Re: [PATCH net 4/5] tcp: prevent bogus undos when SACK is not enabled

On Fri, 9 Mar 2018, David Miller wrote:

From: Ilpo Jï¿œrvinen <redacted>
Date: Fri, 9 Mar 2018 16:11:47 +0200 (EET)

quoted

Unfortunately I don't have now permission to publish the time-seq
graph about it but I've tried to improve the changelog messages so
that you can better understand under which conditions the problem
occurs.

It is indeed extremely unfortunate that you wish to justify a change
for which you cannot provide the supporting data at all.

Here is the time-seqno graph about the issue:

https://www.cs.helsinki.fi/u/ijjarvin/linux/nonsackbugs/recovery_undo_bug.pdf

First the correct CC action (wnd reduction) occurs; then bogus undo 
causes bursting back to the window with which the congestion losses 
occurred earlier; because of the burst, some packets get lost due to 
congestion again.

The sender is actually somewhat lucky here: If only one packet would get 
lost instead of three, the same process would repeat for the next recovery 
(as cumulative ACK to high_seq condition would reoccur).


-- 
 i.