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

[Joel Halpern <jmh@xxxxxxxxxxxxxxx>]

Mostly.  THere is one other aspect.  You may consider it irrelevant, in which case we can simply say so.  Can the inbound notifications coming from a large number of leaves at the same time cause data plane congestion?

Yours,

Joel

On 2/24/2024 8:44 PM, Greg Mirsky wrote:

Hi Joel,

thank you for your quick response. I consider two risks that may stress the root's control plane:

• notifications transmitted by the leaves reporting the failure of the p2mp LSP
• notifications transmitted by the root to every leave closing the Poll sequence

As I understand it, you refer to the former as inbound congestion. The latter - outbound. Is that correct? I agree that even the inbound stream of notifications may overload the root's control plane. And the outbound process further increases the probability of the congestion in the control plane. My proposal is to apply a rate limiter to control inbound flow of BFD Control messages punted to the control plane. 

What would you suggest in addition to the proposed text?

Best regards,

Greg

On Sat, Feb 24, 2024 at 3:28 PM Joel Halpern <jmh.direct@xxxxxxxxxxxxxxx> wrote:

What you say makes sense.  I think we need to acknowledge the inbound congestion risk, even if we choose not to try to ameliorate it.  Your approaches seems to address the outbound congestion risk from the root.

YOurs,

Joel

On 2/24/2024 6:25 PM, Greg Mirsky wrote:

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 <jmh@xxxxxxxxxxxxxxx> 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 <noreply@xxxxxxxx> 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.

-- 
last-call mailing list
last-call@xxxxxxxx
https://www.ietf.org/mailman/listinfo/last-call