Sword Art Online was developed around the Player Killing idea. That was the Game Maker's intent, to trap everyone in a world where you would die in real life if your HP gauge reached Zero. That is the same in Sword Art Online RP. ( you wont die in real life ) but the player killing option is completely possible, even without permission. So there are rules that we have applied, and they are the exact same as the anime.
1.) The HP Gauge can not reach Zero in a safe zone.
Safe zones are typically towns, roadside shops and within player homes.
2.) Duels.
To attack a player in a safe zone, you have to challenge them to a duel, however killing them is not possible.
3.) Sleeping.
Outside of a safe zone, you will not be able to feel an attack. Don't sleep.
4.) Don't get caught.
When a player is outside of a safe zone, they do not have to accept a duel to be attacked ( your player can be killed without permission; if attacker is stronger your character will probably die. )
5.) Rule Breakers.
I am not talking about site rules, but physical Aincrad laws. You break a law you are labeled as a orange player. You kill a player and your icon turns red. Orange & Red players can be identified right away.