Introduction to Websockets¶

The WebSocket specification is a new feature of HTML5. It defines a full-duplex single socket connection over which messages can be sent between client and server. The WebSocket standard simplifies much of the complexity around bi-directional web communication and connection management. The Websocket standard reduces polling and the unnecessary network throughput overhead.

_{Image courtesy of Kaazing}

We can see on this figure the reduction in latency. Once the connection is established, messages can flow from the server to the browser. As the connection remains open, there is no need to send another request to the server. If you want to see some other comparisons, just click on the image above or visit WebSocket.org

The Websocket protocol¶

To establish a WebSocket connection, the client and server upgrade from the HTTP protocol to the WebSocket protocol during their initial handshake. There are several handshake mechanisms. I will just present the basic handshake of one of them in this article (the websocket protocol 76). It's the one which is implemented in mbed websocket library here. For more information, please refer to: protocol-76.

The handshake from the client is as follows:

        GET /demo HTTP/1.1
        Host: example.com
        Connection: Upgrade
        Sec-WebSocket-Key2: 12998 5 Y3 1  .P00
        Sec-WebSocket-Protocol: sample
        Upgrade: WebSocket
        Sec-WebSocket-Key1: 4 @1  46546xW%0l 1 5
        Origin: http://example.com

        ^n:ds[4U

The handshake from the server:

        HTTP/1.1 101 WebSocket Protocol Handshake
        Upgrade: WebSocket
        Connection: Upgrade
        Sec-WebSocket-Origin: http://example.com
        Sec-WebSocket-Location: ws://example.com/demo
        Sec-WebSocket-Protocol: sample

        8jKS'y:G*Co,Wxa-

Once the Websocket connection is established, data can be exchanged according to this format:

• Send a 0x00 byte

• Encode the message using UTF-8 and send the resulting byte stream

• Send a 0xFF byte

Websocket on Mbed¶

There is here, a simple websocket client library which must be used with a Roving Networks wifi module (Wifly RN131). This library has been used in the Internet of Things project.