Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP)

Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP)

Two other standards vital to network communication are Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP). SLIP and PPP were designed to support dial-up access to networks based on the Internet protocols. SLIP is a simple protocol that functions at the Physical layer, whereas PPP is a considerably enhanced protocol that provides Physical layer and Data Link layer functionality. The relationship of both to the OSI model is shown in Figure 2.12.

Developed to provide dial-up TCP/IP connections, SLIP is an extremely rudimentary protocol that suffers from a lack of rigid standardization, which sometimes hinders different implementations from interoperating.

Windows NT supports both SLIP and PPP from the client end using the Dial-Up Networking application. On the server end, Windows NT RAS (Remote Access Service) supports PPP but doesn’t support SLIP. In other words, Windows NT can act as a PPP server but not as a SLIP server.

SLIP is most commonly used on older systems or for dial-up connections to the Internet via SLIP-server Internet hosts.

PPP was defined by the Internet Engineering Task Force (IETF) to improve on SLIP by providing the following features:

Security using password logon
Simultaneous support for multiple protocols on the same link
Dynamic IP addressing
Error control

Different PPP implementations might offer different levels of service and negotiate service levels when connections are made. Because of its versatility, interoperability, and additional features, PPP is presently surpassing SLIP as the most popular serial-line protocol.

Certain dial-up configurations cannot use SLIP for the following reasons:

SLIP supports the TCP/IP transport protocol only. PPP, however, supports TCP/IP, as well as a number of other protocols, such as NetBEUI, IPX, AppleTalk, and DECnet. In addition, PPP can support multiple protocols over the same link.
SLIP requires static IP addresses. Because SLIP requires static IP addresses, SLIP servers do not support Dynamic Host Configuration Protocol (DHCP), which assigns IP addresses dynamically. (DHCP enables clients to share IP addresses so that a relatively small number of IP addresses can serve a larger user base.) If the dial-up server uses DHCP to assign an IP address to the client, the dial-up connection won’t use SLIP.
SLIP does not support dynamic addressing through DHCP. Because of this, SLIP connections cannot dynamically assign a WINS or DNS server. (See Exercise 2.2 at the end of this chapter.)

Windows NT RAS (using PPP) offers a number of other interesting features, including the following:

PPP Multilink Protocol. Multilink enables a single connection to use several physical pathways (such as modems, ISDN lines, and X.25 cards). Utilizing multiple pathways for a single connection increases bandwidth and, therefore, performance.
NetBIOS Gateway. A RAS server can connect a client running the NetBEUI protocol with a TCP/IP or IPX network by serving as a NetBIOS gateway.
IPX or IP Router. A RAS server can act as a router for IPX/SPX and TCP/IP networks. (See Chapter 6 for more information on routers.)