Discriminated unions in MIBs

["Adrian Farrel" <adrian@olddog.co.uk>]

Hi MIB experts,

I am wrestling with the issue of discriminated unions in the GMPLS LSR MIB (latest
published version
http://www.ietf.org/internet-drafts/draft-ietf-ccamp-gmpls-lsr-mib-03.txt)

There is reasonable precedent for simple unions in RFC3291 with
InetAddress/InetAddressType.

What we currently have in the label table (gmplsLabelTable) is a complex union.

   GmplsLabelEntry ::= SEQUENCE {
     gmplsLabelInterface           InterfaceIndexOrZero,
     gmplsLabelIndex               Unsigned32,
     gmplsLabelSubindex            Unsigned32,
     gmplsLabelType                GmplsGeneralizedLabelTypes,
     gmplsLabel                    GmplsLabel,
     gmplsLabelMplsLabel           MplsLabel,
     gmplsLabelPortWavelength      Unsigned32,
     gmplsLabelFreeformLength      Integer32,
     gmplsLabelFreeform            GmplsFreeformLabel,
     gmplsLabelSonetSdhSignalIndex Integer32,
     gmplsLabelSdhVc               Integer32,
     gmplsLabelSdhVcBranch         Integer32,
     gmplsLabelSonetSdhBranch      Integer32,
     gmplsLabelSonetSdhGroupBranch Integer32,
     gmplsLabelWavebandId          Unsigned32,
     gmplsLabelWavebandStart       Unsigned32,
     gmplsLabelWavebandEnd         Unsigned32,
     gmplsLabelRowStatus           RowStatus,
     gmplsLabelStorageType         StorageType
   }

In 'C' the row entry might be written as...

typdef struct
{
  InterfaceIndexOrZero       gmplsLabelInterface;
  Unsigned32                 gmplsLabelIndex;
  Unsigned32                 gmplsLabelSubindex;
  GmplsGeneralizedLabelTypes gmplsLabelType;
  union
  {
    MplsLabel                gmplsLabelMplsLabel;
    Unsigned32               gmplsLabelPortWavelength;
    struct
    {
      Integer32              gmplsLabelFreeformLength;
      GmplsFreeformLabel     gmplsLabelFreeform;
    } freeForm;
    struct
    {
      Integer32              gmplsLabelSonetSdhSignalIndex;
      Integer32              gmplsLabelSdhVc;
      Integer32              gmplsLabelSdhVcBranch;
      Integer32              gmplsLabelSonetSdhBranch;
      Integer32              gmplsLabelSonetSdhGroupBranch;
    } sonetSdh;
    struct
    {
      Unsigned32             gmplsLabelWavebandId;
      Unsigned32             gmplsLabelWavebandStart;
      Unsigned32             gmplsLabelWavebandEnd;
    } waveband;
  } label;
  RowStatus                  gmplsLabelRowStatus;
  StorageType                gmplsLabelStorageType;
} gmplsLabelEntry;


(Ignore the fact that there is no requirement for gmplsLabelFreeformLength given that
gmplsLabelFreeform is an OCTET STRING.)

I could compress the whole into an OCTET STRING and give guidance about interpretation.
This is what the signaling protocol does, but I don't want to do this because I want it to
be simpler to read or write (for example) gmplsLabelSonetSdhBranch.

I could follow the example of InetAddress and define a set of syntaxes for each member of
the union. This is fine for the simple members, but for the more complex members I would
have to give DISPALY HINTs and interpretation clauses in the DESCRIPTION. And it would
still be impossible to read or write a single component.

I could leave the table entry as currently defined. This is most readily accessible, but
it leaves a lot of unused objects in any logical row.

Do you have any advice.

Thanks,
Adrian