4. Operational Overview

A router establishes and keeps open a connection to one or more caches with which it has client/server relationships. It is configured with a semi-ordered list of caches and establishes a connection to the most preferred cache, or set of caches, which accept the connections.

The router MUST choose the most preferred, by configuration, cache or set of caches so that the operator may control load on their caches and the Global RPKI.

Periodically, the router sends to the cache the most recent Serial Number for which it has received data from that cache, i.e., the router's current Serial Number, in the form of a Serial Query. When a router establishes a new session with a cache or wishes to reset a current relationship, it sends a Reset Query.

The cache responds to the Serial Query with all data changes which took place since the given Serial Number. This may be the null set, in which case the End of Data PDU (Section 5.8) is still sent. Note that the Serial Number comparison used to determine "since the given Serial Number" MUST take wrap-around into account; see [RFC1982].

When the router has received all data records from the cache, it sets its current Serial Number to that of the Serial Number in the received End of Data PDU.

When the cache updates its database, it sends a Notify PDU to every currently connected router. This is a hint that now would be a good time for the router to poll for an update, but it is only a hint. The protocol requires the router to poll for updates periodically in any case.

Strictly speaking, a router could track a cache simply by asking for a complete data set every time it updates, but this would be very inefficient. The Serial-Number-based incremental update mechanism allows an efficient transfer of just the data records which have changed since the last update. As with any update protocol based on incremental transfers, the router must be prepared to fall back to a full transfer if for any reason the cache is unable to provide the necessary incremental data. Unlike some incremental transfer protocols, this protocol requires the router to make an explicit request to start the fallback process; this is deliberate, as the cache has no way of knowing whether the router has also established sessions with other caches that may be able to provide better service.

As a cache server must evaluate certificates and ROAs (Route Origin Authorizations; see [RFC6480]), which are time dependent, servers' clocks MUST be correct to a tolerance of approximately an hour.