Hi Tore.
Does anyone know what tricks, if any, the major 6RD deployments (ATT,
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
1480.
In the list of tricks, you might want to add:
* Slightly raise the ICMPv6 rate-limit values for your 6RD BR (we do
50/20)
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).
/JF
