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

Stewart Bryant <stbryant@cisco.com> Thu, 23 July 2015 14:22 UTC

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Date: Thu, 23 Jul 2015 15:21:54 +0100
From: Stewart Bryant <stbryant@cisco.com>
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To: adrian@olddog.co.uk, "'Andrew Qu'" <andrew.qu@mediatek.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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Adrian beat me to it, although I see no reference to the infinite loop 
that may also result.

You are also talking about operations that you do when you change level 
in the LSP hierarchy with operations that you do when forwarding at the 
same level in the hierarchy.

- Stewart


On 23/07/2015 15:08, Adrian Farrel wrote:
>
> Andrew,
>
> Are you mixing "no swap" with "bump in the wire"?
>
> If you are going to not consider TTL then you are doing some far more 
> "interesting" things: like not decrementing the TTL so not being 
> counted as a hop; not appearing in OAM; not checking incoming packets 
> for TTL going to zero.
>
> The discussion of "no swap" as applied *solely* to the label is a 
> black box optimisation. There is no difference at the external 
> interfaces when "no swap" is done or when "pop'n'push" or 
> "swap-inline" is done. The packet coming out has the same features in 
> all cases.
>
> But, if you are doing other special things to the label stack entry 
> (e.g. not looking at it, or not changing it) then IMHO you are not a 
> router, you are a wire.
>
> Adrian
>
> *From:*mpls [mailto:mpls-bounces@ietf.org] *On Behalf Of *Andrew Qu
> *Sent:* 23 July 2015 15:01
> *To:* Alexander Vainshtein; Stewart Bryant; Andrew G. Malis; S. Davari
> *Cc:* mpls@ietf.org
> *Subject:* Re: [mpls] draft-fang-mpls-label-forwarding-no-swap - how 
> much does it really save?
>
> I don’t agree no-swap is the same as “swap with same value”.
>
> I think if standard adds a no-swap operation, that would be great for 
> people to implement
>
> Either SW or HW.
>
> Once the label is defined as swap, I need to consider
>
> 1)TTL
>
> Copy payload TTL ? or using const_value
>
> 2)QoS (pipe mode or not)
>
> If pipe mode => then I need to do something
>
> If not=> then I need to do something else.
>
> However if it is really a “no-swap”,  then it is very clear for HW/SW 
> to implement such operation.
>
> I don’t need to consider any part of above cases.
>
> I don’t think swaping a same value label is NOT the same as “no-swap” 
> at all.
>
> From Standard point of view, we should NOT leave ambiguity here at all.
>
> Thanks,
>
> Andrew
>
> *From:*mpls [mailto:mpls-bounces@ietf.org] *On Behalf Of *Alexander 
> Vainshtein
> *Sent:* Tuesday, July 21, 2015 5:05 PM
> *To:* Stewart Bryant; Andrew G. Malis; S. Davari
> *Cc:* mpls@ietf.org
> *Subject:* Re: [mpls] draft-fang-mpls-label-forwarding-no-swap - how 
> much does it really save?
>
> +1.
>
> Thumb typed on my LG,
> Sasha
>
> ------ Original message ------
> *From: *Stewart Bryant
> *Date: *21/07/2015 17:58
> *To: *Andrew G. Malis;S. Davari;
> *Cc: *mpls@ietf.org <mailto:mpls@ietf.org>;
> *Subject:*Re: [mpls] draft-fang-mpls-label-forwarding-no-swap - how 
> much does it really save?
>
> This draft proposes architectural changes, and as we can show that
> these changes are not needed, in my view, this draft should not go
> forward.
>
> - Stewart
>
>
> On 21/07/2015 09:02, Andrew G. Malis wrote:
>
>     As there are no architectural or protocol changes or IANA
>     considerations, this draft should be informational if it goes
>     forward (it currently says "standards track").
>
>     Cheers,
>
>     Andy
>
>     On Mon, Jul 20, 2015 at 8:52 PM, S. Davari  <davarish@yahoo.com
>     <mailto:davarish@yahoo.com>> wrote:
>
>     Eric
>
>     I agree no standard change is required since this is a local
>     optimization issue.
>
>     Regards,
>     Shahram
>
>
>     > On Jul 20, 2015, at 11:43 AM, Eric C Rosen <erosen@juniper.net
>     <mailto:erosen@juniper.net>> wrote:
>     >
>     >> On 7/20/2015 2:07 PM, Shahram Davari wrote:
>     >> The new swapped labels (The outgoing  label that replaces the
>     >> incoming label) need to be stored in a table. Using this draft
>     >> reduces the number of swapped labels that needs to be stored,
>     >> regardless of  implementation. Don't you agree?
>     >
>     > No.  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.  You could
>     do this today, as a local implementation optimization. There
>     doesn't seem to be any interop issue or any change to the data
>     plane semantics.
>     >
>     >> It also reduces the configuration and management of the new
>     swapped labels.
>     >
>     > We're not discussing whether there are any advantages to the use
>     of domain-wide labels.  We're discussing whether the use of
>     domain-wide labels requires a change in the forwarding plane
>     architecture.  I just don't see that it does.
>     >
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