Re: What do you consider acceptable packet / session modification for a network operator?

[nanog--- via NANOG <nanog () lists nanog org>]

I thought 9000-byte MTU wasn't used on LANs due to the headache of ensuring every single device on the LAN has the same 
MTU.

You don't need PMTUD to work on the internet to use longer packets in your LAN. The "packet too big" reply only has to 
make it from *your* edge router back to *your* server through *your* network.

But every host and switch in an Ethernet must agree on MTU because there's no Ethernet-layer PMTUD.


On 27 December 2025 17:14:19 CET, William Herrin via NANOG <nanog () lists nanog org> wrote:
> On Fri, Dec 26, 2025 at 8:07 AM Marco Moock via NANOG
> <nanog () lists nanog org> wrote:
> > Am 26.12.2025 um 17:47:14 Uhr schrieb Saku Ytti:
> > > You're saying you've never seen an ISP adjust TCP MSS here? I must
> > > have misread, because I've never seen an ISP not adjust here.
> >
> > If that fixes the problem, PMTU discovery (mandatory for IPv6 and IPv4
> > with DF bit) is broken and that means UDP, IPsec, GRE etc. all fail.
>
> Correct: PMTUD on the Internet is broken. ISPs work around this by
> engineering a clean 1500 byte path everywhere they can and clamping
> the MSS the few places that they can't.
>
> There's a reason we haven't moved up to 9kb ethernet frames on the
> server and eyeball LANs. This is that reason.
>
>
> > The ISPs I used emit ICMP packet too big messages.
>
> Everybody emits them. Too many don't make it to the destination.
>
> Regards,
> Bill Herrin
>
>
> -- 
> For hire. https://bill.herrin.us/resume/
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