TCP Flags
As discussed previously, data exchange using TCP does not happen until a three-way handshake has been successfully completed. This handshake uses different flags to influence the way TCP segments are processed. There are 6 bits in the TCP header that are often called flags. In Figure 2-4, six different flags are part of the TCP header: Urgent pointer field (URG), Acknowledgment field (ACK), Push function (PSH), Reset the connection (RST), Synchronize sequence numbers (SYN), and sender is finished with this connection (FIN).
Figure 2-7 illustrates this three-way handshake in a little more detail, elaborating on some of the flags used.
Figure 2-7. Three-Way Handshake Using TCP Flags
Figure 2-7. Three-Way Handshake Using TCP Flags
- Three-Way Handshake
Bob wants to start talking with Alice, so he initiates the TCP session with the SYN bit (flag) set in the first TCP segment. If Alice is happy to talk to Bob, she responds with the SYN flag and ACK flag set to 1. If she is unwilling to talk to Bob, she responds with an RST (reset) flag set to 1.
Abuse of the normal operation or settings of these flags can be used by attackers to launch DoS attacks. This causes network servers or web servers to crash or hang. Table 2-3 illustrates some invalid combinations of these parameters.
Continue reading here: Table 23 TCP Flags
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