WebSocket server data frame anlysis for [RFC6455]

frame.js

    /**
     *  服务器端解析客户端数据
     *  WebSocket Frame格式
      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 ...                |
     +---------------------------------------------------------------+
     */
    socket.ondata = function(frame, start, end) {
	/*
	 * 包首16bit(2byte)
	 * 
	 * first byte解释如下:
	 * FIN + RSV1 + RSV2 + RSV3:0000连续数据  1000分片数据
	 * opcode: 0000 表示连续消息分片
		   0001表示文本消息分片
		   0010表未二进制消息分片
		   1000表示客户端发起的连接关闭
		   1001表示心跳ping
		   1010表示心跳pong
	 * --------------------------------------------	       
	 * second byte解释如下:	       
	 * 掩码位1bit 1加密  0未加密 (客户端发送的数据必须加密  [RFC6455])
	 * 
	 * 剩下7bit需要计算:
	 * 如果该值介于0000 0000 和 0111 1101(0~125)之间,那么该值就代表了实际数据的长度;
	 * 如果该值等于0111 1110(126),那么接下来的2个字节代表数据长度;
	 * 如果该值等于0111 1111(127),那么接下来的8个字节代表数据长度.
	 */

	var firstByte = frame[start],
	    secondByte = frame[start+1],
	    FIN  = Boolean(firstByte & 0x80),
	    RSV1 = Boolean(firstByte & 0x40),
	    RSV2 = Boolean(firstByte & 0x20),
	    RSV3 = Boolean(firstByte & 0x10),
	    MASK = Boolean(secondByte & 0x80),
	    OPCODE  = firstByte & 0x0F,
	    payloadLen = secondByte & 0x7F,
	    // 真实数据长度
	    appDataLen;

	// 检测客户端socket关闭请求
	// Control Frame分片检查(不允许分片)
	if (OPCODE == 0x8) {
	    console.log('socket closed');
	    socket.end();
	} else if(OPCODE > 0x8) {
	    if(!FIN || payloadLen > 125) {
		console.log('Control frames must not be fragmented');
		socket.end();
	    }
	}

	// 获取数据
	var payloadLenBuf, masksBuf = new Buffer(4), dataStartIdx, dataBuf,
	    getDataLen = function(buf) {
		var len = 0;
		for (var i=0;i<buf.length; i++) {
		    len += parseInt(buf[i]);
		}
		return len;
	    };
	if (payloadLen == 126) {
	    // 数据长度buffer
	    payloadLenBuf = new buffer(2);
	    frame.copy(payloadLenBuf, 0, start+2, start+4);
	    appDataLen = getDataLen(payloadLenBuf);

	    // 加密掩码buffer
	    frame.copy(masksBuf, 0, start+4, start+8);

	    // 数据起始索引
	    dataStartIdx = start+8;

	} else if(payloadLen == 127) {
	    payloadLenBuf = new buffer(8);
	    frame.copy(payloadLenBuf, 0, start+2, start+10);
	    appDataLen = getDataLen(payloadLenBuf);

	    frame.copy(masksBuf, 0, start+10, start+14);
	    dataStartIdx = start+14;
	} else {
	    frame.copy(masksBuf, 0, start+2, start+6);
	    dataStartIdx = start+6;
	    appDataLen = payloadLen;
	}
	// 取得加密数据
	dataBuf = frame.slice(dataStartIdx, dataStartIdx + appDataLen);

	// 还原数据
	var temp = [];			
	for (var i=0; i<dataBuf.length; i++) {
	    temp.push(String.fromCharCode(dataBuf[i] ^ masksBuf[i%4]));
	}	
	var data = temp.join('');
	
	// 向客户端发送数据	
	function send(socket, data) {
	    var frames, len = new Buffer(data, 'utf-8').length;
	    if(data.length > 125) {
		frames = new Buffer(4);
		frames[0] = 0x81;
		frames[1] = 0x7e;
		frames[2] = length >> 8;
		frames[3] = length & 0xFF; // 1111 1111
	    } else {
		frames = new Buffer(2);
		frames[0] = 0x81;
		frames[1] = len;   
	    }

	    if (!!socket.writable) {
		socket.write(frames, 'binary');
		socket.write(data, 'utf-8');
	    }
	    
	}	

	// send data to client
	// edit
    };
    });
};