group messaging app (IM client) using WLAN

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Mobile

Android

networking

I am supposed to develop a group messaging app (IM client) using WLAN ,its a term project.This app is supposed to list all the available users that are connected over the wlan and provide the user with the options ,to create a group ,send group messages or send messages individually over the wlan.I have had experience of developing android apps in the past but none of them have ever been network based.

I need to know :
1.what Internet protocols will be used .(sip? ,voip?) for DNS etc ,yeah i have below meager networking knowledge.
2.what areas do i need to research any links to resources will be quite helpfull .
3.any open source app that offers the same functionality
4.do i need to set up a server aswell ?

I would be really thankfull for your advice on this .