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

bruno.decraene@xxxxxxxxxx · Mon, 4 Mar 2024 08:37:18 +0000

Thanks Greg.
Looks good to me(both below email and -07)

Regards,
--Bruno

From: Greg Mirsky <gregimirsky@xxxxxxxxx>

Sent: Saturday, March 2, 2024 9:12 PM

To: DECRAENE Bruno INNOV/NET <bruno.decraene@xxxxxxxxxx>

Cc: rtg-dir@xxxxxxxx; draft-ietf-mpls-p2mp-bfd.all@xxxxxxxx; last-call@xxxxxxxx; mpls@xxxxxxxx; Joel Halpern <jmh@xxxxxxxxxxxxxxx>

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

Hi Bruno,

thank you for sharing your thoughts. I am not sure that increasing the range for jittering the transmission would not result in the increase of the rate of the BFD Control packets transmitted by an egress LSR
 (leave) that detected the failure of a p2mp LSP. I've added a new text in Section 5 with, what I consider, minimalistic jittering mechanism:

NEW TEXT:

   *  these BFD Control packets are transmitted at the rate of one per

      second until either it receives a control packet valid for this

      BFD session with the Final (F) bit set from the ingress LSR or the

      defect condition clears.  However, to improve the likelihood of

      notifying the ingress LSR of the failure of the p2mp MPLS LSP, the

      egress LSR SHOULD initially transmit three BFD Control packets

      defined above in short succession.  The actual transmission of the

      periodic BFD Control message MUST be jittered by up to 25% within

      one-second intervals.  Thus, the interval MUST be reduced by a

      random value of 0 to 25%, to reduce the possibility of congestion

      on the ingress LSR's data and control planes.

I am looking forward to more comments and continued discussion with other experts.

Regards,

Greg

On Tue, Feb 27, 2024 at 5:03 AM <bruno.decraene@xxxxxxxxxx> wrote:

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 <gregimirsky@xxxxxxxxx>

Sent: Monday, February 26, 2024 6:05 PM

To: DECRAENE Bruno INNOV/NET <bruno.decraene@xxxxxxxxxx>

Cc: rtg-dir@xxxxxxxx;
draft-ietf-mpls-p2mp-bfd.all@xxxxxxxx;
last-call@xxxxxxxx; 
mpls@xxxxxxxx; Joel Halpern <jmh@xxxxxxxxxxxxxxx>

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 <bruno.decraene@xxxxxxxxxx> 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 <mpls-bounces@xxxxxxxx>
On Behalf Of Greg Mirsky

Sent: Sunday, February 25, 2024 12:25 AM

To: Joel Halpern <jmh@xxxxxxxxxxxxxxx>

Cc: rtg-dir@xxxxxxxx;
draft-ietf-mpls-p2mp-bfd.all@xxxxxxxx;
last-call@xxxxxxxx; 
mpls@xxxxxxxx

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 <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.

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