What happens when tx is broadcasted whose input is not yet received by mempool or are unknown? lets say, there are 2 transactions  

tx1
address1 -&gt; address2

tx2
address2 -&gt; address3


our mempool doesnt have the tx1 yet, but i try to create a tx2 and broadcast it.

what will happen ?

will tx2 get included in mempool ? and relayed to other nodes ? 

or 

tx2 gets rejected. and wont be included in mempool  and wont be relayed to other nodes ?

what is the actual flow ?

what are such tx are called ?

What happens when tx is broadcasted whose input is not yet received by mempool ?or not in the old blocks i.e. missing/unknown.
Such a transaction is called an orphan transaction.

When you announce a transaction to a peer who misses one or more of its inputs, the transaction will be placed in the orphan pool - where they are temporarily held in the hope that their dependencies arrive. A request will go out to fetch those dependencies from whoever sent us the transaction.

The transaction won't be relayed until it can be validated. Once all dependencies arrive, they will all be subject to normal validation and relay policies, and if acceptable, relayed in dependency order.