CoAP - 适用于物联网的受限制的应用协议
2016-05-06 13:47:09 来源：华启智能
在由PC机组成的世界，信息交换是通过TCP和应用层协议HTTP实现的。但是对于小型设备而言，实现TCP和HTTP协议显然是一个过分的要求。为了让小设备可以接入互联网，CoAP协议被设计出来。CoAP是一种应用层协议，它运行于UDP协议之。CoAP协议非常小巧，最小的数据包仅为4字节。
RFC 7252 Constrained Application Protocol
“The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained networks in the Internet of Things.
The protocol is designed for machine-to-machine (M2M) applications such as smart energy and building automation.”
REST model for small devices
Like HTTP, CoAP is based on the wildly successful REST model: Servers make resources available under a URL, and clients access these resources using methods such as GET, PUT, POST, and DELETE.
Existing skills transfer
From a developer point of view, CoAP feels very much like HTTP. Obtaining a value from a sensor is not much different from obtaining a value from a Web API.
Ready for integration
Since HTTP and CoAP share the REST model, they can easily be connected using application-agnostic cross-protocol proxies. A Web client may not even notice that it just accessed a sensor resource!
Choose your data model
Like HTTP, CoAP can carry different types of payloads, and can identify which payload type is being used. CoAP integrates with XML, JSON,CBOR, or any data format of your choice.
Made for billions of nodes
The Internet of Things will need billions of nodes, many of which will need to be inexpensive. CoAP has been designed to work on microcontrollers with as low as 10 KiB of RAM and 100 KiB of code space (RFC 7228).
Keep waste in check
CoAP is designed to use minimal resources, both on the device and on the network. Instead of a complex transport stack, it gets by with UDP on IP. A 4-byte fixed header and a compact encoding of options enables small messages that cause no or little fragmentation on the link layer. Many servers can operate in a completely stateless fashion.
Discovery integrated
The CoAP resource directory provides a way to discover the properties of the nodes on your network.
Well-designed protocol
CoAP was developed as an Internet Standards Document, RFC 7252. The protocol has been designed to last for decades. Difficult issues such as congestion control have not been swept under the rug, but have been addressed using the state of the art.
Secure
The Internet of Things cannot spread as long as it can be exploited by hackers nilly-willy. CoAP does not just pay lip service to security, it actually provides strong security. CoAP's default choice of DTLS parameters is equivalent to 3072-bit RSA keys, yet still runs fine on the smallest nodes.
