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I have a huge pcap file. I want to know facebook usage in terms of data transfered (upload, download). For that, I am using wireshark to read this file. From a question on stackoverflow , there are many fields that can be used to find bytes.

frame.len==243
ip.len=229
udp.length==209
data.len=201

Now, I have test frame.len and ip.len both gives different results. What I should consider correct ? I am a newbie in networks terminology and I have to just find correct data transfered.

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Hafiz Muhammad Shafiq
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1 Answers1

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What happens, when you connects to server and requests some simple page:

  1. Server application generates requested data (e.g. <body>Hello world</body> string) and passes it to HTTP layer
  2. HTTP layer generates necessary header according to RFC (specifies HTTP version, status code, content type etc), prepends it to generated data and pass everything to TCP layer
  3. TCP layer may break data into more than one pieces (not our case, message is already too small) and prepend necessary info for transport layer to each piece (src/dst port number, sequence number, some flags, checksum etc), then passes it to IP level
  4. IP layer prepends necessary info for routing (source/dest addresses, TTL and other stuff), then passes it to lower layer (e.g. Ethernet)
  5. Ethernet adds its part (MAC addresses, maybe VLAN tags) and pushes all to physical device
  6. Resulted data is sent byte-by-byte from server's NIC to network

So your question is actually up to you. What do you want to measure? Is it "data, which I need to display excluding all auxiliary info"? Or is it "all number of bytes I need to send/receive for getting this lovely cat picture"? Here is a list of fields to get size of each part:

  1. To get data lenght only (as string, unfortunately): http.content_length_header == "606"
  2. To get (data + HTTP header) length: tcp.len == 973
  3. To get (data + HTTP + TCP + IP layers): ip.len=1013
  4. To get every byte sent: frame.len == 1027

If you want to measure bandwidth occupation, use frame.len. If you're interested in "pure site weight", it should be independent from environment, so use http.content_length_header. Things might become more complicated on high level considering the following:

  • Era of HTTPS means you can't easily observe HTTP content in traces, so tcp.len might be the highest option
  • Some data (e.g. audio, video) is transferred over different protocol stack (e.g. IP - UDP - RTP)
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