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Cisco ASDM User Guide
OL-16647-01
Chapter 2 Introduction to the Security Appliance
VPN Functional Overview
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Session lookup
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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
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.
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:
• 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
The security appliance invokes various standard protocols to accomplish these functions.
Security Context Overview
You can partition a single security appliance into multiple virtual devices, known as security contexts.
Each context is an independent device, with its own security policy, interfaces, and administrators.
Multiple contexts are similar to having multiple standalone devices. Many features are supported in
multiple context mode, including routing tables, firewall features, IPS, and management. Some features
are not supported, including VPN and dynamic routing protocols.
In multiple context mode, the security appliance includes a configuration for each context that identifies
the security policy, interfaces, and almost all the options you can configure on a standalone device. The
system administrator adds and manages contexts by configuring them in the system configuration,