R-Pi
Team Emertxe
IoT Protocols
CoAP
CoAP
Introduction
Is one of the latest application layer protocol developed by IETF for smart devices to connect to Internet
Many devices exist as components in vehicles and buildings with constrained resources, it leads a lot of variation in
power computing, communication bandwidth etc
lightweight protocol CoAP is intended to be used and considered as a replacement of HTTP for being an IoT application
layer protocol
CoAP
Features
Constrained web protocol fulfilling M2M requirements
Security binding to DTLS(Datagram Transport Layer Security)
Asynchronous message exchanges
Low header overhead and parsing complexity
URI and content type support
Simple proxy and caching capabilities
UDP binding with optional reliability supporting unicast and multicast requests
CoAP
Structure Model
The bottom layer is Message layer that has been designed to deal with UDP and asynchronous switching
The request/response layer concerns communication method and deal with request/response message
CoAP: Structure Model
Message Layer Model
Message Layer supports 4 types of messages
CON (confirmable)
NON (non-confirmable)
ACK (Acknowledgement)
RST (Reset)
CoAP: Structure Model
Message Layer Model
Reliable message transport
Keep retransmission until get ACK with the same message ID (like 0x8c56 in fig.)
Using default time out and decreasing counting time exponentially when transmitting CON
If recipient fail to process message, it responses by replacing ACK with RST.
Fig shows a reliable message transport
CoAP: Structure Model
Request/Response Layer Model
Piggy-backed
Client sends request using CON type or NON type message and receives response ACK with confirmable
message immediately
fig. shows successful response, ACK contain response message (identify by using token), for failure response,
ACK contain failure response code
CoAP: Structure Model
Request/Response Layer Model
Separate response
If server receive a CON type message but not able to response this request immediately, it will send an empty
ACK in case of client resend this message
When server ready to response this request, it will send a new CON to client and client reply a confirmable
message with acknowledgment
ACK is just to confirm CON message, no matter CON message carry request or response
CoAP: Structure Model
Request/Response Layer Model
Non confirmable request and response
unlike Piggy-backed response carry confirmable message, in Non confirmable request client send NON type
message indicate that Server don't need to confirm
Server will resend a NON type message with response
CoAP: Structure Model
Message Format
CoAP is based on the exchange of compact messages that, by default, are transmitted over UDP (i.e. each CoAP
message occupies the data section of one UDP datagram)
Message of CoAP uses simple binary format
Message= fixed-size 4-byte header plus a variable-length Token plus a sequence of CoAP options plus payload
CoAP: Security + Application
Why use DTLS for CoAP Security
CoAP is now becoming the standard protocol for IoT applications
Security is important to protect the communication between devices
A security protocol DTLS is introduced
There are three main elements when considering security, namely integrity, authentication and confidentiality, DTLS can
achieve all of them
DTLS employ TCP, which is too complex
DTLS solves two problems
reordering and packet lost
It adds three implements
packet retransmission
assigning sequence number within the handshake
replay detection
CoAP: Security + Application
Why use DTLS for CoAP Security
DTLS in application layer protect end-to-end communication
No end-to-end communication protection will make it easy for attacker to access to all text data that passes through a
compromised node
DTLS also avoids cryptographic overhead problems that occur in lower layer security protocols
CoAP: Security + Application
Structure of DTLS
CoAP: Security + Application
Application for Smart Homes
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