In recent years, structured P2P communications is being widely used for its features of self-organization as well as good scalability and flexibility. To make sure that every node can participate in such a network whether it is behind a NAT or not, we must solve the NAT traversal problem. However, existing NAT traversal methods all need the support of a centralized server which will destroy the distributive characteristic of structured P2P. In this paper, we propose a distributed NAT traversal mechanism called SMBR (Selective-Message Buddy Relaying) for structured P2P. SMBR has two main advantages. The first one is that it does not need the support of a server and thus can maintain the characteristics of structured P2P. Secondly, SMBR uses different mechanisms for the control messages and data according to their size. For control messages, it uses the method of buddy's relay while for data direct connections can be built with the help of the buddy. Using this mechanism, SMBR can achieve a balance between the traversal time and the buddies' load.
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