Characterization and measurement of TCP traversal through NATs and firewalls

In recent years, the standards community has developed techniques for traversing NAT/firewall boxes with UDP (that is, establishing UDP flows between hosts behind NATs). Because of the asymmetric nature of TCP connection establishment, however, NAT traversal of TCP is more difficult. Researchers have recently proposed a variety of promising approaches for TCP NAT traversal. The success of these approaches, however, depend on how NAT boxes respond to various sequences of TCP (and ICMP) packets. This paper presents the first broad study of NAT behavior for a comprehensive set of TCP NAT traversal techniques over a wide range of commercial NAT products. We developed a publicly available software test suite that measures the NAT's responses both to a variety of isolated probes and to complete TCP connection establishments. We test sixteen NAT products in the lab, and 93 home NATs in the wild. Using these results, as well as market data for NAT products, we estimate the likelihood of successful NAT traversal for home networks. The insights gained from this paper can be used to guide both design of TCP NAT traversal protocols and the standardization of NAT/firewall behavior, including the IPv4-IPv6 translating NATs critical for IPv6 transition.