IP Quality of Service

94 Chapter 4: Per-Hop Behavior: Resource Allocation I

Case Study 4-11: Packet Prioritization Based on Source Address

A large corporate branch office connects to the head office using a T1 link. One of the

subnets, 213.13.13.0/28 at the branch site, hosts critical applications and servers. The

network administrator for the field office wants to prioritize any traffic coming from this

subnet over the user traffic generated by the rest of the branch office network.

Listing 4-17 lists the configuration to enable priority queuing based on the source IP

address.

Custom Queuing

Whereas priority queuing potentially guarantees the entire bandwidth for mission-critical

data at the expense of low-priority data, custom queuing guarantees a minimum bandwidth

for each traffic classification.

This bandwidth reservation discipline services each nonempty queue sequentially in a

round-robin fashion, transmitting a configurable percentage of traffic on each queue.

Custom queuing guarantees that mission-critical data is always assigned a certain

percentage of the bandwidth, while assuring predictable throughput for other traffic.

You can think of custom queuing as CBWFQ with lots of configuration details.

You can classify traffic into 16 queues. Apart from the 16 queues is a special 0 queue, called

the system queue. The system queue handles high-priority packets, such as keepalive

packets and control packets. User traffic cannot be classified into this queue. Custom

queuing is implemented to classify IP packets into any of the 16 queues based on input

interface, simple and extended IP access lists, packet size, and application type.

A popular use of custom queuing is to guarantee a certain bandwidth to a set of places

selected by an access list. To allocate bandwidth to different queues, you must specify the

byte count for each queue.

Listing 4-17 Priority Queuing Based on Source Address

interface serial 0

priority-group 1

priority-list 1 protocol ip high list 1

priority-list 1 default low

access-list 1 permit 213.13.13.0 0.0.0.15

Custom Queuing 95

How Byte Count Is Used in Custom Queuing

In custom queuing, the router sends packets from a particular queue until the byte count is

exceeded. Even after the byte count value is exceeded, the packet currently being

transmitted is completely sent. Therefore, if you set the byte count to 100 bytes and your

protocol’s packet size is 1024 bytes, every time this queue is serviced, 1024 bytes are sent,

not 100 bytes.

Assume that one protocol has 500-byte packets, another has 300-byte packets, and a third

has 100-byte packets. If you want to split the bandwidth evenly across all three protocols,

you might choose to specify byte counts of 200, 200, and 200 for each queue. This

configuration does not result in a 33/33/33 ratio, however. When the router services the first

queue, it sends a single 500-byte packet; when it services the second queue, it sends a 300-

byte packet; and when it services the third queue, it sends two 100-byte packets. The

effective ratio is 50/30/20. Thus, setting the byte count too low can result in an unintended

bandwidth allocation.

Large byte counts produce a “jerky” distribution, however. That is, if you assign 10 KB, 10

KB, and 10 KB to three queues in the example given, each protocol is serviced promptly

when its queue is the one being serviced, but it might be a long time before the queue is

serviced again. A better solution is to specify 500-byte, 600-byte, and 500-byte counts for

the queue. This configuration results in a ratio of 31:38:31, which might be acceptable.

To service queues in a timely manner, and to ensure that the configured bandwidth

allocation is as close as possible to the required bandwidth allocation, you must determine

the byte count based on each protocol’s packet size. Otherwise, your percentages might not

match what you configure.

Case Study 4-12: Minimum Interface Bandwidth for Different

Protocols

A network administrator wants to allocate bandwidth among protocols A, B, and C as 20

percent, 60 percent, and 20 percent, respectively. It was found that on average, the packet

size for protocol A is 1086 bytes, protocol B is 291 bytes, and protocol C is 831 bytes.

To use custom queuing to achieve this functionality, it is necessary to find the right byte-

count values for each protocol queue leading to the intended bandwidth allocation.

To determine the correct byte counts, perform the following tasks:

Step 1 For each queue, divide the percentage of bandwidth you want to allocate

to the queue by the packet size, in bytes. The ratios are

20:1086, 60:291, 20:831

or

0.01842, 0.20619, 0.02407