1 Figure 3-27: Use of TCP and UDP Port Number Client From: :50047 To: :80 SMTP Server Port 25 Webserver Port 80

2 Figure 3-27: Use of TCP and UDP Port Number Client From: :80 To: :50047 SMTP Server Port 25 Webserver Port 80 From: :50047 To: :80

3 Figure 3-27: Use of TCP and UDP Port Number From: :60003 To: :25 Client SMTP Server Port 25 Webserver Port 80

4 Figure 3-27: Use of TCP and UDP Port Number From: :60003 To: :25 Client From: :50047 To: :80 SMTP Server Port 25 Webserver Port 80 Clients Used Different Ephemeral Ports for Different Connections

5 Figure 3-29: User Data Protocol (UDP) (Study Figure) UDP Datagrams are Simple (Figure 3-30)  Source and destination port numbers (16 bits each)  UDP length (16 bits)  UDP checksum (16 bits) Bit 0 Bit 31 IP Header (Usually 20 Bytes) Source Port Number (16 bits)Destination Port Number (16 bits) UDP Length (16 bits)UDP Checksum (16 bits) Data Field

6 Figure 3-29: User Data Protocol (UDP) (Study Figure) Port Spoofing Still Possible UDP Datagram Insertion  Insert UDP datagram into an ongoing dialog stream  Hard to detect because no sequence numbers in UDP