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1. Before exchanging any network layer datagrams, PPP first opens the
connection through an exchange of LCP configuration packets. As part of this
negotiation process, the PPP processes at each end of the link agree on
various basic link level parameters such as the maximum packet size that can
be transferred and whether the ends must use an authentication mechanism to
identify themselves to their peers before carrying network traffic.
If this negotiation is unsuccessful, the link is considered to be “down” and
incapable of carrying any network traffic. If the negotiation is successful, LCP
goes to an “Open” state and PPP goes on to the next phase.
2. After LCP successfully reaches an Open state, the next step in establishing the
link is to perform authentication where each end of the link identifies itself to the
other end using the “authentication protocol” that the other end dictated as part
of the LCP negotiation.
If authentication fails, the link is marked “down” and cannot carry any network
traffic. If authentication succeeds or if authentication is not required, the PPP
link moves to the next phase.
3. After authentication is negotiated, the peers negotiate encryption for the link.
After authentication phase is complete, the router negotiates the use of
encryption using Encryption Control Protocol (ECP) packets where each end of
the link negotiates which encryption algorithm will be used to encrypt the data
over this PPP link. If ECP did not reach “Open” state then the link is marked
“down” and cannot carry any network traffic. If ECP successfully reaches “Open”
state, or if encryption is not required, the PPP link moves to the next phase,
NCP negotiation (except ECP, which is technically also an NCP). The link is
considered to be “open” or “up” at this time, though it cannot yet carry layer-3
protocol datagrams.
4. Once the link is open, the router negotiates the use of various layer-3 protocols
(for example, IP, IPX, DECnet, Banyan Vines) using Network Control Protocol
(NCP) packets. Each layer-3 protocol has its own associated network control
protocol. For example IP has IPCP and IPX has IPXCP. The basic format and
mechanisms for all these NCP packets is the same for all protocols, and is
basically a superset of the LCP mechanisms as described later in this section.
Each layer-3 protocol is negotiated independently. When a particular NCP
successfully negotiates, the link is “up” for that protocol’s traffic. As with LCP,
configuration information can be exchanged as part of this negotiation; for
example, IPCP can exchange IP addresses or negotiate the use of ″Van
Jacobson IP header compression″.
As with LCP, it is possible for an NCP to fail to negotiate successfully with its
peer. This might happen because the peer does not support a particular
protocol or because some configuration option was unacceptable. If an NCP
fails to reach the “Open” state, no layer-3 protocol packets can be exchanged
for that protocol even though other layer-3 protocols are successfully passing
traffic across the PPP link.
5. Finally, LCP has the ability to terminate the link at any time. This is usually done
at the request of the user but may occur for other reasons such as: an
administrative closing of the link, idle timer expiration, or failure to
re-authenticate on a CHAP rechallenge.
For complete details about PPP LCP, authentication, and the general NCP
negotiation mechanisms, consult RFCs 1331, 1334, 1570, and 1661.
Using PPP
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MRS V3.2 Software User’s Guide