Vpn Functional Overview - Cisco PIX 500 Series Configuration Manual

Chapter 1 Introduction to the Security Appliance
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VPN Functional Overview

A VPN is a secure connection across a TCP/IP network (such as the Internet) that appears as a private
connection. This secure connection is called a tunnel. The security appliance uses tunneling protocols to
negotiate security parameters, create and manage tunnels, encapsulate packets, transmit or receive them
through the tunnel, and unencapsulate them. The security appliance functions as a bidirectional tunnel
endpoint: it can receive plain packets, encapsulate them, and send them to the other end of the tunnel
where they are unencapsulated and sent to their final destination. It can also receive encapsulated
packets, unencapsulate them, and send them to their final destination. The security appliance invokes
various standard protocols to accomplish these functions.
The security appliance performs the following functions:
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The security appliance invokes various standard protocols to accomplish these functions.
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Some packets that require Layer 7 inspection (the packet payload must be inspected or altered) are
passed on to the control plane path. Layer 7 inspection engines are required for protocols that have
two or more channels: a data channel, which uses well-known port numbers, and a control channel,
which uses different port numbers for each session. These protocols include FTP, H.323, and SNMP.
Is this an established connection?
If the connection is already established, the security appliance does not need to re-check packets;
most matching packets can go through the fast path in both directions. The fast path is responsible
for the following tasks:
– IP checksum verification
– Session lookup
TCP sequence number check
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NAT translations based on existing sessions
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Layer 3 and Layer 4 header adjustments
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For UDP or other connectionless protocols, the security appliance creates connection state
information so that it can also use the fast path.
Data packets for protocols that require Layer 7 inspection can also go through the fast path.
Some established session packets must continue to go through the session management path or the
control plane path. Packets that go through the session management path include HTTP packets that
require inspection or content filtering. Packets that go through the control plane path include the
control packets for protocols that require Layer 7 inspection.
Establishes tunnels
Negotiates tunnel parameters
Authenticates users
Assigns user addresses
Encrypts and decrypts data
Manages security keys
Manages data transfer across the tunnel
Manages data transfer inbound and outbound as a tunnel endpoint or router
Cisco Security Appliance Command Line Configuration Guide
VPN Functional Overview
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