Passive instant measurement of packet loss and latency between two locations
If you want to measure a newly installed network, RFC 2544 provides a standardised procedure for commissioning and acceptance. It gives a good indication, but does not always detect problems that occur sporadically.
If multiple locations are connected by dedicated lines and network problems are reported that cannot be easily traced, then two Allegro Network Multimeters (any model) are required to carry out the passive measurement. One Allegro Network Multimeter (the primary) receives traffic information from the remote Allegro Network Multimeter over a standard SSL connection. The path measurement is a useful module in the Allegro Network Multimeter to determine global latency and packet loss between two analyzers.
The appliance web interface
As a packet-based solution, the Allegro Network Multimeter displays network traffic in tables and graphs. The representation of the network traffic is divided into Layers 2, 3, 4 and 7. The display of connections and protocols, live and from the past, are sortable and searchable. However, you can control all settings via this web interface. In the menu under "Generic" you will find the item "Path measurement". If an Allegro Network Multimeter is registered as the main device and a remote Allegro Network Multimeter under the menu item "Settings", both can be configured for path measurement. Please note our instructions in the Product Wiki.
Imagine you have two networks linked via a WLAN bridge. One Allegro analyzer is installed in front of the WLAN bridge, the other behind it. The two devices 'see' each other via the management interface. The primary device fetches the checksums of the analyzed packets from the remote Allegro Network Multimeter. The entire content is anaylzed, a CRC checksum is created and compared between the two devices. This requires around two to five percent of the link bandwidth. Let's say we're starting a large download behind the remote Allegro Network Multimeter. We can now see in live mode how the WLAN bridge behaves based on the checksums on the primary device.
The web interface provides us with information about latency. The first graph shows the latency measured from the primary device to the remote device and back. In our experiment, we see latencies of 1 to 3 seconds; a two-way latency is measured. The Allegro Network Multimeter's web interface shows the average, minimum and maximum time results in the dashboard.
Once the download is complete, we can use the Allegro Network Multimeter's many debugging options to investigate the case retrospectively. For example, the web interface of the analysis solution shows which IP address had the highest data transmission or how many packets were lost in which direction. A diagram shows all the packets that were seen on the remote device but are missing on the main device. This means that these packets were lost on the way to the primary device. For example, in the time interval of the download, 47,000 packets were transmitted, but only 46,000 arrived at the remote end.
This is a passive measurement. Nothing is simulated; real data is used. In contrast to active measurement, the network is not additionally heavily loaded in order to be able to make statements about its behaviour. With two to five percent of the link bandwidth when the checksums are transmitted, our passive measurement has very little impact on the network.
We recommend the video by Klaus Degner, Managing Director of Allegro Packets and the further instructions in our Product Wiki on the subject of path measurement.