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WebSocket 实现原理浅析

Philip-Morton 1年前发布
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背景

之前我们将 CocoaAsyncSocket 作为底层实现，在其上面封装了一套 Socket 通信机制以及业务接口，最近我们开始研究 WebSocket ，并用来替换掉原先的 CocoaAsyncSocket ，简单来说一下两者的关系，WebSocket 和 Socket 虽然名称上很像，但两者是完全不同的东西， WebSocket 是建立在 TCP/IP 协议之上，属于应用层的协议，而 Socket 是在应用层和传输层中的一个抽象层，它是将 TCP/IP 层的复杂操作抽象成几个简单的接口来提供给应用层调用。为什么要做这次替换呢？原因是我们服务端在做改造，同时网页版 IM 已经使用了 WebSocket ，客户端也采用的话对于服务端来说维护一套代码会更好更方便，而且 WebSocket 在体积、实时性和扩展上都具有一定的优势。

WebSocket 最新的协议是 13 RFC 6455 ，要理解 WebSocket 的实现，一定要去理解它的协议！~

WebSocket 的实现分为握手，数据发送/读取

握手

握手要从请求头去理解。

WebSocket 首先发起一个 HTTP 请求，在请求头加上 Upgrade 字段，该字段用于改变 HTTP 协议版本或者是换用其他协议，这里我们把 Upgrade 的值设为 websocket ，将它升级为 WebSocket 协议。

同时要注意 Sec-WebSocket-Key 字段，它由客户端生成并发给服务端，用于证明服务端接收到的是一个可受信的连接握手，可以帮助服务端排除自身接收到的由非 WebSocket 客户端发起的连接，该值是一串随机经过 base64 编码的字符串。

GET /chat HTTP/1.1
Host: server.example.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Origin: http://example.com
Sec-WebSocket-Protocol: chat, superchat
Sec-WebSocket-Version: 13

我们可以简化请求头，将请求以字符串方式发送出去，当然别忘了最后的两个空行作为包结束：

const char * fmt = "GET %s HTTP/1.1\r\n"
                   "Upgrade: websocket\r\n"
                   "Connection: Upgrade\r\n"
                   "Host: %s\r\n"
                   "Sec-WebSocket-Key: %s\r\n"
                   "Sec-WebSocket-Version: 13\r\n"
                   "\r\n";
size = strlen(fmt) + strlen(path) + strlen(host) + strlen(ws->key);
buf = (char *)malloc(size);
sprintf(buf, fmt, path, host, ws->key);
size = strlen(buf);
nbytes = ws->io_send(ws, ws->context, buf, size);

收到请求后，服务端也会做一次响应：

HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=

里面重要的是 Sec-WebSocket-Accept ，服务端通过从客户端请求头中读取 Sec-WebSocket-Key 与一串全局唯一的标识字符串（俗称魔串）“258EAFA5-E914-47DA- 95CA-C5AB0DC85B11”做拼接，生成长度为160字节的 SHA-1 字符串，然后进行 base64 编码，作为 Sec-WebSocket-Accept 的值回传给客户端。

处理握手 HTTP 响应解析的时候，可以用 nodejs 的 http-paser ，解析方式也比较简单，就是对头信息的逐字读取再处理，具体处理你可以看一下它的状态机实现。解析完成后你需要对其内容进行解析，看返回是否正确，同时去管理你的握手状态。

数据发送/读取

数据的处理就要拿这个帧协议图来说明了：

0                   1                   2                   3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-------+-+-------------+-------------------------------+
|F|R|R|R| opcode|M| Payload len |    Extended payload length    |
|I|S|S|S|  (4)  |A|     (7)     |             (16/64)           |
|N|V|V|V|       |S|             |   (if payload len==126/127)   |
| |1|2|3|       |K|             |                               |
+-+-+-+-+-------+-+-------------+ - - - - - - - - - - - - - - - +
|     Extended payload length continued, if payload len == 127  |
+ - - - - - - - - - - - - - - - +-------------------------------+
|                               |Masking-key, if MASK set to 1  |
+-------------------------------+-------------------------------+
| Masking-key (continued)       |          Payload Data         |
+-------------------------------- - - - - - - - - - - - - - - - +
:                     Payload Data continued ...                :
+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +
|                     Payload Data continued ...                |
+---------------------------------------------------------------+

首先我们来看看数字的含义，数字表示位，0-7表示有8位，等于1个字节。

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

所以如果要组装一个帧数据可以这样子：

char *rev = (rev *)malloc(4);
rev[0] = (char)(0x81 & 0xff);
rev[1] = 126 & 0x7f;
rev[2] = 1;
rev[3] = 0;

ok，了解了帧数据的样子，我们反过来去理解值对应的帧字段。

首先0x81是什么，这个是十六进制数据，转换成二进制就是1000 0001， 是一个字节的长度，也就是这一段里面每一位的值：

0 1 2 3 4 5 6 7 8 
+-+-+-+-+-------+
|F|R|R|R| opcode|
|I|S|S|S|  (4)  |
|N|V|V|V|       |
| |1|2|3|     |
+-+-+-+-+-------+
  • FIN 表示该帧是不是消息的最后一帧，1表示结束，0表示还有下一帧。
  • RSV1, RSV2, RSV3 必须为0，除非扩展协商定义了一个非0的值，如果没有定义非0值，且收到了非0的 RSV ，那么 WebSocket 的连接会失效。
  • opcode 用来描述 Payload data 的定义，如果收到了一个未知的 opcode ，同样会使 WebSocket 连接失效，协议定义了以下值：
    • %x0 表示连续的帧
    • %x1 表示 text 帧
    • %x2 表示二进制帧
    • %x3-7 预留给非控制帧
    • %x8 表示关闭连接帧
    • %x9 表示 ping
    • %xA 表示 pong
    • %xB-F 预留给控制帧

0xff 作用就是取出需要的二进制值。

下面再来看126，126则表示的是 Payload len ，也就是 Payload 的长度：

8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                +-+-------------+-------------------------------+
                |M| Payload len |    Extended payload length    |
                |A|     (7)     |             (16/64)           |
                |S|             |   (if payload len==126/127)   |
                |K|             |                               |
+-+-+-+-+-------+-+-------------+ - - - - - - - - - - - - - - - +
|     Extended payload length continued, if payload len == 127  |
+ - - - - - - - - - - - - - - - +-------------------------------+
|                               |Masking-key, if MASK set to 1  |
+-------------------------------+-------------------------------+
| Masking-key (continued)       |           Payload Data         |
+-------------------------------- - - - - - - - - - - - - - - - +
:                     Payload Data continued ...                :
+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +
|                     Payload Data continued ...                |
+---------------------------------------------------------------+
  • MASK 表示Playload data 是否要加掩码，如果设成1，则需要赋值 Masking-key 。所有从客户端发到服务端的帧都要加掩码
  • Playload len 表示 Payload 的长度，这里分为三种情况
    • 长度小于126，则只需要7位
    • 长度是126，则需要额外2个字节的大小，也就是 Extended payload length
    • 长度是127，则需要额外8个字节的大小，也就是 Extended payload length + Extended payload length continued ，Extended payload length 是2个字节，Extended payload length continued 是6个字节
  • Playload len 则表示 Extension data 与 Application data 的和

而数据的发送和读取就是对帧的封装和解析。

数据发送:

int ws_recv(websocket_t *ws) {
    if (ws->state != WS_STATE_HANDSHAKE_COMPLETED) {
        return ws_do_handshake(ws);
    }
    int ret;
    while(TRUE) {
        ret = ws__recv(ws);
        if (ret != OK) {
            break;
        }
    }
    return ret;
}
int ws__recv(websocket_t *ws) {
    int nbytes;
    int ret = OK, i;
    int state = ws->rd_state;
    char *rd_buf;
    uint64_t rd_buf_len = 0;
    switch(state) {
        case WS_READ_IDLE: {
            if (ws->buf_pos < 2) {
                rd_buf_len = 2 - ws->buf_pos;
                rd_buf = malloc(rd_buf_len);
                nbytes = ws->io_recv(ws, ws->context, rd_buf, (size_t) (rd_buf_len));
                if (nbytes < 0) {
                    free(rd_buf);
                    //TODO errono fix
                    ret = nbytes;
                    break;
                }
                ws__enqueue_buf(ws, rd_buf, (size_t)nbytes) ;
                free(rd_buf);
            }
            if (ws->buf_pos < 2) {
                ret = WS_WANT_READ;
                break;
            }
            ws_frame_t * frame;
            if (ws->frame == NULL) {
                frame__alloc(&ws->frame);
                frame = ws->frame;
            } else {
                frame = ws->frame;
            }
            rd_buf = ws->buf;
            frame->fin = (*(rd_buf) & 0x80) == 0x80 ? 1 : 0;
            frame->op_code = *(rd_buf) & 0x0f;
            frame->payload_len = *(rd_buf + 1) & 0x7f;
            if (frame->payload_len < 126) {
                frame->payload_bit_offset = 2;
                ws->rd_state = WS_READ_PAYLOAD;
            } else if (frame -> payload_len == 126) {
                frame->payload_bit_offset = 4;
                ws->rd_state = WS_READ_EXTEND_PAYLOAD_2_WORDS;
            } else {
                frame->payload_bit_offset = 8;
                ws->rd_state = WS_READ_EXTEND_PAYLOAD_8_WORDS;
            }
            ws__reset_buf(ws, 2);
            break;
        }
        case WS_READ_EXTEND_PAYLOAD_2_WORDS: {
#define PAYLOAD_LEN_BITS 2
            if (ws->buf_pos < PAYLOAD_LEN_BITS) {
                rd_buf_len = PAYLOAD_LEN_BITS - ws->buf_pos;
                rd_buf = malloc(rd_buf_len);
                nbytes = ws->io_recv(ws, ws->context, rd_buf, (size_t) (rd_buf_len));
                if (nbytes < 0) {
                    free(rd_buf);
                    ret = nbytes;
                    break;
                }
                ws__enqueue_buf(ws, rd_buf, (size_t)nbytes) ;
                free(rd_buf);
            }
            if (ws->buf_pos < PAYLOAD_LEN_BITS) {
                ret = WS_WANT_READ;
                break;
            }
            rd_buf = ws->buf;
            ws_frame_t * frame = ws->frame;
            //rd_buf[0] = 0; rd_buf[1] = 255
            for (i = 0; i < PAYLOAD_LEN_BITS; i++) {
                *(((char *)&frame->payload_len) + i) = rd_buf[PAYLOAD_LEN_BITS - 1 - i];
            }
            ws__reset_buf(ws, PAYLOAD_LEN_BITS);
            ws->rd_state = WS_READ_PAYLOAD;
#undef PAYLOAD_LEN_BITS
            break;
        }
        case WS_READ_EXTEND_PAYLOAD_8_WORDS: {
#define PAYLOAD_LEN_BITS 8
            if (ws->buf_pos < PAYLOAD_LEN_BITS) {
                rd_buf_len = PAYLOAD_LEN_BITS - ws->buf_pos;
                rd_buf = malloc(rd_buf_len);
                nbytes = ws->io_recv(ws, ws->context, rd_buf, (size_t) (rd_buf_len));
                if (nbytes < 0) {
                    free(rd_buf);
                    ret = nbytes;
                    break;
                }
                ws__enqueue_buf(ws, rd_buf, (size_t)nbytes) ;
                free(rd_buf);
            }
            if (ws->buf_pos < PAYLOAD_LEN_BITS) {
                ret = WS_WANT_READ;
                break;
            }
            rd_buf = ws->buf;
            ws_frame_t * frame = ws->frame;
            for (i = 0; i < PAYLOAD_LEN_BITS; i++) {
                *(((char *)&frame->payload_len) + i) = rd_buf[PAYLOAD_LEN_BITS - 1 - i];
            }
            ws__reset_buf(ws, PAYLOAD_LEN_BITS);
            ws->rd_state = WS_READ_PAYLOAD;
#undef PAYLOAD_LEN_BITS
            break;
        }
        case WS_READ_PAYLOAD: {
            ws_frame_t * frame = ws->frame;
            uint64_t payload_len = frame->payload_len;
            if (ws->buf_pos < payload_len) {
                rd_buf_len = payload_len - ws->buf_pos;
                rd_buf = malloc(rd_buf_len);
                nbytes = ws->io_recv(ws, ws->context, rd_buf, (size_t) (rd_buf_len));
                if (nbytes < 0) {
                    free(rd_buf);
                    ret = nbytes;
                    break;
                }
                ws__enqueue_buf(ws, rd_buf, (size_t)nbytes) ;
                free(rd_buf);
            }
            if (ws->buf_pos < payload_len) {
                ret = WS_WANT_READ;
                break;
            }
            rd_buf = ws->buf;
            frame->payload = malloc(payload_len);
            memcpy(frame->payload, rd_buf, payload_len);
            ws__reset_buf(ws, payload_len);
            if (frame->fin == 1) {
                // is control frame
                if (frame->op_code == OP_CLOSE) {
                    // TODO if should response a close frame
                    // close connection
                    if (ws->close_cb) {
                        ws->close_cb(ws);
                    }
                } else {
                    ws__dispatch_msg(ws, frame);
                    ws->frame = NULL;
                }
            } else {
                ws_frame_t *new_frame;
                frame__alloc(&new_frame);
                frame->next = new_frame;
                new_frame->prev = frame;
                ws->frame = new_frame;
            }
            ws->rd_state = WS_READ_IDLE;
            break;
        }
    }
    return ret;
}

数据解析：

void ws__wrap_packet(_WS_IN websocket_t *ws,
                     _WS_IN const char *payload,
                     _WS_IN unsigned long long payload_size,
                     _WS_IN int flags,
                     _WS_OUT char** out,
                     _WS_OUT uint64_t *out_size) {
    struct timeval tv;
    char mask[4];
    unsigned int mask_int;
    unsigned int payload_len_bits;
    unsigned int payload_bit_offset = 6;
    unsigned int extend_payload_len_bits, i;
    unsigned long long frame_size;
    const int MASK_BIT_LEN = 4;
    gettimeofday(&tv, NULL);
    srand(tv.tv_usec * tv.tv_sec);
    mask_int = rand();
    memcpy(mask, &mask_int, 4);
    /**
     * payload_len bits
     * ref to https://tools.ietf.org/html/rfc6455#section-5.2
     * If 0-125, that is the payload length
     *
     * If payload length is equals 126, the following 2 bytes interpreted as a
     * 16-bit unsigned integer are the payload length
     * 
     * If 127, the following 8 bytes interpreted as a 64-bit unsigned integer (the
     * most significant bit MUST be 0) are the payload length.
     */
    if (payload_size <= 125) {
        // consts of ((fin + rsv1/2/3 + opcode) + payload-len bits + mask bit len + payload len)
        extend_payload_len_bits = 0;
       frame_size = 1 + 1 + MASK_BIT_LEN + payload_size;
        payload_len_bits = payload_size;
    } else if (payload_size > 125 && payload_size <= 0xffff) {
        extend_payload_len_bits = 2;
        // consts of ((fin + rsv1/2/3 + opcode) + payload-len bits + extend-payload-len bites + mask bit len + payload len)
       frame_size = 1 + 1 + extend_payload_len_bits + MASK_BIT_LEN + payload_size;
       payload_len_bits = 126;
       payload_bit_offset += extend_payload_len_bits;
    } else if (payload_size > 0xffff && payload_size <= 0xffffffffffffffffLL) {
        extend_payload_len_bits = 8;
        // consts of ((fin + rsv1/2/3 + opcode) + payload-len bits + extend-payload-len bites + mask bit len + payload len)
       frame_size = 1 + 1 + extend_payload_len_bits + MASK_BIT_LEN + payload_size;
       payload_len_bits = 127;
       payload_bit_offset += extend_payload_len_bits;
    } else {
        if (ws->error_cb) {
            ws_error_t *err = ws_new_error(WS_SEND_DATA_TOO_LARGE_ERR);
            ws->error_cb(ws, err);
            free(err);
        }
       return ;
    }
    *out_size = frame_size;
    char *data = (*out) = (char *)malloc(frame_size);
    char *buf_offset = data;
    bzero(data, frame_size);
    *data = flags & 0xff;
    buf_offset = data + 1;
    // set mask bit = 1
    *(buf_offset) = payload_len_bits | 0x80; //payload length with mask bit on
    buf_offset = data + 2;
    if (payload_len_bits == 126) {
       payload_size &= 0xffff;
    } else if (payload_len_bits == 127) {
       payload_size &= 0xffffffffffffffffLL;
    }
    for (i = 0; i < extend_payload_len_bits; i++) {
        *(buf_offset + i) = *((char *)&payload_size + (extend_payload_len_bits - i - 1));
    }
    /**
     * according to https://tools.ietf.org/html/rfc6455#section-5.3
     * 
     * buf_offset is set to mask bit
     */
    buf_offset = data + payload_bit_offset - 4;
    for (i = 0; i < 4; i++) {
       *(buf_offset + i) = mask[i] & 0xff;
    }
    /**
     * mask the payload data 
     */
    buf_offset = data + payload_bit_offset;
    memcpy(buf_offset, payload, payload_size);
    mask_payload(mask, buf_offset, payload_size);
}

总结

对WebSocket的学习主要是对协议的理解，理解了协议，上面复杂的代码自然而然就会明白~

原文：http://zeeyang.com/2017/07/02/websocket/

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