Re: [mpls] on MIAD ADI requirement draft RE: Agenda for the MPLS Open DT 2021-10-14

[Kireeti Kompella <[e-mail]>]

Hi Matthew,

On Oct 18, 2021, at 01:55, Bocci, Matthew (Nokia - GB) <[e-mail]> wrote:

> “The mechanism to indicate thatAncillary Data is present MUST operate in the context of the top of stack LSE.”

I’m not sure what this means, except possibly, the ADI MUST NOT be the top of stack LSE.  There were some suggestions that the FAI could be at the top of stack, accompanied by some label gymnastics to get the forwarding label.  This rules that option out (which I’m happy about).

Here’s some alternate text:

"The mechanism to indicate that Ancillary Data is present MUST allow for simple forwarding of MPLS packets based on the top of stack LSE.  A node MAY (SHOULD?) go further, though, to find the ADI within the label stack and act upon it."

> MB> This is something we need a discussion on but, in essence, what we are saying is that it must be consistent with the MPLS architecture. An LSR just does a swap of the top label, or a pop and forward, but unless it is doing something like scanning the stack looking for an ELI it doesn’t change the fundamentals of MPLS forwarding behaviour based on labels below the top of stack. Even in the ELI case, it still forwards to an ECMP next hop where that top label is valid. Besides, the ADI is for ancillary data so it is subordinate to the basic MPLS forwarding decision.

I don’t know the full scope of the Requirements doc, but the FAI has a dual purpose: to indicate that certain forwarding actions are to be taken, and to carry relevant data for those actions.  An ADI (and its data) by itself isn’t much use.  Think of the ELI carrying an Entropy Label, but without the semantics of “load balance using this”.  Pretty useless. Or, for a more current example, carrying a GISS without saying, "this is the slice indicator; take the appropriate action”.  Essentially, “here’s a 32-bit field.  Have a nice day!” :)

One can say that the action is implicit in the type of data carried, but I prefer being explicit and more general.  FAI — forwarding actions indicator — focuses on the action, indicates the associated data, and is inclusive of cases (like NFFRR) where there isn’t associated data.

:k