Traffic Shaping Adaption

The rate at which the shaping function shapes traffic can vary over time. The adaption or adaptation process causes the shaper to recognize congestion and reduce the shaping rate temporarily, to help reduce congestion. Similarly, adaption notices when the congestion abates and returns the shaping rate to the original rate.

Two features define how adaption works. First, the shaper must somehow notice when congestion occurs, and when it does not occur. Second, the shaper must adjust its rate downward and upward as the congestion occurs and abates.

Figure 5-9 represents three different ways in which the main router can notice congestion. Three separate lines represent three separate frames sent to the main router, signifying congestion. Two of the frames are data frames with the Frame Relay backward explicit congestion notification (BECN) bit set. This bit can be set inside any Frame Relay frame header, signifying whether congestion has occurred in the direction opposite to the direction of the frame with the bit set. The third (bottom) message is a Foresight message. Stratacom, and later Cisco after they acquired Stratacom, defined Foresight as a signaling protocol in Frame Relay and ATM networks, used to signal information about the network, such as congestion information. If the Frame Relay network consists of Cisco/Stratacom WAN switches, the switch can send Foresight messages, and Cisco routers can react to those messages. Following the figure, each of the three variations for the Main router to recognize that congestion is occurring is explained in detail.

Figure 5-9 FECN, BECN, and Foresight Feedback

All VCs 64 kbps CIR

Figure 5-9 FECN, BECN, and Foresight Feedback

All VCs 64 kbps CIR

First consider the BECN frame. Backward means that the congestion exists in the opposite, or backward, direction, as compared with the direction of the frame. Therefore, if FRS1 notices congestion trying to send frames to R1 (right to left in the figure), on the next frame sent by R1 (left to right in the figure), FRS1 can mark the BECN bit. In fact, any device can set the forward explicit congestion notification (FECN) and BECN bits—however, in some networks, the Frame Relay switches do set the bits, and in some, they do not.

If the BECN bit is set, the Main router, if using adaptive shaping, reduces its shaping rate on the VC to R1. Because the congestion occurs right to left, as signaled by a BECN flowing left to right, router Main knows it can slow down and help reduce the congestion. If Main receives another frame with BECN set, Main slows down more. Eventually, Main slows down the shaping rate until it reaches a minimum rate, sometimes called the minimum information rate (MIR), and other times called the mincir.

Similarly, if Main receives a Frame from R12 with FECN set, the congestion is occurring left to right. It does not help for Main to slow down. It does help for R12 to slow down. Therefore, the Main router can "reflect" the FECN, by marking the BECN bit in the next frame it sends on the VC to R12. R12, receiving a BECN, can reduce the shaping rate.

Finally, Foresight messages are separate, nondata signaling frames. Therefore, when the congestion occurs, Foresight does not need to wait on a data frame to signal congestion. In addition, Foresight sends messages toward the device that needs to slow down. For instance, a switch notices congestion right to left on the VC between Main and R24. The switch generates and sends a Foresight message to Main, using that same VC, so Main knows it needs to slow down its shaping rate on that VC temporarily.

When configuring adaptive shaping, you configure the minimum and maximum shaping rate. The configuration commands refer to the minimum rate as mincir, and the maximum rate as CIR, with mincir defaulting to 50 percent of CIR.

With no congestion, shaping uses the maximum rate. When the shaper receives a BECN or Foresight message, it slows down by 25 percent of the maximum rate. It continues to slow down by 25 percent of the maximum rate per Tc, until the minimum rate is reached. After 16 consecutive intervals occur without a BECN or Foresight congestion message, the shaping rate grows by 1/16 of the maximum rate during each Tc, until the maximum rate is reached again.

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