Packet Sniffing is the process of monitoring and logging packets as they are transmitted across the network between the game client and the server. It is compulsory to packet sniff if you plan to upgrade the game version of your source since packets usually have their headers changed, or worse, their structures when the official servers modify game contents.
You can use a packet sniffer, which is simply a program easily downloadable from the internet, to help you sniff packets.
General Packet Sniffers
These packet sniffers help to intercept and decrypt IP packets into raw data, which can either be non-MapleStory packets (e.g. those you receive or send while browsing the internet) or encrypted MapleStory packets. Hence, it is compulsory for you to identify which are MapleStory packets and process them with decryptors such as
MapleStory Packet Sniffers
These packet sniffers are designed to identify MapleStory so that only MapleStory packets will be logged when intercepted. In addition, the packets are decrypted so you won't have to do it yourself. They are generally preferred over general packet sniffers