于TCP/IP协议栈的TCP协议的重传功能是由在linux内核源码(net/ipv4/tcp_output.c)中的函数tcp_retransmit_skb()实现的

代码如下：

/* This retransmits one SKB.  Policy decisions and retransmit queue

 * state updates are done by the caller.  Returns non-zero if an

 * error occurred which prevented the send.

 */

int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)

{

	struct tcp_sock *tp = tcp_sk(sk);

	struct inet_connection_sock *icsk = inet_csk(sk);

	unsigned int cur_mss;

	int err;

	/* Inconslusive MTU probe */

	if (icsk->icsk_mtup.probe_size) {

		icsk->icsk_mtup.probe_size = 0;

	}

	/* Do not sent more than we queued. 1/4 is reserved for possible

	 * copying overhead: fragmentation, tunneling, mangling etc.

	 */

	 	//说明在发送缓存区中消耗了许多内存去做其他的工作(比如分片等，只有1/4的缓存才是保留给这些工作的)，暂时不能重传

	if (atomic_read(&sk->sk_wmem_alloc) >

	    min(sk->sk_wmem_queued + (sk->sk_wmem_queued >> 2), sk->sk_sndbuf))

		return -EAGAIN;

	//检测重传的段，接收方是否已经收到其部分或者全部，如果收到则说明有bug ，否者就调整TCP段的负荷，即删除SKB缓存区

	//前面部分已经接收到的数据

	if (before(TCP_SKB_CB(skb)->seq, tp->snd_una)) {

		if (before(TCP_SKB_CB(skb)->end_seq, tp->snd_una))

			BUG();

		if (tcp_trim_head(sk, skb, tp->snd_una - TCP_SKB_CB(skb)->seq))

			return -ENOMEM;

	}

	//根据目的地址等条件获取路由，如果获取路由失败就不能发送

	if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk))

		return -EHOSTUNREACH; /* Routing failure or similar. */

	cur_mss = tcp_current_mss(sk);

	/* If receiver has shrunk his window, and skb is out of

	 * new window, do not retransmit it. The exception is the

	 * case, when window is shrunk to zero. In this case

	 * our retransmit serves as a zero window probe.

	 */

	 //如果接收方已经减小了窗口，并且带重传的SKB已经不在新的窗口内，则不能重传该SKB，

	 //有一种情况例外，就是接收方的接受窗口减少为0，在这种情况下会发送0窗口探测段

	if (!before(TCP_SKB_CB(skb)->seq, tcp_wnd_end(tp)) &&

	    TCP_SKB_CB(skb)->seq != tp->snd_una)

		return -EAGAIN;

	if (skb->len > cur_mss) {//如果当前的SKB长度大于MSS，则要进行分段处理

		if (tcp_fragment(sk, skb, cur_mss, cur_mss))

			return -ENOMEM; /* We'll try again later. */

	} else {

		int oldpcount = tcp_skb_pcount(skb);

		if (unlikely(oldpcount > 1)) {

			tcp_init_tso_segs(sk, skb, cur_mss);

			tcp_adjust_pcount(sk, skb, oldpcount - tcp_skb_pcount(skb));

		}

	}

	tcp_retrans_try_collapse(sk, skb, cur_mss);

	/* Some Solaris stacks overoptimize and ignore the FIN on a

	 * retransmit when old data is attached.  So strip it off

	 * since it is cheap to do so and saves bytes on the network.

	 */

	 //有以下Solaris系统的协议栈有时候会忽略重传SKB上带有的FIN标志的payload，将payload全部剥离掉，节省网络流量

	if (skb->len > 0 &&

	    (TCP_SKB_CB(skb)->flags & TCPHDR_FIN) &&

	    tp->snd_una == (TCP_SKB_CB(skb)->end_seq - 1)) {

		if (!pskb_trim(skb, 0)) {

			/* Reuse, even though it does some unnecessary work */

			tcp_init_nondata_skb(skb, TCP_SKB_CB(skb)->end_seq - 1,

					     TCP_SKB_CB(skb)->flags);

			skb->ip_summed = CHECKSUM_NONE;

		}

	}

	/* Make a copy, if the first transmission SKB clone we made

	 * is still in somebody's hands, else make a clone.

	 */

	TCP_SKB_CB(skb)->when = tcp_time_stamp;

	err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);//发送SKB

	if (err == 0) {

		/* Update global TCP statistics. */

		TCP_INC_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS);

		tp->total_retrans++;

#if FASTRETRANS_DEBUG > 0

		if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) {

			if (net_ratelimit())

				printk(KERN_DEBUG "retrans_out leaked.\n");

		}

#endif

		if (!tp->retrans_out)

			tp->lost_retrans_low = tp->snd_nxt;

		TCP_SKB_CB(skb)->sacked |= TCPCB_RETRANS;

		tp->retrans_out += tcp_skb_pcount(skb);

		/* Save stamp of the first retransmit. */

		if (!tp->retrans_stamp)

			tp->retrans_stamp = TCP_SKB_CB(skb)->when;

		tp->undo_retrans += tcp_skb_pcount(skb);

		/* snd_nxt is stored to detect loss of retransmitted segment,

		 * see tcp_input.c tcp_sacktag_write_queue().

		 */

		TCP_SKB_CB(skb)->ack_seq = tp->snd_nxt;

	}

	return err;

}

我们知道，TCP的发送是有一个SKB 队列如图，这样维持一个发送队列，如果收到发送了SKB的ACK，就将对应的SKB从队列中删除掉，在函数

tcp_retransmit_skb中我们可以看到，接受方游有可能只是收到了部分SKB的数据，那么就将收到的SKB的数据删除掉，这样可以节省缓存空间

这里注意的到函数tcp_retransmit_skb()最终的是调用函数tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);将SKB发送出去，而构造TCP的头部信息在函

数tcp_transmit_skb()中，下面是函数tcp_transmit_skb()构造TCP头部的片段 所以也就是说在发送队列中的SKB是没有头部的，这也是方便了选择重传等功能

	/* Build TCP header and checksum it. */

	th = tcp_hdr(skb);

	th->source		= inet->inet_sport;

	th->dest		= inet->inet_dport;

	th->seq			= htonl(tcb->seq);

	th->ack_seq		= htonl(tp->rcv_nxt);

	*(((__be16 *)th) + 6)	= htons(((tcp_header_size >> 2) << 12) |

					tcb->flags);