I originally started building this PacketCast to discuss Path MTU discovery and the implications surrounding not allowing certain types of ICMP traffic to flow freely through a network. I quickly realized that I needed to break the discussion into separate parts and decided to start with a discussion of what IP MTU is.
This PacketCast describes IP MTU and how it can be determined by looking at the characteristics of the traffic on the wire. We look at how an IP stack behaves when using the “Don’t Fragment or DF” bit in the IP header. Additionally, we look at how oversized packets are fragmented with the DF bit is not set. After watching this PacketCast, the definition of IP MTU should be better understood as well as the unexpected consequences when IP MTU is not understood. This PacketCast will help administrators to be able to identify the sending IP MTU size of one host to another by using Wireshark.
As previously mentioned this is only a subset of what my original intention was. The next packet cast will explain and demonstrate the Path MTU Discovery Process. Although it may not seem so, this is a relevant discussion in modern networks that utilize tunneling protocols like PPPoE and VPNs.
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