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):

• RFC 1661, "The Point-to-Point Protocol, LCP."
• RFC 1332, "The PPP Internet Protocol Control Protocol (IPCP)."
• RFC 1662, "PPP in HDLC-like Framing."
• RFC 1334, "PPP Authentication Protocols."
• RFC 1990, "PPP Multilink"

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:

• Add a Link Configuration
• Add a Server Interface (if you are setting up the machine as a PPP server)
• Add a Demand Interface (if you want the machine to support demand connections)
• Manipulate PAP or CHAP Users/Passwords (if you want the machine to support PPP authentication)
• Start PPP to effect your changes (or Stop then Start PPP, if PPP is currently running)

Notes:

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 and SNMP

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:

• retrieval of PPP Link Configuration information (Maximum Receive Unit size, Asynchronous Character Mapping, etc.)
• setting of PPP Link Configuration information
• retrieval of LCP interface information for active LCP links
• changing of the state of active LCP links can be changed to "down" by setting the appropriate ifAdminStatus Management Information Base (MIB) object

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:

• The pppLinkConfigMagicNumber object is read only. In AIX PPP, magic number negotiation is always performed and cannot be disabled.
• The pppLinkConfigFcsSize object is read only. AIX PPP will only support FCS sizes of 16.

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.

• If PPP is currently running,
  1. Stop PPP using the smit stopppp fast path (see the table in Configuring the Asynchronous Point-to-Point Protocol).

  2. Periodically check to see if the subsystem has completed shutdown by entering:

     lssrc -s pppcontrold

     The amount of time it takes to completely stop the subsystem is dependent on the number of links defined in the PPP configuration. The subsystem is completely shut down when the output of this command shows a status of inoperative .

  3. Start PPP using the smit startppp fast path (see the table in Configuring the Asynchronous Point-to-Point Protocol).

• If PPP is not currently running, start PPP using the smit startppp fast path (see the table in Configuring the Asynchronous Point-to-Point Protocol).