3. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

Readers should be familiar with the terminology defined in the offer/answer model [RFC3264], STUN [RFC5389], and NAT Behavioral requirements for UDP [RFC4787].

This specification makes use of the following additional terminology:

Agent: As defined in RFC 3264, an agent is the protocol implementation involved in the offer/answer exchange. There are two agents involved in an offer/answer exchange.
Peer: From the perspective of one of the agents in a session, its peer is the other agent. Specifically, from the perspective of the offerer, the peer is the answerer. From the perspective of the answerer, the peer is the offerer.
Transport Address: The combination of an IP address and transport protocol (such as UDP or TCP) port.
Candidate: A transport address that is a potential point of contact for receipt of media. Candidates also have properties -- their type (server reflexive, relayed or host), priority, foundation, and base.
Component: A component is a piece of a media stream requiring a single transport address; a media stream may require multiple components, each of which has to work for the media stream as a whole to work. For media streams based on RTP, there are two components per media stream -- one for RTP, and one for RTCP.
Host Candidate: A candidate obtained by binding to a specific port from an IP address on the host. This includes IP addresses on physical interfaces and logical ones, such as ones obtained through Virtual Private Networks (VPNs) and Realm Specific IP (RSIP) [RFC3102] (which lives at the operating system level).
Server Reflexive Candidate: A candidate whose IP address and port are a binding allocated by a NAT for an agent when it sent a packet through the NAT to a server. Server reflexive candidates can be learned by STUN servers using the Binding request, or TURN servers, which provides both a relayed and server reflexive candidate.
Peer Reflexive Candidate: A candidate whose IP address and port are a binding allocated by a NAT for an agent when it sent a STUN Binding request through the NAT to its peer.
Relayed Candidate: A candidate obtained by sending a TURN Allocate request from a host candidate to a TURN server. The relayed candidate is resident on the TURN server, and the TURN server relays packets back towards the agent.
Base: The base of a server reflexive candidate is the host candidate from which it was derived. A host candidate is also said to have a base, equal to that candidate itself. Similarly, the base of a relayed candidate is that candidate itself.
Foundation: An arbitrary string that is the same for two candidates that have the same type, base IP address, protocol (UDP, TCP, etc.), and STUN or TURN server. If any of these are different, then the foundation will be different. Two candidate pairs with the same foundation pairs are likely to have similar network characteristics. Foundations are used in the frozen algorithm.
Local Candidate: A candidate that an agent has obtained and included in an offer or answer it sent.
Remote Candidate: A candidate that an agent received in an offer or answer from its peer.
Default Destination/Candidate: The default destination for a component of a media stream is the transport address that would be used by an agent that is not ICE aware. For the RTP component, the default IP address is in the c line of the SDP, and the port is in the m line. For the RTCP component, it is in the rtcp attribute when present, and when not present, the IP address is in the c line and 1 plus the port is in the m line. A default candidate for a component is one whose transport address matches the default destination for that component.
Candidate Pair: A pairing containing a local candidate and a remote candidate.
Check, Connectivity Check, STUN Check: A STUN Binding request transaction for the purposes of verifying connectivity. A check is sent from the local candidate to the remote candidate of a candidate pair.
Check List: An ordered set of candidate pairs that an agent will use to generate checks.
Ordinary Check: A connectivity check generated by an agent as a consequence of a timer that fires periodically, instructing it to send a check.
Triggered Check: A connectivity check generated as a consequence of the receipt of a connectivity check from the peer.
Valid List: An ordered set of candidate pairs for a media stream that have been validated by a successful STUN transaction.
Full: An ICE implementation that performs the complete set of functionality defined by this specification.
Lite: An ICE implementation that omits certain functions, implementing only as much as is necessary for a peer implementation that is full to gain the benefits of ICE. Lite implementations do not maintain any of the state machines and do not generate connectivity checks.
Controlling Agent: The ICE agent that is responsible for selecting the final choice of candidate pairs and signaling them through STUN and an updated offer, if needed. In any session, one agent is always controlling. The other is the controlled agent.
Controlled Agent: An ICE agent that waits for the controlling agent to select the final choice of candidate pairs.
Regular Nomination: The process of picking a valid candidate pair for media traffic by validating the pair with one STUN request, and then picking it by sending a second STUN request with a flag indicating its nomination.
Aggressive Nomination: The process of picking a valid candidate pair for media traffic by including a flag in every STUN request, such that the first one to produce a valid candidate pair is used for media.
Nominated: If a valid candidate pair has its nominated flag set, it means that it may be selected by ICE for sending and receiving media.
Selected Pair, Selected Candidate: The candidate pair selected by ICE for sending and receiving media is called the selected pair, and each of its candidates is called the selected candidate.