Re: [mpls] MPLS label and LSE data models

[Robert Wilton <[e-mail]>]

Hi,

Just as a point of reference, for VLAN tag stacks, the outermost VLAN 
tag is always first in the list.

Regarding relax the ordering or not, I may not be answering exactly the 
same question, but I would say that you want a have a canonical order 
(to allow for easy and efficient comparison), and forcing everyone to 
have to sort the label stack before using it would probably just be 
regarded as a wart.

Thanks,
Rob


On 14/07/2017 22:31, Acee Lindem (acee) wrote:
> Hi Tarek, Jeff,
>
> Typically, YANG indices are added to YANG lists to simply imply 
> ordering. I don’t believe there is absolutely any value in trying to 
> enforce the semantics of a precise label position on this index. It is 
> fairly obvious that the first label in the list is the first label in 
> the stack, the second label in the list is the second label in the 
> stack, and so on… Hopefully, the other YANG model authors will agree 
> with me on this point and the “Index 0 as top” convention should be 
> relaxed. Is there a YANG doctor in the house???
>
> Now, we currently specify the top label as the first label in the list 
> while Jeff has proposed that the bottom label be the first label. 
>  Surely, there is an existing convention within MPLS RFCs and drafts 
> and we should be consistent. I’d research myself but I have a ton of 
> other things to do prior to leaving for Prague tomorrow. When someone 
> refers to the first label, is the top or bottom label? I have always 
> been referring to the first label as the top label (with all due 
> respect to C stack implementations).
>
> Thanks,
> Acee
>
> From: "Tarek Saad (tsaad)" <[e-mail] <mailto:[e-mail]>>
> Date: Thursday, July 13, 2017 at 1:12 PM
> To: Jeff Haas <[e-mail] <mailto:[e-mail]>>, Xufeng Liu 
> <[e-mail] <mailto:[e-mail]>>
> Cc: Greg Mirsky <[e-mail] 
> <mailto:[e-mail]>>, "[e-mail] 
> <mailto:[e-mail]>" 
> <[e-mail] 
> <mailto:[e-mail]>>, "[e-mail] 
> <mailto:[e-mail]>" <[e-mail] <mailto:[e-mail]>>, 
> "[e-mail] 
> <mailto:[e-mail]>" 
> <[e-mail] 
> <mailto:[e-mail]>>, Routing WG 
> <[e-mail] <mailto:[e-mail]>>
> Subject: Re: MPLS label and LSE data models
> Resent-From: <[e-mail] <mailto:[e-mail]>>
> Resent-To: <[e-mail] <mailto:[e-mail]>>, 
> Yingzhen Qu <[e-mail] <mailto:[e-mail]>>, 
> Acee Lindem <[e-mail] <mailto:[e-mail]>>, Christian Hopps 
> <[e-mail] <mailto:[e-mail]>>, <[e-mail] 
> <mailto:[e-mail]>>
> Resent-Date: Thursday, July 13, 2017 at 1:12 PM
>
>     Hi Jeff and Xufeng,
>
>     Sorry, catching up on this thread. Yes, we've made a change for
>     the MPLS label-stack from "leaf-list" to a "list with key index"
>     to address having multiple labels of same value in the same stack.
>
>     We noted an assumption in the description that index 0 is the top
>     of the stack followed by the remainder of the labels in the stack.
>     However, you have a point about enforcing index (n-1) being
>     present before accepting index n. There is some discussion on
>     'preceding-sibling' and 'following-sibling' with some
>     recommendations in rfc6087.. I'll need to check if enforcing such
>     "when" check is good idea in YANG.
>
>     Another idea (not so elegant) is relax this "index 0 as top" and
>     just accept the lowest index of the list as the top followed by
>     the remainder labels (as sorted in index increasing order).
>
>     Regards,
>
>     Tarek
>
>     -----Original Message-----
>
>     From: Jeffrey Haas <[e-mail] <mailto:[e-mail]>>
>
>     Date: Thursday, July 13, 2017 at 12:51 PM
>
>     To: Xufeng Liu <[e-mail] <mailto:[e-mail]>>
>
>     Cc: Greg Mirsky <[e-mail]
>     <mailto:[e-mail]>>,
>     "[e-mail]
>     <mailto:[e-mail]>"
>     <[e-mail]
>     <mailto:[e-mail]>>, "[e-mail]
>     <mailto:[e-mail]>" <[e-mail] <mailto:[e-mail]>>,
>     "[e-mail]
>     <mailto:[e-mail]>"
>     <[e-mail]
>     <mailto:[e-mail]>>, "[e-mail]
>     <mailto:[e-mail]>" <[e-mail] <mailto:[e-mail]>>
>
>     Subject: Re: MPLS label and LSE data models
>
>     Resent-From: <[e-mail] <mailto:[e-mail]>>
>
>     Resent-To: Tarek Saad <[e-mail] <mailto:[e-mail]>>,
>     <[e-mail] <mailto:[e-mail]>>, <[e-mail]
>     <mailto:[e-mail]>>, <[e-mail]
>     <mailto:[e-mail]>>, <[e-mail]
>     <mailto:[e-mail]>>, <[e-mail]
>     <mailto:[e-mail]>>, <[e-mail]
>     <mailto:[e-mail]>>, <[e-mail]
>     <mailto:[e-mail]>>, <[e-mail]
>     <mailto:[e-mail]>>, <[e-mail]
>     <mailto:[e-mail]>>
>
>     Resent-Date: Thursday, July 13, 2017 at 12:42 PM
>
>         Xufeng,
>
>         On Thu, Jul 13, 2017 at 04:14:18PM +0000, Xufeng Liu wrote:
>
>         > Thanks for looking at this. You are right, but we are still
>     discussing various approaches for the static MPLS and the
>     conclusion has not been reached yet.
>
>         > We'd like to hear what you think and appreciate your comments.
>
>         To offer a suggestion, order the stack from bottom (lowest
>     number) to top
>
>         (highest).  Require that bottom of stack be element index zero.
>
>         My yang constraints are a bit weak but I believe you can
>     construct an XPath
>
>         that requires that a node of index 0 must be present.
>
>         The above two suggestions don't help with the issues of
>     needing to sort the
>
>         list by index in order to generate the stack, but it does at
>     least remove
>
>         any possible ambiguity about the critical bottom of stack
>     semantic.
>
>         -- Jeff
>
>
>
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