MQTT - QoS - what does TCP not provide which MQTT does?

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Manju Bharadwaj

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Dec 13, 2015, 12:45:04 AM12/13/15
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I have a question about MQTT QoS. 

MQTT is on top of TCP. So, TCP guarantees that a message will be delivered to the application once and only once. TCP will handle any IP re transmits and will deliver the message to the application only once.

So, what is the additional value in QoS level 2? 

We use TCP over unreliable channels all the time and none of the applications (say an email client) implement any QoS on top of it. Why is it needed in MQTT?

(If MQTT had used UDP then QoS 2 would be easy to understand - but I do not understand why it is needed when TCP is used)

Sumeet Puri

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Dec 13, 2015, 7:39:10 AM12/13/15
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TCP will only guarantee delivery of the messages while the subscriber is connected. When the publisher is publishing, while the subscriber is not connected, the MQTT broker can save messages which can be delivered to the subscriber when it comes back. 

At a high level:

QoS0 - TCP like, deliver on a best effort basis. I.e. if the subscriber is connected, it will get the message, otherwise the message will be discarded 

QoS1 - At least once. I.e. if the subscriber is offline and missed published messages, when it reconnects to the broker, the broker will deliver the messages to the subscriber. This means that there are messaging level ACKs between the subscriber and the MQTT broker, whenever the subscriber gets the message, it can ACK the broker for that message to be discarded. Messages which have not been ACKed before will be redelivered if the subscriber crashed and comes back. However, such boundary conditions, i.e. having multiple messages inflight for throughput reasons without an ACK can cause duplicates, hence Atleast once

QoS2 - Once and only once. As the name suggests, this means that no duplicates are to be delivered. However the definition of duplicates is often related to business logic. I.e. if the same message go delivered twice in 10 seconds, will it be considered a duplicate, or within 2 hours? What if the publisher published a duplicate message? Given the boundary conditions, applications often go with QoS 1 and handle the duplicates at an application level, e.g. with a GUID for message id across all applications.

There are more related capabilities such as RETAIN and WILL which you might find useful. 

HTH, 
Sumeet. 


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Manju Bharadwaj

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Dec 13, 2015, 9:16:16 AM12/13/15
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Thanks Sumeet.
I can understand what you are saying. Here is a follow up question.

You mention that QoS is between the publisher and the subscriber.
From my reading of http://www.hivemq.com/blog/mqtt-essentials-part-6-mqtt-quality-of-service-levels it says that the QoS is between each hop - the publisher to broker and the second one between the broker and subscriber. All the QoS levels there apply (from my reading) even when there are no subscribers. The diagrams there show that the PUBLISH, PUBACK and other messages being between the publisher and broker (in this example). I cannot find any reference to QoS in the manner you described between the publisher and the subscriber. From my understanding, since it is a decoupled system, the publisher does not know if there are any subscribers and that is the way it is designed. If there are any documents / RFCs etc describing what you mentioned, can you please send some pointers?

Thanks
Manju

Manju Bharadwaj

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Dec 13, 2015, 9:31:40 AM12/13/15
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Thanks Francis.

I agree that TCP connects might fail, so retries may be needed. However, from my reading of http://www.hivemq.com/blog/mqtt-essentials-part-6-mqtt-quality-of-service-levels it does not seem to be TCP level retries (which I can totally agree is needed). So, if the client re-tries till the TCP connection is established and send the message and gets a successful TCP ACK, that should be sufficient no?

We can also have a protocol level PUBLISH and PUBACK so that the client knows that the "transaction is done". However once that is over, the client can discard the message. So the question of QoS 2 does not arise at all since the same message will not be sent again since it is discarded (as per my limited understanding). What is wrong with this argument?

Thanks
Manju








On Sun, Dec 13, 2015 at 2:20 PM, Francis Brosnan Blázquez <francis...@aspl.es> wrote:
Hi Manju,

You are comparing two different things: TCP operates at segment level, byte oriented,
which getting into the size, format or meaning of the message,

while QoS provided by MQTT operates at user-message level, providing a confirmed
operation, including retransmission support: many things can go wrong across MQTT
peers (even using TCP),

This question is pretty much as saying we don't need SMTP retry mechanism just
because we are using TCP,

Best Regards,
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Laing, Michael

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Dec 13, 2015, 9:36:41 AM12/13/15
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Sumeet Puri

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Dec 13, 2015, 10:04:15 AM12/13/15
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You are correct that the QoS is at every hop but the core concept of broker based messaging is that the broker provides the QoS facilities to facilitate communication between the Publisher and the Subscriber(s). 

What the blog you have referenced does not cover is the Suback, which is covered in the spec. Almost all popular brokers support it. 

The function of the broker is to losely couple the publishers from the subscribers. Even when there are no subscribers online, when you publish with QoS1, you want to be sure that the message has definitely received and stored at the broker end, for the subscriber to come and consumer later. If you don’t want he broker to store the messages, you use QoS0.

Different brokers implement different kinds of storage for these messages. Most do it via some form of disk, though some such as Solace Appliances (my employer) do it via purpose built hardware in the appliance model, and via disk in the software model. 
Now there could be many reasons this storage may not happen, even through the message may arrive at the broker properly (as per your correct TCP argument). E.g. the disk/storage is full or the queue is out of quota etc. There could be more sophisticated reasons, such as if the message could not be sent over to the DR site and you want to enforce strict replication, such that you reject the message even from the primary site if your business logic so demands. 

In such cases, the publisher should be informed that his message has not been saved successfully at the broker. Hence the need for Puback. 

Suback work conversely, i.e. when the subscribers receive the messages, they let the broker know so that the message can be deleted. 

There is no ACK going between publisher and subscriber, its between publisher and broker, and broker and subscriber. That way one being down/slow does not (should not) impact the other.

HTH, 
Sumeet.
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