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RE: The IPv4 Internet MTU
> > This silly idea gets me to a default MTU for Teredo interfaces in
> > 1384]. I know it looks like cooking but it seems a fair bet and it
> > provides in default cases almost a 10% boost of TCP MSS value for
> > clients without obvious drawbacks.
> Why not just set the Teredo interface MTU to (64KB-ENCAPS)?
> Then, take special care to make sure the small packets get
> through but let the big packets take care of themselves.
The problem is UDP fragmentation. By nature, it is unreliable: the more fragments, the more chances to lose one of them, thus causing the loss of the entire packet. This is aggravated by NAT behavior: some NAT will try to reassemble the packets, but others will just pass the 1st fragment and drop the next ones.
That would not be too much of a problem if TCP implementations could do MTU detection by just reacting to packet losses. But today, many TCP implementations don't. The MTU discovery is triggered by ICMP "packet too big" messages. These messages are not generated if a UDP fragment is lost.
-- Christian Huitema