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

["Rakesh Gandhi (rgandhi)" <[e-mail]>]

Hi Loa,

If we create a hop-by-hop FEC (similar to synonymous label), I think the packet header encoding with ACH could look like following. I am not sure if there would be a need to also add GAL or E(SPL) just before the ACH header.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Synonymous IOAM Label(1)             | TC  |S|  TTL          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Synonymous IOAM Label(n)             | TC  |S|  TTL          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<-+
   |0 0 0 1|Version|   Reserved    |          IOAM G-ACh Type      |  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
   | Reserved      | Block Number  | IOAM-OPT-Type |IOAM HDR Length|  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  I
   |                                                               |  O
   |                                                               |  A
   ~                 IOAM Option and Data Space                    ~  M
   |                                                               |  |
   |                                                               |  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<-+
   |                                                               |
   |                                                               |
   |                 Payload + Padding                             |
   |                                                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Thanks,
Rakesh


From: Loa Andersson <[e-mail]>
Date: Wednesday, January 6, 2021 at 10:50 PM
To: Greg Mirsky <[e-mail]>
Cc: Stewart Bryant <[e-mail]>, Rakesh Gandhi (rgandhi) <[e-mail]>, mpls <[e-mail]>, [e-mail] <[e-mail]>, mpls-chairs <[e-mail]>
Subject: Re: [mpls] MPLS-RT Review of draft-gandhi-mpls-ioam-sr-04
Greg,

(feeling the water) if we create a hop-by-hop FEC, that in itself could
indicate the presence of the ACH-header, right?

/Loa

On 07/01/2021 11:30, Greg Mirsky wrote:
> 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]