PPP, Point-to-Point Protocol, is a data-link protocol created to improve on SLIP for direct links between two endpoints. PPP was standardized through the Internet Engineering Task Force (IETF) and became a common way to carry IP and other network-layer protocols over serial, dial-up, ISDN, leased-line, and later broadband access links.
PPP mattered because it did much more than simply move IP packets across a line. It defined framing, link negotiation, protocol identification, error detection, authentication options, and network-layer configuration. In the dial-up Internet era, PPP was the normal way a modem user became an IP host. In service-provider networks, PPP later survived through PPP over Ethernet, or PPPoE, which remains visible in many DSL, fiber, and broadband access deployments.
The 2002 Embedded Networking Context
The original article described NEC's V850 NetChip modules, jointly developed with Innovada, as embedded networking solutions for equipment makers that wanted to connect products to the Internet through Ethernet LAN or modem over the public switched telephone network. The modules targeted utility meters, motor control, building automation, fire and security systems, heating and air conditioning, and vending machines.
That product story shows why PPP was useful outside ordinary desktop dial-up. Embedded devices often needed a simple serial control interface, a modem option, authentication, and enough TCP/IP support to send data or email without forcing the device manufacturer to build an entire communications stack. PPP provided the dial-up link layer underneath protocols such as IP, ICMP, TCP, UDP, SMTP, POP3, Telnet, and HTTP.
What PPP Provides
RFC 1661 describes PPP as a standard method for transporting multiprotocol datagrams over point-to-point links. PPP has three main pieces:
- Encapsulation: framing for carrying network-layer protocol data over the link, including a protocol field that identifies the payload.
- Link Control Protocol: LCP establishes, configures, tests, and terminates the data-link connection.
- Network Control Protocols: NCPs configure specific network-layer protocols, such as IPCP for IPv4 and IPv6CP for IPv6.
This structure made PPP flexible. The same link mechanism could support IPv4, IPv6, IPX, AppleTalk, compression, authentication, multilink operation, and other options depending on what both endpoints negotiated.
LCP And Link Negotiation
LCP is the part of PPP that brings the link into a usable state. It negotiates options, detects configuration mismatches, and can close the link cleanly. LCP options can include maximum receive unit, authentication protocol, quality monitoring, magic numbers for looped-link detection, and other link behaviors.
That negotiation was important because PPP ran over many kinds of point-to-point media: asynchronous modem links, synchronous serial circuits, ISDN, leased lines, and later Ethernet access through PPPoE. The endpoints needed a common way to agree on how the link should behave before IP packets started moving.
IPCP, IPv6CP, And Addressing
For IPv4, PPP uses the Internet Protocol Control Protocol, or IPCP, defined in RFC 1332. IPCP can negotiate IP-layer options, including address assignment behavior. For IPv6, IPv6 Control Protocol, defined in RFC 5072, enables IPv6 packets to be transported over PPP links.
This is why PPP became so useful for dial-up Internet service. A user did not need to manually configure every link detail. The access server could authenticate the user, negotiate link options, assign or confirm an IP address, and then route traffic.
Authentication: PAP And CHAP
PPP can negotiate authentication before network-layer traffic is allowed. The two classic mechanisms are PAP and CHAP:
- PAP: Password Authentication Protocol is simple but weak because credentials are sent in a form that should not be relied on for modern untrusted environments.
- CHAP: Challenge Handshake Authentication Protocol, defined in RFC 1994, uses a challenge-response exchange so the password itself is not sent over the link in the same direct way.
PPP authentication is not a substitute for modern encryption. If sensitive traffic crosses an untrusted path, use appropriate higher-layer security such as TLS, IPsec, or another current protection mechanism.
PPPoE
PPP over Ethernet, defined in RFC 2516, encapsulates PPP frames inside Ethernet. It was widely adopted by broadband providers because it let them keep PPP-style subscriber sessions, authentication, accounting, and service control while using Ethernet access networks. PPPoE also gave providers a familiar operational model for DSL aggregation and later some fiber access networks.
The main operational caution is overhead. PPPoE adds headers, which commonly reduces the effective IP MTU unless the network supports larger frames end to end. Mismanaged MTU and MSS behavior can cause hard-to-diagnose web, VPN, and application problems. Many broadband troubleshooting stories that look like "the Internet is slow" are really path MTU or fragmentation issues around PPPoE.
PPP Versus SLIP
SLIP was simple and useful for early IP over serial links, but it lacked many features that made PPP operationally better. PPP added link negotiation, authentication, error detection, protocol multiplexing, and network-layer configuration. That is why PPP displaced SLIP for most dial-up and serial IP access.
The comparison remains instructive. A protocol that is easy to implement can still become painful to operate if it lacks negotiation, management, error handling, and security hooks. PPP became popular because it solved those operational problems while staying small enough for modems, routers, and embedded systems.
Where PPP Still Appears
PPP is no longer the everyday user-visible protocol it was during dial-up Internet access, but it has not vanished. It still appears in:
- PPPoE broadband: subscriber authentication and session control in DSL, fiber, and wholesale access networks.
- Serial and leased links: routers, industrial systems, lab networks, satellite links, and specialized point-to-point circuits.
- Embedded devices: modem-connected field equipment, telemetry systems, and constrained devices that need a simple link layer.
- Network labs and education: a clear example of data-link negotiation, authentication, and NCP behavior.
- Legacy remote access: older VPN, dial-in, or access-server environments that still use PPP concepts internally.
Operational Guidance
For networks that still use PPP or PPPoE, focus on the details that cause real outages:
- Document authentication method, username format, RADIUS integration, and accounting behavior.
- Check LCP and NCP negotiation logs before assuming an IP routing problem.
- Validate MTU and MSS behavior, especially with PPPoE, VPNs, tunnels, and IPv6.
- Use CHAP or stronger provider-supported methods rather than PAP where possible.
- Protect sensitive applications with modern encryption above PPP.
- Monitor session drops, authentication failures, address-assignment failures, and access concentrator capacity.
- For embedded systems, plan modem lifecycle, telephone-line retirement, cellular replacement, and remote recovery paths.
PPP's lasting value is architectural. It showed how a point-to-point link could become a negotiated, authenticated, multiprotocol network attachment. In 2026, most users never see PPP directly, but the protocol's design ideas still echo in broadband access, subscriber management, tunneling, embedded connectivity, and the operational need to bring links up cleanly before carrying IP traffic.