Like SLIP, PPP is a protocol used to send datagrams across a serial connection; however, it addresses a couple of the deficiencies of SLIP. First, it can carry a large number of protocols and is thus not limited to the IP protocol. It provides error detection on the link itself, while SLIP accepts and forwards corrupted datagrams as long as the corruption does not occur in the header. Equally important, it lets the communicating sides negotiate options, such as the IP address and the maximum datagram size at startup time, and provides client authorization. This built-in negotiation allows reliable automation of the connection establishment, while the authentication removes the need for the clumsy user login accounts that SLIP requires. For each of these capabilities, PPP has a separate protocol. In this chapter, we briefly cover these basic building blocks of PPP. This discussion of PPP is far from complete; if you want to know more about PPP, we urge you to read its RFC specification and the dozen or so companion RFCs. There is also a comprehensive O’Reilly book on the topic of 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. As opposed to the more primitive SLIP encapsulation, 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 dialup 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 three bytes. It is also used in CSLIP, and is more colloquially referred to as VJ header compression. The use of compression may be negotiated at startup time through IPCP, as well.
On Linux, PPP functionality is split into two parts: a kernel component that handles the low-level protocols (HDLC, IPCP, IPXCP, etc.) and the user space pppd daemon that handles the various higher-level protocols, such as PAP and CHAP. The current release of the PPP software for Linux contains the PPP daemon pppd and a program named chat that automates the dialing of the remote system.
The PPP kernel driver was written by Michael Callahan and reworked by Paul Mackerras. pppd was derived from a free PPP implementation for Sun and 386BSD machines that was written by Drew Perkins and others, and is maintained by Paul Mackerras. It was ported to Linux by Al Longyear. chat was written by Karl Fox.
Like SLIP, PPP is implemented by a special line discipline. To use a serial line as a PPP link, you first establish the connection over your modem as usual, and subsequently convert the line to PPP mode. In this mode, all incoming data is passed to the PPP driver, which checks the incoming HDLC frames for validity (each HDLC frame carries a 16-bit checksum), and unwraps and dispatches them. Currently, PPP is able to transport both the IP protocol, optionally using Van Jacobson header compression, and the IPX protocol.
pppd aids the kernel driver, performing the
initialization and authentication phase that is necessary before
actual network traffic can be sent across the
link. pppd’s behavior may be fine-tuned
using a number of options. As PPP is rather complex, it is impossible
to explain all of them in a single chapter. This book therefore cannot
cover all aspects of pppd, but only gives you an
introduction. For more information, consult Using & Managing
PPP or the pppd manual pages, and
READMEs in the pppd
source distribution, which should help you sort out most questions
this chapter fails to discuss. The PPP-HOWTO might also be of use.
Probably the greatest help you will find in configuring PPP will come
from other users of the same Linux distribution. PPP configuration
questions are very common, so try your local usergroup mailing list or
the IRC Linux channel. If your problems persist even after reading the
documentation, you could try the
comp.protocols.ppp newsgroup. This is
the place where you can find most of the people involved in
 Relevant RFCs are listed in the Bibiliography at the end of this book.
 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.