Notes on PPP

On a Local Area Network (LAN) Internet Protocol (IP) packets are transmitted using the media access protocol for that LAN. The Point to Point Protocol (PPP) is a Data Link layer protocol that supports the Internet Protocol (and other protocols) over a point to point connections. Most commonly PPP is used to provide Internet Protocol packet delivery over dialup lines.

PPP provides several services:

The Link Control Protocol (LCP) is part of PPP used to control the link. There are 11 LCP packet types that can be used to establish, configure, test or terminate a connection.

The Network Control Protocol (NCP) is used to configure the protocol operating at the network level. One of the functions of NCP is to dynamically assign an IP address to the host that is connecting.

The frame format for PPP is very similar to the High-level Data Link Control (HDLC) protocol.  PPP adds a short header and a trailer to the IP packet.

ppppkt.gif (2730 bytes)

PPP is a Data Link layer protocol.  When PPP is run over a phone line, the physical layer is usually RS-232.  While the data link layer considers a byte to be 8 bits, the physical layer is also sending a start bit, parity bit and stop bit to make 11 bits per byte.

An overview of the PPP operation can be described as a Finite State Diagram.

PPPstates.gif (4061 bytes)

PPP is an Internet standard. It is described in detail in RFC1661 and further refined in RFC1662 and RFC1663. These Request For Comments are created by the Internet Engineering Task Force and can be found at


last updated on January 02, 2004