Re: [PATCH net-next 1/7] tcp: apply Kern's check on RTTs used for congestion control
From: Kenneth Klette Jonassen <hidden>
Date: 2016-02-02 19:30:28
On Sat, Oct 17, 2015 at 6:57 AM, Yuchung Cheng [off-list ref] wrote:
Currently ca_seq_rtt_us does not use Kern's check. Fix that by checking if any packet acked is a retransmit, for both RTT used for RTT estimation and congestion control.
*snip* This patch (commit 9e45a3e) puzzles me, because Karn's check was already in effect: http://lxr.free-electrons.com/source/net/ipv4/tcp_input.c?v=4.3#L3117 Since first_ackt/last_ackt is only set when non-retransmitted packets are ACKed (sequentially), we know them to be unambiguous samples for RTTM. Even if a (sequential) ACK covers retransmitted packets, we can still make a valid RTTM if that ACK also covers non-retransmitted packets. But this patch seems to prevent that?