Installing and Administering PPP

96 Chapter 5
Security Techniques
Packets Overview
Packets 2 and 3 are normally combined into a single "SYN ACK" packet
This is called a three-way handshake.
The SYN bit is set, with no ACK bit set, to show a TCP connection
request. By blocking packets with the SYN bit set in a single direction,
you may permit TCP connections in a single direction.
User Datagram Protocol (UDP) Level
Figure 5-4
* RFC-768 [UDP]
The UDP header fields available are the source and destination port
numbers.
UDP does not permit a router to differentiate between inbound packets
requesting new services and inbound packets returning data to outbound
requests. This means that static filters which allow inside users access
to services based on UDP also allow outside users to access the same
service inside your network.