Direct serial connections between two nodes (like dial-up modems) need a protocol that can establish the link, authenticate users, and carry multiple network-layer protocols.
A data link layer protocol suite for direct connections between two nodes, providing link establishment, authentication, and multiprotocol support.
- LCP (Link Control Protocol): Establishes, configures, and tests the link; negotiates options like MRU and authentication method
- Authentication: Optional PAP or CHAP to verify user identity
- NCP (Network Control Protocol): Configures network-layer protocols (e.g., IPCP for IP addresses)
- Data Encapsulation: Frames data with header (flag, address, control) and trailer (FCS for error detection)
- Termination: Gracefully closes the connection when done
- Works over serial links (dial-up, serial cables)
- Supports authentication (PAP, CHAP)
- Multiprotocol: can carry IP, IPv6, IPX via different NCPs
- Includes error detection (FCS) but not correction
- Built from: Data Link Layer — operates at this layer
- Built from: LCP — link control component
- Built from: NCP — network control component
- Related: PPPoE — PPP over Ethernet
- Related: Authentication — PAP and CHAP mechanisms
- Mostly replaced by PPPoE for modern broadband (DSL, fiber)
- No error correction — only detection via FCS
- MRU negotiation can fail if peers disagree on maximum size