Iot sdk thesis

Figure 2.5.5.1.1 DTLS Handshake Messages with Cookie Replay [23] 

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Table 2.5.1.2 can be referred for the description of messages in DTLS handshake, with 
only addition being stateless cookie exchange, the purpose of which has been explained in this 
section. 
Handshake message fragmentation and re-assembly 
All handshake messages must be delivered without loss for a successful session 
negotiation and establishment. Some messages might be too large to be fit in a single UDP 
datagram. In that case, those messages must be fragmented into several datagram and 
reassembled. The responsibility of reassembly lies in the record layer of DTLS protocol. 
To ensure reliability of datagrams, a retransmission mechanism has to be in-place. 
2.5.6
 
Timeout and Retransmission 
As DTLS handshake messages are transported with UDP datagrams, they can get lost 
in transmission. In order to compensate for this reliability, each end-point keeps retransmitting 
the messages that do not get any reply. The retransmission is implemented using a single-timer 
at each end-point.
When messages are being read, if the received fragment is an expected next handshake 
message, the timer is reset. Else, the timer is allowed to tick. Retransmission of last fight of 
messages is triggered as soon as the timer expires [23]. 
There is no fixed formula to decide upon the timer value. The round-trip time (RTT) is 
difficult to estimate given the variety of message lengths and network conditions. The typical 
recommended values for timer are 500 ms to 1000 ms. 
2.5.7
 
DTLS: Role in Thesis 
The LWM2M device management implementation used in thesis is Eclipse Wakaama.
Figure 2.5.7.1 DTLS in LWM2M stack for Thesis 

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Wakaama is written in C and uses Erbium-based CoAP engine for web-transfer. The 
CoAP messages over UDP can be secured by DTLS. Wakaama uses tinyDTLS library to 
provide encryption to the LWM2M traffic between client and the server.
In an ongoing research effort at the telecom vendor, the R&D team partially ported the 
Wakaama to Arduino-like client SODAQ SARA to enable unencrypted device management 
over NB-IoT. However, the ported version is a limited implementation of the protocol stack as 
it lacks DTLS: Wakaama exchanges unencrypted CoAP datagrams, which is not robust-enough. 
In the thesis, I worked on adding tinyDTLS support to the ported Wakaama LWM2M to 
integrate the protocol stack as it appears in Figure 2.5.7.1