Requirements. A feature to return negative acknowledgements (error codes) is under discussion in MQTT-236.

Transcription

1 Requirements 1. A Server MUST advertise its Maximum QoS to the Client during the connection process. 2. Clients SHOULD NOT publish messages with a QoS that exceeds the advertised Maximum QoS. 3. A Server MUST return an error if a Client publishes a message with a QoS that exceeds its Maximum QoS. A feature to return negative acknowledgements (error codes) is under discussion in MQTT-236.

2 Requirement 1: A Server MUST advertise its Maximum QoS The Server MUST return its Maximum QoS in the CONNACK fixed header flags. Modifications to Flags Table Flag Bits Control Packet Fixed header flags Bit 3 Bit 2 Bit 1 Bit 0 CONNECT Reserved CONNACK Used in MQTT QoS 1 QoS 1 0 PUBLISH Used in MQTT DUP 2 QoS 3 QoS 3 RETAIN 4 QoS 1 = Maximum Quality of Service advertised by the server Note: Using Bit 2-Bit 1 does fragment the 4 flag bits in CONNACK but is symmetrical with PUBLISH QoS bits. Modifications to CONNACK Fixed Header Figure 3.8 CONNACK Packet fixed header byte 1 MQTT Control Packet Type (2) Reserved QoS Maximum Reserved byte 2 Remaining Length (2) Table (TBD) Maximum QoS definitions Maximum QoS value Bit 2 bit 1 Description At most once delivery At least once delivery Exactly once delivery Reserved must not be used A CONNACK Packet MUST NOT have both Maximum QoS bits set to 1. If a Client receives a CONNACK Packet which has both Maximum QoS bits set to 1 it MUST close the Network Connection

3 Issue with Will QoS in Connect Flags Figure Connect Flag bits User Name Flag Password Flag Will Retain Will QoS Will Flag Clean Session Reserved byte 8 X X X X X X X 0 Problem: The Client cannot discover the Maximum QoS level until it receives the CONNACK; therefore, the Will QoS in CONNECT may exceed the Maximum QoS. Solution: The Will QoS MUST be reduced if it exceeds the Maximum QoS advertised by the Server in its CONNACK response. Other alternatives: Add a new return code in the CONNACK variable header, fail the operation, and close the Network Connection. Modifications to Connect Return code in variable header Table 3.1 Connect Return code values Value Return Code Response Description 0 0x00 Connection Accepted Connection accepted 6 0x06 Connection Refused, exceeds QoS The Server does not support the level of Will QoS requested by the Client Reserved for future use

4 Requirement 2: Clients SHOULD NOT publish messages with a QoS that exceeds the advertised Maximum QoS Modifications to Response Problem: The client may send a PUBLISH which exceeds the Maximum QoS before it receives the CONNACK: Clients are allowed to send further Control Packets immediately after sending a CONNECT Packet; Clients need not wait for a CONNACK Packet to arrive from the Server Solution: Clients SHOULD NOT send further Control Packets immediately after sending a CONNECT Packet; Clients SHOULD wait for a CONNACK Packet to arrive from the Server Other alternatives: The current behavior could be allowed, but the Server may fail the PUBLISH operation and return an error code. See Requirement 3.

5 Requirement 3: A Server MUST return an error if a Client publishes a message with a QoS that exceeds its maximum QoS. Problem: A Client publishes a message with a QoS that exceeds the Maximum QoS advertised by Server. Solution: Add a PUBLISH return code to the PUBACK and PUBREC variable headers and fail the operation. The return codes are modeled on the CONNECT return codes in the CONNACK variable header. The client SHOULD retry the operation with an appropriate QoS. Modifications to QoS A PUBLISH Packet SHOULD NOT exceed the Maximum QoS advertised by the server. If a Server receives a PUBLISH packet from a Client where the QoS bits exceed the advertised Maximum QoS, it MUST fail the operation and return an error. Modifications to PUBACK Variable header Figure 3.13 PUBACK Packet variable header byte 1 byte 2 byte 3 Packet Identifier MSB Packet Identifier LSB PUBLISH return code Table (TBD) PUBACK Return code values Value Return Code Response Description 0 0x00 Publish Accepted Publish accepted 1 0x01 Publish Refused, exceeds QoS The Server does not support the QoS level requested by the Client Reserved for future use

6 Modifications to PUBREC Variable header Figure 3.15 PUBREC Packet variable header byte 1 byte 2 byte 3 Packet Identifier MSB Packet Identifier LSB PUBLISH return code Table (TBD) PUBREC Return code values Value Return Code Response Description 0 0x00 Publish Accepted Publish accepted 1 0x01 Publish Refused, exceeds QoS The Server does not support the QoS level requested by the Client Reserved for future use