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Transparent SIP communication with NAT
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Mapping internal IP addresses to external IP addresses is essential for Voice over IP (VoIP) communications through network address translation (NAT) gateways and firewalls. Session Initiation Protocol (SIP) is the signaling protocol for establishing VoIP connections; however, SIP-based communications have problems working through firewalls and session border controllers, and all too often, VoIP calls or some unified communications functions fail because of NAT. In this article, I show you how IT managers can resolve these issues with the session traversal utilities for NAT (STUN), traversal using relays around NAT (TURN), and Interactive Connectivity Establishment (ICE) techniques to ensure transparent transitions and improve overall SIP security.
NAT Characteristics
Some years ago, the limited availability of IP addresses led to the development of various strategies by the Internet Engineering Task Force (IETF) for covering a wide environment with the available addresses. One of the intermediate solutions, called NAT (RFC 3022) [1] or PAT (port and address translation), uses conversion between private and public IP addresses.
NAT uses tables to assign the IP addresses of a private (internal) network to public IP addresses (Figure 1). The internal IP addresses remain hidden. NAT services exchange the sender and receiver IP addresses in the IP header. The simplest form of address conversion is known as static NAT. Address translation converts a private IP address sent from a private address space into a public IP address to be received in a public address space. In the reply packet, this conversion takes place in reverse order.
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