RE: Charging for IPs

[Gary Sparkes via NANOG <nanog () lists nanog org>]

I was not aware that much (if any?) IPv4 code cared about PMTUD - I thought that only  gained provenance in IPv6

Outside of setting DF I guess? But I've had no issues with it, but I only accept/transit 1500 MTU so perhaps I don't 
see an issue where one might exist 

-----Original Message-----
From: Saku Ytti <saku () ytti fi> 
Sent: Monday, September 7, 2026 3:42 AM
To: Gary Sparkes <gary () kisaracorporation com>
Cc: North American Network Operators Group <nanog () lists nanog org>; Vasilenko Eduard <vasilenko.eduard () huawei com>
Subject: Re: Charging for IPs

On Mon, 7 Sept 2026 at 10:38, Gary Sparkes <gary () kisaracorporation com> wrote:

> For direct peering, RFC1918 is valid and usable, so that could be an option providers give - I have a few sessions 
> where I'm using RFC1918 addresses that the provider allocated to me for that first hop session.

RFC1918 breaks PTMUD pretty reliably.

You can in many platforms decide to originate ICMP messages from e.g.
loopback instead of the RFC1918 interface. But in that case, why bother with the RFC1918, far-side may route it 
internally and create attack vector to protect against that you have to consider all kinds of attacks towards that 
RFC1918. With RFC8950 you don't have to, you only have to worry about the IPv6.

If you don't have IPv6 and you can originate ICMP from loop, maybe
IPv4 LL address is less risky than RFC1918?

--
  ++ytti
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