Simple Explanation of SMP client-server communication wanted

[Sinhika]

Okay, I get that there are tutorials on how to set up packet handlers.  What I am having trouble wrapping my head around is what information does a mod need to send from client to server and vice versa? Versus what is auto-magically handled by Forge & Vanilla MC parent classes. My brain is failing to make a crucial connection somewhere for this all to make sense.

 

How do you know what you need to set up packet handlers for?

Where does that information come from and when?

Is it related to events? Which events? For what? When?

 

I'm really lost when it comes to figuring out what needs to be done to make sure a mod works in both SSP and SMP modes. Simple explanations and links to open-source/github/etc code that does what you are talking about would be appreciated.

[Sinhika]

So... if I just create new tools, blocks, items and recipes for the standard crafting table or forge, is the client-server communication handled already at the level of the parent classes? i.e., do I need a custom packet handler for placing blocks derived from class Block?

 

I'm sort of getting the idea that packet handlers might only be needed for new GUIs, such as new crafting interfaces and new inventory containers, and for things that move, like vehicles, riding animals, etc. Is that correct?

 

...I probably should look at the packet handler tutorials in more detail.