Re: [Lsr] Martin Duke's Discuss on draft-ietf-lsr-ospf-reverse-metric-08: (with DISCUSS and COMMENT)

[Alvaro Retana <[e-mail]>]

On October 6, 2022 at 5:44:57 AM, Ketan Talaulikar wrote:


Ketan:

Hi!


...
> KT> Added text in the security considerations that cover this issue as well
> as a proposed mitigation. Please let us know if that works.

This is the text that you added:

   A router that is misbehaving or misconfigured, may end up signaling
   varying values of reserve metrics or toggle the state of reserve
   metric.  This can result in a neighbor router having to frequently
   update its Router LSA causing network churn and instability despite
   the LSA rate-limiting behavior in the OSPF protocol.  It is
   RECOMMENDED that implementations support the detection of frequent
   changes in reverse metric signaling and ignore the reserve metric
   (i.e., revert to using their provisioned metric value) during such
   conditions.


Monitoring the changes is the right mitigation.  But the description
of how it would be done is not specific -- for a normative
recommendation.

As I think about this, it occurs to me that even though the originator
of the RM is not sending an LSA, this is an "IGP event", as described
in rfc8405.  Receiving the RM should trigger an SPF and the updated
Router LSA should also trigger SPF events elsewhere.

   IGP event: The reception or origination of an IGP LSDB change
   requiring a new routing table computation.  Some examples are a
   topology change, a prefix change, and a metric change on a link or
   prefix.  Note that locally triggering a routing table computation is
   not considered an IGP event since other IGP routers are unaware of
   this occurrence.


The same back-off mechanism from rfc8405 should be required in this case.


Thanks!

Alvaro.