How to Measure over NAT

NAT changes packet content, which brings a new challenge on passive QoS measurement. However, Qosium can manage this and you can measure QoS over a network path where there is a NAT deployed between.

1. Define the Topology #

There are a couple of specific actions you need to take in the parameterization when measuring over a NATNetwork Address Translation
A technique for remapping an IP address space
or proxies that change the protocol header content of IP packets. The following instructions tell how to do the measurement parameterization with Qosium Scope. Qosium Scopemon and Scope Lite has the corresponding parameters and the procedure is the same than with Scope.

On the bottom of the Topology tab, enable NAT between Probes as shown below.

2. Set the Filter #

Filtering differs from the typical measurement scenario where a NAT is not present. There are two ways how to set the filter. In the most common one, set the filter how the Primary Probe sees the traffic flow(s), while the secondary Probe filter is set automatically. Alternatively, you can define a complex filter that includes parts for both sides of the NAT. This, naturally, requires that you are aware of how the desired traffic looks on the other side of the NAT.

Give Packet filter manually in the Measurement tab. After selecting Manual filter, you have three options in the Secondary Probe filter mode selection.

  • Loose autofilter - Use this when you wish to include all the flows between the two measurement points in the measurement. Include only the IP addresses in the filter. Do this from the primary Probe perspective – the secondary Probe filter is calculated automatically.
  • Strict autofilter - Use this when you wish to include only a single flow between the two measurement points in the measurement. In addition to IP addresses, use source and destination port definitions and also add the transport level protocol in the filter. Do this from the primary Probe perspective – the secondary Probe filter is calculated automatically.
  • Equal filter - This means that exactly the same filter is used in both measurement ends. The filter must thus include something that matches the desired traffic on both ends. Thus, define a filter that matches both ends: ( filter for the desired traffic in one end ) or ( filter for the desired traffic in another end )

Glossary >

Network Address Translation

A technique for remapping an IP address space

Wikipedia article on Network Address Translation