动手学习TCP系列之客户端状态变迁(1)(2)

private static void PacketHandler(PacketCommunicator communicator, EndPointInfo endPointInfo, bool clientToSendFin = true)
{
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 | TcpControlBits.Acknowledgment):
if (tcpStatus == TCPStatus.SYN_SEND)
{
Utils.PacketInfoPrinter(packet);
Packet ack = Utils.BuildTcpResponsePacket(packet, TcpControlBits.Acknowledgment);
communicator.SendPacket(ack);
tcpStatus = TCPStatus.ESTABLISHED;
}
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;
case TcpControlBits.Acknowledgment:
if (tcpStatus == TCPStatus.FIN_WAIT_1)
{
tcpStatus = TCPStatus.FIN_WAIT_2;
Utils.PacketInfoPrinter(packet, tcpStatus);
}
else if (tcpStatus == TCPStatus.LAST_ACK)
{
tcpStatus = TCPStatus.CLOSED;
Utils.PacketInfoPrinter(packet, tcpStatus);
running = false;
}
break;
default:
Utils.PacketInfoPrinter(packet);
break;
}
}
else
{
switch (packet.Ethernet.IpV4.Tcp.ControlBits)
{
case TcpControlBits.Synchronize:
if (tcpStatus == TCPStatus.SYN_SEND)
{
Utils.PacketInfoPrinter(packet, tcpStatus);
}
break;
case TcpControlBits.Acknowledgment:
if (tcpStatus == TCPStatus.ESTABLISHED)
{
Utils.PacketInfoPrinter(packet, tcpStatus);
if (clientToSendFin)
running = false;
}
else if (tcpStatus == TCPStatus.TIME_WAIT)
{
Utils.PacketInfoPrinter(packet, tcpStatus);
running = false;
}
else if (tcpStatus == TCPStatus.CLOSE_WAIT)
{
Utils.PacketInfoPrinter(packet, tcpStatus);
Packet fin = Utils.BuildTcpPacket(endPointInfo, TcpControlBits.Fin | TcpControlBits.Acknowledgment);
communicator.SendPacket(fin);
tcpStatus = TCPStatus.LAST_ACK;
}
break;
case (TcpControlBits.Fin | TcpControlBits.Acknowledgment):
if (tcpStatus == TCPStatus.FIN_WAIT_1)
{
Utils.PacketInfoPrinter(packet, tcpStatus);
}
else if (tcpStatus == TCPStatus.LAST_ACK)
{
Utils.PacketInfoPrinter(packet, tcpStatus);
}
break;
default:
Utils.PacketInfoPrinter(packet);
break;
}
}
break;
default:
throw new InvalidOperationException("The result " + result + " should never be reached here");
}
} while (running);
}

运行效果

下面，将"clientToSendFin"设置为"true"，看看正常情况下客户端的状态变迁。

打开Wireshark监听"VirtualBox Host-Only Network"网卡，并设置filter为"port 8081"。

运行程序，通过console可以看到客户端和服务端之间的包，以及客户端的状态变迁。

下面是Wireshark抓到的包，这七个数据包就表示了TCP连接的建立和终止过程。

总结

本文介绍了TCP状态变迁图，根据客户端的状态变迁过程，得到了客户端的状态变迁表。

然后使用Pcap.Net，基于客户端的状态变迁表，构建了一个简单的客户端，展示了客户端状态变迁的过程。

通过这个实验，一定能够对TCP客户端的状态变迁有个深刻的印象。