why can ping have an unstable response time?

Question

I have a issue with my internet connection and I want to know if the ping response times can be affected by the use of internet. Let me explain, when I turn on my computer and no other device is using the internet connection, and I do nothing more than ping www.google.com y have the next output

$ ping www.google.com 
PING www.google.com (173.194.42.113): 48 data bytes
56 bytes from 173.194.42.113: icmp_seq=0 ttl=49 time=76.820 ms
56 bytes from 173.194.42.113: icmp_seq=1 ttl=49 time=73.923 ms
56 bytes from 173.194.42.113: icmp_seq=2 ttl=49 time=87.032 ms
56 bytes from 173.194.42.113: icmp_seq=3 ttl=49 time=75.011 ms
56 bytes from 173.194.42.113: icmp_seq=4 ttl=49 time=122.664 ms
56 bytes from 173.194.42.113: icmp_seq=5 ttl=49 time=76.388 ms
56 bytes from 173.194.42.113: icmp_seq=6 ttl=49 time=79.530 ms
56 bytes from 173.194.42.113: icmp_seq=7 ttl=49 time=78.308 ms

and so...

But when I start using the browser or the mail client or some other thing, lets see.. opening youtube in the browser the ping response times changes abruptly

56 bytes from 173.194.42.113: icmp_seq=85 ttl=49 time=214.825 ms
56 bytes from 173.194.42.113: icmp_seq=86 ttl=49 time=932.497 ms
56 bytes from 173.194.42.113: icmp_seq=87 ttl=49 time=596.405 ms
56 bytes from 173.194.42.113: icmp_seq=88 ttl=49 time=299.450 ms
56 bytes from 173.194.42.113: icmp_seq=89 ttl=49 time=1628.341 ms
56 bytes from 173.194.42.113: icmp_seq=90 ttl=49 time=1440.310 ms
56 bytes from 173.194.42.113: icmp_seq=91 ttl=49 time=1219.659 ms
56 bytes from 173.194.42.113: icmp_seq=92 ttl=49 time=762.968 ms

And In general the internet experience becomes "slower" for any other activity apart from opening youtube, when youtube is full loaded, the ping times become normal again (under 100 ms)

This is just with my computer, just with my browser just using two tabs, youtube and some other random web page just to test, obviously when other host connect to the router and try to reach some page in internet the results are worst, I have reached 4000 ms of ping when a youtube video is playing.

So my question... It is normal for ping response times (to some internet site) to change in that way when internet connection is used ?

Answer 1

This is usually related to buffering. The delay through a buffer depends on how full that buffer is.

With a simple buffer when there is little traffic the buffer is empty. Packets get through quickly. When there is more traffic than the outgoing interface can handle the buffer fills up. When the buffer is full it starts dropping packets, typically it drops newly arrives packets, a behavious known as "tail drop". When the sender sees packet loss it backs off the speed slightly, the sender then gradually increases the speed again until it once again sees lost packet.

The result of this is that the buffer immediately prior to the bottleneck ends up nearly full while other buffers end up nearly empty. I would guess that the buffer that is causing your problems is on the ISP side where the traffic passes from fixed line to wireless.

So what can be done? There are a couple of things the ISP can do in principle.

• The ISP can make the buffer smaller.
• The ISP can implement mechanisms like random early drop or explicit congestion notififcation which inform senders of congestion in a manner less brutal than "tail drop".

In practice though their equipment may not offer the options needed or they may be unwilling to make changes for you.

Another option is to introduce an artifical bottleneck yourself that is slightly slower than your connection to your ISP. This means giving up some bandwidth and only works well if your bandwdith to your ISP is consistent but it can provide a means to take control of the buffering.

Answer 2

Since you pinged your router and always had a response time 1 ms it is your connection to the internet that is the bottleneck.

When the video has to be downloaded, data can only be sent one by one since your connection is limited to 1Mbit/s. Your network cannot receive all the requested data in this amount of time.

At my home, my flatmate had terrible Round-Trip Times or could not even ping any server when I was downloading files.

Upgrading your connection should help solving this problem.

Another possible solution would be to limit the amount of bandwidth that your browser is allowed to use. But that leads to a longer waiting time for the Youtube videos.