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Interactive Connectivity Establishment (ICE) is a technique used in computer networking to find ways for two computers to talk to each other as directly as possible in peer-to-peer networking. This is most commonly used for interactive media such as Voice over Internet Protocol (VoIP), peer-to-peer communications, video, and instant messaging. In such applications, communicating through a central server would be slow and expensive, but direct communication between client applications on the Internet is very tricky due to network address translators (NATs), firewalls, and other network barriers.
Allocation
During the WebRTC offer/answer process, a client gathers candidates to be used for ICE. Each candidate is a potential address/port to receive media. Generally, three types of candidates get generated in this initial process.
- Host Candidate: Host candidates are generated by the client by binding to its locally assigned IP addresses and port. If you have multiple IP addresses, you can generate multiple host candidates.
- Server Reflex Candidate: Server reflex candidates are generated by sending STUN messages to a STUN/TURN server. A client sends a query message to the STUN server. That query passes through the NAT which creates a binding. The response to the query contains the public IP and port that was generated for the binding. This can now be used as a server reflex candidate.
- Relay Candidate: Relay candidates are generated in the same way as a server reflex candidate. A query message is sent to the TURN server which creates a NAT binding. That binding is used, but the binding will be sent to and from the relay server.
Exchange
As an offer is generated and sent to the end client. The client can also choose to generate their own candidates and send them, too. The candidates can be packed in the original offer or can be sent independently after the offer is sent. The latter is known as trickle ICE. The far-end client, which is now receiving the offer and its accompanying candidates, will now begin to prepare its answer. The answer is generated in a similar manner to the way the offer is generated, and an answer SDP is created. The far-end client can choose to pack their generated candidates into the SDP or send them independently (again, trickle ICE).
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