Re: [mpls] [sfc] Working Group adoption of draft-farrel-mpls-sfc

[Robert Raszuk <[e-mail]>]

Hi Stewart,

1.

> In its simplest form it uses a compact MPLS stack to describe an
arbitarily complex path

What exactly is "compact" in MPLS label stack as defined in
draft-ietf-mpls-sfc ?


     -------------------
    ~   Tunnel Labels   ~
    +-------------------+
    ~     Optional      ~
    ~   Entropy Label   ~
    +-------------------+ - - -
    | SFC Context Label |
    +-------------------+  Basic unit of MPLS label stack for SFC
    |     SF Label      |
    +-------------------+ - - -
    | SFC Context Label |
    +-------------------+  Basic unit of MPLS label stack for SFC
    |     SF Label      |
    +-------------------+ - - -
    ~                   ~
    +-------------------+ - - -
    | SFC Context Label |
    +-------------------+  Basic unit of MPLS label stack for SFC
    |     SF Label      |
    +-------------------+ - - -
    |                   |
    ~    Payload        ~
    |                   |
     -------------------

           Figure 3: The MPLS SFC Label Stack for Label Stacking


2.

> draft-xu-clad-spring-sr-service-chaining unrolls the path and explicitly
calls out each hop
> and each function into the label stack.

That is precisely what draft-ietf-mpls-sfc also does as far as SF's are
concerned.

Other network elements which do not participate in SFs are not aware of
service function states and all they are doing is transporting mpls packets
to and between SFs. Both drafts are identical here.

What differences in network state do you see in transit nodes ? With that
please keep in mind that LDP's network state with locally significant
labels is far greater then SR network state with global/domain wide labels.

Last the core of the comments where focused on using NSH and not to make
any judgement which way of producing alternatives to it is better/worse.

Best,
R.


On Thu, Apr 12, 2018 at 5:04 PM, Stewart Bryant <[e-mail]>
wrote:

>
> Rather than have a process discussion, I think we should go up a level and
> better understand the technical differences between the two drafts.
>
> draft-farrel-mpls-sfc describes the actions at a hop in terms of a tuple
> that mirrors the SFC approach that allows a short indication of potentially
> re-entrant chains. In its simplest form it uses a compact MPLS stack to
> describe an arbitarily complex path that is compatile with simple edge
> routers which are often challenged in terms of the number of labels that
> they can push.
>
> draft-xu-clad-spring-sr-service-chaining unrolls the path and explicitly
> calls out each hop and each function into the label stack. This results in
> a much larger MPLS label stack that will challenge some edge routers. The
> way that we generally deal with imposition limits is through the use of
> binding-SIDs, which in the limiting case resolves to the approach in
> draft-farrel with the limitation that the position on the path is implicit
> in the label stack size rather than explicitly specified by the SI.
>
> Mid-flight path changes (if such things are needed) is clearly simpler
> with draft-farrel.
>
> The short stack in draft-farrel comes at the cost of greater network
> forwarding stack, and the long stack is the price that draft-xu-clad pays
> for the reduction in forwarding state.
>
> The optimal design point between forwarding and control plane state is
> something that is dependent on many parameters, and is dependent on many
> network and operational factors, so much so, that don't think it is wise to
> rule either out of scope at this stage.
>
> The hybrid mode in section 6 of draft-farrel supports the mixed mode in
> section 7 of the draft. This allows the construction of SFCs that are the
> concatination of two or more compacted sub-chains. This allows the operator
> to deploy a solution with the advantages of draft-farrel together with some
> of the flexibility of draft-xu-clad.
>
> At this stage the two drafts are sufficienly different that I think we
> need to proceed with both at least to the point where we fully understand
> the detailed consequences of the two approachs and the scenarios where each
> finds it's niche.
>
> After developing a better understanding the detail of each design, their
> control plane, and operational contexts and how each maps to customer
> network requirements, we can move the drafts to the appropriate IETF track.
> Such tracks may be anything from abandonment to IETF standard for one or
> both of these approaches.
>
> Meanwhile I think that we need to focus our efforts on a deeper
> understanding of the technology and how each might make the Internet work
> better,  rather than spending effort on arguing about IETF process.
>
> - Stewart
>
>