Wireshark-users: [Wireshark-users] Understanding Wireshark / Sniffer Data
Hi, I'd appreciate if someone could recommend a good (cheap/free) resource to understand the data captured using a sniffer like wireshark. For example, the new "Expert Info" seems to pin-point certain important events which helps analysis, but let's say for example it points out a maformed packet with protocol SMPP.... What resource(s) could be used to understand what this means and try and find a solution?

I know there are excellent and not very expensive pay services associated with wireshark, but I'm looking for someone on an extremely tight budget but wants to learn.