[Netalyzr] What is TCP Connection Setup Latency?

Christian Kreibich christian at icir.org
Fri Mar 23 16:13:51 PDT 2012


Hi Cory,

On 03/23/2012 08:10 AM, Cory Riddell wrote:
> We have been having weird problems with Windows remote desktop sessions
> (several times per day the connection is lost but the client is able to
> automatically reconnect). So I ran your tool and found a very high TCP
> connection setup latency. The network latency was good (I think it was
> 54 ms) but the TCP connection setup latency is often well over 1000 ms
> with 3400 ms being the highest value I've seen.

Mhmm ... not good!

> I was guessing the TCP connection setup latency should be roughly 3x the
> network latency because it takes three packets (for ack, syn-ack, syn).
> But today I'm getting 50ms for the network latency and 54 seconds for
> the TCP connection setup latency, so my theory is obviously incorrect.

54 seconds, ouch. Unfortunately your theory is right on, because ...

> What is TCP connection setup latency?

... we indeed measure the time it takes to complete a TCP handshake:  at
the beginning of a test session, we make a sequence of TCP connection
attempts to the backend server the applet uses to conduct its tests, and
record the handshake completion times.  We try this on a number of ports.

The value reported in the summary is then the average of those
completion times.  If you take a look at the client-side transcript and
search for the strings tcpSetupLatency and tcpFirstSetupLatency (we
store the first connection attempt's time separately from subsequent
ones), you can inspect the results (in ms).

I'm not seeing unusual latencies to our backend servers.  If you send me
the ID of your session offline, we'll take a closer look. Perhaps a
horrible outlier is distorting the results.

(It seems we recently messed up the content type when delivering the
client-side transcript -- it may not display immediately in the browser.
Save it to disk and open it in an editor. It's just a text file. We'll
fix this shortly.)

Best,
Christian


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