nanog mailing list archives
Re: Smallest Transit MTU
From: Jerry Pasker <info () n-connect net>
Date: Wed, 29 Dec 2004 17:42:28 -0600
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
Current thread:
- Smallest Transit MTU Jerry Pasker (Dec 29)
- Re: Smallest Transit MTU Joe Abley (Dec 29)
- Re: Smallest Transit MTU Fred Baker (Dec 29)
- Re: Smallest Transit MTU Iljitsch van Beijnum (Dec 29)
- Re: Smallest Transit MTU Jerry Pasker (Dec 29)
- Re: Smallest Transit MTU Fred Baker (Dec 29)
- Re: Smallest Transit MTU Tony Rall (Dec 29)
- Re: Smallest Transit MTU Joe Abley (Dec 29)
- Re: Smallest Transit MTU Daniel Hagerty (Dec 29)
- Re: Smallest Transit MTU Alex Bligh (Dec 29)
- Re: Smallest Transit MTU Joe Abley (Dec 29)
- Re: Smallest Transit MTU Tony Rall (Dec 29)
- Re: Smallest Transit MTU Iljitsch van Beijnum (Dec 29)
- Re: Smallest Transit MTU Joe Abley (Dec 29)
- Re: Smallest Transit MTU Joe Abley (Dec 29)
- Re: Smallest Transit MTU Iljitsch van Beijnum (Dec 29)
- Re: Smallest Transit MTU Edward B. Dreger (Dec 29)