Ripe 181

  > Let me make sure I understand this right. 
  > 
  > There are cascade of two orderings. 
  > A router L1 prefers a route from its peer R1
  > over its other peer R2 if
  >  1) R1's AS != R2's AS 
  >     and the as-in preference of this route via R1's AS
  >     is less than or equal to 
  >     the as-in preference of this route via R2's AS 
  >  2) R1's AS == R2's AS
  >     and the interas-in preference of this route via R1
  >     is less than or equal to 
  >     the interas-in preference of this route via R2.

s/or equal//
Right.

  > This is almost but not equivalent to the following:
  > 	Assume all preference values are in the range 0...Range
  > 	a route is accepted from a peer with gated preference of
  > 		as-in preference * Range + interas-in preference

It is not equivalent because this orders by interas-in preference
across ASes if the as-in preferences are equal. See below.

  > Is this right? The cascade of two orderings is news to me.
  > Having a cascade has pros and cons. 

I do not see the cons. It forces consistency and works well when
interas-in only deals with exceptions, i.e. the normal case.

  > I wish these were the words in Ripe-181. There would not be any
  > confusion. 

Me too, but nothing is perfect. We will put a better explanation
in the PRIDE guide.

  > Ripe-181 does not say they are consistent. It says there is no
  > relevance.

It is good enough for a spec, not for a tutorial.

  > There is one problem that I foresee with your algorithm. You are
  > trying to come-up with one total ordering (per route) whereas your
  > input has cascade of two partial orderings.
  > 
  > What do you do below:
  > 
  > aut-num: AS1
  > as-in: from AS2 10 accept AS100
  > as-in: from AS3 10 accept AS100
  > interas-in: from AS2 L1R1 10 accept AS100
  > interas-in: from AS2 L1R2 11 accept AS100
  > interas-in: from AS3 L1R3 12 accept AS100
  > interas-in: from AS3 L1R4 13 accept AS100
  > 
  > Obviously there is no total ordering with these values.
  > A partial ordering is fine as long as gated is concerned.
  > However, I am not 100% sure that 
  > one can come up with "one" partial ordering
  > unless you compare interas-in preferences for different ASs
  > (i.e.  10 with 12).

The intention of this is clear: 

	Globally accept from AS2 and AS3 with equal preference. 

	Locally prefer L1R1 over L1R2 from AS2.

	Locally prefer L1R3 over L1R4 from AS2.

So the partial ordering is

L1R1 = L1R3 < L1R2 = L1R4

Easy to derive too.

Another way of putting it is to say that the interas-in attributes above are
fully equivalent to:

interas-in: from AS2 L1R1 1 accept AS100
interas-in: from AS2 L1R2 2 accept AS100
interas-in: from AS3 L1R3 1 accept AS100
interas-in: from AS3 L1R4 2 accept AS100

  > Gated does not support two partial orderings.

Nor does any other router configuration language, it does not make much sense.

Is this clear now?

Daniel



-------- Logged at  Fri Oct 14 19:29:15 MET 1994  ---------