SLIP vs PPP: Comparing Dial-Up Internet Protocols

Both SLIP (Serial Line Internet Protocol) and PPP (Point-to-Point Protocol) are common TCP/IP modem protocols used for establishing dial-up internet connections.

Initially, SLIP was introduced, but it had limitations that led to its replacement by PPP. Today, PPP is the more prevalent protocol for establishing internet connections.

SLIP is described in RFC 1055, while PPP is described in RFC 1661.

SLIP

The primary function of SLIP is to transmit IP datagrams across a modem line.

The SLIP data format consists of the data itself, followed by a special END character (decimal 192), which marks the end of the data. To prevent false stops when the data itself contains the END character, SLIP adds an ECS (Escape Character) before the actual END character. This allows the receiver to correctly receive the complete message or data.

RFC 1055 specifies a maximum packet size of 1006 bytes.

Characteristics and Deficiencies of SLIP

• Both the sending and receiving computers must know each other’s IP addresses to communicate. SLIP does not support DHCP (Dynamic Host Configuration Protocol), which is used to assign dynamic IP addresses. Therefore, it’s not suitable for most ISPs (Internet Service Providers) offering dial-up internet services.

• SLIP lacks a type field, making it impossible to specify the protocol type. As a result, SLIP cannot be used for multiple protocol functionalities, a capability that PPP provides.

• SLIP cannot multiplex and demultiplex other protocol systems alongside TCP/IP. PPP supports this functionality.

• SLIP does not handle error correction; this must be managed by upper layers.

• SLIP does not support header compression, making it less than ideal for slow-speed internet connections.

PPP

PPP was designed to address the shortcomings of SLIP. PPP is divided into three fundamental functionalities:

• Method of Encapsulation: PPP encapsulates multi-protocol datagrams.

• LCP (Link Control Protocol): Used by PPP to establish, manage, and terminate dial-up connections.

• NCPs (Network Control Protocols): Used to support upper-layer protocols.

A PPP frame consists of a protocol field, followed by data and padding. The data part carries control packets or upper-layer datagrams.

The protocol field is 1 or 2 bytes long and identifies the protocol type of the enclosed packet. Possible types include LCP packets, NCP packets, IP packets, or OSI network layer protocol packets.