websocket 报文格式

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:

1. Full-Duplex Communication: Both the client and server can send messages to each other independently.
2. Single TCP Connection: After the initial handshake, communication continues over the same TCP connection.
3. 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: 13

Server 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:

1. FIN: 1 bit - Indicates if this is the final fragment in a message.
2. RSV1, RSV2, RSV3: 1 bit each - Reserved for future use.
3. Opcode: 4 bits - Defines the interpretation of the payload data.
4. Mask: 1 bit - Indicates if the payload data is masked.
5. Payload length: 7 bits, 7+16 bits, or 7+64 bits - The length of the payload data.
6. Masking key: 0 or 4 bytes - Used to unmask the payload data (present if Mask is 1).
7. 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 6F

Breakdown:

• 81: FIN=1, Opcode=1 (text frame)
• 05: Payload length is 5 bytes
• 48 65 6C 6C 6F: Payload data "Hello"

Masked Frame:

81 85 37 FA 21 3D 7F 9F 4D 51 58

Breakdown:

• 81: FIN=1, Opcode=1 (text frame)
• 85: Mask=1, Payload length is 5 bytes
• 37 FA 21 3D: Masking key
• 7F 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:

1. 7F ^ 37 = 48 (H)
2. 9F ^ FA = 65 (e)
3. 4D ^ 21 = 6C (l)
4. 51 ^ 3D = 6C (l)
5. 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

1. Install the libwebsockets library. On Ubuntu, you can install it using:

   sudo apt-get install libwebsockets-dev

WebSocket Server Example

#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 libwebsockets library.
• 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:

gcc websocket_server.c -o websocket_server -lwebsockets

Running the Server

Execute the compiled program:

./websocket_server

Testing 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:

npm install -g wscat
wscat -c ws://localhost:7681

After 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