[6bone] non-global address space for IXs (was: 2001:478:: as /48)

[Sabri Berisha]

On Sun, Sep 07, 2003 at 11:30:01PM +0200, Arien Vijn wrote:
> On zondag, september 7, 2003, at 08:41 PM, John Fraizer wrote:
> 
> > RPF, combined with IX address space not being in the routing table will
> > break PMTU-D.
> 
> That remains to be seen. Typically all interfaces in IX peering LANs 
> have the same MTU. How likely is it that a router takes the peering LAN 
> address as source address for a packet too big message? Has anyone ever 
> investigated the behaviour of the various router implementations?

AFAIK most routers router use the IP on the interface the outgoing packet
is originating from as the source IP for the packet. That means that if a
packet is routed through the shared medium, the IXP's prefix will be
used in the ICMP packet. This breaks pmtud in 2 ways: less clueful
admins filtering the IX's prefix as a bogon, and (if the prefix is not
in the global table) on routers which check the source of packets for
a route in their routing table.

My experience comes from having a tunnel at home with a mtu of 1480 for
over 3 years now. Amazing how many networks are improperly configured.. 

-- 
Sabri Berisha 			"I route, therefore you are"

"Wij doen niet aan default gateways" - anonymous engineer bij een DSL klant.