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

Robert Raszuk <robert@raszuk.net> Thu, 23 July 2015 21:11 UTC

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From: Robert Raszuk <robert@raszuk.net>
To: Shahram Davari <davari@broadcom.com>
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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 Shahram,

Labels which are non of a local significance can be distributed by flooding
protocols extensions (ISIS, OSPF) or by direct p2p sessions (BGP 3107,
sessions from the controller, XMPP etc ...)

The important part is that the actual forwarding is computed recursively or
set at the controller.

AFAIK I have not seen any proposal where LDP would play any role in such
distribution.

Regards,
R.





On Thu, Jul 23, 2015 at 11:07 PM, Shahram Davari <davari@broadcom.com>
wrote:

>  Hi Robert,
>
>
>
> How are these labels distributed? Via LDP or via SDN controller?
>
>
>
> Thanks
>
> Shahram
>
>
>
> *From:* rraszuk@gmail.com [mailto:rraszuk@gmail.com] *On Behalf Of *Robert
> Raszuk
> *Sent:* Thursday, July 23, 2015 12:58 PM
> *To:* Eric C Rosen
> *Cc:* Shahram Davari; stbryant@cisco.com; mpls@ietf.org
> *Subject:* Re: [mpls] draft-fang-mpls-label-forwarding-no-swap - how much
> does it really save?
>
>
>
> ​Hi Eric,​
>
>
>
>  ​​
>
> If you notice that the incoming label needs to be 'replaced' by an
> outgoing label of the same value, you could just make the rewrite string
> shorter, so it won't overwrite the top label on the stack.  This seems to
> be what the draft suggests, but it could be done as an optimization for the
> particular case where the incoming and outgoing labels have the same value.
>
>
>
> ​This is precisely ​the crux where your statement fails.
>
>
>
> You use term "incoming label" and "outgoing lable" ... well in the new
> architectures there is no such things.
>
>
>
> It is a "global label" or "path label" with adjacency information.
>
>
>
> So to support legacy hardware new control plane has to make up from single
> label now two (identical) labels to pass it to data plane. Now also data
> plane must be smart to check that and program its state per your
> suggestion.
>
>
>
> Why would we do that other then due to worry about legacy chipsets feared
> to be non compliant to new RFC ?
>
>
>
> Many thx,
>
> R.
>
>
>