Assistance needed on H1 Extension,Control Letter

My H1 is valid till April 2017.I am currently in company’s internal project which is valid till Dec 2017 , running from 2007.
I am planning to file for extension from May 2017 to Apr 2020.

The project end date is as of now till Dec 2017 but it is going to extend further though I cannot confirm now when it is going to be extended.
I am looking for guidance on how to proceed on extension /wording’s to be put in H1 Control letter so to avoid cancellation or RFE.

What is a CONTROL letter ?