title: how to calculate number of bytes going through network with tcpdump?
body: <p>I have tcpdump like this</p>
  
  <pre><code>sudo tcpdmp tcp -n -i eth0 -w test.dmp
  </code></pre>
  
  <p>I want to calculate the number of tcp bytes going through eth0. I capture all the package using tcpdump as above. Is the file size equal the number of bytes or tcpdump add additional information into the dump file?</p>