WebSocket

WebSocket protocol enable the two-way communication between client and server. In 2011, the IETF standardized the WebSocket protocol as RFC 6455. Since then,many browser started implementing and supporting web socket protocol. You can get the browsers that support web socket protocol from below site.

"http://caniuse.com/websockets"

Websocket protocol extends the HTTP Protocol, by upgrading HTTP connection to a websocket. Once the connection is upgraded,then client and server acts as two peers and can able to communicate in both directions and able to transfer 'n' number of messages.

Basic idea of web socket is very simple. Lets say you have a client and a server. Then Websocket communication is established in 2 stages.

    1. Handshake

    2. Data Transfer

Handshake

Clients and server communicate using HTTP. HTTP supports upgrade mechanism using 'Upgrade' header field. By using this field, you can upgrade one version of HTTP to other version. WebSocket also uses this mechanism to set up a connection with a HTTP server in a compatible way.

First, a client requests a websocket connection by using the "Upgrade: websocket" and "Connection: Upgrade" headers, along with a few protocol-specific headers to establish the version being used and set-up a handshake.

The server, if it supports the protocol, replies with the same "Upgrade: websocket" and "Connection: Upgrade" headers and completes the handshake. Once the Handshake completes then, both peers can communicate 'n' number of messages.

Simple terms, Websocket uses HTTP to upgrade the connection to support Websocket protocol. Once the Websocket protocol established, none of the HTTP semantics valid on this connection.

How the Handshake Request looks like

First time client sends simple HTTP Request, and request server to upgrade to websocket protocol.

GET /index HTTP/1.1
Host: localhost:8080
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Sec-WebSocket-Version: 13 

'Upgrade: websocket' tells to the server, that I want to upgrade to websocket.

How the Handshake Response looks like

HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: Nb+FaERLaX6uN3HUNNoFAAU8CPk=

If server able to support websocket protocol, then it switches the protocols from HTTP to websocket and send the response. The server applies a known operation to the value of the Sec-WebSocket-Key (Which send by client in request header) header to generate the value of the Sec-WebSocket-Accept header. Client performs the same operation on Sec-WebSocket-Key and computes a value. Checks the computed value with the value of 'Sec-WebSocket-Accept' sent by server. If the result matches the value received from the server. The client and the server can send messages to each other after a successful handshake.

Data Transfer

Once the Handshake done successfully, then client and server are connected. Once the connection established, both client and server acts as peers, i.e, they are capable of sending and receiving 'n' number of messages. Client or server can initiate the termination of connection at any point of time.

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