On some links it may be desirable to require a peer to authenticate itself before allowing network-layer protocol packets to be exchanged.
By default, authentication is not mandatory. If an implementation desires that the peer authenticate with some specific authentication protocol, then it MUST request the use of that authentication protocol during Link Establishment phase.
Authentication SHOULD take place as soon as possible after link establishment. However, link quality determination MAY occur concurrently. An implementation MUST NOT allow the exchange of link quality determination packets to delay authentication indefinitely.
Advancement from the Authentication phase to the Network-Layer Protocol phase MUST NOT occur until authentication has completed. If authentication fails, the authenticator SHOULD proceed instead to the Link Termination phase.
Only Link Control Protocol, authentication protocol, and link quality monitoring packets are allowed during this phase. All other packets received during this phase MUST be silently discarded.
The implementation responsible for commencing Link Termination phase is the implementation which has refused authentication to its peer.