Test your connectivity for World IPv6 Day

sthaug at nethelp.no sthaug at nethelp.no
Tue Jun 7 17:43:15 CEST 2011

> > Conventional PMTUD only works downwards, so if you start at a
> > default MTU 1280, you'll never discover a larger PMTU than 1280. MSS
> > also works to constrain that, as it tells the other end the maximum
> > segment size to send. 
> This being so, which size do you recommend to start with in IPv6?
> - 1500?
> - 9000 to get a chance to reach jumbo-frame sizes?

Jumbo frames need to be negotiated in any case - there is no official
jumbo frame support in the Ethernet standards. Thus 1500 is a good
place to start - and that's where we'll start.

I fully agree with those who see no reason to start at 1280 because
there *may* be PMTUD problems. Much better to get this out into the
open, the sooner the better. *Yes* there is a difference in that
routers aren't supposed to fragment in IPv6 - however, if you look
at IPv4 traffic today you'll see that a significant amount of it is
sent with the DF bit turned on - in which case IPv4 and IPv6 are in
the same situation, no fragmentation in routers, PMTUD needs to work.

Steinar Haug, Nethelp consulting, sthaug at nethelp.no

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