Re: [mpls] draft-fang-mpls-label-forwarding-no-swap - how much does it really save?

Andrew Qu <> Tue, 28 July 2015 14:57 UTC

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From: Andrew Qu <>
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Thread-Topic: [mpls] draft-fang-mpls-label-forwarding-no-swap - how much does it really save?
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Subject: Re: [mpls] draft-fang-mpls-label-forwarding-no-swap - how much does it really save?
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Hi Stewart,

Agreed that SWAP must be kept because MPLS fundamentals can't be broken.
And I agreed that _if_ there is NO such global label space idea,  no-swap is
optimization of swap as well. 

However I don't agree no-swap become an optimization in terms of network
Behavior when global label idea become a needed thing.

When global Label introduced, then no_swap is the conceptually right thing for the transition [even
Local implementation can use swap to achieve, again, per previous conversation,
the popular wisdom, which I agreed as well,  is that this is local implementation, we should
not get involved, so we must define no-swap as the new primitive at RFC level]

So the question really boils down to that do we need global label?

I believe that is the key thing here.  May be we can put swap/no-swap aside for the moment.



-----Original Message-----
From: Stewart Bryant [] 
Sent: Tuesday, July 28, 2015 4:03 AM
To: Andrew Qu; Alexander Vainshtein; Robert Raszuk; Shahram Davari
Subject: Re: [mpls] draft-fang-mpls-label-forwarding-no-swap - how much does it really save?

> Adding "NO_SWAP" for new/future generation DEVICE does NOT break 
> backward capability.

Existing devices should be assumed to have non-aligned label spaces.
(there is much empirical evidence to support this position)

So if you have a no-swap only device between two existing devices you cannot be sure that you can build an LSP.

A backwards compatible device would therefore need to support swap.

Once you support swap then no-swap become an optimization.

- Stewart

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