ngs of World Multiconference on Systemics, Cybernatics and Informatics, Orlando, FL. 284-89. [35] Stewart, R., Q. Xie, M. Tuexen, S. Maruyama and M. Kozuka. 2007. "Stream Control Transmission Protocol (SCTP) dynamic address recon- figuration". IETF RFC 5061. September. © 2009 by Taylor & Francis Group, LLC NAT Traversal Mario Baldi, Fulvio Risso, and Livio Torrero Technical University of Turin CONTENTS 12.1 Introduction 12.2 NAT Behavior 12.3 Problem Statement 12.4 Generic NAT Traversal Techniques 12.5 SIP-Specific NAT Traversal Techniques 12.6 ALEX: Ensuring End-to-End Connectivity for Both SIP and Media Flows across NATs References 12.1 Introduction The wide diffusion of network address translators (NATs) through the Internet is the result of the increasing demand for hosts capable of communicating across the network although they cannot have routable network addresses due to their shortage in IPv4. The increasing number of mobile devices with wi-fi connectivity has made the demand grow further. The idea behind the NAT is that a restricted minority of the hosts belonging to the same network need to effectively communicate with hosts placed in other networks at any point in time: SO assigning a public routable network address to each host of the network is a waste. On the contrary, a NAT dynamically "assigns" a public network address (and port) only to hosts (and applications) at the moment in which they are actually communicating with other hosts in the Internet. By doing this, the number of public network addresses actually required for a subnetwork is reduced (as it is much lower than the number of hosts connected to such a subnetwork), thus rationalizing this precious resource. Furthermore, NATs are considered by network administrators as a (weak) manner to provide network security, hiding the real topology of the network behind a NAT: since public network addresses (and ports) are assigned dynam- ically, the same host (and application) can be bound to the different network addresses (and ports) in at subseq