Simple definition
Relay service used when direct communication between two customers is not possible.
Technical definition
Relay service used when direct communication between two customers is not possible. In a professional environment, this mechanism usually integrates with other services to make the media pass through an intermediate server when NAT or firewall prevent the direct path.
What is it used for?
make the media pass through an intermediate server when NAT or firewall prevent the direct path.
Practical example
a video conference relays traffic by TURN when pair-to-peer connection fails.
Common issues
- NAT or firewall blocking signage or media
- parameters SIP/RTP inconsistent
- bad address announced to correspondents
Key takeaway: Always adapt the diagnosis to the product version and the architecture actually deployed.