Solutions

Why are there negative values in the %Bytes Saved column in the output of "compression show"?

Solutions ID:    KB2516
Version:    5.0
Status:    Published
Published date:    03/02/2009
Updated:    11/04/2013
 

Problem Description

The %Bytes Saved column in the output of compression show shows negative values for some traffic classes:

PacketShaper# comp show 169.182.26.27
Tunnel Interface: main
Tunnel Partner: 169.182.136.197
Tunnel Status: Normal operation (Up: 1h 10m 10s, Idle: 1s)
Last Shutdown (tunnelProcessShutdownRequest)
Tunnel Quality: 89
Tunnel Savings: 583 Bpm


Compressors Type %Bytes Saved
------------------------------------------------------------------------------
GROUP DICTIONARY cna-1M 2% ( 1 secs old)
ICMP cna-1M 9% ( 1 secs old)
Telnet-Clear cna-1M -5% ( 3 secs old)
LotusNotes cna-1M -18% ( 12 secs old)
pcANYWHERE-D cna-1M 75% ( 15 secs old)
SNMP-Mon cna-1M ----% (196 secs old)
Observed cna-1M ----% (234 secs old)
NetBIOS-IP-NS cna-1M ----% (2757 secs old)
SAP cna-1M ----% (5314 secs old)

Decompressors Type %Bytes Saved
------------------------------------------------------------------------------
DICTIONARY cna-1M 0% ( 1 secs old)

Resolution

Negative compression gains can result when the original packets are not compressible. The post compression IPCOMP packet ends up being larger than the pre-compressed packet. In order for positive compression gains to take place, the original packet must be compressed by a greater number of bytes than are added by the additional IPCOMP packet header. 

The compression show output is a snapshot showing the compression gains for traffic types over the last eight packets compressed.  For a more realistic view of compression gains, use the compression reports for those classes to measure compression gains over a longer period of time.

If class compression reports consistently show low or negative compression gains,  compression for a class can be disabled using the command:

class compress <tclass> off 

 


Rate this Page

Please take a moment to complete this form to help us better serve you.

Did this document help answer your question?
 
 
If you are finished providing feedback, please click the RATE CONTENT button. Otherwise, please add more detail in the following text box and then click RATE CONTENT.
 
 

Your response will be used to improve our document content.

Ask a Question