Other considerations for selecting a compression algorithm to optimize your WAN utilization
folllow:
-- In dial environments, compression can occur in the modem. Two
common modem-compression standards are Microcom Networking
Protocol 5 (MNP5) and the ITU V.42bis. MNP5 and V.42bis offer up
to two times and four times compression, respectively. The two
specifications are not compatible. The modems at both ends of the
connection negotiate the standard to use. If compression is being
done at the modem, do not configure the router to run compression
because some modems, such as a V.42bis and MNP5 modem, cannot
compress a file that is already compressed by software. In the
case of MNP5, it will even try to compress a precompressed file
and actually expand it, thus slowing down the file transfer!
Encrypted data -- Compression
is a Layer 2 function and encryption occurs at Layer 3. When a
data stream is encrypted by the client application, it is then
passed onto the router for routing or compression services. When
the compression engine receives the encrypted data stream, which
by definition has no repetitive patterns, the data expands and
does not compress. LZS then compares the before and after images
to determine which is the smallest, and then sends the
uncompressed data as it was originally received if expansion
occurred. So, if data is encrypted, do not compress the encrypted
data using a Layer 2 compression algorithm.
The solution to this problem is to compress at Layer 3 and then
encrypt at the same layer. Cisco is very active in providing a
solution to this problem with its work on the IP Security (IPSec)
Protocol and the IP Compression Protocol (IPComp). IPSec is a
standards-based method of providing privacy, integrity, and
authenticity to information transferred across IP networks. IPSec
provides IP network-layer encryption. IPComp is a mechanism to
reduce the size of IP datagrams and is especially useful when
encryption is applied to IP datagrams, in which Layer 2 (PPP)
compression is not effective.
CPU cycles versus memory --
The amount of memory that a router must have varies according to
the protocol being compressed, the compression algorithm, and the
number of concurrent circuits on the router. Memory requirements will be
higher for Predictor than for STAC, and payload will use more
memory than link compression. Likewise, link compression utilizes
more CPU cycles.
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