See the full text: Letter by President Donald Tusk to the European Council on the issue of a UK in/out referendum (pdf) and see: The nine labours of Cameron: Analysis of the plans to change
See the full text: Letter by President Donald Tusk to the European Council on the issue of a UK in/out referendum (pdf) and see: The nine labours of Cameron: Analysis of the plans to change EU free movement law (EU Law Analysis, link)