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 are expected to be familiar with [RFC5389] and the terms defined there.
The following terms are used in this document:
TURN: The protocol spoken between the TURN client and the TURN server. It is an extension to the STUN protocol [RFC5389]. The protocol allows a client to allocate and use a relayed transport address.
TURN client: A STUN client that implements this specification.
TURN server: A STUN server that implements this specification. It relays data between a TURN client and its peers.
Peer: A host with which the TURN client wishes to communicate. The TURN server relays traffic between the TURN client and its peers. The peer does not interact with the TURN server using the protocol defined in this document; rather, the peer receives data sent by the TURN server and the peer sends data towards the TURN server.
Transport Address: The combination of an IP address and port.
Host Transport Address: A transport address on a client or a peer.
Server-Reflexive Transport Address: A transport address on the "public side" of a NAT. This address is allocated by the NAT to correspond to a specific host transport address.
Relayed Transport Address: A transport address on the TURN server that is used to relay packets between the client and a peer. A peer sends data to this address on the TURN server, and the data is then relayed to the client.
TURN Server Transport Address: A transport address on the TURN server used to send TURN messages to the server. This is the transport address that the client uses to communicate with the server.
Peer Transport Address: The transport address of the peer as seen by the server. When the peer is behind a NAT, this is the server-reflexive transport address of the peer.
Allocation: A relayed transport address granted to a client through an Allocate request, along with related state, such as permissions and expiration timers.
5-tuple: The combination (client IP address and port, server IP address and port, and transport protocol (currently one of UDP, TCP, or TLS)) used to communicate between the client and the server. The 5-tuple uniquely identifies this communication stream. The 5-tuple also uniquely identifies the allocation on the server.
Channel: A channel number and an associated peer transport address. Once a channel number is bound to a peer's transport address, the client and server can use the more bandwidth-efficient ChannelData message to exchange data.
Permission: The IP address and transport protocol (but not the port) of a peer that is permitted to send traffic to the TURN server and have that traffic relayed to the TURN client. The TURN server will only forward traffic to its client from peers that match an existing permission.
Realm: A string used to describe the server or a context within the server. The realm tells the client which username and password combination to use to authenticate requests.
Nonce: A string that is randomly chosen by the server and included in the message digest. To prevent reply attacks, the server SHOULD change the nonce periodically.