动手学习TCP系列之服务端状态变迁(1)

动手学习TCP系列之服务端状态变迁(1)

上一篇文章介绍了TCP状态机，并且通过实验了解了TCP客户端正常的状态变迁过程。

那么，本篇文章就一起看看TCP服务端的正常状态变迁过程。

服务端状态变迁

根据上一篇文章中的TCP状态变迁图，可以得到服务器的正常状态变迁流程如下：

CLOSED -> LISTEN -> SYN_RECV -> ESTABLISHED -> CLOSE_WAIT -> LAST_ACK -> CLOSED

服务端状态变迁实验

下面就结合上面分析出来的服务端状态变迁表，利用Pcap.Net来模拟服务端正常的状态变迁过程。

代码实现

跟前面几次正好相反，这次我们将在宿主机运行Pcap.Net实现的服务端，然后在虚拟机运行一个客户端。

对于服务端，主程序中设置了源和目的端的连接信息，这次宿主机中的服务端将监听“3333”端口。

然后，程序中设置了服务端TCP初始状态为"LISTENING"，然后就直接运行监听函数了。

// Open the output device
using (PacketCommunicator communicator = selectedDevice.Open(System.Int32.MaxValue, // name of the device
PacketDeviceOpenAttributes.Promiscuous, // promiscuous mode
1)) // read timeout
{
EndPointInfo endPointInfo = new EndPointInfo();
endPointInfo.SourceMac = "08:00:27:00:C0:D5";
endPointInfo.DestinationMac = "";
endPointInfo.SourceIp = "192.168.56.101";
endPointInfo.DestinationIp = "";
endPointInfo.SourcePort = 3333;
endPointInfo.DestinationPort = 0;
using (BerkeleyPacketFilter filter = communicator.CreateFilter("tcp port " + endPointInfo.SourcePort))
{
// Set the filter
communicator.SetFilter(filter);
}
tcpStatus = TCPStatus.LISTENING;
PacketHandler(communicator, endPointInf)
}
 这次的监听函数"PacketHandler"中的逻辑，跟上一次客户端的例子还是有很大差别的。

首先是期待接收和实际发送的TCP包类型有很大的差别，其次就是状态之间的变迁是完全不同的。但是，代码的逻辑依然是根据上面的服务端状态变迁表。

private static void PacketHandler(PacketCommunicator communicator, EndPointInfo endPointInfo)
{
Packet packet = null;
bool running = true;
do{
PacketCommunicatorReceiveResult result = communicator.ReceivePacket(out packet);
switch (result)
{
case PacketCommunicatorReceiveResult.Timeout:
// Timeout elapsed
continue;
case PacketCommunicatorReceiveResult.Ok:
bool isRecvedPacket = (packet.Ethernet.IpV4.Destination.ToString() == endPointInfo.SourceIp) ? true : false;
if (isRecvedPacket)
{
switch (packet.Ethernet.IpV4.Tcp.ControlBits){
case TcpControlBits.Synchronize:
if (tcpStatus == TCPStatus.LISTENING)
{
endPointInfo.DestinationMac = packet.Ethernet.Source.ToString();
endPointInfo.DestinationIp = packet.Ethernet.IpV4.Source.ToString();
endPointInfo.DestinationPort = packet.Ethernet.IpV4.Tcp.SourcePort;
Utils.PacketInfoPrinter(packet);
Packet synAck = Utils.BuildTcpResponsePacket(packet, TcpControlBits.Synchronize | TcpControlBits.Acknowledgment);
communicator.SendPacket(synAck);
tcpStatus = TCPStatus.SYN_RECEIVED;
}break;
case TcpControlBits.Acknowledgment:
if (tcpStatus == TCPStatus.SYN_RECEIVED)
{
tcpStatus = TCPStatus.ESTABLISHED;
Utils.PacketInfoPrinter(packet, tcpStatus);
}
else if (tcpStatus == TCPStatus.LAST_ACK)
{
tcpStatus = TCPStatus.CLOSED;
Utils.PacketInfoPrinter(packet, tcpStatus);
tcpStatus = TCPStatus.LISTENING;
}
else if (tcpStatus == TCPStatus.FIN_WAIT_1)
{
tcpStatus = TCPStatus.FIN_WAIT_2;
Utils.PacketInfoPrinter(packet);
}
break;
case (TcpControlBits.Fin | TcpControlBits.Acknowledgment):
if (tcpStatus == TCPStatus.FIN_WAIT_2)
{
Utils.PacketInfoPrinter(packet);
Packet ack = Utils.BuildTcpResponsePacket(packet, TcpControlBits.Acknowledgment);
communicator.SendPacket(ack);
tcpStatus = TCPStatus.TIME_WAIT;
}
else if (tcpStatus == TCPStatus.ESTABLISHED){
Utils.PacketInfoPrinter(packet);
Packet ack = Utils.BuildTcpResponsePacket(packet, TcpControlBits.Acknowledgment);
communicator.SendPacket(ack);
tcpStatus = TCPStatus.CLOSE_WAIT;
}
break;
default:
Utils.PacketInfoPrinter(packet);
break;
}
}
else
{
switch (packet.Ethernet.IpV4.Tcp.ControlBits)
{
case (TcpControlBits.Synchronize | TcpControlBits.Acknowledgment):
if (tcpStatus == TCPStatus.SYN_RECEIVED)
{
Utils.PacketInfoPrinter(packet, tcpStatus);
}