rity on SIP trunks, you need to take into account different aspects of security. These aspects call for a series of features and capabilities to mitigate the potential threats. Security is always best deployed in a layered architecture, rather than a single box or feature that strives to protect against all possible attacks. Areas worth exploring for SIP trunk security include Determine the level of exposure on the SIP trunk, which depends on how it is de- ployed and who the provider is. Limit the devices that can contact your network via the SIP trunk. Mitigation capa- bilities include features such as access lists, hostname validation, and voice source group definitions. Hide your enterprise network addressing from the outside (which could be Internet- visible) and inspect the validity of traffic that enters your network. Mitigation tech- niques include network address translation (NAT), topology hiding, firewalls, and intrusion protection services (IPS). Determine protocol and session validity. Mitigation techniques include SIP port set- tings, SIP protocol inspection and termination, registration, and authentication methods. Lock down your SIP trunk against toll fraud access using the same methods you used on your TDM gateways. Control the privacy of sessions on the SIP trunk. Mitigation techniques involve the control of originator information available outside the enterprise network with the use of SIP privacy headers, SIP normalization, digit manipulation, and encryption Chapter 7: Design and Implementation Considerations 113 methods of the signaling and the media streams (such as Transport Layer Security [TLS], Secure RTP, and the use of IPSec tunnels or virtual private networks (VPN) on the IP connections). SIP Trunk Levels of Security Exposure The level of security exposure depends on the characteristics of how the SIP trunk con- nects into your network and the strength of security protection your service provider offers. Figure 7-2 illustrates four increasing levels of exposure depending on the connecti