needed); out: return err; }

由于我们时TCP协议,采用的肯定是流的形式,继续看inet_stream_connect,在gdb中打断点,找到inet_stream_connec的定义:

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, 0);
        release_sock(sock->sk);
        return err;
}

采用锁保证操作的原子性,调用的__inet_stream_connect函数源码:

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;
}

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;
}
//判断socket状态
switch (sock->state) {
default:
err = -EINVAL;
goto out;
case SS_CONNECTED:
err = -EISCONN;
goto out;
case SS_CONNECTING:
err = -EALREADY;
/* Fall out of switch with err, set for this state */
break;
case SS_UNCONNECTED://未建立连接,因此发起连接走的是这个流程
err = -EISCONN;
if (sk->sk_state != TCP_CLOSE)
goto out;
//主处理函数,最终调用的是tcp_v4_connect()函数
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.
*/
//如果是非阻塞调用,那么最后返回的就是这个错误码
err = -EINPROGRESS;
break;
}

//如果connect设置的是非阻塞,获取超时时间
//超时时间可以通过SO_SNDTIMEO选项设置
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 */
//非阻塞时,timeo为0,直接返回;否则设置定时器,然后调度出去,等待超时返回
if (!timeo || !inet_wait_for_connect(sk, timeo, writebias))
goto out;

err = sock_intr_errno(timeo);
if (signal_pending(current))
goto out;
}
...
}

  该函数的作用是判断socket的状态,根据状态判断做出动作,sk->sk_prot指向tcp_prot,因此sk->sk_prot->connect最终调用的就是tcp_v4_connect()。

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;
...
    nexthop = daddr = usin->sin_addr.s_addr;//赋值下一跳地址和目的地址,
    inet_opt = rcu_dereference_protected(inet->inet_opt,
                         sock_owned_by_user(sk));
    if (inet_opt && inet_opt->opt.srr) {
        if (!daddr)
            return -EINVAL;
        nexthop = inet_opt->opt.faddr;
    }

    orig_sport = inet->inet_sport;//源地址
    orig_dport = usin->sin_port;//源端口
    fl4 = &inet->cork.fl.u.ip4;
    //根据当前信息,查找路由,并新建路由缓存
    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 (!inet->inet_saddr)
        //如果socket没有绑定ip地址,使用路由查询返回的结果
        inet->inet_saddr = fl4->saddr;
    //inet_rcv_saddr表示的是本地绑定的ip地址,也就是源地址
    inet->inet_rcv_saddr = inet->inet_saddr;

    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);

    inet->inet_dport = usin->sin_port;//目的端口
    inet->inet_daddr = daddr;//目的地址

    inet_csk(sk)->icsk_ext_hdr_len = 0;
    if (inet_opt)
        inet_csk(sk)->icsk_ext_hdr_len = inet_opt->opt.optlen;

    tp->rx_opt.mss_clamp = TCP_MSS_DEFAULT;

    tcp_set_state(sk, TCP_SYN_SENT);//socket进入SYN-SENT状态
    //绑定ip和端口号,并将sock加入哈希表中
    err = inet_hash_connect(&tcp_death_row, sk);
    if (err)
        goto failure;

    sk_set_txhash(sk);
    //使用新的端口号再次做路由查询,
    //因为如果客户端没有用bind()绑定IP地址和端口号,上面inet_hash_connect()
    //就会自动选择一个端口号,因此源端口会不一样
    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))
        //生成序列号
        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;

    //由socket层转入TCP层,构造SYN报文并发送
    err = tcp_connect(sk);
...
}

  客户端自动发起连接,至此第一步完成。

  在分析connect()系统调用时,我们已经发送SYN报文,所以服务端就需要作出回应了,SYN报文到达TCP层由tcp_v4_rcv()接管。

int tcp_v4_rcv(struct sk_buff *skb)
{
    const struct iphdr *iph;
    const struct tcphdr *th;
    struct sock *sk;
    int ret;
    struct net *net = dev_net(skb->dev);
...
    //checksum检查,其实也就是完整性校验
    if (skb_checksum_init(skb, IPPROTO_TCP, inet_compute_pseudo))
        goto csum_error;

    th = tcp_hdr(skb);//获取TCP头部
    iph = ip_hdr(skb);//获取ip头部
    TCP_SKB_CB(skb)->seq = ntohl(th->seq);
    TCP_SKB_CB(skb)->end_seq = (TCP_SKB_CB(skb)->seq + th->syn + th->fin +
                    skb->len - th->doff * 4);
    TCP_SKB_CB(skb)->ack_seq = ntohl(th->ack_seq);
    TCP_SKB_CB(skb)->tcp_flags = tcp_flag_byte(th);
    TCP_SKB_CB(skb)->tcp_tw_isn = 0;
    TCP_SKB_CB(skb)->ip_dsfield = ipv4_get_dsfield(iph);
    TCP_SKB_CB(skb)->sacked  = 0;

    //根据报文的源和目的地址在established哈希表以及listen哈希表中查找连接
    //对于正要建立的连接,返回的就是listen哈希表的连接
    sk = __inet_lookup_skb(&tcp_hashinfo, skb, th->source, th->dest);
    if (!sk)
        goto no_tcp_socket;

process:
    //如果此时socket状态处于time_wait,那就进入对应的处理流程中
    if (sk->sk_state == TCP_TIME_WAIT)
        goto do_time_wait;
...
    th = (const struct tcphdr *)skb->data;
    iph = ip_hdr(skb);

    sk_mark_napi_id(sk, skb);//记录napi的id
    skb->dev = NULL;

    bh_lock_sock_nested(sk);
    tcp_sk(sk)->segs_in += max_t(u16, 1, skb_shinfo(skb)->gso_segs);
    ret = 0;
    if (!sock_owned_by_user(sk)) {//如果sk没有被用户锁定,即没在使用
        //检查是否需要先进入prequeue队列
        if (!tcp_prequeue(sk, skb))
            ret = tcp_v4_do_rcv(sk, skb);//进入到主处理函数

    //如果用户正在使用,则数据包进入backlog中
    //不太理解的是为什么limit入参是sk_rcvbuf和sk_sndbuf之和
    } else if (unlikely(sk_add_backlog(sk, skb,
                       sk->sk_rcvbuf + sk->sk_sndbuf))) {
        bh_unlock_sock(sk);
        NET_INC_STATS_BH(net, LINUX_MIB_TCPBACKLOGDROP);
        goto discard_and_relse;
    }
    bh_unlock_sock(sk);

    sock_put(sk);

    return ret;
...

do_time_wait:
    if (!xfrm4_policy_check(NULL, XFRM_POLICY_IN, skb)) {
        inet_twsk_put(inet_twsk(sk));
        goto discard_it;
    }

    if (skb->len < (th->doff << 2)) {
        inet_twsk_put(inet_twsk(sk));
        goto bad_packet;
    }
    if (tcp_checksum_complete(skb)) {
        inet_twsk_put(inet_twsk(sk));
        goto csum_error;
    }
    //处理在time_wait状态收到报文的情况
    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,
                            iph->saddr, th->source,
                            iph->daddr, th->dest,
                            inet_iif(skb));
        if (sk2) {
            inet_twsk_deschedule(inet_twsk(sk), &tcp_death_row);
            inet_twsk_put(inet_twsk(sk));
            sk = sk2;
            goto process;
        }
        /* Fall through to ACK */
    }
    case TCP_TW_ACK:
        tcp_v4_timewait_ack(sk, skb);
        break;
    case TCP_TW_RST:
        tcp_v4_send_reset(sk, skb);
        inet_twsk_deschedule(inet_twsk(sk), &tcp_death_row);
        inet_twsk_put(inet_twsk(sk));
        goto discard_it;
    case TCP_TW_SUCCESS:;
    }
    goto discard_it;
}

接收到SYN包后要查看下该报文是否之前已建立的连接,通过__inet_lookup_skb()查找是否有匹配的连接。

static inline struct sock *__inet_lookup_skb(struct inet_hashinfo *hashinfo,
                         struct sk_buff *skb,
                         const __be16 sport,
                         const __be16 dport)
{
    //sk_buff结构体里有一个变量指向sock,即skb->sk
    //但是对于尚未建立连接的skb来说,其sk变量为空,因此会走进__inet_lookup()
    struct sock *sk = skb_steal_sock(skb);
    const struct iphdr *iph = ip_hdr(skb);

    if (sk)
        return sk;
    else
        return __inet_lookup(dev_net(skb_dst(skb)->dev), hashinfo,
                     iph->saddr, sport,
                     iph->daddr, dport, inet_iif(skb));
}
static inline struct sock *__inet_lookup(struct net *net,
                     struct inet_hashinfo *hashinfo,
                     const __be32 saddr, const __be16 sport,
                     const __be32 daddr, const __be16 dport,
                     const int dif)
{
    u16 hnum = ntohs(dport);
    //查找established哈希表
    struct sock *sk = __inet_lookup_established(net, hashinfo,
                saddr, sport, daddr, hnum, dif);
    //查找listen哈希表
    return sk ? : __inet_lookup_listener(net, hashinfo, saddr, sport,
                         daddr, hnum, dif);
}

  最终会在listen哈希表中找到该连接,也就是服务端的监听socket。

  之后如果当前这个监听socket没有被使用,就会进入prequeue队列中处理,但是由于这是SYN报文,还没有进程接收数据,所以不会进入prequeue的真正处理中。

bool tcp_prequeue(struct sock *sk, struct sk_buff *skb)
{
    struct tcp_sock *tp = tcp_sk(sk);

    //如果设置了/proc/sys/net/ipv4/tcp_low_latency(低时延)参数,默认为0
    //或者用户还没有调用接收函数接收数据,那么不使用prequeue队列
    //ucopy.task会在接收数据函数recvmsg()中设置为接收数据的当前进程
    //所以对于第一个SYN报文,会从以下分支返回
    if (sysctl_tcp_low_latency || !tp->ucopy.task)
        return false;

    if (skb->len <= tcp_hdrlen(skb) &&
        skb_queue_len(&tp->ucopy.prequeue) == 0)
        return false;

    if (likely(sk->sk_rx_dst))
        skb_dst_drop(skb);
    else
        skb_dst_force_safe(skb);

    //加入到prequeue队列尾部
    __skb_queue_tail(&tp->ucopy.prequeue, skb);
    tp->ucopy.memory += skb->truesize;
    //如果prequeue队列长度大于socket连接的接收缓冲区,
    //将prequeue中的数据报文转移到receive_queue中
    if (tp->ucopy.memory > sk->sk_rcvbuf) {
        struct sk_buff *skb1;

        BUG_ON(sock_owned_by_user(sk));
        //从prequeue中摘链
        while ((skb1 = __skb_dequeue(&tp->ucopy.prequeue)) != NULL) {
            sk_backlog_rcv(sk, skb1);//放入backlog中
            NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPPREQUEUEDROPPED);
        }

        tp->ucopy.memory = 0;
    //如果prequeue中有报文了,那么唤醒睡眠的进程来收取报文
    } else if (skb_queue_len(&tp->ucopy.prequeue) == 1) {
        //唤醒sk上睡眠的进程,这里只唤醒其中一个,避免惊群现象
        //至于怎么唤醒,选择哪个唤醒,暂未研究
        wake_up_interruptible_sync_poll(sk_sleep(sk),
                       POLLIN | POLLRDNORM | POLLRDBAND);
        //没有ACK需要发送,重置延时ACK定时器
        if (!inet_csk_ack_scheduled(sk))
            inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,
                          (3 * tcp_rto_min(sk)) / 4,
                          TCP_RTO_MAX);
    }
    return true;
}

既然不会进入到prequeue队列中,那就进入tcp_v4_do_rcv()的处理,这是个主要的报文处理函数。

int tcp_v4_do_rcv(struct sock *sk, struct sk_buff *skb)
{
    struct sock *rsk;
...
    //SYN报文走的是这里
    if (sk->sk_state == TCP_LISTEN) {
        //查找对应的半连接状态的socket
        struct sock *nsk = tcp_v4_hnd_req(sk, skb);
        if (!nsk)
            goto discard;

        //可知,对于SYN报文,返回的还是入参sk,即nsk=sk
        if (nsk != sk) {
            sock_rps_save_rxhash(nsk, skb);
            if (tcp_child_process(sk, nsk, skb)) {
                rsk = nsk;
                goto reset;
            }
            return 0;
        }
    } else
        sock_rps_save_rxhash(sk, skb);

    //这里是除ESTABLISHED and TIME_WAIT状态外报文的归宿。。。
    if (tcp_rcv_state_process(sk, skb, tcp_hdr(skb), skb->len)) {
        rsk = sk;
        goto reset;
    }
    return 0;
...
}

半连接状态socket通过tcp_v4_hnd_req()查找。

static struct sock *tcp_v4_hnd_req(struct sock *sk, struct sk_buff *skb)
{
    struct tcphdr *th = tcp_hdr(skb);
    const struct iphdr *iph = ip_hdr(skb);
    struct sock *nsk;
    struct request_sock **prev;
    /* Find possible connection requests. */
    //查找半连接队列,对于SYN报文肯定找不到
    struct request_sock *req = inet_csk_search_req(sk, &prev, th->source,
                               iph->saddr, iph->daddr);
    if (req)
        return tcp_check_req(sk, skb, req, prev, false);

    //再一次查找established哈希表,以防在此期间重传过SYN报文且建立了连接
    //对于SYN报文这里也是返回空的
    nsk = inet_lookup_established(sock_net(sk), &tcp_hashinfo, iph->saddr,
            th->source, iph->daddr, th->dest, inet_iif(skb));

    if (nsk) {
        if (nsk->sk_state != TCP_TIME_WAIT) {
            bh_lock_sock(nsk);
            return nsk;
        }
        inet_twsk_put(inet_twsk(nsk));
        return NULL;
    }

#ifdef CONFIG_SYN_COOKIES
    if (!th->syn)
        sk = cookie_v4_check(sk, skb, &(IPCB(skb)->opt));
#endif
    //所以最终返回的还是原来的连接,即该函数对于SYN报文啥都没做
    return sk;
}

加入半连接队列通过icsk->icsk_af_ops->conn_request操作。我们知道icsk->icsk_af_ops指向ipv4_specific。

const struct inet_connection_sock_af_ops ipv4_specific = {
    .queue_xmit    = ip_queue_xmit,
    .send_check    = tcp_v4_send_check,
    .rebuild_header    = inet_sk_rebuild_header,
    .sk_rx_dst_set     = inet_sk_rx_dst_set,
    .conn_request      = tcp_v4_conn_request,
...
};

所以加入半连接的操作就是由tcp_v4_conn_request()操作。

int tcp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
{
    /* Never answer to SYNs send to broadcast or multicast */
    if (skb_rtable(skb)->rt_flags & (RTCF_BROADCAST | RTCF_MULTICAST))
        goto drop;

    return tcp_conn_request(&tcp_request_sock_ops,
                &tcp_request_sock_ipv4_ops, sk, skb);
drop:
    NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_LISTENDROPS);
    return 0;
}

tcp_v4_conn_request()对tcp_conn_request()做了一个简单的封装。

int tcp_conn_request(struct request_sock_ops *rsk_ops,
             const struct tcp_request_sock_ops *af_ops,
             struct sock *sk, struct sk_buff *skb)
{
    struct tcp_options_received tmp_opt;
    struct request_sock *req;
    struct tcp_sock *tp = tcp_sk(sk);
    struct dst_entry *dst = NULL;
    __u32 isn = TCP_SKB_CB(skb)->tcp_tw_isn;
    bool want_cookie = false, fastopen;
    struct flowi fl;
    struct tcp_fastopen_cookie foc = { .len = -1 };
    int err;

    //如果开启了syncookies选项,/proc/sys/net/ipv4/
    if ((sysctl_tcp_syncookies == 2 ||
         //或者此时半连接队列已经满了
         //同时isn不是由tcp_timewait_state_process()函数选择
         //那么判断是否需要发送syncookie
         inet_csk_reqsk_queue_is_full(sk)) && !isn) {
        want_cookie = tcp_syn_flood_action(sk, skb, rsk_ops->slab_name);
        //不需要发送syncookies就直接丢弃报文
        if (!want_cookie)
            goto drop;
    }

    //如果全连接队列满了,同时半连接队列里尚未重传过的SYN报文个数大于1
    //那么就直接丢弃报文
    if (sk_acceptq_is_full(sk) && inet_csk_reqsk_queue_young(sk) > 1) {
        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_LISTENOVERFLOWS);
        goto drop;
    }

    //都没问题的话,那就分配一个request_sock,表示一个请求
    //这个内存分配是从tcp的slab中分配的
    req = inet_reqsk_alloc(rsk_ops);
    if (!req)
        goto drop;

    inet_rsk(req)->ireq_family = sk->sk_family;

    //af_ops即为tcp_request_sock_ipv4_ops,这个结构体比较重要,请留意
    tcp_rsk(req)->af_specific = af_ops;

    tcp_clear_options(&tmp_opt);
    tmp_opt.mss_clamp = af_ops->mss_clamp;
    tmp_opt.user_mss  = tp->rx_opt.user_mss;
    //分析该请求的tcp各个选项,比如时间戳、窗口大小、快速开启等选项
    tcp_parse_options(skb, &tmp_opt, 0, want_cookie ? NULL : &foc);

    if (want_cookie && !tmp_opt.saw_tstamp)
        tcp_clear_options(&tmp_opt);

    tmp_opt.tstamp_ok = tmp_opt.saw_tstamp;//记录时间戳选项开启情况
    //将刚才分析的请求的TCP选项记录到刚刚分配的request_sock中,即req中
    tcp_openreq_init(req, &tmp_opt, skb);

    af_ops->init_req(req, sk, skb);

    if (security_inet_conn_request(sk, skb, req))
        goto drop_and_free;

    //如果不需要发送syncookies
    //同时isn不是由tcp_timewait_state_process()函数选择
    if (!want_cookie && !isn) {
        //如果开启了time_wait状态连接快速回收
        //即设置/proc/sys/net/ipv4/tcp_tw_recycle
        if (tcp_death_row.sysctl_tw_recycle) {
            bool strict;
            //查找路由
            dst = af_ops->route_req(sk, &fl, req, &strict);

            if (dst && strict &&
                //主要用于判断是否会和该IP的旧连接冲突
                //这里就涉及到nat环境下丢包的问题
                !tcp_peer_is_proven(req, dst, true, tmp_opt.saw_tstamp)) {
                NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSPASSIVEREJECTED);
                goto drop_and_release;
            }
        }
        /* Kill the following clause, if you dislike this way. */
        //如果没有开启syncookies选项
        else if (!sysctl_tcp_syncookies &&
             //同时,半连接队列长度已经大于syn backlog队列的3/4
             (sysctl_max_syn_backlog - inet_csk_reqsk_queue_len(sk) <
              (sysctl_max_syn_backlog >> 2)) &&
             //并且当前连接和旧连接有冲突
             !tcp_peer_is_proven(req, dst, false, tmp_opt.saw_tstamp)) {
            //很有可能遭受synflood 攻击
            pr_drop_req(req, ntohs(tcp_hdr(skb)->source), rsk_ops->family);
            goto drop_and_release;
        }
        //生成随机报文序列号
        isn = af_ops->init_seq(skb);
    }
    if (!dst) {
        dst = af_ops->route_req(sk, &fl, req, NULL);
        if (!dst)
            goto drop_and_free;
    }

    tcp_ecn_create_request(req, skb, sk, dst);

    //如果要发送syncookies,那就发送
    if (want_cookie) {
        isn = cookie_init_sequence(af_ops, sk, skb, &req->mss);
        req->cookie_ts = tmp_opt.tstamp_ok;
        if (!tmp_opt.tstamp_ok)
            inet_rsk(req)->ecn_ok = 0;
    }

    tcp_rsk(req)->snt_isn = isn;
    tcp_openreq_init_rwin(req, sk, dst);
    fastopen = !want_cookie && tcp_try_fastopen(sk, skb, req, &foc, dst);
    //这里便是调用tcp_v4_send_synack()发送SYNACK报文了
    err = af_ops->send_synack(sk, dst, &fl, req,
                  skb_get_queue_mapping(skb), &foc);
    if (!fastopen) {
        if (err || want_cookie)
            goto drop_and_free;

        tcp_rsk(req)->listener = NULL;
        //发送报文后将该请求加入半连接队列,同时启动SYNACK定时器
        //调用inet_csk_reqsk_queue_hash_add()完成上述操作
        af_ops->queue_hash_add(sk, req, TCP_TIMEOUT_INIT);
    }

    return 0;
...
}

要加入半连接队列首先要创建一个request_sock,用于表示客户端发起的请求,然后是做一些初始化,其中req->ts_recent后续会用到多次,这个变量表示的就是对端发送报文的时间(前提是对端开启了时间戳选项)。

static inline void tcp_openreq_init(struct request_sock *req,
                    struct tcp_options_received *rx_opt,
                    struct sk_buff *skb)
{
    struct inet_request_sock *ireq = inet_rsk(req);

    req->rcv_wnd = 0;       /* So that tcp_send_synack() knows! */
    req->cookie_ts = 0;
    tcp_rsk(req)->rcv_isn = TCP_SKB_CB(skb)->seq;
    tcp_rsk(req)->rcv_nxt = TCP_SKB_CB(skb)->seq + 1;
    tcp_rsk(req)->snt_synack = tcp_time_stamp;
    tcp_rsk(req)->last_oow_ack_time = 0;
    req->mss = rx_opt->mss_clamp;
    //如果对端开启时间戳,那么记录下这个时间,也就是对方发送SYN报文的时间
    req->ts_recent = rx_opt->saw_tstamp ? rx_opt->rcv_tsval : 0;
    ireq->tstamp_ok = rx_opt->tstamp_ok;//时间戳开启标志
    ireq->sack_ok = rx_opt->sack_ok;
    ireq->snd_wscale = rx_opt->snd_wscale;
    ireq->wscale_ok = rx_opt->wscale_ok;
    ireq->acked = 0;
    ireq->ecn_ok = 0;
    ireq->ir_rmt_port = tcp_hdr(skb)->source;
    //目的端口,也就是当前服务端的监听端口
    ireq->ir_num = ntohs(tcp_hdr(skb)->dest);
}

初始化完这个请求后就要看下这个请求是否有问题。主要检查的就是看是否会和当前ip的上次通讯有冲突。该操作通过tcp_peer_is_proven()检查。

#define TCP_PAWS_MSL    60      /* Per-host timestamps are invalidated
                     * after this time. It should be equal
                     * (or greater than) TCP_TIMEWAIT_LEN
                     * to provide reliability equal to one
                     * provided by timewait state.
                     */
#define TCP_PAWS_WINDOW 1       /* Replay window for per-host
                     * timestamps. It must be less than
                     * minimal timewait lifetime.
                     */
bool tcp_peer_is_proven(struct request_sock *req, struct dst_entry *dst,
            bool paws_check, bool timestamps)
{
    struct tcp_metrics_block *tm;
    bool ret;

    if (!dst)
        return false;

    rcu_read_lock();
    tm = __tcp_get_metrics_req(req, dst);
    if (paws_check) {
        //如果当前ip的上次tcp通讯发生在60s内
        if (tm && (u32)get_seconds() - tm->tcpm_ts_stamp < TCP_PAWS_MSL &&
            //同时当前ip上次tcp通信的时间戳大于本次tcp,或者没有开启时间戳开关
            //从这里看,快速回收打开选项就很容易导致nat环境丢包
            ((s32)(tm->tcpm_ts - req->ts_recent) > TCP_PAWS_WINDOW ||!timestamps))
            ret = false;
        else
            ret = true;
    } else {
        if (tm && tcp_metric_get(tm, TCP_METRIC_RTT) && tm->tcpm_ts_stamp)
            ret = true;
        else
            ret = false;
    }
    rcu_read_unlock();

    return ret;
}

SYNACK报文就是通过tcp_v4_send_synack()发送。

static int tcp_v4_send_synack(struct sock *sk, struct dst_entry *dst,
                  struct flowi *fl,
                  struct request_sock *req,
                  u16 queue_mapping,
                  struct tcp_fastopen_cookie *foc)
{
    const struct inet_request_sock *ireq = inet_rsk(req);
    struct flowi4 fl4;
    int err = -1;
    struct sk_buff * skb;

    //获取路由
    if (!dst && (dst = inet_csk_route_req(sk, &fl4, req)) == NULL)
        return -1;

    //准备synack报文,该报文使用的是用户的send buffer内存
    skb = tcp_make_synack(sk, dst, req, foc);

    if (skb) {
        __tcp_v4_send_check(skb, ireq->ir_loc_addr, ireq->ir_rmt_addr);

        skb_set_queue_mapping(skb, queue_mapping);
        //传到IP层继续处理,组建ip头,然后发送报文
        err = ip_build_and_send_pkt(skb, sk, ireq->ir_loc_addr,
                        ireq->ir_rmt_addr,
                        ireq->opt);
        err = net_xmit_eval(err);
    }

    return err;
}

发送完SYNACK报文,接着就是将该连接放入半连接队列了,同时启动我们的SYNACK定时器。这一动作通过af_ops->queue_hash_add实现,由上面结构体可知,也就是调用inet_csk_reqsk_queue_hash_add()。

void inet_csk_reqsk_queue_hash_add(struct sock *sk, struct request_sock *req,
                   unsigned long timeout)
{
    struct inet_connection_sock *icsk = inet_csk(sk);
    struct listen_sock *lopt = icsk->icsk_accept_queue.listen_opt;
    const u32 h = inet_synq_hash(inet_rsk(req)->ir_rmt_addr,
                     inet_rsk(req)->ir_rmt_port,
                     lopt->hash_rnd, lopt->nr_table_entries);
    //添加到半连接队列
    reqsk_queue_hash_req(&icsk->icsk_accept_queue, h, req, timeout);
    //更新半连接队列统计信息,同时开启SYNACK定时器
    inet_csk_reqsk_queue_added(sk, timeout);
}

   剩下的工作就是将报文封装到ip层,由网卡发出。

   随着SYNACK报文的发送,连接建立随着第二次握手报文来到客户端。客户端接收到这个SYNACK报文,就认为连接建立了。仍然从TCP层开始分析,依然是由tcp_v4_rcv()入手。

int tcp_v4_rcv(struct sk_buff *skb)
{
...
    //根据报文的源和目的地址在established哈希表以及listen哈希表中查找连接
    //由于之前调用connect时已经将连接加入到established哈希表中
    //所以在接收到服务端的SYNACK时,就能从established表中找到对应的连接
    sk = __inet_lookup_skb(&tcp_hashinfo, skb, th->source, th->dest);
    if (!sk)
        goto no_tcp_socket;
...
    ret = 0;
    if (!sock_owned_by_user(sk)) {//如果sk没有被用户锁定,及没在使用
        if (!tcp_prequeue(sk, skb))
            ret = tcp_v4_do_rcv(sk, skb);//进入到主处理函数

    }
...
}

然后还是来到老地方——tcp_v4_do_rcv()。

int tcp_v4_do_rcv(struct sock *sk, struct sk_buff *skb)
{
    struct sock *rsk;

    if (sk->sk_state == TCP_ESTABLISHED) { /* Fast path */
        ...
    }

    if (skb->len < tcp_hdrlen(skb) || tcp_checksum_complete(skb))
        goto csum_err;

    if (sk->sk_state == TCP_LISTEN) {
        ...
    } else
        sock_rps_save_rxhash(sk, skb);

    if (tcp_rcv_state_process(sk, skb, tcp_hdr(skb), skb->len)) {
        rsk = sk;
        goto reset;
    }
    return 0;
...
}

不过因为此时我们socket的状态是SYN_SENT,所以就直接进入tcp_rcv_state_process()的处理流程了。

int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
              const struct tcphdr *th, unsigned int len)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct inet_connection_sock *icsk = inet_csk(sk);
    struct request_sock *req;
    int queued = 0;
    bool acceptable;
    u32 synack_stamp;

    tp->rx_opt.saw_tstamp = 0;

    switch (sk->sk_state) {
...
    case TCP_SYN_SENT:
        //进入到synack报文的处理流程
        queued = tcp_rcv_synsent_state_process(sk, skb, th, len);
        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;
    }
...
}

层层深入,最后进入的归宿是tcp_rcv_synsent_state_process()。

static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
                     const struct tcphdr *th, unsigned int len)
{
    struct inet_connection_sock *icsk = inet_csk(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    struct tcp_fastopen_cookie foc = { .len = -1 };
    int saved_clamp = tp->rx_opt.mss_clamp;
    //分析TCP选项
    tcp_parse_options(skb, &tp->rx_opt, 0, &foc);
    if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr)
        tp->rx_opt.rcv_tsecr -= tp->tsoffset;

    if (th->ack) {//处理带ACK标志的报文
        //如果接收到的确认号小于或等于已发送未确认的序列号,
        //或者大于下次要发送数据的序列号,非法报文,发送RST报文
        if (!after(TCP_SKB_CB(skb)->ack_seq, tp->snd_una) ||
            after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt))
            goto reset_and_undo;

        //如果开启了时间戳选项,且回显时间戳不为空
        if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
            //且回显时间戳不在当前时间和SYN报文发送的时间窗内,就认为该报文非法
            !between(tp->rx_opt.rcv_tsecr, tp->retrans_stamp, tcp_time_stamp)) {
            NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSACTIVEREJECTED);
            goto reset_and_undo;
        }

        if (th->rst) {//ack报文不允许出现rst标志
            tcp_reset(sk);
            goto discard;
        }

        if (!th->syn)//除了上面的几种标志位和SYN标志位,其余报文都丢弃
            goto discard_and_undo;

        TCP_ECN_rcv_synack(tp, th);

        tcp_init_wl(tp, TCP_SKB_CB(skb)->seq);
        //确认ACK的确认号正常
        tcp_ack(sk, skb, FLAG_SLOWPATH);

        /* Ok.. it's good. Set up sequence numbers and
         * move to established.
         */
        tp->rcv_nxt = TCP_SKB_CB(skb)->seq + 1;
        tp->rcv_wup = TCP_SKB_CB(skb)->seq + 1;

        /* RFC1323: The window in SYN & SYN/ACK segments is
         * never scaled.
         */
        tp->snd_wnd = ntohs(th->window);

        if (!tp->rx_opt.wscale_ok) {
            tp->rx_opt.snd_wscale = tp->rx_opt.rcv_wscale = 0;
            tp->window_clamp = min(tp->window_clamp, 65535U);
        }
        //如果连接支持时间戳选项
        if (tp->rx_opt.saw_tstamp) {
            tp->rx_opt.tstamp_ok       = 1;
            tp->tcp_header_len =
                sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
            tp->advmss      -= TCPOLEN_TSTAMP_ALIGNED;
            tcp_store_ts_recent(tp);//记录对端的时间戳
        } else {
            tp->tcp_header_len = sizeof(struct tcphdr);
        }

        if (tcp_is_sack(tp) && sysctl_tcp_fack)
            tcp_enable_fack(tp);

        tcp_mtup_init(sk);//mtu探测初始化
        tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
        tcp_initialize_rcv_mss(sk);

        /* Remember, tcp_poll() does not lock socket!
         * Change state from SYN-SENT only after copied_seq
         * is initialized. */
        tp->copied_seq = tp->rcv_nxt;

        smp_mb();
        //连接建立完成,将连接状态推向established
        //然后唤醒等在该socket的所有睡眠进程
        tcp_finish_connect(sk, skb);
        //快速开启选项
        if ((tp->syn_fastopen || tp->syn_data) &&
            tcp_rcv_fastopen_synack(sk, skb, &foc))
            return -1;

        /* 如果有以下情况,不会马上发送ACK报文
         * 1.有数据等待发送
         * 2.用户设置了TCP_DEFER_ACCEPT选项
         * 3.禁用快速确认模式,可通过TCP_QUICKACK设置
        */
        if (sk->sk_write_pending ||
            icsk->icsk_accept_queue.rskq_defer_accept ||
            icsk->icsk_ack.pingpong) {

            //设置ICSK_ACK_SCHED标识,有ACK等待发送,当前不发送
            inet_csk_schedule_ack(sk);
            //最后一次接收到数据包的时间
            icsk->icsk_ack.lrcvtime = tcp_time_stamp;
            //设置快速确认模式,以及快速确认模式下可以发送的ACK报文数
            tcp_enter_quickack_mode(sk);
            //激活延迟ACK定时器,超时时间为200ms
            //最多延迟200ms就会发送ACK报文
            inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,
                          TCP_DELACK_MAX, TCP_RTO_MAX);

discard:
            __kfree_skb(skb);
            return 0;
        } else {
            tcp_send_ack(sk);//否则马上发送ACK报文,即第三次握手报文
        }
        return -1;
    }
    //没有ACK标记,只有RST标记,丢弃报文
    if (th->rst) {

        goto discard_and_undo;
    }

    /* PAWS check. */
  
    if (tp->rx_opt.ts_recent_stamp && tp->rx_opt.saw_tstamp &&
        tcp_paws_reject(&tp->rx_opt, 0))
        goto discard_and_undo;

    //在SYNSENT状态收到syn报文,说明这是同时打开的场景
    if (th->syn) {
        /* We see SYN without ACK. It is attempt of
         * simultaneous connect with crossed SYNs.
         * Particularly, it can be connect to self.
         */
        //设置连接状态为SYN_RECV
        tcp_set_state(sk, TCP_SYN_RECV);
       
    }
...
}

我们知道,客户端收到这个SYNACK报文后就会进入ESTABLISHED状态,这主要是tcp_finish_connect()里操作的。

 

void tcp_finish_connect(struct sock *sk, struct sk_buff *skb)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct inet_connection_sock *icsk = inet_csk(sk);
    //对于客户端来说,此时连接已经建立,设置连接状态为established
    tcp_set_state(sk, TCP_ESTABLISHED);

    if (skb != NULL) {
        icsk->icsk_af_ops->sk_rx_dst_set(sk, skb);
        security_inet_conn_established(sk, skb);
    }

    /* Make sure socket is routed, for correct metrics.  */
    icsk->icsk_af_ops->rebuild_header(sk);
    //初始化TCP metrics,用于保存连接相关的路由信息 
    tcp_init_metrics(sk);
    //初始化拥塞控制
    tcp_init_congestion_control(sk);

    //记录最后一个数据包发送的时间戳
    tp->lsndtime = tcp_time_stamp;
    //初始化接收缓存和发送缓存
    tcp_init_buffer_space(sk);
    //如果使用了SO_KEEPALIVE选项,那就激活保活定时器
    if (sock_flag(sk, SOCK_KEEPOPEN))
        inet_csk_reset_keepalive_timer(sk, keepalive_time_when(tp));

    if (!tp->rx_opt.snd_wscale)
        __tcp_fast_path_on(tp, tp->snd_wnd);
    else
        tp->pred_flags = 0;

    if (!sock_flag(sk, SOCK_DEAD)) {
        //指向sock_def_wakeup,唤醒该socket上所有睡眠的进程
        sk->sk_state_change(sk);
        //如果进程使用了异步通知,发送SIGIO信号通知进程可写
        sk_wake_async(sk, SOCK_WAKE_IO, POLL_OUT);
    }
}

tcp_finish_connect()中主要有以下几点重要操作:

将socket状态推向ESTABLISHED,也就意味着在客户端来看,连接已经建立
然后是初始化路由和拥塞控制等一下参数
同时如果用户开启了保活定时器,此时开始生效,计算连接空闲时间
最后就是唤醒该socket上所有睡眠的进程,如果有进程使用异步通知,则发送SIGIO信号通知进程可写
最后就是发送第三次握手报文——ACK报文。不过ACK报文并不一定是马上发送,在一下几种情况下会延迟发送。

当前刚好有数据等待发送
用户设置了TCP_DEFER_ACCEPT选项
禁用快速确认模式,可通过TCP_QUICKACK选项设置
不过即使延迟,也最多延迟200ms,这个通过延迟ACK定时器操作。

如果是马上发送ACK报文,则通过tcp_send_ack()发送。

/* This routine sends an ack and also updates the window. */
void tcp_send_ack(struct sock *sk)
{
    struct sk_buff *buff;

    /* If we have been reset, we may not send again. */
    if (sk->sk_state == TCP_CLOSE)
        return;

    tcp_ca_event(sk, CA_EVENT_NON_DELAYED_ACK);

    /* We are not putting this on the write queue, so
     * tcp_transmit_skb() will set the ownership to this
     * sock.
     */
    buff = alloc_skb(MAX_TCP_HEADER, sk_gfp_atomic(sk, GFP_ATOMIC));
    if (buff == NULL) {//分配失败
        //和上面讲到的延迟ACK一样,设置延迟ACK,稍后再发送
        inet_csk_schedule_ack(sk);
        //超时时间为200ms
        inet_csk(sk)->icsk_ack.ato = TCP_ATO_MIN;
        //激活延迟ACK定时器
        inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,
                      TCP_DELACK_MAX, TCP_RTO_MAX);
        return;
    }

    /* Reserve space for headers and prepare control bits. */
    skb_reserve(buff, MAX_TCP_HEADER);
    //初始化无数据的skb
    tcp_init_nondata_skb(buff, tcp_acceptable_seq(sk), TCPHDR_ACK);

    /* We do not want pure acks influencing TCP Small Queues or fq/pacing
     * too much.
     * SKB_TRUESIZE(max(1 .. 66, MAX_TCP_HEADER)) is unfortunately ~784
     * We also avoid tcp_wfree() overhead (cache line miss accessing
     * tp->tsq_flags) by using regular sock_wfree()
     */
    skb_set_tcp_pure_ack(buff);

    /* Send it off, this clears delayed acks for us. */
    skb_mstamp_get(&buff->skb_mstamp);
    //又到了这个路径,往后就是IP层了
    tcp_transmit_skb(sk, buff, 0, sk_gfp_atomic(sk, GFP_ATOMIC));
}

客户端发送第三个握手报文ACK报文后,客户端其实就已经处于连接建立的状态,此时服务端还需要接收到这个ACK报文才算最终完成连接建立。

TCP层接收到ACK还是由tcp_v4_rcv()处理,这就是TCP层的对外接口。

int tcp_v4_rcv(struct sk_buff *skb)
{
...
    //根据报文的源和目的地址在established哈希表以及listen哈希表中查找连接
    //之前服务端接收到客户端的SYN报文时,socket的状态依然是listen
    //所以在接收到客户端的ACK时(第三次握手),依然从listen哈希表中找到对应的连接
    //这里有个疑问就是,既然此时还是listen状态,为啥所有的解释都是说在接收到SYN
    //报文后服务端进入SYN_RECV,连netstat命令查出来的也是。。。
    sk = __inet_lookup_skb(&tcp_hashinfo, skb, th->source, th->dest);
    if (!sk)
        goto no_tcp_socket;
...
    ret = 0;
    if (!sock_owned_by_user(sk)) {//如果sk没有被用户锁定,及没在使用
        if (!tcp_prequeue(sk, skb))
            ret = tcp_v4_do_rcv(sk, skb);//进入到主处理函数
...
}

在SYN报文接收时就会将请求放入半连接队列,因此在第三次握手时就能在半连接队列找到对应的请求连接了。

static struct sock *tcp_v4_hnd_req(struct sock *sk, struct sk_buff *skb)
{
    struct tcphdr *th = tcp_hdr(skb);
    const struct iphdr *iph = ip_hdr(skb);
    struct sock *nsk;
    struct request_sock **prev;
    //在第一次握手时会将连接放入半连接队列,因此这里是能找到对应请求连接的
    struct request_sock *req = inet_csk_search_req(sk, &prev, th->source,
                               iph->saddr, iph->daddr);
    //找到之前的连接
    if (req)
        //使用这个req创建一个socket并返回
        return tcp_check_req(sk, skb, req, prev, false);

...
}

struct request_sock *inet_csk_search_req(const struct sock *sk,
                     struct request_sock ***prevp,
                     const __be16 rport, const __be32 raddr,
                     const __be32 laddr)
{
    const struct inet_connection_sock *icsk = inet_csk(sk);
    struct listen_sock *lopt = icsk->icsk_accept_queue.listen_opt;
    struct request_sock *req, **prev;
    //遍历半连接队列,查找对应连接
    for (prev = &lopt->syn_table[inet_synq_hash(raddr, rport, lopt->hash_rnd,
                            lopt->nr_table_entries)];
         (req = *prev) != NULL;
         prev = &req->dl_next) {
        const struct inet_request_sock *ireq = inet_rsk(req);

        if (ireq->ir_rmt_port == rport &&
            ireq->ir_rmt_addr == raddr &&
            ireq->ir_loc_addr == laddr &&
            AF_INET_FAMILY(req->rsk_ops->family)) {
            WARN_ON(req->sk);
            *prevp = prev;
            break;
        }
    }

    return req;
}

找到这个半连接请求后,就根据这个请求信息创建一个新的socket,由tcp_check_req()操作。

struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
               struct request_sock *req,
               struct request_sock **prev,
               bool fastopen)
{
    struct tcp_options_received tmp_opt;
    struct sock *child;
    const struct tcphdr *th = tcp_hdr(skb);
    __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK);
    bool paws_reject = false;

    BUG_ON(fastopen == (sk->sk_state == TCP_LISTEN));

    tmp_opt.saw_tstamp = 0;
    if (th->doff > (sizeof(struct tcphdr)>>2)) {
        tcp_parse_options(skb, &tmp_opt, 0, NULL);//分析TCP头部选项

        if (tmp_opt.saw_tstamp) {//如果开启了时间戳选项
            //这个时间其实就是客户端发送SYN报文的时间
            //req->ts_recent是在收到SYN报文时记录的
            tmp_opt.ts_recent = req->ts_recent;
            //注释里写ts_recent_stamp表示的是记录ts_recent时的时间
            //这里通过推算的方法得出ts_recent的时间,但是我觉得明显估计的不对
            //按照代码说的,如果SYNACK报文没有重传(req->num_timeout=0)
            //那么ts_recent_stamp即为当前时间减去1
            //但是收到SYN报文的时间肯定不可能是1s前,连接建立也就几毫秒的事。。。
            tmp_opt.ts_recent_stamp = get_seconds() - ((TCP_TIMEOUT_INIT/HZ)<<req->num_timeout);
            //确认时间戳是否回绕,比较第一次握手报文和第三次握手报文的时间戳
            //没有回绕,返回false
            paws_reject = tcp_paws_reject(&tmp_opt, th->rst);
        }
    }

    //如果接收到的报文序列号等于之前SYN报文的序列号,说明这是一个重传SYN报文
    //如果SYN报文时间戳没有回绕,那就重新发送SYNACK报文,然后更新半连接超时时间
    if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
        flg == TCP_FLAG_SYN && !paws_reject) {

        if (!tcp_oow_rate_limited(sock_net(sk), skb, LINUX_MIB_TCPACKSKIPPEDSYNRECV,
                      &tcp_rsk(req)->last_oow_ack_time) &&
                      !inet_rtx_syn_ack(sk, req))//没有超过速率限制,那就重发SYNACK报文
            req->expires = min(TCP_TIMEOUT_INIT << req->num_timeout,
                       TCP_RTO_MAX) + jiffies;//更新半连接的超时时间
        return NULL;
    }

    //收到的ACK报文的确认号不对,返回listen socket
    if ((flg & TCP_FLAG_ACK) && !fastopen &&
        (TCP_SKB_CB(skb)->ack_seq != tcp_rsk(req)->snt_isn + 1))
        return sk;

    /* Also, it would be not so bad idea to check rcv_tsecr, which
     * is essentially ACK extension and too early or too late values
     * should cause reset in unsynchronized states.
     */

    /* RFC793: "first check sequence number". */

    //报文时间戳回绕,或者报文序列不在窗口范围,发送ACK后丢弃
    if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
                      tcp_rsk(req)->rcv_nxt, tcp_rsk(req)->rcv_nxt + req->rcv_wnd)) {
        /* Out of window: send ACK and drop. */
        if (!(flg & TCP_FLAG_RST))
            req->rsk_ops->send_ack(sk, skb, req);
        if (paws_reject)
            NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
        return NULL;
    }

    /* In sequence, PAWS is OK. */
    //开启了时间戳,且收到的报文序列号小于等于期望接收的序列号
    if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt))
        req->ts_recent = tmp_opt.rcv_tsval;//更新ts_recent为第三次握手报文的时间戳

    //清除SYN标记  
    if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
        /* Truncate SYN, it is out of window starting
           at tcp_rsk(req)->rcv_isn + 1. */
        flg &= ~TCP_FLAG_SYN;
    }

    /* RFC793: "second check the RST bit" and
     *     "fourth, check the SYN bit"
     */
    if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
        TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
        goto embryonic_reset;
    }

    /* ACK sequence verified above, just make sure ACK is
     * set.  If ACK not set, just silently drop the packet.
     *
     * XXX (TFO) - if we ever allow "data after SYN", the
     * following check needs to be removed.
     */
    if (!(flg & TCP_FLAG_ACK))
        return NULL;

    /* For Fast Open no more processing is needed (sk is the
     * child socket).
     */
    if (fastopen)
        return sk;

    /* While TCP_DEFER_ACCEPT is active, drop bare ACK. */
    //设置了TCP_DEFER_ACCEPT,即延迟ACK选项,且该ACK没有携带数据,那就先丢弃
    if (req->num_timeout < inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
        TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
        inet_rsk(req)->acked = 1;//标记已经接收过ACK报文了
        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPDEFERACCEPTDROP);
        return NULL;
    }

    /* OK, ACK is valid, create big socket and
     * feed this segment to it. It will repeat all
     * the tests. THIS SEGMENT MUST MOVE SOCKET TO
     * ESTABLISHED STATE. If it will be dropped after
     * socket is created, wait for troubles.
     */
    //这里总算是正常ACK报文了,创建一个新的socket并返回
    child = inet_csk(sk)->icsk_af_ops->syn_recv_sock(sk, skb, req, NULL);
    if (child == NULL)
        goto listen_overflow;
    //将老的socket从半连接队列里摘链
    inet_csk_reqsk_queue_unlink(sk, req, prev);
    //删除摘除的请求,然后更新半连接队列的统计信息
    //如果半连接队列为空,删除SYNACK定时器
    inet_csk_reqsk_queue_removed(sk, req);

    //将新创建的新socket加入全连接队列里,并更新队列统计信息
    inet_csk_reqsk_queue_add(sk, req, child);
    return child;
...
}

找到半连接队列里的请求后,还需要和当前接收到的报文比较,检查是否出现时间戳回绕的情况(timestamps选项开启的前提下),通过tcp_paws_reject()检测。

static inline bool tcp_paws_reject(const struct tcp_options_received *rx_opt,
                   int rst)
{
    //检查时间戳是否回绕
    if (tcp_paws_check(rx_opt, 0))
        return false;

    //第三次握手报文一般都不会有rst标志
    //另一个条件是,当前时间和上次该ip通信的时间间隔大于TCP_PAWS_MSL(60s),
    //即一个time_wait状态持续时间
    if (rst && get_seconds() >= rx_opt->ts_recent_stamp + TCP_PAWS_MSL)
        return false;
    return true;
}

static inline bool tcp_paws_check(const struct tcp_options_received *rx_opt,
                  int paws_win)
{
    //rx_opt->ts_recent是SYN报文发送时间,
    //rx_opt->rcv_tsval是客户端发送第三次握手报文的时间
    //也就是要保证时间戳没有回绕,正常情况下这里就满足返回了
    if ((s32)(rx_opt->ts_recent - rx_opt->rcv_tsval) <= paws_win)
        return true;
    //距离上一次收到这个ip的报文过去了24天,一般不可能
    if (unlikely(get_seconds() >= rx_opt->ts_recent_stamp + TCP_PAWS_24DAYS))
        return true;
    //没有开启时间戳
    if (!rx_opt->ts_recent)
        return true;
    return false;
}

确认时间戳未发生回绕后,看下是不是重传的SYN报文,如果是那就重发SYNACK报文,并重置SYNACK定时器。

接下来一个比较重要的点就是,如果开启了TCP_DEFER_ACCEPT选项,即延迟ACK选项,但是接收到的这个ACK没有携带数据,那就先丢弃,标记收到过ACK报文,等待后续客户端发送数据再做连接建立的真正操作。

重重检查后,总算是要创建新的socket了,因此inet_csk(sk)->icsk_af_ops->syn_recv_sock上场了。我们熟悉的icsk_af_ops又来了,它指向的是ipv4_specific,

const struct inet_connection_sock_af_ops ipv4_specific = {
    .queue_xmit    = ip_queue_xmit,
    .send_check    = tcp_v4_send_check,
    .rebuild_header    = inet_sk_rebuild_header,
    .sk_rx_dst_set     = inet_sk_rx_dst_set,
    .conn_request      = tcp_v4_conn_request,
    .syn_recv_sock     = tcp_v4_syn_recv_sock,
    ...
};

所以创建新socket就是tcp_v4_syn_recv_sock()完成的。

/*
 * The three way handshake has completed - we got a valid synack -
 * now create the new socket.
 */
struct sock *tcp_v4_syn_recv_sock(struct sock *sk, struct sk_buff *skb,
                  struct request_sock *req,
                  struct dst_entry *dst)
{
    struct inet_request_sock *ireq;
    struct inet_sock *newinet;
    struct tcp_sock *newtp;
    struct sock *newsk;
#ifdef CONFIG_TCP_MD5SIG
    struct tcp_md5sig_key *key;
#endif
    struct ip_options_rcu *inet_opt;
    //如果全连接队列已经满了,那就丢弃报文
    if (sk_acceptq_is_full(sk))
        goto exit_overflow;
    //创建一个新的socket用于连接建立后的处理,原来的socket继续监听新发起的连接
    newsk = tcp_create_openreq_child(sk, req, skb);
    if (!newsk)
        goto exit_nonewsk;

...
    //处理新创建的socket的端口,一般就是和原来监听socket使用同一个端口
    if (__inet_inherit_port(sk, newsk) < 0)
        goto put_and_exit;
    //将新创建的新socket加入established哈希表中
    __inet_hash_nolisten(newsk, NULL);

    return newsk;
...
}

检查全连接队列是否满了,满了就丢弃报文,否则请出tcp_create_openreq_child()创建新socket。

struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb)
{
    //创建子socket
    struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC);
    //接下来就是新socket的各种初始化了
    if (newsk != NULL) {
        const struct inet_request_sock *ireq = inet_rsk(req);
        struct tcp_request_sock *treq = tcp_rsk(req);
        struct inet_connection_sock *newicsk = inet_csk(newsk);
        struct tcp_sock *newtp = tcp_sk(newsk);
        ...
        //初始化新socket的各个定时器
        tcp_init_xmit_timers(newsk);
        ...
        //如果开启时间戳
        if (newtp->rx_opt.tstamp_ok) {
            //记录第三次握手报文发送的时间,上面的流程已经将ts_recent更新
            newtp->rx_opt.ts_recent = req->ts_recent;
            newtp->rx_opt.ts_recent_stamp = get_seconds();
            newtp->tcp_header_len = sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
        } else {
            newtp->rx_opt.ts_recent_stamp = 0;
            newtp->tcp_header_len = sizeof(struct tcphdr);
        }
        ...
    }
    return newsk;
}

struct sock *inet_csk_clone_lock(const struct sock *sk,
                 const struct request_sock *req,
                 const gfp_t priority)
{
    struct sock *newsk = sk_clone_lock(sk, priority);

    if (newsk != NULL) {
        struct inet_connection_sock *newicsk = inet_csk(newsk);

        //新创建的socket状态设置为SYN_RECV
        newsk->sk_state = TCP_SYN_RECV;
        newicsk->icsk_bind_hash = NULL;
        //记录目的端口,以及服务器端端口
        inet_sk(newsk)->inet_dport = inet_rsk(req)->ir_rmt_port;
        inet_sk(newsk)->inet_num = inet_rsk(req)->ir_num;
        inet_sk(newsk)->inet_sport = htons(inet_rsk(req)->ir_num);
        newsk->sk_write_space = sk_stream_write_space;

        inet_sk(newsk)->mc_list = NULL;
        newicsk->icsk_retransmits = 0;//重传次数
        newicsk->icsk_backoff     = 0;//退避指数
        newicsk->icsk_probes_out  = 0;

        /* Deinitialize accept_queue to trap illegal accesses. */
        memset(&newicsk->icsk_accept_queue, 0, sizeof(newicsk->icsk_accept_queue));

        security_inet_csk_clone(newsk, req);
    }
    return newsk;
}

从inet_csk_clone_lock()中我们终于看到socket的状态进入SYN_RECV了,千呼万唤始出来啊,这都是第三次握手报文了,说好的接收到SYN报文就进入SYN_RECV的呢,骗得我好苦。

创建好新的socket后,需要将该socket归档,处理其使用端口,并且放入bind哈希表,这样之后我们才能查询得到这个新的socket。

int __inet_inherit_port(struct sock *sk, struct sock *child)
{
    struct inet_hashinfo *table = sk->sk_prot->h.hashinfo;
    unsigned short port = inet_sk(child)->inet_num;
    const int bhash = inet_bhashfn(sock_net(sk), port,
            table->bhash_size);
    struct inet_bind_hashbucket *head = &table->bhash[bhash];
    struct inet_bind_bucket *tb;

    spin_lock(&head->lock);
    tb = inet_csk(sk)->icsk_bind_hash;
    //一般新socket和原先的socket端口都是一样的
    if (tb->port != port) {
        /* NOTE: using tproxy and redirecting skbs to a proxy
         * on a different listener port breaks the assumption
         * that the listener socket's icsk_bind_hash is the same
         * as that of the child socket. We have to look up or
         * create a new bind bucket for the child here. */
        inet_bind_bucket_for_each(tb, &head->chain) {
            if (net_eq(ib_net(tb), sock_net(sk)) &&
                tb->port == port)
                break;
        }
        if (!tb) {
            tb = inet_bind_bucket_create(table->bind_bucket_cachep,
                             sock_net(sk), head, port);
            if (!tb) {
                spin_unlock(&head->lock);
                return -ENOMEM;
            }
        }
    }
//将新的socket加入bind哈希表中 inet_bind_hash(child, tb, port); spin_unlock(&head->lock); return 0; }

但是加入bind哈希表并不足够,bind哈希表只是存储绑定的ip和端口信息,还需要以下几个动作:

将新建的socket加入establish哈希表
将原先老的socket从半连接队列里拆除并更新半连接统计信息
将新建的socket加入全连接队列
加入全连接队列由inet_csk_reqsk_queue_add()操作,

static inline void inet_csk_reqsk_queue_add(struct sock *sk,
                        struct request_sock *req,
                        struct sock *child)
{
    reqsk_queue_add(&inet_csk(sk)->icsk_accept_queue, req, sk, child);
}

static inline void reqsk_queue_add(struct request_sock_queue *queue,
                   struct request_sock *req,
                   struct sock *parent,
                   struct sock *child)
{
    //将请求req的sk指针指向新建立的sock,这样在后面调用accept()时
    //就能通过全连接队列上的这个请求找到这个新创建的sock,然后进行通信
    req->sk = child;
    sk_acceptq_added(parent);

    if (queue->rskq_accept_head == NULL)
        queue->rskq_accept_head = req;
    else
        queue->rskq_accept_tail->dl_next = req;

    queue->rskq_accept_tail = req;
    req->dl_next = NULL;
}

static inline void sk_acceptq_added(struct sock *sk)
{   
    //全连接队列里连接数量统计更新
    sk->sk_ack_backlog++;
}

有一点要注意的就是,加入半连接队列的函数是inet_csk_reqsk_queue_added(),和加入全连接队列的函数就差一个单词,一个是add,一个是added,别混淆了。

返回这个新创建的socket后,就进入tcp_child_process()函数继续深造。

int tcp_child_process(struct sock *parent, struct sock *child,
              struct sk_buff *skb)
{
    int ret = 0;
    int state = child->sk_state;
    //新的socket没有没用户占用
    if (!sock_owned_by_user(child)) {
        //对,又是它,就是它,处理各种状态socket的接口
        ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb),
                        skb->len);
        /* Wakeup parent, send SIGIO */
        if (state == TCP_SYN_RECV && child->sk_state != state)
            parent->sk_data_ready(parent, 0);
    } else {
        /* Alas, it is possible again, because we do lookup
         * in main socket hash table and lock on listening
         * socket does not protect us more.
         */
        //用户占用则加入backlog队列
        __sk_add_backlog(child, skb);
    }

    bh_unlock_sock(child);
    sock_put(child);
    return ret;
}

接着socket就要从SYN_RECV进入ESTABLISHED状态了,这就又要tcp_rcv_state_process()出马了。

int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
              const struct tcphdr *th, unsigned int len)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct inet_connection_sock *icsk = inet_csk(sk);
    struct request_sock *req;
    int queued = 0;
    bool acceptable;
    u32 synack_stamp;

    tp->rx_opt.saw_tstamp = 0;
...

    req = tp->fastopen_rsk;//快速开启选项相关
...
    /* step 5: check the ACK field */
    //检查ACK确认号的合法值
    acceptable = tcp_ack(sk, skb, FLAG_SLOWPATH |
                      FLAG_UPDATE_TS_RECENT) > 0;

    switch (sk->sk_state) {
    //这个是新创建的socket,所以此时状态是SYN_RECV
    case TCP_SYN_RECV:
        if (!acceptable)
            return 1;

        /* Once we leave TCP_SYN_RECV, we no longer need req
         * so release it.
         */
        if (req) {//快速开启走这个流程
            synack_stamp = tcp_rsk(req)->snt_synack;
            tp->total_retrans = req->num_retrans;
            reqsk_fastopen_remove(sk, req, false);
        } else {
            //非快速开启流程
            synack_stamp = tp->lsndtime;
            /* Make sure socket is routed, for correct metrics. */
            icsk->icsk_af_ops->rebuild_header(sk);
            tcp_init_congestion_control(sk);//初始化拥塞控制
            //mtu探测初始化
            tcp_mtup_init(sk);
            tp->copied_seq = tp->rcv_nxt;
            //初始化接收和发送缓存空间
            tcp_init_buffer_space(sk);
        }
        smp_mb();
        //服务端连接状态终于抵达终点,established
        tcp_set_state(sk, TCP_ESTABLISHED);
        //调用sock_def_wakeup唤醒该sock上等待队列的所有进程
        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.
         */
        //对于服务端是被动开启socket,所以不会走这个流程
        if (sk->sk_socket)
            sk_wake_async(sk, SOCK_WAKE_IO, POLL_OUT);
        ...
    }

    /* step 6: check the URG bit */
    tcp_urg(sk, skb, th);

    /* step 7: process the segment text */
    switch (sk->sk_state) {
    ...
    case TCP_ESTABLISHED:
        //这时socket已经是established状态了,可以处理及接收数据了
        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:
        __kfree_skb(skb);
    }
    return 0;
}

最终,服务端也到达established状态,建立了可靠连接。

posted @ 2019-12-26 15:52  LittleTurtle  阅读(829)  评论(0编辑  收藏  举报