Re: MTU of the Internet?
-----Original Message----- From: Robert E. Seastrom <rs@bifrost.seastrom.com> To: peterf@microsoft.com <peterf@microsoft.com> Cc: nanog@merit.edu <nanog@merit.edu> Date: Wednesday, February 04, 1998 1:40 PM Subject: Re: MTU of the Internet? <SNIP> </SNIP>
I have no idea where they came up with this "576 internally" nonsense. Generally whenever one runs into that number it is as a result of creaky old software that expects to be running over milnet or arpanet.
IPX used 576 forever whenever you had to cross IPX "subnets". The reason was simple. They were lazy. 576 was the least common denominator between Ethernet, TR, and _Arcnet_ Large IPX (LIPX) allowed them to do basically what IP calls Path MTU discovery. Only took them a day short of forever to figure out how to do it.
Are Microsoft stacks known to be broken in the packet fragmentation/reassembly department? Or are just acknowledging deficiencies in their path mtu discovery code by setting the MSS in the basement? I knew they had problems with window length (this from my friends with long fat pipes)...
With all the paranoids trying to block all ICMP, not just ICMP_ECHO, doesn't that essentially break PMTUD. 576 may not be efficient, but its probably the safest to assume.
On Wed, 4 Feb 1998, Eric Germann wrote:
With all the paranoids trying to block all ICMP, not just ICMP_ECHO, doesn't that essentially break PMTUD. 576 may not be efficient, but its probably the safest to assume.
No, it just means that these paranoids should get a clue. Even servers coloed at large NSPs sometimes have suck bogus filters applied by the NSP that should know better. On a side note, from what I understand Win95 implements blackhole detection for this situation (don't know of many Unixes that do that...), although it isn't enabled by default and you probably need patch mumble and mumble to get it. Not sure if it works or not.
participants (2)
-
Eric Germann -
Marc Slemko