3.1 PPP Architecture
3.1.2 PPP functional components
PPP is a standard encapsulation protocol for the transport of different network-layer protocols (including, but not limited to, IP) across serial, point-to-point links.

PPP also describes mechanisms for the following:

  • Network-protocol multiplexing
  • Link configuration
  • Link-quality testing
  • Authentication
  • Header compression
  • Error detection
  • Link-option negotiation

PPP has the following three main functional components:

  • It has a method for encapsulating datagrams over serial links, based on the International Organization for Standardization (ISO) High-Level Data Link Control (HDLC) protocol (this is not Cisco HDLC), as shown in Figure .
  • LCP establishes, configures, authenticates, and tests the data-link connection.
  • NCPs establish and configure different network-layer protocols (such as IP, Internetwork Packet Exchange [IPX], and AppleTalk). For example, Internet Protocol Control Protocol (IPCP) is the NCP for IP.

HDLC is the basis of PPP frame format. Figure shows the difference between the two frame formats. RFC 1662 describes PPP in HDLC framing in detail. 

Related RFCs

The following is a partial list of RFCs of interest for access products:

  • RFC 1055-Nonstandard for Transmission of IP Datagrams over Serial Lines: SLIP
  • RFC 1144-Compressing TCP/IP Headers for Low-Speed Serial Links
  • RFC 1220-Point-to-Point Protocol Extensions for Bridging
  • RFC 1334-PPP Authentication Protocols
  • RFC 1378-PPP AppleTalk Control Protocol (ATCP)
  • RFC 1492-Access Control Protocol, sometimes called TACACS
  • RFC 1549-PPP in HDLC Framing
  • RFC 1552-PPP Internetworking Packet Exchange Control Protocol (IPXCP)
  • RFC 1570-PPP LCP Extensions
  • RFC 1661-Point-to-Point Protocol (PPP)
  • RFC 1662-PPP in HDLC-Like Framing (obsoletes RFC 1549)
  • RFC 1717-PPP Multilink Protocol (MLP)
  • RFC 1990-PPP Multilink Protocol (MLP) (obsoletes RFC 1717)