Should I Give A Promice Ring?

I want to get my girlfriend a promise ring but I don`t know if I should, we have been dating for almost six months. One day, one of her friends asked me if we were serious and were going to get married? So I talk to my girlfriend about this. She said she would marry me if I asked, but she said since I have to leave for 2 years she would want a promise ring, and that we would talk about marriage when I get back. Is a promise ring the rigt solution? And as typical marriage should I ask the father or just give it to her and let her explaine it to her parents.