Installing and Administering PPP

Chapter 5 95
Security Techniques
Packets Overview
Transmission Control Protocol (TCP) Level
Figure 5-3
* RFC-793 [TCP]
The TCP header fields available for matching include port numbers for
the source or destination port, the presence of the SYN bit without ACK,
and the ACK, FIN and RST bits.
PPP does not provide a method for filtering on TCP options, the presence
of URG/EOM bits in the TCP options, or other TCP header fields.
Establishing a TCP connection requires synchronization. Each side
must send its own initial sequence number, receive a confirming
acknowledgment from the other end, receive the other end’s initial
sequence number and send the confirming acknowledgment.
The steps in the sequence look like this:
Step Path TCP Bit Message
1
2
3
4
A -> B
A <- B
A <- B
A -> B
SYN
ACK
SYN
ACK
My sequence number is X.
Your sequence number is X.
My sequence number is Y.
Your sequence number is Y.