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

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Hi Greg,

Please see inline [Bruno]



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From: Greg Mirsky <[e-mail]>
Sent: Friday, April 1, 2022 10:38 PM
To: DECRAENE Bruno INNOV/NET <[e-mail]>
Cc: mpls <[e-mail]>; detnet WG <[e-mail]>; [e-mail]; Tony Li <[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)

Hi Bruno,
thank you for pointing me to the text in Section 4.1. I agree that for the case described in RFC 6790, i.e., a single ELI, EL in the stack, that text sets clear requirements for disposing of ELI,EL.
[Bruno] Good. Thank you.

What I wanted to bring to the discussion is how a transit node does that in the SR-MPLS scenario (RFC 8662<https://datatracker.ietf.org/doc/html/rfc8662>).
[Bruno] The SR-MPLS scenario is just a scenario of MPLS LSP hierarchy (with a different name). When we have a hierarchy of LSPs, the MPLS architecture defines the term Ingress, Transit, Egress on a per LSP basis https://datatracker.ietf.org/doc/html/rfc3031#section-3.15
So in you example there are two cases for LSP/segment level m ("m" refers to RFC3031 section 3.15):
- if the LSR is swapping (or PHP) the top label, this LSR is a transit LSR for that LSP.
- if the LSR is the end of the LSP (i.e. the originator of the FEC), this LSR is an Egress for that LSP.
In both cases, RFC 6790 applies on that LSP.

Regards,
--Bruno


I think that there's no text in RFC 8662 that establishes equivalency between a transit node disposing of ELI,EL if ELI is at the top of the stack and the LER per RFC 6790. That might cause different interpretations resulting in different handling. How practically we can collect information about deployed solutions?

Regards,
Greg

On Fri, Apr 1, 2022 at 9:06 AM <[e-mail]<mailto:[e-mail]>> wrote:
Greg,

> From:  Greg Mirsky
> I agree that the wording in RFC 6790 is open to interpretation. It is quite possible that a more pedantic developer would put a check for the zero value of the EL TTL field


RFC 6790 says

< 4.1<https://datatracker.ietf.org/doc/html/rfc6790#section-4.1>.  Egress LSR >
[...]

"The EL's TTL MUST be ignored."
https://datatracker.ietf.org/doc/html/rfc6790#section-4.1


To me, that does not read like open to interpretation.

> And I'm surprised that the authors of the draft claim precisely the opposite

Are you now feeling better about the authors?

--Bruno



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From: mpls <[e-mail]<mailto:[e-mail]>> On Behalf Of Greg Mirsky
Sent: Thursday, March 31, 2022 6:00 PM
To: Tony Li <[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)

I agree that the wording in RFC 6790 is open to interpretation. It is quite possible that a more pedantic developer would put a check for the zero value of the EL TTL field "to ensure that it is not used inadvertently for forwarding". Is it possible to check all existing implementations that support ELI/EL? And I'm surprised that the authors of the draft claim precisely the opposite:
   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.

Regards,
Greg

On Thu, Mar 31, 2022 at 8:43 AM Tony Li <[e-mail]<mailto:[e-mail]>> wrote:

Gentlebeings,

On Mar 31, 2022, at 8:29 AM, Greg Mirsky <[e-mail]<mailto:[e-mail]>> wrote:

my interpretation of bullet 4 in Section 4.2 RFC 6790 "The TTL for the EL MUST be zero to ensure that it is not used inadvertently for forwarding" leads me to believe that any other than zero value in the EL TTL field is invalid per RFC 6790. Consequently, that packet MUST be dropped. If that is not breaking the existing network, please help me understand what is it.


Normally, we write clauses that describe such fields as "must be transmitted as zero and ignored upon receipt" just to avoid such ambiguity. It is unfortunate that RFC 6790 did not utilize this phrase. As it stands, it has certainly specified that the TTL field must be transmitted as zero. Yes, that implies that any other value is invalid. However, that does not guarantee that implementations will check.  In fact, the Law of Lethargy (people will do the least amount of work possible) suggests that most implementations will not check and will simply ignore the TTL field completely.

However, this is not a guarantee. Any design that attempts to reuse this TTL field does run a non-zero risk of being impacted by designs that do check and reject such entries.

IMHO, this by itself is not a serious risk, but risk evaluation is always subjective.

Designs should always acknowledge and articulate the risks that they undertake. It is then up to the collective wisdom of the group to weigh and evaluate the risks, benefits, and tradeoffs when making a decision.

Regards,
Tony


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