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Latency and Bufferbloat

Chrisrn
Hooked
Posts: 9
Registered: 30-05-2016

Latency and Bufferbloat

 

 

So I'm seeing huge amounts of latency on my connection. Not just a few hundred ms, but  pinging my local plusnet exchange often takes 10-15 seconds! It's very easy to reproduce: any time I upload anything, my line gets clogged and becomes unusable. This makes things like online gaming etc impossible, and generally results in very poor service. My download speeds aren't too bad, I can get 3Mb/s, but I never really get more than 100kb/s upload, usually closer to 10kb/s . 

 

I have talked to plusnet support several times about this but they don't seem to understand. My line has been checked repeatedly by BT engineers and it is fine. I am fairly sure the problem is not  with the line since I see very little packet loss. 

 

This is a typical ping report against my local exhange:

 

Ping statistics for 195.166.130.216:
Packets: Sent = 48, Received = 47, Lost = 1 (2% loss),
Approximate round trip times in milli-seconds:
Minimum = 70ms, Maximum = 17820ms, Average = 1097ms

There were no lost packets (except the last one when I interrupted) but at least one packet took 17 seconds to arrive! 

 

I don't see the same delay on my internal network (e.g. pinging another computer conencted to the same router) so I don't think it is a problem with my router. 

So I am almost certain that this is a problem with plusnet's exchange.  

Perhaps I am on the receiving end of traffic shaping? 

 

After many calls to plusnet and listening to hours of awful 90s hold music they made some changes, sent me a new router and things were ok for about a month. But now the problem is back! It's frustrating because I know it can be ok. 

 

Does anyone know how to fix this? 

 

 

21 REPLIES
Community Veteran
Posts: 471
Thanks: 35
Fixes: 2
Registered: 26-09-2015

Re: Latency and Bufferbloat

Are you sure you don't have anything doing automatic uploads, iPhoto or Google Drive Sync or anything?   It's really easy to max out the upload path on ADSL.

Chrisrn
Hooked
Posts: 9
Registered: 30-05-2016

Re: Latency and Bufferbloat

Yes, I've done things like turn off wifi and connect only one machine via ethernet while monitoring traffic, so I'm pretty sure it's not that. Good idea though. 

 

 

I'll also note that I can trigger this behaviour by simply starting an upload. 

I'll see ping times go from 100ms to 10s as soon as I upload anything. 

Community Veteran
Posts: 471
Thanks: 35
Fixes: 2
Registered: 26-09-2015

Re: Latency and Bufferbloat

I'll also note that I can trigger this behaviour by simply starting an upload. 

I'll see ping times go from 100ms to 10s as soon as I upload anything. 

Do you know what rate you were uploading at compared to your line rate?  For example on our ADSL 1 connection I could trigger that behaviour with an upload rate of 393K (line synch rate is 448K).   How much latency was introduced depends on the router, the Plusnet 2704N seems particularly bad, however any sort of router will show some effect since that rate was exceeding the speed that the router can transmit - the surplus either has to be queued or dropped.

Having said that I also saw episodes of high latency triggered sometimes by low upload speeds of 90 or 100K, in those cases the issue clearly wasn't in my router but must lie elsewhere.  Plusnet never found a resolution, nor indeed expressed an opinion on those symptoms.

Chrisrn
Hooked
Posts: 9
Registered: 30-05-2016

Re: Latency and Bufferbloat

Well, from the look of it, my line speed is only 120k upsream, so I guess that's pretty easy to max out.
I'm also using the 2704n.
It's annoying that it buffers so much though, since it screws up tcp rate limiting. There doesn't seem even seem go be a QOS option exposed to limit the upstream rate on the 2704n...
Interestingly, I don't see LAN latency, even when uploading, which suggests to me that it isn't my router at all.
Do you know any tests I can perform to pin down the problem?


There was a period about a month ago where things worked fine, so I know it can work!
Community Veteran
Posts: 471
Thanks: 35
Fixes: 2
Registered: 26-09-2015

Re: Latency and Bufferbloat

I can't really think of any.   The issue of excessive queuing, the so-called buffer bloat, is unlikely to arise on LAN to LAN traffic as there's no particular bottleneck between Ethernet connected devices.   In contrast you have a host connected at 100meg on the LAN sending upstream to the Internet where the router sees only a 448k (or less) connection, and for some bizarre reason the designers of the 2704N thought it best to stick that all in a huge queue meaning that everything is delayed.  This of course makes VoIP unusable, and also affects downloads as the small upstream TCP packets get delayed meaning the sender window empties.

I haven't looked at QoS settings on the 2704N, I suspect if they have any they will be very simplistic.   A better router is the real answer, a decent router won't introduce so much delay in any case, and may have QoS policies that you can apply.

However note that this won't fix the "latency episode even when upload is not maxed out" issue that I saw from time to time.  That might be something to do with Plusnet's gateways, but as I say we never got that resolved.

Community Veteran
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Registered: 10-06-2010

Re: Latency and Bufferbloat

If the ADSL is only connecting at 120kbit/s for the upload, then something is broken.

Community Veteran
Posts: 471
Thanks: 35
Fixes: 2
Registered: 26-09-2015

Re: Latency and Bufferbloat

This speed test here gives Buffer Bloat figures for both upload and download.

http://speedtest.dslreports.com/

The download figure may be biased in your case because of Plusnet prioritising icmp, if in fact they do for Echo and not just Echo Reply.  However it's upload we're interested in.   For comparison using my Billion router that test gives an upload speed of 0.443 and "bloat" of 217ms.

Chrisrn
Hooked
Posts: 9
Registered: 30-05-2016

Re: Latency and Bufferbloat

That report shows pretty much what I expect:

3.33Mb/s up, 0.096Mb/s down and 16000ms delay under load. 

http://www.dslreports.com/speedtest/4046004

 

Thanks for all your help. It seems that I just have a terrible connection and the cheap plusnet router is simply doesn't deal well in these conditions. 

Community Veteran
Posts: 38,460
Thanks: 1,027
Fixes: 62
Registered: 15-06-2007

Re: Latency and Bufferbloat

Ignoring speed test results for the present what are the stats for your line as taken from the router (noise margin, speeds and attenuation)

Chrisrn
Hooked
Posts: 9
Registered: 30-05-2016

Re: Latency and Bufferbloat

Well, those aren't available on the 2704N, but I do have some recent numbers from plusnet:

 

  Upstream Downstream
Loop loss 17.8 30.5
SNR Margin 5.0 5.8
Errored Seconds 0 1
Cell Count 1627683 26585383
Speed 122 7286

 

Community Veteran
Posts: 38,460
Thanks: 1,027
Fixes: 62
Registered: 15-06-2007

Re: Latency and Bufferbloat

Actually they are https://www.plus.net/help/broadband/router-user-guides/#2704n-log-into-my-router

You can find all the info you need on how your line is working by going to the Helpdesk section after logging in to your router (the step we mentioned in How do I log into my router and check my settings?).

The helpdesk has all sorts of useful information and statics, they’re split up into the following sections:

  • 1-5: Router model and firmware.
  • 6-20: Line stats and connection details.
  • 21-24: wi-fi settings.
  • 25-28: Firewall and router software details.

However those stats indicate that something is badly wrong

As a first step do you have a master socket where you can remove the lower half of the faceplate and connect the router to the test socket behind it via a filter and report the stats

This explains https://community.plus.net/t5/Broadband/ADSL-Speed-faults/m-p/1270245#U1270245

Chrisrn
Hooked
Posts: 9
Registered: 30-05-2016

Re: Latency and Bufferbloat

Ah, so they are. Apologies, this is my second 2704N and the helpdesk page on the first one never displayed anything. I never checked that page on the replacement. 

 

Anyway,  the stats are: 

- Data rate: 115/7162
8 - Maximum data rate: 100/7784
9 - Noise margin: 4.5/6.0
10 - Line attenuation: 17.8/30.0
12 - Data sent/received: 99.5M/2.9G

 

 

Community Veteran
Posts: 38,460
Thanks: 1,027
Fixes: 62
Registered: 15-06-2007

Re: Latency and Bufferbloat

Is that in the test socket

Chrisrn
Hooked
Posts: 9
Registered: 30-05-2016

Re: Latency and Bufferbloat

Yes