Subject: Valid Operations on MPLS Labels

From: Bora Akyol <[email-masked]>
Date: Fri, 28 Jan 2000 14:46:38 -0800

What are valid operations that can be performed on MPLS labels?

Yes sounds simple, but is it really?

1. SWAP: Lookup on incoming label, determine egress interface, outgoing
label, swap labels, update TTL.

2. PUSH: Look up on incoming label, determine egress interface, swap
topmost label and push one or more labels. Fix TTL in top most label.

3. POP: This is where the MPLS arch document is confusing. Assuming that
Penultimate hop popping is not used:
 So far we know that we have to lookup on incoming label, determine
operation (in this case POP), pop topmost label.

If it reveals an IP packet, we do a route lookup and everything is OK.

If it reveals an MPLS packet underneath, then what are valid operations
that can be performed on the label that emerges after the topmost label
is popped. I would like to know whether a swap or a push can follow a
pop. I am willing to do another lookup on the resultant packet, but
don't want to get into a recursive pop or push paradigm.

Any thoughts?

Bora

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From: "Kullberg, Alan" <[email-masked]>
Date: Mon, 31 Jan 2000 09:56:15 -0500

Hi Jana,

I agree that the label SWAP and PUSH on the outgoing side could
be done in a single operation.  I misunderstood your original
message.

Alan

> -----Original Message-----
> From: Pathangi N Janardhanan [mailto:[email-masked]]
> Sent: Monday, January 31, 2000 9:51 AM
> To: Kullberg, Alan
> Cc: [email-masked]
> Subject: Re: Valid Operations on MPLS Labels
> 
> 
> Hi,
> 
> > 
> > Since multiple LSPs (and hence, multiple different labels) can share
> > an LSP for tunneling between 2 LSRs, I think the label stack must be
> > processed label by label.
> 
>   Yes, the processing of incoming label stack must be performed on a 
> label by label basis, what I was refering to was the outgoing labels,
> we could model a SWAP and PUSH on the outgoing side as a single SWAP
> of the appropriate label stack.
> 
>   considering a stack of depth greater than 2, we may have to
> recurisvely POP labels if we are the egress for more than one level.
> 
> Jana
>