於TCP/IP協議棧的TCP協議的重傳功能是由在linux內核源碼(net/ipv4/tcp_output.c)中的函數tcp_retransmit_skb()實現的
代碼如下：
/* 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.
*/
//說明在發送緩存區中消耗了許多內存去做其他的工作(比如分片等，只有1/4的緩存才是保留給這些工作的)，暫時不能重傳
if (atomic_read(&sk->sk_wmem_alloc) >
min(sk->sk_wmem_queued + (sk->sk_wmem_queued >> 2), sk->sk_sndbuf))
return -EAGAIN;
//檢測重傳的段，接收方是否已經收到其部分或者全部，如果收到則說明有bug ，否者就調整TCP段的負荷，即刪除SKB緩存區
//前面部分已經接收到的數據
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;
}
//根據目的地址等條件獲取路由，如果獲取路由失敗就不能發送
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.
*/
//如果接收方已經減小了窗口，並且帶重傳的SKB已經不在新的窗口內，則不能重傳該SKB，
//有一種情況例外，就是接收方的接受窗口減少為0，在這種情況下會發送0窗口探測段
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) {//如果當前的SKB長度大於MSS，則要進行分段處理
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系統的協議棧有時候會忽略重傳SKB上帶有的FIN標志的payload，將payload全部剝離掉，節省網絡流量
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);//發送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;
}
我們知道，TCP的發送是有一個SKB 隊列如圖，這樣維持一個發送隊列，如果收到發送了SKB的ACK，就將對應的SKB從隊列中刪除掉，在函數這裡注意的到函數tcp_retransmit_skb()最終的是調用函數tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);將SKB發送出去，而構造TCP的頭部信息在函
數tcp_transmit_skb()中，下面是函數tcp_transmit_skb()構造TCP頭部的片段 所以也就是說在發送隊列中的SKB是沒有頭部的，這也是方便了選擇重傳等功能
/* 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);
