Hello!
> 2. When a packet was SACKed but not retransmitted, the linux also
> calculate the seq_rtt from it when there is no unSACKed packet in the queue.
> I cannot understand the second situation: The packet was SACKed before,
> that means it arrived the receiver and triggered a SACK sometime before. The
> interval between when packet is sent and when the SACK is received should be
> the RTT for experienced this packet. Even now the packet is ACKed, I don't
> think this ACK is triggered by this packet. Why is it used to calculate the
> RTT?
It is not used. When a segment fills a hole, tcp uses skb->when of the segment
which _filled_ the hole. See?
What's about using SACKs to give additional feed to rtt estimator,
even when ACK is duplicate, it is intersting idea, I even read about
this somewhere. But we do not use this.
Alexey
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