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

Alexander Vainshtein <> Thu, 23 July 2015 14:15 UTC

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From: Alexander Vainshtein <>
To: "" <>, 'Andrew Qu' <>
Thread-Topic: [mpls] draft-fang-mpls-label-forwarding-no-swap - how much does it really save?
Date: Thu, 23 Jul 2015 14:13:42 +0000
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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 all,
I concur with Adrian.

I also think that the authors of the draft did not intend to change the TTL or TC processing rules with their “no-swap” option.


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From: mpls [] On Behalf Of Adrian Farrel
Sent: Thursday, July 23, 2015 5:09 PM
To: 'Andrew Qu'
Subject: Re: [mpls] draft-fang-mpls-label-forwarding-no-swap - how much does it really save?


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.


From: mpls [] On Behalf Of Andrew Qu
Sent: 23 July 2015 15:01
To: Alexander Vainshtein; Stewart Bryant; Andrew G. Malis; S. Davari
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.



From: mpls [] On Behalf Of Alexander Vainshtein
Sent: Tuesday, July 21, 2015 5:05 PM
To: Stewart Bryant; Andrew G. Malis; S. Davari
Subject: Re: [mpls] draft-fang-mpls-label-forwarding-no-swap - how much does it really save?


Thumb typed on my LG,

------ Original message ------
From: Stewart Bryant
Date: 21/07/2015 17:58
To: Andrew G. Malis;S. Davari;
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

- 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").


On Mon, Jul 20, 2015 at 8:52 PM, S. Davari  <<>> wrote:

I agree no standard change is required since this is a local optimization issue.


> On Jul 20, 2015, at 11:43 AM, Eric C Rosen <<>> 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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