Understanding the network.pdf - Back to Home

PPP
PPP can provide multiprotocol data-link transmissions over any full-duplex (and
some half-duplex) asynchronous or synchronous link. Along with standard PSTN
DSO (and higher) and SONET transports, PPP can also operate over ISDN, frame
relay, and ATM. PPP is largely a creation of the Internet, much like TCP/IP, and is
modeled on SLIP and the ISO's HDLC. The protocol itself is described in more than
40 RFCs that define PPP's general operation, LCP operation, authentication
mechanisms, NCP implementations, data compression, and encryption schemes.
Everything you want or need to know about PPP is in the RFCs. The following is
intended merely as an overview to PPP's operation.
In addition to providing data-link framing, PPP supports two protocol-level
authentication schemes: Challenge Handshake Authentication Protocol (CHAP) and
Password Authentication Protocol (PAP). CHAP is a three-way authentication
method that utilizes a shared secret between two hosts. Throughout the duration of
the connection, the two hosts send periodic CHAP authentication requests that
require the responding host to send its local identifier and shared password used by
the two communicating hosts. PAP uses a standard username and password
authentication scheme for authentication when the connection is first established.
Along with transmission framing and authentication services, PPP also provides the
following:
• Link quality monitoring and error detection—PPP employs a verbose Link
Control Protocol (LCP) that establishes and tests the link configuration,
provides link quality monitoring, data compression negotiation, error
detection, and link state reporting.
• Network layer address negotiation—PPP provides a network control protocol
to manage the various facilities needed to operate the Layer 3 protocols it
supports (IP, AppleTalk, IPX, DecNet, and so on). Among the supported
services are network address configuration, multiprotocol traffic
multiplexing, and TCP header compression.
• Multilink operation—This is a mode of operation that uses a collection of
ISDN B-channels or DS1 circuits as a single -high-speed logical transmission
path. Packets are bit-striped across the channels, ensuring that the
bandwidth is evenly utilized. After the connection is established, channels
can be added or removed depending on bandwidth requirements. Dynamic
allocation is not a function of PPP. It is handled through the use of the
Bandwidth Allocation Control Protocol (BACP).
PPP link negotiation is a rather complex process. It uses a primary/secondary link
establishment model similar to the one used by HDLC. One station negotiates the
link establishment. There are three phases of PPP link establishment: