Serial Line Internet Protocol

Serial Line Internet Protocol
Name	Serial Line Internet Protocol
Developer	Rick Adams
Based on	Internet Protocol
Purpose	Point-to-Point Protocol encapsulation over serial links
Osi layer	Data link layer
Port	N/A
Rfc	1055

Serial Line Internet Protocol. It is a simple data link layer protocol used for transmitting Internet Protocol datagrams over serial point-to-point links, such as RS-232 connections or dial-up modem lines. Developed in the late 1980s, primarily by Rick Adams for the 4.2BSD and 4.3BSD operating systems, it was formally documented in RFC 1055 in 1988. It provided a crucial, early method for connecting individual computers or terminal servers to TCP/IP networks before being largely superseded by the more robust Point-to-Point Protocol.

Overview

The protocol was created to address the need for a simple encapsulation method to run Internet Protocol over low-speed, asynchronous serial lines common in the 1980s. It operates directly on top of a serial connection, framing raw IP packets for transmission between two endpoints, such as a personal computer and a network access server. Its design prioritized minimalism and ease of implementation over advanced features like error correction, multiplexing, or link configuration, making it a staple in early Unix systems and Internet service provider offerings. The protocol's primary use case was for dial-up Internet access, allowing home computers to establish a direct network layer connection to the burgeoning Internet.

Technical details

Functioning at the data link layer, it encapsulates Internet Protocol datagrams within its simple frame structure for transmission over a serial line. It uses a form of byte stuffing with special control characters to delineate frame boundaries, specifically employing the END character (0xC0) as a frame delimiter. The protocol itself provides no addressing mechanism, as it is designed strictly for point-to-point links, and it lacks any native authentication, compression, or error detection capabilities beyond what the underlying serial hardware might provide. Its operation is stateless and connectionless, simply passing IP packets between the serial interface and the network stack.

Frame format

A standard frame begins and ends with the END character (0xC0). To ensure this delimiter character does not appear in the encapsulated IP datagram's data field, the protocol employs a simple escape mechanism using the ESC character (0xDB). Within the data field, any occurrence of the END character is replaced by the two-byte sequence ESC-ESC_END (0xDB 0xDC), and any occurrence of the ESC character itself is replaced by ESC-ESC_ESC (0xDB 0xDD). This technique, known as character stuffing, ensures transparency but adds minimal overhead. The frame's payload consists solely of the raw Internet Protocol datagram, with no additional header fields for sequence numbers, checksums, or protocol identification.

Configuration and operation

Configuration typically involved setting the correct serial port parameters, such as baud rate, data bits, parity, and stop bits, on both connected devices, often a client computer and a terminal server or router. On Unix and Linux systems, a network interface such as `sl0` would be configured using tools like ifconfig to associate it with a TTY device like `/dev/ttyS0`. Once configured, the link would establish a direct network pathway, with the protocol driver handling the encapsulation and de-encapsulation of IP traffic. Operation was straightforward but manual, requiring matching settings on both ends and offering no negotiation for parameters like MTU.

Related protocols

Its direct successor and replacement is the Point-to-Point Protocol, defined in RFC 1661, which introduced essential features like Link Control Protocol negotiation, Network Control Protocol support for multiple protocols, and optional components for authentication (e.g., Password Authentication Protocol), compression, and error detection. Other contemporary or competing early serial protocols included Compressed SLIP, a variant that attempted rudimentary TCP and IP header compression, and proprietary systems from vendors like Cisco Systems. For different physical media, protocols like Ethernet using EtherType or the Point-to-Point Protocol over Ethernet served analogous framing roles.

Historical significance and legacy

It played a foundational role in the early commercialization and expansion of the Internet by enabling the first wave of dial-up Internet access for individuals and small businesses. Its implementation in major Berkeley Software Distribution releases and its support in early Microsoft Windows versions, including Windows 95, cemented its place in 1990s computing. However, its technical limitations—such as lack of authentication, error detection, and support for protocols other than IP—led the Internet Engineering Task Force to standardize the Point-to-Point Protocol as its official successor. Today, it is considered obsolete but is remembered as a key stepping stone in the evolution of Internet access technologies, preceding the widespread adoption of DSL, cable modem, and fiber-optic broadband.

Category:Internet protocols Category:Data link layer protocols Category:Obsolete Internet standards Category:Point-to-point protocols