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.
[1] In fact, HDLC is a much more general protocol devised by the International Standards Organization (ISO) and is also an essential component of the X.25 specification.
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 server to authenticate itself. These authentication procedures are totally independent of each other. There are two protocols for different types of authorization, which we will discuss further in this chapter: Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP).
Each network protocol that is routed across the data link (like IP and AppleTalk) is configured dynamically using a corresponding Network Control Protocol (NCP). To send IP datagrams across the link, both sides running PPP must first negotiate which IP address each of them uses. The control protocol used for this negotiation is the Internet Protocol Control Protocol (IPCP).
Besides sending standard IP datagrams across the link, PPP also supports Van Jacobson header compression of IP datagrams. This technique shrinks the headers of TCP packets to as little as 3 bytes. It is more colloquially referred to as VJ header compression. The use of compression may be negotiated at startup time through IPCP, as well.