Re: NAT stops forwarding ACKs after PMTU discovery

Christoph Paasch
2013-08-19T15:33:35Z

On 19/08/13 - 07:35:09, Phil Oester wrote:
> On Mon, Aug 19, 2013 at 06:58:05AM -0700, Eric Dumazet wrote:
> > Yeah, but here, this is conntrack who is blocking the thing.
> >
> > TCP receiver has no chance to 'fix' it.
> >
> > See conntrack is one of those buggy middle box as well.
> >
> > So if you want to properly handle this mess, you'll also have to fix
> > conntrack.
>
> Better to fix the box which is randomizing the acks and ignoring
> the SACKs. Usually these are older Cisco PIX/ASA devices which just
> need a code upgrade.

I agree, the problem are these horrid middleboxes...

Unfortunately, they will hardly go away in the near futur. Rather the
opposite is the case.

If you have a public server running, I would be interested in the count of
invalid SACK-blocks received (netstat -s | grep TCPSACKDiscard). This is an
indication for such kind of middlebox between your server and the client,
implying that these connections cannot benefit from TCP-FastRetransmission
and each packet-loss will require an RTO to recover.

Cheers,
Christoph