Chapter 10. Traffic Shaping

Several objects in the FB2500's configuration allow you to specify the name of a graph, by setting the value of the graph attribute. This causes the traffic flow that is associated with that object (a firewall rule, an interface, or whatever the attribute is attached to) to be recorded on a graph with the specified name. For connections that have a defined state, such as a PPP link, the graph will also show the link state history. Other information, such as packet loss and latency may also be displayed, depending on whether it can be provided by the type of object you are graphing.

For example, the XML snippet below shows the graph attribute being set on an interface. As soon as you have set a graph attribute (and saved the configuration), a new graph with the specified name will be created.

 <interface name="LAN"
            port="LAN"
            graph="LAN">
       

The graph is viewable directly from the FB2500 via the web User Interface - to view a graph, click the "PNG" or "SVG" item in the "Graphs" menu. This will display all the graphs that are currently configured - it is not currently possible to show a single graph within the web User Interface environment.

It is possible to access the graph data in many ways, using the URL to control what information is shown, labels, and colours, and also allowing graphs to be archived. See Appendix K for more details.

Alternatively, the underlying graph data is available in XML format, again via the FB2500's built-in HTTP server. The XML version of the data can be viewed in the web User Interface by clicking the "XML" item in the "Graphs" menu, and then clicking on the name of the graph you're interested in.

Both directions of traffic flow are recorded, and are colour-coded on the PNG image generated by the FB2500. The directions are labelled "tx" and "rx", but the exact meaning of these will depend on what type of object the graph was referenced from - for example, on a graph for an interface, "tx" will be traffic leaving the FB2500, and "rx" will be traffic arriving at the FB2500.

Each data point on a graph corresponds to a 100 second interval ; where a data point is showing a traffic rate, the rate is an average over that interval. For each named graph, the FB2500 stores data for the last 24 hours.

Once you have graphed a (possibly bi-directional) traffic flow, you can then also define speed restrictions on those flows. These can be simple "Tx" and "Rx" speed limits or more complex settings allowing maximum average speeds over time.

You define the speed controls associated with the graphed traffic flow(s) by creating a shaper top-level object. To create or edit a shaper object in the web User Interface, first click on the "Shape" category icon. To create a new object, click the "Add" link. To edit an existing object, click the appropriate "Edit" link instead.

The shaper object specifies the parameters (primarily traffic rates) to use in the traffic shaping process, and the shaper is associated with the appropriate existing graph by specifying the name attribute of the shaper object to be the same as the name of the graph.

You can define a shaper object and set the speed controls for that shaper, and then define the graph attribute on something, e.g. an interface, to apply that shaper to the interface.

It is also possible, in most cases, to simply set a speed attribute on some object. This creates an un-named shaper (so no graph) which has the specified speed for egress (tx). This is unique to that object unlike named shapers which are shared between all objects using the same named shaper.

It is also possible to set graph and speed attributes to create a named shaper with the specified speed, without having to create a separate shaper object.

If you set a graph attribute without a speed attribute or creating a shaper object then that simply creates a graph without traffic shaping. Multiple objects can share the same graph.

Graphs can sometimes be created automatically and may have speeds applied. For L2TP sessions the circuit ID (which may be overridden by RADIUS auth responses) is used to make a graph for the session.

If defining a shaper using the shaper object there are a number of extra options which allow a long term shaper to be defined. A long term shaper is one that changes the actual speed applied dynamically to ensure a long term usage level that is within a defined setting.

The key parameters for the long term shaper are the target speed (e.g. tx), the minimum speed (e.g. tx-min) and maximum speed (e.g. tx-max). The target speed is the maximum rate that should apply over a long term, but actual speed is set dynamically between min and max rates.

The rate is initially set to the maximum. Actual usage below the target builds up credit. This credit is limited to allow a defined minimum burst time at the maximum rate. So if not used for a while the maximum rate can be used for the minimum burst time. However continued use at more than the target will accumulate over usage.

Once this credit is used up the rate starts to drop. It will drop all the way to the minimum set if necessary.

Once the over usage is cleared, but less usage or usage capped to below the target rate, the rate increases to the target rate.

Once a further credit is accumulated by lower usage than the target, and this is at a level to allow the minimum burst time at maximum rate, the rate increases to the maximum rate again.

Shapers can be shared between FireBricks with a common broadcast domain.

The interface to broadcast and listen on for sharing is set via the share-interface setting in the global cqm options. Once that is configured then setting the share option on a shaper will combine its state with that of other FireBricks when calculating any damping required.