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Understanding multicast and QoS in IP routers

Jul 1, 2007 12:00 PM, BY CIPRIAN POPOVICIU

Quality of service

So what do today's routers do? They transfer data between hosts over the shortest path and for certain applications with minimal or no packet drops. They support telephony services where packets cannot be delayed significantly. They deliver video content — sensitive to both drops and delays — and replicate it to multiple hosts or routers. And of course, they must handle the control plane messages essential to the proper operation of the network. This is a lot, both in terms of amount of traffic and the variety of service requirements. It is tempting to throw resources at the problem, more memory, CPU and bandwidth. But aside from the inevitable cost increase, congestion is a fact of life in a connectionless, best-effort environment such as IP networks. Routers need help to use their resources smartly in handling the various traffic types.

There are two architectures that enable IP to provide quality of service. The integrated service (IntServ) architecture brings IP forwarding closer to a circut-oriented-type protocol. In this case, before the traffic is exchanged between the source and the destination, the necessary resources are reserved across the entire path. The reservation is done with the help of signaling protocols.

The alternative architecture is called differentiated service (DiffServ). In this case, all routers are preconfigured with a set of classes and the resources that should be allocated to each class.

When IP packets enter the network, they are assigned — based on the service or application they support — to one of these classes, and their headers are marked with a 6 bits pattern corresponding to each class. Routers recognize the marking and handle the packet according to the policy defined for that class.

While IntServ provides more granular control of resource allocation, DiffServ does not require state maintenance in the network, making it simpler, more scalable and more resilient.

This IP QoS brief overview represents one aspect of an overall QoS architecture, which involves, for example, optimizations of the media layer as well. Leaving the details to dedicated references (see Reference 2 on page 55), we can summarize the additional functions performed by a QoS-enabled router:

Figure 3. Conceptual operation of DiffServ
Click image to enlarge.

Classify IP packets.
Mark packets based on classification.
Apply the appropriate resources to handle the packets.
Perform signaling if necessary.

Figure 3 depicts some of these functions in the context of DiffServ, which is the most commonly deployed QoS architecture.

With an increasing portfolio of services and an increase in users' expectations with respect to these services, QoS is becoming an important component of IP networks. It is particularly important in the lower bandwidth access portion of networks.

Summary

Routers have evolved beyond mere forwarding of packets across IP networks. They discover users that share a common interest, and they optimally deliver the traffic of interest to them. Routers allocate the right amount of resources to each traffic type in order to meet its service requirements. The complexity and capability of today's routers meet the needs of a diverse set of services and applications, which have one thing in common: They run over IP.

Ciprian Popoviciu, PhD, CCIE, is a technical leader within the Networked Solutions Integration Test Engineering group at Cisco Systems. He is also a senior member of the IEEE.