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I wonder it it is tied to my NIC at all or if the OS or driver intercepts and immediately returns data sent to the loopback address?

Do the electrons actually travel to my NIC then the NIC returns them?

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More trivia the entire 127 /8 is dedicated to loop back – Jacob Feb 11 '11 at 11:33
It would be interesting to see some real-world throughput and/or latency figures for the loopback if anyone has them. – NPE Feb 11 '11 at 12:25
Even if the electrons travel there, the NIC probably wouldn't return the same electrons :) – Halil Özgür Oct 6 '11 at 20:31
:) yeah i should say signal.. – user55029 Nov 25 '11 at 0:49
there's no place like – user130370 Dec 5 '12 at 21:08
up vote 22 down vote accepted

You don't mention a particular OS but for most all that happens is that the data travels down the stack until it gets to IP at which point it's pretty much sent back. That's a massive oversimplification but means that the entire process is usually CPU bound so its performance is therefore directly linked to CPU speed plus stack efficiency. In practical terms modern CPUs and OSs should be able to 'bounce' loopback traffic considerably faster than 40Gbps - which is the fastest NIC I think I'm capable of buying today. Hope this helps.

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No all traffic never hits the physical network, it gets processed by a loop back adapter in the kernel.

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so that means super fast right :) – user55029 Feb 11 '11 at 4:45
@Mrk Mnl yes you bet. For my ping 00ms using XAMP 1.7.4 – Benny Feb 11 '11 at 11:17
any local address never hits the physical network. just one of them – alvosu Feb 11 '11 at 11:27
"hitting the physical network" is a bit ambiguous as it could get to your NIC without being sent and not hit the physical network, anyway if it is returned by the kernel, i.e. the OS, obviously it doesnt get to the NIC. – user55029 Feb 21 '11 at 7:46

You may be interested in the Loopback Fast Path feature we added in Windows 8 and Windows Server 2012. I've written a short blog about it here

It also provides an illustration of the path taken by the loopback with and without the optimization

With Best Regards

Ed Briggs Microsoft Corporation

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It depends on what OS you're running, but Windows 2000 had a bug in it where requests on the loopback adaptor would be slow. There's some useless trivia for you!

Please see this kb article for more information.

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iperf -c 1111 -t 5
iperf: ignoring extra argument -- 1111
Client connecting to, TCP port 5001
TCP window size: 49.7 KByte (default)
[  3] local port 32840 connected with port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0- 5.0 sec  4.72 GBytes  8.12 Gbits/sec

$ iperf -c 1111 -t 5
iperf: ignoring extra argument -- 1111
Client connecting to, TCP port 5001
TCP window size: 49.7 KByte (default)
[  3] local port 56482 connected with port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0- 5.0 sec  4.62 GBytes  7.94 Gbits/sec
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There's an awful lot wrong with this answer. It is faster. If you aturate the collision window then the limiting factor will be driven by context switching. – symcbean Feb 11 '11 at 13:02

I'd rather say that you find your answer in it's name itself. The name says: "Local Loopback Address", which in itself means that the network never intervenes in the process and the request is in itself looped back locally.

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I was about to post the same when I noticed you beat me to it. Indeed 'loopback address' should give a hint :) – luis.espinal Feb 11 '11 at 12:35
the question was how fast, i know what it does – user55029 Feb 12 '11 at 8:22
The name doesn't actually tell you anything. For example, a physical RS-232 loop back will be just as slow as your serial port, whereas if you were to implement one at the driver level it would be faster. Of course localhost will be handled in software, but you can't tell that just from the name "loopback". In case you're too young to know of these things, I give you – aij Nov 18 '15 at 4:00

If you are on Linux...

type in "ifconfig -a" as root...

Notice the "interrupt" line on all NICs...notice there is no "interrupt" line on "lo"...that means it does not even have an interrupt assigned to it, so pretty darn fast ;-)

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