Which service does an ICE-capable SIP UAC start to help receive signaling and media?

An ICE-capable SIP User Agent Client (UAC) starts a Local STUN (Session Traversal Utilities for NAT) service to facilitate the receiving of signaling and media. The role of STUN is to help the client discover its public IP address and the type of NAT (Network Address Translation) it is behind, which is critical for establishing peer-to-peer communications.

By utilizing STUN, the UAC can perform the necessary checks to determine its network environment, enabling it to send its public address to other SIP endpoints. This is especially important in scenarios where NAT devices are in use because NAT can interfere with the ability to establish direct communication paths between devices on different networks.

While TURN (Traversal Using Relays around NAT) is also relevant in scenarios where direct peer-to-peer communication is not possible, it is used as a fallback mechanism. Session Border Controllers and firewall services play roles in managing and securing SIP sessions but are not primarily responsible for the initial steps of establishing NAT traversal using ICE (Interactive Connectivity Establishment) techniques. Therefore, starting with a Local STUN service is the correct approach for an ICE-capable SIP UAC.