On Tue, Nov 24, 2015 at 05:39:04AM -0500,
 Shumon Huque <shu...@gmail.com> wrote 
 a message of 234 lines which said:

> > That was exactly my point, and in that sense I'd say "SHOULD
> > delete" is redundant (and possibly imposes unnecessary
> > restrictions on implementations).
> 
> 
> Yes, I agree. The current description is a bit too implementation specific.

My concern is that some implementations may have a cache composed of a
tree structure *plus* a hashed index for speed. When receiving a
query whose answer is in the cache, such implementation may not
perform a "downward search" in the cache.

May be something like: "After the reception of a NXDOMAIN answer for a
given name, the resolver SHOULD/MUST? reply NXDOMAIN for every name
under the denied name." (There are details, such as TTL and such as
RFC 6604, see the draft for these.)

That way, we just specify a behaviour, with zero implementation
detail.

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