Compression Troubleshooting
If you are experiencing problems with the compression feature, this document
can help you troubleshoot your problem. Select your problem below:
Graphs Don't Show Compression
If the Compression Summary, Top Ten Compression, or other compression graphs
don't show compression data, check the table below for possible problems and
solutions.
| Problem |
Solution |
|
Are your cables connected backwards?
The two Xpress units must be configured outside port to outside port
(drawing).
If one of the units is connected to the WAN or Internet through the inside
port, compression tunnels cannot be created.
|
Refer to the PacketShaper Quick Start Guide for instructions on connecting your PacketShaper to
the network.
If you discover that you have miscabled the unit, reconnect the cables
properly, and then turn compression off and back on.
|
|
Are you sending flows to a host on the other side of a compression-enabled
PacketShaper?
If the host does not have a compression-enabled PacketShaper Xpress in
front of it, data cannot be compressed.
|
Make sure there is a PacketShaper Xpress in front of the host and that
compression is enabled. |
|
Is compression enabled on the partner unit(s)?
Xpress can’t create a compression tunnel if the partner unit has
not turned on compression also.
|
Log into each of the partner units and make sure Compression is
set to on. |
|
Have you applied a never-admit policy to the RSVP class?
The compression tunnel discovery process will not function if the RSVP
class has a never-admit policy.
|
Delete the policy. |
|
Do intermediate routers or firewalls disallow RSVP or IPComp traffic?
The compression tunnel discovery process will not function if RSVP or
IPComp protocols are blocked.
|
Remove the restriction. |
|
Is there a compression tunnel?
Compression cannot take place without a compression
tunnel. Compression Tunnel
A communications link between two points; the data in the encapsulated link is compressed between the two end points of the tunnel.
|
Check the tunnel status
with the setup compression show CLI command.
The following commands are useful for troubleshooting tunnel setup problems:
|
|
Does your Xpress unit have LEMs?
Xpress requires that the LEMs be configured with Xpress-IP addresses
so that compression tunnels can be established with each interface.
|
Assign
Xpress-IP addresses to each device. |
I am not Getting Desired Compression Results
If your compression results are not what you expected, there could be a number
of reasons:
- Your traffic consists mainly of non-compressible traffic. Because
Xpress only compresses traffic that is likely to achieve useful gains, if
most of your traffic is considered to be non-compressible, your link won't
show as much savings from compression. Flows that are already compressed (such
as images, zipped attachments, and streaming media) and encrypted data are
examples of non-compressible services. To see a list of non-compressible services,
use the setup compression show services command. To see which types
of traffic are currently being compressed, use the setup compression show
command. The output lists the services being compressed for each host.
Note: Compression graphs include only compressible traffic by default
(although there is an option to include non-compressible traffic as well).
It may also be useful to export compression statistics for each class. For
example, to see the breakdown of compressible and non-compressible bytes in
each class, use this CLI command:
measure dump class all outbound by element 1d sort tunneled-precompression-bytes
non-compressible-bytes
- Poor tunnel quality can affect compression results. To check the
quality of the compression tunnels, look for the Tunnel Quality field in the
setup compression show output. Tunnel quality can range between 0 and
100, with a value of 100 indicating best tunnel quality. A value that is less
than 95 could indicate problems with the network, connectivity problems with
your service provider, or inadequate link size (that is, your link is saturated).
Tunnel quality is derived from underlying metrics such as packet loss. Because
the tunneling feature uses a reliable protocol, lost packets must be retransmitted.
If the same packet is transmitted twice, this will reduce the number of bytes
saved by compression. Any steps you can take to minimize the likelihood of
compressed packets being dropped will improve compression performance. Check
all the network devices between the tunnel end points to determine where the
packet loss is occurring. For example, if you determine that a particular
router is being overloaded, you could adjust some of the router settings in
order to increase bandwidth capacity.
- The traffic on your link is pushing the limits of your PacketShaper unit.
Because traffic shaping takes precedence over compression, if your network
gets swamped, PacketShaper will temporarily send traffic without compressing
it and less traffic will go through the compression tunnel. Upgrading to a
higher capacity unit will allow compression to take place in these periods
of heavy traffic load.
Compression is Slowing Down Traffic
Because PacketShaper Xpress combines traffic management with compression,
it is able to minimize the natural latency caused by compression. PacketShaper's
TCP rate control, UDP rate control, and advanced queuing technologies allow
it to manage flows of data through the compression engine to ensure consistent,
predictable responses with minimal delays. However, if you do notice a traffic
delay when compression is enabled, try the following:
- Use PacketShaper's control capabilities to improve application performance.
Set appropriate policies and partitions in order to protect mission critical
traffic, contain recreational and unsanctioned traffic, and smooth peaks in
bursty traffic. Make sure traffic shaping is turned on!
- Check to see if packing is enabled. Packing is a feature of compression that combines multiple uncompressed packets into
a single compressed "super packet" in order to save on overhead, and thereby increases
compression rates. However, packing can add latency. To check your packing setting, go to the
System Variables setup page. If the Enable Packing variable is on, you can turn it off. Alternatively, you can enable packing but decrease the value for the Packing hold time variable. Keep in mind, though, that disabling
packing or decreasing the hold time will possibly reduce compression rates.
Compression is Not Working on a Few Applications or Hosts
If you notice that a few of your applications or hosts are not getting compressed, there are several possible reasons why this may be happening.
- You may have disabled compression for a class using the class
compress off command. The "d" flag in the class show
output will indicate those classes for which compression has been disabled.
In the example below, the Outbound/NTP class has the "d" flag.
- Host or partner restrictions may be preventing the application from getting
compressed. For example, if the application is on a server that is on the
list of outside hosts excluded from compression, the application will not
get compressed. To see the hosts and partners that are under restriction,
use the setup compression show hosts command.
- It's possible that Xpress was not able to properly identify which side
of PacketShaper the host is on. This can happen when using Xpress in a direct
standby configuration or with a load-balanced network. When Xpress cannot
identify a host's side, the traffic will not get compressed. When you determine
the IP addresses and subnets that were set to the wrong side, you can use
the hostdb side commands to manually configure the side of these hosts.
- Use the hl new command to create
a host list for the IP addresses, ranges, and/or subnets that should be
on the inside.
- Create a host list for the outside hosts.
- Use the hostdb side set
command to assign the host lists to the inside and outside. For example:
hostdb side set inside list:inside_list
hostdb side set outside list:outside_list
Note: In addition to (or instead of) host lists, the hostdb side
set command allows you to set individual IP addresses, ranges, and subnets.
- Change the side mode to manual using the hostdb
side manual command.
- Use the hostdb show command to confirm the side of your manually-set
hosts.
PacketGuide™ for PacketWise® 7.5