User`s manual

SIP User's Manual 9. Networking Capabilities
Version 5.0 233 October 2006
9.4 Point-to-Point Protocol over Ethernet (PPPoE)
PPPoE is a method of sending the Point-to-Point Protocol over Ethernet network.
9.4.1 Point-to-Point Protocol (PPP) Overview
Point-to-Point Protocol (PPP) provides a method of transmitting data over serial point-to-
point links. The protocol defines establishing, configuring and testing the data link
connection and the network protocol.
The PPP standard describes a state machine used to establish a valid connection between
two hosts over a serial connection. There are three major stages described, helping to
establish a network layer (such as an IP) connection over the point-to-point link: LCP (Link
Configuration Protocol) Authentication and NCP (Network Control Protocol). Once the
network protocol is configured, the two hosts can communicate, sending network layer
protocol (such as IP) over the PPP connection (a small PPP header is added at the
beginning of each packet).
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
).