Minimum Internet MTU

Jun-ichiro itojun Hagino itojun at
Mon Dec 22 23:33:24 UTC 2003

> I'm working with a few folks on firewall and IDS rules that will flag
> suspicious fragmented traffic. I know the legal minimum of a
> non-terminal fragment is 28 bytes, but given non-terminals should
> reflect the MTU of the topologies along the link, this number is far
> lower than what I expect you should see for legitimate fragmentation in
> the wild.
> A few years back I noted some 512-536 MTU links in ASIA. I've been doing
> some testing and can't seem to find them anymore. Is is safe to assume
> that 99.9% of the Internet is running on 1500 MTU or higher these days? 

	there are many deployment of DSL-based layer 2 providers, which
	use L2TP (or whatever) tunnelling as well as PPPoE to associate
	end clients to layer 3 ISPs.  they enforce MTU like 1450 or lower.  
	in Japan, NTT east/west (NTT is a previously-government-owned telco)
	provide such service and enforce MTU of 1454.


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