Re: [Isis-wg] draft-ietf-isis-segment-routing-extensions-02

"Stefano Previdi (sprevidi)" <> Fri, 06 June 2014 12:05 UTC

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From: "Stefano Previdi (sprevidi)" <>
To: Hannes Gredler <>
Thread-Topic: [Isis-wg] draft-ietf-isis-segment-routing-extensions-02
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Date: Fri, 6 Jun 2014 12:05:21 +0000
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Cc: DECRAENE Bruno IMT/OLN <>, " Tantsura" <>, " list" <>, "Clarence Filsfils \(cfilsfil\)" <>
Subject: Re: [Isis-wg] draft-ietf-isis-segment-routing-extensions-02
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On Jun 6, 2014, at 1:02 PM, Hannes Gredler wrote:

> hi stefano,
> +ISIS-WG as this is now a WG-item.

sure. I usually try to have, at least, agreement with co-authors 
before presenting the change to the WG and ask for consensus but 
this way is fine for me too.

> a comment n the 'V' flag -
> what is IMO missing is the clarification when and when not to use the 'V' flag.
> i.e. for which particular data plane -

indeed, we could add some clarification such as:

. V (value flag): is set when the SID is a label value (i.e.: not an index).
  The V flag is set when:
    . SID carries MPLS labels (i.e.: not indexes)
    . SID carries IPv6 addresses
. L (local flag): is set when the SID has local significance (local means 
  router/node context). It applies to both MPLS and IPv6 dataplanes:
  . MPLS: prefix-SID being global, the L flag is unset
          adj-SID being local, the L flag is set
  . IPv6: Adj-SID can be eithr local or global. When used with a globally 
          unique ipv6 address, the L flag is unset.

This, I believe reflects what you and me agreed a ouple of weeks ago, right ?

> to play devils advocate:
>  as generic as the draft is right now,
>  the V-flag may be abused for prefix-SIDs on a MPLS data plane
>  as an attempt to-reintroduce the notion of global labels,
>  which as you may know i do reject.

there are maybe use-cases out there where a prefix would be better 
advertised with a global label. Also, at some point in the future 
we may want to support global labels without having to go through 
a complete re-spin of the drafts, so adding a bit to govern the 
significance of a sid is a good thing to have.

> may i ask you to add some clarifying notes that V-flag=1 applies only for the
> Prefix SIDs assuming IPv6-SR dataplane.

Well, I'd prefer to stick with what the co-authors already agreed 
to propose at the WG so anyone, feel free to express your opinion.

> furthermore i am not really sure what the V-flag shall mean for Adjacency-SIDs.
> (the adjacency-SID always has been a label and never an index).

at some point, one may want to have a global index for adj-sid's


> thanks,
> /hannes
> On Jun 6, 2014, at 10:47 AM, Stefano Previdi (sprevidi) wrote:
>> Co-authors,
>> this is the latest version we are ready to submit. It contains the different 
>> updates we've discussed in the past weeks.
>> . Addition of flags in prefix-sid and adj-sid subtlv. We added E, V and L flags. 
>> At some point we removed the R flag but is now back.
>> . use of prefix-sid-tlv for mapping server
>> . fixed typo's and inconsistencies
>> Bruno and others already sent comments that I have tryed to address (e.g.: I 
>> moved the SID/Label SubTLV section after PrefixSID and Adj-SID sections.
>> Thanks for letting me know any comment.
>> s.
>> <draft-ietf-isis-segment-routing-extensions-01.txt>
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