[Sidrops] ASPA validation of paths received from customers and providers

[Maria Matejka <[e-mail]>]

Hello all,

while preparing ASPA testing and teaching, we (in the BIRD team) ran
into an uncertainty.

The specification currently mandates to validate the AS Path as
received, and it's expected that it's complemented by BGP Roles for the
neighboring hop. While this makes perfect sense for ingestion of the
routes, there is a misleading gap in our view in not including the local
ASN if the neighbor is our customer or provider.

In the case of our Customer, if the Customer forgets to include our ASN
in their ASPA provider list, we may end up advertising their routes with
ASPA-invalid AS Paths to our neighbors.

In the case of our Provider, if we forget to include their ASN in our
ASPA provider list, we may end up advertising routes with ASPA-invalid
AS Paths to our customers.

This is not the case of Peers or route servers where the Upstream
verification algorithm is applied to the whole upstream portion of the
path.

We suggest that if the neighbor is Customer or Provider, the validation
procedure at least strongly recommends (SHOULD) prepending the local ASN
before executing the Upstream or Downstream Validation Procedure. This
behavior would extend the validated path by a part which is expected to
be valid anyway, so we expect no harm / risk from this update.

The benefit from this would be that if anything is broken regarding the
ASPA records, then the failure points to its actual source much more
clearly and noticeably than with the current wording.

We haven't been following all the discussions thoroughly so there may be
something on this topic we have missed in the process. Feel free to reply
with a pointer to an older discussion if something like this has already
been resolved.

Thanks!  
Maria

-- 
Maria Matejka (she/her) | BIRD Team Leader | CZ.NIC, z.s.p.o.