You're right, you are using it wrong. An LLM can read code faster than you can, write code faster than you can, and knows more things than you do. By "you" I mean you, me, and anyone with a biological brain.

Where LLMs are behind humans is depth of insight. Doing anything non-trivial requires insight.

The key to effectively using LLMs is to provide the insight yourself, then let the LLM do the grunt work. Kind of like paint by numbers. In your case, I would recommend some combination of defining the API of the library you want yourself manually, thinking through how you would implement it and writing down the broad strokes of the process for the LLM, and collecting reference materials like a format spec, any docs, the code that's creating these packets, and so on.