11.11 Exercises
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:
- A method for encapsulating multiprotocol datagrams.
AIX PPP supports the TCP/IP network layer protocols.
- A Link Control Protocol (LCP) for establishing,
configuring, and testing the data-link connection. AIX PPP implements this
through the streams kernel extensions.
- 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 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 and
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.
The Asynchronous Point-to-Point Protocol on AIX utilizes three user-level
processes:
- 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.
- 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.
- 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.
12.1 Configuring
Asynchronous PPP Server