AS3 TCP Socket 拆包

• 作者：柳大·Poechant（鍾超）
• 郵箱：zhongchao.ustc#gmail.com（# -> @）
• 部落格：Blog.CSDN.net/Poechant
• 微博：weibo.com/lauginhom
• 日期：June 2^nd, 2012

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在 AS3 中的 Socket 只有 TCP 一種方式，而 TCP 就必然面臨拆包的問題。對於 Windows、Linux、Mac等作業系統的不同網路模型，Flash 程式在不拆包下的執行糟糕程度不一樣，我的親身體會是 Windows 的網路模型會積攢更大的緩衝區資料讓 Flash TCP Socket 每次收到的資料量都比 Mac 上的多，所以情況就更糟糕。這時候拆包的效果就十分明顯。

下面有一段更形象化的解釋，摘自 Netty 的

One Small Caveat of Socket Buffer

In a stream-based transport such as TCP/IP, received data is stored into a socket
receive buffer. Unfortunately, the buffer of a stream-based transport is not a queue of
packets but a queue of bytes. It means, even if you sent two messages as two independent
packets, an operating system will not treat them as two messages but as just a bunch of
bytes. Therefore, there is no guarantee that what you read is exactly what your remote
peer wrote. For example, let us assume that the TCP/IP stack of an operating system has
received three packets:

+-----+-----+-----+
| ABC | DEF | GHI |
+-----+-----+-----+

Because of this general property of a stream-based protocol, there's high chance of reading them in the following fragmented form in your application:

+----+-------+---+---+
| AB | CDEFG | H | I |
+----+-------+---+---+

Therefore, a receiving part, regardless it is server-side or client-side, should defrag the
received data into one or more meaningful frames that could be easily understood by the
application logic. In case of the example above, the received data should be framed like the
following:

+-----+-----+-----+
| ABC | DEF | GHI |
+-----+-----+-----+

下面例項中的 _socket 和 lastReservedLength 都是在函式外部定義的，其定義不影響程式的理解。各段含義我寫在了程式的註釋中。

部分程式碼如下：

private function onResponse(e:ProgressEvent):void
{
    // Data available from socket
    while (_socket.bytesAvailable > 0)
    {
        var tmpLength:int = 0;
        
        // Last package received is incomplete, and package length maintains last reserved length
        if (lastReservedLength > 0)
        {
            // CASE 1: Package is still incomplete after receiving data from socket at least once
            if (lastReservedLength > _socket.bytesAvailable + 4) break;
                
                // CASE 2: The last part of a package
            else tmpLength = lastReservedLength;
        }
            // Last package received is complete, and package length should be read from socket 
        else
        {
            var lengthBytes:ByteArray = new ByteArray();
            lengthBytes.endian = Endian.LITTLE_ENDIAN;
            
            _socket.readBytes(lengthBytes, 0, 4); // TODO Crash!
            //Error: Error #2030: End of file was encountered.
            //at flash.net::Socket/readBytes()
            
            tmpLength = lengthBytes.readInt();
            
            // CASE 3: The first part of a package
            if (_socket.bytesAvailable < tmpLength - 4)
            {
                lastReservedLength = tmpLength;
                break;
            }
        }
        
        // CASE 4: A complete package received once
        
        // Generate ByteArray of a complete package
        var resultBytes:ByteArray = new ByteArray();
        resultBytes.writeInt(tmpLength);
        _socket.readBytes(resultBytes, 4, tmpLength - 4);
        lastReservedLength = 0;
        
        // parse the ByteArray
        resultBytes.position = 0;
        var resProto:Object = CppLibUtil.parseMediaProto(resultBytes);
        
        // Dispatch the ByteArray
        if (resProto != null) {
            
            var uri:String;
            
            try {
                uri = resProto.uri;
            } catch (e:Event) {
                trace(e);
            }
                
            dispatchEvent(new InternalEvent(uri, resProto));
        }
    }
}

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轉載請註明來自柳大的CSDN部落格：Blog.CSDN.net/Poechant，微博：weibo.com/lauginhom

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