FAQ: Test Traffic Measurements
- What is the purpose of the Test Traffic Measurements service (TTM)?
TTM's main purpose is to provide standardized metrics for one-way delay and one-way packet loss between measurement devices in a format, which is easily understood by users. One-way measurements are vital, since more and more of Internet routing is becoming asymetrical, i.e. packets use different paths between two nodes depending on the direction of traffic. The TTM project complies with standards that are maintained and developed by the IETF's IP Performance Metrics Working Group (RFC's 2330, 2678 through 2681).
- What are TTM Test Boxes?
Test Boxes are measurement devices with GPS (Global Positioning System) antennas connected to them for synchronization and increased time accuracy (approx. 100 ns). They generate test-traffic, receive test-traffic and perform trace routes to determine path vectors between boxes. The traffic generated resembles "real" traffic as much as possible to avoid preferential treatment by network devices in the path between two measurement points.
- How does TTM work?
One-way measurements imply that information for calculating metrics and plotting data will scattered over any two measurement devices. Consequently, central servers at the RIPE NCC must regularly collect the information, make calculations and generate the plots in order to present the "Big Picture". They also manage the TTM operations by sending control messages to individual Test Boxes and acting as a central repository for measurement software.
- What is IP Packet Delay Variation (IPDV), also called "jitter"?
IPDV measures the variation in delay of uni-directional, consecutive packets (packet 1 and 2, 2 and 3 etc.) which flow between between two hosts over an IP path. Low IPDV is especially important for applications requiring timely delivery of packets, e.g. multimedia applications, VoIP, video etc. The maximum delay variation is useful for determining the optimal buffer sizes for such applications.
See URL for more information:
- What are "valid packets"?
Valid packets are packets that have good timestamps and can therefore be used for TTM statistics. Bad packets are usally sent during unreliable GPS conditions, meaning that their timestamps are inaccurate. Bad packets are discarded when calculating TTM metrics.
- Why is it important plotting the 2.5th and 97.5th percentiles for delay measurements?
The 2.5th and 97.5th percentiles represent the one-way delay between two Test Boxes in a best-case and worst-case scenario respectively, just as the median (50th percentile) would indicate a normal value between two measurement points. In many cases, the so-called standard deviation can be used to describe this as well. However, for some of the delay distributions, it can be argued that the value cannot be properly calculated. Percentiles do not suffer from this problem
- What does "RMS" mean?
Physical scientists often use the term "root-mean-square" as a synonym for "standard deviation" when referring to the square root of the mean squared deviation of a signal from a given baseline. In our case, high values could be caused either by frequent path vector changes, unstable/congested links or a combination of both. Low values would naturally imply the opposite. Of course, there may be other interpretations too.
It can be argued that the RMS or standard deviation should not be used to describe the data considering the nature of the delay distributions.
- How much traffic do Test Boxes generate?
By default, approximately 300 kB per day to every destination Test Box. The defaults can be changed on series C/D.
Defaults per Destination Test Box:
Test-Traffic Packet Payload Size: 100 bytes
Test-Traffic Packet Tx Rate: 2 packets/min
- We received and installed our Test Box, but why does it not give a login prompt?
It occasionally happens that a connector came lose during transport. Please, open the cover of the Test Box ( 4 screws) and press all connectors firmly into their sockets, in particular the flat ribbon cables for the disks and the cards on the motherboard. Also make sure that the disk trays are in the locked position.
If the problem persists, then swap the disks and try again. Please contact us if still unresolved.
- How can the IP address of a Test Box be changed?
Please send us the new IP address, netmask & default router at least two working days before the renumbering should be effective.
TT-ops will notify you when the change has been configured. When your new addressing schema is ready, press the reset button to reboot the machine. After rebooting, ping the new IP address to make sure that the box is there. Data-taking will restart automatically.
What kind of access do Test Boxes need to and from the outside world?
We require the following (see also RIPE 179).
- Outgoing mail (SMTP)
- Incoming SSH (from 126.96.36.199/23)
- Incoming/outgoing NTP (port 123)
- Incoming/outgoing DNS (local resolver)
- Incoming/outgoing TestTraffic (ports 1024 and higher).
- We changed our filter rules, what should we check?
Telnet to the box and check if you still see lines starting with RCDP there. Then contact TT-Ops and inform them about the change.
The Test Box operators at the NCC should check:
- RCDP data files continue to collect data
- Other boxes continue to receive data from this box
- Operational mails still arrive at email@example.com
- The local named can resolve DNS queries (try "host tt01 127.0.0.1" and "host tt01 188.8.131.52")
- What are "monitoring ranges"?
Customers can specify ranges of IP addresses which may access the Test Box. In other words, "monitoring ranges" can be defined as a kind of "permit access-list" for allowing source hosts access to the measurement device. TTM operators are naturally included in the monitoring ranges. Please beware that hosts not within these ranges will have access denied.
- Why do I receive alarm messages?
If the the delay between two Test Boxes changes significantly, then an email will be sent to the operators of those boxes.
The alarm uses a "long-term average, short-term average" (LTA-STA) algorithm, which compares certain percentiles and medians for a 30-day interval with dito for a 30-minute interval. More specifically, if the 5th percentile for the shorter interval exceeds the 95th percentile for the longer one, i.e. 95% of recent observations are higher (or worse) than 95% of values registered over the past 30 days, then an alarm will be sent to the specified operators every 30 minutes until the condition disappears. The LTA-STA algorithm will adjust to daily descepancies in traffic patterns by deviding the day into four 6-hour-intervals (0.00-6.00, 6.00-12.00 etc) and then comparing them separately to avoid false alarms.
Click here for more information about Test Box alarms.
- We received an invoice but something is wrong.
For technical questions about invoices, such as:
We need more information before we can pay,
The invoice was sent to the wrong person,
There is a mistake in adding up the various amounts,
Can you send us a new copy of the invoice,
please contact us by email at firstname.lastname@example.org.
If you believe that we made a mistake with the contents of the invoice (for example: we charged you for equipment that was not delivered), please write to email@example.com.
In all cases, please mention the number of your test-box and the number of the invoice.
- Why did you include VAT on our invoice?
Even though most customers in the EU are do not have to pay VAT, local regulations force us to charge VAT unless they have provided us with a valid EU VAT number. To avoid being charged VAT at all, please mail us your EU VAT number.
If your organization is exempt from VAT and your invoice mentions VAT, then send us your VAT number and ask for a new invoice.
In all cases, please mention both the number of your test-box and the invoice number.