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

[Marco Moock via NANOG <nanog () lists nanog org>]

Am 26.12.2025 um 06:08:34 Uhr schrieb William Herrin:

> That's not really on the list of Internet problems with PMTUD. Not a
> lot of packets without the DF bit set any more.
>
> No, the problem is there's lots of reasons for that ICMP packet to
> get dropped.
>
> * No valid route from the complaining router to the packet origin.

> IP is end-to-end. You're only supposed to have to guarantee routes
> between the endpoints, not between the midpoints and endpoints.

I do not understand that. If the router has a public routable address
and either a default route to a router with full table, the packet
should arrive. Otherwise a general routing problem exist.
I am aware of such situations, but PMTU issues are just one of the
many issues that are caused by this.

> * Complaining router's interface is numbered with RFC1918.

Then the NAT mechanism is failing, as there must not be non-global
addresses traveling AS borders. The NAT ACL must include all used
addresses that are non-global.

> And I haven't even touched the stupid firewall admins who erroneously
> block all ICMP "because it's ping." There are a lot of them.

I know, but they create there own problems and there is no need that
ISPs circumvent their self-made problems.

> No, if you don't want the headache of having to deal with every goofy
> little situation where PMTUD doesn't work and you _know_ you have a
> link with an MTU under 1500 (common with ISPs using PPPOE to the
> customer premise equipment) then you clamp the TCP MSS. You don't like
> it. But you do it anyway because tech support hours are expensive and
> that results in fewer of them.

I've never seen that yet at the ISPs I use.

-- 
Gruß
Marco

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