netdev - Why we discard all rtt samples when only some of the acked skbs have been retransmited in processing ack?

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Message-ID: <CAAA3+BpsxtyM2uAvX6B4ys73ZC6fX5K1ib3Acsk0fp5cQBNgWg@mail.gmail.com>
Date:	Tue, 17 Sep 2013 12:01:28 +0800
From:	LovelyLich <lovelylich@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Why we discard all rtt samples when only some of the acked skbs have
 been retransmited in processing ack?

Hi Eric,

In tcp_clean_rtx_queue(), we set the flag FLAG_RETRANS_DATA_ACKED when we

encounter one ever retransmited skb A. But if there is one( or more) skb B

after this retransmited skb, and we calculate the rtt for skb B. The question

is because we have set the flag FLAG_RETRANS_DATA_ACKED, and we will just

return in tcp_ack_no_tstamp() !

Two questions:

1. if we will just ignore all packets in this ack, we do not need to calculate

skb B's rtt sample.

2. what I want to know, even if A's rtt sample is not reliable, but B's rtt

sample can be trusted. Why we discard it ?

Thanks in advanced.

regards,

Yi
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