[mpls] 答复: [OSPF] WG Last Call for "Signalling ELC using OSPF"

[Xuxiaohu <[e-mail]>]

Hi Bruno,

Thanks for sharing the following information. However, it seems that it has nothing to do with the EL-based MPLS load-balancing mechanism. The ELC and RLDC as described in draft-ietf-ospf-mpls-elc are applicable to the EL-based MPLS load-balancing mechanism.

Best regards,
Xiaohu

发件人: [e-mail] [mailto:[e-mail]]
发送时间: 2016年11月10日 16:56
收件人: Xuxiaohu
抄送: OSPF WG List; Carlos Pignataro (cpignata)
主题: RE: [OSPF] WG Last Call for "Signalling ELC using OSPF"

Hi all,

(Hopefully) For the benefit of the discussion, I’m sharing a real world MPLS (transit LSR) load balancing algo on one plateform:

Label <=3: 2-tuple <source IP(rightmost 30bit), destination IP (rightmost 30bit)>
Label = 4: bottom-of-stack label (rightmost 20bit)
Label>=5: Fifth(rightmost 20bit) label from the top

This is a more or less random pick, and in general the behaviors are very diverse depending on the plateform (router, line card, os version, MPLS encapsulated traffic).

In general, it’s not clear to me how the RLDC advertisement really helps the ingress to make an informed decision and improve the load balancing of its flow across the MPLS network, as the current advertisement seems a bit simplistic to me.
More specifically, with the current definition of the RLDC, it’s hard for me to guess which RLDC value should be advertised. So I would really expect interop issues and endless discussion about who is right.

Thanks,
Regards,
-- Bruno

From: Xuxiaohu [mailto:[e-mail]]
Sent: Thursday, November 10, 2016 5:29 AM
To: Carlos Pignataro (cpignata); DECRAENE Bruno IMT/OLN
Cc: OSPF WG List
Subject: ??: [OSPF] WG Last Call for "Signalling ELC using OSPF"

Hi Carlos,

Thanks for your comments. Please see my response inline.

发件人: OSPF [mailto:[e-mail]] 代表 Carlos Pignataro (cpignata)
发送时间: 2016年11月8日 23:20
收件人: Bruno Decraene
抄送: OSPF WG List
主题: Re: [OSPF] WG Last Call for "Signalling ELC using OSPF"

Jumping a bit mid-thread, but with somewhat similar or related comments.

Summary: I am concerned with the lack of precision in the definitions and usage on these set of documents.

Acee, if this document needs to depend on Section 4 of draft-ietf-mpls-spring-entropy-label-04.txt, then the issue is compounded. I believe draft-ietf-mpls-spring-entropy-label-04has its own set of problems (for example, placing EL/ELI independent of whether the underlying Label is for a Node SID or a Segment SID is not optimum).

So the main question is: Is draft-ietf-ospf-mpls-elc-03 to be treated independently form draft-ietf-mpls-spring-entropy-label-04? That is a fundamental question for advancing and reviewing this/these doc(s). We have the advertisement of a capability — what is it used for? Is it the right variable to advertise (based on the application using it)?

[Xiaohu] draft-ietf-ospf-mpls-elc-03 defines an approach of advertising the RLDC of an LSR via OSPF. The definition of the RLDC is described in the following text (i.e., each LSR's capability of reading the maximum label stack depth). As for how to use this capability, it’s not limited by the draft although one use case (i.e., the application as described in Section 4 of draft-ietf-mpls-spring-entropy-label-04.txt) is mentioned in this draft.

Specifically, I think that RLDC is largely underspecified. The doc says:
"
   it would be useful for ingress LSRs to know each LSR's capability of
   reading the maximum label stack deepth.  This capability, referred to
   as Readable Label Deepth Capability (RLDC)
“

That definition simple does not make sense. And it does not explain the applicability (A, b, c, d).

[Xiaohu]Could you speak more specifically? Is the definition of the RLDC (i.e., what is the capability of reading the maximum label stack depth) itself not clear enough? If so, could you please suggestion any more specific text? Or the applicability of the RLDC not clear enough? If so, we co-authors currently only think about one use case of this RLDC that is described in Section 4 of draft-ietf-mpls-spring-entropy-label-04.txt) , please feel free to tell us if you find any other use cases of this RLDC.

Then the doc continues:
“
   Readable Label Deepth Capability (RLDC) can be used by ingress
   LSRs to determine whether it's necessary to insert an EL for a given
   LSP tunnel in the case where there has already been at least one EL
   in the label stack [I-D.ietf-mpls-spring-entropy-label] .
"

So this only is useful when there’s an EL (I assume ELI/EL pair) already?

[Xiaohu] In the EL insertion use case, yes, your understand is correct.

And is “[I-D.ietf-mpls-spring-entropy-label]” normative?

[Xiaohu] Currently yes, if it’s widely believed that this reference should be informational, it can be moved to the informational reference section.

Overall, I think a WGLC is premature, since some basics are not well covered. Is it RLD or RLDC? And what exactly is a “Readable Label Deepth Capability (RLDC)” The capability to do what? Is this a Capability or a scalar?

[Xiaohu] RLDC (Readable Label Deeth Capability) means a given LSR's capability of reading the maximum label stack depth. If it’s widely believed that “RLD” is more suitable than “RLDC” from the expression perspective, it’s fine to make a change.

Another point comment — although I think this needs a thorough review before WGLC.

The doc says:
   Of course,
   even it has been determined that it's neccessary to insert an EL for
   a given LSP tunnel, if the egress LSR of that LSP tunnel has not yet
   indicated that it can process ELs for that tunnel, the ingress LSR
   MUST NOT include an entropy label for that tunnel as well.

This is good and consistent with RFC 6790:
   c.  X MUST NOT include an entropy label for a given tunnel unless the
       egress LSR Y has indicated that it can process entropy labels for
       that tunnel.

So it’s better to point to it instead of re-MUST NOT independently.

[Xiaohu] Good suggestion. Will fix it.

But, what is the egress of a tunnel in this context? Not the final egress, but the SID egress. ?

[Xiaohu] IMHO, the egress of an LSP tunnel is clear enough and easy to understand. In contrast, what’s the mean of SID egress, could you please help point to a reference for this term defincation?

Best regards,
Xiaohu

Thanks,

—
Carlos Pignataro, [e-mail]<mailto:[e-mail]>

“Sometimes I use big words that I do not fully understand, to make myself sound more photosynthesis."

On Nov 8, 2016, at 6:15 AM, [e-mail]<mailto:[e-mail]> wrote:

Hi Acee,

Thanks for your reply. Please see inline [Bruno]

From: Acee Lindem (acee) [mailto:[e-mail]]
Sent: Tuesday, November 08, 2016 2:27 AM
To: DECRAENE Bruno IMT/OLN; OSPF WG List
Subject: Re: [OSPF] WG Last Call for "Signalling ELC using OSPF"

Hi Bruno,

From: Bruno Decraene <[e-mail]<mailto:[e-mail]>>
Date: Monday, November 7, 2016 at 6:43 AM
To: Acee Lindem <[e-mail]<mailto:[e-mail]>>, OSPF WG List <[e-mail]<mailto:[e-mail]>>
Subject: RE: [OSPF] WG Last Call for "Signalling ELC using OSPF"

Hi authors, all,

Please find below some comments on the RLDC definition.

1) I’d like to see a more specific definition of RLDC.
e.g. load-balancing hashing could be done based on (hashing):
-a- a (stack of) N  “regular” MPLS labels (i.e. there is no ELI in this stack)
-b- the (IP) header below the stack of N  labels
-c- the EL label in the stack of N labels (i.e. there is one ELI in this stack)

I’d like the specification to be clear on the applicability of the RLDC. Does it apply on these 3 cases, on only a subset?
Personally, I’d like at least a and c be in scope of the RLDC definition, such that an ingress with limited push capability could have enough information to decide that it could avoid pushing an ELI,EL pair if the stack of MPLS labels that it pushes has enough entropy within the first RLDC labels.


I think that the signaling document should reference section 4 of https://www.ietf.org/id/draft-ietf-mpls-spring-entropy-label-04.txt. However, I don’t think -b- is covered there.
[Bruno] The above section 4 has indeed more details, but I’m not certain that the definition is clear enough for everyone to agree on which “a”, “b”, “c” cases are covered by the RLDC advertisement. My reading would be that “a” and “c” are covered, not “b”. Reading your email, I’d say that you have the same understanding. But it bothers me that one of the authors has a different understanding.
Also draft-ietf-mpls-spring-entropy-label-04.txt is an informational document, so I’m not sure how much it would qualify to be a normative definition of what draft-ietf-ospf-mpls-elc advertise.

Thanks,
Regards,
-- Bruno


2) Current text seems to limit the use of the RLDC to the insertion of a _second_ EL. Why is the RLD not applicable to the first EL?


§1.  Introduction

“This capability, referred to

   as Readable Label Deepth Capability (RLDC) can be used by ingress

   LSRs to determine whether it's necessary to insert an EL for a given

   LSP tunnel in the case where there has already been at least one EL

   in the label stack [I-D.ietf-mpls-spring-entropy-label<https://tools.ietf.org/html/draft-ietf-ospf-mpls-elc-03#ref-I-D.ietf-mpls-spring-entropy-label>]”



§6 Usage and Applicability

“The RLDC is used by ingress LSRs

   to determine whether it's necessary to insert an EL for a given LSP

   tunnel in the case where there has already been at least one EL in

   the label stack. »




I think that this is an unnecessary restriction of the RLDC usage. Indeed, an ingress with a limited capability in term of label push, could be forced to push a single EL label. It should be able to use the RLDC info in order to choose the best location for the EL, even if it pushes a single one.
But both sentences seems to restrict RLDC usage for the additional EL push, not the first one.

I would tend to agree.

Thanks,
Acee





Thanks,
Regards,
--Bruno

From: OSPF [mailto:[e-mail]] On Behalf Of Acee Lindem (acee)
Sent: Saturday, November 05, 2016 5:46 PM
To: OSPF WG List
Subject: [OSPF] WG Last Call for "Signalling ELC using OSPF"

This begins the WG last call for  "Signalling ELC using OSPF”, https://www.ietf.org/id/draft-ietf-ospf-mpls-elc-03.txt. Please review the document and send comments to this list prior to Nov 27th, 2016. Due to the IETF week, the last call period is going to be 3 weeks rather than usual 2 weeks.

Thanks,
Acee

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