Abstract: To solve the problem that the free-riding behavior of a node will seriously affect the download progress of the common node and the whole network performance in the BT network， a new economic model (TTEM) was proposed based on the tit-for-tat mechanism. According to the commodity trading and credit system in the social life， the economic model was presented that consisted of the node wealth， the file blocks pricing and the node overdraft. Finally， the TTEM was proposed by deploying the economic model on the tit-for-tat. Resources spread between nodes as a transaction in TTEM. Nodes were traded by the price of the file blocks， making the wealth of the node change， if the node’s wealth is greater than the overdraft. The simulation results show TTEM is able to reduce the impact which the free-rider nodes make on the BT network than the pure tit-for-tat mechanism under the similar propagation velocity of the file resources.

Key words: BitTorrent, free-riding, tit-for-tat, economic model