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.
