Handling NAT traversal and mobility for multimedia traffic

Network Address Translator (NAT) traversal is a problem for many applications because a host in a private network cannot be contacted by a node which is outside this network unless the communication is initiated by the host itself. This restriction poses further problems when one of the hosts moves during an active session to a new location which is also behind a NAT. In this study we have proposed solutions to the above problems based on Mobile IP and SIP messaging. Our solutions are based on hole punching and the assumption that the hosts already know the details of each other. Hence it will not be required to consult the proxy or location servers when reestablishing the communication and this should reduce the required handoff time. I. INTRODUCTION The increase in the number of hosts connected to the Internet has almost exhausted the address space available in IPv4 and users are forced to employ private addresses at their office locations and home networks. The global IP addresses they need to access the Internet are provided by Network Address Translators (NAT) which map the private address to a global IP address:port pair. NATs allow users to share a limited number of global IP addresses by assigning different port numbers to different connections. This approach could be preferred even if there were enough addresses available, as it provides a security mechanism keeping unwanted traffi ca way from the local network. Using NATs, however, can be a problem when it comes to applications like Voice over IP (VoIP). As the hosts behind a NAT do not have global IP addresses, they are not accessible from the Internet unless they initiate a session. Hence calling users behind NATs requires extra effort and many different approaches have been proposed to solve the problems intro- duced due to NAT usage. These will be summarized in the proceeding sections. The other issue related to the NAT problem arises when the terminals are mobile. When a user who is behind a NAT moves to a new location which is also served by a NAT, it is a problem to continue the communication without an interruption. There has been various proposals to solve this problem of seamless handoff in case of mobility but they usually assume that at least one of the parties are using a global IP address. The case where the mobile host moves to a new network also served by a NAT is usually avoided. In this study we propose a mechanism to reestablish a connection when the nodes are moving between networks. Our study involves the case when the visited network is also using a NAT. Hence both parties are behind different NATs even after the move. We propose two solutions; one based on Session Initiation Protocol (SIP) messages and the other Mobile IP. In both cases, as the mobile node knows the location and the IP address of the stationary node, session reestablishment is expected to take less time than the initial connection setup. We assume that UDP packets are being exchanged throughout the study. The rest of the paper is organized as follows: section 2 describes the problems associated with NAT traversal and various proposed solutions. Section 3 introduces Mobile IP and section 4 Session Initiation Protocol. Section 5 describes hole punching process and section 6 explains our proposed solutions followed by conclusions in section 7.