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Table Of Contents
TCP Out-of-Order Packet Support for Cisco IOS Firewall and Cisco IOS IPS
Prerequisites for TCP Out-of-Order Packet Support for Cisco IOS Firewall and Cisco IOS IPS
Restrictions for TCP Out-of-Order Packet Support for Cisco IOS Firewall and Cisco IOS IPS
Information About TCP Out-of-Order Packet Support for Cisco IOS Firewall and Cisco IOS IPS
How TCP Out-of-Order Packet Support Works
How to Configure Cisco IOS Firewall or IPS to Handle TCP Out-of-Order Packets
Changing Default TCP Out-of-Order Packet Parameters
Configuration Examples for TCP Out-of-Order Packet Parameters
Verifying TCP Out-of-Order Packets: Example
Feature Information for TCP Out-of-Order Packet Support for Cisco IOS Firewall and IPS
TCP Out-of-Order Packet Support for Cisco IOS Firewall and Cisco IOS IPS
First Published: November 17, 2006Last Updated: November 17, 2006This feature allows out-of-order packets in TCP streams to be cached and reassembled before they are inspected by Cisco IOS Intrusion Prevention System (IPS) or Cisco IOS Firewall.
Finding Feature Information in This Module
Your Cisco IOS software release may not support all of the features documented in this module. To reach links to specific feature documentation in this module and to see a list of the releases in which each feature is supported, use the "Feature Information for TCP Out-of-Order Packet Support for Cisco IOS Firewall and IPS" section.
Finding Support Information for Platforms and Cisco IOS and Catalyst OS Software Images
Use Cisco Feature Navigator to find information about platform support and Cisco IOS and Catalyst OS software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Contents
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Prerequisites for TCP Out-of-Order Packet Support for Cisco IOS Firewall and Cisco IOS IPS
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Prerequisites for TCP Out-of-Order Packet Support for Cisco IOS Firewall and Cisco IOS IPS
Cisco IOS IPS or Cisco IOS Firewall must be configured on your router.
Restrictions for TCP Out-of-Order Packet Support for Cisco IOS Firewall and Cisco IOS IPS
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Information About TCP Out-of-Order Packet Support for Cisco IOS Firewall and Cisco IOS IPS
Before reassembling TCP out-of-order packets, you should understand the following concept:
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How TCP Out-of-Order Packet Support Works
Cisco IOS Firewall and IPS track packets in TCP connections. If configured to look into the application data of the packets, Cisco IOS Firewall and IPS expect the TCP packets to arrive in the correct order because some data items are split across segments. When packets arrive out of order, they are dropped by the firewall or IPS. Dropping out-of-order packets can cause significant delays in end applications because packets are dropped only after the retransmission timer expires (on behalf of the sender).
Out-of-order TCP packet support enables Cisco IOS Firewall and IPS to hold a copy of the out-of-order packet in a buffer (whose size is configurable with a maximum of 1024 packets per session). The original packet passes through the router and reaches its destination, but the firewall or IPS do not execute on the packet. When the next packet arrives, the firewall or IPS look for that packet to "fill the hole," providing a consecutive sequence of segments. If this packet does not fulfill that requirement, it is processed as an out-of-order packet; when another packet arrives and provides a consecutive sequence of segments, it is processed by the firewall or IPS.
How to Configure Cisco IOS Firewall or IPS to Handle TCP Out-of-Order Packets
This section contains the following procedure:
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Changing Default TCP Out-of-Order Packet Parameters
Use this task to change any of the predefined parameters that instruct Cisco IOS Firewall application inspection or Cisco IOS IPS how to handle out-of-order TCP packets.
SUMMARY STEPS
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DETAILED STEPS
Configuration Examples for TCP Out-of-Order Packet Parameters
This sections contains the following configuration example:
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Verifying TCP Out-of-Order Packets: Example
The following example shows how to instruct Cisco IOS IPS how to handle out of order packets for TCP connections:
Router(config)# ip inspect tcp reassembly queue length 18Router(config)# ip inspect tcp reassembly memory limit 200The following sample output displays the configured out-of-order packet parameters:
Router# show ip ips statisticsSignature Statistics [process switch:fast switch]Signature 1000: 324 packets checked: [124:200]Signature 1024: 100 packets checked: [0:100]Interfaces configured for ips 0Session creations since subsystem startup or last reset 0Current session counts (estab/half-open/terminating) [0:0:0]Maxever session counts (estab/half-open/terminating) [0:0:0]Last session created neverLast statistic reset neverTCP reassembly statisticsreceived 200 packets out-of-order; dropped 25peak memory usage; 200 KB; current usage: 154 KBpeak queue length 18Additional References
The following sections provide references related to the TCP Out-of-Order Packet Support for Cisco IOS Firewall and Cisco IOS IPS feature.
Related Documents
Standards
MIBs
None
To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:
RFCs
Technical Assistance
Command Reference
This section documents the following new command:
ip inspect tcp reassembly
To set parameters that define how Cisco IOS Firewall application inspection and Cisco IOS Intrusion Prevention System (IPS) will handle out-of-order TCP packets, use the ip inspect tcp reassembly command in global configuration mode. To disable at least one defined parameter, use the no form of this command.
ip inspect tcp reassembly {[queue length packet-number] [timeout seconds] [memory limit size-in-kb] [alarm {on | off}]}
no ip inspect tcp reassembly {[queue length] [timeout] [memory limit]}
Syntax Description
Command Default
Queue length: 16
Timeout value:
Memory Limit: 1024 kilobytes
Alarm: on
Command Modes
Global configuration
Command History
Usage Guidelines
The queue length Value
The value specified for the queue length is applicable for two queues per session: one queue is for the initiator traffic and the other queue is for the responder traffic. For example, the default queue size is 16. Thus, up to 16 packets can be held per queue, so 16 packets per queue results in a maximum of 32 packets per session.
When the maximum queue length value is reached, the packet being switched is dropped unless it is the packet that will be processed by a firewall or IPS. If the packet is dropped, a syslog message, which explains why the packet was dropped, will be generated. (To generate syslog messages, you must have the alarm option set to "on.")
The timeout Value
When a timer expires for the first time, the packets in the queue are not deleted. However, after the retry timer expires, the session is deleted, a syslog message is generated, and all unprocessed, out-of-order packets still in the queue are deleted.
The memory limit Value
When the limit for TCP reassembly memory is reached, packets from the reassembly queue of the current session are released so incoming packets can be accepted. Packets from the end of the queue are released to ensure that they are farthest away from the hole that is to be filled. However, if the queue is empty and the maximum memory has been reached, the incoming packet is dropped.
The alarm Value
If an alarm value is not configured, the value is set to "on," unless the ip inspect alarm command is enabled and set to off; thus, syslog messages related to TCP connections will not be generated. However, if the alarm value for this command is set to "on" and the ip inspect alarm command is set to "off," the value of the ip inspect alarm command is ignored and syslog messages are generated.
The alarm value is independent of and in addition to the syslog messages that can be enabled for a Cisco IOS Firewall or Cisco IOS IPS.
Examples
The following example shows how to instruct Cisco IOS IPS how to handle out-of-order packets for TCP connections:
Router(config)# ip inspect tcp reassembly queue length 18Router(config)# ip inspect tcp reassembly memory limit 200Feature Information for TCP Out-of-Order Packet Support for Cisco IOS Firewall and IPS
Table 1 lists the release history for this feature.
Not all commands may be available in your Cisco IOS software release. For release information about a specific command, see the command reference documentation.
Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS and Catalyst OS software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Note
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.
© 2006 Cisco Systems, Inc. All rights reserved.
