[Sidrops] ASPA false leak

"Jakob Heitz (jheitz)" <jheitz@cisco.com> Tue, 15 October 2019 23:45 UTC

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From: "Jakob Heitz (jheitz)" <jheitz@cisco.com>
To: SIDR Operations WG <sidrops@ietf.org>
Thread-Topic: ASPA false leak
Thread-Index: AdWDr4IJUqd9dgFSRaS/zGHcilvp8Q==
Date: Tue, 15 Oct 2019 23:45:00 +0000
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Subject: [Sidrops] ASPA false leak
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Consider the topology:

   AS5      AS3
     \     /   \
      \   /     \
       AS4     AS2
         \     /
          \   /
           AS1

AS1 has providers AS2 and AS4.
AS2 has provider  AS3.
AS4 has providers AS3 and AS5.

AS5 receives a route with AS-path (4 3 2 1).
ASPA would declare that AS4 leaked the route from AS3 to AS5.
However, AS4 is an authorized provider for AS1.
Even though AS4 has a path to AS1, it chose to use an alternative
valid path to reach AS1. This might be to relieve congestion or to
avoid an outage. Regardless of the reason, I don't think this
route should be rejected.

I think we need an additional rule:
If an AS is found to be leaking a route, but an alternative valid
path can be found from that AS to the originator AS, then the AS
is not leaking. A path (X(1), ..., X(n)) is a valid alternative
if for each X(i), X(i+1) attests that X(i) is its provider.

Thoughts?

Regards,
Jakob.