Subject: Re: A question on RTT estimation of SACKed packet.

From: "Xiaoliang \(David\) Wei" <[email-masked]>
Date: Thu, 29 Aug 2002 15:19:08 -0700

Thanks, Alexey.
> >     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.
>
> It is not used. When a segment fills a hole, tcp uses skb->when of the
segment
> which _filled_ the hole. See?

Yes, this is exactly my question: When this segment fills a hole, it may be
SACKed before.
If it's SACKed before, that means its "round trip" had already finished
before this ACK came-- Now it's filling the hole just because some strange
things (such as ack loss) happened. So, I don't think we should use its
skb->when to estimate RTT. But in Linux, such a packet (SACKed) may still
provide RTT estimation.

>
> 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.
Actually, I am going to do that: Can I get a safe RTT estimation from SACK,
when the SACKed packet (in rtx queue) is never SACKed or Retransmitted?

-David