Compression

In addition to queuing, data compression is a useful way to increase network performance over a WAN link. By reducing the size of the frame to be transmitted, throughput can be increased. This section discusses the various kinds of compression. The commands needed to configure compression also are shown.

The kinds of compression supported by Cisco routers are as follows:

Link compression
Payload compression
TCP header compression
Microsoft Point-to-Point Compression (MPPC)

These methods are discussed briefly in the following sections.

NOTE

By default, frames are transmitted across a link uncompressed.

Link Compression

Also known as per-interface compression, this technique involves compressing both the header and a data frame's payload.

Two main algorithms are used to compress the traffic:

Predictor This algorithm is based on predicting the next sequence of characters in the data stream. This method is memory-intensive.
STAC This algorithm searches for redundant strings and replaces them with tokens, which are shorter than the original strings. This method is CPU-intensive.

Payload Compression

This technique involves compressing the data portion of a data frame. This is especially useful in an internetwork made up of different WAN networks, such as X.25, Frame Relay, and ATM.

It is also called per-virtual circuit compression. Payload compression uses the STAC compression algorithm.

TCP Header Compression

This technique is based on the Van Jacobson algorithm detailed in RFC 1144. This method is protocol-specific. Because only the TCP/IP header is compressed, the Layer 2 header is left unchanged.

This method is CPU-intensive and is good for protocols that have a small payload size, such as Telnet.

Microsoft Point-to-Point Compression

This technique, based on RFC 2118, uses an LZ compression mechanism. It can be used when communicating with a host using MPPC across a WAN link.

Configuring Compression

The following interface mode commands enable compression.

This command configures compression for an LAPB, PPP, or HDLC link:

Router(config-if)#compress [predictor |stac |mppc]

This command enables STAC compression on a Frame Relay point-to-point interface or subinterface:

Router(config-if)#frame-relay payload-compress

The following command enables TCP header compression. The passive option compresses outgoing TCP packets only if incoming TCP packets are compressed. If the passive option is not specified, all packets are compressed.

Router(config-if)#ip tcp header-compression [passive]

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