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

[Paul Wilkins]

My understanding is well behaved hosts will send DF traffic only for PMTUD.
If you're sending regular traffic with DF, you're pretty much asking for it.

Paul Wilkins

On 3 June 2015 at 19:23, Jeremy Visser <jeremy at sunriseroad.net> wrote:

> On 03/06/15 17:03, Paul Wilkins wrote:
> > 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?
>
> Most hosts speaking TCP set the DF (Don’t Fragment) bit, thereby
> encouraging MTU boundaries to send ICMP fragmentation reports.  (Feature,
> not a bug.)
>
> The alternative is to speak TCP without the DF bit set, thereby causing
> fragmentation at an MTU boundary, leading to worse efficiency.
>
> Both situations suck, because both ICMP and fragmented packets are subject
> to senseless amounts of filtering by uninformed admins.
>
> In IPv6, fragmentation is end-to-end, making it even more crucially
> important for intermediate hops to not block vital ICMP messages.  (Dear
> Internet: ICMP is not optional.)
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