IPv6 fc00::/7 ??? Unique local addresses

Sun Oct 24 17:46:26 UTC 2010

> Probably no reason at all, though probably little perceived benefit.
> 1492 is common enough that google/youtube already runs lower MTU's
just
> to avoid common broken PPPoE setups (which often could run higher MTU,
> but weren't configured that way).

I run into that already with people doing various things inside their
net (MPLS, GRE, IPIP?) that shorten the effective MTU but they block the
ICMP unreachable packets and break PMTU discovery.  That blanket
blocking of ICMP unreachable type 3 code 4 is evil, in my opinion.

If your traffic passes through a Cisco ASA series device (and maybe
other vendors, too) your MTU is effectively 1380 anyway as that is the
maximum MSS that it advertises (or can even be configured to advertise)
when it establishes an outbound connection and in some versions of its
code will drop a packet from an endpoint that doesn't honor the
advertised MSS.

It is a real performance killer across the Internet in my opinion and
better performance could be had, particularly for long distance links
where you are limited by the number of "in flight" packets if those
packets could be bigger. The problem is that even if you have two end
points that are jumbo capable, the networks in the path don't seem to
support >1500 MTU.  If everyone configured their peering and internal
gear to support a 9216 byte frame size and set their MTUs to 9000, that
change would be transparent to the connections flowing though it and
people who wanted to send larger frames could do so without impacting
anyone using a shorter size.