We are starting to work on the standardization of MQTT-SN – MQTT for Sensor Networks. The current specification for MQTT-SN is in a similar position to that for MQTT before it became a standard at OASIS; it is published by IBM, freely available with a liberal license, and has been in use for several years. It is not as widely used as MQTT was at the same point, but there are several existing users and implementations. To this end, I propose that in the process of standardization we take the approach that was adopted for MQTT 3.1.1 – minimal changes with the existing specification to allow standardization to proceed as quickly as these things ever do. In the case of MQTT 3.1.1, that was about two years.
While there is a general agreement to get things moving quickly, a concern has been raised from a couple of quarters. That is, the current MQTT-SN specification was written before MQTT 5.0 existed. One of the primary goals of MQTT-SN is to extend MQTT to non-TCP networks – to do so, it must allow the easy interoperation of the two protocols. Messages from MQTT must be able to flow to MQTT-SN and vice versa. The concern is that the current MQTT-SN specification might more closely align with MQTT 3.1.1 rather than 5.0, and we should really be aiming at 5.0 as that is likely to be more frequently used in the future.
Several features in MQTT 5.0 were influenced by MQTT-SN in fact, so the flow of concepts might be towards MQTT rather than MQTT-SN. In this article, I’ll go through the aspects of MQTT-SN and see how they match up to the two MQTT standards, 3.1.1 and 5.0.
MQTT-SN is both a client/server and peer to peer protocol. An MQTT-SN server can be a broker in the MQTT sense, or a gateway which does little more than act as a mediator between MQTT-SN and MQTT and the underlying transports. Here I will use the term server to refer to both brokers and gateways for MQTT-SN.
Every MQTT packet has a header byte followed by a variable-length remaining length field. Some of the packets have multiple variable-length fields (string or binary) as part of their construction. Although MQTT packet sizes are still kept as small as is feasible, MQTT-SN is intended to be suitable for even lower power devices and used over networks with fewer reliability characteristics than TCP. Each MQTT-SN packet, apart from one possible exception, has a single variable-length field within it, so the only one length field is needed for each packet, helping to reduce their size. As a result, for instance, the MQTT connect packet has been split into several MQTT-SN packets:
- WILLTOPICREQ – sent by the server to request that a client sends the will topic name
- WILLTOPIC – sent by the client to tell the server its will topic name
- WILLMSGREQ – sent by the server to request that a client sends the will message
- WILLMSG – sent by the client to tell the server its will message
- WILLTOPICUPD – sent by the client to update its will topic name stored in the server
- WILLLTOPICRESP -sent by the server to confirm the will topic name has been updated
- WILLMSGUPD – sent by the client to update its will message stored in the server
- WILLMSGRESP – sent by the server to confirm the will message has been updated
Connect and Disconnect
The fields in the connect packet are:
- Will flag – request will topic and message prompting
- CleanSession – as in MQTT 3.1.1
- ProtocolId – corresponds to protocol name and version as in both MQTT 3.1.1 and 5.0
- Duration – keep alive timeout as in both MQTT 3.1.1 and 5.0
- ClientId – as in both MQTT 3.1.1 and 5.0
The clean session flag operates in a similar manner to MQTT 3.1.1 in that the cleanup operates at both the start and end of the session. In MQTT 5.0, the clean session flag becomes the clean start flag, and a separate property session expiry dictates when the session state is removed on the server. The MQTT 5.0 facilities are much more flexible and I would advocate changing MQTT-SN to match. One way to achieve this would be to add a session expiry 2 byte integer field (matching the duration field) to the CONNECT packet.
The clientid variable length field is allowed to be 0-length in both MQTT 3.1.1 and 5.0, indicating that the server should assign a clientid itself. If we do allow this behaviour in MQTT-SN I think it’s important that that clientid is returned to the client, as it is in MQTT 5.0. This could be done by including the server assigned clientid in the MQTT-SN CONNACK packet, which currently has no variable length field.
The MQTT-SN DISCONNECT packet has a duration field, which operates in a similar way to the MQTT 5.0 session expiry property. In this case MQTT-SN is already closer MQTT 5.0. MQTT-SN also allows the DISCONNECT packet to be sent by the server to the client, so that the client has more information about the reason for the disconnection. This is forced on MQTT-SN anyway, as unlike for MQTT there may be no underlying session (TCP for MQTT) to break, as in UDP for instance. Again, this is closer to MQTT 5.0 than 3.1.1. The latter does not allow the server to send a DISCONNECT packet.
In fact the MQTT-SN behaviour on disconnect is more sophisticated than MQTT 5.0 (see Sleeping Clients in the MQTT-SN specification), but that doesn’t alter the fact that it is closer to 5.0 than 3.1.1.
The will message processing in MQTT-SN uses 8 packets as listed above, so is equally removed from both MQTT 3.1.1 and 5.0. Section 6.3 of the MQTT-SN specification lists the combination of interactions between the clean session and will flags on the connect packet – I think these would remain intact if we changed the clean session flag to clean start to match MQTT 5.0. So I feel there is no need to change the will processing in MQTT-SN to align with MQTT 5.0 more. There may be other reasons but this isn’t one.
In both MQTT 5.0 and MQTT-SN topic ids can be used instead of a full topic string. However, in MQTT-SN this is almost compulsory, because the PUBLISH packet’s one variable length field is the payload. The topic data is limited to a two byte field to hold a topic id (2 byte integer) or a short topic string (2 bytes). In MQTT 5.0 the topic id is registered by including it in the publish packet along with the long topic string. In MQTT-SN this registration is delegated to a separate packet, REGISTER, which must be sent before sending a PUBLISH packet. This applies to both clients and servers.
This does lead to a problem when using the PUBLISH packet in a QoS -1 mode, which is exclusive to MQTT-SN. QoS -1 in MQTT-SN means that a client can send a message to a server outside of the familiar CONNECT/DISCONNECT session start and end range. Typically this could be used in a multi-cast environment where the client is not sure of the location of the server. There are a number of proposals to allow a variable length topic name to be included in a PUBLISH packet. At least one has already been implemented, which is to use the spare topic id type indicator to specify that the topic name is variable length field, and the topic id integer holds the length. This would mean the PUBLISH packet is the only one with two variable length fields (topic name and payload). I would advocate allowing this format for all PUBLISH QoS.
MQTT-SN also has the concept of pre-registered topic ids – there is no parallel in either version of MQTT.
I feel there is no need to change to particularly align with MQTT 5.0 – any problems with the existing MQTT-SN implementation of topic ids should be fixed for their own sake.
The MQTT-SN SUBACK packet includes a return code. As in MQTT 3.1.1, the UNSUBACK packet does not. It would seem sensible to add a return code to every ack. I think the UNSUBACK is the only one without. This would mirror MQTT 5.0 too.
Other MQTT-SN Features
Other capabilities of MQTT-SN are straightforward mirrors of simple MQTT function, favouring neither one nor the other, or have no analog in MQTT at all. These include:
- PING and PINGRESP
- Keep alive
- Gateway advertisement and discovery
- Forwarder encapsulation
It turns out there are more changes that I would like to see than I was expecting before writing this article. However, I feel they are more to do with fixing up some of the more irritating MQTT-SN aspects for themselves, rather than aligning with MQTT 5.0 per se.