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[802.3_YANG] YANGsters 14 Jan 2020 Agenda

Agenda for the YANGsters call tomorrow.

The text for the List Size Node discussion is included below.


14 Jan 2020

    • Review List Size Node note to go to YANG Doctors
    • IEEE YANG Structure
      • Review Structure Notes from 7 Jan meeting
      • Discussion
      • Plan for the discussion at the face-to-face
    • Information about Workshop on 25 Jan
    • Status of Sponsor Ballots
    • Status of YANG Documents
    • AoB
  • Web conference:
  • Phone numbers:
  • By phone:
  • United States – San Francisco, CA   +1.415.594.5500
  • Canada – Ottawa   +1.613.699.9318
  • Germany – Frankfurt   +49.69.9753.3131
  • Hungary – Budapest   +36.1.700.8899
  • China – Beijing +86.10.8783.3389
  • United Kingdom – National   +44.33.0088.2634
  • Access Code   684-645-640#


<list node discussion>

The YANG people from IEEE 802.1 discovered a symmetric modelling issue related

to YANG lists, and kindly request for feedback about the common practices in

YANG modules beyond those from IEEE 802.1.

Consider the following YANG code example:

01: leaf my-list-size {

02:   type uint32;

03:   config false;

04:   description “The number of elements in ‘my-list’”;

06: }

07: leaf-list my-list {

08:   type my-list-element-type;

09: }


The state value of leaf my-list-size (L01) needs to be consistent with the

number of elements in leaf-list my-list (L07). Therefore, IEEE 802.1

identified the following options:

A)           Delete the size leaf (i.e., my-list-size).

B)            Formulate consistency requirements in IEEE standards documents

C)            Formulate consistency requirements in YANG description nodes (e.g., L04).

D)           Formulate consistency requirements in YANG must statement(s)


The rationale of option A) is that a leaf-list implicitly provides the number of

contained elements. NETCONF <get-config> or <get> operations can be used to

retrieve all list elements content from a server, and thus the number of

elements is also known to the client. A redundant size leaf is entirely avoided.

The main concern in this approach is the associated amount of data to be

transferred from server to client. As a practical example, a client requiring

the number of entries in a Filtering Database (FDB) entries would read

potentially thousands of entries (IEEE 802.1Q does not specify a limit here).

One might argue that clients don’t have to know the number of elements in the

FDB. However, the question whether there is a need or not is not the point, it’s

just an example, and there are several other, less commonly known, lists

specified by IEEE 802.1 standards for which monitoring the size information is

inevitable in practical use. Coexisting size leafs like leaf my-list-size

appears more efficient for this purpose.


As soon as a size leafs for list elements (or specific subsets, such as FDB

entries of a particular type) exist, it is required to ensure consistency

amongst a size leaf and the associated list in an unambiguous manner. Network

equipment specified by IEEE 802(.1) standards is often configured by machines.

For example, a centralized controller device might compute and upload

network-consistent configurations to all devices in the network. As a result,

configuration of such networks is lacking human intelligence – unambiguous and

deterministic behavior is required, and thus it is also required that the

relationship between lists and associated list size nodes is specified cleanly.


Considering option B), such specification can (and is in many cases) be located

in IEEE standards documents for the associated managed objects. However, there

are significant concerns from IEEE 802.1 that implementers might accidentally

oversee such statements (e.g., IEEE 802.1Q-2018 is a document with more than

2000 pages). It is better to locate such information in YANG.


Option C) sketches such a specification in the YANG code in a human readable

manner,  but it has some drawbacks. First, unambiguous normative verbal

formulation can become long, YANG files become large, such strong normative

specification is not helpful for client-side users and thus make the

descriptions more heavy-weight. Second, machines (e.g., YANG compilers) cannot

interpret this specification.


Option D) appears to address these drawbacks: must statements allow machine

readable XPathexpressions (and also readable by humans). Moreover, must

statements are separated from information in description nodes relevant for

client-side users. However, IEEE 802.1 is not fully aware of the feasibility,

limitations, and implications of this option. IEEE 802.1 could not discover such

a use of must statements in IETF’s published YANG files, such that feedback from

other SDOs on this option would be highly appreciated.

</list node discussion>

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