For interfaces with PPPoE encapsulation, you can configure interfaces to support the PPP Password Authentication Protocol (PAP). PAP is a simple protocol that authenticates a user to a network access server using a two-way handshake. Authentication is done only upon initial link establishment. After the link is established, an ID and password pair is repeatedly sent by the peer to the authenticator until authentication is acknowledged or the connection is terminated.
For more information, see the JUNOS System Basics Configuration Guide