The ONL NPR Tutorial

NPR Tutorial >> Filters, Queues and Bandwidth

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Auxilliary Filters

Auxilliary filters are normally used in a monitoring application in conjunction with a plugin. But they have other uses in an experimental environment. For example, they have been used to generate a heavy traffic load.

The main features of auxilliary filters are:

• Priority Selection: When more than one auxilliary filter matches a packet, the one with the highest priority (lowest number) is selected.
• Packet Copying: If a packet matches atleast one auxilliary filter, a copy of the packet is directed to the component indicated by the filter. If the packet also matches a route table entry or a primary filter, the packet is directed according to the selected entry. Thus, it is possible to have two copies of the packet.

To demonstrate the packet-copying nature of auxilliary filters, we use the dumbbell network shown to the right and send ping packets from n1p3 to n2p2. At input port 1.3, we install both a route table and an auxilliary filter so that the route table will direct packets over the solid red top path and the auxilliary filter will direct packets over the dashed red bottom path. The route table directs packets destined for n2p2 to go out port 1.4. But the auxilliary filter directs matching packets to go out port 1.0. The effect of the auxilliary filter can be demonstrated by watching the bandwidth going over the two paths and the bandwidth going out of port 2.2.

The recipe for installing the auxilliary filter at input port 1.3 is:

• RLI Window:
  Select Port 1.3 => Configuration => Add Filter.

  The Add Filter dialogue box appears.

• Add Filter Dialogue Box:
  
  • Select * from the protocol menu to match any protocol.
  • Check the aux box to make the filter an auxilliary filter
  • Enter 0 in the output ports box
  • Select Add

The bandwidth chart (right) shows that the traffic going out port 2.2 (solid red curve) is double the bandwidth received at port 2.1 (green 2.1 rx) and bandwidth transmitted at port 1.0 (brown 1.0 tx). The chart clearly shows that there is traffic going over both the two links that connect NPR 1 and NPR 2.

The output of the ping command (below) also shows that the there are duplicate (DUP!) packets detected at the receiver n2p2.

Sampling Filters

Sampling filters are a type of auxilliary filter that allows you to probabilistically select a fraction of packets that match an auxilliary filter. Although a sampling filter can be used without a plugin, it becomes much more useful in conjunction with a plugin that can process the sampled packets. The main things to remember are:

• The filter must be an auxilliary filter.
  Remember that an auxilliary filter is denoted by selecting the aux checkbox in a filter. This means that a duplicate packet will be generated if there is another matching filter or route table entry, and it is this duplicate packet that may be selected from sampling. Of course, if the auxilliary filter is the only matching filter, a duplicate packet is not made.
• The sampling probability menu offers you only one of four choices: 100%, 50%, 25% or 12.5%.
  For example, if you choose 25%, 25% of the matching packets will be selected and 75% will be dropped.
• The selection is probabilistic.
  This means that every time there is a matching packet, the NPR flips a coin based on the sampling probability to determine if the packet should be dropped or not. This means that if n packets are considered for sampling when the sampling percentage is 25%, N packets will be selected where N follows the Bernoulli probability law with a success probability of 0.25. For example, if 100 packets are matched, the actual number selected by sampling at the 25% level might not be 25.

We could have converted the auxilliary filter at the beginning of this page to a sampling filter by just selecting a sampling percentage other than 100%. But since auxilliary filters and sampling filters in particular are usually used in conjunction with plugins, we partially show how that can be done here. We leave the explanation of how to install a plugin to a later page. The figure (right) shows a 50% sampling filter that directs packets to microengine 0 where a plugin must be installed. The only differences are:

• port/plugin selection: We selected plugin only instead of port only.
• output plugins: We entered 0 to indicate that matching packets should be directed to microengine 0.
• sampling type: We selected 50% instead of accepting the default 100% value.

NPR Tutorial >> Filters, Queues and Bandwidth	TOC