[OASIS Issue Tracker] (MQTT-543) Allow long topic names for publish of all QoS

[OASIS Issues Tracker <workgroup_mailer@lists.oasis-open.org>]

Ian Craggs created MQTT-543:
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             Summary: Allow long topic names for publish of all QoS
                 Key: MQTT-543
                 URL: https://issues.oasis-open.org/browse/MQTT-543
             Project: OASIS Message Queuing Telemetry Transport (MQTT) TC
          Issue Type: Improvement
          Components: MQTT-SN
    Affects Versions: MQTT-SN-1.2
            Reporter: Ian Craggs


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 packetâs 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.



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