WebSocket is a communication protocol providing full-duplex communication channels over a single TCP connection. It is designed to be used in web applications to enable interactive communication between a client (typically a web browser) and a server. WebSockets are initiated by a handshake using the HTTP protocol, which is then upgraded to the WebSocket protocol.
Key Features of WebSocket:
- Full-Duplex Communication: Both the client and server can send messages to each other independently.
- Single TCP Connection: After the initial handshake, communication continues over the same TCP connection.
- Efficient: Unlike HTTP, which requires a new connection for each request, WebSocket maintains a single connection, reducing overhead.
WebSocket Handshake
The WebSocket handshake is the process that initiates a WebSocket connection. It starts with the client sending an HTTP request to the server with an Upgrade header indicating the desire to upgrade to a WebSocket connection. The server then responds with headers confirming the upgrade.
Example of a WebSocket Handshake:
Client Request:
GET /chat HTTP/1.1
Host: example.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Sec-WebSocket-Version: 13Server Response:
HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=WebSocket Frame Format
After the handshake, the client and server exchange data frames. WebSocket frames have a specific format that includes an opcode, payload length, and the actual data. Below is the structure of a WebSocket frame:
- FIN: 1 bit - Indicates if this is the final fragment in a message.
- RSV1, RSV2, RSV3: 1 bit each - Reserved for future use.
- Opcode: 4 bits - Defines the interpretation of the payload data.
- Mask: 1 bit - Indicates if the payload data is masked.
- Payload length: 7 bits, 7+16 bits, or 7+64 bits - The length of the payload data.
- Masking key: 0 or 4 bytes - Used to unmask the payload data (present if Mask is 1).
- Payload data: (x+y) bytes - The actual data being sent.
Example of a WebSocket Frame (Hexdump)
Below is an example of a WebSocket text frame containing the message "Hello".
Unmasked Frame:
81 05 48 65 6C 6C 6FBreakdown:
81: FIN=1, Opcode=1 (text frame)05: Payload length is 5 bytes48 65 6C 6C 6F: Payload data "Hello"
Masked Frame:
81 85 37 FA 21 3D 7F 9F 4D 51 58Breakdown:
81: FIN=1, Opcode=1 (text frame)85: Mask=1, Payload length is 5 bytes37 FA 21 3D: Masking key7F 9F 4D 51 58: Masked payload data
To unmask the payload data, each byte of the payload is XOR'd with the corresponding byte of the masking key. Here's how to unmask the payload data in the example above:
7F ^ 37 = 48(H)9F ^ FA = 65(e)4D ^ 21 = 6C(l)51 ^ 3D = 6C(l)58 ^ 7F = 6F(o)
The unmasked payload data is "Hello".
Conclusion
WebSocket is a protocol based on TCP that enables full-duplex communication between a client and server over a single connection. It starts with an HTTP handshake and then switches to the WebSocket protocol for continuous communication. The protocol uses a specific frame format to exchange messages.
Below is a simple demo of WebSocket communication in C. This example will include a basic WebSocket server using the libwebsockets library, which is a lightweight C library for WebSocket clients and servers.
Prerequisites
-
Install the
libwebsocketslibrary. On Ubuntu, you can install it using:bashsudo apt-get install libwebsockets-dev
WebSocket Server Example
c
#include <libwebsockets.h>
#include <string.h>
#include <signal.h>
#include <stdio.h>
static int interrupted;
static const struct lws_protocols protocols[] = {
{
"example-protocol",
[](struct lws *wsi, enum lws_callback_reasons reason, void *user, void *in, size_t len) {
switch (reason) {
case LWS_CALLBACK_ESTABLISHED:
lwsl_user("Connection established\n");
break;
case LWS_CALLBACK_RECEIVE:
lwsl_user("Received data: %s\n", (char *)in);
lws_write(wsi, (unsigned char *)in, len, LWS_WRITE_TEXT);
break;
default:
break;
}
return 0;
},
0, 0, 0, 0
},
{ NULL, NULL, 0, 0, 0, 0 }
};
static void sigint_handler(int sig) {
interrupted = 1;
}
int main(void) {
struct lws_context_creation_info info;
struct lws_context *context;
signal(SIGINT, sigint_handler);
memset(&info, 0, sizeof info);
info.port = 7681;
info.protocols = protocols;
info.options = LWS_SERVER_OPTION_DO_SSL_GLOBAL_INIT;
context = lws_create_context(&info);
if (!context) {
lwsl_err("lws_create_context failed\n");
return 1;
}
while (!interrupted) {
lws_service(context, 1000);
}
lws_context_destroy(context);
return 0;
}Explanation
- Libraries and Headers : The example uses the
libwebsocketslibrary. - Signal Handling : A signal handler to cleanly exit the server on
Ctrl+C. - Protocols: Define the WebSocket protocols, in this case, "example-protocol".
- Callback Function: This function handles WebSocket events. It prints messages when a connection is established and when data is received. It also echoes the received data back to the client.
- Context Creation: Setup and creation of the WebSocket context.
- Service Loop: The main loop where the server waits for and processes WebSocket events.
- Cleanup: Clean up and free resources on exit.
Compilation
To compile the code, save it to a file (e.g., websocket_server.c) and use the following command:
bash
gcc websocket_server.c -o websocket_server -lwebsocketsRunning the Server
Execute the compiled program:
bash
./websocket_serverTesting the WebSocket Server
You can use a WebSocket client (e.g., a browser or a WebSocket client tool like wscat) to connect to the server:
bash
npm install -g wscat
wscat -c ws://localhost:7681After connecting, you can send messages, and the server will echo them back.
This example demonstrates a simple WebSocket server that can be expanded upon to handle more complex interactions and protocols.
握手请求
GET /ws/v2?aid=35&device_id=2256622095183710&access_key=4c344a03d3473afa881d8377f45dbeb6&fpid=1&sdk_version=3&iid=1896027950179704&pl=0&ne=1&version_code=91710&sid=11f8f93ec399434ca20a1b992abad1b7 HTTP/1.1
Host: frontier100-toutiao.toutiaoapi.com
Connection: Upgrade
Pragma: no-cache
Cache-Control: no-cache
x-support-ack: 1
Upgrade: websocket
Origin: wss://frontier-toutiao.snssdk.com
Sec-WebSocket-Version: 13
x-tt-store-region: cn-bj
x-tt-store-region-src: did
x-tt-request-tag: s=-1;p=1
X-SS-DP: 35
x-tt-trace-id: 00-2ac33dc5010c3808c8136bd3d7d70023-2ac33dc5010c3808-01
User-Agent: Dalvik/2.1.0 (Linux; U; Android 13; SM-E5260 Build/TP1A.220624.014) NewsArticle/9.1.7 cronet/TTNetVersion:437fcb60 2022-11-09 QuicVersion:22f74f01 2022-10-11
Accept-Encoding: gzip, deflate, br
X-Cylons: jQ1ABID3KlKFZuGKLcqhMHVY
Sec-WebSocket-Key: lSdFzLlNEFY+wI+VBlbOTA==
Sec-WebSocket-Extensions: permessage-deflate; client_max_window_bits
Sec-WebSocket-Protocol: pbbp2
握手响应
HTTP/1.1 101 Switching Protocols
Server: nginx
Date: Tue, 07 Feb 2023 07:24:43 GMT
Content-Type: application/octet-stream
Connection: upgrade
Upgrade: websocket
Sec-WebSocket-Accept: 7cCulc1ObP4idNxTd4sftSwszOU=
Sec-Websocket-Protocol: pbbp2
Handshake-Status: 0
Handshake-Msg: OK
Handshake-Options: ping-interval=30;
tt-idc-switch: 10000@20230130174459
Access-Control-Expose-Headers: tt-idc-switch
server-timing: inner; dur=3
x-tt-trace-host: 01bfda85b24fa255716a400b95f35865e801de7aa0619c3c851990fc82fb61148404fccbeeeebebdba77ee9c062919ed5fd23f4b558e9975cd3763c12d0676afcee3b1d537faa85513bf792c004d971b2b7e12cb70b9902274abf845399cff1539
x-tt-trace-tag: id=00;cdn-cache=miss
x-tt-trace-id: 00-2ac33dc5010c3808c8136bd3d7d70023-2ac33dc5010c3808-01