Security 11-171Basic IP packet componentsAll IP packets contain the same basic header information, as follows:This header information is what the packet filter uses to make filtering decisions. It is important to note that apacket filter does not look into the IP data stream (the User Data from above) to make filtering decisions.Basic protocol typesTCP: Transmission Control Protocol. TCP provides reliable packet delivery and has a retransmissionmechanism (so packets are not lost). RFC 793 is the specification for TCP.UDP: User Datagram Protocol. Unlike TCP, UDP does not guarantee reliable, sequenced packet delivery. If datadoes not reach its destination, UDP does not retransmit the data. RFC 768 is the specification for UDP.There are many more ports defined in the Assigned Addresses RFC. The table that follows shows some of theseport assignments.Example TCP/UDP PortsSource IP Address 163.176.132.18Destination IP Address 163.176.4.27Source Port 2541Destination Port 80Protocol TCPACK Bit YesDATA User DataTCP Port Service20/21 FTP23 Telnet25 SMTP80 WWW144 NewsUDP Port Service161 SNMP
