Moving MSR from a QNAP container to RP 5 - some issues

[Crille]

"The receiver of a PUBLISH Packet MUST respond according to Table 3.4 - Expected Publish Packet response as determined by the QoS in the PUBLISH Packet."

So even though the server only supports QoS 0 it's obligated to send a PUBACK on a QoS 1 level packet as described in table 3.4, correct?

[toggledbits]

Yes, PUBACK for QoS 1, and PUBREC for QoS 2. I don't think that would be a big problem for @akbooer when he gets the time, because all of the information contained in the response can be sourced from the request (i.e. topic, packet identifier, etc.). And otherwise the treatment of the PUBLISH packet can be the same (no further changes beyond sending those ACKs). Not actually having guaranteed delivery behind that is, in my view, an acceptable variance. On the sending side (repeat to subscribers), even though a subscriber may request QoS 1 or 2 for packets from the broker, it still must accept QoS 0 packets (because the requested QoS is a maximum, not an absolute), so everything outbound at QoS 0 isn't likely going to cause problems, especially in this world.

To summarize: if he just provides the PUBACK and PUBREC responses to fix that layer of the protocol, that's good enough. No further actions required above that layer. Not fully compliant (no guaranteed delivery), but at that point, few if any would ever notice.

The other question I don't have an answer for (or I've forgotten; and haven't dug through code to figure out)... @akbooer, does it support retain? If so, what's the storage mechanism, and is it persistent?

[akbooer]

Well, I must be reading the MQTT 3.1.1 spec all wrong.

When I read:

"The SUBACK Packet sent by the Server to the Client MUST contain a return code for each Topic Filter/QoS pair. This return code MUST either show the maximum QoS that was granted for that Subscription or indicate that the subscription failed [MQTT-3.8.4-5]. The Server might grant a lower maximum QoS than the subscriber requested. The QoS of Payload Messages sent in response to a Subscription MUST be the minimum of the QoS of the originally published message and the maximum QoS granted by the Server. The server is permitted to send duplicate copies of a message to a subscriber in the case where the original message was published with QoS 1 and the maximum QoS granted was QoS 0 [MQTT-3.8.4-6]."

...I understood it to mean that, since I only ever grant QoS 0, then no message would require a PUBACK or a PUBREC.

I realise that this is my bad for writing my own MQTT broker, but it made sense in the context of openLuup, especially in support of Shelly devices which was the reason I did it in the first place. I apologise if this has led to some difficulties, and I'm starting to look at an MQTT validation suite to check out my implementation further. I also realize that Mosquitto is the de-facto standard, but it turns out that having an internal server confers some significant benefits in terms of the internal openLuup architecture.

However, if anyone can clarify the about QoS response issue further, I'm very happy to comply. If a simple fix is to send PUBACK or PUBREC, then I'll do it, but I want to know the reason why. I do understand that the original CONNECT request contains a LWT QoS, per this paragraph:

3.1.2.6 Will QoS

"Position: bits 4 and 3 of the Connect Flags."

"These two bits specify the QoS level to be used when publishing the Will Message."

"If the Will Flag is set to 0, then the Will QoS MUST be set to 0 (0x00) [MQTT-3.1.2-13]."

"If the Will Flag is set to 1, the value of Will QoS can be 0 (0x00), 1 (0x01), or 2 (0x02). It MUST NOT be 3 (0x03) [MQTT-3.1.2-14]."

...but I had assumed that QoS to be overridden by the actual level established in SUBSCRIBE / SUBACK. However, now that I write that, it seems the LWT actually may have a separate life from standard messages?

[akbooer]

@toggledbits said in latest-22328 restart fails:

The other question I don't have an answer for (or I've forgotten; and haven't dug through code to figure out)... @akbooer, does it support retain? If so, what's the storage mechanism, and is it persistent?

Yes, it supports retained messages.

Is it persistent? Not across openLuup restarts.

The openLuup console page: openLuupIP:3480/console?page=mqtt shows all current subscriptions and also (at the bottom) retained messages.

[toggledbits]

@akbooer said in latest-22328 restart fails:

...I understood it to mean that, since I only ever grant QoS 0, then no message would require a PUBACK or a PUBREC.

This section is about your response to a SUBSCRIBE (SUBACK)and what QoS you use to PUBLISH. If you only grant QoS 0 on subscribe, then you will never expect a PUBACK or PUBREC because you never publish anything to a subscriber other than QoS 0.

It in no way limits what a client may publish to the broker, which could include any QoS.

[akbooer]

Well, that would explain it.

[akbooer]

Latest development release should have a fix for this...

  -- 2022.12.02  Fix response to QoS > 0 packets, thanks @Crille and @toggledbits
  if QoS == 1 then
    ack = MQTT_packet.PUBACK (PacketId)
  elseif QoS == 2 then
    ack = MQTT_packet.PUBREC (PacketId)
  end

[toggledbits]

That appears to do the trick! Client library is much happier! Thanks, @akbooer

[akbooer]

Thank you! It changed my understanding of MQTT.

[toggledbits]

It's a big help to all of us. Thanks for taking the time; I know you're focused on ZigBee right now, so I appreciate your willingness to be distracted.

[akbooer]

It’s what a community is for!

[Crille]

Thank you both for solving this issue. I can also confirm it's working as intended now.