On Thu, Jan 25, 2001, David S. Miller <[EMAIL PROTECTED]> wrote:
>
> H. Peter Anvin writes:
> > > RFC793, where is lists the unused flag bits as "reserved".
> > > That is pretty clear to me. It just has to say that
> > > they are reserved, and that is what it does.
> > >
> >
> > Is the definition of "reserved" defined anywhere? In a lot of specs,
> > "reserved" means MBZ.
> >
> > Note, that I'm not arguing with you. I'm trying to pick this apart.
>
> It says "reserved for future use, must be zero".
>
> I think the descrepency (and thus what the firewalls are doing) comes
> from the ambiguous "must be zero". I cannot fathom the RFC authors
> meaning this to be anything other than "must be set to zero by current
> implementations" or else what is the purpose of the "reserved for
> future use part" right?
>
> Honestly, is there anyone here who can tell me honestly that when they
> see the words "reserved" in the description of a bit field description
> (in a hardware programmers manual of some device, for example) that
> they think it's ok the read the value and interpret it in any way?
>
> To me it's always meant "we want to do cool things in the future,
> things we haven't thought of now, so don't interpret these bits so we
> can do that and you will still work".
Generally it's to ensure that all implementations set those bit by
default to 0 as well.
Then in the future, 0 means "I don't support this feature either by
choice or by not implementing it yet".
JE
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