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Firewalled Ports – a TCP connect or half-
open scan should receive either a SYN/ACK or
a RST/ACK. However, a third possibility exists:
No response. This is often due to a firewalled
port being filtered, or possibly the packets
being lost due to network congestion.

TCP versus UDP

Transport layer protocols within TCP/IP:

• Transmission Control Protocol (TCP) –
Reliable, connection-oriented,
performs a setup handshake, error
detection and correction, and
windowing
• User Datagram Protocol (UDP) –
Unreliable, connectionless, no
handshake is performed, and “best
effort” protocol

UDP Port Scan – open UDP ports can be
identified with port scans, even though UDP is
connectionless. Sending a UDP request to a
particular port will result in no response for
an open port, ICMP port unreachable for a
closed port.

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