Policing but Not Discarding

Shapers queue excess packets, and policers discard excess packets. However, policers allow a sort of compromise, where the packets are not discarded, but they are marked so that if congestion occurs later, this particular packet is more likely to be discarded. Consider Figure 5-17, for instance, and the policing function on R1.

Figure 5-17 Marking Down Packets with Policing

Policing, Marking Down Excess Traffic

Figure 5-17 Marking Down Packets with Policing

Policing, Marking Down Excess Traffic

In the figure, two packets travel over a route marked with dotted lines. Each is marked with DSCP AF11 as they enter R1. R1's policer decides that Packet1 conforms, but that Packet2 exceeds the policing rate. R1's policer re-marks Packet2 as AF13. DiffServ suggests that AF13 should be in the same queuing class as AF11, but with a higher drop preference than AF11. If no congestion occurs, both packets get through the network. If some congestion occurs, however, Packet2 has a higher chance of being discarded, because it has a DSCP that implies a higher preference to be dropped than does Packet1.

Policing by marking down the packet provides a compromise option. The ISP can protect the network from being overrun by aggressively discarding marked-down packets at the onset of congestion. Customer satisfaction can be improved by letting the customers take advantage of the capacity when it is available.

The same concept, on a limited basis, can be applied in Frame Relay and ATM networks. Frame Relay headers include the discard eligibility (DE) bit, and ATM headers include the cell loss priority (CLP) bit. Each of these single bits can be used to imply that the frame or cell is a better frame or cell to discard when congestion occurs.

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