Re: IP over FDDI comments

[[e-mail] (Vernon Schryver)]

> From: Steve Deering <[e-mail]>
> Subject: Re:  IP over FDDI comments
> In-Reply-To: vjs%rhyolite.wpd's message of Tue, 30 Jan 90 123216 PST
> 
> Without the bridge, you end up with a (usually) partitioned IP subnet,
> which violates a basic architectural requirement of IP routing. ...

Not if you outlaw single MAC stations from the secondary or "back" ring.
(I'm trying to reform.  It seems "back" is not a permitted word
for the "secondary" ring.)

Also not if you use the dual-IP network scheme.
 
> >	What happens when the ring wraps?  What would a bridge between
> >	the rings do then?  Please recall that an individual station
> >	including any bridge or concentrator cannot know when the ring
> >	has wrapped within any usefully small time.
> 
> How small a time do you need?  Current bridges (e.g., DEC LANbridge)
> multicast their routing packets once a second, which means that a
> bridge can detect a wrap (by hearing its own multicast from interface A
> arrive on interface B) and stop forwarding within 500 ms on average.

As I understand SMT, you don't hear of topology changes more quickly
than 30 seconds.

500ms is how many FDDI packets?  Each one "teaching" the bridge that
the source address is on the wrong ring?  I suppose you could have
bridges dump their tables on every reconfiguration.

> >	The 2nd problem has caused me to worry about all multi-packet ARP
> >	schemes.  There seem to me to be a lot of cases to cover, and some
> >	cases where a sub-optimimal answer is inevitible  (e.g. when the
> >	new-fangled ARP packet between dual-MAC stations is lost in the
> >	fiber).
> I don't believe this is any problem at all, at least under my basic
> proposal, but I probably can't convince you of that without discussing
> it in person, or typing a lot more than I have time for.  Oh well.

I bet you could convince me without much trouble.  If you want, I
could do it by myself.  It's not that I think its impossible, but
non-trivial to get right.

> Under Vernon's scheme, you could end up with an Ethernet plus one ring
> forming a single IP subnet, and the other ring forming a second IP
> subnet.  Yuck.

Hey, that's a feature!  If you care, you put an IP router somewhere.

If you outlaw secondary-ring-single-MAC's, you don't need a router
anywhere, since the only hosts in the universe who need a route
to the secondary ring are on it.  

This assumes you put all bridges on the main ring.  If not, you have
problems that (I modestly claim) are easily solved only if you use
the dual-IP network hack.


(I also sympathize with those of us who are trying build ether bridges.
Workstations are not going to use 1500 byte frames.  Customers who connect
pairs of FDDI dual-rings with ethernet bridges are going to make big stinks.)


vjs