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/* 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;}
   

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