Differentiated Services: Using QoS to Manage Internet Traffic
The Internet and the Internet Protocol (IP) were designed to provide a best-effort, fair datagram delivery service. With the tremendous increase in traffic volume, and the introduction of new real-time multimedia and multicasting applications, the traditional Internet protocols and services are woefully inadequate. The first step in fixing this problem was the introduction of the Integrated Services Architecture (ISA) and the related RSVP reservation protocol.
ISA and RSVP support quality of service (QoS) offerings on the Internet and in private internets. Although ISA in general and RSVP in particular are useful tools, their features are relatively complex to deploy. Further, they may not scale well to handle large volumes of traffic because of the amount of control signaling required to coordinate integrated QoS offerings and because of the maintenance of state information required at routers.
As the burden on the Internet grows, and as the variety of applications grow, there is an immediate need to provide differing levels of QoS to different traffic flows. The differentiated services (DS) architecture (RFC 2475) is designed to provide a simple, easy-to-implement, low-overhead tool to support a range of network services that are differentiated on the basis of performance.
Several key characteristics of DS contribute to its efficiency and ease of deployment:
IP packets are labeled for differing QoS treatment using the existing IPv4 Type of Service octet or IPv6 Traffic Class octet (see Figure 1). Thus, no change is required to IP.
Figure 1 IP headers.
A service level agreement (SLA) is established between the service provider (internet domain) and the customer prior to the use of DS. This setup avoids the need to incorporate DS mechanisms in applications. Thus, existing applications need not be modified to use DS.
DS provides a built-in aggregation mechanism. All traffic with the same DS octet is treated the same by the network service. For example, multiple voice connections are not handled individually but in the aggregate. This provides for good scaling to larger networks and traffic loads.
DS is implemented in individual routers by queuing and forwarding packets based on the DS octet. Routers deal with each packet individually and don't have to save state information on packet flows.