Traffic shaping

Traffic shaping relieves bottlenecks in topologies with high-speed connections to the central site, and low-speed connections at remote sites. Committed information rate enforcement and quality of service are the major components of Nortel Networks traffic shaping. The traffic shaping parameters are defined in CCITT I.370.

The committed information rate (CIR) is the rate at which the network supports data transfer under normal operations. Its name is descriptive: you have a contract with your carrier, who has committed to providing a given throughput, here called the committed information rate. The CIR is measured in bits per second. You configure this value that the carrier provides per virtual circuit.

When configuring the CIR, consider the following:

· CIR of 0

You can contract with a carrier for a CIR of 0, which yields best-effort service at low cost. The carrier transmits data, but does not commit to providing a specified throughput. To configure a CIR of 0, set both the throughput (which is the CIR) and the committed burst (B_c) to 0, and set the excess burst (B_e) to a value greater than 0. For more information about burst rates, see the next section, "Committed burst rate and excess burst rate."

· Maximum CIR

The maximum CIR should not be greater than the speed of the access line on the slower end of a virtual circuit. In a big pipe/little pipe topology, likely CIRs at the remote sites would be 32 Kb/s, 56 Kb/s, or 64 Kb/s. If you configure CIRs for these virtual circuits at the central site, you can use CIR enforcement (described in the next topic) to prevent the big pipe from sending traffic that exceeds the PVC CIRs.

· CIR enforcement

CIR enforcement means restricting the speed of outbound traffic to a rate no faster than the CIR. It is the major component of traffic shaping. You can configure CIR enforcement to operate over Synchronous, High-Speed Serial Interface (HSSI), T1, E1, and Integrated Services Digital Network (ISDN) lines, for frame relay backup, demand, bandwidth-on-demand, and leased lines at the virtual circuit level. CIR enforcement operates on whole frames only. It controls congestion either by bringing down the virtual circuit, or by throttling the traffic.

The committed burst size (B_c) defines the number of bits that the router can transmit over a specified time interval (T_c) when congestion is occurring. The excess burst size (B_e) defines the number of extra bits that the router attempts to send over the T_c when there is no congestion. Both the B_c and the B_e are values that you configure.

The sum of the B_c and the B_e is the maximum amount of traffic that can travel across the network per T_c when there is no congestion. If you set the B_e to a value greater than zero, the router can send traffic exceeding the CIR. To enforce the CIR, that is, to limit traffic that the router can send to the amount of the CIR, set the B_e to 0.

If you enable congestion control and set the congestion method to throttle, the virtual circuit sends only B_c bits of data over the time interval T_c when congestion occurs, even if you have configured the B_e to a value greater than 0. It queues the excess data until congestion abates. If you set the congestion method to throttle-then-shutdown, the virtual circuit first queues traffic when congestion occurs, and then terminates the virtual circuit if throttling does not alleviate congestion.

Traffic shaping is best used at central offices to prevent the "big pipe" from sending too much data too quickly to remote sites with "little pipes." This concept should guide your decisions about how to configure traffic shaping.

Consider the following when you configure traffic shaping:

• In general, the value you assign to the B_c should equal 1/4 of the CIR to avoid excessive queuing and dropped packets.

If, however, you are sending frames that exceed the size of the Bc, data travels slowly because the router must use multiple time periods to accommodate the packet size and avoid exceeding the CIR. If setting the Bc to 1/4 of the CIR yields a value lower than the packet size, set the Bc to 1/3 or even 1/2 of the CIR.

For example, a typical TFTP frame is 548 bytes. If the CIR is 16,000 bits, the Bc configured according to the 1/4 guideline would be 4,000 bits, or 500 bytes, which is not big enough to accommodate a TFTP frame.

If you set the Bc to 16,000/2, or 1/2 CIR, the result is 8,000 bits, or a packet size of 1,000 bytes, which works, but may result in excessive queuing because the Tc is 1/2 second. If you set the Bc to 16,000/3, or 1/3 CIR, the result is a Bc of 5,333 bits or 666 bytes, much closer to the 548 TFTP frame size.

• If you cannot predict the typical frame size, monitor frame relay shaping statistics for numbers of large frames and dropped frames. If either of these numbers is increasing constantly or dramatically, adjust the B_c to a higher value in small increments.