102 lines
4.4 KiB
HTML
102 lines
4.4 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
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<!--Converted with LaTeX2HTML 96.1-c (Feb 29, 1996) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds -->
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<HTML>
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<HEAD>
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<TITLE>Untangling the P's</TITLE>
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</HEAD>
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<BODY LANG="EN">
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<A HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
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<B> Next:</B> <A HREF="node109.html">PPP on </A>
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<B>Up:</B> <A HREF="node107.html">The Point-to-Point Protocol</A>
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<B> Previous:</B> <A HREF="node107.html">The Point-to-Point Protocol</A>
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<BR> <P>
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<H1><A NAME="SECTION0010100000">Untangling the P's</A></H1>
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<P>
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<A NAME="4060"></A>
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<P>
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Just like SLIP, PPP is a protocol to send datagrams across a serial
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connection, but addresses a couple of deficiencies of the former. It
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lets the communicating sides negotiate options such as the IP address
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and the maximum datagram size at startup time, and provides for client
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authorization. For each of these capabilities, PPP has a separate
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protocol. Below, we will briefly cover these basic building blocks of
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PPP. This discussion is far from complete; if you want to know more
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about PPP, you are urged to read its specification in RFC-1548, as well
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as the dozen or so companion RFCs.<A HREF="footnode.html#4068"><IMG ALIGN=BOTTOM ALT="gif" SRC="foot_motif.gif"></A>
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<P>
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<A NAME="4069"></A>
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At the very bottom of PPP is the <em>High-Level Data Link Control</em>
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Protocol, abbreviated HDLC,<A HREF="footnode.html#4071"><IMG ALIGN=BOTTOM ALT="gif" SRC="foot_motif.gif"></A> which defines the boundaries around the individual PPP frames, and
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provides a 16-bit checksum. As opposed to the more primitive SLIP
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encapsulation, a PPP frame is capable of holding packets from other
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protocols than IP, such as Novell's IPX, or Appletalk. PPP achieves
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this by adding a protocol field to the basic HDLC frame that
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identifies the type of packet is carried by the frame.
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<P>
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LCP, the Link Control Protocol, is used on top of HDLC to negotiate
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options pertaining to the data link, such as the Maximum Receive Unit
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(MRU) that states the maximum datagram size one side of the link agrees
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to receive.
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<P>
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An important step at the configuration stage of a PPP link is client
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authorization. Although it is not mandatory, it is really a must for
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dial-up lines. Usually, the called host (the server) asks the client to
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authorize itself by proving it knows some secret key. If the caller fails
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to produce the correct secret, the connection is terminated. With PPP,
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authorization works both ways; that is, the caller may also ask the server
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to authenticate itself. These authentication procedures are totally
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independent of each other. There are two protocols for different types of
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authorization, which we will discuss further below. They are named
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Password Authentication Protocol, or PAP, and Challenge Handshake
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Authentication Protocol, or CHAP.
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<P>
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Each network protocol that is routed across the data link, like IP,
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AppleTalk, etc, is configured dynamically using a corresponding Network
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Control Protocol (NCP). For instance, to send IP datagrams across the
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link, both PPPs must first negotiate which IP-address each of them uses.
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The control protocol used for this is IPCP, the Internet Protocol Control
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Protocol.
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<P>
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Beside sending standard IP-datagrams across the link, PPP also supports
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Van-Jacobson header compression of IP-datagrams. This is a technique to
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shrink the headers of TCP packets to as little as three bytes. It is
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also used in CSLIP, and is more colloquially referred to as VJ-header
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compression. The use of compression may be negotiated at startup time
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through IPCP as well.
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<P>
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<HR><A HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
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<B> Next:</B> <A HREF="node109.html">PPP on </A>
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<B>Up:</B> <A HREF="node107.html">The Point-to-Point Protocol</A>
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<B> Previous:</B> <A HREF="node107.html">The Point-to-Point Protocol</A>
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<P><ADDRESS>
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<I>Andrew Anderson <BR>
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Thu Mar 7 23:22:06 EST 1996</I>
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</ADDRESS>
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</BODY>
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</HTML>
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