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 classifica-tion 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 some-how 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)88 Chapter 2: QoS Tools and Architectures
Figure 2-1 Sample Network, with Queuing and Shaping Tools Enabled
Classification and marking tools simplify the classification process of the other QoS tools. Even with seemingly simple requirements, the classification functions can require many comparisons to every packet. Rather than have each tool do extensive packet matching, classification and marking tools do the extensive classification once, and mark a field in a packet header. The remaining QoS tools just need to look for this marked field, simplifying the repetitive classi-fication work.
The two most commonly used marking fields in the IP header are the IP Precedence field, and the Differentiated Services Code Point (DSCP) field. You will see the details of these two fields, along with the other fields that can be used for marking, later in this chapter. Consider Figure 2-2, where classification and marking is performed on input of R3.
The queuing and shaping features can now classify more efficiently. Queuing is still performed on R3 and R2, and shaping is still performed on R3. However, the extensive matching logic for each packet done for all incoming traffic can be performed once on R3’s FA0/0 interface, or once on one of the LAN switches, such as SW3. Once marked, the other QoS tools can react to the marked value, which each QoS tool can efficiently match in the end-to-end path through the network.
Queuing Policy (R2 and R3):
Q1: Voice Q2: Voice Signaling Q3: Web Traffic from Server 1 Q4: All Other Traffic
Queuing Figure Shows QoS for Packets Flows Right-to-Left
Bit Rate
40kbps for all traffic from Server1 20kbps for all other Data Do not shape voice (let it have the rest of the bandwidth)
Introduction to IOS QoS Tools 89
Figure 2-2 Sample Network, with Simplified Classification as a Result of Classification and Marking
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. Mark Voice with Precedence 5 Mark Voice Signaling with Precedence 3 Mark Server1 Traffic with Precedence 2 All Else Is Marked IP Precedence 0 Bit Rate Mark on Switch if Possible Figure Shows QoS for Packets Flows Right-to-Left
90 Chapter 2: QoS Tools and Architectures
Queuing
Queuing, also occasionally called “scheduling,” provides the ability to reorder packets when congestion occurs. Whereas queuing sometime occurs at the ingress interface, called “input queuing”, most queuing methods only implement output queuing. The general idea is simple, but the details can be a little overwhelming. Consider Figure 2-3, with a simple two-queue output queue system.
Table 2-2 Comparison of Classification and Marking Tools
Tool
Routing packets based on something besides recog-nition of applications that use the dynamic port
VoIP dial peers Call routing for VoIP None IP Precedence
* All claims about features/functions that may be affected by IOS versions assume version 12.2, unless otherwise stated.
Introduction to IOS QoS Tools 91
Figure 2-3 Simple Output Queuing, Two Queues
In the figure, four packets arrived in order, at about the same time. The queuing tool’s classifi-cation feature classified packets 1 through 3 as belonging in Queue 1, and packet 4 as belonging in Queue 2. The figure implies that Queue 2 should receive 75 percent of the bandwidth. But which packet is sent next? In what order do these four packets leave the router? If packet 5 shows up a little later, could it be sent before some of packets 1 through 4? Could the tool sup-port more than two queues? Well, the answers to these questions define the key comparison points between the various queuing tools. You should look for the following when comparing queuing tools:
•
Classification capabilities, particularly the packet header fields that can be matched to clas-sify a packet into a particular queue. In some cases, the queuing tool automatically classifies traffic, whereas other tools require you to configure the values to be matched in the packets explicitly.•
The maximum number of queues (sometimes called the maximum number of classes). If you need to distinguish between x different types of traffic for queuing, you need at least x queues.•
The queue service algorithm. For some queuing tools, Cisco publishes the algorithms used to decide what packet is taken from which queue next; for other tools, Cisco publishes the net effect of the algorithm. In either case, you can still make a good choice as to which tool to use.Ultimately, you use these queuing features, and other less-obvious features, when choosing the right queuing tool for a particular need in a particular network.
Queuing Tools
QoS queuing tools provide you with a variety of queuing methods. Queuing tools define a number of queues. Cisco publishes the queue service algorithm in some cases; in others, Cisco publishes only the end result (the “what”), but not the algorithm (the “how”). Table 2-3 outlines the key features of IOS queuing methods.
4 X 1500 Byte Packets
R1
Output Queue 2 R2 25%Bandwidth
75%
Bandwidth
Which Packet Goes Next?
Output Queue 1 1 2 3
4
92 Chapter 2: QoS Tools and Architectures
Table 2-3 Comparison of Queuing Tools
Tool
Strict service; always serves higher-priority queue over lower queue.
Serves a configured number of bytes per queue, per round-robin pass through the queues. Result:
Rough percentage of the bandwidth given to each queue under load.
Weighted Fair Queuing (WFQ)
4096 Automatic, based on flows. (Flow identified by source/destination address and port numbers, plus protocol type.)
Each flow uses a different queue.
Queues with lower volume and higher IP precedence get more service; high volume, low prece-dence flows get less service.
Class-Based
Service algorithm not published;
results in set percentage band-width for each queue under load.
Low Latency Queuing
N/A Same as CBWFQ LLQ is a variant of CBWFQ,
which makes some queues
“priority” queues, always getting served next if a packet is waiting in that queue. It also polices traffic.
IP RTP Priority N/A Even UDP ports between 16384 and 32767 (all VoIP payload ports)
An added feature with WFQ or CBWFQ, all VoIP payload is placed in a special “priority”
queue, always getting served next if a packet is waiting in that queue.
Modified Deficit Round-Robin (MDRR)
8 IP precedence Similar to CQ, but each queue
gets an exact percentage of bandwidth. Supports LLQ mechanism as well.
* Some queuing tools support different configuration tools that allow matching the same fields that an ACL can match. In these cases, only the IP ACL method of matching is listed in this summary table.
Introduction to IOS QoS Tools 93
Chapter 4, “Congestion Management,” covers each of the queuing tools in detail.