Why do failed attempts of Socket.connect take 1 sec on Windows?

Question

Working with .net, I noticed that an attempt to connect to a port that is not listened on always takes one 1 second. To check whether this is a problem in the .net abstraction or whether it is a lower level issue, and to compare it with linux (where an unsuccessful telnet takes something like 3ms), I used a node.js script to connect to port

12345, a port no process is listening on
80, a port that is being listened on

Results for Windows:

Connecting to 127.0.0.1:12345
#3: error elapsed: 1000ms, Error: connect ECONNREFUSED
#2: error elapsed: 1002ms, Error: connect ECONNREFUSED
#4: error elapsed: 1003ms, Error: connect ECONNREFUSED
#1: error elapsed: 1007ms, Error: connect ECONNREFUSED
#0: error elapsed: 1015ms, Error: connect ECONNREFUSED

Connecting to 127.0.0.1:80
#0: connect elapsed: 8ms
#1: connect elapsed: 1ms
#2: connect elapsed: 3ms
#3: connect elapsed: 4ms
#4: connect elapsed: 6ms

Results for Linux:

Connecting to 127.0.0.1:12345
#4: error elapsed: 0ms, Error: connect ECONNREFUSED
#3: error elapsed: 1ms, Error: connect ECONNREFUSED
#2: error elapsed: 1ms, Error: connect ECONNREFUSED
#1: error elapsed: 1ms, Error: connect ECONNREFUSED
#0: error elapsed: 3ms, Error: connect ECONNREFUSED

Connecting to 127.0.0.1:80
#4: connect elapsed: 0ms
#3: connect elapsed: 0ms
#2: connect elapsed: 0ms
#1: connect elapsed: 1ms
#0: connect elapsed: 2ms

Node.js source:

var net = require('net');
var host = process.argv[2];
var port = Number(process.argv[3]);
console.log("Connecting to %s:%d", host, port);
for (i = 0; i < 5; i++)
{
  (function(i) {
    var date = +new Date;
    var client = net.connect({host: host, port: port});
    client.on('error', function(msg)
    {
      console.log("#%d: error elapsed: %dms, %s", i, new Date - date, msg);
    });
    client.on('connect', function()
    {
      console.log("#%d: connect elapsed: %dms", i, new Date - date);
    });
  })(i);
}

A couple of notes

I also tried remote IP addresses, the results are similar to those shown above for localhost
in the case of node.js and the way I wrote it, the (asynchronous) connection attempts run in parallel, but in case of .net, I was testing synchronously, i.e. one attempt after the other, and the result was the same: 1sec every time

Any idea why Windows is slow here? Is that some kind of deliberate throttling?

Interesting. Have you tried seeing what happens if another program starts listening on that port during that one second delay? (Also, I take it the remote addresses you were testing with did not have a firewall configured?) — Harry Johnston, Oct 18 '13 at 03:07

Evgeniy Berezovsky · Accepted Answer · 2013-10-18T06:50:55.620

@HarryJohnston's comment lead me to rewrite the program to see

what happens if another program starts listening on that port during that one second delay

And it seems, on Windows, a call to connect() attempts to connect to an unbound port up to 3 times in 500ms intervals. If that third attempt also fails, it gives up.

Here's the log of a test run, where I occasionally and randomly started and stopped a process that listens on port 12345:

Connecting to 192.168.77.11:12345
#1: error elapsed: 1008ms, Error: connect ECONNREFUSED
#2: error elapsed: 1001ms, Error: connect ECONNREFUSED
#3: error elapsed: 1004ms, Error: connect ECONNREFUSED
#4: connect elapsed: 1001ms
#5: connect elapsed: 1000ms
#6: connect elapsed: 500ms
#7: error elapsed: 1000ms, Error: connect ECONNREFUSED
#8: connect elapsed: 1001ms
#9: error elapsed: 1000ms, Error: connect ECONNREFUSED
#10: connect elapsed: 500ms
#11: error elapsed: 1000ms, Error: connect ECONNREFUSED
#12: error elapsed: 1000ms, Error: connect ECONNREFUSED
#13: connect elapsed: 500ms
#14: error elapsed: 1000ms, Error: connect ECONNREFUSED
#15: connect elapsed: 500ms
#16: error elapsed: 1000ms, Error: connect ECONNREFUSED
#17: connect elapsed: 500ms
#18: connect elapsed: 1000ms
#19: connect elapsed: 1000ms
#20: connect elapsed: 500ms
#21: connect elapsed: 1011ms
#22: connect elapsed: 1000ms
#23: error elapsed: 1000ms, Error: connect ECONNREFUSED

And the modified source code:

var net = require('net');
var host = process.argv[2];
var port = Number(process.argv[3]);
console.log("Connecting to %s:%d", host, port);
var connect = function(i) {
    var date = +new Date;
    var client = net.connect({host: host, port: port});
    client.on('error', function(msg)
    {
      console.log("#%d: error elapsed: %dms, %s", i, new Date - date, msg);
      connect(i + 1);
    });
    client.on('connect', function()
    {
      console.log("#%d: connect elapsed: %dms", i, new Date - date);
      connect(i + 1);
    });
  };
connect(1);

You'll find that it does that for all error codes, but with much bigger timeouts for the other ones, totaling around a minute, something like 6+12+32 or 8+16+32. — user207421, Oct 18 '13 at 04:37
@EJP You mean errors of `connect()` other than *connection refused* result in those timeouts totaling almost a minute? Any examples you can share? — Evgeniy Berezovsky, Oct 18 '13 at 05:23
A connection timeout takes a minute, but if you can set it up you will see it consists of three separate attempts. — user207421, Oct 18 '13 at 05:48

score 4 · Answer 2 · edited Jun 03 '21 at 12:28

4

According to this article by Microsoft, the reason is that the RFC leaves it up to the implementation to decide what to do exactly when a connection is refused, and on Windows the default is to retry 3 times before failing.

edited Jun 03 '21 at 12:28

PhilR

5,375
1
21
27

answered Feb 08 '19 at 20:49

Lauri Nurmi

566
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3
14

Why do failed attempts of Socket.connect take 1 sec on Windows?

2 Answers2

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