Tuesday, August 13th, 2019 (8:24 am) - Score 18,176

Over the past few months some customers of BT’s broadband service have been contacting ISPreview.co.uk to highlight an unusual issue, which causes high ping (latency / lag) spikes while playing online video games at the same time as somebody is also streaming a video (Netflix, YouTube, Twitch, Sky on Demand etc.).

At this point we should say that there are all sorts of natural reasons why latency can spike on slower lines (e.g. trying to enjoy multiplayer when all the bandwidth is being sucked up by somebody else streaming a video is rarely fun). Nevertheless in this case the issue is also occurring on much faster connections, where the spare bandwidth isn’t being even remotely taxed by streaming (i.e. there’s ample upstream and downstream capacity).

The same issue, which seems to cause significant and sporadic lag (latency) spikes of up to 200-300ms (milliseconds) in multiplayer games (often occurring every few seconds), does NOT crop up when customers on affected connections have switched to a different ISP with the same connection type. We should add that changing router made no difference, which again points back to BT’s side of the network.

A large number of related topics about this issue can also be found on BT’s Community Forum (here, here, here, here and here among many others) and many of those who complain say they first spotted the issue near the start of 2019, although experiences do vary. Likewise it doesn’t seem to matter what video game or platform is being used (Xbox Live, Steam etc.) as the same issue will still occur when streaming.

Example Complaint (Tmillo) “I’ve had BT for years now and never had any problems. I can normally game happily at an optimal (20ms) ping and my girlfriend can happily watch Netflix/Prime on the BT Youview box. However, since early January her usage has caused my ping to rise to 120ish. This just makes playing unbearable. Now, I’d normally just agree that it’s too much strain but I have no idea why it went from being perfect to this seemingly random. I tried contacting BT, running speed checks but everything looked fine. It’s just the second she starts playing, my Xbox becomes unbearable.”

Example Complaint (hawicky) “It’s honestly ridiculous. The forum is riddled with posts DAILY of this issue, and BT is somehow completely oblivious? It’s honestly unusable. Living in a house with 4/5 other people means this issue is constant, even until as late as 2am. None of the support from BT has helped the issue one bit. They were going to send an engineer to our house, but then cancelled the engineer as they thought they could fix the problem from the cabinet. Receive a text a couple days later saying “issue is fixed” which is a complete lie, ping spikes all the way up to 250-300ms every 2 seconds. Ludicrous.”

Example Complaint (GazzaLFC) “I have been with BT for a long time and all of a sudden (last couple of months’ maybe) whenever someone else in the house uses Netflix my games lag. I don’t know why, we have not changed/introduced anything so I can only assume BT have changed something?”

The problem does not appear to be affecting everyone but it’s clearly not terribly isolated either, although in researching this we’ve struggled to find a common dominator that might help to identify the cause (outside of the general video streaming connection of course). Various configuration problems with network routing / peering, traffic management, DNS, Content Delivery Networks (CDN) / caching and so forth come to mind.

Most of those who have complained are also upset at the seeming lack of response or acknowledgement from BT on the issue. A couple of people claim that the problem went away after they threatened to cancel their service and switch to a different ISP, although this may be merely coincidental.

In fairness even we didn’t pay much attention to it until recently, when more complaints started to fall our way. Mainly because it’s hard to distinguish such gripes from the normal issues that may occur on a congested connection. However, as above, most of those complaining aren’t in this boat and have faster services.

Naturally we have been badgering BT about this since last week and they admit that it’s a complicated problem, albeit without offering much insight into the cause.

A BT Spokesperson told ISPreview.co.uk: “We’re currently investigating reports from a small number of customers who are experiencing increased latency when video streaming and gaming simultaneously using the same connection. We’re working closely with our network partners to resolve the issue as soon as possible and we apologise to those affected for any inconvenience caused.”

The good news is that BT now seem to acknowledge that “the issue” does exist but sadly there’s no indication of when it might be resolved. We’ll be keeping a close eye on this one to see if there are any changes or improvements to the situation over the next few days, weeks or months.

UPDATE 14th Aug 2019:

Some of the feedback we’ve had about this since yesterday is increasingly leading us to suspect that the culprit might, possibly, have to do with how BT caches internet traffic / content for video providers closer to their network and also prioritises different kinds of traffic. At first we didn’t think this would cause the kind of latency spikes seen but our view is starting to shift.

In a somewhat oversimplified explanation. We think that video gaming traffic (multiplayer) may be finding itself a little stuck behind video content because the two types of traffic are being given a different priority; where as other ISPs tend to put them into the same priority (e.g. best efforts normal).

The way BT’s system works might thus account for the spikes in latency but I must stress, this is only a theory based on feedback from users and somebody familiar with the technology. It may be possible to test this by tweaking the DNS but that’s a work-in-progress train of thought.