Side_1_360

the GenSyn traffic generator processes while
DAG monitors (probes) collect the IP packet
traces.

3.2 Using Passive Measurements

and DAG Cards

Passive measurement data is collected by
observing real packets at selected measurement
points. The method captures information con-
tained in the various fields of the packet header.
As opposed to active measurements, this
approach is non-intrusive and ideally the mea-
surement process does not disturb the operation
of the network. Unlike active measurements,
passive measurements can gather detailed infor-
mation about every packet by tracing (taking a
copy of) every packet sent and received. The
obvious drawback of this method is that the col-
lection of raw packet traces from high capacity
networks creates huge data volumes.

The measurement instrumentation for QoS test-
ing of IP networks deploys dedicated PCs placed
at strategic locations that passively collect mea-
surement data. These PCs are synchronized by
GPS. The motivations for using dedicated moni-
tors with specialized interface boards to pas-
sively capture synchronized packet traces were
as follows:

• The measurements should not interfere with
  the traffic generation or the operation of the
  network being tested.


• Highly accurate timestamps are needed to
  measure unidirectional delay precisely.


• The monitor must be able to capture packet
  traces without losing any information even at
  a high load.

Optionally, each traffic generator could have run
software like tcpdumpto capture packet traces.
However, this approach has several limitations
including:

• The generation of timestamps in software is
  inaccurate;


• The measurements would impact the traffic
  generation;
  
  
  • The packet capturing software (tcpdump) has
    been observed to lose information when the
    load is high.
  
  
  
  

Each monitor PC has two DAG3.2E Fast Ether-

net interface boards [Dag] specialized for captur-

ing packet traces. The DAG cards generate a 64

byte record for each packet received. The record

contains Ethernet, IP and transport layer header

information together with a timestamp as illus-

trated in Figure 16.

Packets are tapped from a Fast Ethernet inter-

faces to a DAG interface board using an Ether-

net switch, as shown in Figure 17. The clocks

of the DAG interface boards have a very high

clock precision and are synchronized by GPS

receivers. Hence, synchronisation of the time-

stamp clocks in the microsecond range is

achieved.

Figure 16 DAG data format

over 10/100 Mb/s Ethernet

Figure 17 Configuration for

attaching traffic generating

and monitoring PCs

8 byte

timestamp

14 byte

Ethernet header

20 byte

IP-header

20 byte

Transport

protocol

6 byte header

SRC

6 byte

DST

2

byte

Prot

Monitoring

PC

DAG0

Switch

DAG1

Convert

GenSyn PC GenSyn PC

GPS antenna

SPAN port

Copy of

packet