Date: Fri, 22 Nov 1996 04:42:14 -0500 From: Edward Ing <edward.ing@utoronto.ca> To: isp@freebsd.org Cc: questions@freebsd.org Subject: IP routing thoery: ping pong effect. Message-ID: <32957576.2CE5@utoronto.ca>
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The following is my theoretical network. H's are hosts, N's are
networks.
x.x.n.n are the interface IPs. Ha is the highest node. N1 is like the
backbone.
So the internetwork is asymmetrical, and not quite like the
picture.
|---Ha --------|
x.x.3.1 | | x.x.1.4
| |
N3---------- -----------N1
| |
| |
x.x.3.5 | | x.x.1.5
Hc Hd
x.x.4.1 | | x.x.2.1
| |
N4------------ ------------N2
| |
| |
x.x.4.5 | x.x.2.5 |
He Hb
Assume that these hosts are only a fraction of the hosts on each network
so
no fix networking table is used for routing because it would be
too complicated. Defautroute (0.0.0.0) for He is
x.x.4.1, for Hc is x.x.3.1, for Hb is x.x.2.1 for Hd is x.x.1.4.
If Ha is given a default route, we will get a ping-pong effect.
E.g. If default route of Ha is x.x.3.5 (Hc) and if He sends packet to Hb
this effect shows up. He routes to Hc on defaultroute, Hc routes to Ha
on defaultroute. Ha routes back to Hc on its default route and thus the
hosts
play ping-pong. A packet from He will never find Hb. Nor can Ha have
default
to Hd, otherwise the same effect occurs on the other side.
Thus, Ha cannot have a defaultroute. But if that is the case, Hb and He
can
never send packets to each other in anycase.
My question is: how is it possible for Hb and He to route packages to
each
other? Obviously theoretically they can because it happens on the
Internet.
Thus, how can this theoretically be accomplished.
The answer I think must have to do with dynamic routing tables in Ha.
Or is it done with distributed routing tables (like DNS)?
Is this why Ha is a router rather than a mere gateway?
How would this work? I would appreciate a concise answer.
Edward Ing.
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