When using NKP, resource representations may be cached at both the server and client side of a connection if the expiry metadata associated with a response permits and each cache is managed independently.
Expiry metadata is transmitted transparently with a response over NKP. Expiries of ALWAYS, NEVER and CONSTANT can be sent without need for a remoted expiry function but all other expiration methods require one. A remoted expiry function involves making a call back to the server to check if a representation's validity has expired. In NKP2 expiry functions are requested asynchronously - this means your application will never block due to network latencies.
Because it may be expensive to constantly poll a server for expiry status of every response it has served, especially
for geographically remote servers a compromise must be made. Servers may specify a minimum poll period in
which to check remote expiries. This limits checks to a maximum of once every so many milliseconds. Servers
can specify a global