关于MQTT 3.1.1连接报文Keep Alive字节格式的技术问询

Let's break this down clearly—your confusion about translating time intervals to the 16-bit Keep Alive field is totally reasonable, since binary byte splitting can feel opaque at first.

Core Background on the Keep Alive Field #

First, remember: the Keep Alive value is a 16-bit unsigned integer stored in big-endian format (most significant byte, or MSB, comes first). To get the byte structure for any interval, you just need to split the total number of seconds into two 8-bit segments: the high byte (MSB) and low byte (LSB).

How to Represent a 10-Minute Interval #

10 minutes equals 600 seconds. Here's how to convert that to the required byte structure:

1. Convert 600 to hexadecimal: 600 = 0x258
2. Split this into the high and low 8-bit parts:
   • MSB (first byte): 0x02 → binary 00000010
   • LSB (second byte): 0x58 → binary 01011000

So your Keep Alive bytes would look like this:

• Keep Alive MSB: 0 0 0 0 0 0 1 0
• Keep Alive LSB: 0 1 0 1 1 0 0 0

Confirming the Maximum Keep Alive Interval #

You’re spot-on about the maximum value! 65535 is the largest number a 16-bit unsigned integer can hold (since 2^16 - 1 = 65535). This translates to 16 consecutive 1s in binary, which splits into two 8-bit bytes of all 1s:

• Keep Alive MSB: 1 1 1 1 1 1 1 1 (decimal 255)
• Keep Alive LSB: 1 1 1 1 1 1 1 1 (decimal 255)

As you noted, this equals roughly 18 hours (65535 ÷ 3600 ≈ 18.2), which aligns perfectly with the MQTT 3.1.1 specification.

内容的提问来源于stack exchange，提问作者Boris Deletic