From the official NHL release:
SENATORS’ CARKNER SUSPENDED FOR ONE GAME
NEW YORK (April 15, 2012) -- Ottawa Senators forward Matt Carkner has been suspended for one game for continuing to inflict punishment upon an opponent who was an unwilling combatant in an altercation during Game 2 of the team's Eastern Conference Quarterfinal series with the New York Rangers in New York on Saturday, April 14, the National Hockey League’s Department of Player Safety announced today.
Carkner instigated an altercation with New York forward Brian Boyle, who did not respond. After knocking Boyle to the ice with two punches, Carkner continued to throw punches at his opponent, who was an unwilling combatant. Carkner's actions classified him as an "aggressor" under NHL rules.
The incident occurred at 2:15 of the first period. Carkner was assessed a minor penalty for instigating, a major penalty for fighting and a game misconduct.
For a full explanation of the decision, complete with video, please click on the following link: