Re: Working Group Last Call on BFD YANG model, RFC 9127-bis ending 20 December, 2021

[Reshad Rahman <[e-mail]>]

 Should RFC9127-bis obsolete RFC9127?
Regards,Reshad.
    On Tuesday, December 7, 2021, 08:43:44 AM EST, Jeffrey Haas <[e-mail]> wrote:  
 
 Working Group,

While some explanation is required, the request to the Working Group is very
simple: Please review this minor update to the recently published BFD YANG
module and offer your comments whether we should head to rapid publication.

This last call ends 20 December.

---

The history:
This module defines a YANG grouping, "client-cfg-parms".  The intent of that
grouping is to provide a common user experience in IETF YANG modules that
want to use BFD.  It provides a consistent set of leaf nodes that can be
used by those client protocols so you don't have to remember whether 
it's "enable" or "enabled", what a multiplier is called, and where the
timers live.

This grouping is currently present in the RIP YANG model.  It is also in the
RFC Editor's queue for the PIM, OSPF, and ISIS modules.  The BGP model
intends to use this grouping.

A small issue was noted shortly after publication that even though the
grouping is correct, its structure in RFC 9127 was awkward for
implementations that do not use per-client configuration of BFD parameters.

Using the YANG tree for ietf-bfd-mpls included in the module from the -bis,
consider the following:
          |  +--rw enabled?                          boolean
          |  +--rw local-multiplier?                multiplier
          |  +--rw (interval-config-type)?
          |  |  +--:(tx-rx-intervals)
          |  |  |  +--rw desired-min-tx-interval?    uint32
          |  |  |  +--rw required-min-rx-interval?  uint32
          |  |  +--:(single-interval) {single-minimum-interval}?
          |  |    +--rw min-interval?              uint32

There are two commonly deployed styles of BFD provisioning in the industry:
- Fully centralized.  In this case, BFD clients only need to indicate that
  they have "enabled" BFD to be used in that case.  The sessions are
  configured at global scope.  (E.g. "protocols bfd")
- Per-client configuration.  In this case, the client will also want to
  indicate that it supports local configuration of parameters such as the
  multiplier, and intervals.

In the current structure of RFC 9127, an implementation that uses fully
centralized mode will need to create a YANG deviation for each use of BFD's
client-cfg-parms.  While this was considered acceptable during the original
drafting of the grouping in the BFD YANG module, current practices have
evolved.

The fix, and the very small change to RFC 9127 in this -bis, is to add a new
YANG feature, "client-base-cfg-parms", and take the client configuration
parameters and predicate it on that feature.

This small change permits all of the client YANG modules listed above to
inherit this feature behavior with no changes to those client modules.

The following section from 9127-bis states the change as well:

 : Updates since RFC 9127
 : 
 :    This version of the draft updates the 'ietf-bfd-types' module to
 :    define a new feature called 'client-base-cfg-parms and a 'if-feature'
 :    statement that conditionally includes definition of parameters such
 :    as 'multiplier' or 'desired-min-tx-interval'.  The feature statement
 :    allows YANG implementations of protocol such as OSPF, ISIS, PIM and
 :    BGP, to support both a model where such parameters are not needed,
 :    such as when multiple BFD sessions are supported over a given
 :    interface, as well as when they need to be defined per session.

-- Jeff

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Date: Tue, 07 Dec 2021 04:26:39 -0800
From: [e-mail]
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Subject: I-D Action: draft-ietf-bfd-rfc9127-bis-00.txt


A New Internet-Draft is available from the on-line Internet-Drafts directories.
This draft is a work item of the Bidirectional Forwarding Detection WG of the IETF.

        Title          : YANG Data Model for Bidirectional Forwarding Detection (BFD)
        Authors        : Reshad Rahman
                          Mahesh Jethanandani
                          Lianshu Zheng
                          Santosh Pallagatti
                          Greg Mirsky
    Filename        : draft-ietf-bfd-rfc9127-bis-00.txt
    Pages          : 70
    Date            : 2021-12-06

Abstract:
  This document defines a YANG data model that can be used to configure
  and manage Bidirectional Forwarding Detection (BFD).

  The YANG modules in this document conform to the Network Management
  Datastore Architecture (NMDA) (RFC 8342).


The IETF datatracker status page for this draft is:
https://datatracker.ietf.org/doc/draft-ietf-bfd-rfc9127-bis/

There is also an htmlized version available at:
https://datatracker.ietf.org/doc/html/draft-ietf-bfd-rfc9127-bis-00


Internet-Drafts are also available by rsync at rsync.ietf.org::internet-drafts


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