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Firewall Tutorial
This header information is what the packet filter uses to make filtering decisions. It is
important to note that a packet filter does not look into the IP data stream (the User Data
from above) to make filtering decisions.
Basic protocol types
TCP: Transmission Control Protocol. TCP provides reliable packet delivery and has a
retransmission mechanism (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 data does 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 these port assignments.
Protocol TCP
DATA User Data