Appendix 6.B Limits
| Bound | Mainline | Defined in |
|---|---|---|
| Message ceiling, native channel | 65 536 bytes | §6.4 |
| Request delivery bound, native channel | 5 s | §6.4 |
| Per-server timeout | 2 s | §6.7 |
| Attempts per question | 3 | §6.7 |
| Demotion period | 30 s | §6.7 |
| Positive TTL cap | 86 400 s | §6.7 |
| Negative TTL cap | 300 s | §6.7 |
| EDNS0 buffer advertised | 1 232 bytes | §6.7 |
| Upstream transactions in flight (synthetic and cached answers are never refused) | 4 096 | §6.7 |
| Native connections delivering a request or awaiting resolution | 256 | §6.4 |
| Stub TCP clients delivering a query or awaiting resolution | 256 | §6.8 |
| Stub UDP queries awaiting resolution | 256 total; 16 per source address and port | §6.8 |
| Cache entries | 8 192 | §6.7 |
| Stub TCP client bound | 10 s | §6.8 |
| Manager reconnect backoff | 0.5 s doubling to 10 s | §6.9 |
The upstream ceiling leaves synthetic and cached answers unaffected once a query has passed its door's admission limits. Native and stub TCP reply writers have separate 256-frame queues and one-second write deadlines.
An implementation MAY adjust these; it MUST document the values it
uses. The relations that must hold: the shim's own timeout exceeds
attempts × per-server timeout, so a slow upstream is reported by the
resolver as unavailable rather than by the shim as a timeout.