[AusNOG] MTU debugging? (Or possibly just a fault with Amazon SES?)

[Paul Wilkins]

I wonder if Amazon are not RFC 791 compliant, because 'This usually
signifies an MTU misconfiguration on the remote end, we cannot help with
this' suggests a problem with packets dropping when oversize, rather than
fragmenting?

Ordinarily, I'd suspect something above the network layer that's doing
packet inspection. Any compliant device at the network layer should
fragment and forward the packets, but that costs CPU and it may be cheaper
to drop the packet on the floor, RFCs notwithstanding.

Paul Wilkins

On 3 June 2015 at 16:04, Jeremy Visser <jeremy at sunriseroad.net> wrote:

> On 03/06/15 14:47, Damien Gardner Jnr wrote:
> > But sure enough, as expected the router IS sending the correct ICMP
> reply back to amazon:
> >
> > However the next packet in is then again over sized:
> >
> > So the router again sends a frag needed:
> >
> > And what do you know, another packet that is too large?
>
> To me that looks like Amazon are ignoring your ICMP "frag needed" message,
> and as such their TCP is retransmitting over and over until timeout.
>
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