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

[Ketan Talaulikar <[e-mail]>]

Hi Alvaro,

Please check inline below for response with KT2.

On Thu, Oct 6, 2022 at 4:42 PM Alvaro Retana <[e-mail]> wrote:

> 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.
>

KT2> Indeed I was referencing RFC8405 but indirectly. Let us get into the
details. The reception of RM changes the operational value of its local
link metric. However, this MUST NOT be seen as an IGP Event that directly
triggers the SPF on the neighbor. What needs to happen is that this metric
change should trigger an update of the Router LSA (which is subject to
rate-limiting - MinLSInterval). This Router LSA change would then be the
one that triggers the SPF event subject to the back-off and details in
RFC8405. This is the base protocol behavior - we are not changing anything
here. Directly triggering local SPF on RM changes can result in aggravating
microloops.

KT2> The recommendation was to ignore RM - i.e., suspend the RM feature
during instability and revert to using the provisioned metric.

Thanks,
Ketan


>
>
> Thanks!
>
> Alvaro.
>