Set Keep-Alive Values---C到C#代码的转换

一、什么是Keep-Alive模式？

我们知道HTTP协议采用“请求-应答”模式，当使用普通模式，即非KeepAlive模式时，每个请求/应答客户和服务器都要新建一个连接，完成 之后立即断开连接（HTTP协议为无连接的协议）；当使用Keep-Alive模式（又称持久连接、连接重用）时，Keep-Alive功能使客户端到服 务器端的连接持续有效，当出现对服务器的后继请求时，Keep-Alive功能避免了建立或者重新建立连接。
http 1.0中默认是关闭的，需要在http头加入"Connection: Keep-Alive"，才能启用Keep-Alive；http 1.1中默认启用Keep-Alive，如果加入"Connection: close "，才关闭。目前大部分浏览器都是用http1.1协议，也就是说默认都会发起Keep-Alive的连接请求了，所以是否能完成一个完整的Keep- Alive连接就看服务器设置情况。

二、代码使用

SetKeepAliveValues 方法启用或禁用 TCP 保持选项指定的 TCP 保持超时和间隔用于保持 TCP 数据包的每个连接设置。保持选项的详细信息，请参阅 4.2.3.6 节要求的互联网 HostsCommunication 图层在 IETF 网站上可用的 RFC 1122 中指定。

 optionInValue 参数，传递给 Socket.IOControl 应指向 Mstcpip.h 头文件中定义的 tcp_keepalive 结构。这种结构的定义如下：

/* Argument structure for SIO_KEEPALIVE_VALS */
struct tcp_keepalive {
u_long onoff;
u_long keepalivetime;
u_long keepaliveinterval;
};

将 tcp_keepalive C 转换为 C# struct结构

// Convert tcp_keepalive C struct To C# struct
[
       System.Runtime.InteropServices.StructLayout
       (
           System.Runtime.InteropServices.LayoutKind.Explicit
       )
]
unsafe struct TcpKeepAlive
{
    [System.Runtime.InteropServices.FieldOffset(0)]
    [
          System.Runtime.InteropServices.MarshalAs
           (
               System.Runtime.InteropServices.UnmanagedType.ByValArray,
               SizeConst = 12
           )
    ]
    public fixed byte Bytes[12];

    [System.Runtime.InteropServices.FieldOffset(0)]
    public uint On_Off;
        
    [System.Runtime.InteropServices.FieldOffset(4)]
    public uint KeepaLiveTime;
        
    [System.Runtime.InteropServices.FieldOffset(8)]
    public uint KeepaLiveInterval;
}

public int SetKeepAliveValues
      (
           System.Net.Sockets.Socket Socket,
           bool On_Off,
           uint KeepaLiveTime,
           uint KeepaLiveInterval
       )
{       
    int Result = -1;

    unsafe
    {
        TcpKeepAlive KeepAliveValues = new TcpKeepAlive();

        KeepAliveValues.On_Off = Convert.ToUInt32(On_Off);
        KeepAliveValues.KeepaLiveTime = KeepaLiveTime;
        KeepAliveValues.KeepaLiveInterval = KeepaLiveInterval;

        byte[] InValue = new byte[12];

        for (int I = 0; I < 12; I++)
            InValue[I] = KeepAliveValues.Bytes[I];

        Result = Socket.IOControl(IOControlCode.KeepAliveValues, InValue, null);
    }

    return Result;
}

调用方法：

System.Net.Sockets.Socket Socket = new System.Net.Sockets.Socket
    (
        System.Net.Sockets.AddressFamily.InterNetwork,
        System.Net.Sockets.SocketType.Stream,
        System.Net.Sockets.ProtocolType.Tcp
    );
    
// Set 10 Hours: 10 * 60 * 60 * 1000 = 36,000,000 every 1 Second 1000
SetKeepAliveValues(Socket, true, 36000000, 1000);