Point-to-Point Protocol

Since IP packets cannot be transmitted over a modem line on their own without some data link protocol that can identify where the transmitted frame starts and where it ends, Internet service providers (ISPs) have used PPP for customer dial-up access to the Internet.

[3] Two derivatives of PPP, Point-to-Point Protocol over Ethernet (PPPoE) and Point-to-Point Protocol over ATM (PPPoA), are used most commonly by ISPs to establish a digital subscriber line (DSL) Internet service LP connection with customers.

The people who had designed PPP included many additional features that had been seen only in proprietary data-link protocols up to that time.

LCP provides automatic configuration of the interfaces at each end (such as setting datagram size, escaped characters, and magic numbers) and for selecting optional authentication.

RFC 1994 describes Challenge-Handshake Authentication Protocol (CHAP), which is preferred for establishing dial-up connections with ISPs.

PPP permits multiple network layer protocols to operate on the same communication link.

It negotiates network-layer information, e.g. network address or compression options, after the connection has been established.

NCPs include fields containing standardized codes to indicate the network layer protocol type that the PPP connection encapsulates.

The previous section introduced the use of LCP options to meet specific WAN connection requirements.

The Protocol field indicates the type of payload packet: 0xC021 for LCP, 0x80xy for various NCPs, 0x0021 for IP, 0x0029 AppleTalk, 0x002B for IPX, 0x003D for Multilink, 0x003F for NetBIOS, 0x00FD for MPPC and MPPE, etc.

PPP on serial links is usually encapsulated in a framing similar to HDLC, described by IETF RFC 1662.

The FCS is calculated over the Address, Control, Protocol, Information and Padding fields after the message has been encapsulated.

Some of them, like SSL, SSH, or L2TP create virtual network interfaces and give the impression of direct physical connections between the tunnel endpoints.

RFC 1547 (Requirements for an Internet Standard Point-to-Point Protocol, December 1993) provides historical information about the need for PPP and its development.

A series of related RFCs have been written to define how a variety of network control protocols, including TCP/IP, DECnet, AppleTalk, IPX, work with PPP; they can be found on the Datatracker IETF website.