How do you collect the dog tags of another player in multiplayer mode of battlefield 3?

in battlefield 3 online mode how do you collect the dog tags of another player?How do you collect the dog tags of another player in multiplayer mode of battlefield 3?
Knife 'em and you automatically collect that person's dog tags.How do you collect the dog tags of another player in multiplayer mode of battlefield 3?
Sneak behind their back and knife them, sometimes it won't take his tags and won't even kill him so you can try to knife him again but you be better of shooting him in case he turns around and kills youHow do you collect the dog tags of another player in multiplayer mode of battlefield 3?
Use your knife to kill your enemy thus you collect his tag, you can then see who's tags you have collected and also unlock more along the way, have fun