On Mon, Sep 9, 2019 at 10:56 PM Neal Cardwell <ncardw...@google.com> wrote:
>
> Fix tcp_ecn_withdraw_cwr() to clear the correct bit:
> TCP_ECN_QUEUE_CWR.
>
> Rationale: basically, TCP_ECN_DEMAND_CWR is a bit that is purely about
> the behavior of data receivers, and deciding whether to reflect
> incoming IP ECN CE marks as outgoing TCP th->ece marks. The
> TCP_ECN_QUEUE_CWR bit is purely about the behavior of data senders,
> and deciding whether to send CWR. The tcp_ecn_withdraw_cwr() function
> is only called from tcp_undo_cwnd_reduction() by data senders during
> an undo, so it should zero the sender-side state,
> TCP_ECN_QUEUE_CWR. It does not make sense to stop the reflection of
> incoming CE bits on incoming data packets just because outgoing
> packets were spuriously retransmitted.
>
> The bug has been reproduced with packetdrill to manifest in a scenario
> with RFC3168 ECN, with an incoming data packet with CE bit set and
> carrying a TCP timestamp value that causes cwnd undo. Before this fix,
> the IP CE bit was ignored and not reflected in the TCP ECE header bit,
> and sender sent a TCP CWR ('W') bit on the next outgoing data packet,
> even though the cwnd reduction had been undone. After this fix, the
> sender properly reflects the CE bit and does not set the W bit.
>
> Note: the bug actually predates 2005 git history; this Fixes footer is
> chosen to be the oldest SHA1 I have tested (from Sep 2007) for which
> the patch applies cleanly (since before this commit the code was in a
> .h file).
>
> Fixes: bdf1ee5d3bd3 ("[TCP]: Move code from tcp_ecn.h to tcp*.c and tcp.h &
> remove it")
> Signed-off-by: Neal Cardwell <ncardw...@google.com>
> Acked-by: Yuchung Cheng <ych...@google.com>
> Acked-by: Soheil Hassas Yeganeh <soh...@google.com>
> Cc: Eric Dumazet <eduma...@google.com>
Signed-off-by: Eric Dumazet <eduma...@google.com>
Thanks Neal
