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2 Measures of Interest

Network services offered have various traffic

characteristics and QoS requirements. Real-time

services (e.g. voice) have strict delay and loss

requirements while non real-time services (e.g.

file transfer) have other performance require-

ments (e.g. high throughput). Thus, a full service

IP network carries services with a wide range of

performance requirements.

A set of performance parameters is required to

characterize the network performance experi-

enced at each protocol layer, as illustrated in

Figure 2.1.

Network-levelmeasurements focus on the basic

components of a network domain, namely nodes

and links, and the packets traversing this com-

munication infrastructure. Network-level mea-

surements is a common building block from

which the performance of every service carried

in an IP network depends. The fundamental per-

formance parameters for any resource sharing

system are delay, throughput, loss and resource

utilization.

At the transportlevel the performance of the

transport protocols is addressed. The dominating

transport protocols in IP networks are currently

TCP and UDP. Examples of transport level met-

rics for TCP are number of retransmissions, the

time to set up and close a TCP connection, aver-

age segment size and throughput. The relations

between the network and transport level perfor-

mance metrics depend on the behavior of the

actual transport protocol.

At the applicationlevel, the performance as

observed by various applications is studied.

Examples of applications used in the Internet are

WWW, FTP, TELNET, VoIP, video and audio

streaming. Application level performance met-

rics for file transfers are for instance the time to

establish a connection, mean file size and mean

download time.

At the userlevel the human perception of a

given service or system is important. Obviously,

human perception depends on a wide range of

factors that are difficult to measure. Ultimately,

the human perception of a given service deter-

mines the success and acceptance of the service.

This perception is again closely associated with

expectation and costs.

The performance parameters for the transport

and application layer are determined by the char-

acteristics of actual protocols and services, e.g.

for VoIP the performance parameters include the

time to set up a call and the call blocking proba-

bility.

The performance measured at a higher protocol

layer depends on the performance of lower lay-

ers 2). However, the relationship between perfor-

mance measures at different protocol layers is

not straightforward. Apparently, the mapping

from network level measurements of delay, loss

and throughput to human perception is very dif-

ficult and depends on many aspects.

This article addresses traffic measurements.

However, note that dependability measures can

in some cases be derived from performance mea-

sures, e.g. the service availability can be defined

as the state where the end-to-end delay is less

than a specified limit, and the measurement

accumulates the fraction of the time the system

is in this state.

Obviously, measuring the performance of vari-

ous protocol layers is vital for IP traffic engi-

neering. The focus of this article is on network-

level performance measurements from the per-

spective of a network operator. However, the

remainder of the article is also applicable to per-

formance measurements at the transport protocol

and application layers.

Figure 2.1 Various levels of
performance

2)Performance can also be measured at lower protocol layers than the IP layer, e.g. MAC and

physical layer.

IPv6

User perception of performance

Application level performance

VoIP FTP VoD Web • • • SMTP

• call blocking prob


• time to set up call
  .....

Transport level performance

TCP • UDP

• throughput


• number of retransmission


• round-trip time
  .....
  
  
  • •
    
    
    • throughput
    
    
    • loss
    
    
    • delay
      .....
    
    
    
    
  
  
  
  

Network-level performance

IPv4

• delay


• loss


• throughput


• utilization
  .....
  
  
  • delay
  
  
  • loss
  
  
  • throughput
  
  
  • utilization
    .....