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C.9 The anatomy of a TCP connection

  TCP is equipped with 6 ``flags'', which may be ON or OFF. These flags are:
FIN
``Controlled'' connection close
SYN
Open new connection
RST
``Immediate'' connection close
PSH
Instruct receiver host to push the data up to the application rather than just queue it
ACK
``Acknowledge'' a previous packet
URG
``Urgent'' data which needs to be processed immediately

In this example, your client is 5.6.7.8, and the port assigned to you dynamically is 1049. The server is 1.2.3.4, port 80.

You begin the connection attempt:

5.6.7.8:1049 -> 1.2.3.4:80 SYN=ON

The server receives this packet and understands that someone wants to form a new connection. A response is sent:

1.2.3.4:80 -> 5.6.7.8:1049 SYN=ON ACK=ON

The client receives the response, and informs that the response is received

5.6.7.8:1049 -> 1.2.3.4:80 ACK=ON

Here, the connection is opened. This is called a three-way handshake. Its purpose is to verify to BOTH hosts that they have a working connection between them.

The internet being what it is, unreliable and flooded, there are provisions to compensate for packet loss.

If the client sends out the initial SYN without receiving a SYN+ACK within a few seconds, it'll resend the SYN.

If the server sends out the SYN+ACK without receiving an ACK in a few seconds, it'll resend the SYN+ACK packet.

The latter is actually the reason that SYN flooding works so well. If you send out SYN packets from lots of different ports, this will tie up a lot of resources on the server. If you also refuse to respond to the returned SYN+ACK packets, the server will KEEP these connections for a long time, resending the SYN+ACK packets. Some servers will not accept new connections while there are enough connections currently forming; this is why SYN flooding works.

All packets transmitted in either direction after the three-way handshake will have the ACK bit set. Stateless packet filters make use of this in the so called ``established'' filters: They will only let packets through that have the ACK bit set. This way, no packet may pass through in a certain direction that could form a new connection. Typically, you don't allow outside hosts to open new connections to inside hosts by requiring the ACK bit set on these packets.

When the time has come to close the connection, there are two ways of doing it: Using the FIN flag, or using the RST flag. Using FIN flags, both implementations are required to send out FIN flags to indicate that they want to close the connection, and then send out acknowledgements to these FINs, indicating that they understood that the other end wants to close the connection. When sending out RST's, the connection is closed forcefully, and you don't really get an indication of whether the other end understood your reset order, or that it has in fact received all data that you sent to it.

The FIN way of closing the connection also exposes you to a denial-of-service situation, since the TCP stack needs to remember the closed connection for a fairly long time, in case the other end hasn't received one of the FIN packets.

If sufficiently many connections are opened and closed, you may end up having ``closed'' connections in all your connection slots. This way, you wouldn't be able to dynamically allocate more connections, seeing that they're all used. Different OSes handle this situation differently.

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