- val = tcp_metric_get(tm, TCP_METRIC_RTT);
- if (val == 0 || tp->srtt == 0) {
- rcu_read_unlock();
- goto reset;
- }
- /* Initial rtt is determined from SYN,SYN-ACK.
- * The segment is small and rtt may appear much
- * less than real one. Use per-dst memory
- * to make it more realistic.
- *
- * A bit of theory. RTT is time passed after "normal" sized packet
- * is sent until it is ACKed. In normal circumstances sending small
- * packets force peer to delay ACKs and calculation is correct too.
- * The algorithm is adaptive and, provided we follow specs, it
- * NEVER underestimate RTT. BUT! If peer tries to make some clever
- * tricks sort of "quick acks" for time long enough to decrease RTT
- * to low value, and then abruptly stops to do it and starts to delay
- * ACKs, wait for troubles.
- */
- val = msecs_to_jiffies(val);
- if (val > tp->srtt) {
- tp->srtt = val;
- tp->rtt_seq = tp->snd_nxt;
- }
- val = tcp_metric_get_jiffies(tm, TCP_METRIC_RTTVAR);
- if (val > tp->mdev) {
- tp->mdev = val;
- tp->mdev_max = tp->rttvar = max(tp->mdev, tcp_rto_min(sk));
- }