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 |
CoAP Request and Response Rules
原文：http://www.cnblogs.com/tifnan2015/p/5364839.html