This is something that gets repeated on this site often - that terminology (particularly IT terminology) is not always grammatical, and may have a completely different meaning in that context to any dictionary definition.
In this particular case, a packet "cap" pretty much mirrors the dictionary definition. A cap is a limit.
However, a packet "overhead" is the time it takes to transmit data on a packet-switched network. In English, an "overhead" is a cost or an expense, particularly one which you are obliged to pay regularly, for example, your rent is an "overhead". You may find the IT-related definition in some dictionaries but I couldn't find it. Terminology only really finds its way into the dictionary if it becomes part of everyday speech.
If you want to know the meanings of terminology that are not in the dictionary you would be best asking on a site dedicated to that particular field such as IT, or networking. Similarly, the part of your question about the difference between the two is an IT technical question, not an English grammar question.