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I am very new to this mqtt-sn stuff. So my question is - how does a mqtt-sn message look like. So I mean the raw data format. I do not clearly understand what octet means. So as I understand an octet is a byte.

So is the data transferred binary?

Or what means octet exact?

Please can someone give me a sample message. It is really bad that there is not example message in the specification.

Thanks, Mathias

mgiaco
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2 Answers2

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An octet is just a collection of 8 bit so just another name for a byte

So a message consists of a header made up of 2 or 4 bytes and the the message body.

The header is sub divided into 1 OR 3 bytes for the length and 1 byte for the type. If the first byte is 0x01 then the next 2 bytes are the length, else the value of the first byte is the length.

The next byte is a type, a table of valid message types can be found in sections 5.2.2 in the spec

The Message body varies depending on the type.

But to publish a message with payload HelloWorld on Topic ID of AB (0x41, 0x42) would look something like this:

0x0F  - length (15 bytes total, including length)
0x0C  - msg type (publish)
0x02  - flags (QOS 0, topic name)
0x41  - topic ID 1
0x42  - topic ID 2
0x00  - MsgID (00 for QOS 0)
0x48  - H
0x65  - e
0x6C  - l
0x6C  - l
0x6F  - o
0x57  - W
0x6F  - o
0x72  - r
0x6C  - l
0x64  - d

Where the topic id is the output from a topic register message (Section 6.5 in the spec)

hardillb
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An octet is a byte.

There isn't an example payload because the spec doesn't dictate a payload format. You can use whatever you want.

ralight
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