Why is Fiddler adding 1 second latency to some TCP connections?

[Karl Ivar Dahl]

I use Fiddler for performance evalutions, and noticed that some HTTP calls consistently takes ~1000ms.

This is not random; out of 3 calls in a scenario, it is consistently the same call that is slow, but this may change after a restart. 

I have used a lot of time finding the culprit in my code, but after disabling Fiddler the problem immediately disappeared, the request then takes 80ms as it should.

Note from the screenshot that the first call is slow, the TCP connect to localhost takes 1001ms, but I have also observed this issue on the second call instead of the first, and in other scenarios.

I connect to an Apache Tomcat server on Windows 10.

I have disabled any "modem simulations". The target server and Fiddler itself is on localhost.

I am using the latest release (version 4.6.2.2).

Is there any way to avoid this latency?

Request Count: 1

Bytes Sent: 1　519 (headers:385; body:1　134)

Bytes Received: 5　090 (headers:518; body:4　572)

ACTUAL PERFORMANCE

--------------

ClientConnected: 09:44:00.885

ClientBeginRequest: 09:44:00.886

GotRequestHeaders: 09:44:00.886

ClientDoneRequest: 09:44:00.886

Determine Gateway: 0ms

DNS Lookup: 0ms

TCP/IP Connect: 1001ms    <============

HTTPS Handshake: 0ms

ServerConnected: 09:44:01.888

FiddlerBeginRequest: 09:44:01.888

ServerGotRequest: 09:44:01.888

ServerBeginResponse: 09:44:01.964

GotResponseHeaders: 09:44:01.964

ServerDoneResponse: 09:44:01.964

ClientBeginResponse: 09:44:01.964

ClientDoneResponse: 09:44:01.964

Overall Elapsed: 0:00:01.078

RESPONSE BYTES (by Content-Type)

--------------

application/xml: 4　572

~headers~: 518

[Eric Lawrence]

A 1 second TCP/IP Connect time for localhost suggests a problem in your network stack. Is your local server listening on IPv6? If not, do you see a change if you use "127.0.0.1:8080" instead, or when you disable IPv6 inside Tools > Fiddler Options?

[Karl Ivar Dahl]

-Disabling IPV6 did not help-

-Using 127.0.0.1 instead of localhost:8080 did however solve the problem.

Thank you very much!

I suppose something is not set up correctly on my computer, I had no idea that 127.0.0.1 vs localhost could be an issue at all.

What could cause such a problem?

[Karl Ivar Dahl]

I suppose this discussion found  on stack exchange covers most of the problem in the context of MySQL.

I am still curious why disabling IPv6 in Fiddler did not help, though.

[Eric Lawrence]

I'd expect that disabling IPv6 would work just fine. 

Did you restart Fiddler after you did so?

[Karl Ivar Dahl]

No, I did not restart. I can now see the reminder of this at the bottom of the Options page.

After restarting, I have not been able to recreate the problem.

Thanks for your help!