CCIE or Null!

I don’t know about you but when I find myself performing packet captures and analyzing PCAPs I usually only know the symptoms of the issue I am attempting to troubleshoot. IE: Connection timeouts, slow response, long transfer times, etc. I usually don’t know much more than that, only in rare occasions do I get a heads up and insight into the behaviors of the application I am trying to troubleshoot. For all the other situations I need to rely on the PCAPs and interpret what and how the applications are communicating. Whether or not the application is behaving properly and performance is as it should be or if there is indeed something amiss somewhere.

Now for me the easiest way to do this is by using the reviewing the ‘Summary’ page under the ‘Statistics’ menu. A sample summary page is below:

A few great call-outs from this screen:

• Packet Size Limit -Knowing whether or not the packets within the capture were sliced after the first so many bytes is important to know, as sometimes you might not see the entire TCP header or wireshark will start classifying the packets as malformed. Although you will also see a ‘Truncated’ message within the packet indicating the packet was sliced.
• First Packet, Last Packet, & Elapsed time -Matching up the time of a packet capture with when the particular issue occurred is crucial, after all you don’t want to find yourself analyzing the wrong capture. The Elapsed time is important to make note of as this give you the ability to establish a baseline, knowing how long a process takes can you help you identify an issue or identify expected behavior in the future.
• Avg. Packet Size – Depending on what you are trying to troubleshoot the average packet size can be a quick indicator in regards to whether or not your fully using the MTU an your network. If you are troubleshooting data transfers normally you would expect the Avg. Packet to be quite large. If you see exceptionally small packet sizes data transfers may take a lengthy amount of time due to the increase TCP overload and normal L3 forwarding. Same goes for the Avg Mbit/sec, if you have large packets flowing you can expect to see a higher throughput rate, and the opposite for lower packet size rate.

The next spot that is worth checking out is the ‘Conversations’ which is also found under ‘Statistics’ this quaint little window gives you a brief overview of any Source/Destination devices identified within the capture. From an L2 Ethernet perspective up to a L4 TCP/UDP Perspective allowing you see what end points are really involved with this communication along with how much data was sent, the length of time the connection, etc. It’s not completely unheard of for applications to communicate with other devices (Web Servers, DB Servers, File Servers, Other App Servers) to perform whatever tasks it is trying to perform and it could be very possible this third server may or may not be slowing down the process.

So by using these two windows in Wireshark you’ve identified the following:

1. The length of time the process take. – Found in the elapsed time of the capture, as long as the entire process was captured that is.
2. The endpoints involved with this communication. – Remember it is important to cut down as much background noise as possible.
3. How much data is transferred and at what size & rate. – This can very helpful when working data transfers.