Re: [Idr] Martin Duke's No Objection on draft-ietf-idr-bgp-ls-segment-routing-msd-16: (with COMMENT)

Alvaro Retana <> Fri, 24 April 2020 18:55 UTC

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From: Alvaro Retana <>
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Date: Fri, 24 Apr 2020 11:55:01 -0700
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To: Martin Duke via Datatracker <>, The IESG <>, Martin Duke <>
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Subject: Re: [Idr] Martin Duke's No Objection on draft-ietf-idr-bgp-ls-segment-routing-msd-16: (with COMMENT)
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On April 24, 2020 at 12:54:53 PM, Martin Duke wrote:



> This is not a DISCUSS-level concern, but I found it odd that the Node MSD TLV
> must be the minimum of all configured interfaces without regard for the
> presence of any Link MSD TLVs.
> For example, if all node interfaces have an MSD of 20 except one with an MSD
> of 10, it would be much more compact to advertise a Node MSD of 20 and a
> single Link MSD of 10. Section 5 says the Link MSD would take precedence, so
> there would be no information loss. As I understand the spec, this would not
> be allowed, and each link would have to be advertised separately to gain that
> level of granularity.
> If this is not the intent, then in Section 3, extending a sentence to say
> "Node MSD is the smallest MSD supported by the node on the set of interfaces
> configured for use, [excepting links advertised with their own Link MSD TLV]"
> would avoid the problem.

This could have been a good optimization.

However, this ship has sailed.  BGP-LS is a mechanism to carry
topology related information to a central controller.  This
information, as in this case, is obtained from the IGPs (OSPF or
IS-IS).  IOW, BGP-LS is simply carrying the same information that
IS-IS/OSPF generated...with the same semantics, which is what the
controller is expecting.  The corresponding IGP specifications
(rfc8491/rfc8476) contain the same language...if we change this
document then we would have to change the IGP documents...and all the
implementations. :-(