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SubjectRe: A router bug?
On Sun, Jan 31, 1999 at 08:57:50AM +0100, David wrote:
> On 31 Jan 1999, Andi Kleen wrote:
> > This explains it. Your other box is connected to an ethernet with MTU of
> > 1500. Thus it sends a MSS option of 1500-X into the initial SYN of the
> > TCP connection. Now the other server correctly sets its MSS to 1500-X
> > and sends packets with that size with the Dont-Fragment bit set for PMTU
> > discovery. Once they reach the other end of your slip link they get dropped
> > and the router sends back a ICMP frag-needed to tell the other box to lower
> > its pmtu - but because of the ICMP blocking firewalls set up by people that
> > were so nicely described by Alan it never sees them. It does not happen
> > from your router box, because there the first interface has a 576 MTU which
> > means that TCP only puts a small MSS option in the first SYN, and all packets
> > that are exchanged are small and no pmtu discovery is needed.
>
> This describes the problem more accurately. However, I didn't notice any
> outbound frag-needed icmps. That was several kernels ago and we had since
> simply put up with it.
>
> > One workaround: set the mtu on the gateway route you use on the other box
> > (route add default gw ROUTER mss 576), then the MSS will contain small values
> > from the beginning. It would be better to complain to the sites that don't
> > work, because it is clearly a misconfiguration on their part. Another workaround
> > is to increase the sl0 MTU to 1500bytes.
>
> Doing the above works. We are ecstatic.

Complain to the other end. It is their fault and they break the RFCs.

>
> Being naive that I am and not getting feedback *hint* *hint :), why did
> setting -all- interfaces on the net devs to an MTU of 576 still fail but
> setting the mss on the route work?

It should have worked, assuming you down the interface first so that all
routes are flushed and updated. TCP gets its initial MSS from the route,
and there is no mechanism currently in the kernel to broadcast device MTU
changes to all routes.

-Andi

--
This is like TV. I don't like TV.

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