Real-time systems - networks and protocols

The previous article described how a real-time system is one in which timing is a key characteristic. The real-world consists of many events that can occur randomly and inherent properties of the specific system can take an unpredictable amount of time to process. Real-time system design consists of ensuring that events are processed within a predictable timeframe, so that any timing deadlines are met.

To explore real-time systems in the context of the web or the IoT, it is necessary to look at the properties of the underlying system, specifically the network infrastructure and the protocols used to manage the transfer of data. This allows an assessment of the real-time properties of these systems.

Network infrastructure

The network infrastructure underlying the Internet is potentially unreliable. Packets can be lost in transmission, traffic is variable, protocols add varying degrees of overhead, servers can vary in performance and response time, network nodes and routers can fail, and bandwidth varies considerably from low-speed ADSL to high-speed fibre links. This makes the Internet inherently unpredictable, and as has been described, determinism (predictability) is a desirable property of real-time systems - predictable timing is desirable so it is possible to meet any timing constraints.

There's not much that can be done by the application developer to chnage this inherently problematic state of affairs, it is the nature of the beast. The good news however is that it was known that the Internet would be unreliable since its conception, and so its design has taken this into account.

The "oil" that keeps the Internet running smoothly despite its unreliable nature are protocols. These protocols keep packets moving over this unreliable and unpredictable infrastructure.

Protocols

The main protocol stack running on the Internet today is the TCP/IP protocol stack. This is a layered stack that consists of the following layers:

  1. Application (Sockets, HTTP)
  2. Transport (TCP/UDP)
  3. Internet (IP)
  4. Link / physical (Ethernet, PPP)

The application layer is the highest layer.

These protocols attempt to deal with the problems of an unreliable network by adding different levels of overhead. For example, Transmission Control Protocol (TCP) offers a connection-oriented protocol where an end-to-end connection is established. Any lost or corrupted packets, or packets that arrive out of order, are dealt with by the protocol, and may require a handshake that resends the required packets. TCP therefore provides a reliable connection, where packet delivery is guaranteed.

User Datagram Protocol (UDP) on the other hand does not guarantee delivery of packets. Packets may arrive out of order and may also be lost or corrupted. UDP is known as a connectionless protocol, it does not establish a reliable end-to-end transmission stream. However, compared to TCP, it is a relatively lightweight protocol. It is used in applications where the occasional loss or corruption of packets can be handled by the application or ignored on some cases. UDP is used for example as a way a game server can update clients. Should network conditions deteriorate, then packets will be lost and the client application can use techniques such as client-side prediction to compensate. If conditions deteriorate further it could mean the game drops frame and becomes jerky. This may not be deemed to be a "show stopper" and it may be that once the network conditions improve the client simply resynchromizes with the server and reliable gameplay continues. TCP is deemed to add unnecessary overhead to the network connection here and so is not used in this scenario.

Protocols at the application level can be quite varied and examples are FTP, Telnet, HTTP, web sockets, and the low-level sockets API. Much of the web today is based on the HTTP protocol. This is essentially a round-trip, stateless, request-response protocol. The client, typically a web browser, will send an HTTP request packet to a remote server. The remote server will process the request, which may consist of loading a web page or performing a database query and generating HTML to return to the client in a response. The web browser will then display the response as a web page.

This round-trip type protocol is relatively slow and unpredictable due to network latencies and variable performance over the connection. This is also a half-duplex mode of communication - in HTTP you have to wait for a response - the client cannot transmit and receive a stream of packets at the same time using HTTP. HTTP is therefore not an ideal protocol where speedy full-duplex high speed data streams are involved. To improve matters some techniques have appeared over the years - AJAX being one. This allows the browser to communicate with a server and obtain a response using JavaScript and XML, without the roundtrip overhead usually associated with full HTTP.

To improve matters further, and allow high-volume data streams to be exchanged between a server and a client in full-duplex communication, new protocols such as web sockets have been established. Web sockets sits on TCP, so is designed for connection-oriented, reliable, high volume, full-duplex data transfers. This protocol will be described further in upcoming articles in the series in the context of the "real-time web".