fix-rx-rtt-computation-when-delay-is-involved-20010326

fix-rx-rtt-computation-when-delay-is-involved-20010326

"   rx: attempt to fix incorrect rtt computation, possibly winning
    on performance for links with some packet loss.

    Currently, the rtt is computed as the time from the transmission
    of a packet until the receipt of the corresponding ack packet.
    But this includes a potential delay (rx_{soft,hard,last}AckDelay)
    on the client which results in an abnormally high rtt, and even
    more erroneous rtt_dev and retransmit timeout values.

    The proposed solution is to ignore RX_ACK_DELAY type ack packets
    for the purposes of rtt computations, and introduce another ack
    packet type for RX_ACK_DELAY-type acks which are sent immediately
    by the client.

    This should of course be backwards-compatible with existing clients.
"