HttpClient explained using netstat

November 26, 2019

Learn http client once again this time via netstat

This is not a guide to:

How to use HttpClient in your app ?

But rather how to use netstat and how TCP connections are behaving in different code scenarios.

Below examples will showcase dotnet HttpClient running next to nestat being refreshed every 1sec.
Keep-alive is set to true on default so connections will be reused. Output from netstat is filtered to show only relevant data.

sudo watch -n 1 "netstat --TCP -pnW | grep 104.27"

Netstat explained

Every TCP connection is a pair of local and foreign address.

Address is combination of IP/port, where:

  • IP is used to locate machine
  • Port is used to determine application on that machine.

If machine is talking to the server, port for local address will be picked from pool of available ports. Port for foreign address will be of course port on what remote server is listening (in this case 443).


Netstat columns:

  • Local address IP/port of local machine
  • Foreign address IP/port of remote machine
  • Every connection is in some state, here are the most important:
    • ESTABLISHED connection is opened after 3-way handshake and ready to transmit the data
    • TIME_WAIT client side (HttpClient) closed the connection, but there can be still something flowing from server, because of that connection is still open and will be closed after some time
    • CLOSE_WAIT same as above, but from server side
    • FIN_WAIT2 remote server has sent acknowledgement of receiving connection termination request (from httpclient)
    • LISTENING local server is waiting for incoming remote connections
Output of `netstat --TCP -pnW`

Example 1

In code:

  • HttpClient is instantiated per single request
  • HttpClient is not disposed

In netstat:

  • Each request gets its own TCP connection
  • TCP connections are not closed after completion of requests
  • TCP connections are closed after program is terminated

If program is continuously running:

  • After some time of idleing, foreign address sends connections termination requests and local connections go to CLOSE_WAIT state
  • System is waiting for dotnet program to close the connections if not after some time connections will be closed by operating system.

We created overhead by not-reusing existing TCP connection to ( Make this loop iterate over 65,535 time and you will reach maximum number of ports available, making this machine completely unresponsive.

Example 2

In code:

  • HttpClient is instantiated per single request
  • HttpClient is disposed after each request In netstat:
    • Each call gets its own TCP connection (see port changing on client side (
    • Connection immediately goes to TIME_WAIT state after disposing HttpClient

We still have overhead by not reusing existing connection, on the other hand port deplation is much less likely to happen. Although make this loop parallel Parallel.ForEach and see machine becoming unresponsive.

Example 3

In code:

  • HttpClient is instantiated only once
  • HttpClient is disposed once it executed all requests In netstat:
    • Each http call goes through single TCP connection
    • All connections immediately go to TIME_WAIT state after disposing HttpClient

TCP connections are finally reused but remember this is only example.
In real-life scenarios I recommend using HttpClient Factories, which takes care of HttpClients life-cycle, more over here.

Keep in my mind everything above was conducted on Linux Mint, on Windows situation in netstat might be slightly different.