The Motonori Problem

Martha Steenstrup <> Wed, 25 November 1992 21:11 UTC

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Subject: The Motonori Problem
Date: Wed, 25 Nov 92 15:31:57 -0500
Sender: ietf-archive-request@IETF.CNRI.Reston.VA.US
From: Martha Steenstrup <>
Message-ID: <9211251612.aa29188@CNRI.Reston.VA.US>

Hello Yuko,

Within IDPR, one cannot directly express a source policy such as
"use AD A as much as possible" nor can a route server confirm that
it is obeying such a source policy.  However, one can associate
distance-related costs to intra-domain routes and achieve a
similar result indirectly.

In your example, suppose that the intra-domain paths X1-A1, X1-A2, and
X1-A3 in AD A and intra-domain paths B1-Y1, B2-Y1, and B3-Y1, each
have associated with them a cost directly proportional to distance
between the entry and exit virtual gateways.  This cost may be, for
example, monetary or delay.  Moreover, suppose that distance
differences in your picture reflect those in the real network.

If both of the following hold:

(1) AD A and AD B advertise the distance-related costs as part
of the transit policies that apply to their respective domains.

(2) The source policy corresponding to the host in AD X and applied to
communication with a host in AD Y requests minimization of the
distance-related cost over the route.

then a route server in AD X will automatically generate the "correct
route" using SPF according to the advertised costs.  Here the "correct"
route is the one exhibiting the minimum distance-related cost.

The route server in AD X will always know of the existence of the
three separate virtual gateways connecting AD A and AD B, because
these are alsways advertised as part of IDPR transit policies.
However, if AD A or AD B does not advertise different costs for
exiting through different virtual gateways as part of its transit
policies, then the route server in AD X will not be able to determine
which route is minimum-cost, based on information learned solely
through IDPR.  In this case, the route server would have to be able to
obtain this cost information through some other mechanism (perhaps
private communicate between domain administrators).

Normally, however, we expect that domains will include most of their
attributes in their advertised transit policies.  So in your example,
if there are different costs associated with the different
intra-domain routes and if these costs are expected to be of interest
to users of these routes, then they should be advertised in a domain's
transit policies.

If a source domain wants to make a route selection based upon criteria
that are not advertised in transit policies, then it must be able to
obtain this information through other means.