Remote Access Server

PPP Connection Process

There are four distinct phases of negotiation of a PPP connection. Each of these four phases must complete successfully before the PPP connection is ready to transfer user data. The four phases of a PPP connection are:

PPP configuration
Authentication
Callback
Protocol configuration

Phase 1: PPP Configuration

PPP configures PPP protocol parameters using LCP (Link Control Protocol). During the initial LCP phase, each peer negotiates communication options that are used to send data and include:

PPP parameters, such as MRU, address and control field compression, and protocol ID compression.
Which authentication protocols are used to authenticate the remote access client. An authentication protocol is selected but not implemented until the authentication phase.
Multilink options.

Phase 2: Authentication

After LCP is complete, the authentication protocol agreed upon by the remote access server and the remote access client is implemented. The nature of this traffic is specific to the PPP authentication protocol. For more details, see "PPP Authentication Protocols" later in this chapter.

Phase 3: Callback

The Microsoft implementation of PPP includes an optional callback phase using the Callback Control Protocol (CBCP) immediately after authentication. In order for a remote access client user to get called back, the dial-in properties of the user account must be enabled for callback and either the remote access client can specify the callback number or the remote access server must specify the callback number.

If a connection is implementing callback, both PPP peers hang up and the remote access server calls the remote access client at the negotiated number.

Phase 4: Protocol Configuration

Once the PPP is configured and callback is complete (optional), network layer protocols can be configured. With remote access on Windows 32-bit operating systems, the remote access server sends the remote access client Configuration-Request messages for all of the LAN protocols enabled for remote access on the remote access server. The remote access client either continues the negotiation of the LAN protocols enabled at the remote access client or sends an LCP Protocol-Reject message containing the Configuration-Request message.

IPCP, IPXCP, ATCP, and NBFCP each go through a negotiation process very similar to LCP negotiation to configure their corresponding network layer protocol. CCP packets are exchanged to configure MPPC and MPPE.