Early Review of draft-ietf-l2sm-l2vpn-service-model-08
review-ietf-l2sm-l2vpn-service-model-08-yangdoctors-early-lhotka-2018-02-26-00

Request Review of draft-ietf-l2sm-l2vpn-service-model-05
Requested rev. 05 (document currently at 10)
Type Early Review
Team YANG Doctors (yangdoctors)
Deadline 2018-01-31
Requested 2018-01-17
Requested by Adrian Farrel
Other Reviews Genart Telechat review of -08 by Joel Halpern (diff)
Comments
This document is approaching WG last call. Lots of lessons have been from the progress of rfc-to-be-8299 (draft-wu-l3sm-rfc8049bis) but we would like to flush out YANG issues before entering last call so that the whole WG can look at something that is fully stable.

(Detailed review provided by Jan Lindblad who is also a YANG doctor.)
Review State Completed
Reviewer Ladislav Lhotka
Review review-ietf-l2sm-l2vpn-service-model-08-yangdoctors-early-lhotka-2018-02-26
Posted at https://mailarchive.ietf.org/arch/msg/yang-doctors/SkbrM_o6nSpCWmxxWvLQ9xcKN84
Reviewed rev. 08 (document currently at 10)
Review result Ready with Issues
Draft last updated 2018-02-26
Review completed: 2018-02-26

Review
review-ietf-l2sm-l2vpn-service-model-08-yangdoctors-early-lhotka-2018-02-26

**** General comments

     - The 'ietf-l2vpn-svc' module contained in this document is
       rather large: it defines 386 schema nodes, 35 features and 175
       identities. It is therefore natural to ask whether the authors
       considered splitting the definitions into multiple
       modules. This would make the data model more modular and
       probably also make some of the features unnecessary. See also
       draft-ietf-netmod-rfc6087bis-18, sec. 4.17.

     - The module defines most containers, lists and even individual
       leaves in a grouping that is then used only once. This approach
       has its pros nad cons but it is not the common practice in YANG
       modelling - groupings are mostly defined only if they are used (or
       expected to be used) repeatedly and/or in different modules.

     - Some of the groupings (e.g. 'site-device') reference nodes
       outside the grouping, which is discouraged, see
       draft-ietf-netmod-rfc6087bis-18, sec. 4.13.

     - Names are sometimes overloaded in that the same name is used
       for different purposes. For example, 'cloud-access' is used as
       the name of both a list and a feature. In one case
       ('bum-frame-delivery'), the same name is used for a list, the
       container that encloses it, and the grouping in which the
       container is defined. This should be avoided.

**** Specific comments

     - Most when expressions correctly use the XPath function
       'derived-from-or-self()'. In two cases ('vpn-id' and 'cos-id'
       leaves) identities are compared in XPath expressions using
       plain string equality. These XPath expressions need to be
       changed to use 'derived-from-or-self()' as well.

     - The when expression for container 'devices' user a disjunction
       of two 'derived-from-or-self()' checks. This usually indicates
       that a new identity can be defined from which both or-ed
       identities are derived.

**** Instance examples

     The example instance documents contained in the document require
     to be carefully checked because they are often invalid or even
     not well-formed. I would also suggest to use examples in JSON
     encoding that is easier to read:

     The problems that I discovered include:

     - extra space in opening and closing XML tags
       < network-access-id> and </ network-access-id>

     - p. 28: wrong namespace URI
       "urn:ietf:params:xml:ns:yang:ietf-l3vpn-svc"

     - p. 28: in the first 'vpn-service' entry (key 'VPNB'), two
       mandatory leaves are missing: 'ce-vlan-preservation' and
       'ce-vlan-cos-perservation'.

**** Nits

     - The name of 'ce-vlan-cos-perservation' should most likely be
       'ce-vlan-cos-preservation'

     - Inconsistent indentation: features 'cfm' and 'y-1731' have less
       spaces of indentation than other features

     - When concatenating literal strings, the widely accepted
       convention is to put the '+' sign ot the second line, and align
       the string components. That is, instead of

       path "/l2vpn-svc/vpn-profiles/valid-provider-identifiers"+
         "/cloud-identifier/id";

       use

       path "/l2vpn-svc/vpn-profiles/valid-provider-identifiers"
          + "/cloud-identifier/id";