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

<bruno.decraene@orange.com> Thu, 24 November 2016 09:57 UTC

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From: bruno.decraene@orange.com
To: "Acee Lindem (acee)" <acee@cisco.com>
Thread-Topic: [OSPF] ??: WG Last Call for "Signalling ELC using OSPF"
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Date: Thu, 24 Nov 2016 09:57:46 +0000
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Subject: Re: [mpls] [OSPF] ??: WG Last Call for "Signalling ELC using OSPF"
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Hi Acee

Please see inline [Bruno3]

From: Acee Lindem (acee) [mailto:acee@cisco.com]
Sent: Monday, November 21, 2016 6:41 PM
To: DECRAENE Bruno IMT/OLN
Cc: OSPF WG List; mpls@ietf.org; Carlos Pignataro (cpignata); Xuxiaohu
Subject: Re: [OSPF] ??: WG Last Call for "Signalling ELC using OSPF"

Hi Bruno,

From: Bruno Decraene <bruno.decraene@orange.com<mailto:bruno.decraene@orange.com>>
Date: Monday, November 21, 2016 at 12:21 PM
To: Acee Lindem <acee@cisco.com<mailto:acee@cisco.com>>
Cc: OSPF WG List <ospf@ietf.org<mailto:ospf@ietf.org>>, "mpls@ietf.org<mailto:mpls@ietf.org>" <mpls@ietf.org<mailto:mpls@ietf.org>>, "Carlos Pignataro (cpignata)" <cpignata@cisco.com<mailto:cpignata@cisco.com>>, Xiaohu Xu <xuxiaohu@huawei.com<mailto:xuxiaohu@huawei.com>>
Subject: RE: [OSPF] ??: WG Last Call for "Signalling ELC using OSPF"

Hi Acee,

From: Acee Lindem (acee) [mailto:acee@cisco.com]
Sent: Monday, November 21, 2016 6:02 PM
To: DECRAENE Bruno IMT/OLN; Xuxiaohu
Cc: OSPF WG List; mpls@ietf.org<mailto:mpls@ietf.org>; Carlos Pignataro (cpignata)
Subject: Re: [OSPF] ??: WG Last Call for "Signalling ELC using OSPF"

Hi Bruno,

From: Bruno Decraene <bruno.decraene@orange.com<mailto:bruno.decraene@orange.com>>
Date: Monday, November 21, 2016 at 9:43 AM
To: Acee Lindem <acee@cisco.com<mailto:acee@cisco.com>>, Xiaohu Xu <xuxiaohu@huawei.com<mailto:xuxiaohu@huawei.com>>
Cc: OSPF WG List <ospf@ietf.org<mailto:ospf@ietf.org>>, "mpls@ietf.org<mailto:mpls@ietf.org>" <mpls@ietf.org<mailto:mpls@ietf.org>>, "Carlos Pignataro (cpignata)" <cpignata@cisco.com<mailto:cpignata@cisco.com>>
Subject: RE: [OSPF] ??: WG Last Call for "Signalling ELC using OSPF"

Hi Acee,

From: Acee Lindem (acee) [mailto:acee@cisco.com]
Sent: Saturday, November 19, 2016 12:33 AM
To: DECRAENE Bruno IMT/OLN; Xuxiaohu
Cc: OSPF WG List; mpls@ietf.org<mailto:mpls@ietf.org>; Carlos Pignataro (cpignata)
Subject: Re: [OSPF] ??: WG Last Call for "Signalling ELC using OSPF"

Hi Bruno,

From: OSPF <ospf-bounces@ietf.org<mailto:ospf-bounces@ietf.org>> on behalf of Bruno Decraene <bruno.decraene@orange.com<mailto:bruno.decraene@orange.com>>
Date: Friday, November 18, 2016 at 11:30 AM
To: Xiaohu Xu <xuxiaohu@huawei.com<mailto:xuxiaohu@huawei.com>>
Cc: OSPF WG List <ospf@ietf.org<mailto:ospf@ietf.org>>, "mpls@ietf.org<mailto:mpls@ietf.org>" <mpls@ietf.org<mailto:mpls@ietf.org>>, "Carlos Pignataro (cpignata)" <cpignata@cisco.com<mailto:cpignata@cisco.com>>
Subject: Re: [OSPF] ??: WG Last Call for "Signalling ELC using OSPF"

Hi Xiaohu,

Please see inline [Bruno]

From: Xuxiaohu [mailto:xuxiaohu@huawei.com]
Sent: Monday, November 14, 2016 1:00 PM
To: DECRAENE Bruno IMT/OLN
Cc: OSPF WG List; Carlos Pignataro (cpignata); mpls@ietf.org<mailto:mpls@ietf.org>
Subject: ??: [OSPF] ??: WG Last Call for "Signalling ELC using OSPF"


Hi Bruno,



Could you please explain why the defination of the RLDC should be specific to the LB behavior of the transit LSR?

[Bruno] The whole purpose of EL and ELC is to improve load balancing of MPLS packets on transit LSR.

According to §6 of your draft, RLDC is also used to improve load-balancing : "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. »



What would be the point for the ingress to add an additional EL, within the RLDC of LSR A, if LSR A do not use this EL to improve the load balancing?

cf my example below where a LSR can read 5 labels, yet do not use those 5 labels for the load-balancing hence would not benefit from adding an EL within those 5 labels.

BTW, it would be useful for the discussion if you could reply to the content of my email sent: Monday, November 14, 2016 (also included below). As this was already the second time I send this example on the OSPF mailing list.









If I understand correctly, it seems that the text proposed by you conflict with Acee's take (see blow):


"   1. The standards track IGP drafts should have a precise definition of RLD and so not require a normative reference to the MPLS entropy draft (which is informational). The IGP drafts need not precisely specify how the information is used - this can be specified via a reference to the MPLS draft.
   2. The MPLS draft should precisely specify the initial use case of entropy label insertion at the ingress of the LSP. It should not limit the applicability of RLDC to this use case. "

[Bruno] I'm not seeing any conflict. I agree with both points. In this thread, I'm working on 1, i.e. having a clear definition of RLD.  But I would also like that this RLD advertisement be effective in improving the load-balancing of MPLS packets.

I think Readable Label Depth (RLD) should be independent of EL Capability (ELC). It allows advertisement of the the maximum number of labels an OSPF router will examine in a received MPLS encapsulated packet.
 If an OSPF Router supports ELC, it would imply that it support the EL Capability within RLD labels.
[Bruno] Would work for me, assuming that this is stated in the document, and :s/support the EL Capability within RLD labels/for load-balancing purpose, use the EL within RLD labels.
I would propose the following text: "RLDC is the maximum number of labels, from the top of the stack, where the MPLS transit LSR searches for the ELI,EL pair and load-balance based on the EL if present."

I would completely decouple the two capabilities. Here is the text I would recommend.

The Readable Label Depth (RLD) is the maximum number of labels, starting with top or first label in the stack, that an LSR can examine in a received MPLS packet.  The supported RLD can be important when searching for an entropy label for purpose of load-balancing as the <ELI, EL> pair must be included in the first RLD labels in the stack.

[Bruno] What would you use this RLD information for?
Current OSPF draft proposes to use it to make a decision on whether/where adding an additional ELI, EL pairs in the stack of labels. But if we don't know whether that additional EL, within the RLD, will be used for load-balancing purpose, the ingress can't know whether adding this ELI, EL is useful or not. Since we are in a context where the number of labels that can be pushed is limited, we may be wasting 2 label push for zero benefit.

As the name would imply, Entropy Label Capability (ELC) would be used to determine if the LSR supports load-balancing. The RLD capability, also as as the name would imply, would be used solely to determine the number of labels an LSR will read.
[Bruno3] "read" is one thing but it does not say much about the result/behavior. That's why I'm calling for "read and use" or "use" for short.


Am I the only one who thinks this is obvious?



think ELC should be defined in RFC 6790 and the SPRING Entropy label draft as opposed to the IGP advertisement drafts.
[Bruno] I tend to agree that the definition of RLD, or the load-balancing behavior of a transit LSR supporting EL, would be better specified by the MPLS WG. Then the value advertised by control plane protocols/signaling.
https://tools.ietf.org/html/rfc7325#section-2.4.5 talks about this, but the document is informational, and the text is a bit too large/ open to have the LSR behavior advertised in the IGP using a single integer.
But this option may delay a lot the IGP draft, unless it is splitted in 2 parts (as ELC is ready). Alternatively, I'm ok with your above proposition.

Why can't we simply use the definition of entropy processing included in RFC 6790 section 4?
[Bruno] Basically, section 4.3 says that the LSR can load-balance based on whatever it wants, and not necessarily based on the EL label. If it does not use the EL label, there is no point in the ingress trying to add additional ELI,EL labels within the RLD of this LSR.

I'd say that a router supporting ELC recognizes the <ELI, EL> label in the first RLD labels and MAY load-balance as described in RFC 6790. An OSPF document should not update the MPLS data plane behavior.
[Bruno3] I agree that an OSPF document should not update the MPLS data plane behavior. ELC is defined in RFC 6790 as the capability of the egress (of the LSP/segment). It does not advertise any transit capability.
(Now we could argue that a sensible implementation would implement both features at the same time, but I would not a priori bet on this for all implementation, especially since those are two different forwarding plane capabilities and some hardware could be able to do one and not the other)


Which is the purpose of advertising the RLD (AFAIK).

It is one usage of RLD - however, the maximum number of labels an LSR can examine must not be inextricably tied to the Entropy Label processing.
[Bruno3] It's currently the only usage, so I'd rather have the RLD useful for this usage. If there are more usages, we may take them into account. But there is not free lunch: if we want to advertise 2 (slightly) different thing, we'll need to advertise two capabilities)

Thanks,
--Bruno

Thanks,
Acee



Thanks
--Bruno

Thanks,
Acee




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