To facilitate a connection between a remote system and a remote access server, common protocols must be used between the systems. Two types of protocols are required to establish a remote connection. You first need to have the protocols that communicate at the data-link layer, including the following:
Point-to-Point Protocol (PPP) PPP is actually a family of protocols that work together to provide connection services. PPP enables remote clients and servers to negotiate authentication between devices. PPP can employ a variety of encryption methods to secure transmissions.
Serial Line Internet Protocol (SLIP) SLIP is an older connection protocol than PPP, and it was originally designed to enable data to be transmitted via Transmission Control Protocol/Internet Protocol (TCP/IP) over serial connections in a UNIX environment. Unfortunately, SLIP does not support encryption or authentication and therefore has largely fallen out of favor. If you have users that employ SLIP to connect from remote systems, you should move them to PPP connections as soon as possible.
Point-to-Point Protocol over Ethernet (PPPoE) PPPoE is a method of using PPP connections over Ethernet. Using PPPoE and a broadband connection such as xDSL or cable Internet access, it is possible for individual users to have authenticated access to high-speed data networks, which provides an efficient way to create a separate connection to a remote server for each user. This strategy allows Internet access and billing on a per-user basis rather than a per-site basis. Users accessing PPPoE connections require the same information as required with standard dial-up phone accounts, including a username and password combination. As with a dial-up PPP service, an Internet service provider (ISP) will most likely automatically assign configuration information such as the IP address, subnet mask, default gateway, and DNS server information.
After a data link has established the connection between the devices, LAN protocols are used. This includes TCP/IP, AppleTalk, and IPX/SPX.