Networking

Use Wireshark to inspect packets on your network

Scott Reeves illustrates how you can use Wireshark to inspect packets, looking specifically at various points in the OSI layer, to troubleshoot network issues.

I've been using Wireshark in a number of my posts to show aspects of network performance and to illustrate areas of TCP/IP such as the three way TCP handshake. This week, I thought I'd delve a little more into Wireshark itself and illustrate how the OSI layer works.

Two previous posts have shown the TCP three-way handshake (see, for example, my previous post "Using jperf and Wireshark for troubleshooting network issues"), so I will omit covering it in this post. What I will do is draw attention to the way you can look inside a packet captured using Wireshark. For this example, I used a capture of a network session run using a jperf client and a server. Figure A is a sample from this capture.

Figure A

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Looking at Figure A, you can see that the first line shows a summary of the frame. The other lines show the data link layer, the network layer, the transport layer, and finally, the actual data contained within the frame. I will step through each line in order.

One note before beginning: Wireshark will highlight the bytes in hex in the bottom pane (as shown in Figure A). You can change this to binary if desired.

Data Link

Figure B

Moving to the Ethernet layer (shown in Figure B), we can see that it is pretty simple. It contains a destination address and a source address. The data link layer is relatively simple in that it is only concerned with getting a frame to the next adjacent node on the physical medium. The main point of interest is the Organizational Unique Identifier (OUI), which takes up the first three bytes of an Ethernet address. The IEEE assigns the OUI to manufacturers of Ethernet equipment. If you want to take a look at assigned OUIs, the list is maintained at http://standards.ieee.org/develop/regauth/oui/oui.txt.

Wireshark makes it easy for you to check on the manufacturer of a card; you can highlight the field (as shown in Figure B) and extract the OUI. You can then plug the number into the website, http://www.wireshark.org/tools/oui-lookup.html, and obtain the manufacturer.

Network

The Ethernet layer is concerned with node to node. The IP layer is concerned with moving between networks, hence the original meaning of the term internetwork, from whence Internet was derived. Highlighting the network layer shows more details. From Figure C, we can see the source and destination IP addresses as well as the IP header length (20 bytes in this case).

Figure C

We can also see the Differentiated Services (DiffServ) area. This would be where extra information relating to the packet's type of service goes. For most packets on a LAN this is set to zero, which means best effort. There are several other levels, such as Expedited Forwarding, which generally has low loss, low delay, and low jitter. Diff Serv levels can be used when looking to implement a Quality of Service on IP networks.

Transport

The transport layer is where applications communicate via the use of ports. Looking at the capture shown in Figure D, we can see that the source port is 40519, while the destination port is 5001.

Figure D

Other areas of interest are the header length (32 bytes in this case) and the sequence number.  The sequence number generally will change for each packet.

Data

The final part of the capture is the data part; this is shown in Figure E. Here you will see what data is being sent over the medium. This is where you will find applications such as ssh are very important: using ssh means that a login name and password are encrypted.  In contrast, rsh and rlogin are not encrypted; data is sent in clear text, meaning that you can capture a packet and read the login name and/or password from the data section of the captured packet.

Figure E

Overhead

The size of the data part of the frame is 1448 bytes in this case. There is some overhead due to the headers used by TCP, IP and Ethernet. From Figure E, we can see that the actual frame size is 1514 bytes, whereas the size of the data is 1448 bytes. The overhead in this case is 64 bytes. Of this, 12 bytes is Ethernet, 20 bytes is IP and 32 bytes is TCP.

Wireshark offers many other features, other than the ability to inspect frames. This post touches on how to look at various points in the OSI layers, which can act as an aid in troubleshooting network problems. It can also help you to understand what sort of traffic is going over the network.

About

Scott Reeves has worked for Hewlett Packard on HP-UX servers and SANs, and has worked in similar areas in the past at IBM. Currently he works as an independent IT consultant, specializing in Wi-Fi networks and SANs.

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