Traversal of the customer edge with NAT-unfriendly protocols

Abstract

Customer Edge Switching (CES) provides policy based reachability to hosts in a private network without the disadvantages caused by traditional mechanisms for traversing Network Address Translators (NAT). The solution enables transparent communication across address realms without keep-alive signalling and application layer code in end systems as required by the current recommended approach to NAT traversal. Although most protocols traverse the customer edge correctly, we identify a few protocols that require special processing because of the IP addresses carried in the user data. This paper first presents the results of protocol compatibility testing with CES and selects two protocols, SIP and FTP, for further study. The paper then reports the implementation of Application Layer Gateways for these two protocols and provides guidelines for processing other protocols. The proposed approach significantly cuts the session establishment delays typical in SIP. The presented work is a part of a larger project that proposes the Customer Edge Switching to replace NATs and introduce co-operative firewalls for protecting customer networks.