# Re: \[PATCH net 2/5\] tcp: prevent bogus FRTO undos with non-SACK flows

- From: Neal Cardwell \<ncardwell@google.com\>
- Date: 2018-03-07 19:24:48 
- List: netdev
- Page: https://ratatoskr.run/netdev/2018/03/11090561
- Thread: https://ratatoskr.run/netdev/2018/03/11090376/t.md

```text
On Wed, Mar 7, 2018 at 7:59 AM, Ilpo Järvinen [ref] wrote:
>
> In a non-SACK case, any non-retransmitted segment acknowledged will
> set FLAG_ORIG_SACK_ACKED in tcp_clean_rtx_queue even if there is
> no indication that it would have been delivered for real (the
> scoreboard is not kept with TCPCB_SACKED_ACKED bits in the non-SACK
> case). This causes bogus undos in ordinary RTO recoveries where
> segments are lost here and there, with a few delivered segments in
> between losses. A cumulative ACKs will cover retransmitted ones at
> the bottom and the non-retransmitted ones following that causing
> FLAG_ORIG_SACK_ACKED to be set in tcp_clean_rtx_queue and results
> in a spurious FRTO undo.
>
> We need to make the check more strict for non-SACK case and check
> that none of the cumulatively ACKed segments were retransmitted,
> which would be the case for the last step of FRTO algorithm as we
> sent out only new segments previously. Only then, allow FRTO undo
> to proceed in non-SACK case.

Hi Ilpo - Do you have a packet trace or (even better) packetdrill
script illustrating this issue? It would be nice to have a test case
or at least concrete example of this.

Thanks!
neal
```