Table Of Contents

Cisco IOS Firewall Performance Improvements

Feature Overview

Throughput Improvement

Connections Per Second Improvement

CPU Utilization Improvement

Benefits

Restrictions

Supported Platforms

Supported Standards, MIBs, and RFCs

Configuration Tasks

Changing the Size of the Hash Table

Verifying CBAC Configurations

Configuration Examples

Changing the Size of the Hash Table Example

Command Reference

Cisco IOS Firewall Performance Improvements

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Feature History

Release	Modification
12.2(8)T	This feature was introduced.

This document describes the Cisco IOS Firewall Performance Improvements feature in Cisco IOS Release 12.2(8)T. It includes the following sections:

•Feature Overview

•Supported Platforms

•Supported Standards, MIBs, and RFCs

•Configuration Tasks

•Configuration Examples

•Command Reference

Feature Overview

The Cisco IOS Firewall Performance Improvements feature introduces three performance metrics for Context-Based Access Control (CBAC)—Throughput Improvement, Connections Per Second Improvement, and CPU Utilization Improvement.

CBAC is a context-based firewall that performs the following:

•Inspects traffic in one direction for network, transport, and application layer information

•Extracts relevant port information

•Dynamically creates access list entries for return traffic

•Closes ports at the end of a connection

CBAC also forces protocol conformance for some protocols, has a limited vulnerability signature detection mechanism, and extensive denial-of-service (DOS) prevention mechanisms. However, many of these features are CPU intensive, thereby, adversely affecting the performance of the router. The router is also affected during times of heavy traffic due to the processing of the base engine itself. With this feature, the performance of the router running CBAC is no longer subdued.

Throughput Improvement

Throughput is a metric defined by the number of packets transferred from the input interface to the output interface within 1 second by a router running CBAC. When the CBAC base engine inspects packets that belong to an existing session, it must find out which session the packet belongs to; thus, the base engine implements a hash table to search for the session of the packet.

Collisions in a hash table result in poor hash function distribution because many entries are hashed into the same bucket for certain patterns of addresses. Even if a hash function distribution evenly dispenses the input across all of the buckets, a small hashtable size will not scale well if there are a large number of sessions. As the number of sessions increase, the collisions increase, which increases the length of the linked lists, thereby, deteriorating the throughput performance.

The Cisco IOS Firewall Performance Improvements feature allows users to dynamically change the size of the session hash table without reloading the router by using the ip inspect hashtable command. By increasing the size of the hash table, the number of sessions per hash bucket can be reduced, which improves the throughput performance of the base engine.

Connections Per Second Improvement

Connections per second is a metric defined by the number of short-lived connections that can be created and deleted within 1 second by a router running CBAC. (These connections apply only to TCP connections because UDP is a connectionless protocol.)

Initially, CBAC had several restrictions that limited the connections per second metric. While a packet was being processed for connection setup and connection teardown of TCP connections, the base engine (which allocates and de-allocates memory while processing packets) would "bump up" several packets to the process switching path. Bumping up these packets drastically slowed down their processing. Also, the base engine had to process each packet again when it was bumped up into the process switching path, which also contributed to the degrading performance.

The Cisco IOS Firewall Performance Improvements feature prevents these restrictions by allowing only the first packet of any connection to be bumped up to the process switching path while the remaining packets are processed by the base engine in the fast path. Thus, the base engine is no longer slowed down by bumping up several packets or by processing packets twice.

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Note In this document, a connection is defined as creating a session, sending a data packet, and immediately deleting a session.

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CPU Utilization Improvement

The CPU utilization of the router running CBAC can be measured while a specific throughput or connections per second metric is maintained. This improvement is used in conjunction with the throughput and connections per second metrics.

Benefits

Layer 4 Processing Performance Improvement

This enhancement improves the connections per second metric and the CPU utilization. The code path for connection initiation and teardown was rewritten, thereby, enabling quicker creation of the connections per second metric, which reduces CPU utilization per connection.

Hash Table Function Performance Improvement

With this enhancement, the hash function has been rewritten to ensure better distribution. This newly improved feature allows users to dynamically configure the size of the session hash table from 1K to 8K. When a packet belonging to an existing session comes into the router, a hash table is used to map the packet to an existing firewall session. As the number of sessions increases, the number of sessions hashing into the same bucket increases if the size of the hash table is fixed. By allowing the user to change the size of the hash table when the number of concurrent sessions increases or to reduce the search time for the session, the throughput performance of the base engine is greatly improved.

Application Module Tuning Performance Improvement

This enhancement makes changes to application modules, ensuring that only the connection-initiating packet from all the packets belonging to the connection initiation and teardown is bumped up to the process switching path. Thus, the connections per second metric is significantly improved.

Restrictions

To benefit from the performance enhancements, your router must be running CBAC.

Supported Platforms

Cisco IOS Firewall is not supported on all hardware platforms. Use Cisco Feature Navigator to find information about platform support and Cisco IOS, Catalyst OS, and Cisco IOS XE software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Supported Standards, MIBs, and RFCs

Standards

None

MIBs

None

To obtain lists of supported MIBs by platform and Cisco IOS release, and to download MIB modules, go to the Cisco MIB website on Cisco.com at the following URL:

http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml

RFCs

None

Configuration Tasks

See the following sections for configuration tasks for the Cisco IOS Firewall Performance Improvements feature. Each task in the list is identified as either required or optional.

•Changing the Size of the Hash Table (required)

•Verifying CBAC Configurations (optional)

Changing the Size of the Hash Table

You can increase the hash table to improve packet distribution. To change the size of the session hash table, use the following command in global configuration mode:

Command

Purpose

Router# ip inspect hashtable number

Changes the size of the hash table. number specifies the size of the hash table in terms of buckets. Possible values for the hash table are 1024, 2048, 4096, and 8192; the default value is 1024. Note You should increase the hash table size when the total number of sessions running through the CBAC router is approximately twice the current hash size; decrease the hash table size when the total number of sessions is reduced to approximately half the current hash size. Essentially, try to maintain a 1:1 ratio between the number of sessions and the size of the hash table.

Verifying CBAC Configurations

To verify all CBAC configurations and all existing sessions that are currently being tracked and inspected by CBAC, use the show ip inspect all command in EXEC mode.

Configuration Examples

This section provides the following configuration example:

•Changing the Size of the Hash Table Example

Changing the Size of the Hash Table Example

The following example shows how to change the size of the hash table to 2048 buckets:

ip inspect hashtable 2048

Command Reference

The following commands are introduced or modified in the feature or features documented in this module. For information about these commands, see the Cisco IOS Security Command Reference at http://www.cisco.com/en/US/docs/ios/security/command/reference/sec_book.html. For information about all Cisco IOS commands, go to the Command Lookup Tool at http://tools.cisco.com/Support/CLILookup or to the Cisco IOS Master Commands List.

•ip inspect hashtable

Any Internet Protocol (IP) addresses used in this document are not intended to be actual addresses. Any examples, command display output, and figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses in illustrative content is unintentional and coincidental.

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