Specification | API (on MDN)
https://www.websocket.org/ : Collection of demos, articles, and products
"Writing a WebSocket server in C# / Java / Javascript (Deno)":
WebSockets communicate over a TCP (Transmission Control Protocol) connection. When a client connects to a server, it sends a GET request to upgrade the connection to a WebSocket from a simple HTTP request. This is known as handshaking.
You must,
Sec-WebSocket-Key request header without any leading and trailing whitespace258EAFA5-E914-47DA-95CA-C5AB0DC85B11Sec-WebSocket-Accept response header as part of an HTTP response.After a successful handshake, client can send messages to the server, but now these are encoded.
If we send "abcdef", we get these bytes:
129 134 167 225 225 210 198 131 130 182 194 135
| FIN (Is this the whole message?) | RSV1 | RSV2 | RSV3 | Opcode |
|---|---|---|---|---|
| 1 | 0 | 0 | 0 | 0x1=0001 |
FIN: You can send your message in frames, but now keep things simple. Opcode 0x1 means this is a text. Full list of Opcodes
134: If the second byte minus 128 is between 0 and 125, this is the length of the message. If it is 126, the following 2 bytes (16-bit unsigned integer), if 127, the following 8 bytes (64-bit unsigned integer, the most significant bit MUST be 0) are the length.
Note: It can take 128 because the first bit is always 1.
167, 225, 225 and 210 are the bytes of the key to decode. It changes every time.
Decoding algorithm: decoded byte = encoded byte XOR (position of encoded byte BITWISE AND 0x3)th byte of key
//
// csc wsserver.cs
// wsserver.exe
using System;
using System.Net;
using System.Net.Sockets;
using System.Text;
using System.Text.RegularExpressions;
class Server {
public static void Main() {
string ip = "127.0.0.1";
int port = 80;
var server = new TcpListener(IPAddress.Parse(ip), port);
server.Start();
Console.WriteLine("Server has started on {0}:{1}, Waiting for a connection...", ip, port);
TcpClient client = server.AcceptTcpClient();
Console.WriteLine("A client connected.");
NetworkStream stream = client.GetStream();
// enter to an infinite cycle to be able to handle every change in stream
while (true) {
while (!stream.DataAvailable);
while (client.Available < 3); // match against "get"
byte[] bytes = new byte[client.Available];
stream.Read(bytes, 0, bytes.Length);
string s = Encoding.UTF8.GetString(bytes);
if (Regex.IsMatch(s, "^GET", RegexOptions.IgnoreCase)) {
Console.WriteLine("=====Handshaking from client=====\n{0}", s);
// 1. Obtain the value of the "Sec-WebSocket-Key" request header without any leading or trailing whitespace
// 2. Concatenate it with "258EAFA5-E914-47DA-95CA-C5AB0DC85B11" (a special GUID specified by RFC 6455)
// 3. Compute SHA-1 and Base64 hash of the new value
// 4. Write the hash back as the value of "Sec-WebSocket-Accept" response header in an HTTP response
string swk = Regex.Match(s, "Sec-WebSocket-Key: (.*)").Groups[1].Value.Trim();
string swka = swk + "258EAFA5-E914-47DA-95CA-C5AB0DC85B11";
byte[] swkaSha1 = System.Security.Cryptography.SHA1.Create().ComputeHash(Encoding.UTF8.GetBytes(swka));
string swkaSha1Base64 = Convert.ToBase64String(swkaSha1);
// HTTP/1.1 defines the sequence CR LF as the end-of-line marker
byte[] response = Encoding.UTF8.GetBytes(
"HTTP/1.1 101 Switching Protocols\r\n" +
"Connection: Upgrade\r\n" +
"Upgrade: websocket\r\n" +
"Sec-WebSocket-Accept: " + swkaSha1Base64 + "\r\n\r\n");
stream.Write(response, 0, response.Length);
} else {
bool fin = (bytes[0] & 0b10000000) != 0,
mask = (bytes[1] & 0b10000000) != 0; // must be true, "All messages from the client to the server have this bit set"
int opcode = bytes[0] & 0b00001111, // expecting 1 - text message
offset = 2;
ulong msglen = bytes[1] & (ulong)0b01111111;
if (msglen == 126) {
// bytes are reversed because websocket will print them in Big-Endian, whereas
// BitConverter will want them arranged in little-endian on windows
msglen = BitConverter.ToUInt16(new byte[] { bytes[3], bytes[2] }, 0);
offset = 4;
} else if (msglen == 127) {
// To test the below code, we need to manually buffer larger messages — since the NIC's autobuffering
// may be too latency-friendly for this code to run (that is, we may have only some of the bytes in this
// websocket frame available through client.Available).
msglen = BitConverter.ToUInt64(new byte[] { bytes[9], bytes[8], bytes[7], bytes[6], bytes[5], bytes[4], bytes[3], bytes[2] },0);
offset = 10;
}
if (msglen == 0) {
Console.WriteLine("msglen == 0");
} else if (mask) {
byte[] decoded = new byte[msglen];
byte[] masks = new byte[4] { bytes[offset], bytes[offset + 1], bytes[offset + 2], bytes[offset + 3] };
offset += 4;
for (ulong i = 0; i < msglen; ++i)
decoded[i] = (byte)(bytes[offset + i] ^ masks[i % 4]);
string text = Encoding.UTF8.GetString(decoded);
Console.WriteLine("{0}", text);
} else
Console.WriteLine("mask bit not set");
Console.WriteLine();
}
}
}
}
Deno.serve({
port: 80,
handler: async (request) => {
// If the request is a websocket upgrade,
// we need to use the Deno.upgradeWebSocket helper
if (request.headers.get("upgrade") === "websocket") {
const { socket, response } = Deno.upgradeWebSocket(request);
socket.onopen = () => {
console.log("CONNECTED");
};
socket.onmessage = (event) => {
console.log(`RECEIVED: ${event.data}`);
socket.send("pong");
};
socket.onclose = () => console.log("DISCONNECTED");
socket.onerror = (error) => console.error("ERROR:", error);
return response;
} else {
// If the request is a normal HTTP request, we serve the client HTML file.
const file = await Deno.open("./index.html", { read: true });
return new Response(file.readable);
}
},
});
Last modified 18 May 2025