3Com VCX V7111, User Manual, Page: 287 / 464

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VCX V7111 VoIP Gateway User Guide
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At the initial phase, the hosts use LCP (link configuration protocol) to negotiate for link
characteristic and parameters. Packets sent in this phase have two octets of PPP header
followed by LCP message with variable length. Various parameters and options are
negotiable at this phase, including MRU (maximum receive unit), Authentication Protocol,
and others.
Once the link is established (each side sends a configure ack message to the other side), the
authentication phase may begin. The authentication phase is not mandatory. However, it is
negotiated in the link configuration phase. A host may ask other hosts for authentication
using Password Authentication Protocol (PAP) or Challenge Handshake Authentication
Protocol (CHAP).
The PAP sends the username and password to the remote host unencrypted.
The CHAP is a more sophisticated method of authentication. The two hosts share a secret.
The authenticator sends a challenge to the host requesting authentication. The host
performs a calculation (one-way hash) using the challenge received from the authenticator
and the shared secret, and sends the result to the authenticator. The authenticator verifies
the host if the result of the calculation is correct; otherwise it is rejected.
The last configuration phase, immediately after the authentication phase (or after the Link
Configuration) is the Network Control Protocol. There is a family of control protocols for
establishing and configuring different network-layer protocols, for example, IPCP (PPP
Internet Protocol Control Protocol), IPv6CP (PPP IP v6 Control Protocol), and BCP (PPP
Bridging Control Protocol). Each of them handles and manages the specific needs required
by their respective network-layer protocol.
When working in an IP network, IPCP is used as the Network Configuration Protocol. The
IPCP is used to configure the network layer of the hosts, requesting/declaring on IP
Addresses.
Further information on PPP Protocol is available on the IETF website
(
http://www.ietf.org/rfc/rfc1661.txt ). Further information on Password Authentication Protocol
is available on the IETF website ( http://www.ietf.org/rfc/rfc1334.txt ). Further information on
Challenge Handshake Authentication Protocol is available on the IETF website
(
http://www.ietf.org/rfc/rfc1994.txt ). Further information on PPP Internet Protocol Control
Protocol (IPCP) is available on the IETF website ( http://www.ietf.org/rfc/rfc1332.txt ).
PPPoE Overview
PPPoE is a method of sending the Point-to-Point Protocol over Ethernet network. PPPoE
provides the ability to connect a network of hosts over a simple bridging access device to a
remote Access Concentrator. Access control, billing and type of service can be done on a
per-user, rather than a per-site, basis.
A common use of the PPPoE is in the ADSL market: The home PC is connected to a modem
using Ethernet, and the PC uses the PPPoE to simulate as if it was directly connected to the
remote host on a point-to-point connection.
Since PPPoE frames are sent over Ethernet, each PPP session must learn the Ethernet
address of the remote peer, as well as establish a unique session identifier. The PPPoE