Re: Smallest Transit MTU

[Jerry Pasker <info () n-connect net>]

> Regardless of this, it's probably a good idea to obsolete the 
> original meaning of the DF bit.

So my next question is: Is it safe for the entire internet to ignore 
the DF bit entirely?  Sounds like it would save plenty of router 
manufactures, plenty of time/effort.

Apparently Cisco's official recommendation for solving the problem 
for packets destined to any network with an MTU less than 1500 bytes 
due to ICMP "Fragmentation Needed But DF Set" packets not making it 
back to the original pMTUd server (for whatever reason......) is to 
clear the DF bits with policy routing, and fragment anyway.

"Let's break the internet some more to fix something that someone 
else* broke!  Fun!"

*as in: an idiot ICMP blocking firewall admin who thinks that "ICMP" 
means ping.

Maybe they think they can use pMTUd to make up the speed lost from 
the possible increase in congestion/dropped packets caused by the 
lack of ICMP source-quench messages reaching their server.

I hate to think how many people-hours were wasted on the 
implementation of anything to do with the DF flag, routers kicking 
back ICMPs when encountering smaller networks,  everything pMTUd, the 
router code to flip DF bits, and the implementation of all of it to 
arrive back at the way life was pre-pMTUd+bad firewall.

/rant.

-Jerry