- if (tp->snd_cwnd <= tp->snd_ssthresh) {
- /* In "safe" area, increase. */
- if (tp->snd_cwnd < tp->snd_cwnd_clamp)
- tp->snd_cwnd++;
- } else {
- /* In dangerous area, increase slowly.
- * In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd
- */
- if (tp->snd_cwnd_cnt >= bictcp_cwnd(tp)) {
- if (tp->snd_cwnd < tp->snd_cwnd_clamp)
- tp->snd_cwnd++;
- tp->snd_cwnd_cnt=0;
- } else
- tp->snd_cwnd_cnt++;
- }
- tp->snd_cwnd_stamp = tcp_time_stamp;
-}
-
-/* This is based on the congestion detection/avoidance scheme described in
- * Lawrence S. Brakmo and Larry L. Peterson.
- * "TCP Vegas: End to end congestion avoidance on a global internet."
- * IEEE Journal on Selected Areas in Communication, 13(8):1465--1480,
- * October 1995. Available from:
- * ftp://ftp.cs.arizona.edu/xkernel/Papers/jsac.ps
- *
- * See http://www.cs.arizona.edu/xkernel/ for their implementation.
- * The main aspects that distinguish this implementation from the
- * Arizona Vegas implementation are:
- * o We do not change the loss detection or recovery mechanisms of
- * Linux in any way. Linux already recovers from losses quite well,
- * using fine-grained timers, NewReno, and FACK.
- * o To avoid the performance penalty imposed by increasing cwnd
- * only every-other RTT during slow start, we increase during
- * every RTT during slow start, just like Reno.
- * o Largely to allow continuous cwnd growth during slow start,
- * we use the rate at which ACKs come back as the "actual"
- * rate, rather than the rate at which data is sent.
- * o To speed convergence to the right rate, we set the cwnd
- * to achieve the right ("actual") rate when we exit slow start.
- * o To filter out the noise caused by delayed ACKs, we use the
- * minimum RTT sample observed during the last RTT to calculate
- * the actual rate.
- * o When the sender re-starts from idle, it waits until it has
- * received ACKs for an entire flight of new data before making
- * a cwnd adjustment decision. The original Vegas implementation
- * assumed senders never went idle.
- */
-static void vegas_cong_avoid(struct tcp_opt *tp, u32 ack, u32 seq_rtt)
-{
- /* The key players are v_beg_snd_una and v_beg_snd_nxt.
- *
- * These are so named because they represent the approximate values
- * of snd_una and snd_nxt at the beginning of the current RTT. More
- * precisely, they represent the amount of data sent during the RTT.
- * At the end of the RTT, when we receive an ACK for v_beg_snd_nxt,
- * we will calculate that (v_beg_snd_nxt - v_beg_snd_una) outstanding
- * bytes of data have been ACKed during the course of the RTT, giving
- * an "actual" rate of:
- *
- * (v_beg_snd_nxt - v_beg_snd_una) / (rtt duration)
- *
- * Unfortunately, v_beg_snd_una is not exactly equal to snd_una,
- * because delayed ACKs can cover more than one segment, so they
- * don't line up nicely with the boundaries of RTTs.
- *
- * Another unfortunate fact of life is that delayed ACKs delay the
- * advance of the left edge of our send window, so that the number
- * of bytes we send in an RTT is often less than our cwnd will allow.
- * So we keep track of our cwnd separately, in v_beg_snd_cwnd.
- */
-
- if (after(ack, tp->vegas.beg_snd_nxt)) {
- /* Do the Vegas once-per-RTT cwnd adjustment. */
- u32 old_wnd, old_snd_cwnd;
-
-
- /* Here old_wnd is essentially the window of data that was
- * sent during the previous RTT, and has all
- * been acknowledged in the course of the RTT that ended
- * with the ACK we just received. Likewise, old_snd_cwnd
- * is the cwnd during the previous RTT.
- */
- old_wnd = (tp->vegas.beg_snd_nxt - tp->vegas.beg_snd_una) /
- tp->mss_cache_std;
- old_snd_cwnd = tp->vegas.beg_snd_cwnd;
-
- /* Save the extent of the current window so we can use this
- * at the end of the next RTT.
- */
- tp->vegas.beg_snd_una = tp->vegas.beg_snd_nxt;
- tp->vegas.beg_snd_nxt = tp->snd_nxt;
- tp->vegas.beg_snd_cwnd = tp->snd_cwnd;
-
- /* Take into account the current RTT sample too, to
- * decrease the impact of delayed acks. This double counts
- * this sample since we count it for the next window as well,
- * but that's not too awful, since we're taking the min,
- * rather than averaging.
- */
- vegas_rtt_calc(tp, seq_rtt);
-
- /* We do the Vegas calculations only if we got enough RTT
- * samples that we can be reasonably sure that we got
- * at least one RTT sample that wasn't from a delayed ACK.
- * If we only had 2 samples total,
- * then that means we're getting only 1 ACK per RTT, which
- * means they're almost certainly delayed ACKs.
- * If we have 3 samples, we should be OK.
- */
-
- if (tp->vegas.cntRTT <= 2) {
- /* We don't have enough RTT samples to do the Vegas
- * calculation, so we'll behave like Reno.
- */
- if (tp->snd_cwnd > tp->snd_ssthresh)
- tp->snd_cwnd++;
- } else {
- u32 rtt, target_cwnd, diff;
-
- /* We have enough RTT samples, so, using the Vegas
- * algorithm, we determine if we should increase or
- * decrease cwnd, and by how much.
- */
-
- /* Pluck out the RTT we are using for the Vegas
- * calculations. This is the min RTT seen during the
- * last RTT. Taking the min filters out the effects
- * of delayed ACKs, at the cost of noticing congestion
- * a bit later.
- */
- rtt = tp->vegas.minRTT;
-
- /* Calculate the cwnd we should have, if we weren't
- * going too fast.
- *
- * This is:
- * (actual rate in segments) * baseRTT
- * We keep it as a fixed point number with
- * V_PARAM_SHIFT bits to the right of the binary point.
- */
- target_cwnd = ((old_wnd * tp->vegas.baseRTT)
- << V_PARAM_SHIFT) / rtt;
-
- /* Calculate the difference between the window we had,
- * and the window we would like to have. This quantity
- * is the "Diff" from the Arizona Vegas papers.
- *
- * Again, this is a fixed point number with
- * V_PARAM_SHIFT bits to the right of the binary
- * point.
- */
- diff = (old_wnd << V_PARAM_SHIFT) - target_cwnd;
-
- if (tp->snd_cwnd < tp->snd_ssthresh) {
- /* Slow start. */
- if (diff > sysctl_tcp_vegas_gamma) {
- /* Going too fast. Time to slow down
- * and switch to congestion avoidance.
- */
- tp->snd_ssthresh = 2;
-
- /* Set cwnd to match the actual rate
- * exactly:
- * cwnd = (actual rate) * baseRTT
- * Then we add 1 because the integer
- * truncation robs us of full link
- * utilization.
- */
- tp->snd_cwnd = min(tp->snd_cwnd,
- (target_cwnd >>
- V_PARAM_SHIFT)+1);
-
- }
- } else {
- /* Congestion avoidance. */
- u32 next_snd_cwnd;
-
- /* Figure out where we would like cwnd
- * to be.
- */
- if (diff > sysctl_tcp_vegas_beta) {
- /* The old window was too fast, so
- * we slow down.
- */
- next_snd_cwnd = old_snd_cwnd - 1;
- } else if (diff < sysctl_tcp_vegas_alpha) {
- /* We don't have enough extra packets
- * in the network, so speed up.
- */
- next_snd_cwnd = old_snd_cwnd + 1;
- } else {
- /* Sending just as fast as we
- * should be.
- */
- next_snd_cwnd = old_snd_cwnd;
- }
-
- /* Adjust cwnd upward or downward, toward the
- * desired value.
- */
- if (next_snd_cwnd > tp->snd_cwnd)
- tp->snd_cwnd++;
- else if (next_snd_cwnd < tp->snd_cwnd)
- tp->snd_cwnd--;
- }
- }
-
- /* Wipe the slate clean for the next RTT. */
- tp->vegas.cntRTT = 0;
- tp->vegas.minRTT = 0x7fffffff;
- }
-
- /* The following code is executed for every ack we receive,
- * except for conditions checked in should_advance_cwnd()
- * before the call to tcp_cong_avoid(). Mainly this means that
- * we only execute this code if the ack actually acked some
- * data.
- */
-
- /* If we are in slow start, increase our cwnd in response to this ACK.
- * (If we are not in slow start then we are in congestion avoidance,
- * and adjust our congestion window only once per RTT. See the code
- * above.)
- */
- if (tp->snd_cwnd <= tp->snd_ssthresh)
- tp->snd_cwnd++;
-
- /* to keep cwnd from growing without bound */
- tp->snd_cwnd = min_t(u32, tp->snd_cwnd, tp->snd_cwnd_clamp);
-
- /* Make sure that we are never so timid as to reduce our cwnd below
- * 2 MSS.
- *
- * Going below 2 MSS would risk huge delayed ACKs from our receiver.
- */
- tp->snd_cwnd = max(tp->snd_cwnd, 2U);
-
- tp->snd_cwnd_stamp = tcp_time_stamp;
-}
-
-static inline void tcp_cong_avoid(struct tcp_opt *tp, u32 ack, u32 seq_rtt)
-{
- if (tcp_vegas_enabled(tp))
- vegas_cong_avoid(tp, ack, seq_rtt);
- else
- reno_cong_avoid(tp);