Today, peer-to-peer traffic, boosted with file-sharing applications, becomes an ever-increasing part of Internet traffic. There is no clear answer to what to do with peer-to-peer traffic from an ISP perspective. Basically, it depends on the billing model the ISP offers. Some ISPs consider peer-to-peer as just another sort of traffic, increasing the revenues if the billing model is based on the total amount of traffic. Other ISPs consider peer-to-peer traffic an unprofitable increase of the customer bandwidth requirements, especially in case of a flat-rate billing model. In the latter case, the possible options are to block or rate-limit the peer-to-peer traffic. An alternative is moving to special billing models, such as offering special tariffs for peer-to-peer traffic. In any case, it is desirable to monitor the peer-to-peer traffic. The issue in identifying the latest peer-to-peer applications is that their protocols are embedded in the payload of HTTP port 80, preventing the use of any traffic classification based on the Layer 4 port. The payload inspection for peer-to-peer recognition naturally implies the use of Cisco Network-Based Application Recognition (NBAR). As explained in Chapter 10, "NBAR," it inspects not only the Layer 4 port to classify the traffic, but also the payload, offering a new dimension of traffic monitoring. The section "Limiting Peer-to-Peer Traffic" in Chapter 10 explains monitoring and rate-limiting Gnutella peer-to-peer traffic.