Re: [mpls] MPLS-RT Review of draft-gandhi-mpls-ioam-sr-04

[Eric Gray <[e-mail]>]

Loa,

	As a clarification, did you mean to say "... would not _ONLY_ solve the immediate problem but also be useful for the future"?

	As a further clarification, does this align with Stewart's suggestion (using a separate FEC) below?

	As a final question, is it not the case that having a separate FEC for any sort of OAM introduces a non-trivial risk of resulting in the OAM PDU being treated differently than a corresponding (possibly even otherwise identical) non-OAM PDU - thus reducing the usefulness of OAM?

	For instance, if you have two FEC entries - one for "normal" forwarding and another for "special" processing of PDUs intended to otherwise be forwarded identically - there is a possibility (not much point in speculating how likely/unlikely) that the NHLFE for the normal forwarding case might be lost at some point along an LSP and the loss not be detected because the "special" NHLFE remains...

--
Eric

-----Original Message-----
From: mpls <[e-mail]> On Behalf Of Loa Andersson
Sent: Thursday, January 7, 2021 12:13 AM
To: Mach Chen <[e-mail]>; Greg Mirsky <[e-mail]>
Cc: mpls-chairs <[e-mail]>; mpls <[e-mail]>; [e-mail]; Rakesh Gandhi (rgandhi) <rgandhi=[e-mail]>
Subject: Re: [mpls] MPLS-RT Review of draft-gandhi-mpls-ioam-sr-04

Maach,

If it is a requirement from iOAM to do hop by hop processing, then the SPL/eSPL is a very blunt tool, there is is always a risk that the (e)SPL label that indicate the special behavior is below the maximum stack depth that can be scanned.

Right?

If we create a FEC that says "this packet has a hop by hop processing requirement, go find the ACH-header immediately after the label stack to see what you need to do." That would not solve the immediate problem but also be useful for the future.

/Loa


On 07/01/2021 11:50, Mach Chen wrote:
> Hi all,
> 
> IMHO, I think the key issue is that there is no hop-by-hop option in 
> MPLS, but the iOAM requires that.
> 
> There are three potential options:
> 
> 1)Scan the stack and find the (e)SPL label that indicate the special 
> behavior;
> 
> 2)Introduce an (e)SPL and always keep it on the top of the label 
> stack;
> 
> 3)Use the way as Stewart suggested, a new FEC, just like the SFL;
> 
> Best regards,
> 
> Mach
> 
> *From:*Greg Mirsky [mailto:[e-mail]]
> *Sent:* Thursday, January 7, 2021 11:30 AM
> *To:* Loa Andersson <[e-mail]>
> *Cc:* Stewart Bryant <[e-mail]>; Rakesh Gandhi 
> (rgandhi) <rgandhi=[e-mail]>; mpls <[e-mail]>; 
> [e-mail]; mpls-chairs <[e-mail]>
> *Subject:* Re: [mpls] MPLS-RT Review of draft-gandhi-mpls-ioam-sr-04
> 
> Hi Loa, et al.,
> 
> RFC 8169 uses TTL expiration to achieve processing at each RTM-capable 
> node. That approach creates a state in transient nodes and may not fit 
> with the "no state" paradigm of the Segment Routing.
> 
> And I've got a question. AFAIK, the presence of ACH in an MPLS LSP is 
> indicated by GAL. Is there an intention to introduce another (E)SPL 
> for that?
> 
> Regards,
> 
> Greg
> 
> On Wed, Jan 6, 2021 at 7:20 PM Loa Andersson <[e-mail] 
> <mailto:[e-mail]>> wrote:
> 
>     Stewart,
> 
>     If we want to make sure that packets are processed at evey node, is the
>     new FEC complementary to the ACH-header or an alternative?
> 
>     /Loa
> 
>     On 05/01/2021 00:30, Stewart Bryant wrote:
>     > 
>     > 
>     >> On 4 Jan 2021, at 14:38, Rakesh Gandhi (rgandhi) 
>     >> <rgandhi=[e-mail] <mailto:[e-mail]>
>     >> <mailto:rgandhi <mailto:rgandhi>=[e-mail]
>     <mailto:[e-mail]>>> wrote:
>     >>
>     >> <RG> Yes, this is similar to the entropy label where a mid-point node 
>     >> needs to scan the label stack to find the indicator label. We can add 
>     >> some text to clarify this. There is already an optimization to use a 
>     >> different indicator label for HbH compared to E2E case to 
>     >> unnecessarily avoid parsing the IOAM data fields.
>     > 
>     > The EL is entirely optional to process. It is no more than a hint to use 
>     > in ECMP and there is no architectural requirement to find it to operate 
>     > correctly.
>     > 
>     > If iOAM is purely a option then you could scan the stack and hope that 
>     > you can reach down far enough to find it. Though there is a fight to see 
>     > who gets to be the lowest label in range of the forwarding parser.
>     > 
>     > If you want to be sure that iOAM is processed HxH then you really need 
>     > to run it on a new FEC with that behaviour built into the FEC. That 
>     > would be the architected way of doing this in MPLS.
>     > 
>     > - Stewart
>     > 
>     > 
>     > 
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> 
>     --
> 
>     Loa Andersson                        email: [e-mail] <mailto:[e-mail]>
>     Senior MPLS Expert [e-mail] <mailto:[e-mail]>
>     Bronze Dragon Consulting             phone: [phone]
> 
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-- 

Loa Andersson                        email: [e-mail]
Senior MPLS Expert                          [e-mail]
Bronze Dragon Consulting             phone: [phone]

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