Alcatel Omni Switch/Router, User Manual

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30 Point-to-Point Protocol
The Point-to-Point Protocol (PPP) provides a standard method for transporting multi-protocol
datagrams over point-to-point links. The base protocol is specified in RFC 1661. Many other
RFCs define additional capabilities for network protocol negotiation, management information
databases (MIBs), and PPP operation over different kinds of serial channels.
PPP is comprised of three main components. The first component is a method of
encapsulating multi-protocol datagrams so that the underlying protocol can be identified; the
second component is the Link Control Protocol (LCP) that is used for establishing,
configuring, and testing the datalink connection; the third component is a family of Network
Control Protocols (NCPs) that are used for establishing and configuring different network-
layer protocols such as IP and IPX.
The implementation of PPP for the Omni Switch/Router WAN Switching Modules supports
bridging, IP routing and IPX routing. Data compression of the PPP packets is also supported
when the WSX module contains a STAC 9705 Data Compression Coprocessor.
PPP Connection Phases
There are five phases to a PPP connection: Dead, Establish, Authenticate, Network, and
Terminate:
Dead.
The first phase is called the “Dead” phase because the physical channel has not yet
been activated.
Establish.
After the physical channel has been activated, the PPP connection enters the second
phase, called “Establish,” wherein it attempts to negotiate link-level parameters and options
using the Link Control Protocol (LCP). This phase ends when the LCP enters its own “open”
state.
Authenticate.
After LCP has reached its “open” state, the PPP connection enters the phase
called “Authenticate” wherein it tries to identify the peer with which it is attempting to
establish a connection. If the authentication option is enabled, either the Password
Authentication Protocol (PAP) or the Challenge Handshake Authentication Protocol (CHAP) is
used to perform the authentication. If authentication is not enabled, the PPP connection
proceeds to the next phase, “Network.”
Network.
After the “Authenticate” phase is successful (or when it is not enabled), the PPP
connection proceeds to the next phase, called “Network,” wherein the network protocols are
negotiated using the appropriate Network Control Protocol (NCP). For example, to negotiate
the use of IP over the PPP connection, the Internet Protocol Control Protocol (IPCP) is used.
The details of the negotiation are specific to each network protocol, but may include such
tasks as assigning network layer addresses. A network layer protocol must be negotiated
successfully before the exchange of protocol packets can proceed; but, once negotiated, the
protocol can begin to freely exchange packets. The PPP connection spends most of its time in
the “Network” phase, because this is where the active transmission of data occurs.
Terminate
. The final phase of a PPP connection is called the “Terminate” phase. This phase
begins when authentication is unsuccessful or the channel becomes inoperative. Very often,
this phase is simply bypassed, and PPP will return to the idle (Dead) phase when a channel is
disconnected.