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

[Xuxiaohu <[e-mail]>]

Hi Acee and Bruno,

Thanks for your comments. Please see my response inline.

发件人: OSPF [mailto:[e-mail]] 代表 Acee Lindem (acee)
发送时间: 2016年11月8日 9:27
收件人: [e-mail]; OSPF WG List
主题: 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.

[Xiaohu] The RLDC is only applicable to c, as it had been stated in section 6 (see below).

§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.

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.

[Xiaohu] I agree that the goal (i.e., use the RLDC info in order to choose the best location for the EL, even if it pushes a single one.) is perfect. And it seems in align with the second of the recommendations for inserting EL pairs


   o  An LSR that is limited in the number of <ELI, EL> pairs that it
      can insert SHOULD insert such pairs deeper in the stack.

   o  An LSR SHOULD try to insert <ELI, EL> pairs at positions so that
      for the maximum number of transit LSRs, the EL occurs within the
      RLD of the incoming packet to that LSR.

   o  An LSR SHOULD try to insert the minimum number of such pairs while
      trying to satisfy the above criteria.

However, it would make the EL insertion decision process on the ingress much complex. For instance, the ingress need to determine the total number of transit LSRs within each LSP hierarchy when that LSP is the outermost one. In fact, the EL insertion decision process in the recommended solution as described in section 4 of https://www.ietf.org/id/draft-ietf-mpls-spring-entropy-label-04.txt has  already been a bit complex. For instance, the ingress needs to determine the minimum of the RLDs of transit LSRs of each LSP hierarchy when that LSP is the outermost one. How to do that in the inter-area/inter-level scenario? IMHO, the most practice way is to insert a single EL under the innermost LSP☺

Best regards,
Xiaohu

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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