PPP basics and the PPP protocol field

PPP provides several basic but important functions that are useful on a leased line that connects two devices, as reviewed in the following list:

* Definition of a header and trailer that allows delivery of a data frame over the link.

* Support for both synchronous and asynchronous links.

* A protocol type field in the header, allowing multiple Layer 3 protocols to pass over the same link.

* Built-in authentication tools: Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP).

* Control protocols for each higher-layer protocol that rides over PPP, allowing easier integration and support of those protocols.

The PPP Protocol Field

One of the most important features included in the PPP standard, but not in the HDLC standard, is the protocol field.  The protocol field identifies the type of packet inside the frame.  When PPP was created, this field allowed packets from many different Layer 3 protocols to pass over a single link.  Today, the protocol type field still provides the same function, even for the support of two different versions of IP (4 & 6).

PPP defines a set of Layer 2 control messages that perform various link control functions.  These control functions fall into two main categories:

* Those needed regardless of the Layer 3 protocol sent across the link.

* Those specific to each Layer 3 protocol.

The PPP Link Control Protocol (LPC) implements the control functions that work the same regardless of the Layer 3 protocol.  For features related to any higher-layer protocols, typically Layer 3 protocols, PPP uses a series of PPP control protocols (CP), such as IP Control Protocol (IPCP).  PPP uses one instance of LCP per link, and one CP for each Layer 3 protocol defined on the link.  On a PPP link using IPv4, IPv6, and Cisco Discovery Protocol (CDP), the link uses one instance of LCP, plus IPCP (for IPv4), IPv6CP, and CDPCP (CDP)