cancel
Showing results for 
Search instead for 
Did you mean: 

DNS Resolution Slow

SuperZoom
Grafter
Posts: 353
Registered: 17-05-2013

DNS Resolution Slow

Having moved from Be*, I am finding DNS resolution times to be a lot slower on PlusNet.
Specifically, using the ISP's own DNS servers (as assigned to the router) takes over 80% longer on average, and using OpenDNS servers takes over 90% longer on average.
It seems to be something to do with the way the network is set up rather than the servers themselves. As far as I can tell there is nothing wrong with PlusNet's DNS servers and the same kind of delay is present when using public servers eg. Google or OpenDNS.
All my hardware has remained the same (including the router,) so the only change is PlusNet network vs Be* network.
Can anyone explain this?
Could it be the traffic management system? How is DNS traffic prioritised?
Or interleaving on the ADSL upstream? (Be* doesn't error-correct the upstream).
I'm at a loss and tech support just keep saying there is no problem with the servers (I agree: there isn't). It does have a significant impact on web browsing speeds on older hardware, however, so I'd like to get it improved if possible.
Thanks for any insights or suggestions.
49 REPLIES
Community Veteran
Posts: 4,969
Thanks: 362
Fixes: 16
Registered: 10-06-2010

Re: DNS Resolution Slow

You can check traffic prioritisation in Wireshark if you like, but as expected the IP layer of the packets had 0xa0 (a high priority):
Differentiated Services Field: 0xa0 (DSCP 0x28: Class Selector 5; ECN: 0x00: Not-ECT (Not ECN-Capable Transport))
I find the Plusnet DNS servers can typically deliver a "fast" response in about 20ms. There is some general investigation of the DNS servers in this thread.
Could it be that your latency in general has increased (almost doubled?), and not just DNS?
SuperZoom
Grafter
Posts: 353
Registered: 17-05-2013

Re: DNS Resolution Slow

Yup:
Differentiated Services Field: 0xa0 (DSCP 0x28: Class Selector 5; ECN: 0x00)
Hmm. You may be right about latency in general. I don't have detailed records (not a gamer) but ping times to www.bbc.co.uk have gone from around 10ms to around 30ms and pings to servers hosted in Germany have gone from around 30ms to around 40ms. Still 35ms to the nearest speedtest.net server.
So perhaps DNS is doing better than the average (although, of course, ping is ICMP not UDP).
Would interleaving on the U/S account for the difference? (Will soon find out when move to FTTP, I suppose.)
Extra hops?
Latency introduced by passing through traffic management even though the traffic is assigned a high priority?
Is there any hope?...  Cry
Community Gaffer
Community Gaffer
Posts: 5,004
Thanks: 268
Fixes: 4
Registered: 04-04-2007

Re: DNS Resolution Slow

Interleaving adds to your latency immediately by a small amount (variable depending on interleaving depth)
Kelly Dorset
Broadband Service Manager
Community Veteran
Posts: 4,969
Thanks: 362
Fixes: 16
Registered: 10-06-2010

Re: DNS Resolution Slow

I don't know much about Be's backhaul network to say if the difference between that and BT's infrastructure could account for all the extra latency. My connection has interleaving on, both up and down, but apparently living within commuting distance of London, where all the Plusnet gateways and DNS servers are, makes things a bit quicker. It might depend on where you live, if Be had things more distributed around the country, or just if the Be infrastructure connecting their equipment in the telephone exchange to elsewhere introduced less latency.
35ms sounds quite typical for an interleaved connection. You can ask for interleaving to be switched off, but this would reduce the stability of your connection, reducing the bandwidth, so switching interleaving off isn't a good idea on longer lines, and also not during the initial 10 day line training period.
SuperZoom
Grafter
Posts: 353
Registered: 17-05-2013

Re: DNS Resolution Slow

Thanks for the replies.
@Kelly
I don't know what the depth being used on my line is - presumably that is determined by DLM?
The D/S was certainly interleaved when the connection was with Be* (I didn't have fastpath on). Again, I don't know the depth setting, though.
So only the upstream is different.
As I say, FTTP will get rid of that factor altogether, up and down, so maybe I just have to wait a bit and see.
The upshot at the moment, though, is that web browsing is noticeably slower for me on this machine on PlusNet than on Be*, even though the sync speed is slightly higher. That's disappointing.
@ejs
Yeah, I think there are slightly more hops to the PlusNet servers, but the extra delay is very similar for public DNS servers outside the network too. It's sensible to leave interleaving on as the line isn't that great historically. In fact, the reason I have a faster sync speed now is that I had the target noise margin set to 9db on Be* to make sure it never lost sync, whereas the line training on 21CN starts out at 6db. But are PlusNet able to turn off interleaving on the upstream only, in fact, or is it all or nothing?
Community Gaffer
Community Gaffer
Posts: 5,004
Thanks: 268
Fixes: 4
Registered: 04-04-2007

Re: DNS Resolution Slow

Quote from: SuperZoom
The upshot at the moment, though, is that web browsing is noticeably slower for me on this machine on PlusNet than on Be*, even though the sync speed is slightly higher. That's disappointing.

What difference are you seeing between the latencies?  You really shouldn't be able to noticed 10-15ms difference in web browsing.  That suggests to me you are seeing a different issue.
Kelly Dorset
Broadband Service Manager
Community Veteran
Posts: 1,606
Thanks: 20
Registered: 29-06-2010

Re: DNS Resolution Slow

Is your upstream capped at just under 448kb? If you are on ADSL2+ (G.992.5) and you have a capped upstream, requesting it to be uncapped should show an improvement.
SuperZoom
Grafter
Posts: 353
Registered: 17-05-2013

Re: DNS Resolution Slow

@Kelly
Well, I think the reason it is so noticeable is that this particular machine is rather old. You are right that (for example) on my Galaxy Note II everything feels very snappy.
But the reason I looked into it was precisely because from immediately after the transition to PlusNet, the Telegraph website was a lot slower (and still is). Generally, browsing feels more sluggish than before when it shouldn't, because nothing else (including the router) has changed (apart from the relevant PlusNet connection settings).
So, cached name resolution using Be*'s servers a couple of days before the switch was 18.66ms as an average of 20 requests to each of the two servers assigned to the connection by DHCP, whilst it is now 34.03ms on the same basis using PlusNet's assigned servers. The figures for the fastest tested OpenDNS server on the same basis are 18.12ms vs 35.55ms.
I only know this because I thought it might be useful to take some performance stats before switching over. They don't matter as such. What matters is that the difference is noticeable: and I know that because I noticed it! Otherwise, I would just have thought "This is fine, jolly good" and carried on...
@Jaggies
No, it's uncapped now. One of the first things I did was request that. It hasn't made a difference to the DNS RTT. Interestingly, the U/S sync speed is still around 200Kbps lower than it was on Be* though.
Community Veteran
Posts: 4,969
Thanks: 362
Fixes: 16
Registered: 10-06-2010

Re: DNS Resolution Slow

The Telegraph website, like many other big websites, is hosted on Akamai. When you resolve its IP address using Plusnet's DNS servers, you get an IP address specially for Plusnet, with a very short traceroute. This is supposed to make it faster, but I'm not sure it actually is. If you use Google DNS or OpenDNS it would resolve to a different IP address, with a longer traceroute.
Community Gaffer
Community Gaffer
Posts: 5,004
Thanks: 268
Fixes: 4
Registered: 04-04-2007

Re: DNS Resolution Slow

Have you used a web browser plugin like firebug to time the page load times?  Might not be useful now you've moved though :/
Kelly Dorset
Broadband Service Manager
Community Veteran
Posts: 1,829
Thanks: 93
Fixes: 5
Registered: 21-01-2013

Re: DNS Resolution Slow

I moved from O2 to plusnet fibre a few months back.
I agree Plusnets DNS resolvers are slower than O2's for cached lookups, in fact I find them slower than most other public DNS resolvers.
Uncached lookups are ok here using PN's DNS.
Disagree about public DNS speeds, I find them slightly faster here but then my latency is about 10ms faster here too.
SuperZoom
Grafter
Posts: 353
Registered: 17-05-2013

Re: DNS Resolution Slow

Quote from: Kelly
Have you used a web browser plugin like firebug to time the page load times?   Might not be useful now you've moved though :/


Well, using cURL just now, I get this:
URL Fetched:            http://www.bbc.co.uk/news/
Name resolved after:    0.070 Secs
Connected after:        0.100 Secs
Transfer started after: 0.150 Secs
Total transfer size:    118676 Bytes
Total time taken:      4.016 Secs
Perceived Speed:        29550.000 Bytes/Sec

URL Fetched:            http://www.telegraph.co.uk
Name resolved after:    0.070 Secs
Connected after:        0.100 Secs
Transfer started after: 0.140 Secs
Total transfer size:    202659 Bytes
Total time taken:      5.107 Secs
Perceived Speed:        39682.000 Bytes/Sec

But I'm not sure what that tells us that's new, other than that Ofcom might want to know why a 6Mbps connection is only perceived as being around 2-300Kbps!!!  Wink
I don't want to sound prickly, but the implication is that if I see an elephant in my garden then I can only be sure that it isn't a giraffe if I have professional verification of its head height... And once the plants are trampled, a DNA test will be the only sure way to tell whether they were petunias or gladioli...

@npr
I think cached lookups are the closest to removing other factors. What figures were and are you getting, out of interest?
SuperZoom
Grafter
Posts: 353
Registered: 17-05-2013

Re: DNS Resolution Slow

Quote from: ejs
The Telegraph website, like many other big websites, is hosted on Akamai. When you resolve its IP address using Plusnet's DNS servers, you get an IP address specially for Plusnet, with a very short traceroute. This is supposed to make it faster, but I'm not sure it actually is. If you use Google DNS or OpenDNS it would resolve to a different IP address, with a longer traceroute.

Yes, that's correct.
The Telegraph was only an example, really. And, in fact, with both ISPs I have used their own DNS servers for browsing, so the route to www.telegraph.co.uk probably was optimal for Be* as well. The inclusion of the public servers was to show that the issue is not with the PlusNet name servers themselves (an assertion emphasised by tech support to the exclusion of all else) but with the network in some way (whether because of traffic management or otherwise,) because name resolution times are longer by a similar factor whatever server is used.
Community Veteran
Posts: 4,969
Thanks: 362
Fixes: 16
Registered: 10-06-2010

Re: DNS Resolution Slow

The 4 seconds to transfer 118676 bytes and 5 seconds to transfer 202659 bytes seems a bit odd. I was thinking perhaps an MTU/RWIN issue, but then again maybe not, because the transfer sizes were so small. I'm also not sure how you got those numbers from cURL.