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RE: draft-wbeebee-ipv6-cpe-router-04 comments
On Mon, 13 Jul 2009, Templin, Fred L wrote:
As you say, MSS clamping can be applied to TCP the same as for any link
and in fact is a common operational practice. However, TCP headers are
not always available in-the-clear, and as you say not all traffic is
TCP. IMHO, operators can already do MSS clamping w/o the need for
additional text in these documents.
In the case of 6to4, a common platform to do this on is Cisco 7600 (it
does it pretty much wirespeed), in that platform you cannot do MSS adjust.
Therefore it would make more sense to do this in the CPE.
The idea with setting the tunnel MTU is to set a size that is highly
unlikely to fragment, as sustained fragmentation is dangerous in any
case. An additional alternative under development you may not be aware
of is SEAL, which fixes fragmentation to the point that larger MTUs can
be realized - even up to jumbogram size if that is desired. See:
Yes, I was a bit unclear, myself (since I control both ends of the tunnel
(I don't use 6to4 but a statically configured ipv6-in-ip) I have actually
set the MTU of this tunnel to 1350 which means this is the lowest common
denominator for PMTUD (is my guess anyway) so this solves my "problem".
I still think it would make most sense to do the MSS clamping in the 6to4
tunnel CPE because it knows for sure whether traffic is going to a tunnel
Mikael Abrahamsson email: email@example.com