86 Chapter 2: QoS Tools and Architectures

Foundation Topics

This chapter introduces the long list of QoS tools and two important QoS architectures— differentiated services (DiffServ) and integrated services (IntServ). You can think of QoS implementation as building a house. To properly build a house, you certainly need (and use) a large variety of tools. An experienced builder might be able to use the tools to build a house without an architectural plan. With the architectural plans, however, an experienced house builder can build a better house! Similarly, an experienced network engineer can use QoS tools with good results, without considering the DiffServ and IntServ architectures. By considering these architectures, however, a network engineer can use the tools to implement the QoS policies more consistently and efficiently, and thereby guarantee better QoS.

This chapter begins with an introduction to the various QoS tools in IOS. Then a section titled “The Good-Old Common Sense QoS Model” identifies where you might use a tool in a network, without considering the two formal QoS models. The chapter closes with one section on each of the two formal specifications for QoS architectures and conventions, namely the DiffServ and IntServ architectures.

Introduction to IOS QoS Tools

Ultimately, this book will help you pass one or two QoS exams (the CCIP QoS exam and the Cisco Channel Partner DQOS exam). Both exams cover a wide variety of types of QoS tools. This chapter lists the tools covered on the DQOS exam; any tools only covered on the QOS exam are listed in Appendix B, “Topics on the CCIP QoS Exam.”

Of particular note, neither exam covers the implementation details of these tools on LAN switches, so the configurations and features of QoS tools on LAN switches is not listed in this section. Look to Chapter 10, “LAN QoS,” for further details about how these QoS tools operate in the LAN. Also make sure to check www.cisco.com, and www.ciscopress.com/1587200589, for the latest information about any changes to the exams.

The coverage here begins with an explanation of classification and marking tools, followed by queuing tools, shaping and policing tools, congestion-avoidance tools, link-efficiency tools, call admission control (CAC), and QoS management tools.

Classification and Marking

Almost every QoS tool uses classification to some degree. To put one packet into a different queue than another packet, the IOS must somehow differentiate between the two packets. To perform header compression on RTP packets, but not on other packets, the IOS must determine which packets have Real Time Protocol (RTP) headers. To shape data traffic going into a Frame Relay network, so that the voice traffic gets enough bandwidth, the IOS must differentiate

Introduction to IOS QoS Tools 87

between Voice over IP (VoIP) and data packets. If an IOS QoS feature needs to treat two packets differently, you must use classification.

Classification involves differentiating one packet from another, typically by examining fields inside the headers. After classification, a QoS tool can treat packets in one class differently than others. To just give all VoIP traffic preference over all other traffic, the queuing tool would need to classify traffic into one of two categories: VoIP or not-VoIP.

Because most QoS tools need to differentiate between packets, most QoS tools have classification features. In fact, you may already know something about several of the QoS tools described in this book. You may realize that you already know how to perform classification using some of those tools. For instance, many QoS tools enable you to classify using access-control lists (ACLs)—for instance, if ACL 101 “permits” a packet, the packet falls into one queue; if ACL 102 permits a packet, it is placed in a second queue; and so on. In one way of thinking, queuing could instead be called “classification and queuing,” because the queuing feature must somehow decide which packets end up in which queue. Similarly, traffic shaping could be called “classification and traffic shaping,” policing could be called “classification and policing,” and so on. Because most QoS tools classify traffic, however, the names of most QoS tools never evolved to mention the classification function of the tool.

Only one category of QoS tool, called classification and marking, highlights the classification feature in the name of the tool. For other tools, the classification function is just part of the story; with classification and marking tools, classification is the whole point of the tool. To appreciate the need for classification and marking tools, consider Figure 2-1.

The figure shows the QoS policies for traffic flowing right to left. R3 performs queuing and shaping, and R2 performs queuing only. However, for both sets of queues, and for the shaping function, classification must occur. The classification part of the effort seems to be a simple task, but it may cause many comparisons to be made. For instance, each packet exiting R3’s S0 and R2’s S0 interfaces might be compared for the following:

•From source address 10.1.1.1, TCP source port 80 (Server1 web traffic)

•Using User Datagram Protocol (UDP), port number range 16384 to 32767 (voice pay- load)—may also want to check IP address ranges to match IP Phones’ voice subnets, or voice gateway IP addresses

•Using TCP port 1720 (H.323 voice signaling)

•Using TCP port range 11000 to 11999 (Voice signaling)

•Using TCP port 1719 (Voice signaling)

•Using TCP port 2000 to 2002 (Skinny voice signaling)

•Using UDP port 2427 and 2428 (MGCP voice signaling)

Introduction to IOS QoS Tools 89

Figure 2-2 Sample Network, with Simplified Classification as a Result of Classification and Marking

Figure Shows QoS for Packets Flows Right-to-Left

Server 1

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Classification and Marking Tools

A variety of classification and marking tools exist. Classification and marking tools first classify by looking at something inside each packet; you can compare these tools by listing the fields the tool can examine. Classification and marking tools mark the frame or packet based on the earlier comparisons; you can compare these tools by listing the fields that can be marked. Some classification and marking tools also perform other functions, as noted in Table 2-2.

Chapter 3 explains the details of each of the tools, all the marked fields, and the configuration of each tool.