Contrary to [INTRO:1], the Internet does have a standard point to point line protocol: the Point-to-Point Protocol (PPP), defined in [LINK:2], [LINK:3], [LINK:4], and [LINK:5].
A point to point interface is any interface that is designed to send data over a point to point line. Such interfaces include telephone, leased, dedicated or direct lines (either 2 or 4 wire), and may use point to point channels or virtual circuits of multiplexed interfaces such as ISDN. They normally use a standardized modem or bit serial interface (such as RS-232, RS-449 or V.35), using either synchronous or asynchronous clocking. Multiplexed interfaces often have special physical interfaces.
A general purpose serial interface uses the same physical media as a point to point line, but supports the use of link layer networks as well as point to point connectivity. Link layer networks (such as X.25 or Frame Relay) use an alternative IP link layer specification. Routers that implement point to point or general purpose serial interfaces MUST IMPLEMENT PPP.
PPP MUST be supported on all general purpose serial interfaces on a router. The router MAY allow the line to be configured to use point to point line protocols other than PPP. Point to point interfaces SHOULD either default to using PPP when enabled or require configuration of the link layer protocol before being enabled. General purpose serial interfaces SHOULD require configuration of the link layer protocol before being enabled.