Subject: Optional elements?

From: "Joel M. Halpern" <[email-masked]>
Date: Sat, 12 Feb 2005 09:30:22 -0500

In revising the model document, I have been trying to capture items which 
were discussed, and tentatively agreed.  One such item is that LFB 
attributes and structure elements could be "optional".  That is, the LFB 
definition indicated which elements were required, and which elements not 
all FE were mandated to support.

This makes sense to me.

It does have the effect of complicating the encoding of structures (in what 
we have been calling a DATARAW.)  The receiver of an encoded structure 
needs to know what fields are present.  One could mandate that the sender 
and receiver agree (via an earlier exchange) on what fields are used.  But 
since this difference might be on a per instance level, that seems ineffective.
Thus, support for optional fields seems to mandate that the encoding of 
structures use a per-field TLV mechanism.
This is a small efficiency cost for a significant flexibility gain.

Confirmation from the working group that optional elements should be 
supported would be helpful.

Yours,
Joel M. Halpern