Re: [mpls] Rtgdir last call review of draft-ietf-mpls-p2mp-bfd-06

[[e-mail]]

Hi Greg,

Thanks for considering my comment and for your reply.
I’m not following the draft but a priori my understand is that the reporting from the egress to the root may happen at the discretion of the egress, with no constraint in term of respecting any timing. If so, I don’t see why we would need to be within 75% of a specific time limit. We could a priori choose any value for this time limit (configured on the egress or any default value) and pick a random number within 0% to 100% of this limit. But you are likely to know much better than me, so up to you.

Regards,
--Bruno

From: Greg Mirsky <[e-mail]>
Sent: Monday, February 26, 2024 6:05 PM
To: DECRAENE Bruno INNOV/NET <[e-mail]>
Cc: [e-mail]; [e-mail]; [e-mail]; [e-mail]; Joel Halpern <[e-mail]>
Subject: Re: [mpls] Rtgdir last call review of draft-ietf-mpls-p2mp-bfd-06

Hi Bruno,
thank you for your interest and the suggestion. BFD spec (RFC 5880) includes the mechanism intended to avoid synchronization of BFD Control messages:
   The periodic transmission of BFD Control packets MUST be jittered on
   a per-packet basis by up to 25%, that is, the interval MUST be
   reduced by a random value of 0 to 25%, in order to avoid self-
   synchronization with other systems on the same subnetwork.  Thus, the
   average interval between packets will be roughly 12.5% less than that
   negotiated.
Do you think that the same randomization mechanism applied to the transmission of notifications to the root of p2mp LSP would be useful?

Regards,
Greg

On Mon, Feb 26, 2024 at 2:45 AM <[e-mail]<mailto:[e-mail]>> wrote:
Hi Greg,

My 2 cents (not following the draft).
Another typical option may be to allow the network operator to configure, on the egress, an acceptable delay before reporting to the root. The egress would then pick a random value in this range. Statiscally, the more egress the more spread the reports to the root, which a priori would be good for scaling.
It would be up to the network operator to configure the right delay depending on the number of the leaves and the need for fast reporting (or not).

Totally up to you, but that would have my vote as this is a typical issue. (granted this is more likely an issue with protocols handling thousands of customers, but even for MPLS LSR scaling, RSVP-TE scaling issues are not unheard)

Regards,
--Bruno

From: mpls <[e-mail]<mailto:[e-mail]>> On Behalf Of Greg Mirsky
Sent: Sunday, February 25, 2024 12:25 AM
To: Joel Halpern <[e-mail]<mailto:[e-mail]>>
Cc: [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>
Subject: Re: [mpls] Rtgdir last call review of draft-ietf-mpls-p2mp-bfd-06

Hi Joel,
thank you for the clarification. My idea is to use a rate limiter at the root of the p2mp LSP that may receive notifications from the leaves affected by the failure. I imagine that the threshold of the rate limiter might be exceeded and the notifications will be discarded. As a result, some notifications will be processed by the headend of the p2mp BFD session later, as the tails transmit notifications periodically until the receive the BFD Control message with the Final flag set.  Thus, we cannot avoid the congestion but mitigate the negative effect it might cause by extending the convergence. Does that make sense?

Regards,
Greg

On Sat, Feb 24, 2024 at 2:39 PM Joel Halpern <[e-mail]<mailto:[e-mail]>> wrote:

That covers part of my concern.  But....  A failure near the root means that a lot of leaves will see failure, and they will all send notifications converging on the root.  Those notifications themselves, not just the final messages, seem able to cause congestion.  I am not sure what can be done about it, but we aren't allowed to ignore it.

Yours,

Joel
On 2/24/2024 3:34 PM, Greg Mirsky wrote:
Hi Joel,
thank you for your support of this work and the suggestion. Would the following update of the last paragraph of Section 5 help:
OLD TEXT:
   An ingress LSR that has received the BFD Control packet, as described
   above, sends the unicast IP/UDP encapsulated BFD Control packet with
   the Final (F) bit set to the egress LSR.
NEW TEXT:
   As described above, an ingress LSR that has received the BFD Control
   packet sends the unicast IP/UDP encapsulated BFD Control packet with
   the Final (F) bit set to the egress LSR.  In some scenarios, e.g.,
   when a p2mp LSP is broken close to its root, and the number of egress
   LSRs is significantly large, the control plane of the ingress LSR
   might be congested by the BFD Control packets transmitted by egress
   LSRs and the process of generating unicast BFD Control packets, as
   noted above.  To mitigate that, a BFD implementation that supports
   this specification is RECOMMENDED to use a rate limiter of received
   BFD Control packets passed to processing in the control plane of the
   ingress LSR.

Regards,
Greg

On Thu, Feb 22, 2024 at 4:10 PM Joel Halpern via Datatracker <[e-mail]<mailto:[e-mail]>> wrote:
Reviewer: Joel Halpern
Review result: Ready

Hello,

I have been selected as the Routing Directorate reviewer for this draft. The
Routing Directorate seeks to review all routing or routing-related drafts as
they pass through IETF last call and IESG review, and sometimes on special
request. The purpose of the review is to provide assistance to the Routing ADs.
For more information about the Routing Directorate, please see
https://wiki.ietf.org/en/group/rtg/RtgDir

Although these comments are primarily for the use of the Routing ADs, it would
be helpful if you could consider them along with any other IETF Last Call
comments that you receive, and strive to resolve them through discussion or by
updating the draft.

Document: draft-name-version
Reviewer: your-name
Review Date: date
IETF LC End Date: date-if-known
Intended Status: copy-from-I-D

Summary:  This document is ready for publication as a Proposed Standard.
    I do have one question that I would appreciate being considered.

Comments:
    The document is clear and readable, with careful references for those
    needing additional details.

Major Issues: None

Minor Issues:
    I note that the security considerations (section 6) does refer to
    congestion issues caused by excessive transmission of BFD requests.   I
    wonder if section 5 ("Operation of Multipoint BFD with Active Tail over
    P2MP MPLS LSP") should include a discussion of the congestion implications
    of multiple tails sending notifications at the rate of 1 per second to the
    head end, particularly if the failure is near the head end.  While I
    suspect that the 1 / second rate is low enough for this to be safe,
    discussion in the document would be helpful.

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