MTU handling in 6RD deployments

Jean-Francois.TremblayING at Jean-Francois.TremblayING at
Tue Jan 7 14:46:19 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

Our managed CPEs (D-Links) send (IPv4 MTU) - 20 bytes in RAs, usually 

In the list of "tricks", you might want to add: 
* Slightly raise the ICMPv6 rate-limit values for your 6RD BR (we do 

I haven't seen IPv6 MSS clamping in the wild yet (it was discussed on 
this list a year ago). 

> 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.

>From off-the-shelf, we see mostly D-Links and Cisco/Linksys/Belkin 
with option 212 support. A few Asus models started showing up in the 
stats in 2013 I believe. Last time I checked, all models supporting 
option 212 also reduced their MTU properly (YMMV here, that was almost a 
year ago).

Too bigs remain quite common however... 
#sh ipv6 traffic | in too
               11880 encapsulation failed, 0 no route, 3829023354 too big
#sh ver | in upt
uptime is 2 years, 4 weeks, 5 days, 4 hours, 3 minutes

If 6lab's data is right, roughly half of Canada's IPv6 users go through 
that box (50k users).


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