[Teas] Yangdoctors early review of draft-ietf-teas-yang-te-types-01

Jan Lindblad <janl@tail-f.com> Tue, 30 October 2018 13:56 UTC

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Subject: [Teas] Yangdoctors early review of draft-ietf-teas-yang-te-types-01
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Reviewer: Jan Lindblad
Review result: Ready with Issues

This is my YANG Doctor early review of draft-ietf-teas-yang-te-types, or more
specifically ietf-te-types.yang . I find the module ready with a couple of
issues. I have also noted a few nits. Generally speaking, I'm not sure if
reviewing a -types module with plenty of groupings on its own is the best way
to go about it. The groupings defined here may be used in good or not so good
ways later.

General questions:

#1: There are many locations in the YANG talking about an "ERO subobject index"
(and once RRO index, record route subobject). What is this, and how is it
supposed to be used? The document is silent on this matter, and I have seen
modules with problems around numeric index leafs much like this earlier. Are
these numbers stable, i.e. remains the same forever?

#2: There are few leafs (5) with default values given, and none with mandatory.
Probably needs to increase before we get to last call.

#3: There are several choices in the module that are meant to be augmented with
additional cases. In many instances, this is explicitly spelled out, very good.
If this is meant to happen in all choices, it would be nice to point this out
in every instance. Also, if there are any specific assumptions or
considerations to keep in mind when augmenting in a new technology, please note
that in the description as well.

Issues and nits:

#4: Unclear data type
419:
  typedef admin-group {
    type binary {
      length 4;

What is the format of this binary? If this is always a 4-byte binary, wouldn't
a numeric type be more user friendly, e.g. uint32?

#5: identifier in container with optional leafs
1496:
  grouping te-topology-identifier {

The name suggests this is used as an identifier, but all the leafs are
optional. This is not typical. They are also in a container, precluding them
from being used as list keys. Is that as intended?

#6: Optional -id leafs again
1700:
          leaf node-id {
          leaf link-tp-id {
1768:
        leaf address {
1783:
        leaf node-id {
        leaf link-tp-id {

Leafs that appear to be used as identifiers are optional

#7: binary length in bits?

1731:
           leaf as-number {
            type binary {
              length 16;
1773:
        leaf ip-flags {
          type binary {
            length 8;
1805:
          leaf label-flags {
            type binary {
              length 8;

It appears to me the modeler might have thought the length is given in bits.
The value of length is in bytes, however.

#8: Must expression copy paste
1852:
    container label-end {
      must "not(../label-end/te-label/direction) or "
        + "not(te-label/direction) "
        + "or ../label-end/te-label/direction = te-label/direction" {

This must expression appears to have been copied from label-start. In any case,
it always evaluates to true and has no effect.

#9: Unclear bit field
1885:
    leaf range-bitmap {
      type binary;
      description
        "When there are gaps between label-start and label-end,
         this attribute is used to specify the positions
         of the used labels.";
    }

Need more information on how to interpret this leaf. Which bits map to what,
and what does the bit field values 0 and 1 indicate?

#10: Canonical representation
67:
  typedef te-bandwidth {

The type is based on a string with a pattern allowing hex characters and an
upper or lowercase P. Since the pattern allows multiple representations of the
same underlaying value (0x1p10 presumably means the same as 0x1p0xa and
0x1P0XA) the question comes up if there is a canonical representation of this
value, e.g. using all lowercase and all hex, or if the string must be
remembered exactly as given by the client. The description could answer this
question.

#11: Mix of upper and lowercase
The module specifies many enumeration and identity values. Some are all
lowercase. Some are all uppercase. The principle of least astonishment suggests
to pick one and stick with it. YANG recommendations suggest to use all
lowercase when in doubt.

  typedef te-link-direction {
  typedef te-label-direction {
  typedef te-hop-type {
  identity LSP_METRIC_TYPE {
  identity LSP_METRIC_RELATIVE {
  identity LSP_METRIC_ABSOLUTE {
  identity LSP_METRIC_INHERITED {

Best Regards,
/jan