RE: [rohc] FN: default_methods and control fields clarification/n ew usage

["Lars-Erik Jonsson \(LU/EAB\)" <lars-erik.jonsson at ericsson.com>]

> Here are my current thoughts:
> 
> * It seems like a useful thing to be able to indicate the need for
> context, but I don't understand why we are only interested in doing it
> for fields which don't appear in the uncompressed header. Is 
> there some motivation that I'm missing? What problem are we solving
> here that doesn't need solving for fields which reside in the 
> uncompressed header?

To me it looks really useful to make this distinction. Fields that 
are "created" and used only on a per-packet basis do not require an
explicit control-field declaration, but would be declared as part of
the compressed header, with a let declaration that creates the
mapping between the created field and original uncompressed field(s).

However, control fields are different because they are not
necessarily created directly out of uncompressed fields, at least not
on a per-packet basis, and they do require context. This is why we
agreed they should be explicitly declared as "control fields".

I see three kind of fields, original fields, per-packet created fields,
and context-requiring created fields (control-fields).

 
> * It strikes me that there are quite a few different types of field,
> (e.g. "inferred" fields), which we are currently not being explicit
> about, either in the FN or in the TCP draft. The notation 
> (and hence the TCP draft) would be easier to read if we were
> explicit about these things. At present we are disguising them as
> other things, which is confusing (to me at least). To bring this
> discussion full circle one such example is a non-default statement
> appearing in a default list.

I see what you mean, but this also worries me, as I think we would
open up the classic can of worms if we start doing these things. We
are *very* close to finalizing the FN, and I do not want to run
another round of redefining things that we have actually agreed are
ok. Just sort out the control-fields and default-methods stuff, and
then let's try to close this document once and for all.

/L-E

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