"There are three kinds of zones;
treating these zones differently may allow lookup protocol
optimizations:
Zones containing only delegation assertions are delegation-only zones.
Delegation-only zones are not relevant as part of an assertion lookup, other
than for discovering and verifying authority. Top-level domains are
generally delegation-only.
Zones containing no delegation assertions are final zones. Final zones are
not relevant as part of an authority discovery.
Zones containing at least one delegation assertion and at least one
assertion that is not a delegation assertion are mixed zones. No
optimizations are available for mixed zones."
For bootstrapping and recursive lookup to work, a zone needs to have a delegation, ip and redirect assertions. Thus, the first kind of zone cannot exist. The final one cannot be used to optimize. For the second one, there is no guarantee that it will not start delegating in the future. Therefore, a querier first has to make sure that the zone is still final in which case he could have sent directly a delegation query.
Remove this part?
"There are three kinds of zones; treating these zones differently may allow lookup protocol optimizations:
For bootstrapping and recursive lookup to work, a zone needs to have a delegation, ip and redirect assertions. Thus, the first kind of zone cannot exist. The final one cannot be used to optimize. For the second one, there is no guarantee that it will not start delegating in the future. Therefore, a querier first has to make sure that the zone is still final in which case he could have sent directly a delegation query.