WebSockets Internals

aorgish

Problem

Historically, creating web applications that need bidirectional communication between a client and a server (e.g., instant messaging and gaming applications) has required an abuse of HTTP to poll the server for updates while sending upstream notifications as distinct HTTP calls

Lot of connections

HTTP overhead

RFC 6202 Known Issues and Best Practices for the Use of Long Polling and Streaming in Bidirectional HTTP

A simpler solution would be to use a single TCP connection for traffic in both directions. This is what the WebSocket Protocol provides. Combined with the WebSocket API [WSAPI], it provides an alternative to HTTP polling for two-way communication from a web page to a remote server.

The main goal of the protocol is to allow bidirectional communication between an application running within a UA such as a web browser, and an application running on a distant server, not limited by the domain-origin or any other policy.

SubGoals

Coexist with HTTP servers on ports 80 and 443 Differentiate from HTTP enough to cause WS clients to know a WS server from HTTP server Make sure that intermediaries do not confuse WS data content for valid HTTP requests

Handshake

ws://www.domain.com/path

wss://www.domain.com/path

Client request

GET /chat HTTP/1.1
Host: server.example.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Origin: http://example.com
Sec-WebSocket-Protocol: chat, superchat
Sec-WebSocket-Version: 13

Server response

HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=
Sec-WebSocket-Protocol: chat

WS Frame

Opcodes

x0 denotes a continuation frame
x1 denotes a text frame
x2 denotes a binary frame
x3-7 are reserved for further non-control frames
x8 denotes a connection close
x9 denotes a ping
xA denotes a pong
xB-F are reserved for further control frames

Ping-Pong

SubProtocols

Sec-WebSocket-Protocol: protocol-list

WebSocket Subprotocol Name Registry

WAMP

"The WebSocket Application Messaging Protocol"

Publish & Subscribe (PubSub)
Remote Procedure Calls (RPC)

STOMP

"Simple Text Orientated Messaging Protocol."

STOMP provides an interoperable wire format so that STOMP clients can communicate with any STOMP message broker to provide easy and widespread messaging interoperability among many languages, platforms and brokers.

WebSockets Internals – Problem – Handshake

aorgish

WebSockets Internals – Problem – Handshake

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