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System Administration Guide, Volume 3

How the Components Work Together

This section describes how the components of PPP function for outbound and inbound connections.

Outbound Connections Scenario

Outbound communications begin when a user on one endpoint host initiates an activity involving the peer host on the other end of the PPP link. The following activities take place when a user types an rcp command to copy a file from a host on the other side of the link.

  1. rcp sends the data through the levels of the TCP/IP protocol stack.

  2. A virtual network interface (ipdn or ipdptpn) receives the data in the form of IP packets.

  3. The interface sends the aspppd link manager a connection request that initiates an outbound connection.

  4. The link manager then:

    1. Verifies that the connection request corresponds to a configured path in the /etc/asppp.cf configuration file.

    2. Consults the UUCP database files (/etc/uucp/Systems, /etc/uucp/Devices, and /etc/uucp/Dialers) for specific information about the modem and destination system.

    3. Places a phone call to the destination host or attaches to the appropriate hardwired serial line.

  5. The physical link to the peer host is established.

  6. The link manager configures and initiates PPP.

  7. The data-link layer is established, and the PPP modules on the peer host start communicating.

  8. The link manager enables IP over the link.

The link manager then monitors the connection until an event, such as an idle timeout, line disconnect, or error condition, occurs. When any of these events occurs, the link manager disconnects from the peer host and returns to the idle state.

Inbound Connections Scenario

The host initiating the inbound communication logs in, which invokes the /usr/sbin/aspppls login service. Then the following events occur:

  1. The login service connects to the link manager through the /tmp/.asppp.fifo file.

  2. The login service provides the link manager with information such as the login name used by the endpoint at the other end of the link.

  3. The link manager uses this login name to find a corresponding configured path in the configuration file.

  4. The link manager then configures and initiates PPP.

  5. The data-link layer is established, and the PPP modules on the peer hosts start communicating.

  6. The link manager enables IP over the link.

The link manager then monitors the connection until an event occurs such as an idle time out, line disconnect, or error condition. When any of these events occur, the link manager disconnects from the peer and returns to the idle state.