The TCP protocol resides on top of the IP protocol. It is a stateful protocol
and has built-in functions to see that the data was received properly by the
other end host. The main goals of the TCP protocol is to see that data is
reliably received and sent, that the data is transported between the Internet
layer and Application layer correctly, and that the packet data reaches the
proper program in the application layer, and that the data reaches the program
in the right order. All of this is possible through the TCP headers of the
packet.
The TCP protocol looks at data as an continuous data stream with a start and a
stop signal. The signal that indicates that a new stream is waiting to be opened
is called a SYN three-way handshake in TCP, and consists of one packet sent with
the SYN bit set. The other end then either answers with SYN/ACK or SYN/RST to
let the client know if the connection was accepted or denied, respectively. If
the client receives an SYN/ACK packet, it once again replies, this time with an
ACK packet. At this point, the whole connection is established and data can be
sent. During this initial handshake, all of the specific options that will be
used throughout the rest of the TCP connection is also negotiated, such as ECN,
SACK, etcetera.
While the datastream is alive, we have further mechanisms to see that the
packets are actually received properly by the other end. This is the
reliability part of TCP. This is done in a simple way, using a Sequence number
in the packet. Every time we send a packet, we give a new value to the Sequence
number, and when the other end receives the packet, it sends an ACK packet back
to the data sender. The ACK packet acknowledges that the packet was received
properly. The sequence number also sees to it that the packet is inserted into
the data stream in a good order.
Once the connection is closed, this is done by sending a FIN packet from either
end-point. The other end then responds by sending a FIN/ACK packet. The FIN
sending end can then no longer send any data, but the other end-point can still
finish sending data. Once the second end-point wishes to close the connection
totally, it sends a FIN packet back to the originally closing end-point, and
the other end-point replies with a FIN/ACK packet. Once this whole procedure is
done, the connection is torn down properly.
As you will also later see, the TCP headers contain a checksum as well. The
checksum consists of a simple hash of the packet. With this hash, we can with
rather high accuracy see if a packet has been corrupted in any way during
transit between the hosts.
