The NAT and Firewall Traversal Problem
for VoIP
Figure 1 illustrates the NAT and firewall traversal problem for VoIP. Home users connect to the Internet using broadband routers from various vendors, such as Belkin, D-Link, LinkSys and NetGear. Users connect using wireless hotspots at Internet cafes or hotels, which also use routers from vendors, such as NetGear and SonicWall.
Also, business users connect to the Internet using firewall products such as Cisco PIX, Juniper NetScreen, and CheckPoint FW1. Businesses sometimes also set up web-proxies such as Squid or Microsoft ISA for accessing the Web by their employees. Now suppose all the users behind all of these NAT and firewall devices/solutions want to make VoIP calls between each other. Will all of these calls work using traditional VoIP technologies? The answer is no. Most VoIP calls will not work through these NATs and firewalls. This is referred to as the NAT and firewall traversal problem - or simply the NAT traversal problem.
Figure 1: Broadband users connect to Internet using a plethora
of NATs and firewalls. For widespread adoption, VoIP calls needs
to work through them seamlessly.
Recently, there have been a lot of mobile phone products with Wi-Fi VoIP features (single-mode Wi-Fi or dual mode cellular plus Wi-Fi). According to Infonetics Research,
the number of Wi-Fi phones would double or triple each year until 2009 reaching a worldwide market of $3.7 billion [12]. Now suppose a lot of users have these Wi-Fi phones, and they move around with these phones to use them from wherever they are - at homes, Internet cafes and offices. They will
not only face the same NAT traversal problem, but worse, their connections
and their network configurations, may change frequently. This
further emphasizes the NAT traversal problem as it will severely
limit users capability to communicate.
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