Policing When and Where

Whenever the physical clock rate exceeds the traffic contract, policing may be needed. Suppose, for instance, that ISP1 has 1000 customers, just like PB Tents, each with a 100-Mbps connection, and a contract for support of 2 Mbps. What happens over time? Well, without something to prevent it, each customer will send and receive more and more traffic. For a while, all the customers are happy, because their packets make it through the overbuilt ISP1 core. Even if ISP1 has enough capacity to support 10 Mbps of traffic from every customer, eventually, ISP1's network will become overrun, because their customers keep sending more and more traffic, so eventually all traffic will suffer. Queues become congested frequently, causing dropped packets. Multimedia traffic suffers through the poor performance as a result of high delay and jitter. TCP sessions continually decrease their window sizes because of the lost packets, causing synchronization effects inside ISP1. ISP1 can add capacity, but that probably means that ISP1 should start charging more to their customers, who may not be willing to upgrade to a higher-traffic contract.

In actual ISP networks, the network engineers design the core of the network expecting some degree of oversubscription. The term "oversubscription" means that the customer has sent and received more traffic than was contracted, or subscribed. As in the example of ISP1 in the preceding paragraph, ISPs and Frame Relay providers build their network expecting some oversubscription. However, they certainly do not build the core expecting every customer to send traffic at full access rate, all the time.

Policing protects a network from being overrun by traffic. If ISP1 just policed traffic from each customer, discarding packets that exceed the traffic contract, it would protect itself from being overrun. However, the decision to add policing to a network can be politically difficult. Suppose that ISP1 has these 1000 customers, each of whom contracted for 2 Mbps of traffic. Each customer sends and receives more, averaging 10 Mbps, so that ISP1's network is becoming too congested. ISP1 chooses to implement policing, using the contracted rate, discarding packets that exceed 2 Mbps of traffic. Of course, most of their customers will be very unhappy! Such a move may be a career-ending, if not business-ending, choice.

Policers can also just mark down the traffic, instead of discarding it. To do so, the policer marks the packet with a different IP precedence or DSCP value when the traffic rate is exceeded, but it still lets the packet through. Later QoS functions, including policers and packet-drop tools such as Weighted Random Early Detection (WRED), can more aggressively discard marked-down packets as compared with those that have not been marked down. Essentially, the policer can increase the chance that a packet will get discarded somewhere else in the network if that packet causes the traffic rate to be exceeded. Generally speaking, when policers mark down packets, if the network is not currently congested, the packet can get through the network; if congested, the packet is much more likely to be discarded.

ISPs make the business choice of whether to police, and how aggressively to police. The options reduce to the following three basic options:

• Do not police. To support the traffic, build the network to support the traffic as if all customers will send and receive data at the clock rate of the access link. From a sales perspective, close deals by claiming that no policing will be done, but encourage customers who exceed their contracts to pay for more bandwidth.

• Police at the contracted rate. To support these traffic levels, the network only needs to be built to support the collective contracted rates, although the core would be overbuilt to support new customers. From a sales perspective, encourage customers that are beginning to exceed their contracts to upgrade, and give incentives.

• Police somewhere in between the contracted rate and the access-link clock rate. For instance, ISP1 might police PB Tents at 5 Mbps, when the contract reads 2 Mbps. The network can be built to support the collective policed rates. The sales team can encourage customers to buy a larger contracted rate when they consistently exceed the contracted rate, but keep customer satisfaction higher by pointing out their generosity by only policing at rates much higher than the contracted rates.

Policing can be useful in multiaccess WANs (Frame Relay and ATM networks) for the same reason that it was useful for the ISP connection described earlier. Whenever data can be sent faster than the contracted rate, the danger exists that a network will be overrun when many sites exceed their contract at the same time. An example will help you understand a few of the issues. Figure 5-2, the network diagram for PB Tents network, has been expanded to show 12 branches, with a single central site.

Figure 5-2 PB Tents Network, 12 Frame Relay Branches, 1 Central Site

Figure 5-2 PB Tents Network, 12 Frame Relay Branches, 1 Central Site

Each branch can send traffic at 128 kbps, but each branch only has a contracted 64-kbps CIR on their respective VCs to the main site. If all 12 sites conform to their CIRs, the Frame Relay network should be able to handle the load. If all 12 sites offer 128 kbps of traffic for long periods, however, the provider may still go ahead and try to forward all the traffic, because most Frame Relay providers overbuild their core networks. They also like to imply in their sales pitch that the customer gets to send excess packets for free.

Of course, at some point, if every customer of this provider sent traffic at full line rates for a period of time, the network would probably congest. The same options exist for the Frame Relay network as for an ISP—not to police but build more capacity; police to CIR, and deal with the sales and customer satisfaction issues; or police at something over CIR, and deal with the sales and customer satisfaction issues in slightly different ways.

To police the network in Figure 5-2, the Frame Relay switches can be configured to perform the policing, or the routers can be used. Traditionally, policing is performed as packets enter a network, which would suggest policing as packets enter the Frame Relay switches from the customer. If the service provider actually controls the edge routers in the enterprise network, however, the policing feature can be performed as packets exit the routers, going toward the Frame Relay cloud. If the customer controls the routers at the edge of the cloud, policing in these routers may be risky for the service provider, just because of the possibility that some customers might turn off policing to get more capacity for free.

The Cisco QoS exams cover policing in IOS routers. The exams do not cover policing in Frame Relay switches, or in LAN switches, although the basic concepts are the same.

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