CTCP (Copper Transmission Control Protocol)

CTCP is used for reliably exchanging data. It is contained in a [[CNP]] packet’s payload. CTCP resembles the TCP Protocol, but is very simplified.

Connection Handshake

Each CTCP session starts with a three-way handshake. First, the client sends a SYN packet to the host looking like this.

{
	Sequence number = random
	Acknowledgment number = 0
	Flags = {"SYN"}
}

Then the server answers with a SYN-ACK packet.

{
	Sequence number = random
	Acknowledgment number = SYN sequence number + 1
	Flags = {"SYN", "ACK"}
}

Finally, the client answers with a ACK packet looking like the following.

{
	Sequence number = SYN-ACK acknowledgment number
	Acknowledgment number = SYN-ACK sequence number + 1
	Flags = {"ACK"}
}

Data Exchange

Now the data exchange can begin The coordination of a CTCP data exchange works by counting up two values P and Q. After the handshake, P1 is the same as the sequence number and Q1 as the acknowledgment number of the ACK packet.

{
	Sequence number = P1
	Acknowledgment number = Q1
	Flags = {},
	Payload = any
}

The recipient acknowledges by sending a packet with the ACK flag and an acknowledgment number of P1 + 1 this is now our P2.

{
	Sequence number = Q1
	Acknowledgment number = P2
	Flags = {"ACK"}
}

Then he sends the actual answer. In this answer, the sequence number is P2 and the acknowledgment number is Q1 + 1 this is now Q2.

{
	Sequence number = P2
	Acknowledgment number = Q2
	Flags = {}
	Payload = any
}

The original sender acknowledges the answer by sending a packet with the ACK flag and an acknowledgment number of Q2 + 1 this is now Q3.

{
	Sequence number = P2
	Acknowledgment number = Q3
	Flags = {"ACK"}
}

For the next message, both the sequence and acknowledgment numbers would be P2 and Q3 again.

{
	Sequence number = P2
	Acknowledgment number = Q3
	Flags = {}
	Payload = string
}

Closing the connection

Closing the connection works again with a three-way conversation. First, the initiator sends a FIN packet with sequence and acknowledgment number, like during a data exchange.

{
	Sequence number = ?
	Acknowledgment number = ?
	Flags = {"FIN"}
}

Then the receiver answers with a FIN-ACK packet.

{
	Sequence number = FIN acknowledgment number
	Acknowledgment number = FIN sequence number + 1
	Flags = {"FIN", "ACK"}
}

Finally, the client answers with a ACK packet looking like the following.

{
	Sequence number = FIN-ACK acknowledgment number
	Acknowledgment number = FIN-ACK sequence number + 1
	Flags = {"ACK"}
}