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NOTE

2.2 WebSocket

WebSocket's full-duplex message channel, HTTP/1.1 upgrade handshake, framing, masking, ping/pong, and comparison with polling, SSE, and ordinary HTTP.

Computer NetworksCreated Updated 1 min readhistorical

This is a historical learning note and may contain outdated or incomplete understanding.

1. What Is WebSocket?

WebSocket is a protocol for long-lived, full-duplex message exchange between client and server.

It is useful when both sides need to send data independently without creating a new HTTP request for every server-originated event.

2. Why Use It?

Before or instead of WebSocket, applications may use:

  • periodic polling;
  • long polling;
  • Server-Sent Events (SSE) for server-to-client streams;
  • streaming HTTP responses.

WebSocket is most appropriate when bidirectional low-latency messaging is valuable, such as chat, collaborative applications, games, or control channels.

It is not automatically better than HTTP for every real-time workload; SSE or HTTP streaming can be simpler for one-way updates.

3. Handshake

With HTTP/1.1, WebSocket commonly begins with an Upgrade request:

GET /chat HTTP/1.1
Host: server.example.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: ...
Sec-WebSocket-Version: 13

The server confirms with:

HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: ...

After the handshake, communication follows WebSocket framing rather than ordinary HTTP request/response messages.

Modern HTTP/2 environments can also establish WebSocket semantics through extended CONNECT rather than the classic HTTP/1.1 Upgrade mechanism when supported.

4. Frames

Important frame fields include:

  • FIN — final fragment of a message;
  • opcode — continuation, text, binary, close, ping, or pong;
  • payload length;
  • mask bit and masking key;
  • payload data.

Client-to-server frames are masked; server-to-client frames are not.

Large logical messages can be fragmented across frames.

5. Liveness

WebSocket defines ping and pong control frames. Applications commonly combine them with timeouts to detect dead peers and intermediaries.

Do not confuse WebSocket’s persistent connection with HTTP keep-alive: HTTP keep-alive reuses a transport connection across multiple request/response transactions, while WebSocket switches to a long-lived bidirectional message protocol.