Fundamentals of InfoSecadvanced
Explain the three-way TCP handshake and why an attacker who leaves it incomplete can cause resource starvation.
Answer
The client sends a SYN request, the server replies with SYN-ACK, and the client returns ACK. The connection is established after the final acknowledgment. A SYN flood sends many requests without completing the handshake, leaving half-open connections that consume server resources.
Explanation
### What a good answer covers
- The client begins with a SYN request.
- The server replies with SYN-ACK to acknowledge and synchronize.
- The client sends ACK, completing the three-way handshake.
- In a SYN flood, many requests remain incomplete, leaving half-open state that consumes resources.
### The concept
The handshake establishes a TCP connection before application data is exchanged. A server must keep enough information about pending connections to manage the process. Repeated incomplete requests can therefore create resource pressure. Firewalls and filtering may help distinguish or limit suspicious connection attempts, but no single measure guarantees prevention.
### Terms
- **Transmission Control Protocol (TCP)**: a connection-oriented protocol.
- **SYN flood**: an attack that creates many incomplete connection requests.
- **Resource starvation**: depletion of resources needed for legitimate work.
### Remember
The final ACK closes the handshake; without it, the server may retain pending state.
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