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For specific MESSAGE example, refer to SMS over IMS page


According to RFC 3428


Section 4. UAC Processing

  • MESSAGE requests do not initiate dialogs.  User Agents MUST NOT insert Contact header fields into MESSAGE requests
  • If the UAC receives a 200 OK response to a MESSAGE request, it may assume the message has been delivered to the final destination.  It MUST NOT assume that the recipient has actually read the instant message.  If the UAC receives a 202 Accepted response, the message has been delivered to a gateway, store and forward server, or some other service that may eventually deliver the message.  In this case,  the UAC MUST NOT assume the message has been delivered to the final destination.  If confirmation of delivery is required for a message that has been responded to with a 202 Accepted, that confirmation  must be delivered via some other mechanism


Section 7. UAS Processing

  • A UAS receiving a MESSAGE request SHOULD respond with a final response immediately. Note, however, that the UAS is not obliged to display the message to the user either before or after responding with a 200 OK. That is, a 200 OK response does not necessarily mean the user has read the message.
  • A 2xx response to a MESSAGE request MUST NOT contain a body.
  • A UAS MUST NOT insert a Contact header field into a 2xx response. A UAS which is, in fact, a message relay, storing the message and forwarding it later on, or forwarding it into a non-SIP domain, SHOULD return a 202 (Accepted) response indicating that the message was accepted, but end to end delivery has not been guaranteed.
  • A 4xx or 5xx response indicates that the message was not delivered successfully. A 6xx response means it was delivered successfully, but refused.