CMDH and CoAP both define retry policies, which should be used?

Question

When CMDH processing is in use to retry communications between CSEs, and that communication link uses CoAP (which defines its own retry logic), which retry policy should be used: CMDH or CoAP?

Answer 1

CMDH processing - level retries happens on a higher layer than the transport protocol's retransmission. This is the same as with any transport protocol, e.g. with TCP/IP retransmissions. Those can actually be quite dynamic, depending also on various external factors (network defaults, congestion, etc).

CMDH retries are application-level settings and should be set to a higher value, but this depends on the use case, e.g. when does it actually still make sense to process an old event.