IOT-MQTT protocol - Control packets -PUBLISHREC

3.5 PUBREC - release has been received (already received QoS 2 release, Part 1)

PUBREC PUBLISH packet is a response to packet data having a QoS 2. It is a protocol for exchanging a second QoS 2 data packets.

3.5.1 Fixed Header fixed title

FIG 3.14 - PUBREC fixed packet header

Place	7	6	5	4	3	2	1	0
Byte 1	MQTT control packet type (5)				reserved
	0	1	0	1	0	0	0	0
Byte 2	The remaining length (2)
	0	0	0	0	0	0	1	0

The remaining length field

This is a variable length header. For PUBREC packet, a value of 2.

3.5.2 Variable Header variable head

Variable header contains a packet identifier being acknowledged PUBLISH package.

FIG 3.15 - PUBREC packet header variables

Place	7	6	5	4	3	2	1	0
Byte 1	MSB packet identifier
Byte 2	LSB packet identifier

3.5.3 Payload Payload

PUBREC packet no load.

3.5.4 Actions Action

This is described in detail in Section 4.3.3.