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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.[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.
