TCP连接建立系列 — 服务端接收ACK段（二）
本文主要分析：三次握手中最后一个ACK段到达时，服务器端的处理路径。
内核版本：3.6
Author：zhangskd @ csdn blog
创建新sock
协议族相关的操作函数，我们要看的是TCP/IPv4的实例ipv4_specific。
const struct inet_connection_sock_af_ops ipv4_specific = {
...
.conn_request = tcp_v4_conn_request, /* 处理SYN段 */
.syn_recv_sock = tcp_v4_syn_recv_sock, /* 创建和初始化一个新的sock */
...
};
三次握手完成以后，要为新的连接创建一个传输控制块，并初始化传输控制块。
一个TCP传输控制块是由多层组成的，包括：
tcp_sock
inet_connection_sock
inet_sock
sock
sock_common
所以，初始化要做的工作比较多。
/* 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;
/* 如果全连接队列满了，那么返回NULL */
if (sk_acceptq_is_full(sk))
goto exit_overflow;
/* 根据监听sock和req，为新连接创建一个传输控制块，并初始化 */
newsk = tcp_create_openreq_child(sk, req, skb);
if (! newsk)
goto exit_nonewsk;
newsk->sk_gso_type = SKB_GSO_TCPV4;
inet_sk_rx_dst_set(newsk, skb); /* 保存接收路由缓存 */
newtp = tcp_sk(newsk);
newinet = inet_sk(newsk);
ireq = inet_rsk(req);
newinet->inet_daddr = ireq->rmt_addr; /* 目的IP */
newinet->inet_rcv_saddr = ireq->loc_addr;
newinet->inet_saddr = ireq->loc_addr; /* 源IP */
inet_opt = ireq->opt;
rcu_assign_pointer(newinet->inet_opt, inet_opt); /* IP选项 */
ireq->opt = NULL;
newinet->mc_index = inet_iif(skb);
newinet->mc_ttl = ip_hdr(skb)->ttl;
newinet->rcv_tos = ip_hdr(skb)->tos;
inet_csk(newsk)->icsk_ext_hdr_len = 0;
if (inet_opt)
inet_csk(newsk)->icsk_ext_hdr_len = inet_opt->opt.optlen;
newinet->inet_id = newtp->write_seq ^ jiffies;
if (! dst) {
dst = inet_csk_route_child_sock(sk, newsk, req);
if (! dst)
goto put_and_exit;
} else {
/* syncookie case: see end of cookie_v4_check() */
}
sk_setup_caps(newsk, dst);
tcp_mtup_init(newsk); /* MTU probe init */
tcp_sync_mss(newsk, dst_mtu(dst));
newtp->advmss = dst_metric_advmss(dst);
if (tcp_sk(sk)->rx_opt.user_mss && tcp_sk(sk)->rx_opt.user_mss < newtp->advmss)
newtp->advmss = tcp_sk(sk)->rx_opt.user_mss;
tcp_initialize_rcv_mss(newsk);
if (tcp_rsk(req)->snt_synack) /* 第一个RTT样本 */
tcp_valid_rtt_meas(newsk, tcp_time_stamp - tcp_rsk(req)->snt_synack);
newtp->total_retrans = req->retrans;
#ifdef CONFIG_TCP_MD5SIG
/* Copy over the MD5 key from the original socket */
key = tcp_md5_do_lookup(sk, (union tcp_md5_addr *) &newinet->inet_daddr, AF_INET);
if (key != NULL) {
/* We're using one, so create a matching key on the newsk structure.
* If we fail to get memory, then we end up not copying the key across. Shucks.
*/
tcp_md5_do_add(newsk, (union tcp_md5_addr *) &newinet->inet_daddr, AF_INET,
key->key, key->keylen, GFP_ATOMIC);
sk_nocaps_add(newsk, NETIF_F_GSO_MASK);
}
#endif
/* 把newsk链入使用端口的哈希链表中，更新端口的统计信息 */
if (__inet_inherit_port(sk, newsk) < 0)
goto put_and_eixt;
/* 把newsk链入ESTABLISHED状态的哈希表中 */
__inet_hash_nolisten(newsk, NULL);
return newsk;
exit_overflow:
NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_LISTENOVERFLOWS);
exit_nonewsk:
dst_release(dst);
exit:
NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_LISTENDROPS);
return NULL;
put_and_exit:
tcp_clear_xmit_timers(newsk);
tcp_cleanup_congestion_control(newsk);
bh_unlock_sock(newsk);
sock_put(newsk);
goto exit;
}
根据监听传输控制块sock、连接请求块req，为新的连接创建一个传输控制块sock。
初始化此传输控制块对应的inet_sock、inet_connection_sock、tcp_sock结构中的变量。
struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req, struct sk_buff *skb)
{
/* 克隆一个传输控制块，并对新的传输控制块上锁 */
struct sock *newsk = inet_csk_clone_lock(sk, req, GFP_ATOMIC);
if (newsk != NULL) {
cosnt 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);
struct tcp_sock *oldtp = tcp_sk(sk);
struct tcp_cookie_values *oldcvp = oldtp->cookie_values;
/* 由于TCPCT选项已被废弃，此处不做分析 */
if (oldcvp != NULL) { ... }
/* Now setup tcp_sock,初始化tcp_sock实例 */
newtp->pred_flags = 0;
/* 接收序号、发送序号相关变量初始化 */
newtp->rcv_wup = newtp->copied_seq = newtp->rcv_nxt = treq->rcv_isn + 1;
newtp->snd_sml = newtp->snd_una = newtp->snd_nxt = newtp->snd_up
= treq->snt_isn + 1 + tcp_s_data_size(oldtp);
tcp_prequeue_init(newtp); /* prequeue队列初始化 */
INIT_LIST_HEAD(&newtp->tsq_node);
tcp_init_wl(newtp, treq->rcv_isn); /* 上次更新发送窗口的ACK段序号 */
/* 时延相关变量初始化 */
newtp->srtt = 0;
newtp->mdev = TCP_TIMEOUT_INIT;
newicsk->icsk_rto = TCP_TIMEOUT_INIT;
/* 拥塞控制相关变量初始化 */
newtp->packets_out = 0;
newtp->retrans_out = 0;
newtp->sacked_out = 0;
newtp->fackets_out = 0;
newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
tcp_enable_early_retrans(newtp);
newtp->snd_cwnd = TCP_INIT_CWND;
newtp->snd_cwnd_cnt = 0;
newtp->bytes_acked = 0;
newtp->frto_counter = 0;
newtp->frto_highmark = 0;
/* 如果拥塞控制算法不为Reno，则把使用的拥塞控制算法模块引用计数加1。
* 如果该模块还没插入内核，则使用Reno。
*/
if (newicsk->icsk_ca_ops != &tcp_init_congestion_ops &&
! try_module_get(newicsk->icsk_ca_ops->owner))
newicsk->icsk_ca_ops = &tcp_init_congestion_ops;
tcp_set_ca_state(newsk, TCP_CA_Open);
tcp_init_xmit_timers(newsk); /* 初始化几个定时器 */
skb_queue_head_init(&newtp->out_of_order_queue);
newtp->write_seq = newtp->pushed_seq = treq->snt_isn + 1 + tcp_s_data_size(oldtp);
/* TCP选项相关 */
newtp->rx_opt.saw_tstamp = 0;
newtp->rx_opt.dsack = 0;
newtp->rx_opt.num_sacks = 0;
newtp->urg_data = 0;
/* 如果用户设置了SO_KEEPALIVE选项 */
if (sock_flag(newsk, SOCK_KEEPOPEN))
inet_csk_reset_keepalive_timer(newsk, keepalive_time_when(newtp));
newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) {
if (sysctl_tcp_fack)
tcp_enable_fack(newtp);
}
newtp->window_clamp = req->window_clamp;
newtp->rcv_ssthresh = req->rcv_wnd;
newtp->rcv_wnd = req->rcv_wnd;
newtp->rx_opt.wscale_ok = ireq->wscale_ok;
if (newtp->rx_opt.wscale_ok) {
newtp->rx_opt.snd_wscale = ireq->snd_wscale;
newtp->rx_opt.rcv_wscale = ireq->rcv_wscale;
} else {
newtp->rx_opt.snd_wscale = newtp->rx_opt.rcv_wscale = 0;
newtp->window_clamp = min(newtp->window_clamp, 65535U);
}
newtp->snd_wnd = (ntohs(tcp_hdr(skb)->window) << newtp->rx_opt.snd_wscale);
newtp->max_window = newtp->snd_wnd;
if (newtp->rx_opt.tstamp_ok) {
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);
}
#ifdef CONFIG_TCP_MD5SIG
newtp->md5sig_info = NULL;
if (newtp->af_specific->md5_lookup(sk, newsk))
newtp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
#endif
if (skb->len >= TCP_MSS_DEFAULT + newtp->tcp_header_len)
newicsk->icsk_ack.last_seg_size = skb->len - newtp->tcp_header_len;
newtp->rx_opt.mss_clamp = req->mss;
TCP_ECN_openreq_child(newtp, req);
TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_PASSIVEOPENS);
}
return newsk;
}
克隆一个传输控制块，并对新的传输控制块上锁。
/* inet_csk_clone_lock - clone an inet socket, and lock its clone.
* @sk: the socket to clone
* @req: request_sock
* @priority: for allocation (%GFP_KERNEL, %GFP_ATOMIC, etc)
*
* Caller must unlock socket even in error path (bh_unlock_sock(newsk))
*/
struct sock *inet_csk_clone_lock(cons struct sock *sk, const struct request_sock *req, const gfp_t priority)
{
struct sock *newsk = sk_clone_lock(sk, priority); /* 从缓存中分配一个sock，并克隆sk */
if (newsk != NULL) {
struct inet_connection_sock *newicsk = inet_csk(newsk);
newsk->sk_state = TCP_SYN_RECV; /* 新sock的状态为SYN_RECV */
newicsk->icsk_bind_hash = NULL; /* 端口绑定的哈希桶 */
inet_sk(newsk)->inet_dport = inet_rsk(req)->rmt_port; /* 目的端口 */
inet_sk(newsk)->inet_num = ntohs(inet_rsk(req)->loc_port); /* 源端口 */
inet_sk(newsk)->inet_sport = inet_rsk(req)->loc_port; /* 源端口 */
newsk->sk_write_space = sk_stream_write_space; /* write_space callback */
newicsk->icsk_retransmits = 0;
newicsk->icsk_backoff = 0;
newicsk->icsk_probes_out = 0;
memset(&newicsk->icsk_accept_queue, 0, sizeof(newicsk->icsk_accept_queue));
security_inet_csk_clone(newsk, req);
}
return newsk;
}
把newsk链入使用端口的哈希链表中，更新端口的统计信息。
int __inet_inherit_port(struct sock *sk, struct sock *child)
{
struct inet_hashinfo *table = sk->sk_prot->h.hashinfo; /* 指向tcp_hashinfo */
unsigned short port = inet_sk(child)->inet_num; /* 端口 */
const int bhash = inet_bhashfn(sock_net(sk), port, table->bhash_size); /* 哈希值 */
struct inet_bind_hashbucket *head = &table->bhash[bhash]; /* 哈希桶 */
struct inet_bind_bucket *tb; /* 端口实例 */
spin_lock(&head->lock); /* 对哈希桶上锁 */
tb = inet_csk(sk)->icsk_bind_hash;
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.
*/
struct hlist_node *node;
inet_bind_bucket_for_each(tb, node, &head->chain) {
if (net_eq(ib_net(tb), sock_net(sk)) && tb->port == port)
break;
}
if (! node) {
/* 申请和初始化一个inet_bind_bucket */
tb = inet_bind_bucket_create(table->bind_bucket_cachep, sock_net(sk), head, port);
if (! tb) {
spin_unlock(&head->lock);
return -ENOMEM;
}
}
}
inet_bind_hash(child, tb, port); /* 把child链入该端口的哈希链表中，更新相关变量 */
spin_unlock(&head->lock);
return 0;
}
void inet_bind_hash(struct sock *sk, struct inet_bind_bucket *tb, const unsigned short snum)
{
struct inet_hashinfo *hashinfo = sk->sk_prot->h.hashinfo; /* 指向tcp_hashinfo */
atomic_inc(&hashinfo->bsockets); /* 增加总的绑定次数 */
inet_sk(sk)->inet_num = snum; /* 保存绑定的端口 */
sk_add_bind_node(sk, &tb->owners); /* 把此sock链入tb->owners哈希链表中 */
tb->num_owners++; /* 增加端口绑定次数 */
inet_csk(sk)->icsk_bind_hash = tb; /* 把此tb作为icsk成员icsk_bind_hash */
}
把newsk链入ESTABLISHED状态的哈希表中。
int __inet_hash_nolisten(struct sock *sk, struct inet_timewait_sock *tw)
{
struct inet_hashinfo *hashinfo = sk->sk_prot->h.hashinfo;
struct hlist_nulls_head *list;
spinlock_t *lock;
struct inet_ehash_bucket *head;
int twrefcnt = 0;
WARN_ON(! sk_unhashed(sk)); /* 要求sk不能已经链入哈希链表中 */
sk->sk_hash = inet_sk_ehashfn(sk); /* 连接的哈希值 */
head = inet_ehash_bucket(hashinfo, sk->sk_hash); /* 哈希桶 */
list = &head->chain;
lock = inet_ehash_lockp(hashinfo, sk->sk_hash);
spin_lock(lock);
__sk_nulls_add_node_rcu(sk, list); /* 把sk链入到哈希链表中 */
if (tw) {
WARN_ON(sk->sk_hash != tw->tw_hash);
twrefcnt = inet_twsk_unhash(tw);
}
spin_unlock(lock);
sock_prot_inuse_add(sock_net(sk), sk->sk_prot, 1);
return twrefcnt;
}
static inline int inet_sk_ehashfn(const struct sock *sk)
{
const struct inet_sock *inet = inet_sk(sk);
const __be32 laddr = inet->inet_rcv_saddr;
const __u16 lport = inet->inet_num;
const __be32 faddr = inet->inet_daddr;
const __be16 fport = inet->inet_dport;
struct net *net = sock_net(sk);
return inet_ehashfn(net, laddr, lport, faddr, fport);
}
唤醒监听进程
调用tcp_child_process()来做最后的处理：
1. tcp_ack()处理接收到的ACK，更新child的状态为ESTABLISHED。
唤醒child上的等待进程，初始化子传输控制块的一些字段。
2. 唤醒监听sock上的等待进程，以便监听进程执行accept()。
3. 如果child被用户进程占用，则先把ACK段添加到backlog队列中。
/* Queue segment on the new socket if the new socket is active,
* otherwise we just shortcircuit this and continue with the new socket.
*/
int tcp_child_process(struct sock *parent, struct sock *child, sk_buff *skb)
{
int ret = 0;
int state = child->sk_state;
/* child没被用户进程占用 */
if (! sock_owned_by_user(child)) {
/* 调用tcp_ack()处理接收的ACK，设置新状态ESTABLISHED，唤醒child上的等待进程，
* 初始化child的一些字段。
*/
ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb), skb->len);
/* Wakeup parent, send SIGIO.
* 实例为sock_def_readable，唤醒调用accept()的进程。
*/
if (state == TCP_SYN_RECV && child->sk_state != state)
parent->sk_data_ready(parent, 0);
} else { /* 如果child被用户进程占用，则先把skb添加到backlog队列中 */
__sk_add_backlog(child, skb);
}
bh_unlock_sock(child);
sock_put(child);
return ret;
}
把数据包添加到backlog队列中。
static inline void __sk_add_backlog(struct sock *sk, struct sk_buff *skb)
{
/* dont let skb not refcounted, we are going to leave rcu lock */
skb_dst_force(skb);
/* backlog队列为空时 */
if (! sk->sk_backlog.tail)
sk->sk_backlog.head = skb;
else
sk->sk_backlog.tail->next = skb;
sk->sk_backlog.tail = skb;
skb->next = NULL;
}
子传输控制块调用tcp_ack()处理收到的ACK，把子传输控制块的状态从TCP_SYN_RECV更新为TCP_ESTABLISHED，
并唤醒子传输控制块上的等待进程，更新子传输控制块的一些字段。
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);
int queued = 0;
tp->rx_opt.saw_tstamp = 0;
switch(sk->sk_state) {
...
}
if (! tcp_validate_incoming(sk, skb, th, 0))
return 0;
/* step 5: check the ACK field */
if (th->ack) {
int acceptable = tcp_ack(sk, skb, FLAG_SLOWPATH) > 0; /* 进入ACK处理路径 */
switch (sk->sk_state) {
case TCP_SYN_RECV:
if (acceptable) {
tp->copied_seq = tp->rcv_nxt;
smp_mb();
/* 在这里，才从TCP_SYN_RECV变为TCP_ESTABLISHED */
tcp_set_state(sk, TCP_ESTABLISHED);
sk->sk_state_change(sk); /* 实例为sock_def_wakeup()，唤醒sk上的等待进程*/
/* 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;
/* Make sure socket is routed, for correct metrics. */
icsk->icsk_af_ops->rebuild_header(sk);
tcp_init_metrics(sk); /* 根据路由缓存信息初始化控制块 */
tcp_init_congestion_control(sk); /* 初始化拥塞控制算法 */
/* Prevent spurious tcp_cwnd_restart() on first data packet. */
tp->lsndtime = tcp_time_stamp;
tcp_mtup_init(sk);
tcp_initialize_rcv_mss(sk);
tcp_init_buffer_space(sk);
tcp_fast_path_on(tp);
} else
return 1;
break;
...
}
} else
goto discard;
...
discard:
__kfree_skb(skb);
}
return 0;
}static void sock_def_wakeup(struct sock *sk)
{
struct socket_wq *wq;
rcu_read_lock();
wq = rcu_dereference(sk->sk_wq);
if (wq_has_sleeper(wq)) /* 如果sock上有等待任务 */
wake_up_interruptible_all(&wq->wait); /* 唤醒全部的等待任务 */
rcu_read_unlock();
}
/* check if there are any waiting processes. */
static inline bool wq_has_sleeper(struct socket_wq *wq)
{
smp_mb();
return wq && waitqueue_active(&wq->wait);
}
static inline int waitqueue_active(wait_queue_head_t *q)
{
return ! list_empty(&q->task_list);
}
#define wake_up_interruptible_all(x) __wake_up(x, TASK_INTERRUPTIBLE, 0, NULL)
void __wake_up(wait_queue_head_t *q, unsigned int mode, int nr_exclusive, void *key)
{
unsigned long flags;
spin_lock_irqsave(&q->lock, flags);
__wake_up_common(q, mode, nr_exclusive, 0, key);
spin_unlock_irqrestore(&q->lock, flags);
}
static void __wake_up_common(wait_queue_head_t *q, unsigned int mode, int nr_exclusive,
int wake_flags, void *key)
{
wait_queue_t *curr, *next;
list_for_each_entry_safe(curr, next, &q->task_list, task_list) {
unsigned flags = curr->flags;
if (curr->func(curr, mode, wake_flags, key) && (flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
break;
}
}
