Step 1 — study popular protocols to understand how client/server interactions typically work.
Step 2 — deploy the network appliance in question to your LAN and intercept its packets with wireshark.
Step 3 — begin inference of protocol from observed behavior and test hypothesis by sending hand-crafted payloads to the server in question.
Step 4 — rinse and repeat until assumptions are proven to be correct with a high degree of reliability.
A good way to ensure you’ve captured the major parts of the protocol is to record about 72 hours of traffic and then replay it through a proxy that directs traffic to your newly created service.
If you can interpret the vast majority of the messages without error, you’re getting close to a reliable implementation.
Step 5 — use this strategy to develop a deep understanding of both protocols in question.
Step 6 — write an “adapter” that can translate protocol A to protocol B and vice versa.
Step 7 — implement the adapter towards whatever use case you have in mind.