MQTT packet fragmentation

[Paridhika Kayal]

Hi,

MQTT supports maximum packet size of 268,435,455 bytes. And I read that it does not allow fragmentation. Still when I am trying to send large files of 30-40 MB. I see packets getting fragmented into 64k bytes. Can you please tell me the reason for this.

Thanks

Paridhika

[Dan Anderson]

Hi,

I assume what you are seeing is fragmentation by the underlying network protocols - like with a sniffer or something.

i.e., while MQTT might support giant packets, it can run fine over IP which has a 64k max datagram size, which in turn is sent over something like ethernet with another smaller packet size (like 1508 mtu).  These other layers are abstract though (from MQTT's perspective) - the ethernet frames are reassembled into IP packets which are assembled into TCP streams which are reassembled into giant MQTT packets - all normally automatically and transparently to you.

Dan

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