Maybe I'm misreading this but...

Steve Gibbard scg at wwnet.net
Fri Oct 16 21:39:46 UTC 1998


On Fri, 16 Oct 1998, Dean Anderson wrote:

> But Path MTU discovery is a bit more complicated.  I'm not looking at any
> docs at the moment, so I hope I'm not completely wrong about this, but as I
> recall the discovery process tries to send packets to each hop. First
> discovering the route path, and then trying to determine the mtu of each
> hop.   While the intermediate RFC1918 addresses can reply to things they
> happen to get, you can't directly send a packet to them to see if they will
> want to fragment it.  

Path MTU discovery works by trying to send packets all the way through to
the destination, with the MTU of the first hop.  If it receives an ICMP
fragmentation needed and DF bit set host unreachable message, it then
tries successively smaller packet sizes until it stops getting the
destination unreachable messages. 

> But perhaps it would work if everyone accepted rfc1918 sourced packets.
> However this isn't the case either.  So I think it is safe to assume that
> PMTU is broken whenever RFC1918 networks are touched.

If a router with an IP addrses in RFC1918 space is sending back
destination unreachable messages, and somebody along the path is filtering
out RFC1918 sourced packets, then it will break PMTU discovery.  For that
reason, using RFC1918 addresses on large MTU interfaces of routers that
also have small MTUs would probably be a bad idea.  However, unless
there's something I'm not thinking of here, using RFC1918 addresses on a
router where all interfaces have the same MTU should not break PMTU
discovery.

-Steve

--
Steve Gibbard
WWNet System Administration
+1 734 513-7707 x 2009
http://www.wwnet.net





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