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PPP Components and General Operation
(Page 1 of 2)
PPP stands for the Point-to-Point
Protocol, but even the standard that defines PPP immediately
starts describing protocols that comprise it, which is why I consider
it a protocol suite. At the highest level, the functions of PPP can
be broken down into several components. Each of these encompasses a
general class of PPP functionality, and is represented by either one
protocol in the suite or a set of protocols.
Main PPP Components
The PPP standard itself describes
three main components of PPP:
- PPP Encapsulation Method: The primary
job of PPP is to take higher-layer messages such as IP datagrams and
encapsulate them for transmission over the underlying physical layer
link. To this end, PPP defines a special frame format for encapsulating
data for transmission, based on the framing used in the HDLC protocol.
The PPP frame has been specially designed to be small in size and contain
only simple fields, to maximize bandwidth efficiency and speed in processing.
- Link Control Protocol (LCP): The PPP Link
Control Protocol (LCP) is responsible
for setting up, maintaining and terminating the link between devices.
It is a flexible, extensible protocol that allows many configuration
parameters to be exchanged to ensure that both devices agree on how
the link will be used.
- Network Control Protocols (NCPs): PPP
supports the encapsulation of many different layer three datagram types.
Some of these require additional setup before the link can be activated.
After the general link setup is completed with LCP, control is passed
to the PPP Network
Control Protocol (NCP) specific to the
layer three protocol being carried on the PPP link. For example, when
IP is carried over PPP the NCP used is the PPP Internet Protocol Control
Protocol (IPCP). Other NCPs are defined for supporting the IPX protocol,
the NetBIOS Frames (NBF) protocol, and so forth.
The PPP encapsulation method and
LCP are defined in the main PPP standard and some support standards;
the NCPs are described in separate standard documents, one per NCP.
Additional PPP Functional Groups
While the three components above
do constitute much of the total package of PPP, I would add to the list
of components in the standard two additional functional groups. These
represent some of the many extra protocols that have been added over
time to the suite to support or enhance the basic operation of PPP:
- LCP Support Protocols: Several protocols
are included in the PPP suite that are used during the link negotiation
process, either to manage it or to configure options. Examples include
protocols CHAP and PAP, which are used
by LCP during the optional authentication phase.
- LCP Optional Feature Protocols: A number
of protocols have been added to the basic PPP suite over the years to
enhance its operation after a link has been set up and datagrams are
being passed between devices. For example, the PPP
Compression Control Protocol (CCP) allows
compression of PPP data, the PPP
Encryption Control Protocol (ECP) enables
datagrams to be encrypted for security, and the PPP
Multilink Protocol (PPP MP) allows a single
PPP link to be operated over multiple physical links. The use of these
features often also requires additional setup during link negotiation,
so several define extensions (such as extra configuration options) that
are negotiated as part of LCP.
Each of these additional protocols
is generally defined by a different standards document. You can find
a list of some of these in the topic on PPP standards.
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Version 3.0 - Version Date: September 20, 2005
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