1. PRO-2015-0739-Discussion_on_fragmentation_&_content_format_negotiation_in_MQTT_binding
Source: Sung-Chan Choi, Yong-Suk Park, Jaeho Kim
Data:

2. Fragmentation Issue TS-0010 describes
Note: MQTT packets are subjected to a theoretical maximum message size of 256MB, but it is good practice not to send packets that are bigger than a 100kB. If a larger amount of data needs to be sent, it should be segmented into multiple PUBLISH packets.

3. MQTT Fragmentation
Fixed
Variable
Payload
MQTT Message Format
In order to reassemble fragmented application messages which are delivered over MQTT payload, when the application message is to be fragmented, there need to express the information about fragmented sequence number, flag indicating more fragmented message following or not, ..
And this information may be included in MQTTT Payload part

4. Each Fragmentation data may need following information
MQTT Fragmentation
MQTT Client
MQTT Broker
MQTT Client
Frag4
Frag3
Frag4
Frag3
Frag2
Frag1
Frag2
Frag1
Each Fragmentation data may need following information
Identification
Fragmented Data Sequence Number
More Fragmented Data Indication Flag
Fragmented Data Size

5. ID/SequenceNumber/MoreFlag/FragmentDataSize
Example
MQTT Broker
MQTT Client
MQTT Client
Application data size: 200KB
Fragmentation data size: 50KB
PUBLISH #w77e/0/1/51200
PUBLISH #w77e/1/1/51200
PUBLISH #w77e/2/1/51200
PUBLISH #w77e/3/0/51200
ID/SequenceNumber/MoreFlag/FragmentDataSize

6. Content Format Negotiation
In HTTP binding, for negotiation, it uses “Accept” and “Content-type” in HTTP Header Fields
In MQTT binding TS-0010, section content format negotiation describes the similar concept as in HTTP binding
To this end, we can define parameters for example,
MQ-M2M-ACCEPT
MQ-M2M-DATATYPE
And this information could be included in MQTT payload

7. Example MQTT Broker MQTT Client MQTT Client PUBLISH
Request
PUBLISH
MQ-M2M-ACCEPT=“application/onem2m-res+xml”
MQ-M2M-DATATYPE=“application/onem2m-res+xml”
Response
PUBLISH
MQ-M2M-DATATYPE=“application/onem2m-res+xml”

8. Protocol Binding Option1
Allocate bytes in MQTT payload for fragmentation & content format negotiation information and save the information as string data format
MQTT Payload
Fragmentation Part
Negotiation Part
“#w77e/0/1/51200”
“MQ-M2M-ACCEPT=application/onem2m-res+xml,
MQ-M2M-DATATYPE=application/onem2m-res+xml”

9. Protocol Binding Option2
Allocate MQTT payload byte in more detail for each fragmentation & content format negotiation information
MQTT Payload
Fragmentation Part
Negotiation Part ID
Sequence Number
More Flag
FragmentDataSize
Example
10bytes
20bytes
1bytes
30bytes

10. Protocol Binding MQTT Payload Fragmentation Part Negotiation Part
MQ-M2M-ACCEPT
MQ-M2M-DATATYPE
Example
30bytes
30bytes