129 lines
5.1 KiB
HTML
129 lines
5.1 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>SLIP Operation</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="node95.html">Using dip</A>
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<B>Up:</B> <A HREF="node92.html">Serial Line IP</A>
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<B> Previous:</B> <A HREF="node93.html">General Requirements</A>
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<BR> <P>
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<H1><A NAME="SECTION009200000">SLIP Operation</A></H1>
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<A NAME="slipoperation"></A>
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<A NAME="3734"></A>
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<A NAME="3735"></A>
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<A NAME="3736"></A>
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Dial-up IP-servers frequently offer SLIP service through special user
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accounts. After logging into such an account, you are not dropped into
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the common shell; instead a program or shell script is executed that
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enables the server's SLIP driver for the serial line and configures the
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appropriate network interface. Then you have to do the same at your end
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of the link.
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<P>
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On some operating systems, the SLIP driver is a user-space program;
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under , it is part of the kernel, which makes it a lot faster.
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This requires, however, that the serial line be converted to SLIP mode
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explicitly. This is done by means of a special tty line discipline,
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SLIPDISC. While the tty is in normal line discipline (DISC0), it will
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exchange data only with user processes, using the normal read(2)
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and write(2) calls, and the SLIP driver is unable to write to or
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read from the tty. In SLIPDISC, the roles are reversed: now any
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user-space processes are blocked from writing to or reading from the
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tty, while all data coming in on the serial port will be passed directly
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to the SLIP driver.
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<P>
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<A NAME="3739"></A>
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<A NAME="3740"></A>
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<A NAME="3741"></A>
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<A NAME="3742"></A>
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The SLIP driver itself understands a number of variations on the SLIP
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protocol. Apart from ordinary SLIP, it also understands CSLIP, which
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performs the so-called Van Jacobson header compression on outgoing
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IP-packets.<A HREF="footnode.html#3743"><IMG ALIGN=BOTTOM ALT="gif" SRC="foot_motif.gif"></A> This improves throughput for interactive sessions noticeably.
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Additionally, there are six-bit versions for each of these protocols.
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<P>
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<A NAME="4001"></A>
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A simple way to convert a serial line to SLIP mode is by using the
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slattach tool. Assume you have your modem on /dev/cua3,
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and have logged into the SLIP server successfully. You will then
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execute:
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<pre>
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# slattach /dev/cua3 &
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</pre>
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This will switch the line discipline of cua3 to SLIPDISC, and
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attach it to one of the SLIP network interfaces. If this is your first
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active SLIP link, the line will be attached to sl0; the second
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would be attached to sl1, and so on. The current kernels support
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up to eight simultaneous SLIP links.
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<P>
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<A NAME="3752"></A>
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The default encapsulation chosen by slattach is CSLIP. You may
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choose any other mode using the -p switch. To use normal SLIP
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(no compression), you would use
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<pre>
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# slattach -p slip /dev/cua3 &
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</pre>
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Other modes are cslip, slip6, cslip6 (for the
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six-bit version of SLIP), and adaptive for adaptive SLIP. The
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latter leaves it to the kernel to find out which type of SLIP
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encapsulation the remote end uses.
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<P>
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Note that you must use the same encapsulation as your peer does. For
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example, if cowslip uses CSLIP, you have to do so, too. The
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symptoms of a mismatch will be that a ping to the remote host
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will not receive any packets back. If the other host pings you,
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you may also see messages like ``Can't build ICMP header'' on
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your console. One way to avoid these difficulties is to use
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adaptive SLIP.
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<P>
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In fact, slattach does not only allow you to enable
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SLIP, but other protocols that use the serial line as well,
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like PPP or KISS (another protocol used by ham radio people).
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For details, please refer to the slattach(8) manual page.
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<P>
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After turning over the line to the SLIP driver, you have to configure
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the network interface. Again, we do this using the standard
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ifconfig and route commands. Assume that from
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vlager, we have dialed up a server named cowslip. You
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would then execute
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<PRE>
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# ifconfig sl0 vlager pointopoint cowslip
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# route add cowslip
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# route add default gw cowslip
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</PRE>
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The first command configures the interface as a point-to-point link to
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cowslip, while the second and third add the route to
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cowslip and the default route using cowslip as a gateway.
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<P>
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When taking down the SLIP link, you first have to remove all routes
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through cowslip using route with the del option,
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take the interface down, and send slattach the hangup signal.
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Afterwards you have to hang up the modem using your terminal program
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again:
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<PRE>
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# route del default
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# route del cowslip
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# ifconfig sl0 down
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# kill -HUP 516
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</pre>
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<HR><A HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A>
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<BR>
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<B> Next:</B> <A HREF="node95.html">Using dip</A>
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<B>Up:</B> <A HREF="node92.html">Serial Line IP</A>
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<B> Previous:</B> <A HREF="node93.html">General Requirements</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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