add

doc/design/ncache

@@ -0,0 +1,34 @@
+
+Negative Caching
+
+
+The non-DNSSEC case is pretty easy.
+
+	foundname = soa name
+	rdataset = soa
+	node = NULL
+
+DNSSEC complicates things a lot, because we have to return one or more NXT
+records (if we have them) as proof.  Another tricky bit here is that we may
+have an NXT record so we know the answer is NODATA, but we don't have the SOA
+so we can't make a NODATA response that a non-DNSSEC-aware server could
+cache.  Life would sure be easier if we knew if the client understood DNSSEC.
+Not sure what to do in this case.  Probably return delegation to force client
+to ask authority.
+
+
+Perhaps we should just create some kind of meta-rdata, the "negative cache 
+rdata type"?
+
+Or maybe something like:
+
+dns_rdataset_ncachefirst()
+dns_rdataset_ncachenext()
+dns_rdataset_ncachecurrent()
+
+dns_db_ncachenew(db, type)		/* type can be any */
+dns_db_ncachesoa(name, rdataset)
+dns_db_ncachenxt(name, rdataset)
+dns_db_ncacheadd(db, name, version)
+
+Ick.  I favor the former.