Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!elroy.jpl.nasa.gov!swrinde!ucsd!dog.ee.lbl.gov!ucbvax!NSIPO.NASA.GOV!medin
From: medin@NSIPO.NASA.GOV ("Milo S. Medin", NASA ARC NSI Project Office)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Help designing address allocation in a metronet
Message-ID: <9103222223.AA00621@cincsac.arc.nasa.gov>
Date: 22 Mar 91 22:22:59 GMT
References: <77489@bu.edu.bu.edu>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 76


Kent,

...
	 	The router only has to worry about variable subnet masks for
	 the subnetted net that it participates in.  For distant nets it uses
	 the netmask that it derives from the net class.
	 
	 	Within the variably subnetted net, the router must have the
	 mask for each route.  This is most easily gotten from the routing
	 protocol and most easily stored in the routing table.

This isn't quite true.  There is no requirement that I know of that subnets
have to be connected.  In fact, inside an OSPF routing domain, things can
be configured to work quite well in this way (we actually take advantage
of this for a couple of unique situations).  Since the router doesn't
know that all subnets are connected , it shouldn't make assumptions about
the class of routes.  In fact, OSPF is designed without any hardwiring
of Class A, B, or C routes in it at all; it's just mask and match.  Router
vendors should not make distinctions between net and subnet routes in their
routing table data structures.  There is no need to do it this way.  Routing
on best match is the really preferred way to go.  It's more flexible and
doesn't need be any slower...
	 
...
	 	They will all have to support OSPF or some other protocol with
	 variable length subnet support or else don't vary the subnet mask
	 length.  (Try gated.)

If you don't support variable length masks, I wouldn't consider you a 
compliant OSPF router.  The RENO code can support variable length masks,
and I think Jeff Honig will take advantage of this when porting the UMD OSPF
code to gated...   People need to understand that variable length subnets 
are a real solution to user's problems, and they ought to be firmly
supported.  Don't let your vendor hedge on this!
	 
...
	 	Another argument for OSPF, which is link state and not
	 distance vector.  Milo Medin has documentary evidence of reduced
	 routing bandwidth in the NASA internet to prove that link state is
	 better in this regard.

In the normal case, you are only flooding deltas, not the full routing
table.  Jeff Burgan (also from NASA) gave a very nice summary of operational
experience with OSPF at the last IETF, not just in NSI but also BARRNET
and OARNet, and it's clear that OSPF is a big win here.  Additionally,
John Moy has calculated that in steady state, OSPF could support passing around
~2000 external net routes on a 9.6 Kbps link using only about 5% of the link
bandwidth.  If routers are being attacked and destroyed and routing is having
to converge to new paths, then this figure will go up, but it's piles better
than the utilization of the old style DV protocols in use in various places
today.


	 	The way most people do this sort of thing is to have one
	 network for the WAN and each site has its own independent network
	 space.  Is Class C big enough for RAINnet itself?  Set the subnet mask
	 all the way down to two nodes per subnet.  That's two bits for the
	 host part, 0 and 3 are reserved, 1 and 2 are the endpoints of the
	 link.  Make every subnet in the RAINnet net the same length.  Every
	 host or router not directly on RAINnet simply routes to the netmasked
	 route.  That should work and it won't waste subnet space.  If the mask
	 size is constant you don't need OSPF.  If you are passing around all
	 RIP routes, try judicious use of default routes.  Prefer dynamic
	 default via RIP.  If that isn't good enough then try OSPF or some
	 other link state algorithm to reduce traffic levels.
	 
KA9Q supports variable length masks, so I don't think this is that big 
of a problem.  Who knows, it may even support OSPF one day.  

						Thanks,
						    Milo

" OSPF: Ask for it by name.  Accept no substitutes. "

PS Usual disclaimers apply...