c# send TCP packet without TCP handshake

Question

I am attempting to interop with a custom hardware device.

It requires the first TCP packet to contain a data payload

I tried to achieve this using TcpClient in System.Net.Sockets

   TcpClient tcpclnt = new TcpClient();
   tcpclnt.Connect("192.168.1.11", 1500);
   Stream stm = tcpclnt.GetStream();
   byte[] ba = messagestr.StringToByteArray();                   
   stm.Write(ba, 0, ba.Length);

However wireshark shows that this code sends a number of TCP/IP packets to establish a connection and the data packet (containing messagestr) is the 4-5th packet.

How can I make C# send the data in the FIRST TCP packet?

If you don't want TCP to establish a connection, are you sure that your requirement is to create a TCP socket? Don't you need a raw socket? — CodeCaster
– CodeCaster, Commented Oct 2, 2015 at 13:30
wiresharking the correct packets I am trying to imitate shows them to be well formed TCP packets (though just 1 sent / 1 received) — GreyCloud
– GreyCloud, Commented Oct 2, 2015 at 13:32
If that really is the protocol that the hardware device implements, I suspect the creators to have been high on something. How does the SYN/ACK/sequence flow work then? Anyway check TCP/IP Raw Sockets, How do you get raw TCP packet in C#?, Send TCP packet in C#. — CodeCaster
– CodeCaster, Commented Oct 2, 2015 at 13:33
"I suspect the creators to have been high on something" yes I fully agree(!) this is a RS232 protocol embedded into TCP packets :( — GreyCloud
– GreyCloud, Commented Oct 2, 2015 at 13:37
If you want to send hand-crafted TCP packets you need to use the WinPcap library. There are Pcap wrappers for C# like SharpPcap available. — Robert
– Robert, Commented Oct 2, 2015 at 14:00

Remus Rusanu · Accepted Answer · 2015-10-02 13:31:22Z

3

Use UDP to send only one packed without establishing a session. You are literally asking TCP not to be TCP.

answered Oct 2, 2015 at 13:31

Remus Rusanu

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1 Comment

GreyCloud Over a year ago

wiresharking the correct packets I am trying to imitate shows them to be well formed TCP packets (though just 1 sent / 1 received) ... should I therefore be trying to create a TCP packet using the UDP protocol?

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