SIP Firewall and NAT Traversal
TUTORIALS
What is NAT (Network Address Translation?) Why do we need ALG (Application Layer Gateway?)
964kb Zipped Powerpoint tutorial 
By SIPKnowledge 
A very well written and descriptive tutorial on NAT and the traversal problems for SIP Networks.
View the powerpoint tutorial here >>>
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WHITEPAPERS
(www.ingate.com) Solving the Firewall/NAT Traversal Issue of SIP: Who Should Control Your Security Infrastructure?
Best practices for SIP NAT traversal
Paradial Whitepaper - Connectivity Challenges for Real-time IP Communications - How NATs and firewall issues are solved efficiently and securely
Media Proxy - NAT traversal for geographically distributed Voice over IP networks
LINKS AND RESOURCES
IETF RFC 3489 - STUN - Simple Traversal of User Datagram Protocol (UDP) Through Network Address Translators (NATs)
Using Asterisk as a SIP/IAX Gateway - The Asterisk open source telephony server can be used as a gateway in order to avoid SIP NAT Traversal.
SIPfoundry - The NAT & Firewall Traversal Problem
SIP Foundry - SIP NAT Traversal using the Astaro Security Linux Firewall
SIP Foundry - Introduction to STUN Servers
Sourceforge Project: STUN Client and Server library - This project implements a simple STUN (Simple Traversal of UDP through NATs) server and client on Windows, Linux, and Solaris.