Maybe I'm misreading this but...

I Am Not An Isp patrick at ianai.net
Fri Oct 16 19:08:58 UTC 1998


At 01:12 PM 10/16/98 -0400, tvo wrote:
>Doesn't this break MTU path discovery though?

No, it wouldn't.  Those addresses are not routable on the global 'Net,
however, there is nothing stopping a device with an RFC1918 address from
sending a packet onto the 'Net.

Assume you have a T1 between two routers, but you are out of 'normal'
addresses.  So you make the two serial ports 10.1.1.1 and 10.1.1.2.
Simple, huh?  Well, anyone outside your network attempting to reach those
addresses will fail - there is no route to 10.x.x.x on the Internet.  (At
least not in *my* network. :)  Let's further assume there's a LAN on the
other side of this T1 with a MTU below 1500 bytes.  Now, when you attempt
PMTU discovery to a device on that small MTU LAN (going through the T1 of
course), what happens?  When the router on the opposite end of the T1 gets
the packet, it sees that the packet is "too big" and DF bit is set (as per
PMTU rules) and cannot send the packet.  So what should it do?  Why send an
ICMP "Datagram Too Big" error message, of course.  (Type 3, "Destination
Unreachable", Code 4, "fragmentation needed and DF set".  See RFC 1191,
http://www.pmg.lcs.mit.edu/cgi-bin/rfc/view-plain?number=1191, and RFC 792,
http://www.pmg.lcs.mit.edu/cgi-bin/rfc/view?792, for more info.)  Just like
any other (properly functioning) router on the Internet.  But what's the
source address?  Why 10.1.1.1 of course.  So you get a packet from RFC1918
space.  Perfectly normal, perfectly acceptable, perfectly legitimate.

See, when a router gets a packet, it does not look at the source IP for
routing info.  (Unless you're doing something weird like policy routing.)
So the source IP is irrelevant to a router.  Hell, even the destination IP
is irrelevant as long as it's 32 bits and matches something in the router's
table (including a possible default route).  I have never seen a router
vendor treat a packet with RFC1918 space in source or destination any
differently than any other packet.  Unless, of course, the user manually
modifies the default behavior with filters or something - which can be done
to any address just as easily.

The point is, there is magical router fairy looking for RFC1918 space and
removing all those packets from your network.  Those addresses are treated
by your router just like *any other address*.  Forwarded, filtered,
whatever depending upon the rules you set up and the information it
receives dynamically.

I hope that explains everything.

TTFN,
patrick

I Am Not An Isp
www.ianai.net
"Think of it as evolution in action." - Niven & Pournelle



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