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WAN Compression Techniques 309
The Cisco IOS header compression strategy subscribes to the Van Jacobson Algorithm defined
in RFC 1144. It is highly effective on TCP/IP traffic, which consists of small packets with a few
bytes of data, such as Telnet. TCP/IP header compression removes some of the redundant fields
in the header of a TCP/IP connection. By keeping a copy of the original header on either side
of the link, removing the redundant fields, and coding the remaining fields, the header can be
compressed from 40 bytes to 5.
Compression Design Issues
Compression is effective in improving WAN transmission only when an application can accept
the characteristics of compression being applied to it. When considering whether to use
compression, keep these network characteristics in mind:
·
Number of remote sites
·
Increase in latency
·
Router memory requirements
·
Router CPU utilization
Each point-to-point connection must have dedicated memory to support the compression
overhead. The greater the number of remote sites, the greater the requirement for memory. Each
additional line increases CPU utilization, and the additional burden can increase latency. When
compression is applied on the original data stream, the information must be processed and
analyzed. As a result, latency is added to the input data before transmission. An increase in
latency could affect transmission when protocols sensitive to added network latency are used
over the WAN.
The amount of memory that the router must have and that the network manager must plan on
varies according to the protocol being compressed, the compression algorithm, and the number
of concurrent circuits on the router.
Cisco devices use the STAC (LZS) and Predictor data compression algorithms. The Cisco IOS
software uses an optimized version of STAC that provides good compression ratios but is
processor-intensive. The Predictor data compression algorithm was obtained from the public
domain and optimized by Cisco engineers. When compared with STAC, it makes more efficient
use of CPU cycles but requires more memory. The network manager must plan on additional
memory for routers in a network utilizing data compression, regardless of the type of operation
used.
TIP
The most recent releases in the Cisco IOS are recommended to ensure the best compression
efficiency. Run the same version of IOS code on both sides of the WAN link to ensure compatibility.
87200333.book Page 309 Wednesday, August 22, 2001 2:53 PM