MTU handling in 6RD deployments

Mark Townsley mark at
Tue Jan 7 13:25:18 CET 2014

On Jan 7, 2014, at 12:56 PM, Emmanuel Thierry wrote:

> Hello,
> Le 7 janv. 2014 à 12:37, Tore Anderson a écrit :
>> Hi list,
>> 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
> For your information, i see an advertised mtu of 1480 on my WiFi interface with the Free CPE.

Section 9.1 of RFC 5969:

   If the MTU is well-managed such that the IPv4 MTU on the CE WAN side
   interface is set so that no fragmentation occurs within the boundary
   of the SP, then the 6rd Tunnel MTU should be set to the known IPv4
   MTU minus the size of the encapsulating IPv4 header (20 bytes).  For
   example, if the IPv4 MTU is known to be 1500 bytes, the 6rd Tunnel
   MTU might be set to 1480 bytes.  Absent more specific information,
   the 6rd Tunnel MTU SHOULD default to 1280 bytes.

Note I've heard some ISPs consider running Jumbo Frames under the covers so that IPv4 could carry 1520 and 1500 would be possible for IPv6, but have not yet seen that confirmed to me in practice. 

- Mark

>> * Several (or all) of the above in combination
>> Also, given that some ISPs offer [only] Layer-2 service and expect/allow
>> their customers to bring their own Layer-3 home gateway if they want
>> one, I would find it interesting to learn if any of the most common
>> off-the-shelf home gateway products (that enable 6RD by default) also
>> implement any such tricks by default or not.
> Best regards
> Emmanuel Thierry

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