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

Robert Raszuk <robert@raszuk.net> Thu, 23 July 2015 21:36 UTC

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Date: Thu, 23 Jul 2015 23:36:30 +0200
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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,

Both IGP and BGP extensions are proposed.

As described they use the trick of locally defining a NO_SWAP semantics
(for example by defining "CONTINUE" action as "swap to the same label") and
using this notion in their architecture.

They do it as there is no MPLS architecture semantics for that.

Is it possible .. sure.

Is it optimal and good practice going forward .. I do not think so.

Best,
R.


On Thu, Jul 23, 2015 at 11:28 PM, Shahram Davari <davari@broadcom.com>
wrote:

>  Which control planes are already defined for this global label? IS-IS?
> BGP? If it is defined then what do you need?
>
>
>
> Thx
> SD
>
>
>
> *From:* rraszuk@gmail.com [mailto:rraszuk@gmail.com] *On Behalf Of *Robert
> Raszuk
> *Sent:* Thursday, July 23, 2015 2:22 PM
>
> *To:* Shahram Davari
> *Cc:* Eric C Rosen; stbryant@cisco.com; mpls@ietf.org
> *Subject:* Re: [mpls] draft-fang-mpls-label-forwarding-no-swap - how much
> does it really save?
>
>
>
>
>
> The point is that new control plane is already defined. In fact we already
> have two :)
>
>
>
> As I mentioned in my first mail to the list the concept of
> NO_SWAP/CONTINUE is common to both H-SDN and SEGMENT ROUTING architectures.
>
>
>
> Ref: https://goo.gl/3oxRbl
>
>
>
> Thx,
> R.
>
>
>
>
>
> On Thu, Jul 23, 2015 at 11:16 PM, Shahram Davari <davari@broadcom.com>
> wrote:
>
> Robert,
>
>
>
> So instead of calling it no-swap probably you should call it global label
> or so, and then define new control plane for it. But seems the data-pane
> behavior does not change and existing hardware can support this global
> label.  So maybe you just need new control plane.
>
>
>
> Thx
> Shahram
>
>
>
> *From:* rraszuk@gmail.com [mailto:rraszuk@gmail.com] *On Behalf Of *Robert
> Raszuk
> *Sent:* Thursday, July 23, 2015 2:12 PM
> *To:* Shahram Davari
> *Cc:* Eric C Rosen; stbryant@cisco.com; mpls@ietf.org
>
>
> *Subject:* Re: [mpls] draft-fang-mpls-label-forwarding-no-swap - how much
> does it really save?
>
>
>
> 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.
>
>
>
>
>
>
>