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AIX Version 4.3 System Management Guide: Communications and Networks

Asynchronous Point-to-Point Protocol (PPP) Subsystem

The Asynchronous Point-to-Point Protocol (PPP) subsystem provides an alternative to SLIP. PPP provides a standard method for transporting multiprotocol datagrams over point-to-point media. PPP is comprised of three main layers:

  1. A method for encapsulating multiprotocol datagrams. AIX PPP supports the TCP/IP network layer protocols.
  2. A Link Control Protocol (LCP) for establishing, configuring, and testing the data-link connection. AIX PPP implements this through streams kernel extensions.
  3. A family of Network Control Protocols (NCPs) for establishing and configuring different network layer protocols. AIX PPP supports Internet Protocol Control Protocol (IPCP) for negotiating a TCP/IP connection.

AIX PPP supports the following Request for Comments (RFCs):

AIX PPP differentiates between client and server. An AIX system can act as both a client and a server. The distinction is made to simplify configuration. PPP servers tend to allocate a pool of IP addresses among the connections that are being made. There is some correlation between the media devices. AIX PPP breaks this correlation. All server PPP connections are allocated on a first-available basis. This facilitates the separation of PPP from the media. The attachment process must request to be linked to the proper type of link.

User-Level Processes

The Asynchronous Point-to-Point Protocol on AIX utilizes three user-level processes:

  1. A control daemon (pppcontrold) executed by root under the System Resource Controller (startsrc -s pppcontrold). The control daemon's function encompasses loading and configuring all kernel extensions associated with the subsystem. It remains running as long as PPP function is required by the operating system.
  2. An attachment process (pppattachd) that binds a TTY stream to an instance of the Link Control Protocol, Network Control Protocol, and a datagram protocol. An instance of pppattachd exists for each active PPP connection in the system. Any user of the attachment process must belong to the uucp group and contain /usr/sbin within their PATH environment variable.
  3. A dialer process (pppdial) that establishes an outgoing connection. The dialer is intended to be executed by pppattachd as the connector program. Its purpose is to interact over the asynchronous device prior to PPP negotiation. This interaction is defined similarly to the UUCP chat dialog format. The dialer capability is provided to assist in establishing a connection with a remote system. The actual session establishment is out of the scope of PPP.

Configuring the Asynchronous Point-to-Point Protocol

You can use Web-based System Manager or SMIT to configure the Asynchronous Point-to-Point Protocol. The following table shows all tasks that you may need when configuring your system. You must have root privileges to perform the tasks in this table.

At a minimum, when you initially configure your system, you will choose the following tasks from the table:

Configuring the Asynchronous PPP Tasks
Web-based System Manager:    wsm network fast path
(Network application)

Task SMIT Fast Path
Create Link Control Configuration smit ppplcp
   Add a Link Configuration smit addlcp
   Change/Show a Link Configuration smit chglcp
   Remove a Link Configuration1 smit rmlcp
Create PPP IP Interfaces smit pppip
   Add a Server Interface smit addpppserver
   Change/Show a Server Interface smit listserver
   Remove a Server Interface1 smit rmlistserver
   Add a Demand Interface smit addpppdemand
   Change/Show a Demand Interface smit listdemand
   Remove a Demand Interface1 smit rmlistdemand
Manipulate PAP users/passwords smit ppppap
   Add a PAP User smit addpapuser
   Change/Show a PAP User smit listpapuser
   Remove a PAP User smit rmpapuser
Manipulate CHAP users/passwords smit pppchap
   Add a CHAP User smit addchapuser
   Change/Show a CHAP User smit listchapuser
   Remove a CHAP User smit rmchapuser
Start PPP2 smit startppp
Stop PPP3 smit stopppp
  1. Selecting this task destroys the existing information.
  2. An alternative way to start PPP is to issue the startsrc -s pppcontrold command. However, the SMIT interface also allows you to set PPP to start at boot time.
  3. An alternative way to stop PPP is to issue the stopsrc -s pppcontrold command. However, the SMIT interface also allows you to have PPP not start at boot time.


PPP can interact with the TCP/IP SNMP daemon to report PPP link layer configuration information as well as information about active Link Control Protocol (LCP) interfaces. Providing that both the TCP/IP SNMP and the SNMP management software are configured correctly, PPP SNMP enables:

Not all objects defined by RFC1471 for the PPP MIB are supported. Only the pppLink table applies to the AIX PPP subsystem, thus the pppLqr and pppTests portions are not supported. The pppLink portion is supported with the following exceptions:

Enabling PPP SNMP

By default, SNMP for PPP is disabled. To enable PPP SNMP, you can use the Web-based System Manager fast path wsm network or use the following procedure. You must have root privileges to perform this procedure.

Note: The following procedure assumes that PPP Link Configuration is already set. If not, perform the procedure in "Configuring the Asynchronous Point-to-Point Protocol" before enabling PPP SNMP.
  1. Start the SMIT Interface and display the Change/Show a Link Configuration screen by entering:
    smit chglcp
  2. Toggle the Enable PPP SNMP subagent field to yes.

  3. Accept your changes and exit SMIT.

PPP SNMP will not be enabled, however, until PPP is restarted.

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