The ONL NPR Tutorial

NPR Tutorial >> Filters, Queues and Bandwidth

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Setting Port Rate

The rate at which IP datagrams are allowed to leave a port is controlled by the Queue Manager. This port rate can be set by selecting Configuration => Queue Table from the port menu and then entering the rate in the Port Rate dialogue box. "Port Rate" is somewhat of a misnomer since enterring a value in the box sets only the maximum outgoing rate, not the incoming rate. If you ask for a port rate of R (e.g., 300 Mbps), the outgoing traffic will have the following properties:

• The long-term output rate will be R if there is always a packet to be transmitted. The implication is that a packet of length L will suffer a queueing delay of L/R.
• If N back-to-back packets each of length L arrive to an empty queue, the first packet will appear to be transmitted out at a rate equal to the actual capacity of the physical link (over 1 Gbps), and subsequent packets will appear to be transmitted out at an ever smaller rate until it reaches R.

• If you overload the output (input rate is greater than the port rate), the link will appear to have a long-term average output rate of R.
• If you send a small burst of small packets to the output port, the output rate may appear closer to the capacity of the physical link (over 1 Gbps).

Setting the Port Rate

Here is how to set the port rate of port 1.4 to 300 Mbps:

• RLI Window:
  Select the port in the NPR icon.

  A Configuration-Monitoring menu for the port.

• Select Configuration => Queue Table.

  The Queue Table window for that port will appear showing the Port Rate entry box with a default value of 1000 Mbps (1 Gbps).

• Queue Table:
  Enter 300 in the Port Rate entry box.
• RLI Window:
  Select File => Commit.

• default: The initial capacity is 1000 Mbps.
• desired: We enter 300 into the Port Rate entry box.
• actual: After entering File => Commit, the Port Rate box shows that the port rate was set to 299.837 Mbps. The acual port rate differs from what we wanted because the port rate is set to an integer multiple of 64 Kbps.

The Port Rate is the maximum long-term IP packet output rate. The entire packet including the IP header and transport level headers is considered when implementing the traffic regulator.

The Queue Table window shown above also contains a number of other parameters related to packet scheduling that will be covered later.

Recap

• Port Rate:
  Sets the long-term output rate of a port. Because the actual value in the Port Rate entry box is an integer multiple of 64 Kbps, you may get slightly less than what you asked for.
• Traffic Regulator:
  The Queue Manager implements a traffic regulator that guarantees that the long-term average output rate will not exceed the port rate. But short bursts of small packets arriving to an empty queue will look like they are transmitted out at a rate near the capacity of the physical link (over 1 Gbps).

NPR Tutorial >> Filters, Queues and Bandwidth	TOC