How does latency work?

[JohnJSal]

Hi everyone. Quick question about latency.

I always assumed that latency was the time it took from when you initiate a command on your end (e.g. typing in a URL and pressing enter) to when the server responds (in this case, displays the Web page).

It may be that this is correct after all, but it doesn't seem to be working that way in my experience. When I had cable internet my latency was practically non-existent, around 20ms I think. And so naturally the time between pressing enter and seeing the web page was essentially instantaneous.

But now that I have satellite internet (cue the weeping and gnashing), it seems different. My latency is in the range of 600-700ms (eesh), but rather than take about this long for the server to respond and load the web page, it often seems to take several seconds before I see anything -- I'd say about 3 seconds.

So what gives here? If latency works as I think it does, why does it take so long to get a web page to appear? Or does it work differently than I've been thinking?

Thanks!

P.S. When I say "load the web page" and the like, I mean only for the page to appear initially. I'm not referring to how long it takes the page to fully load, because I know this is dependent on the speed of the connection and not the latency.

 

[beers]

I always assumed that latency was the time it took from when you initiate a command on your end (e.g. typing in a URL and pressing enter) to when the server responds (in this case, displays the Web page).

That's pretty much it.

But now that I have satellite internet (cue the weeping and gnashing), it seems different. My latency is in the range of 600-700ms

Yep, the issue is that your browser doesn't make just one request, but multiple requests to the destination that all take 600-700ms each. If you have any dependencies stacked on top of each other such as 'dns to snarf.com takes 700 ms to resolve, request to webpage takes 700 ms to come back, request for an image file takes 700 ms after you receive the data from the webpage' you're already looking at 2.1 seconds of pure wait time.

 

[JohnJSal]

That's pretty much it.


Yep, the issue is that your browser doesn't make just one request, but multiple requests to the destination that all take 600-700ms each. If you have any dependencies stacked on top of each other such as 'dns to snarf.com takes 700 ms to resolve, request to webpage takes 700 ms to come back, request for an image file takes 700 ms after you receive the data from the webpage' you're already looking at 2.1 seconds of pure wait time.

Ouch. I thought the latency number was a round-trip time. Are you saying that it takes 700ms just to contact the site and then another 700ms for the site to respond? Or just that multiple call/responses are happening one after the other?

 

[Geoff]

Latency in a ping is round trip, it's the time it takes an ICMP packet to travel from the sending device, to the pinged device, and back. What you are experiencing is due to the TCP handshaking process, there isn't just one packet that goes from your computer to the server that says "request this website", you send multiple packets to first initiate the handshake, then you receive acknowledgements back, then you send back an acknowledgement back, then the delay for the data to be processed on the server and sent back to you, then another round of acknowledgements before the connection is closed. This is why you see several seconds, as this all adds up. The reason it's so high is because instead of traveling at the speed of light from say your house to your ISP, to the likely nearby server, it's going from your house, to an orbiting satellite, to their receiving station, then to the server.

 

[JohnJSal]

Thanks very much! Very informative!