RTT.md

This file displays the process flow for RTT calculation in the kernel.

Transmitting a packet

tcp_sendmsg()
-> tcp_sendmsg_locked()
-> tcp_push_one()
-> tcp_write_xmit()
-> |- tcp_mstamp_refresh()
  |- __tcp_transmit_skb()

static inline void tcp_mstamp_refresh(struct tcp_sock *tp)
{
	u64 val = tcp_clock_us();

	if (val > tp->tcp_mstamp)
		tp->tcp_mstamp = val;
}

static int __tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, ...)
: skb->skb_mstamp = tp->tcp_mstamp;

Receiving a packet

tcp_v4_rcv()
-> tcp_v4_do_rcv()
-> tcp_rcv_state_process()
-> |- tcp_mstamp_refresh()
  |- tcp_ack()
    -> tcp_clean_rtx_queue()
    -> |- tcp_stamp_us_delta()
      |- tcp_ack_update_rtt()
      -> tcp_rtt_estimator()

static int tcp_clean_rtx_queue(struct sock *sk, ...)
: last_ackt = skb->skb_mstamp;
  WARN_ON_ONCE(last_ackt == 0);
  if (!first_ackt)
	  first_ackt = last_ackt;

  seq_rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, first_ackt);
  rtt_update = tcp_ack_update_rtt(sk, flag, seq_rtt_us, ...);

static void tcp_rtt_estimator(struct sock *sk, long mrtt_us)
: m -= (srtt >> 3);
  srtt += m;		/* rtt = 7/8 rtt + 1/8 new */