HTTP Upgrade Handshake

WebSockets start as HTTP/1.1 on TCP port 80 or 443, then switch to a binary framing protocol. The client sends:

c
Loading…

The server replies 101 Switching Protocols with Sec-WebSocket-Accept set to Base64(SHA1(key + 258EAFA5-E914-47DA-95CA-C5AB0DC85B11)).

Frame Format

After the handshake, messages use 2-14 byte headers plus payload:

FIN (1 bit): final fragment flag
Opcode (4 bits): 1 = text, 2 = binary, 8 = close, 9 = ping, 10 = pong
MASK (1 bit): client frames must set this
Payload length: 7, 16, or 64 bits depending on size
Masking key: 4 bytes, XORed with payload (client-to-server only)

Payload length uses a compact encoding: values under 126 fit in 7 bits, 126 means the next 2 bytes hold the length, and 127 means the next 8 bytes hold it. This lets a single text frame carry a short chat message or a large binary blob over the same TCP connection.

Why This Exercise

This task asks you to document the handshake and parse a WebSocket frame. RFC 6455 mandates client-side masking specifically to prevent cache-poisoning attacks against misbehaving HTTP proxies. nginx's proxy_pass WebSocket support and every browser's WebSocket API perform this same Sec-WebSocket-Accept SHA-1 handshake before switching protocols on port 443 or 80.

Write a C program that documents the WebSocket handshake and parses a WebSocket frame. Show key generation, accept calculation, and frame decode.

Requirements:

Generate a random 16-byte nonce and Base64-encode it to create Sec-WebSocket-Key
Compute Sec-WebSocket-Accept by concatenating the key with the GUID, computing SHA-1, and Base64-encoding
Print the client handshake request showing the Upgrade and Sec-WebSocket-Key headers
Print the server handshake response showing the 101 status and Sec-WebSocket-Accept
Define the WebSocket frame structure (FIN, opcode, MASK, length, masking key, payload)
Parse a sample frame byte array and extract each field
If the frame is masked, show how to unmask the payload with XOR
Print a breakdown of each frame field with printf

HTTP Upgrade Handshake

WebSockets start as HTTP/1.1 on TCP port 80 or 443, then switch to a binary framing protocol. The client sends:

c
Loading…

The server replies 101 Switching Protocols with Sec-WebSocket-Accept set to Base64(SHA1(key + 258EAFA5-E914-47DA-95CA-C5AB0DC85B11)).

Frame Format

After the handshake, messages use 2-14 byte headers plus payload:

FIN (1 bit): final fragment flag
Opcode (4 bits): 1 = text, 2 = binary, 8 = close, 9 = ping, 10 = pong
MASK (1 bit): client frames must set this
Payload length: 7, 16, or 64 bits depending on size
Masking key: 4 bytes, XORed with payload (client-to-server only)

Why This Exercise

Write a C program that documents the WebSocket handshake and parses a WebSocket frame. Show key generation, accept calculation, and frame decode.

Requirements:

Generate a random 16-byte nonce and Base64-encode it to create Sec-WebSocket-Key
Compute Sec-WebSocket-Accept by concatenating the key with the GUID, computing SHA-1, and Base64-encoding
Print the client handshake request showing the Upgrade and Sec-WebSocket-Key headers
Print the server handshake response showing the 101 status and Sec-WebSocket-Accept
Define the WebSocket frame structure (FIN, opcode, MASK, length, masking key, payload)
Parse a sample frame byte array and extract each field
If the frame is masked, show how to unmask the payload with XOR
Print a breakdown of each frame field with printf

Implement a WebSocket Handshake and Frame Parser

Topic Overview

HTTP Upgrade Handshake

Frame Format

Why This Exercise

Implementation Task

Progressive Hints

Resources

Learn by building

Implement a WebSocket Handshake and Frame Parser

Topic Overview

HTTP Upgrade Handshake

Frame Format

Why This Exercise

Implementation Task

Progressive Hints

Resources

Learn by building