Implementation of RIPE-181++

 Daniel Karrenberg <Daniel.Karrenberg at ripe.net> writes:
  * 
Okay, thanks for your changes. I have incorperated them and added a
small note on the slitting ASes. Find below he revised text. 

	
		--Tony.

P.S. Today is really the last day to comment before we send this
document out as ripe-181.

The interaction of interas-in/interas-out with as-in/as-out

Although formally defined above, the rules  associated  with  policy
described  in  terms  of  interas-in and interas-out with respect to
as-in and as-out are worthy of clarification for implementation.

When using interas-in or interas-out policy descriptions,  one  must
always  make  sure the set of policies described between two ASes is
always equal to or a sub-set of the policy described in  the  global
as-in  or  as-out  policy.  When a sub-set is described remember the
remaining routes are implicitly shared across all connections. It is
an error for the interas policies to describe a superset of the glo-
bal policies, i.e. to announce or accept more routes than the global
policies.

When defining complex interas based  policies  it  is  advisable  to
ensure  that  any possible ambiguities are not present by explicitly
defining your policy with respect to the  global  as-in  and  as-out
policy.

If we look at a simple example, taking just  in-bound  announcements
to simplify things. If we have the following global policy:


aut-num: AS1
as-in: from AS2 10 accept AS100 OR {10.0.0.0/8}

Suppose there are three peerings between AS1 and AS2, known  as  L1-
R1, L2-R2 and L3-R3 respectively. The actual policy of these connec-
tions is to accept AS100 equally on these three links and just route
10.0.0.0/8  on L3-R3. The simple way to mention this exception is to
just specify an interas policy for L3-R3:

interas-in: from AS2 L3 R3 (pref=100) accept {10.0.0.0/8}


The implicit rule that all routes not mentioned in interas  policies
are  accepted on all links with equal preference ensures the desired
result.

The same policy can be written explicitly as:

interas-in: from AS2 L1 R1 (pref=100) accept AS100
interas-in: from AS2 L2 R2 (pref=100) accept AS100
interas-in: from AS2 L3 R3 (pref=100) accept AS100 OR {10.0.0.0/8}




ripe-181.txt                                           October, 1994
^L                               - 29 -


Whilst this may at first sight seem obvious, the problem arises when
not all connections are mentioned. For example, if we specified only
an interas-in line for L3-R3 as below:

aut-num: AS1
as-in: from AS2 10 accept AS100 OR {10.0.0.0/8}
interas-in: from AS2 L3 R3 (pref=100) accept AS100 OR {10.0.0.0/8}


then the policy for the other links according  to  the  rules  above
would mean they were equal to the global policy minus the sum of the
local  policies  (i.e.  ((AS100  OR  {10.0.0.0/0})   /   (AS100   OR
{10.0.0.0/0}))  =  empty)  which  in this case would mean nothing is
accepted on connections L1-R1 and L2-R2 which is incorrect.

Another example: If we only registered  the  policy  for  link   L2-
R2:

interas-in: from AS2 L2 R2 (pref=100) accept AS100

The implicit policy for both L1-R1 and L3-R3 would be as follows:

interas-in: from AS2 L1 R1 (pref=100) accept {10.0.0.0/8}
interas-in: from AS2 L3 R3 (pref=100) accept {10.0.0.0/8}


This is derived as the set of  global  policies  minus  the  set  of
interas-in  policies  (in  this case just accept AS100 as it was the
L2-R2 interas-in policy we  registered)  with  equal  cost  for  the
remaining connection. This again is clearly not what was intended.

We strongly recommend that you always mention all policies  for  all
interas  connections explicitly, to avoid these possible errors. One
should always ensure the set of the interas policies is equal to the
global policy. Clearly if interas policies differ in complex ways it
is worth considering splitting the  AS  in  question  into  separate
ASes.  However, this is beyond the direct scope of this document.

It should also be noted there is no direct relationship between  the
cost used in as-in and the preference used in interas-in.




-------- Logged at  Wed Sep 7 01:08:13 MET DST 1994  ---------