Linux Network Administrator's Guide, 3rd Edition

Chapter 6. The Point-to-Point Protocol

Point-to-point protocol (PPP) is a protocol used to send datagrams across a serial connection. In this chapter, we briefly cover its basic building blocks. We will also cover PPP over Ethernet (PPPoE), which is now commonly used by telecom providers to establish DSL sessions. There is also a comprehensive O’Reilly book on the topic, Using & Managing PPP, by Andrew Sun.

At the very bottom of PPP is the High-Level Data Link Control (HDLC) protocol, which defines the boundaries around the individual PPP frames and provides a 16-bit checksum.^[1] A PPP frame is capable of holding packets from protocols other than IP, such as Novell’s IPX or Appletalk. PPP achieves this by adding a protocol field to the basic HDLC frame that identifies the type of packet carried by the frame.

The Link Control Protocol (LCP) is used on top of HDLC to negotiate options pertaining to the data link. For instance, the Maximum Receive Unit (MRU) states the maximum datagram size that one side of the link agrees to receive.

An important step at the configuration stage of a PPP link is client authorization. Although it is not mandatory, it is really a must for dial-up lines in order to keep out intruders. Usually the called host (the server) asks the client to authorize itself by proving it knows some secret key. If the caller fails to produce the correct secret, the connection is terminated. With PPP, authorization works both ways; the caller may also ask the ...