Cisco Systems 78-11741-02 manual Compressing TCP Packet Headers, Expressing TCP Header Compression

Configuring IP Services

Configuring TCP Performance Parameters

Compressing TCP Packet Headers

You can compress the headers of your TCP/IP packets in order to reduce their size, thereby increasing performance. Header compression is particularly useful on networks with a large percentage of small packets (such as those supporting many Telnet connections). To enable TCP header compression, use the following command in interface configuration mode:

Command	Purpose
Router(config-if)# ip tcp	Enables TCP header compression.
header-compression [passive]

The ip tcp header-compressioninterface configuration command only compresses the TCP header; it has no effect on UDP packets or other protocol headers. The TCP header compression technique is supported on serial lines using High-Level Data Link Control (HDLC) or PPP encapsulation. You must enable compression on both ends of a serial connection.

By using the passive keyword, you can optionally specify outgoing packets to be compressed only if TCP incoming packets on the same interface are compressed. If you specify the command without the passive keyword, the software will compress all traffic. Without the command, the default is no compression.

Note Fast processors can handle several fast interfaces, such as T1 lines, that are running header compression. However, you should think carefully about the traffic characteristics of your network before compressing TCP headers. You might want to use the monitoring commands to compare network utilization before and after enabling TCP header compression.

Expressing TCP Header Compression

Before Cisco IOS Release 12.0(7)T, if compression of TCP headers was enabled, compression was performed in the process switching path. Compression performed in the process switching path meant that packets traversing interfaces that had TCP header compression enabled were queued and passed up to the process to be switched. This procedure slowed down transmission of the packet, and therefore some users preferred to fast switch uncompressed TCP packets.

In Cisco IOS Release 12.1, if TCP header compression is enabled, it occurs by default in the fast-switched path or the CEF-switched path, depending on which switching method is enabled on the interface.

If neither fast switching nor CEF switching is enabled, then if TCP header compression is enabled, it will occur in the process-switched path as before.

The Express TCP Header Compression feature reduces network overhead and speeds up transmission of TCP packets. The faster speed provides a greater benefit on slower links than faster links.

In order for Express TCP Header Compression to work, the following conditions must be in place:

•CEF switching or fast switching must be enabled on the interface.

•HDLC, PPP, or Frame Relay encapsulation must be configured.

•TCP header compression must be enabled.

Cisco IOS IP Configuration Guide

IPC-111