MTU handling in 6RD deployments

Martin.Gysi at Martin.Gysi at
Tue Jan 14 16:25:52 CET 2014

Hi Tore,

>Does anyone know what tricks, if any, the major 6RD deployments (AT&T,
>Free, Swisscom, others?) are using to alleviate any problems stemming
>from the reduced IPv6 MTU? Some possibilities that come to mind are:

>* Having the 6RD CPE lower the TCP MSS value of SYN packets as they
>enter/exit the tunnel device
>* Having the 6RD BR lower the TCP MSS value in the same way as above
>* Having the 6RD CPE advertise a lowered MTU to the LAN in RA Options
>* Several (or all) of the above in combination

We advertise an MTU of 1472 in the RA Options. We still have a small
number of PPPoE users and the max-payload-tag is not working reliably
enough on third-party devices.

Hence: 1500 Bytes - 20 Bytes IPv4 - 8 Bytes PPPoE = 1472 Bytes


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