90 Chapter 2: QoS Tools and Architectures
Table 2-2 |
Comparison of Classification and Marking Tools |
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Other Functions |
Fields That Can |
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Besides Class and |
Be Examined for |
Fields That Can |
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Tool |
Mark |
Classification |
Be Marked* |
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Policy-based routing |
Routing packets based on |
ACLs indirectly through |
IP ToS field |
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(PBR) |
something besides desti- |
route maps |
IP Precedence field |
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nation address |
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QoS Group |
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Committed access |
Policing |
IP ACLs |
IP Precedence |
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rate (CAR) |
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QoS Group |
IP DSCP |
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IP DSCP |
QoS Group |
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MPLS Experimental |
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Class-based marking |
None |
IP ACLs |
IP precedence |
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(CB marking) |
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Any markable fields |
DSCP |
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Input interface |
802.1P CoS |
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MAC addresses |
ISL Priority |
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All NBAR-enabled fields |
ATM CLP |
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Frame Relay DE |
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MPLS Experimental |
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QoS Group |
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Network based appli- |
Statistical information |
Extensive list (see |
None; used in con- |
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cation recognition |
about traffic mix; recog- |
Chapter 3, “Classification |
junction with CB |
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(NBAR) |
nition of applications that |
and Marking”) |
marking |
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use the dynamic port |
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VoIP dial peers |
Call routing for VoIP |
None |
IP Precedence |
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*All claims about features/functions that may be affected by IOS versions assume version 12.2, unless otherwise stated.
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.
Introduction to IOS QoS Tools 91
Figure 2-3 Simple Output Queuing, Two Queues
4 X 1500
Byte Packets
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R1 |
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25% |
Output Queue 1 |
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Bandwidth |
3 2 1 |
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Which Packet Goes Next? |
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Output Queue 2 |
R2 |
75% |
4 |
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Bandwidth |
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In the figure, four packets arrived in order, at about the same time. The queuing tool’s classification 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 support 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 classify 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.
92 Chapter 2: QoS Tools and Architectures
Table 2-3 |
Comparison of Queuing Tools |
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Maximum |
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Queue Service |
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Number of |
Classification |
Algorithm/End |
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Tool |
Queues |
Capabilities |
Result of Algorithm |
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Priority Queuing |
4 |
IP ACL* |
Strict service; always serves |
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(PQ) |
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Input interface |
higher-priority queue over lower |
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queue. |
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Fragments |
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Custom Queuing |
16 |
IP ACL* |
Serves a configured number of |
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(CQ) |
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Input interface |
bytes per queue, per round-robin |
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pass through the queues. Result: |
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Fragments |
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Rough percentage of the |
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bandwidth given to each queue |
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under load. |
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Weighted Fair |
4096 |
Automatic, based on |
Each flow uses a different queue. |
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Queuing (WFQ) |
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flows. (Flow identified |
Queues with lower volume and |
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by source/destination |
higher IP precedence get more |
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address and port |
service; high volume, low prece- |
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numbers, plus protocol |
dence flows get less service. |
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type.) |
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Class-Based |
64 |
IP ACL* |
Service algorithm not published; |
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Weighted Fair |
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NBAR |
results in set percentage band- |
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Queuing (CBWFQ) |
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width for each queue under load. |
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Same as CB marking |
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Low Latency |
N/A |
Same as CBWFQ |
LLQ is a variant of CBWFQ, |
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Queuing |
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which makes some queues |
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“priority” queues, always getting |
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served next if a packet is waiting |
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in that queue. It also polices |
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traffic. |
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IP RTP Priority |
N/A |
Even UDP ports between |
An added feature with WFQ or |
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16384 and 32767 (all |
CBWFQ, all VoIP payload is |
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VoIP payload ports) |
placed in a special “priority” |
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queue, always getting served next |
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if a packet is waiting in that |
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queue. |
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Modified Deficit |
8 |
IP precedence |
Similar to CQ, but each queue |
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Round-Robin |
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gets an exact percentage of |
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(MDRR) |
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bandwidth. Supports LLQ |
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mechanism as well. |
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*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.