This paper presents an argumentative version of the well known alternating offers negotiation protocol. The negotiation mechanism is based on an abstract preference based argumentation framework where both epistemic and practical arguments are taken into consideration in order to decide about different strategic issues. Such issues are the offer that is proposed at each round, acceptance or refusal of an offer, concession or withdrawal from the negotiation. The argumentation framework shows clearly how offers are linked to practical arguments that support them, as well as how the latter are influenced by epistemic arguments. Moreover it illustrates how agents' argumentative theories evolution, due to the exchange of arguments, influences the negotiation outcome. Finally, a generic algorithm that implements a concession based negotiation strategy is presented.