/* 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 = tcp_current_mss(sk, 0);
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.
*/ // 如果消耗很多的内存做其他事，那么就没有多余的来做队列的处理了~
if (atomic_read(&sk->sk_wmem_alloc) > // sk_wmem_alloc：传输队列大小
min(sk->sk_wmem_queued + (sk->sk_wmem_queued >> 2), sk->sk_sndbuf)) // sk_wmem_queud：固定的队列大小
return -EAGAIN;
if (before(TCP_SKB_CB(skb)->seq, tp->snd_una)) {// 若这样，说明是有一部分数据才需要重传，形如：seq---snd_una---end_seq，前面一半已收到ACK
if (before(TCP_SKB_CB(skb)->end_seq, tp->snd_una)) // 若这样，说明全部ACK，无需重传，BUG
BUG();
if (tcp_trim_head(sk, skb, tp->snd_una - TCP_SKB_CB(skb)->seq)) // 一些控制信息检查
return -ENOMEM;
}
/* 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.
*/
if (!before(TCP_SKB_CB(skb)->seq, tcp_wnd_end(tp)) // 如果数据在窗口后面，不会发送
&& TCP_SKB_CB(skb)->seq != tp->snd_una)
return -EAGAIN;
if (skb->len > cur_mss) { // 如果skb长度 > MSS
if (tcp_fragment(sk, skb, cur_mss, cur_mss)) // 先分片。再传送
return -ENOMEM; /* We'll try again later. */
}
/* Collapse two adjacent packets if worthwhile and we can. */ // 我*，这么多条件
if (!(TCP_SKB_CB(skb)->flags & TCPCB_FLAG_SYN) && // SYN包
(skb->len < (cur_mss >> 1)) && // 长度<半个MSS
(tcp_write_queue_next(sk, skb) != tcp_send_head(sk)) && // 不是结尾
(!tcp_skb_is_last(sk, skb)) && // 不是最后一个
(skb_shinfo(skb)->nr_frags == 0 && // 没有分页数据
skb_shinfo(tcp_write_queue_next(sk, skb))->nr_frags == 0) &&
(tcp_skb_pcount(skb) == 1 && // gso_segs=1
tcp_skb_pcount(tcp_write_queue_next(sk, skb)) == 1) &&
(sysctl_tcp_retrans_collapse != 0))
tcp_retrans_try_collapse(sk, skb, cur_mss); // 这个函数不是很明白，待看~~~~~~~~~~~~~~~~~~~~~~~~~
if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk)) // 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
return -EHOSTUNREACH; /* Routing failure or similar. */
/* 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.
*///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 & TCPCB_FLAG_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); // 这个才是正在的传输函数~~~~~~~~~~~~~~~~~~~~后面再说~~~~~~~~~~~~~~~
// 这个函数就是将数据包发送到下面一层，再慢慢传输出去~~~~~~~~~~~~~~
if (err == 0) { // 发送成功，那么就需要更新TCP统计信息
/* Update global TCP statistics. */
TCP_INC_STATS(TCP_MIB_RETRANSSEGS);
tp->total_retrans++; // 整体重传数量++
#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); // 重传出去的数量+=。。。
/* Save stamp of the first retransmit. */
if (!tp->retrans_stamp)
tp->retrans_stamp = TCP_SKB_CB(skb)->when; // 第一次重传时间戳
tp->undo_retrans++;
/* 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;
}
