Knowledge base:
Troubleshooting negative latencies
Posted by Pieter Vandercammen, Last modified by Pieter Vandercammen on 14 January 2020 10:50 AM

Latency is the time a packet takes to travel from source to destination. Hence, seeing negative latency values in the report is always a surprise, it's as if packets arrived even before they were sent! The figure below is one such an example. This article explains how this is possible and what actions can be taken to prevent it.

Intro: Measuring latency with ByteBlower

This section explains how ByteBlower performs the latency measurement. This helps to understand the cause of negative latency values.

The picture below shows two FrameBlasting flows. The one at the top is a regular flow with packets going from PORT_1 to PORT_2, the one at the bottom is a latency flow.

Contrary to the regular flow, the ByteBlower server will modify traffic for the latency flow! Part of the payload content is replaced with a realtime timestamp. This value represents the moment the packet leaves the ByteBlower, so the current local time at the source ByteBlower port.

The receiving ByteBlower port only needs to inspect the packet and compare the timestamp in the packet to its current time. The difference (local time at the destination/receiver ByteBlower port minus the timestamp value in the packet) represents how long the packet was in transit, i.e. the latency of the packet.

The advantage of this approach is that the only communication between PORT_1 and PORT_2 is through the test traffic itself, no other protocols are needed. The ports don't have to be on the same interface, same server or even in the same lab.

The above approach is used both by ByteBlower servers and Wireless Endpoints. All information is available in the traffic itself. This makes it very flexible to measure the latency between ports docked to the same server, between a server and Wireless Endpoint or between different ByteBlower servers.

Since we rely on local clocks on the ports to generate and compare the timestamps, and since the timestamps are carried in the packet payload, there are two major reasons for problems with measuring latency:

  • The sending and receiving side measure the local time differently (clocks not synchronized)
  • The packets have been corrupted

We'll have a look at both problems in more detail.

Synchronized clocks

In the section above we've explained how the transmitting side adds a timestamp to the frames The receiving end compares this value to its local time. We expect to measure a difference between the value in the frame and the time at the receiving end, this is how long the packet was under way. Hence the clocks need to be synchronized, otherwise we're just measuring the difference in clocks rather than the packet transit time. Clock differences where the receiver port's clock is trailing the sender port's clock is the major cause of negative latency values!

Within the same ByteBlower

Not a problem here, since source and destination port use the same clock!

If the setup allows you to, using a single ByteBlower server is the preferred way to measure latency!

ByteBlower to ByteBlower

As mentioned above both ByteBlowers needs to be time-synced. The article below offers more info on how to configure this:

https://support.excentis.com/index.php?/Knowledgebase/Article/View/15

Note that for latency measures, it is important to keep the clocks of the different ByteBlower servers in sync (using NTP or PTP), since otherwise they can drift away from each other. If you only sync them once, the first measurement can give good latency results, but repeating that same test a couple of months later could provide wrong results.

Wireless Endpoint to ByteBlower

Measuring latency using a Wireless Endpoint is a challenge. Contrary to regular ByteBlower traffic, here it is not a ByteBlower server port that sends or receives traffic, but it's the Wireless Endpoint (your phone e.g.). Key question: which timestamp to use?

When registering with a Meeting Point, the Wireless Endpoint tries and synchronizes its local time to the time of the Meeting Point (the latter taking the time of the ByteBlower server it is connected to). Important to note that this time synchronization is not as good as two ByteBlower servers using the same NTP server! Whereas the ByteBlower servers can use a (typically stable) management network to synchronize, the Wireless Endpoint uses the same connection as the traffic itself.

Furthermore, there is no update of time synchronization during a test, so a Wireless Endpoint moving from one AP to another AP during a test could experience a severe time drift without its clock being updated.

Bottom line: latency measures for a Wireless Endpoint are provided as a best effort service. Be careful when interpreting these results!

Packet corruption

Since the timestamps are carried in the packet payload, there is another reason why latency measurements can go wrong: corrupted packets.

Most likely to happen in case of a corrupted packet is:

  1. packet is dropped due to CRC failure
  2. CRC is valid, but timestamp was altered, packet is counted as invalid (no latency) because the calculated time offset is too large (>1 minute)
  3. CRC is valid, but timestamp was altered, latency shows unexpected peaks (multiple seconds, but less than 1 minute)

Below is an example of such a latency measurement (simulated through an impairment node).

Next to the obvious spikes in latency, you can have a look at the packet loss measurements to know whether or not the negative latency was caused by packet corruption.

To solve these problems, you will need to solve the packet corruption itself (network or device).

Another case?

If your tests matches none of the above cases,then it's a good idea to contact us at support.byteblower@excentis.com . We'll help you further from there.

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