IBM SC30-3681-08 manual Using PPP

Using PPP

1.Before exchanging any network layer datagrams, PPP ®rst opens the connection through an exchange of LCP con®guration 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 ªdownº and incapable of carrying any network traffic. If the negotiation is successful, LCP goes to an ªOpenº 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 identi®es itself to the other end using the ªauthentication protocolº that the other end dictated as part of the LCP negotiation.

If authentication fails, the link is marked ªdownº 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 ªOpenº state then the link is marked ªdownº and cannot carry any network traffic. If ECP successfully reaches ªOpenº 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 ªopenº or ªupº 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 ªupº for that protocol's traffic. As with LCP, con®guration information can be exchanged as part of this negotiation; for example, IPCP can exchange IP addresses or negotiate the use of ²Van Jacobson IP header compression².

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 con®guration option was unacceptable. If an NCP fails to reach the ªOpenº 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.

452MRS V3.2 Software User's Guide