nanog mailing list archives
Re: MTU of the Internet?
From: Phil Howard <phil () charon milepost com>
Date: Thu, 5 Feb 1998 15:47:04 -0600 (CST)
Marc Slemko writes...
On Thu, 5 Feb 1998, Phil Howard wrote:IMHO setting DF should not be allowed where the MTU is greater than 576, or whatever number today constitutes the "minimum reasonable requirement" which I would say isn't larger than 1006. Maybe in a few years we can kiss SLIP bye-bye and make sure everything is 1500.Erm... no. The whole point of setting the DF bit is to avoid fragmentation. Read up on path MTU discovery to see why it is a good thing. Just because braindead filters cause problems is no reason to suggest that PMTU discovery is bad. It becomes even more critical in IPv6 where routers don't fragment period, so people had better get used to it. Trying to force everyone to have the same MTU simply is not practical. You will always have systems with higher path MTUs that can get a gain from knowing it and you will always have systems with lower MTUs for whatever reason.
I agree MTU needs to be flexible. But in a protocol where MTU discovery is based on ICMP, and where filters are often implemented for ICMP without detailing, then I think DF is just as uncivilized as other bad behaviour. What's the mechanism for negotiating packet size in IPv6 and how to does it deal with minimal routers in between? Does it have an MTU discovery? And is its MTU discovery poorly designed like in IPv4 (which looks to me like it's an afterthought). Of course MTU discovery is something that needs to take place across TCP, UDP, and whatever else is just above IP, but putting that over in ICMP is, IMHO, part of the problem. It would not violate the statelessness principle to allow a transit router to send back something entirely outside of the concept of ICMP to do MTU discovery since that doesn't involve storing any state of any connections in transit routers. -- Phil Howard | crash359 () nowhere0 edu blow5me1 () noplace4 edu no8way61 () spam9mer edu phil | eat91me8 () no8where org end6it70 () anywhere edu eat7this () nowhere6 net at | no1spam0 () s4p0a2m7 org blow2me2 () nowhere3 net stop2570 () s8p8a0m8 com milepost | eat8this () anyplace edu blow3me8 () nowhere8 net w2x1y6z4 () no5place com dot | stop2ads () no9where com stop5ads () no3place org stop7it4 () anyplace org com | no58ads6 () no13ads5 com eat2this () s5p6a5m0 net stop4790 () spam7mer edu
Current thread:
- Re: The 'tude [Was MTU of the Internet?], (continued)
- Re: The 'tude [Was MTU of the Internet?] Perry E. Metzger (Feb 05)
- Re: MTU of the Internet? George Swallow (Feb 04)
- Re: MTU of the Internet? Matthew Petach (Feb 05)
- Re: MTU of the Internet? Dennis Simpson (Feb 04)
- Re: MTU of the Internet? Jeff Stehman (Feb 05)
- Re: MTU of the Internet? Marc Slemko (Feb 05)
- Re: MTU of the Internet? Phil Howard (Feb 05)
- Re: MTU of the Internet? Kevin A. Smith (Feb 05)
- Re: MTU of the Internet? Jeff Stehman (Feb 05)
- Re: MTU of the Internet? Phil Howard (Feb 05)
- Re: MTU of the Internet? Marc Slemko (Feb 05)
- Re: MTU of the Internet? Phil Howard (Feb 05)
- Re: MTU of the Internet? Perry E. Metzger (Feb 05)
- Re: MTU of the Internet? Dan Foster (Feb 05)
- Re: MTU of the Internet? Marc Slemko (Feb 04)
- Re: MTU of the Internet? George Swallow (Feb 05)
- Re: MTU of the Internet? ken emery (Feb 05)
- Re: MTU of the Internet? Frank Kastenholz (Feb 06)
- Re: MTU of the Internet? Phil Howard (Feb 06)