Re: [Detnet] [mpls] [Pals] draft-decraene-mpls-slid-encoded-entropy-label-id (was RE: Please review the PALS/MPLS/DetNet Joint Session minutes)

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Hi Gyan,

Thanks for the clarifications questions.
I’ll take the liberty to top post to re-organize in two distinct points.


  1.  RFC 6790 specification of the use the EL’s TTL field.

You have correctly highlighted the two relevant text:

On the receiver side, §4.1 says “The EL's TTL MUST be ignored.”

On the sender side, §4.2 says “The TTL for the EL MUST be zero”

“a” So do we agree that RFC 6790 says “MUST be sent as zero and MUST ignored on receipt” ?
“b” Do we agree that this how reserved field are defined at the IETF (e.g. https://datatracker.ietf.org/doc/html/rfc7176#section-2.1.1) ?
“c” Do we agree that the way this field is specified (cf “a”) has always allowed the IETF to further extend this field?


2)    Implicit null /PHP
In case of PHP, the transport label is removed by the PHP and the ultimate node (egress LER) receives a label stack with the ELI, EL as top two labels.
As per MPLS architecture, LER looks at the top label (ELI) and either:

- supports RFC 6790 and then applies “Y then recognizes the ELI and pops two labels: the ELI and the EL.” In no way the EL is exposed and used for forwarding.
- does not support RFC 6790 and hence will drop the packet as per RFC 3031 (§3.18). In no way the EL is exposed and used for forwarding.

So in summary, the EL is never exposed (as top label) and can never be used for forwarding.

Regards,
--Bruno



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From: Gyan Mishra <[e-mail]>
Sent: Thursday, March 31, 2022 4:58 PM
To: DECRAENE Bruno INNOV/NET <[e-mail]>
Cc: Henderickx, Wim (Nokia - BE/Antwerp) <[e-mail]>; John E Drake <[e-mail]>; detnet WG <[e-mail]>; mpls <[e-mail]>; [e-mail]
Subject: Re: [Detnet] [mpls] [Pals] draft-decraene-mpls-slid-encoded-entropy-label-id (was RE: Please review the PALS/MPLS/DetNet Joint Session minutes)

Hi Bruno

Please provide clarification on how existing implementations using Entropy Label ELI/EL RFC 6790 with your proposal to reuse the entropy label.

In section 2 you talk about the new entropy label control field in your proposal to reuse the TTL field as the entropy label control field.

I have some questions related below that are concerning with your proposal.

RFC 6790 section 4 talks about the TTL processing below excerpt.


   If an ingress LSR X chooses to impose an EL, then Y will receive a

   tunnel termination packet with label stack <TL, ELI, EL> <remaining

   packet header>.  Y recognizes TL as the label it distributed to its

   upstreams for the tunnel and pops it.  (Note that TL may be the

   implicit null label, in which case it doesn't appear in the label

   stack.)  Y then recognizes the ELI and pops two labels: the ELI and

   the EL.  Y then processes the remaining packet header as normal; this

   may require further processing of tunnel termination, perhaps with

   further ELI+EL pairs.  When processing the final tunnel termination,

   Y MAY enqueue the packet based on that tunnel TL's or ELI's TC value

   and MAY use the tunnel TL's or ELI's TTL to compute the TTL of the

   remaining packet header.  The EL's TTL MUST be ignored.


So the TL or ELI is used to compute the TTL of the remaining packet header.  States that EL’s TTL is ignored.

Section 4.2 mentions that the TTL for the EL MUST be set to 0 so it’s not used for forwarding.  The issue here is related to implicit null PHP case where the TL is popped and ELI,EL are exposed and to ensure that the EL is not used for forwarding the EL MUST be set to 0.


   4.  If, for the chosen tunnel, Y has not indicated that it can

       process ELs, push <TL> onto the packet.  If Y has indicated that

       it can process ELs for the tunnel, push <TL, ELI, EL> onto the

       packet.  X SHOULD put the same TTL and TC fields for the ELI as

       it does for TL.  X MAY choose different values for the TTL and TC

       fields if it is known that the ELI will not be exposed as the top

       label at any point along the LSP (as may happen in cases where

       PHP is used and the ELI and EL are not stripped at the

       penultimate hop (see Section 4.4<https://datatracker.ietf.org/doc/html/rfc6790#section-4.4>).  The BoS bit for the ELI MUST

       be zero (i.e., BoS is not set).  The TTL for the EL MUST be zero

       to ensure that it is not used inadvertently for forwarding.  The

       TC for the EL may be any value.  The BoS bit for the EL depends

       on whether or not there are more labels in the label stack.

The EL is not used for forwarding as long as the field is set to 0 which is a MUST.  However if you reuse the TTL field as the entropy label control field it will not be set to 0 and thus that could break implementations in the PHP case where the ELI/EL are exposed.

Also the TTL being set to 0 is different then the field being actually a Reserved or not applicable field.

I disagree with sentence below in section 2.


   Hence essentially the TTL field of the EL behaves as a reserved field

   which must be set to zero when sent and ignored when received.

The TTL field as I stated MUST be set to 0 so it’s not used for forwarding.  So it’s not reserved and it’s read by the LSR looking for the field to be set to 0 so it’s not used for forwarding.  I can’t see how that won’t break existing implementations.

Kind Regards

Gyan

On Thu, Mar 31, 2022 at 12:30 AM Gyan Mishra <[e-mail]<mailto:[e-mail]>> wrote:

I like Bruno’s idea of reusing the entropy label as indicator of MEH in the label stack and is backwards compatibility for devices not supporting can continue to use for ECMP load balancing.

I think this is a solid interim solution to get the ball rolling with minimal software updates and being able to support ancillary data in the label stack and as other solutions are progressed that may take longer or implement and deploy at least in the near term we have a quick solution that could be promising for operators.

I think we do have to vett out the backwards compatibility and scenario I can think of is if you want to be able to use the entropy label for ECMP load balancing and simultaneously want to also use as ancillary data indicator I am guessing won’t work and that is something we would have to be cognizant of if deployed.

Kind Regards

Gyan

On Wed, Mar 30, 2022 at 4:04 PM John E Drake <jdrake=[e-mail]<mailto:[e-mail]>> wrote:
Wim,

I think I would term it a thought experiment.  An RFC 6790 compliant node will take the value in the EL label field and use it to select an outgoing interface.  If the value in the EL field is a slice ID, such an node will select an outgoing interface which is not necessarily part of the slice in question and that outgoing interface will be to a node which is not necessarily part of the slice in question.

Yours Irrespectively,

John



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From: Henderickx, Wim (Nokia - BE/Antwerp) <[e-mail]<mailto:[e-mail]>>
Sent: Wednesday, March 30, 2022 3:21 PM
To: John E Drake <[e-mail]<mailto:[e-mail]>>; [e-mail]<mailto:[e-mail]>
Cc: mpls <[e-mail]<mailto:[e-mail]>>; detnet WG <[e-mail]<mailto:[e-mail]>>; [e-mail]<mailto:[e-mail]>
Subject: Re: [mpls] [Pals] draft-decraene-mpls-slid-encoded-entropy-label-id (was RE: Please review the PALS/MPLS/DetNet Joint Session minutes)

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John, do you have evidence of this or is this a theoretical claim ?

From: mpls <[e-mail]<mailto:[e-mail]>> on behalf of John E Drake <jdrake=[e-mail]<mailto:jdrake=[e-mail]>>
Date: Wednesday, 30 March 2022 at 19:13
To: [e-mail]<mailto:[e-mail]> <[e-mail]<mailto:[e-mail]>>
Cc: mpls <[e-mail]<mailto:[e-mail]>>, detnet WG <[e-mail]<mailto:[e-mail]>>, [e-mail]<mailto:[e-mail]> <[e-mail]<mailto:[e-mail]>>
Subject: Re: [mpls] [Pals] draft-decraene-mpls-slid-encoded-entropy-label-id (was RE: Please review the PALS/MPLS/DetNet Joint Session minutes)
Except that putting a slice ID in the Entropy Label field will break existing  ELI/EL Implementations because their hashing of the slice ID won’t necessarily place a packet on the correct outgoing I/F
Sent from my iPhone

On Mar 30, 2022, at 1:00 PM, [e-mail]<mailto:[e-mail]> wrote:

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From: Tony Li <[e-mail]<mailto:[e-mail]>> On Behalf Of Tony Li
Sent: Wednesday, March 30, 2022 4:08 PM
> [Kireeti]: suggest attending talk by Tony on danger of reusing ELI before making any decision.
https://notes.ietf.org/notes-ietf-113-pals<https://urldefense.com/v3/__https:/notes.ietf.org/notes-ietf-113-pals__;!!NEt6yMaO-gk!Sw9ofU9AyD7Z-JKwyAqMlHk5xhNLxZNMSu31Yt6-K7yh-6JehvlSPLDcqrP3gOo$>

Done. The talk raised no “danger of reusing ELI” for draft draft-decraene-mpls-slid-encoded-entropy-label-id; quite the contrary.
I quote: “claims of backward compatibility apply to draft-decraene-mpls-slid-encoded-entropy-label-id-03”. With more details on slide 18
https://datatracker.ietf.org/meeting/113/materials/slides-113-mpls-05-policy-on-mpls-special-purpose-labels-reuse-00<https://urldefense.com/v3/__https:/datatracker.ietf.org/meeting/113/materials/slides-113-mpls-05-policy-on-mpls-special-purpose-labels-reuse-00__;!!NEt6yMaO-gk!Sw9ofU9AyD7Z-JKwyAqMlHk5xhNLxZNMSu31Yt6-K7yh-6JehvlSPLDcNEC7QKk$>


Yes, the issue with this proposal is that it has no space for in-stack data and not enough space for possible expansion of additional actions.

[Bruno] There are two steps:
- This proposal allows for carrying 8 Indicators and a slice ID while been backward compatible with egress LER hance providing faster deployment with incremental benefit.
- If more in-stack data is required the proposal is extensible (e.g. draft-jags-mpls-ext-hdr) but at the cost of losing the above benefits for the ASes & uses-cases requiring more than 8 Indicators per AS or In-Stack Data.
So we can have both worlds: simple first step and extensibility for those who need it.

Independently, we also/already have the post stack data option to carry ancillary data, which may limit the need for In-Stack data extension.

--Bruno

Tony




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Gyan Mishra

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