What is an ICE candidate?

An ICE candidate is one possible network address — IP, port and transport — at which a WebRTC peer might be reachable. ICE gathers several candidates per peer, pairs them and runs connectivity checks to find the best working path.

What is the difference between srflx and relay?

A server-reflexive (srflx) candidate is the public address a STUN server observes for you, allowing a direct peer-to-peer path through a cone NAT. A relay candidate is an address on a TURN server that forwards media when no direct path works, at the cost of extra latency.

What does typ host mean in a candidate line?

It marks a host candidate — a local interface address such as a LAN IP or, in modern browsers, an mDNS .local hostname that masks the real IP. Host candidates have the highest priority and are tried first.

When is a peer-reflexive candidate created?

A prflx candidate is not signalled in advance. It is discovered during connectivity checks when a NAT maps a new port for an incoming STUN probe, revealing an address neither STUN nor TURN had reported.

Does this tool send my candidate anywhere?

No. The candidate line is parsed entirely in your browser with JavaScript. Nothing is uploaded, logged or stored.

WebRTC ICE Candidate Types

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How WebRTC finds a path between peers

Before media can flow, WebRTC’s ICE (Interactive Connectivity Establishment) agent gathers every address at which each peer might be reachable, called candidates, then probes pairs of them to pick the best working route. This reference explains the four candidate types, the SDP attribute that encodes them, and how each fits into NAT traversal — plus a parser for any candidate line.

How it works

Each candidate is encoded as a single SDP attribute line:

a=candidate:842163049 1 udp 1677729535 203.0.113.5 54123 typ srflx raddr 192.168.1.7 rport 54123

The fields are foundation, component (1 = RTP, 2 = RTCP), transport, priority, connection address, port, the keyword typ, and the candidate type. Server-reflexive and relayed candidates also carry raddr/rport pointing back at the base host candidate. ICE assigns each pair a priority and runs STUN connectivity checks, preferring direct host and srflx paths and only falling back to a TURN relay when symmetric NATs or firewalls block everything else.

Tips and notes

Host candidates win when peers share a network; browsers now mask the LAN IP behind an mDNS .local name.
A srflx candidate needs a STUN server; a relay candidate needs a TURN server with credentials.
Peer-reflexive candidates appear only during the handshake and are never signalled ahead of time.
Always provide a TURN server for reliability — a meaningful share of real-world calls cannot establish a direct path.