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The Differentiated Services QoS Model

If you understood the Good-Old Common Sense (GOCS) model for QoS, you already understand at least half of the concepts behind DiffServ. DiffServ goes into a lot more depth, includes a large number of terms, but the core concepts of DiffServ can be summarized as follows:

•Takes advantage of the scaling properties of class-based QoS tools to differentiate between types of packets, with the goal of “scalable service differentiation in the Internet.”

•In a single network, packets should be marked at the ingress point into a network, with other devices making QoS choices based on the marked field.

•The marked field will be in the IP header, not a data-link header, because the IP header is retained throughout the network.

•Between networks, packets can be reclassified and re-marked at ingress into another network.

•To facilitate marking, the IP header has be redefined to include a 6-bit Differentiated Services Code Point (DSCP) field, which allows for 64 different classifications.

To some extent, DiffServ formally defines a QoS architecture using common sense, or “best practices,” for QoS design today. Along with the formal definitions comes a lot of terminol- ogy—terminology that is purposefully not vendor specific. So, after learning the DiffServ terms, you need to relate them to Cisco tools and terms. But DiffServ is more than just recording some good ideas about QoS—DiffServ defines another useful field in the IP header (DSCP), as well as some conventions for usage of the new DSCP field. Finally, DiffServ defines general categories of QoS functions and the purpose of the tools in each category. This book has already covered those same concepts and terms from Cisco’s perspective, so in this chapter, you will read about the DiffServ terms for categories or types of QoS tools and how they relate to Cisco’s terms.

DiffServ Specifications and Terminology

DiffServ is defined by the RFCs listed in Table 2-11.

Table 2-11 DiffServ RFCs

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Table 2-12 DiffServ Terminology and Their Definitions (Continued)

Table 2-12 contains material reprinted from RFC 2475.

DiffServ terminology overwhelms most people when first learning the architecture. Not all the DiffServ terms are even listed in the table. In fact, I wouldn’t be surprised if you are already wondering which of these terms you really need to know when using QoS and which of these terms you need to know for the Cisco QoS exams. Neither of the exams covered by this book focus on DiffServ as an end to itself. If you glance over the table, and read this section, you should become familiar enough with the terms to do well on those questions on the exams.

The rest of this section explores some examples of usage of DiffServ terminology. The first two terms are “behavior aggregate” and “per-hop behavior.” If you read the previous section about the fictitious “GOCS model” for QoS, you already know the concepts behind the terms.

Figure 2-14 shows the terms in a figure that is a duplicate of Figure 2-12.