MQTT Essentials Part 6: Quality of Service 0, 1 & 2

mqttessentials_part6

Welcome to part 6 of MQTT Essentials. A ten-part blog series on the core features and concepts of the MQTT protocol. In this post, we explain the different Quality of Service levels in MQTT. The term ‘quality of service’ has come up a few times in previous posts, let’s look into exactly what this term means.

Quality of Service

What is Quality of Service?

The Quality of Service (QoS) level is an agreement between the sender of a message and the receiver of a message that defines the guarantee of delivery for a specific message.

There are 3 QoS levels in MQTT:

  • At most once (0)
  • At least once (1)
  • Exactly once (2).

When you talk about QoS in MQTT, you need to consider the two sides of message delivery:

  1. Message delivery form the publishing client to the broker.
  2. Message delivery from the broker to the subscribing client.

We will look at the two sides of the message delivery separately because there are subtle differences between the two. The client that publishes the message to the broker defines the QoS level of the message when it sends the message to the broker. The broker transmits this message to subscribing clients using the QoS level that each subscribing client defines during the subscription process. If the subscribing client defines a lower QoS than the publishing client, the broker transmits the message with the lower quality of service.

Why is Quality of Service important?

QoS is a key feature of the MQTT protocol. QoS gives the client the power to choose a level of service that matches its network reliability and application logic. Because MQTT manages the re-transmission of messages and guarantees delivery (even when the underlying transport is not reliable), QoS makes communication in unreliable networks a lot easier.

How does it work?

Let’s take a closer look at how each QoS level is implemented in the MQTT protocol and how it functions:

QoS 0 – at most once

The minimal QoS level is zero. This service level guarantees a best-effort delivery. There is no guarantee of delivery. The recipient does not acknowledge receipt of the message and the message is not stored and re-transmitted by the sender. QoS level 0 is often called “fire and forget” and provides the same guarantee as the underlying TCP protocol.

publish_qos0_flow

QoS 1 – at least once

QoS level 1 guarantees that a message is delivered at least one time to the receiver. The sender stores the message until it gets a  PUBACK packet from the receiver that acknowledges receipt of the message. It is possible for a message to be sent or delivered multiple times.

publish_qos1_flow

puback_packet

The sender uses the packet identifier in each packet to match the PUBLISH packet to the corresponding PUBACK packet. If the sender does not receive a PUBACK packet in a reasonable amount of time, the sender resends the PUBLISH packet. When a receiver gets a message with QoS 1, it can process it immediately. For example, if the receiver is a broker, the broker sends the message to all subscribing clients and then replies with a PUBACK packet.
If the publishing client sends the message again it sets a duplicate (DUP) flag. In QoS 1, this DUP flag is only used for internal purposes and is not processed by broker or client. The receiver of the message sends a PUBACK, regardless of the DUP flag. 

QoS 2 – exactly once

QoS 2 is the highest level of service in MQTT. This level guarantees that each message is received only once by the intended recipients. QoS 2 is the safest and slowest quality of service level. The guarantee is provided by at least two request/response flows (a four-part handshake) between the sender and the receiver. The sender and receiver use the packet identifier of the original PUBLISH message to coordinate delivery of the message.

publish_qos2_flow

When a receiver gets a QoS 2 PUBLISH packet from a sender, it processes the publish message accordingly and replies to the sender with a PUBREC packet that acknowledges the PUBLISH packet. If the sender does not get a PUBREC packet from the receiver, it sends the PUBLISH packet again with a duplicate (DUP) flag until it receives an acknowledgement. 

pubrec_packet

Once the sender receives a PUBREC packet from the receiver, the sender can safely discard the initial PUBLISH packet. The sender stores the PUBREC packet from the receiver and responds with a  PUBREL packet.

pubrel_packet

After the receiver gets the PUBREL packet, it can discard all stored states and answer with a PUBCOMP packet (the same is true when the sender receives the PUBCOMP). Until the receiver completes processing and sends the PUBCOMP packet back to the sender, the receiver stores a reference to the packet identifier of the original PUBLISH packet. This step is important to avoid processing the message a second time.

After the sender receives the PUBCOMP packet, the packet identifier of the published message becomes available for reuse.

pubcomp_packet

When the QoS 2 flow is complete, both parties are sure that the message is delivered and the sender has confirmation of the delivery..

If a packet gets lost along the way, the sender is responsible to retransmit the message within a reasonable amount of time. This is equally true if the sender is an MQTT client or an MQTT broker. The recipient has the responsibility to respond to each command message accordingly.

Good to know

Some aspects of QoS are not very obvious at first glance. Here are a few things to keep in mind when you use QoS:

Downgrade of QoS

As we already mentioned, the QoS definition and levels between the client that sends (publishes) the message and the client that receives the message are two different things. The QoS levels of these two interactions can also be different. The client that sends  the PUBLISH message to the broker defines the QoS of the message. However, when the broker delivers the message to recipients (subscribers), the broker uses the QoS that the receiver (subscriber) defined during subscription. For example, client A is the sender of the message. Client B is the receiver of the message. If client B subscribes to the broker with QoS 1 and client A sends the message to the broker with QoS 2, the broker delivers the message to client B (receiver/subscriber) with QoS 1. The message can be delivered more than once to client B, because QoS 1 guarantees delivery of the message at least one time and does not prevent multiple deliveries of the same message.

Packet identifiers are unique per client

The packet identifier that MQTT uses for QoS 1 and QoS 2 is unique between a specific client and a broker within an interaction. This identifier is not unique between all clients. Once the flow is complete, the packet identifier is available for reuse. This reuse is the reason why the packet identifier does not need to exceed  65535. It is unrealistic that a client can send more than this number of messages without completing an interaction.

Best Practice

We are often asked for advice about how to choose the correct QoS level. Here are some guidelines that can help you in your decision making process. The QoS that is right for you depends heavily on your use case.

Use QoS 0 when …

  • You have a completely or mostly stable connection between sender and receiver. A classic use case for QoS 0 is connecting a test client or a front end application to an MQTT broker over a wired connection.
  • You don’t mind if a few messages are lost occasionally. The loss of some messages can be acceptable if the data is not that important or when data is sent at short intervals
  • You don’t need message queuing. Messages are only queued for disconnected clients if they have QoS 1 or 2 and a  persistent session.

Use QoS 1 when …

  • You need to get every message and your use case can handle duplicates. QoS level 1 is the most frequently used service level because it guarantees the message arrives at least once but allows for multiple deliveries. Of course, your application must tolerate duplicates and be able to process them accordingly.
  • You can’t bear the overhead of QoS 2. QoS 1 delivers messages much faster than QoS 2.

Use QoS 2 when …

  • It is critical to your application to receive all messages exactly once. This is often the case if a duplicate delivery can harm application users or subscribing clients. Be aware of the overhead and that the QoS 2 interaction takes more time to complete.

Queuing of QoS 1 and 2 messages

All messages sent with QoS 1 and 2 are queued for offline clients until the client is available again. However, this queuing is only possible if the client has a  persistent session.


That’s the end of part six in our MQTT Essentials series. We hope that you find this information useful. In the next post, we’ll cover a subject that is closely associated with Quality of Service levels: persistent sessions in MQTT.

Have a great week and we look forward to seeing you for the next MQTT Monday!

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54 comments

  1. Fernando says:

    Hi,
    I have one question about QoS.

    As mentioned before MQTT protocol is above the Transport Layer or TCP. my understanding about TCP is that every time it sends a datagram, the receiver acknowledge it and also provides QoS. So why to use the QoS when protocols like TCP already have it? Does not make it redundant?

    Thank you.

    1. Hi Fernando,

      this is a great question! A very extensive and in-depth answer by Andy Stanford-Clark (one of the inventors of MQTT) can be found here in the following link, make sure to check it out: https://groups.google.com/forum/#!searchin/mqtt/stanford/mqtt/6JZTV5XIwEQ/9K6QolImMdUJ

      To summarize the excellent link: TCP offers reliability on top of the TCP / IP stack, MQTT offers application-to-application data reliability. This differentiation is very important if you are dealing with unreliable networks (mobile, wifi, …) since in-flight packets can get lost if your TCP connection drops.

      Hope this helps!
      Dominik from the HiveMQ Team

    2. Francesco Bianchi says:

      I think that you should correct the paragraph about QoS level 0; in particular, quote: “This is often called “fire and forget” and provides the same guarantee as the underlying TCP protocol.”
      This is not true, TCP provides “guaranteed delivery”.

    3. Hallo Francesco,

      Thank you for joining the conversation and raising this point. The point has been asked and answered before.
      Here’s what I said:

      TCP itself is of course not ‘fire and forget’ and guarantees delivery, as long as the connection is not lost. Unlike UDP for example.
      So as long as there is no connection loss, message delivery is guaranteed on TCP layer.
      QoS 0 is ‘fire and forget’ and therefore does not provide any additional guarantees on MQTT layer.
      Quality of Service levels higher than 0 provide extra guarantees for failure cases.
      As MQTT is at times used in unreliable mobile network scenarios or similar environments, it is often useful to take advantage of these additional guarantees.

      Kind regards,
      Florian from The HiveMQ Team.

  2. Rohit says:

    Hi,

    Is there a way to browse a given mqtt topic,without messages to be deleted/removed. Kind of non destructive read?

    1. Hi,

      each client (with persistent session) has its own queue if the client is offline, Topics can’t be browsed per se. If you are looking for an approach to read all messages on a specific topic, you can just spawn a new client which subscribes to that topic. All other clients won’t be affected by that.

      All the best,
      Dominik from the HiveMQ Team

  3. Eddie says:

    Hi all,
    I have 2 questions regarding reliability with QoS:
    – if a publisher send a message to a subscriber using a broker, how can publisher knows when receiver receives the message.
    – What is the purpose of the packetid from the publish command, since receiver will not see it (i’ve played with emqtt…an erlang mqtt server) and :
    sender publish to a topic with a packetid=5 ….broker ..receive it and send back the pub ack…and the subscriber receive it but with a different packet id

    Thanks,
    Eddie

    1. Hi Eddie,

      the publisher does not know that the message was delivered to a subscriber. The publisher only knows when the message was published successfully to the broker. The publishers and subscribers are completely de-coupled. For more details about that I can recommend: http://www.hivemq.com/blog/mqtt-essentials-part2-publish-subscribe

      The packet identifier is used for identifying associated messages in a message flow so. A message flow is always Client -> Broker or Broker -> Client. Each Tuple [Client,Broker] use their own packet ids due to the de-coupling in a pub/sub system. So the packet identifier is needed for the client and broker implementations to collate messages. Packet ids are usually a implementation detail and most client APIs don’t expose the packet identifier.

      Hope this helps,
      Dominik from the HiveMQ Team

  4. Iain. says:

    Hi,

    Regarding receiving a PUBLISH with a QoS of 2 you say the following:

    “After the receiver gets the PUBREL it can discard every stored state and answer with a PUBCOMP. The same is true when the sender receives the PUBCOMP.”

    However, what happens if the PUBCOMP goes missing? The broker will resend the PUBREL but if the client has discarded every stored state, then the sequence which lead to the PUBREL will have been forgotten and the client will behave as if it has just received a PUBREL out of the blue. I am struggling to find an elegant way to handle this, but maybe I am missing something?

    Thanks for creating this series. I’ve found it very useful.

    BR.
    Iain.

    1. Rob says:

      Obviously a bit late but just in case…

      The sender doesn’t discard their state after sending the PUBREL, they discard state only after receiving the PUBCOMP. See figure 4.3 in the MQTT specification.

      If the PUBCOMP is lost and the sender resends PUBREL, the receiver must respond by sending another PUBCOMP packet containing the same packet identifier as the PUBREL (MQTT 4.3.3-1). Ownership of the message is transferred to the receiver after the sender receives the first PUBREC message and the sender is supposed to discard the message.

      Any new PUBLISH with the same packet identifier is treated as a new publication and the receiver must send a PUBREC (MQTT 4.3.3-1). It’s the receiver’s responsibility to guarantee that duplicates are not forwarded in this case.

  5. Iain says:

    Hi,

    Is your commenting system working correctly? For instance on this page (http://www.hivemq.com/blog/mqtt-essentials-part-6-mqtt-quality-of-service-levels) it states there are 8 comments, but I can only see 6. Other pages have similar discrepancies. I posted a comment on this page a few days ago and after an initial “waiting for moderation” message it disappeared.

    Thanks.
    Iain.

    1. Hi Iain,

      sorry for the late reply. The comment should now be available.

      Regarding the comment number: Thanks for the heads up! We’ll check if there’s a problem with the displayed number.

      Thanks so much,
      Dominik from the HiveMQ Team

  6. tayyab says:

    great tutorials. thanks

  7. AlekSi says:

    > The duplicate (DUP) flag, which is set in the case a PUBLISH is redelivered, is only for internal purposes and won’t be processed by broker or client in the case of QoS 1. The receiver will send a PUBACK regardless of the DUP flag.

    What about QoS 2?

    1. Hi AlekSi,

      the same goes for QoS 2 but with PUBREC instead of PUBACK.

      The HiveMQ Team

  8. William says:

    Hi,
    Thanks for these great articles. I have questions about the process of QoS 2. Regarding this article: “After the receiver gets the PUBREL it can discard every stored state and answer with a PUBCOMP. The same is true when the sender receives the PUBCOMP.”

    What happens if the PUBCOMP message missing? Is there any mechanism for the receiver to resend the PUBCOMP message?

    Because this article says that: After the sender received the PUBREC message, the sender can safely discard the initial publish and It will store the PUBREC and respond with a PUBREL.

    When the PUBCOMP message got lost, the sender is still stored the PUBREC message. It can not discard every stored state, and keep it forever.

    Thanks,
    William Luo

  9. sujithra says:

    Hi Williams,
    Regarding your response for the “What happens if the PUBCOMP message goes missing? ” .. The question is all about… When PUBCOMP gets lost, the sender will have PUBREC messag by which is might resend the PUBREC message, but the receiver has already discared all the stored state as said above. Then how will it recognize the sender and resend PUBCOMP when it doesnt have any info about the sender.????

    1. Adr says:

      Does any one have an answer to this ?
      “What happens if the PUBCOMP message goes missing?”

      Adr

    2. Hallo Adrien,

      Let’s take a look at the MQTT 3.1.1 spec

      When a Client reconnects with CleanSession set to 0, both the Client and Server MUST re-send any unacknowledged PUBLISH Packets (where QoS > 0) and PUBREL Packets using their original Packet Identifiers [MQTT-4.4.0-1]. This is the only circumstance where a Client or Server is REQUIRED to redeliver messages.

      What this means is the following:
      It is the sender’s * job to resend unacknowledged packages, when a connection was lost and reconnection happened. (This is true for both QoS 1 and QoS 2)
      Since TCP is the underlying transport protocol for MQTT, when a message ‘goes missing’ a connection loss will inevitably get registered.

      * For clarification:
      Sender does not mean broker or client – both function as a sender in different scenarios. When publishing, the client is the sender. When forwarding a published message to a subscriber, the broker is the sender.

      I hope this answers your question.
      Best regards,
      Florian, from The HiveMQ Team.

  10. arijit says:

    Hi,
    Thanks for these great articles. I have questions about uses of Quality of Service . It’s mentioned in article that for critical application we should use Quality of Service 2 but for which kind of application we should use Quality of Service 2 and for which we should use Quality of Service 1 ?Please provide some specific example.

    Thanks

    1. In general we recommend to go with QoS 1, since the guarantees (at least once) are often perfect for most scenarios. If you have a scenario where you can make sure that the persistent state of a client gets never corrupted, QoS 2 might be a good option if you really want exactly once delivery. Most clients chose to use QoS 1 with manual de-duplication (if needed by the scenario) for best performance and overhead ratio.

      Hope this helps,
      Dominik

  11. Bipal Shakya says:

    Hi,
    I would like to know what you mean by the following extract regarding QoS-0.:
    “This is often called “fire and forget” and provides the same guarantee as the underlying TCP protocol.”

    Is TCP fire-and-forget?? Or do you mean to say that although MQTT QoS-0 is fire-and-forget, it will provide the guarantee provided by TCP?

    1. Hallo Bipal,

      To clarify:
      TCP itself is of course not ‘fire and forget’ and guarantees delivery, as long as the connection is not lost. Unlike UDP for example.
      So as long as there is no connection loss, message delivery is guaranteed on TCP layer.
      QoS 0 is ‘fire and forget’ and therefore does not provide any additional guarantees on MQTT layer.
      Quality of Service levels higher than 0 provide extra guarantees for failure cases.
      As MQTT is at times used in unreliable mobile network scenarios or similar environments, it is often useful to take advantage of these additional guarantees.

      I hope this helps.
      Kind regards,
      Florian, from The HiveMQ Team.

  12. Dheeraj says:

    When QoS>0 and Publisher publishes a message, the messageId starts with 1. Is there any use of messageId = 0 for publisher?

    1. Hallo Dheeraj,

      When publishing messages with Quality of Service = 0, the messageID will equal 0. For QoS > 1 the messageID will always be greater than 0 as well.

      Kind regards,
      Florian from the HiveMQ Team.

    2. Dheeraj says:

      Thanks Florian for your response. Just reiterating, when publishing message over QoS>0, the message ID will always be greater than 0.
      Message ID 0 is not used by Publisher.
      Please confirm.

      Regards,
      Dheeraj

    3. Dheeraj,
      Once again: When a message gets published with QoS=0, the messageId will be 0.
      For QoS > 0 the messageId will never be 0.
      Hope this answers your question.

      Kind regards,
      Florian from The HiveMQ Team.

  13. Gerardo Toledo says:

    Hello ! excelents tutorials, thank you very much.
    I have a question regardings QoS 1 or 2. I speak spanish, I hope to write well…
    Supossing there is a client “1” with persistant session suscribed to a topic “A” with QoS 1 or 2, and suddendly disconnect (for example connections problems) and in the meanwhile a client”2″ publish with Qos 1 or 2 in the same topic “A” more than one message, for example the payload “Msg number 1” and later “Msg number 2” , and the last one: “Msg number 3” .
    After that, client “1” reconnects. What payload receive? ” :
    “Msg number 1” or
    “Msg number 2” or
    “Msg number 3” ?
    or all of them? If it is the case, it is not clear for me how is the process because succesive payload have to erase previous one, etc.

    Thank you very much ! Regards from Argentina !

    1. Hi Gerardo,

      Thank you for the kind words. It’s nice to see you are taking an interest in MQTT and HiveMQ.
      Your English is very well understandable by the way 🙂
      Regarding your question:
      The answers can be found in the next part of this blog series.
      In short:
      When your client “1” uses the setting cleanSession=false on connection, the message received on the topics it was subscribed on will be stored on the broker and delivered once it reconnects.

      Hope this helps.

      Kind regards,
      Florian from The HiveMQ Team.

  14. Ashish Shukla says:

    Hello!
    I was new to MQTT but not any more thanks to these awesome tutorials…!!
    Much appreciated! 🙂

    I have a question hope you could help me understand:

    Is there any connection/relation between the QoS set by the publisher of a message on a topic and the QoS requested by a subscriber for the same topic or are they independent?
    In other words, does the QoS at which a message is published governs the QoS it can be subscribed with?

    1. Hi Ashish,

      the QoS used for the PUBLISH message is independent from the QoS from the SUBSCRIBE. Keep in mind that the QoS of a message can be downgraded for a specific subscriber if the QoS in its subscription is smaller than the QoS in the PUBLISH message. MQTT does not “upgrade” subscriptions.
      In other words: the QoS with which the message is published defines the maximum QoS with which the message is delivered to the subcriber. And the QoS in a SUBSCRIBE defines the maximum QoS for a subscription which a client wants to receive.

      Example:
      Publish QoS 1
      Subscribe with QoS 2 <- receives this messages with QoS 1 Subscribe with QoS 1 <- receives this message with Qos 1 Subscribe with QoS 0 <- receives this message with QoS 0

  15. Omid says:

    Hello
    I need resend message if broker could not sent message when user offline
    How qos i use?0 or 1 or 2?

    1. Hi Omid,

      Nice to see you’re taking an interest in MQTT and HiveMQ.
      The topic of message queuing is covered in the part 7 of this blog series.
      Short answer: You need to utilize both the cleanSession=false flag for your client’s connection and use a Quality of Service Level that is greater than zero.
      I hope this helps.

      Kind regards,
      Florian from The HiveMQ Team.

  16. tim Rowledge says:

    I have a problem related to the ‘pubcomp goes missing’ issue above.

    In this case, I sometimes get a repeated PUBREL despite having sent the PUBCOMP. Given that I’ve followed my understanding of the spec and cleared the msgID as having been dealt with fully, what is the expected response to the second and any subsequent PUBRELs? The best I can come up with right now is to simply respond with another PUBCOMP with the msgID copied from the PUBREL. It’s somehow not very satisfying though.

  17. Giorgio says:

    Hello, thank you for all but i have a quastion.
    If i subscribe with a certain QoS on a certain Topic Name, how can the broker reply with a SUBACK where is specified the QoS allowed if the broker can understand the QoS allowed when a publusher publish on same Topic Name!?!?

    1. Hi Giorgio,

      if I understand you correctly, you want to know how the broker decides if a client can subscribe to a topic with QoS 1 or 2.

      For detailed information on authorization with MQTT please read the MQTT Security Fundamentals: Authorization (https://www.hivemq.com/blog/mqtt-security-fundamentals-authorization)

      Hope this helps,
      Abdullah from the HiveMQ Team.

  18. Nancy says:

    Hello,
    I have few questions regarding QoS and there response time.
    1. If a client doesn’t specify the QoS level, what will be the default QoS level?
    2. In case of QoS 1 and 2 approximately how much time will be taken by Broker to respond back and forth to publish?

    Thank You
    Kind Regards
    Nancy

    1. Hi Nancy,

      Nice to see you’re taking an interest in MQTT and HiveMQ.

      1. MQTT itself doesn’t specify a default QoS level. Each client implementation uses individual default values.

      2. HiveMQ will acknowledge incoming PUBLISH or PUBREL packets immediately.

      Kind regards,
      Abdullah from the HiveMQ Team

  19. Shlomy says:

    Hi,

    Thanks for the helpful information!
    I wonder what will happen if the subscriber use QOS 2 but the publisher use OQS 0/1 ?
    That means each message thats published successfully to the broker will read exactly once by the subscriber ?

    1. Hi Shlomy,

      Thank you for the nice words. We appreciate them.

      The QoS level of publishes is determined by the publisher and will be downgraded.
      That means for your example that the subscriber will receive the message with QoS 0 or 1 which was selected by the publisher.

      Kind regards,
      Abdullah from the HiveMQ Team.

  20. Mihai says:

    Very helpful information, thanks for the article. I would like to ask a question (maybe it’s a dumb question).

    When you say: “the message is guaranteed to be delievered […]”, does it mean that the message is just going to be sent to the reciver or also received according to the QoS of its link to the broker?

    Maybe with an example I can explain myself better: Imagine we have a “sender”, connected to the broker with a QoS of 1 and a receiver with a QoS of 1 too. When the sender sends a message to the receiver, will the broker PUBACK to the sender once the message is pushed on the link to the receiver or only when the receiver has acknowledged that it has received the message?

    Thank you!

    1. Hi Mihai,

      nice to see you’re taking an interest in MQTT.
      Your question is actually anything but stupid. It is a very good question.
      Delivery guarantees apply to each individual sender to receiver communication. (1. Publisher -> Broker 2. Broker -> Subscriber)
      Without adding additional business logic the Publisher will not get an acknowledgement when a Subscriber receives the message it sends.
      This is typical for Publish/Subscribe patterns as there is no way of telling when all interested parties have received an original message, due to the decoupling and multicasting.

      I hope this helps.

      Kind regards,
      Florian from The HiveMQ Team.

    2. Mihai says:

      Thank you, I think I understand. Due to decoupling senders is not informed when receiver gets the message, but thanks to QoS, the broker can guarantee it. (correct me if I’m wrong) Just another small question :=) Is the QoS requested by the sender always enforced regardless of the QoS of the subscription of the receiver?

    3. Hi Mihai,

      the QoS of a message will be downgraded in the following scenarios:

        The subscriber has a higher QoS than the publisher -> QoS of the publisher will be used.
        The subscriber has a lower QoS than the publisher -> QoS of the subscriber will be used.

      Hope that clears it up a bit.

      Kind regards,
      Abdullah from the HiveMQ Team.

  21. Gyungsoo says:

    Hi,

    What is the Retry policy when Sender does not receive PUBACK in Qos 1?

    Does the server need to disconnect the client, set the clean session to 0, and resend the “unacknowledged ” message?
    or
    Does not the server need to disconnect the client, after timeout, and resend the “unacknowledged” message?

    Thank you.

    1. Hi Gyungsoo,

      nice to see you’re taking an interest in MQTT and HiveMQ.

      In QoS 1 is the sender of the publish responsible that the message arrives at the receiver.
      If the sender notices that no PUBACK has come back, it will re-send the publish after a reconnection with CleanSession 0.

      MQTT 3.1.1 specification (chapter 4.4):

      “When a Client reconnects with CleanSession set to 0, both the Client and Server MUST re-send any unacknowledged PUBLISH Packets (where QoS > 0) and PUBREL Packets using their original Packet Identifiers [MQTT-4.4.0-1]. This is the only circumstance where a Client or Server is REQUIRED to redeliver messages.”

      Kind regards,
      Abdullah from the HiveMQ Team.

  22. Paul says:

    Why is the 4th control packet needed in QoS2? I seem to miss the point as it seems redundant to me.
    When a receiver gets a message, it replies with an ack.
    Now the sender knows the message got delivered.
    To ensure that this ack got delivered the receiver awaits an ack from the sender that the sender will not try to publish the message again.

    Now for me the conversation is done, everyone is assured that the message is delivered and there is no need of resending.

    Also what purpose does the dupe bit serve if its not to be processed by the receiver?

    1. Hi Paul,

      Great to see that you’re taking an interest in MQTT and sharing your thoughts on the mechanisms in question with us.

      Let me try to clarify the decisions made during the specification of the MQTT protocol with you.

The two-way acknowledgement has the purpose of ensuring EXACTLY ONCE delivery (Quality of Service Level 2). This is why the second round of “send and acknowledge” has been introduced. 
For cases, where a problem with the client connection during message delivery occurs, this is necessary to make sure the delivery guarantee of exactly once can be upheld.
      A transaction with only 1 acknowledgement leaves room for multiple delivery, when the connection is lost between the receiver sending the PUBACK and the sender receiving that PUBACK.

      The final PUBCOMP packet is needed, so the sender is aware that the PUBREL has been received by the receiver and the transaction is done.
      When no PUBCOMP is received by the sender it has to re-send the the PUBREL packet after connection re-establishment. Otherwise, when the connection is lost between the sender sending the PUBREL and the receiver receiving it, the transaction would never finish.

      I hope this helps your understanding of the Quality of Service Level 2 transaction flow.

      In regards to the DUP flag I can say this:
      From a pure MQTT protocol level stand point there is no use for the DUP flag per se.
      Its purpose comes from the possibility to interpret it on application level. An application that tracks the received messages for example might take the existence of a DUP flag as a trigger to check that message against the record of already received messages.

      Let me know if you have additional questions.

      Kind regards,
      Florian from the HiveMQ Team.

  23. Andrea says:

    Hello and thanks for the great article!
    I would like to ask a question about the way the broker identifies each message received as unique.
    Is the packet identifier the only parameter the broker uses to differenciate two received messages?
    My question is because using an mqtt client lib not thread-safe I faced the following problem: 2 different messages were published by 2 different threads with the same packet identifier on the same topic, and the broker loses the first of the to 2 messages because it acknoledges only the second one.
    In other words, is there a chance that the broker uses a second parameter to differenciate the messages, for example the topic?
    Thank you!
    Kind Regards,
    Andrea

    1. Hi Andrea,
      Thank you for the kind words. We’re always happy to see when people are interested in MQTT.
      The behavior you describe is expected and conform with the MQTT specification.

      SUBSCRIBE, UNSUBSCRIBE, and PUBLISH (in cases where QoS > 0) Control Packets MUST contain a non-zero 16-bit Packet Identifier [MQTT-2.3.1-1]. Each time a Client sends a new packet of one of these types it MUST assign it a currently unused Packet Identifier

      I hope this helps.
      Kind regards,
      Florian from the HiveMQ Team.

  24. Ayanle says:

    I was wondering how I can set the QoS when I am trying to pub/sub a topic?

    1. Hello Ayanle,

      nice to see your interest in MQTT and HiveMQ.
      The QoS level is part of the PUBLISH or SUBSCRIBE packet respectively.
      Check out our Client Library Encyclopedia to find examples on how it is done for the most common MQTT libraries.

      Kind regards,
      Florian from the HiveMQ Team.

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