MQTT

MQTT is a machine-to-machine (M2M)/"Internet of Things" connectivity protocol. It was designed as an extremely lightweight publish/subscribe messaging transport. It is useful for connections with remote locations where a small code footprint is required and/or network bandwidth is at a premium. For example, it has been used in sensors communicating to a broker via satellite link, over occasional dial-up connections with healthcare providers, and in a range of home automation and small device scenarios. It is also ideal for mobile applications because of its small size, low power usage, minimised data packets, and efficient distribution of information to one or many receivers (more...)

Specification

http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc398718008

Server

Implementation notes of building a MQTT 3.1.1 compliant server.

CONNECT/CONNACK

Create super basic server that supports a client connection.

Create server application using a TCP/IP Socket listening to port 1883. When a connection is requested open the connection and wait for the first packet of data to be sent from the client. A timeout should be enabled to close the connection if no data is received.

Per the specification the first data received over the new connection must be a CONNECT Control Packet. The following is the simplest possible type of connection, using a new session, and no other connection flags, keep alive, or payload.

On any data received from a client, read and parse the bytes, it should be structured as follows.

CONNECT Control Packet

Byte Bits       Dec  Description
--------------------------------
0    00010000   16   Control Packet Type
1    00001010   10   Packet Length
2    00000000   0    Protocol Name Length MSB
3    00000100   4    Protocol Name Length LSB
4    01001101   M*   Protocol Name 1st Byte
5    01010001   Q*   Protocol Name 2nd Byte
6    01010100   T*   Protocol Name 3rd Byte
7    01010100   T*   Protocol Name 4th Byte
8    00000100   4    Protocol Level
9    00000010   2    Connect Flags
10   00000000   0    Keep Alive MSB
11   00000000   0    Keep Alive LSB

*UTF-8 equivalent of decimal value

The first 4-bits of the packet are the Control Packet Type, here it is "0001" (1) for a CONNECT, the following 4-bits are reserved and must be "0000".

The next byte defines the packet length, not including the header, which makes up the first two bytes of the packet. Here it is "00001010" in binary, or (10), which indicates there are 10 more bytes in the packet. The remaining number of bytes received should be checked and equal to this number to insure data was properly received.

The next two bytes indicate the length of the protocol name, in this case (4).

Following that is the Protocol Name as UTF-8 encoded bytes spelling "MQTT".

Next is the Protocol Level which is (4) for MQTT Version 3.1.1.

The next byte consists of six flags specifying in order the use of: Username, Password, Will Retention, Will QoS (as 2-bits) and Clean Session. The 8th and last bit is reserved and must be "0".

The last two bytes specify a keep alive time in seconds, when set to zero this functionality is not used.

Upon receiving and verifying a well formed CONNECT, the server must respond with a CONNACK Control Packet. If the packet isn't processed correctly the connection must be closed, otherwise a CONNACK can be returned with a refused error message followed by closing teh connection.

The following packet is sent from the server to the client as a CONNACK acknowledging a successful connection without error.

CONNACK Control Packet

Byte Bits       Dec  Description
--------------------------------
0    00100000   32   Control Packet Type
1    00000010   2    Packet Length
2    00000000   0    Connect Acknowledge Flags
3    00000000   0    Connect Response Code

The first 4-bits of the packet are the Control Packet Type, here it is "0010" (2) for a CONNACK, the following 4-bits are reserved and must be "0000".

The next byte defines the remaining length of the packet "00000010" (2), this will always be the case as there is no payload for this type.

The following byte is for flags, the fist 7-bits are reserved and always "0000000". The last bit indicates Session Presence, since this is a "clean" session, it will be (0).

The last byte is the response code, (0) indicates connection accepted with no errors. A value of (1-5) means it was refused, and values from (6-255) are reserved.

The client and server have now established a active session and information can be sent to and from client and server.