Distributed Systems and IoT Architecture

By accessing communication peripherals, such as network controllers and radio interfaces, microcontrollers are able to establish data communication with nearby devices and even with remote servers through the internet.

A set of embedded targets connected together and interacting with each other can be seen as a self-contained distributed system. Homogeneous machine-to-machine communication can be implemented using non-standard, and even proprietary, protocols.

Depending on the set of standard protocols it implements, an embedded system may be able to successfully communicate with heterogeneous, remote systems. Implementing standard protocols that are standardized or widely supported introduces the possibility to interact with gateways in the same geographic area, and with remote cloud servers across the internet.

The connectivity range of small, embedded devices may include remote coordination using Information Technology (IT) systems...

Technical requirements

In this chapter, we assume that you are familiar with general concepts of modern computer networking, although no previous experience with distributed applications is required. For a more complete background on network programming, which is relevant to the content of this chapter, we suggest, as further reading, Hands On Network Programming with C (L. Van Winkle – Packt Publishing 2019). There are no specific examples provided in the book’s repository for this chapter. More complete examples of TCP and Transport Layer Security (TLS) client/server communication can be found in the source code distribution of the open source projects presented here.

Network interfaces

Embedded devices often integrate one or more communication interfaces. Many microcontrollers integrate the Media Access Control (MAC) portion of an Ethernet interface, so connecting a Physical Layer Transceiver (PHY) would enable LAN access. Some devices are coupled with radio transceivers, operating at fixed frequency ranges and implementing one or more protocols to communicate over wireless links. Frequently used frequencies for wireless communication are the 2.4 GHz band, in use by Bluetooth and 802.11 Wi-Fi, and some specific ISM ranges of frequency below 1 GHz, which depend on local regulations. Usable sub-GHz frequencies include the 868 MHz ISM band in the European Union and the 915 MHz ISM band in the US. Transceivers are usually designed to access the physical layer according to specific link protocols, regulating shared access to the physical media among two or more devices. While two interfaces accessing the same media can have different configurations...

The Internet protocols

Standardized at the beginning of the 1980s, the IP stack, mostly referred to nowadays as TCP/IP, is a family of network, transport, and application protocols providing standard communication over a wide range of technologies and interfaces. In the upcoming subsections, we will discuss the integration of these standard protocols into embedded systems, describe the interfaces that embedded applications use to communicate with remote endpoints, and learn how to interact with the different layers of the stack, from the network interfaces up to the socket abstraction to establish connections or connectionless sessions with a remote peer.

Standard protocols, custom implementations

Designing distributed communication using non-standard protocol stacks is, in almost all cases, not worth the effort required to reinvent state-of-the-art technology. TCP/IP standards have been the subject of extensive research for many decades, and have been the main building block...

TLS

Link-layer protocols often provide some basic security mechanisms to guarantee the authentication of the client connecting to a specific network and encrypt data by using symmetric keys such as AES. In most cases, authentication at the link layer is sufficient to guarantee a basic level of security. Nevertheless, pre-shared, well-known keys often used in LR-WPAN network stacks may be vulnerable to multiple kinds of attacks, and using a pre-shared key would allow an attacker to decipher any traffic that has been previously captured on the same link if the key was compromised. In other scenarios, encryption alone is not sufficient to guarantee that the other endpoint is what it claims to be, or that the data flow has not been altered during transmission.

A device that takes part in an IoT-distributed system is required to implement a higher grade of security, especially in embedded devices that do not protect the memory in any way and where any backdoor means that attackers can...

Application protocols

In order to be able to communicate with remote devices and cloud servers in a distributed scenario, embedded systems must implement standard protocols that are compatible with the existing infrastructure. Two of the most common approaches taken when designing remote services are as follows:

• Web-based services
• Message protocols

The former is mainly the classic, client-server, Representational State Transfer (REST)-based communication that is popular in web services accessed through personal computers or portable devices. Web services require no adaptation in particular on the cloud side to support embedded systems, except for the choice of an embedded-friendly cipher set, as described in the Securing socket communication section. However, the request-reply communication model introduces some restrictions on the design of distributed applications. The HTTP protocol can be upgraded by common agreement on the two HTTP endpoints, and support WebSocket...

Summary

This chapter has given us an overview of the design of machine-to-machine distributed systems and IoT services, including connected embedded devices, with a focus on security elements that are too often overlooked or underestimated in embedded development. The technology proposed allows full, professional-grade, secure, and fast TCP/IP connectivity on very small targets and uses state-of-the-art technology, such as the most recent version of the TLS cipher suites. Several approaches have been considered, both in terms of hardware and software technologies available for microcontroller-based targets, for a broader view of the technologies, protocols, and security algorithms available for building distributed embedded systems.

The next chapter will illustrate the multitasking possibilities of modern embedded microcontrollers by explaining how to write a small scheduler for Cortex-M microprocessors from scratch, and will summarize the key roles of a real-time operating system...