MQTT-v5 blockwise transfer

[Julien Christophe]

Hello,

We are currently using MQTTv5 properties in order to implement a request-response mechanism (using 0x09 Correlation Data). [1]

Our broker have the above limitation: maximum payload size of 128k.

We are facing a problem with request of large objects, who could be bigger than the maximum payload size.

We where seeking of a solution in order to implement block-wise transfer (we saw coap implementation [2], and were potentially doing something similar)

I was thinking about adding block-wise transfer flags in the MQTTv5 philosophy, something like:

0x30 | variable byte integer | block number
0x31 | byte ("boolean")      | more (=not last block)
0x32 | byte                  | block size exponent

We were thinking about adding user-properties (0x26) like NUM=0; M=0; SZX=256, but, as user-properties are utf8-encoded strings, and are more oriented "application-side" and not "transport-side", I think it is worst :/

I didn't find any work speaking about block-wise transfer in mqtt, so, for now, we have no idea of how to implement that...

If you have any idea, let me know :)

Thanks,

Julien

[1] http://docs.oasis-open.org/mqtt/mqtt/v5.0/cs01/mqtt-v5.0-cs01.html#_Toc502667250
[2] https://tools.ietf.org/html/rfc7959

[Paul Fremantle]

Julien

Given that MQTT5 doesn't support block transfer, I would think that the user properties would be the right place to do this. 

In the minds of the designers, I imagine one might see block transfer as an "application" of MQTT5 (if that makes sense).

However, I like your suggestion for adding this in future to the core standard - it might be worth sending a comment into the committee:

https://www.oasis-open.org/committees/comments/index.php?wg_abbrev=mqtt

Paul

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Paul Fremantle

Doctoral Researcher, University of Portsmouth, School of Computing

Visiting Scientist, Institute of the Architecture of Application Systems, Stuttgart

Visiting Lecturer, Software Engineering Programme, Oxford University

Co-Founder, WSO2
Apache Member and Committer

email: paul.fr...@port.ac.uk, pa...@fremantle.org

twitter: pzfreo / skype: paulfremantle / blog: http://pzf.fremantle.org

[Julien Christophe]

Thank you Paul !
So we are currently testing this protocole with new properties (0x30...) instead of the user properties.
As we want to be as close as possible to the spec, we will try to make a draft and/or submit this suggestion to the commitee.
Thank you for your answer,
Julien

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