Subject: RtgDir Early review: draft-ietf-isis-mpls-elc-08

Hello

I have been selected to do a routing directorate “early” review of this draft.
​https://datatracker.ietf.org/doc/draft-ietf-isis-mpls-elc/

The routing directorate will, on request from the working group chair, perform an “early” review of a draft before it is submitted for publication to the IESG. The early review can be performed at any time during the draft’s lifetime as a working group document. The purpose of the early review depends on the stage that the document has reached.

As this document is in working group last call, my focus for the review was to determine whether the document is ready to be published. Please consider my comments along with the other working group last call comments.

For more information about the Routing Directorate, please see ​http://trac.tools.ietf.org/area/rtg/trac/wiki/RtgDir

Document: draft-ietf-isis-mpls-elc-08
Reviewer: Dhruv Dhody
Review Date: 12-09-2019
Intended Status: Standards Track

Summary: I have some minor concerns about this document that I think should be resolved before it is submitted to the IESG.

The draft is focused and straightforward, the reader needs to be aware of RFC6790 and draft-ietf-mpls-spring-entropy-label beforehand. I have reviewed this and the OSPF I-D together and you will find similar comments for both I-Ds. 

Minor
*****
(1) Could you mark that the codepoints mentioned in the draft are early allocated by IANA? Currently it says the value are desired. I also suggest following change in Section 7 (IANA Considerations) - 

OLD:
   IANA is requested to allocate the E-bit (bit position 3 is desired)
   from the "Bit Values for Prefix Attribute Flags Sub-TLV" registry.

   IANA is requested to allocate a MSD type (the type code of 2 is
   desired) from the "IGP MSD Types" registry for ERLD.
NEW:
   IANA is requested to confirm the early allocation of the E-bit (Bit
   position 3) in the "Bit Values for Prefix Attribute Flags Sub-TLV" 
   registry.

   IANA is requested to confirm the early allocation of the ERLD (type
   code of 2) in the "IGP MSD Types" registry.
END

(2) Section 3 talks about ERLD in Node MSD sub-TLV. But what happens if one receives ERLD in the Link MSD sub-TLV? As per my understanding this is not allowed, better to add normative text for the case then. 

Also we have this text in draft-ietf-mpls-spring-entropy-label - 

   In a distributed switching architecture, each linecard may have a
   different capability in terms of ERLD.  For simplicity, an
   implementation MAY use the minimum ERLD of all linecards as the ERLD
   value for the system.

   There may also be a case where a router has a fast switching path
   (handled by an ASIC or network processor) and a slow switching path
   (handled by a CPU) with a different ERLD for each switching path.
   Again, for simplicity's sake, an implementation MAY use the minimum
   ERLD as the ERLD value for the system.

   The drawback of using a single ERLD for a system lower than the
   capability of one or more specific component is that it may increase
   the number of ELI/ELs inserted.  This leads to an increase of the
   label stack size and may have an impact on the capability of the
   ingress node to push this label stack.

If we are deviating from this and opting for the node (marked 'MAY' above) as the only possibility, we need to handle this properly. Maybe check with chairs/AD on this!    

(3) Section 4, can we add some more description on what the 'E' flags means, in the similar style of other flags [https://tools.ietf.org/html/rfc7794#section-2.1]

(4) Section 8, suggest to also add one sentence for the impact of advertising incorrect ERLD. If there isn't any, that can also be stated. 

Nits
****
(1) Suggested ordering of sections - ..ELC/ERLD/BGP-LS/ACK..  [matching between OSPF/ISIS]
(2) Section 2, add [I-D.ietf-mpls-spring-entropy-label] for terminology reference
(3) Section 3, Add reference to draft-ietf-mpls-spring-entropy-label for the definition and usage of ERLD
(4) Section 6,

   The ERLD MSD-type introduced for IS-IS in Section 3 is advertised
   using the Node MSD TLV (TLV 266) of the BGP-LS Node NLRI Attribute as
   defined in section 3 of [I-D.ietf-idr-bgp-ls-segment-routing-ext].

I think you mean draft-ietf-idr-bgp-ls-segment-routing-msd here! 

Also, maybe change the title "BGP-LS Extension" as there is no 'extension' required, ELC/ERLD is BGP-LS would be automatically supported.   

(5) Expand MSD on first use. 
(6) The first figure is titled Figure 2!  
(7) Section 4, mark the figure as "Prefix Attribute Flags"
(8) All references are marked Normative, please re-check if this is intentional. 

Thanks! 
Dhruv