RFC 3235 (rfc3235) - Page 2 of 13
Network Address Translator (NAT)-Friendly Application Design Guidelines
Alternative Format: Original Text Document
RFC 3235 NAT Friendly Application Design Guidelines January 2002 that the ALG design may be simple and automated. ALGs typically operate inside small routers along with the NAT component. Ideally, the ALG should be simple and not require excessive computation or state storage. Many of the same issues in application design that create issues for NAT (and thus can require ALG support) are also issues for firewalls. An application designer would do well to keep this in mind, as any protocol that does require special handling by NAT or firewall products will be more difficult to deploy than those that require no special handling. 2. Discussion Network Address Translation presents a challenge to some existing applications. In many cases, it should be possible for developers of new applications to avoid problems if they understand the issues. This document aims to provide the application designer with information on what things they can do and what to avoid when trying to build applications that are able to function across NAT. The proliferation of NAT, especially in homes and small offices cannot be dismissed. The marketing of these technologies to homes and small businesses is often focused on a single-computer environment, and thus providers only give out a single IP address to each user. NAT has become a popular choice for connecting more than a single system per location. Clearly the most common problem associated with NAT implementations is the passing of addressing data between stations. Where possible, applications should find alternatives to such schemes. Studying a few existing protocols will serve to highlight the different approaches possible. Two common forms of Traditional NAT exist. With Basic NAT, only the IP addresses of packets are altered by the NAT implementation. Many applications will operate correctly with Basic NAT. The other common form is Network Address Port Translation. With NAPT, both the IP addresses and the source and destination ports (for TCP and UDP) are potentially altered by the gateway. As such, applications passing only port number information will work with Basic NAT, but not with NAPT. Application designers should strive for compatibility with NAPT, as this form of NAT is the most widely deployed. This is also the form of NAT that will likely see the greatest penetration in homes and small offices. Not all applications lend themselves to the architectural model imposed by NAPT. Senie Informational
