The asymmetric NAT and its traversal method

摘要

In In this paper, the new type of NAT is firstly discovered and the associated traversal method is also firstly proposed. This new type is named as ldquoasymmetric conerdquo. The behavior of ldquoasymmetric conerdquo is different from ldquofull conerdquo, ldquorestricted conerdquo, ldquoport-restricted conerdquo and ldquosymmetric conerdquo. However, the behavior of the ldquoasymmetric conerdquo fully follows and matches the NAT standard (RFC-1631, 3022). In the ldquoasymmetric conerdquo, the outbound packet is strictly inspected. Its behavior is like the ldquosymmetric conerdquo. Nevertheless, the inbound packet is loosely inspected. It is like the ldquofull conerdquo, ldquoport restricted conerdquo or ldquorestricted conerdquo. Thus, the NAT type can be increased to 7 types. Except for the traditional 4 types, those extra 3 types are ldquoasymmetric full conerdquo, ldquoasymmetric restricted conerdquo, and ldquoasymmetric port restricted conerdquo. In the NAT traversal approach, the STUN algorithm was proposed by IETF to traversal the ldquofull conerdquo and ldquoport restricted conerdquo. However, the STUN algorithm can be worked on the ldquoasymmetric full conerdquo and ldquoasymmetric port restricted conerdquo. In this paper, a NAT traversal algorithm for the ldquoasymmetric conerdquo is also firstly proposed in this paper. The theoretic analysis and experimental results confirm the performance of our NAT traversal algorithm in ldquoasymmetric conerdquo.