深入理解TCP协议及其源代码

 1 /* Slow start is used when congestion window is no greater than the slow start
 2  * threshold. We base on RFC2581 and also handle stretch ACKs properly.
 3  * We do not implement RFC3465 Appropriate Byte Counting (ABC) per se but
 4  * something better;) a packet is only considered (s)acked in its entirety to
 5  * defend the ACK attacks described in the RFC. Slow start processes a stretch
 6  * ACK of degree N as if N acks of degree 1 are received back to back except
 7  * ABC caps N to 2. Slow start exits when cwnd grows over ssthresh and
 8  * returns the leftover acks to adjust cwnd in congestion avoidance mode.
 9  */
10 u32 tcp_slow_start(struct tcp_sock *tp, u32 acked)
11 {
12     u32 cwnd = min(tp->snd_cwnd + acked, tp->snd_ssthresh);
13     acked -= cwnd - tp->snd_cwnd;
15     tp->snd_cwnd = min(cwnd, tp->snd_cwnd_clamp);
16 
17     return acked;
18 }
19 EXPORT_SYMBOL_GPL(tcp_slow_start);

 1 /* In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd (or alternative w),
 2  * for every packet that was ACKed.
 3  */
 4 void tcp_cong_avoid_ai(struct tcp_sock *tp, u32 w, u32 acked)
 5 {
 6     /* If credits accumulated at a higher w, apply them gently now. */
 7     if (tp->snd_cwnd_cnt >= w) {
 8         tp->snd_cwnd_cnt = 0;
 9         tp->snd_cwnd++;
10     }
11 
12     tp->snd_cwnd_cnt += acked;
13     if (tp->snd_cwnd_cnt >= w) {
14         u32 delta = tp->snd_cwnd_cnt / w;
15 
16         tp->snd_cwnd_cnt -= delta * w;
17         tp->snd_cwnd += delta;
18     }
19     tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_cwnd_clamp);
20 }
21 EXPORT_SYMBOL_GPL(tcp_cong_avoid_ai);

 1 /*
 2  * TCP Reno congestion control
 3  * This is special case used for fallback as well.
 4  */
 5 /* This is Jacobson's slow start and congestion avoidance.
 6  * SIGCOMM '88, p. 328.
 7  */
 8 void tcp_reno_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 9 {
10     struct tcp_sock *tp = tcp_sk(sk);
11 
12     if (!tcp_is_cwnd_limited(sk))
13         return;
14 
15     /* In "safe" area, increase. */
16     if (tcp_in_slow_start(tp)) {
17         acked = tcp_slow_start(tp, acked);
18         if (!acked)
19             return;
20     }
21     /* In dangerous area, increase slowly. */
22     tcp_cong_avoid_ai(tp, tp->snd_cwnd, acked);
23 }
24 EXPORT_SYMBOL_GPL(tcp_reno_cong_avoid);
25 
26 /* Slow start threshold is half the congestion window (min 2) */
27 u32 tcp_reno_ssthresh(struct sock *sk)
28 {
29     const struct tcp_sock *tp = tcp_sk(sk);
30 
31     return max(tp->snd_cwnd >> 1U, 2U);
32 }
33 EXPORT_SYMBOL_GPL(tcp_reno_ssthresh);
34 
35 u32 tcp_reno_undo_cwnd(struct sock *sk)
36 {
37     const struct tcp_sock *tp = tcp_sk(sk);
38 
39     return max(tp->snd_cwnd, tp->prior_cwnd);
40 }
41 EXPORT_SYMBOL_GPL(tcp_reno_undo_cwnd);