Given the current near-universal deployment of NATs (Network Address Translators) in the public Internet, the lack of standards for NAT behavior has given rise to a crisis. While it is widely acknowledged that NATs create problems for numerous Internet applications, our inability to describe precisely what a NAT is or how it behaves leaves us few solutions for compensating for the presence of NATs. The behavior of NATs varies dramatically from one implementation to another. As a result it is very difficult for applications to predict or discover the behavior of these devices. Predicting and/or discovering the behavior of NATs is important for designing application protocols and NAT traversal techniques that work reliably in existing networks. This situation is especially problematic for end- to-end interactive applications such as multiuser games and interactive multimedia. NATs continue to proliferate and have seen an increasing rate of deployment. IPv6 deployments can eliminate this problem, but there is a significant interim period in which applications will need to work both in IPv4 NAT environments and with the IPv6 to IPv4 transition mechanisms. This working group proposes to generate requirements documents and best current practices to enable NATs to function in as deterministic a fashion as possible. It will consider what is broken by these devices and document approaches for characterizing and testing them. The NAT behavior practices will be application independent. The group will also advise on how to develop applications that discover and reliably function in environments with NATs that follow the best current practices identified by this working group. This will include the development of protocol-independent toolkits usable by application protocols for NAT traversal. This will include a revision of RFC 3489 for NAT binding discovery and a relay protocol that focuses on security. The work will be done with the goal of encouraging eventual migration to IPv6 and compliance with the UNSAF [RFC 3424] considerations. It will not encourage the proliferation of NATs. The behavior that will be considered includes IP fragmentation and parameters that impact ICMP, UDP, TCP, IGMP, MLD, and multicast. The proposed WG will coordinate with v6ops, midcom and nsis. The work is largely limited to examining various approaches that are already in use today and providing suggestions about which ones are likely to work best in the internet architecture.