WebSocket is defined in RFC 6455 (2011). Unlike HTTP which sends data as plain text headers and body, WebSocket communicates through compact frames — binary packets with a precise bit-level structure.
Every WebSocket connection starts as a normal HTTP/1.1 request. The client sends special headers asking to "upgrade" the protocol:
Step 1 — Client sends HTTP Upgrade request:
GET /chat HTTP/1.1
Host: server.example.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Sec-WebSocket-Version: 13
Step 2 — Server responds with 101 Switching Protocols:
HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=
The Sec-WebSocket-Accept value is computed as:
base64( SHA-1( Sec-WebSocket-Key + "258EAFA5-E914-47DA-95CA-C5AB0DC85B11" ) )
This proves the server genuinely understands the WebSocket protocol.
After the 101 response, the TCP connection is repurposed. The HTTP handshake is over — both sides now speak WebSocket frames directly over the same socket. No more HTTP.
Each message is broken into one or more frames. Here is the binary structure of a frame:
| Field | Size | Description |
|---|---|---|
| FIN | 1 bit | 1 = final fragment of the message |
| RSV1-3 | 3 bits | Reserved for extensions (usually 000) |
| Opcode | 4 bits | Type of frame (text, binary, close, ping, pong) |
| MASK | 1 bit | Client→Server MUST always be masked |
| Payload len | 7 bits | 0–125 = length; 126 = next 2 bytes; 127 = next 8 bytes |
| Masking key | 32 bits | 4-byte XOR key (client frames only) |
| Payload | variable | Message bytes, XOR'd with masking key |
| Opcode | Name | Description |
|---|---|---|
| 0x0 | Continuation | A fragment continuing a previous fragmented message |
| 0x1 | Text | UTF-8 encoded text data (most common) |
| 0x2 | Binary | Raw binary — images, files, audio, etc. |
| 0x8 | Close | Initiates graceful connection teardown |
| 0x9 | Ping | Heartbeat request — peer MUST reply with Pong |
| 0xA | Pong | Heartbeat reply to a Ping frame |