tcp connection setup的实现(一)

bind的实现: 



先来介绍几个地址结构. 

struct sockaddr 其实相当于一个基类的地址结构,其他的结构都能够直接转到sockaddr.举个例子比如当sa_family为PF_INET时,sa_data就包含了端口号和ip地址(in_addr结构). 

struct sockaddr {
    sa_family_t sa_family;  /* address family, AF_xxx   */
    char        sa_data[14];    /* 14 bytes of protocol address */
};

接下来就是sockaddr_in ,它表示了所有的ipv4的地址结构.可以看到他也就相当于sockaddr 的一个子类. 

struct sockaddr_in {
  sa_family_t       sin_family; /* Address family       */
  __be16        sin_port;   /* Port number          */
  struct in_addr    sin_addr;   /* Internet address     */
  /* Pad to size of `struct sockaddr'. */
  unsigned char     __pad[__SOCK_SIZE__ - sizeof(short int) -
            sizeof(unsigned short int) - sizeof(struct in_addr)];
};

这里还有一个内核比较新的地质结构sockaddr_storage,他可以容纳所有类型的套接口结构,比如ipv4,ipv6..可以看到它是强制对齐的,相比于sockaddr. 

struct __kernel_sockaddr_storage {
    unsigned short  ss_family;      /* address family */
///每个协议实现自己的地址结构.
    char        __data[_K_SS_MAXSIZE - sizeof(unsigned short)];
                /* space to achieve desired size, */
                /* _SS_MAXSIZE value minus size of ss_family */
} __attribute__ ((aligned(_K_SS_ALIGNSIZE)));   /* force desired alignment */

接下来看几个和bind相关的数据结构: 

第一个是inet_hashinfo,它主要用来管理 tcp的bind hash bucket(在tcp的初始化函数中会将tcp_hashinfo初始化.然后在tcp_prot中会将tcp_hashinfo付给结构体h,然后相应的我们就可以通过sock中的sock_common域来存取这个值).后面我们会分析这个流程. 

struct inet_hashinfo {
    /* This is for sockets with full identity only.  Sockets here will
     * always be without wildcards and will have the following invariant:
     *
     *          TCP_ESTABLISHED <= sk->sk_state < TCP_CLOSE
     *
     * TIME_WAIT sockets use a separate chain (twchain).
     */
///下面会分析这个结构.
    struct inet_ehash_bucket    *ehash;
    rwlock_t            *ehash_locks;
    unsigned int            ehash_size;
    unsigned int            ehash_locks_mask;

    /* Ok, let's try this, I give up, we do need a local binding
     * TCP hash as well as the others for fast bind/connect.
     */
///表示所有的已经在使用的端口号的信息.这里bhash也就是一个hash链表,而链表的元素是inet_bind_bucket,紧接着我们会分析这个结构.
    struct inet_bind_hashbucket *bhash;

    unsigned int            bhash_size;
    /* Note : 4 bytes padding on 64 bit arches */

    /* All sockets in TCP_LISTEN state will be in here.  This is the only
     * table where wildcard'd TCP sockets can exist.  Hash function here
     * is just local port number.
     */
///listening_hash表示所有的处于listen状态的socket.
    struct hlist_head       listening_hash[INET_LHTABLE_SIZE];

    /* All the above members are written once at bootup and
     * never written again _or_ are predominantly read-access.
     *
     * Now align to a new cache line as all the following members
     * are often dirty.
     */
    rwlock_t            lhash_lock ____cacheline_aligned;
    atomic_t            lhash_users;
    wait_queue_head_t       lhash_wait;
    struct kmem_cache           *bind_bucket_cachep;
};

struct inet_ehash_bucket管理所有的tcp状态在TCP_ESTABLISHED和TCP_CLOSE之间的socket.这里要注意,twchain表示处于TIME_WAIT的socket. 

struct inet_ehash_bucket {
    struct hlist_head chain;
    struct hlist_head twchain;
};

inet_bind_bucket结构就是每个使用的端口的信息,最终会把它链接到bhash链表中. 

struct inet_bind_bucket {
    struct net      *ib_net;
///端口号
    unsigned short      port;
///表示这个端口是否能够被重复使用.
    signed short        fastreuse;
///指向下一个端口的inet_bind_bucket 结构.
    struct hlist_node   node;
///也就是使用这个端口的socket链表
    struct hlist_head   owners;
};

最后一个结构是tcp_hashinfo他在 tcp_init中被初始化,而tcp_init是在inet_init中被初始化的.然后tcp_hashinfo会被赋值给tcp_proto和sock的sk_prot域. 

struct inet_hashinfo __cacheline_aligned tcp_hashinfo = {
    .lhash_lock  = __RW_LOCK_UNLOCKED(tcp_hashinfo.lhash_lock),
    .lhash_users = ATOMIC_INIT(0),
    .lhash_wait  = __WAIT_QUEUE_HEAD_INITIALIZER(tcp_hashinfo.lhash_wait),
};

然后来看bind的实现,bind对应的系统调用是sys_bind: 

asmlinkage long sys_bind(int fd, struct sockaddr __user *umyaddr, int addrlen)
{
    struct socket *sock;
    struct sockaddr_storage address;
    int err, fput_needed;

///通过fd查找相应的socket,如果不存在则返回错误.
    sock = sockfd_lookup_light(fd, &err, &fput_needed);
    if (sock) {
///用户空间和内核的地址拷贝.
        err = move_addr_to_kernel(umyaddr, addrlen, (struct sockaddr *)&address);
        if (err >= 0) {
            err = security_socket_bind(sock,
                           (struct sockaddr *)&address,
                           addrlen);
            if (!err)
///调用inet_bind方法.
                err = sock->ops->bind(sock,
                              (struct sockaddr *)
                              &address, addrlen);
        }
///将socket对应的file结构的引用计数.
        fput_light(sock->file, fput_needed);
    }
    return err;
}

sockfd_lookup_light主要是查找fd对应的socket 

static struct socket *sockfd_lookup_light(int fd, int *err, int *fput_needed)
{
    struct file *file;
    struct socket *sock;

    *err = -EBADF;
///通过fd得到对应的file结构
    file = fget_light(fd, fput_needed);
    if (file) {
///我们在sock_map_fd通过sock_attach_fd中已经把file的private域赋值为socket,因此这里就直接返回socket.
        sock = sock_from_file(file, err);
        if (sock)
            return sock;
        fput_light(file, *fput_needed);
    }
    return NULL;
}

然后来看inet_bind的实现. 

int inet_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
{
///取得绑定地址.以及相关的socket和inet_sock.
    struct sockaddr_in *addr = (struct sockaddr_in *)uaddr;
    struct sock *sk = sock->sk;
    struct inet_sock *inet = inet_sk(sk);
    unsigned short snum;
    int chk_addr_ret;
    int err;

    /* If the socket has its own bind function then use it. (RAW) */
    if (sk->sk_prot->bind) {
        err = sk->sk_prot->bind(sk, uaddr, addr_len);
        goto out;
    }
    err = -EINVAL;
    if (addr_len < sizeof(struct sockaddr_in))
        goto out;
///得到地址类型,比如广播地址之类的.
    chk_addr_ret = inet_addr_type(sock_net(sk), addr->sin_addr.s_addr);

    err = -EADDRNOTAVAIL;

///主要是判断绑定的地址不是本地时的一些条件判断.
    if (!sysctl_ip_nonlocal_bind &&
        !inet->freebind &&
        addr->sin_addr.s_addr != htonl(INADDR_ANY) &&
        chk_addr_ret != RTN_LOCAL &&
        chk_addr_ret != RTN_MULTICAST &&
        chk_addr_ret != RTN_BROADCAST)
        goto out;
///得到端口号.
    snum = ntohs(addr->sin_port);
    err = -EACCES;
///主要是端口号小于prot_sock(1024)必须得有root权限.如果没有则退出.capable就是用来判断权限的.
    if (snum && snum < PROT_SOCK && !capable(CAP_NET_BIND_SERVICE))
        goto out;

    /*      We keep a pair of addresses. rcv_saddr is the one
     *      used by hash lookups, and saddr is used for transmit.
     *
     *      In the BSD API these are the same except where it
     *      would be illegal to use them (multicast/broadcast) in
     *      which case the sending device address is used.
     */
    lock_sock(sk);

    /* Check these errors (active socket, double bind). */
    err = -EINVAL;
///检测状态是否为close.如果是close状态,说明这个socket前面已经bind过了.而num只有当raw socket时才会不为0
    if (sk->sk_state != TCP_CLOSE || inet->num)
        goto out_release_sock;

///设置相应的地址.rcv_saddr是通过hash查找的源地址,而saddr是ip层使用的源地址(ip头的源地址).
    inet->rcv_saddr = inet->saddr = addr->sin_addr.s_addr;
///如果是多播或者广播,设置saddr.
    if (chk_addr_ret == RTN_MULTICAST || chk_addr_ret == RTN_BROADCAST)
        inet->saddr = 0;  /* Use device */

///这里get_port用来发现我们绑定的端口,是否被允许使用.而get_port在tcp中,被实例化为inet_csk_get_port,接近着我们会分析它的实现.
    if (sk->sk_prot->get_port(sk, snum)) {
        inet->saddr = inet->rcv_saddr = 0;
        err = -EADDRINUSE;
        goto out_release_sock;
    }
///这两个锁不太理解.不知道谁能解释下.
    if (inet->rcv_saddr)
        sk->sk_userlocks |= SOCK_BINDADDR_LOCK;
    if (snum)
        sk->sk_userlocks |= SOCK_BINDPORT_LOCK;
///设置源端口
    inet->sport = htons(inet->num);
///目的地址和目的端口,暂时设为0
    inet->daddr = 0;
    inet->dport = 0;
    sk_dst_reset(sk);
    err = 0;
out_release_sock:
    release_sock(sk);
out:
    return err;
}

这里我先来介绍下inet_csk_get_port的流程. 

当绑定的port为0时,这时也就是说需要kernel来分配一个新的port. 
1 首先得到系统的port范围. 

2  随机分配一个port. 

3 从bhash中得到当前随机分配的端口的链表(也就是inet_bind_bucket链表). 

4 遍历这个链表(链表为空的话,也说明这个port没有被使用),如果这个端口已经被使用,则将端口号加一,继续循环,直到找到当前没有被使用的port,也就是没有在bhash中存在的port. 

5 新建一个inet_bind_bucket,并插入到bhash中. 

当指定port时. 

1 从bhash中根据hash值(port计算的)取得当前指定端口对应的inet_bind_bucket结构. 

2 如果bhash中存在,则说明,这个端口已经在使用,因此需要判断这个端口是否允许被reuse. 

3 如果不存在,则步骤和上面的第5部一样. 

int inet_csk_get_port(struct sock *sk, unsigned short snum)
{
    struct inet_hashinfo *hashinfo = sk->sk_prot->h.hashinfo;
    struct inet_bind_hashbucket *head;
    struct hlist_node *node;
    struct inet_bind_bucket *tb;
    int ret;
    struct net *net = sock_net(sk);

    local_bh_disable();
    if (!snum) {
///端口为0,也就是需要内核来分配端口.
        int remaining, rover, low, high;
///得到端口范围.
        inet_get_local_port_range(&low, &high);
        remaining = (high - low) + 1;
        rover = net_random() % remaining + low;

///循环来得到一个当前没有使用的端口.
        do {
///通过端口为key,来得到相应的inet_bind_bucket
            head = &hashinfo->bhash[inet_bhashfn(net, rover,
                    hashinfo->bhash_size)];
            spin_lock(&head->lock);
            inet_bind_bucket_for_each(tb, node, &head->chain)
                if (tb->ib_net == net && tb->port == rover)
///说明这个端口已被使用,因此需要将端口加1,重新查找.
                    goto next;
            break;
        next:
            spin_unlock(&head->lock);
///如果端口大于最大值,则将它赋值为最小值(这是因为我们这个端口是随机值,因此有可能很多端口就被跳过了),重新查找.
            if (++rover > high)
                rover = low;
        } while (--remaining > 0);

        /* Exhausted local port range during search?  It is not
         * possible for us to be holding one of the bind hash
         * locks if this test triggers, because if 'remaining'
         * drops to zero, we broke out of the do/while loop at
         * the top level, not from the 'break;' statement.
         */
        ret = 1;
        if (remaining <= 0)
            goto fail;
///将要分配的端口号.
        snum = rover;
    } else {
///指定端口号的情况.和上面的方法差不多,只不过只需要一次.
        head = &hashinfo->bhash[inet_bhashfn(net, snum,
                hashinfo->bhash_size)];
        spin_lock(&head->lock);
        inet_bind_bucket_for_each(tb, node, &head->chain)
            if (tb->ib_net == net && tb->port == snum)
                goto tb_found;
    }
    tb = NULL;
    goto tb_not_found;
tb_found:
///用来处理端口号已经被使用的情况.他被使用的socket不为空的情况.
    if (!hlist_empty(&tb->owners)) {
///fastreuse大于0说明其他的socket允许另外的socket也使用这个端口,而reuse表示当前的端口也允许和其他的端口分享这个port.并且socket的状态必须是TCP_LISTEN,才能做这个判断.
        if (tb->fastreuse > 0 &&
            sk->sk_reuse && sk->sk_state != TCP_LISTEN) {
            goto success;
        } else {
            ret = 1;
///如果出错,调用inet_csk_bind_conflict.主要是有可能一些使用这个端口的socket,有可能使用不同的ip地址.此时,我们是可以使用这个端口的.
            if (inet_csk(sk)->icsk_af_ops->bind_conflict(sk, tb))
                goto fail_unlock;
        }
    }
tb_not_found:
    ret = 1;
///重新分配一个inet_bind_bucket,并链接到bhash.
    if (!tb && (tb = inet_bind_bucket_create(hashinfo->bind_bucket_cachep,
                    net, head, snum)) == NULL)
        goto fail_unlock;
    if (hlist_empty(&tb->owners)) {
///设置当前端口的fastreuse,这个域也只能是处于listen的socket才能设置.
        if (sk->sk_reuse && sk->sk_state != TCP_LISTEN)
            tb->fastreuse = 1;
        else
            tb->fastreuse = 0;
    } else if (tb->fastreuse &&
           (!sk->sk_reuse || sk->sk_state == TCP_LISTEN))
        tb->fastreuse = 0;
success:
///将这个socket加到这个端口的ower中.
    if (!inet_csk(sk)->icsk_bind_hash)
        inet_bind_hash(sk, tb, snum);
    WARN_ON(inet_csk(sk)->icsk_bind_hash != tb);
    ret = 0;

fail_unlock:
    spin_unlock(&head->lock);
fail:
    local_bh_enable();
    return ret;
}

在看listen的代码之前.我们也先来看相关的数据结构: 

其中inet_connection_sock我们先前已经介绍过了,它包含了一个icsk_accept_queue的域,这个域是一个request_sock_queue类型,.我们就先来看这个结构: 

request_sock_queue也就表示一个request_sock队列.这里我们知道,tcp中分为半连接队列(处于SYN_RECVD状态)和已完成连接队列(处于established状态).这两个一个是刚接到syn,等待三次握手完成,一个是已经完成三次握手,等待accept来读取. 

这里每个syn分节到来都会新建一个request_sock结构,并将它加入到listen_sock的request_sock hash表中.然后3次握手完毕后,将它放入到request_sock_queue的rskq_accept_head和rskq_accept_tail队列中.这样当accept的时候就直接从这个队列中读取了. 

struct request_sock_queue {
///一个指向头,一个指向结尾.
    struct request_sock *rskq_accept_head;
    struct request_sock *rskq_accept_tail;
    rwlock_t        syn_wait_lock;
    u8          rskq_defer_accept;
    /* 3 bytes hole, try to pack */
///相应的listen_socket结构.
    struct listen_sock  *listen_opt;
};

listen_sock 表示一个处于listening状态的socket. 

struct listen_sock {
///log_2 of maximal queued SYNs/REQUESTs ,这里不太理解这个域的作用.
    u8          max_qlen_log;
    /* 3 bytes hole, try to use */
///当前的半连接队列的长度.
    int         qlen;
///也是指当前的半开连接队列长度,不过这个值会当重传syn/ack的时候(这里要注意是这个syn/ack第一次重传的时候才会减一)自动减一.
    int         qlen_young;
    int         clock_hand;
    u32         hash_rnd;
///这个值表示了当前的syn_backlog(半开连接队列)的最大值
    u32         nr_table_entries;
///半连接队列.
    struct request_sock *syn_table[0];
};

最后来看下request_sock,它保存了tcp双方传输所必需的一些域,比如窗口大小,对端速率,对端数据包序列号等等这些值. 

struct request_sock {
    struct request_sock     *dl_next; /* Must be first member! */
///mss值.
    u16             mss;
    u8              retrans;
    u8              cookie_ts; /* syncookie: encode tcpopts in timestamp */
    /* The following two fields can be easily recomputed I think -AK */
    u32             window_clamp; /* window clamp at creation time */
///窗口大小.
    u32             rcv_wnd;      /* rcv_wnd offered first time */
    u32             ts_recent;
    unsigned long           expires;
///这个域包含了发送ack的操作集合.
    const struct request_sock_ops   *rsk_ops;
    struct sock         *sk;
    u32             secid;
    u32             peer_secid;
};

listen的对应的系统调用是sys_listen,它首先通过sockfd_lookup_light查找到相应的socket,然后调用inet_listen,大体流程和bind差不多,只不过中间调用的是inet_listen罢了. 

这里还有一个概念那就是backlog,在linux中,backlog的大小指的是已完成连接队列的大小.而不是和半连接队列之和.而半开连接的大小一般是和backlog差不多大小. 

而半开连接队列的最大长度是根据backlog计算的,我们后面会介绍这个. 

因此我们直接来看inet_listen的实现,这个函数主要是进行一些合法性判断,然后调用inet_csk_listen_start来对相关域进行处理: 

int inet_listen(struct socket *sock, int backlog)
{
    struct sock *sk = sock->sk;
    unsigned char old_state;
    int err;

    lock_sock(sk);

    err = -EINVAL;
///判断状态(非连接状态)以及socket类型.
    if (sock->state != SS_UNCONNECTED || sock->type != SOCK_STREAM)
        goto out;

    old_state = sk->sk_state;
///状态必须为close或者listen.
    if (!((1 << old_state) & (TCPF_CLOSE | TCPF_LISTEN)))
        goto out;

    /* Really, if the socket is already in listen state
     * we can only allow the backlog to be adjusted.
     */
///非listen状态,需要我们处理.
    if (old_state != TCP_LISTEN) {
        err = inet_csk_listen_start(sk, backlog);
        if (err)
            goto out;
    }
///将backlog赋值给sk_max_ack_backlog,也就是完全连接队列最大值.
    sk->sk_max_ack_backlog = backlog;
    err = 0;

out:
    release_sock(sk);
    return err;
}

然后来看inet_csk_listen_start的实现. 

它的主要工作是新分配一个listen socket,将它加入到inet_connection_sock的icsk_accept_queue域的listen_opt中.然后对当前使用端口进行判断.最终返回: 

int inet_csk_listen_start(struct sock *sk, const int nr_table_entries)
{
    struct inet_sock *inet = inet_sk(sk);
    struct inet_connection_sock *icsk = inet_csk(sk);
///新分配一个listen socket.
    int rc = reqsk_queue_alloc(&icsk->icsk_accept_queue, nr_table_entries);

    if (rc != 0)
        return rc;
///先将这两个ack_backlog赋值为0.
    sk->sk_max_ack_backlog = 0;
    sk->sk_ack_backlog = 0;
    inet_csk_delack_init(sk);

    /* There is race window here: we announce ourselves listening,
     * but this transition is still not validated by get_port().
     * It is OK, because this socket enters to hash table only
     * after validation is complete.
     */
///设置状态.
    sk->sk_state = TCP_LISTEN;
///get_port上面已经分析过了.这里之所以还要再次判断一下端口,是为了防止多线程,也就是另一个线程在我们调用listen之前改变了这个端口的信息.
    if (!sk->sk_prot->get_port(sk, inet->num)) {
//端口可用的情况,将端口值付给sport,并加入到inet_hashinfo(上面已经分析过)的listening_hash hash链表中.
        inet->sport = htons(inet->num);

        sk_dst_reset(sk);
///这里调用__inet_hash实现的.
        sk->sk_prot->hash(sk);

        return 0;
    }
///不可用,则返回错误.
    sk->sk_state = TCP_CLOSE;
    __reqsk_queue_destroy(&icsk->icsk_accept_queue);
    return -EADDRINUSE;
}

最后我们来看下reqsk_queue_alloc的实现: 

///半开连接的最大长度.
int sysctl_max_syn_backlog = 256;

int reqsk_queue_alloc(struct request_sock_queue *queue,
              unsigned int nr_table_entries)
{
    size_t lopt_size = sizeof(struct listen_sock);
    struct listen_sock *lopt;
///在当前的nr_table_entries(也就是listen传进来的backlog)和sysctl_max_syn_backlog取一个较小的值.
    nr_table_entries = min_t(u32, nr_table_entries, sysctl_max_syn_backlog);

///也就是说nr_table_entries不能小于8.
    nr_table_entries = max_t(u32, nr_table_entries, 8);

///其实也就是使nr_table_entries更接近于2的次幂
    nr_table_entries = roundup_pow_of_two(nr_table_entries + 1);
///最终所要分配的listen_sock 的大小.
    lopt_size += nr_table_entries * sizeof(struct request_sock *);
    if (lopt_size > PAGE_SIZE)
        lopt = __vmalloc(lopt_size,
            GFP_KERNEL | __GFP_HIGHMEM | __GFP_ZERO,
            PAGE_KERNEL);
    else
        lopt = kzalloc(lopt_size, GFP_KERNEL);
    if (lopt == NULL)
        return -ENOMEM;
///计算max_qlen_log的值,他最小要为3,最大为对nr_table_entries求以2为低的log..
    for (lopt->max_qlen_log = 3;
         (1 << lopt->max_qlen_log) < nr_table_entries;
         lopt->max_qlen_log++);

    get_random_bytes(&lopt->hash_rnd, sizeof(lopt->hash_rnd));
    rwlock_init(&queue->syn_wait_lock);
    queue->rskq_accept_head = NULL;
///给nr_table_entries赋值.
    lopt->nr_table_entries = nr_table_entries;

    write_lock_bh(&queue->syn_wait_lock);
///将listen_socket赋值给queue->listen_opt
    queue->listen_opt = lopt;
    write_unlock_bh(&queue->syn_wait_lock);

    return 0;
}