Re: Mail Bridge Performance


J. Noel Chiappa (JNC@XX.LCS.MIT.EDU)
Thu 6 Mar 86 18:16:01-EST


        Nope. The problem is GGP, not EGP. (Not that EGP doesn't have
its little brain bubbles, which certain people will remember me
attacking brutally, but this time it's innocent.) I explain this every
six months, but new people keep making the same mistake.

        There are plenty of links between the MILNet and the ARPANet
in California, but the primary reason you aren't routing through them
is that GGP, which is the (ancient) routing protocol used for passing
info among core gateways, is being used in a way it was *not
designed* to work in. The GGP protocol is throwing away of lot of
information, but that *was legitimate* if you use GGP the way it was
supposed to be.
        Specifically, when a GGP routing update from gateway Y says
that it can get to net X, it doesn't say what the 'next hop' is, even
if that 'next hop' is on the *same net* as the two gateways which are
communicating. Why? Well, the way GGP was supposed to work, all the
gateways on a net were supposed to communicate with all the other
agetways, i.e. N^2 communication. In such a scenario, you'd be hearing
from the direct gateway to net N (gateway X) , as well as gateway Y,
and you'd find the direct connected one was closer.
        This model is no longer applicable; all gateways do not
communicate directly; many talk only to their EGP peers, and the only
path that EGP peer (gateway Y) has to give routing info to its core
neighbours is GGP. GGP drops the information about the next hop
gateway (gateway X) being on the same net, with the result that all
the other gateways on that net take an *extra hop* through the EGP
peer (gateway Y) to get to network N.
        Even if they put in EGP speaking gateways on the West coast,
that *still* won't fix the problem unless you are an EGP peer with the
core gateway which is the EGP peer of the local net you are trying to
get to. The traffic will still take an extra hop from your core EGP
peer to the other core EGP peer. If the gateway to that net is still
only peering with an EGP gateway on the East cost, *all traffic* to
that net from *everywhere* has to go across the country and through
that gateway. There's nothing anyone can do to fix that extra hop
except replace GGP.

        The information needed to fix all this is there in EGP; it's
the protocols *between) the core gateways that's broken. Once the
Butterflys go in, this problem will clear up *without any* changes
to EGP. (Not to say that there aren't lots of other problems with
EGP that won't be cleared up so easily!) EGP is not the problem.
        To the extent that people switch to a West Coast peer once one
goes in, the problem will diminish, it's true. So your suggested fix
would be a help, although you're pointing your finger at the wrong
culprit.

        Noel
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