MQTT-SN TopicIdType flag

[AlemV]

I am a little bit confused with the TopicIdType flag.

Due to MQTT-SN Protocol Specification (5.3.4 Flags) TopicIdType means:
0x00 - normal topic id
0x01 - pre-defined topic id
0x02 - short topic name

Paragraph 3 tells that:
Predefined topic ids are also a two-byte long replacement of the topic name, their mapping to the topic names is however known in advance by both the client’s application and the gateway/server.

So imagine a situation where a sensor registers any topic with the REGISTER message and receives the REGACK message with the registered topic_id value. Then the sensor wants to subscribe and publish to this topic. What TopicIdType flag should I use in the SUBSCRIBE and PUBLISH messages?

PUBLISH has no TopicName field, so should I set TopicIdType to "normal" or "pre-defined" to use already received topic_id value?
SUBSCRIBE has "TopicName or TopicId" field, so should I set TopicIdType to "pre-defined" to use already received topic_id value(I don't want to use topic name string)?

And what means "known in advance by both the client’s application and the gateway/server"? Hard-coded or after the client sent the REGISTER message?

Thanks,
Vladimir Alemasov

[Seth Hoenig]

Once you register a topic, you receive a topic_id that will from then on be associated with it. Use that topic_id, along with topicIdType=0x00 (normal topic id) to publish/subscribe to that topic.

Short topic name can be used only if you are subscribing/publishing to a 2 byte long topic - and want to avoid the registration step. In that case you can set topicIdType=0x02 and use the 2 bytes reserved for the topic to be the actual topic string.

A predefined topic name is something hard coded into both your gateway and your client - there is no registration or dynamic configuration. There are also no predefined topic names mandated in the MQTT-SN spec, so anything you implement is up to you. To use one, set topicIdType=0x01. Most likely, you'll want to configure some predefined topic names for interesting topics in the $SYS/ tree (which many MQTT brokers support only by convention).

Seth

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[AlemV]

Once you register a topic, you receive a topic_id that will from then on be associated with it. Use that topic_id, along with topicIdType=0x00 (normal topic id) to publish/subscribe to that topic.

Ok, I can use topicIdType=0x00 (normal topic id) in the Publish message, but how to use it in the Subscribe message is not clear for me.

"5.4.15 SUBSCRIBE" tells:
TopicIdType: indicates the type of information included at the end of the message, namely “0b00” topic name, “0b01” pre-defined topic id, “0b10” short topic name, and “0b11” reserved.

There is no "normal topic id" option. When I choose topicIdType=0x00 I should use a topic name instead of the registered topic id.

 

Short topic name can be used only if you are subscribing/publishing to a 2 byte long topic - and want to avoid the registration step. In that case you can set topicIdType=0x02 and use the 2 bytes reserved for the topic to be the actual topic string.

A predefined topic name is something hard coded into both your gateway and your client - there is no registration or dynamic configuration. There are also no predefined topic names mandated in the MQTT-SN spec, so anything you implement is up to you. To use one, set topicIdType=0x01. Most likely, you'll want to configure some predefined topic names for interesting topics in the $SYS/ tree (which many MQTT brokers support only by convention).

Ok, thanks. It's clear for me.

Vladimir Alemasov

[Seth Hoenig]

I'm sorry, you are right. In the SUBSCRIBE (and UNSUBSCRIBE) case, you do not use REGISTER to attain a "normal topic id" association first. You simply set TopicIdType=0x00, and the full topic name you wish to subscribe to.

I believe the reasoning for this is to minimize the number of messages going across the network - it's an unusual case for a client to both publish and subscribe to the same topic, and using the REGISTER protocol just to set yourself up for a  SUBSCRIBE would just involve sending extra messages that aren't really necessary.

Seth

[AlemV]

Seth,

Thank you for your comments. They are very useful to me.

Vladimir