Specifications

CHAPTER 3. SYSTEM ARCHITECTURE 13
This measurement system is currently used (with up to 32 Gigabit Ethernet ports) to
characterize switches and LANs for the ATLAS Data Collection. The parameters that can
be measured are throughput, one-way latency and packet loss.
3.2 Testing Wide Area Networks
Initially the tester was designed to run on a Lo c al Area Network (LAN) with all the agents
placed nearby and being interconnected by switches, but then we had to extend it to a
Wide Area Network (Internet). In a LAN setup we only have to deal with Ethernet traffic,
but for the Internet we have the IP layer and some other issues to consider.
The basic requirements for a network performance measuring system that is able to char-
acterize connections over the Internet are:
• to compute throughput and packet loss, taking into account the fact that packets may
be re-ordered
• to compute one-way latency
Network Under Test
Packet loss
One−way latency
Throughput
Histograms
Agent Agent
Manager
Figure 3.2: The network tester in a WAN environment
The one-way latency is important because the path through the Internet between two end-
points may go through different routers for the two directions. The best approach would
be to compute this value for each and every packet being sent. The exact value of the jitter
on a particular connection for any time interval and traffic pattern can thus be obtained.
To adapt our LAN-geared network performance measuring system to an Internet environ-
ment we had to add the following improvements:
• Generate and receive IP packets
• Take into account arrival of out-of-order packets in the packet loss calculation
• Develop a completely new global c lock system able to synchronize traffic generators
located at distances of hundreds of kilometers (for one-way latency computation)