MQTT has a type of packet used to simulate heartbeats between the client and server to maintain the connection. They are PINGREQ and PINGRESP packets, which we often call heartbeat packets.
In the previous article, we introduced the PINGREQ and PINGRESP packets in MQTT 5.0. Now, we will introduce the control packet used when disconnecting: DISCONNECT. In MQTT, both the client and the server can send a DISCONNECT packet to the other party before disconnecting the network ...
the tcp send buffer maybe full before any PINGREQ is sent, so ping_due will always 1, so this code will never run in else if (client->ping_due == 0) { Log(TRACE_PROTOCOL, -1, "Couldn't send PINGREQ for client %s on socket %d, noting", client->clientID, client->net.socket)...
MQTT is a binary based protocol were the control elements arebinary bytesand nottext strings. MQTT uses acommandandcommand acknowledgementformat. That means each command has an associated acknowledgement. Topic names, Client ID, User names and Passwords are encoded asUTF-8 strings. ThePayloadexcludin...
It is possible for a Client to send a PUBLISH packet with Packet Identifier 0x1234 and then receive a different PUBLISH packet with Packet Identifier 0x1234 from its Server before it receives a PUBACK for the PUBLISH packet that it sent. ...