CoAP Request and Response Rules

CoAP specification (at the time of this writing, draft-18) does not clearly set out rules that define what would be a response for a given CON or a NON request. Embedded within the specification are various rules that are outlined. While many rules are still open and not clearly defined, the table below is a guidance that we have started following. Please note, this should be treated only as a guidance and not as something that is clearly defined by the CoAP Specifications.

Client Sends	Message Successfully Parsed and Understood By Server	Server Has all Information to Process the Request and can Successfully Process	Server Sends Message Type	Server Sends Message Code	Remarks
CON	YES	YES	ACK	One of success response codes	Happy day scenario
CON	YES	NO	ACK	One of failed response codes	URI path in request is wrong and 4.04 not found is sent in ACK
CON	NO	NO	RST	One of failed response codes	e.g. Unknown message code
NON	YES	YES			No response sent back
NON	YES	YES	NON	One of success response codes	Response sent back as NON message
NON	YES	NO	RST	One of failed response codes	URI path in request is wrong and 4.04 not found is sent in RST
NON	NO	NO	RST	One of failed response codes	e.g. Unknown message code
NON	YES	YES	CON	One of request codes	e.g. Previous NON request was for a data that requires confirmation from the sender on whether it reached the client or not