Find delegation cuts and restore the original query
02The published cold-cache sequence probes labels before asking for the requested mail records.
With org NS data already cached, Table 3 starts at A example.org. The root query is absent. This is a new starting state, not a sixth mandatory message after the answer.
Source for this drawing: RFC 9156 · Read RFC section 4 ↗
New to this? Start hereA quick picture + helpful background
A message needs an address.
Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.
Helpful first, if these ideas are unfamiliar
These pictures are analogies to help you learn. The linked RFC explains the actual rules.
Terms in this drawing
Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
The RFC’s Table 2 asks for MX a.b.example.org. and uses the selected minimising QTYPE A. A cold cache has no entries; a warm cache has some (§§1.2,4). Bootstrap root-server information is assumed available; this is not a diagram of learning root addresses.
- The RFC’s Table 2 asks for MX a.b.example.org. and uses the selected minimising QTYPE A. A cold cache has no entries; a warm cache has some (§§1.2,4). Bootstrap root-server information is assumed available; this is not a diagram of learning root addresses.
- First ask root for A org., then the org server for A example.org.; cache referral NS (name-server) record sets and restart from the closest known delegation (§3 steps1,6a). The example assumes these delegations and usable server addresses.
- The same example.org server receives A b.example.org. and A a.b.example.org. A dot does not prove a zone cut. Our chosen NOERROR/NODATA probe replies keep that server as the ancestor while adding labels (§§2.2,3 steps3–6c). NODATA means the requested record type is absent, not that the name or its descendants do not exist.
- Only after reaching the full name does this MX example send MX a.b.example.org. as the original query. The selected A probe cannot itself answer a different MX question. If the incoming type were A, the cached full-name A result could save a query; this is not a rule that every lookup needs five messages (§4).
- Table 3’s separate warm-cache run starts with the org delegation already known and omits the root query. Cache entries may also answer the original query immediately (§3 step 0). Cache presence, contents, expiry and extra name-server address lookups are not implemented here.
Documentation-domain examples. Suffixes, supplied response events and label budgets are bounded teaching models, not a DNS wire codec, validating resolver or live privacy measurement.
Read the RFC section ↗