Implement the MQTT Protocol for IoT Messaging

Topic Overview

Fixed Header

MQTT runs over TCP port 1883 (or 8883 with TLS). Every packet starts with a fixed header of 2-5 bytes. For example, a CONNECT packet has type nibble 1 in the high bits of byte 0, plus a variable-length "remaining length" field encoding how many bytes follow.

Packet Types

Common control packet types:

• CONNECT (1): client requests connection, carries protocol name "MQTT" and level 4 for v3.1.1
• CONNACK (2): server acknowledges
• PUBLISH (3): message to a topic like sensors/temp
• SUBSCRIBE (8): client requests topic filters with QoS levels

The variable header adds fields like packet identifier, connect flags, and keep-alive interval (e.g. 60 seconds). The payload carries client ID, credentials, topic names, and message data.

Remaining Length Encoding

Each byte uses 7 data bits plus a continuation bit. Value 321 encodes as two bytes: 0xC1 0x02.

Why This Exercise

This task requires you to build MQTT CONNECT and PUBLISH packets. Mosquitto and AWS IoT Core parse this exact fixed-header format from millions of battery-powered sensors where minimal framing overhead matters on cellular links.

QoS levels add delivery guarantees: 0 is fire-and-forget, 1 is at-least-once with PUBACK, 2 is exactly-once with a four-step handshake.

Write a C program that encodes an MQTT 3.1.1 PUBLISH packet (spec section 3.3) from parameters read on stdin.

Input (whitespace/newline separated):

1. qos: 0, 1, or 2
2. dup: 0 or 1 (DUP flag, only meaningful for QoS > 0)
3. retain: 0 or 1 (RETAIN flag)
4. topic: the topic name (no spaces)
5. payload: the application message: all remaining characters up to the end of its line (may contain spaces); empty line means empty payload

Output:

• Line 1: fixed_header= the fixed header byte(s) in hex: PUBLISH type 3 in the high nibble, then DUP/QoS/RETAIN flags in the low nibble (bit3=DUP, bits2-1=QoS, bit0=RETAIN), followed by the remaining-length varint bytes in hex. MQTT's variable-length encoding: 7 data bits per byte, least-significant-group first, continuation bit 0x80 set on every byte except the last (e.g. 321 encodes as C1 02)
• Line 2: remaining_length=N bytes=K where N is the remaining length in bytes, 2-byte big-endian topic length + topic + (2-byte packet identifier only when QoS > 0) + payload, and K is how many varint bytes encode N (1 to 4)
• Line 3: topic_len=N: topic length in bytes

No sockets needed: this is pure packet-encoding arithmetic. MQTT 3.1.1 spec section 2.2.3 defines the remaining-length encoding.