Please give me a summary of the current rule set for roller derby.
A roller derby bout consists of two 30 minute halves.
Players will be on the track for a series of two minute intervals.Each team will have 5 players on the track at one time. 
Four players from each are designated blockers, and will play both offense and defense at the same time.
Each team will field one jammer who is the main point scoring skater.
Jammers earn points for passing blockers from the opposing team.