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The Point-to-Point
Protocol (PPP) originally emerged as an encapsulation
protocol for transporting IP traffic over point-to-point links. PPP also
established a standard for the 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 supports these functions by providing an extensible Link
Control Protocol (LCP) and a family of Network
Control Protocols (NCPs)
to negotiate optional configuration parameters and facilities. In
addition to IP, PPP supports other protocols, including Novell's
Internetwork Packet Exchange (IPX) and DECnet. This chapter provides a
summary of PPP's basic protocol elements and operations.
PPP provides a method for transmitting
datagrams over serial point-to-point links. PPP contains three main
components:
To establish communications over a
point-to-point link, the originating PPP 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. When each of the chosen
network-layer protocols has been configured, packets from each
network-layer protocol can be sent over the link. The link will remain
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 is capable of
operating across any DTE/DCE
interface. Examples include EIA/TIA-232-C (formerly
RS-232-C), EIA/TIA-422 (formerly RS-422), EIA/TIA-423
(formerly RS-423),) and International Telecommunication Union
Telecommunication Standardization Sector (ITU-T)
(formerly CCITT) 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.
PPP uses the principles, terminology, and
frame structure of the International Organization for
Standardization (ISO) HDLC procedures (ISO 3309-1979),
as modified by ISO 3309:1984/PDAD1
"Addendum 1: Start/stop transmission." ISO 3309-1979 specifies
the HDLC frame structure for use in synchronous environments. ISO
3309:1984/PDAD1 specifies proposed modifications to ISO 3309-1979 to
allow its use in asynchronous environments. The PPP control procedures
use the definitions and control field encodings standardized in
ISO 4335-1979/Addendum 1-1979. The PPP frame format appears in Figure
13-1 .
Figure 13-1: Six fields make up the PPP frame.
The following descriptions summarize the
PPP frame fields illustrated in Figure
13-1 :
The LCP can negotiate modifications to
the standard PPP frame structure. Modified frames, however, always
will be clearly
distinguishable from standard frames.
The PPP LCP
provides a method of establishing, configuring, maintaining, and
terminating the point-to-point connection. LCP goes through four
distinct phases:
- First, link establishment and
configuration negotiation occurs. Before any network-layer datagrams
(for example, IP) can be exchanged, LCP first must open the
connection and negotiate configuration parameters. This phase is
complete when a configuration-acknowledgment frame has been both
sent and received.
- This is followed by link-quality
determination. LCP allows an optional link-quality determination
phase following the link-establishment and configuration-negotiation
phase. In this phase, the link is tested to determine whether the
link quality is sufficient to bring up network-layer protocols. This
phase is optional. LCP can delay transmission of network-layer
protocol information until this phase is complete.
- At this point, network-layer protocol
configuration negotiation occurs. After LCP has finished the
link-quality determination phase, network-layer protocols can be
configured separately by the appropriate NCP and can be brought up
and taken down at any time. If LCP closes the link, it informs the
network-layer protocols so that they can take appropriate action.
- Finally, link termination occurs. LCP
can terminate the link at any time. This usually will be done at the
request of a user but can happen because of a physical event, such
as the loss of carrier or the expiration of an idle-period timer.
Three classes of LCP frames exist.
Link-establishment frames are used to establish and configure a link.
Link-termination frames are used to terminate a link, while
link-maintenance frames are used to manage and debug a link.
These frames are used to accomplish the
work of each of
the LCP phases.
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