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A limited number of point-to-point WAN
links were possible in the 1980s due to a lack of a standard
Internet encapsulation protocol. The Point-to-Point Protocol (PPP)
was designed to solve this problem. In addition to solving the
problem of standardized Internet encapsulation of IP over
point-to-point links, PPP was also designed to address the issues of
assignment and management of IP addresses, asynchronous (start/stop)
and bit-oriented synchronous encapsulation, network protocol
multiplexing, link configuration, link quality testing, error
detection, and option negotiation for such capabilities as
network-layer address negotiation and data compression negotiation.
PPP addresses these issues by providing an extensible Link Control
Protocol (LCP) and a family of Network Control Protocols (NCPs) to
negotiate optional configuration parameters and facilities. Today,
PPP supports other protocols besides IP, including IPX and DECnet.
PPP provides a method for
transmitting datagrams over serial point-to-point links. It has
three main components:
- A method for encapsulating
datagrams over serial links-PPP uses the High-Level Data Link
Control (HDLC) protocol as a basis for encapsulating datagrams
over point-to-point links.
- An extensible LCP to establish,
configure, and test the data-link connection.
- A family of NCPs for establishing
and configuring different network-layer protocols. PPP is
designed to allow the simultaneous use of multiple network-layer
protocols.
In order to establish communications
over a point-to-point link, the originating PPP station first sends
LCP frames to configure and (optionally) test the data link. After
the link has been established and optional facilities have been
negotiated as needed by the LCP, the originating PPP sends NCP
frames to choose and configure one or more network-layer protocols.
Once each of the chosen network-layer protocols has been configured,
packets from each network-layer protocol can be sent over the link.
The link remains configured for communications until explicit LCP or
NCP frames close the link or until some external event occurs (for
example, an inactivity timer expires or a user intervenes).
PPP can operate across any DTE/DCE
interface (for example, EIA/TIA-232, EIA/TIA-422, EIA/TIA-423, and
ITU-T V.35). The only absolute requirement imposed by PPP is the
provision of a duplex circuit, either dedicated or switched, that
can operate in either an asynchronous or synchronous bit-serial
mode, transparent to PPP link-layer frames. PPP does not impose any
restrictions regarding transmission rate, other than those imposed
by the particular DTE/DCE interface in use.
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