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Chapter 9 - Quality of Service on the Internet

Although the Internet has been traditionally a best-effort network, the ability to provide a certain level of QoS for certain packet flows is essential for some applications. For example, while the user of a file transfer application may accept a longer transfer delay when the network is congested, a multimedia user may find trying to maintain a conversation with a long round-trip delay irritating. Such users would probably request a higher QoS for their multimedia flows than for the rest of their flows. However, QoS is not only about requesting a better treatment for certain flows; users also want. | Chapter 9 Quality of Service on the Internet Although the Internet has been traditionally a best-effort network the ability to provide a certain level of QoS for certain packet flows is essential for some applications. For example while the user of a file transfer application may accept a longer transfer delay when the network is congested a multimedia user may find trying to maintain a conversation with a long round-trip delay irritating. Such users would probably request a higher QoS for their multimedia flows than for the rest of their flows. However QoS is not only about requesting a better treatment for certain flows users also want to know if the network will be able to provide them with the requested QoS. If there is a long delay or a high packet loss rate some users may prefer to exchange instant messages instead of having a VoIP Voice over IP conversation. There are two models that provide QoS on the Internet the Integrated Services model and the Differentiated Services DiffServ model. We cover the former in Section 9.1 and the latter in Section 9.2. 9.1 Integrated Services The Integrated Services architecture specified in RFC 1633 89 was designed to provide end-to-end QoS. Endpoints request a certain level of QoS for their packet flows and if the network grants it their routers treat those flows accordingly. There are two different services available in this architecture the controlled load service and the guaranteed service. The controlled load service ensures that packets are treated as if the network was under moderate load. Flows using this service are not affected by network congestion when this appears. Nevertheless the network does not guarantee a certain bandwidth or a certain delay. This service can be seen as a better-than-best-effort service. The guaranteed service guarantees a certain bandwidth or a certain delay threshold. In practice it is not common to see this service in use because the controlled load service is often good enough and is .