On Wed, May 31, 2017 at 11:47 AM, Stephen Hemminger
[off-list ref] wrote:

On Wed, 31 May 2017 11:21:27 -0700
Yuchung Cheng [off-list ref] wrote:

quoted

When the sender switches its congestion control during loss
recovery, if the recovery is spurious then it may incorrectly
revert cwnd and ssthresh to the older values set by a previous
congestion control. Consider a congestion control (like BBR)
that does not use ssthresh and keeps it infinite: the connection
may incorrectly revert cwnd to an infinite value when switching
from BBR to another congestion control.

This patch fixes it by disallowing such cwnd undo operation
upon switching congestion control.  Note that undo_marker
is not reset s.t. the packets that were incorrectly marked
lost would be corrected. We only avoid undoing the cwnd in
tcp_undo_cwnd_reduction().

Signed-off-by: Yuchung Cheng <redacted>
Signed-off-by: Soheil Hassas Yeganeh <redacted>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>

That looks correct. Are there other values of congestion state
that should be reset?

Nothing in particular comes up. We found this bug by internally
checking insane cwnd values on Google hosts. We might find more :-)