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SLIP(1M)							      SLIP(1M)

NAME    [Toc]    [Back]

     slip, if_sl, if_sl.o - Serial Line	IP

SYNOPSIS    [Toc]    [Back]

     slip [-dcI] [-m netmask] [-M metric] [-s MTU] [-A atime]
	 [-P port] [-T icmp_type] [-R "routecmd"] [-S smod]
	 [-p proto] [-u	uucp] [-l local] -r remote {-i|-o|-q}

DESCRIPTION    [Toc]    [Back]

     SLIP or SL/IP stands for Serial Line Internet Protocol.  It is used to
     extend a local area network using low speed telephone lines.  It can be
     used to transfer data between machines which are using TCP/IP or UDP/IP.
     The modems	and communications lines used with SLIP	are relatively
     inexpensive, and operate at 2400 to 38,400	bits/second.  This makes SLIP
     appropriate for remote sessions and modest	file transfers.

     The slip program is used to connect to a remote machine.  It does
     everything	necessary to permit network data to reach the modem, and to
     connect the TTY port to the rest the network system.  It does things that
     are done by ifconfig(1M) for other	network	interfaces.  The result	is a
     "point-to-point" link that	can be part of an existing IP interwork.

     There are three main modes	of the slip program.  When simply dialing a
     remote machine, the -o or "output"	argument should	be used.  When simply
     answering the telephone or	using an existing link,	the -i or "input" flag
     must be used.

     When starting a "demand-dialing" link, the	-q or "quiet" flag must	be
     used.  "Demand-dialing" or	"quiet"	mode is	one in which the link is made
     only when needed by network activity. When	there is no activity, the link
     is	turned off.  When activity begins the modem link is established.
     Activity is defined as packets that are not of certain types.  By
     default, UDP and TCP packets to or	from ports "daytime" (13), "time"
     (37), "route" (520), 123 (ntp), and "timed" (525) (see /etc/services) and
     ICMP_TSTAMPREPLY are not counted as activity.  These lists	can be changed
     with the -P and -T	arguments.

     While a quiet mode	connection can be started at the receiving end of a
     connection, it may	not have the desired effect.  The daemon does not
     expect to use the serial connection to its	standard input in quiet	mode.
     If	started	in quiet mode as the result of the remote system dialing in,
     the daemon	will ignore the	incoming serial	connection.  It	expects	to
     wait quietly until	it sees	locally	generated traffic and the need to dial
     its own new link.	One might conceivably start a quiet mode daemon
     remotely for a simple kind	of traffic driven "dial-back."

     A slip program using demand-dialing can call another slip program which
     is	in either output or demand-dialing mode.  In case the other end	is
     also using	demand-dialing,	demand-dialing uses random binary exponential
     backoffs after failed attempts to call.

									Page 1

SLIP(1M)							      SLIP(1M)

     The -P argument can either	clear or augment the list of ports not counted
     as	activity.  If the string following "-P"	is a single dash (-), the list
     of	ports is cleared.  Otherwise, the program tries	to interpret the
     string as the name	of a TCP port, the name	of a UDP port, or a number, in
     that order.  The -T argument is similar, except that it is	given either a
     single dash (-) or	a decimal number to clear or augment the list of ICMP

     -A	is used	to specify a number of seconds of no activity after which the
     link is turned off.  Two numbers separated	by a comma can be specified,
     with the first one	the number of seconds of inactivity while no TCP
     connections are open, and the second the number of	seconds	while a
     connection	is open.  The lower layers snoop on packets to infer the
     number of open TCP	connections that go over the link.  This snooping
     cannot be made entirely reliable, because the end of the connection may
     be	a distant machine that forwards	only some of its packets through this
     machine.  The snooping works only when "cslip" protocol is	used (see
     below).  Still, "30,300" can limit	many telephone calls for quick,
     automatic transactions like email to less than a minute, without making
     remote interactive	sessions painful.

     If	-A is not specified but	"quiet"	mode is, a value of "30,300" is	the

     The -r argument must be used to specify the host name or IP address of
     the remote	end of the link.  If the -l argument is	not used, the slip
     program assumes the local end of the link has the same IP address as the
     hostname of the machine.  This default is usually appropriate, whether
     connecting	two ethernets or extending an ethernet to a distant, isolated
     workstation.  In the latter case, it is best to use a single network
     number and	allocate host numbers on that network for remote workstations.

     The -p argument selects the variety of SLIP protocol.  The	common or
     standard variety is the default or	specified with std.  A protocol	with
     adds a checksum to	the standard variety is	specified with cksum.  A
     version which compresses TCP/IP to	3 bytes	consisting of framing,
     checksum and TCP/IP protocol information in addition to user data bytes
     is	selected with comp.  A compressing protocol is useful with modems such
     as	the Telebit Trailblazer.

     A version which compresses	TCP/IP to a varying number of bytes, sometimes
     as	few as 4 counting framing is selected with cslip.  This	is what	is
     commonly known as "Van Jacobson Compression," described in	RFC 1144.

     The standard (-p std) and "Van Jacobson Compression" ( -p cslip)
     protocols do not have extra error detection or correction facilities.
     This makes	them inappropriate for NFS in heterogeneous networks, because
     other NFS vendors often use UDP with checksums turned off.	 However,
     because the "comp"	and "cksum" protocols include link error detection,
     they can be used reliably with NFS.  The low speed	communications lines
     commonly used make	NFS slightly hard to use.

									Page 2

SLIP(1M)							      SLIP(1M)

     The -u argument specifies a name in the /etc/uucp/Systems file for
     dialing. Its default value	is the remote machine name.  UUCP hostnames
     can be at most 7 or sometimes 8 characters	long.  It is useful to use one
     name for dialing and another for TCP/IP when the more public, harder to
     change TCP/IP name	is long.

     The -d flag requests additional debugging information.  Additional
     instances of -d produce more information.	The debugging information is
     sent to the system	log, /var/adm/SYSLOG, if its standard error file
     descriptor	is not a tty.  The signals SIGUSR1 and SIGUSR2 increase	and
     decrease, respectively, the amount	of debugging information.

     Avoid increasing the debugging level to more than 1, because entire
     packets will be logged, including those containing	PPP PAP	usernames and
     passwords,	which can let anyone who can read the system log discover the

     Another reason to avoid increasing	the debugging level to more than 1 is
     that turns	on messages from the IRIX kernel.  While the kernel is
     displaying	the message, it	has all	interrupts turned off, which can cause
     input to be lost, which often causes more messages	from the kernel, and
     so	on.

     The -c flag tells the program to "camp" on	the telephone, to redial the
     remote machine whenever the link is broken.  It can only be used in the
     "output" mode.  It	is not needed in "quiet" mode..	 The slip program must
     be	killed to finally terminate a link that	is "camping" or	in "quiet"
     mode (see kill(1) or killall(1M)).

     The -R or "route" argument	can be used to install a default, static
     route.  When the link is first established	and later each time it is
     restored when -c is used, the string is given to the route(1M) command.
     The command can be	"-", in	which case "route add default remote" is

     The -S argument "pushes" the named	STREAMS	module below the if_sl STREAMS
     module.  If there are more	than one -S arguments, the named modules are
     pushed in the order they are named.

     The -m, -M, and -s	arguments modify default parameters.  The default
     netmask is	the one	implied	by the IP address.  The	default	metric is 1.
     The default maximum transfer unit,	or "MTU" is 512	bytes.	Such a small
     MTU is less efficient, but	reduces	latency	for interactive	applications.
     The MTU can be reduced to 256 bytes.

     The -I option causes the driver to	discard	all ICMP packets.  This
     defends a very low	speed link against ping	packets	which can otherwise
     saturate the link.	It is not recommended for links	as fast	or faster than
     9600 bits/second.

									Page 3

SLIP(1M)							      SLIP(1M)

   Installation	Notes
     The program uses the dialing information on each appropriate line of the
     /etc/uucp/Systems file until it succeeds.	This can be useful if there is
     more than one telephone number that might be used to contact the remote
     machine.  A /etc/uucp/Systems line	like the following works well to call
     an	IRIS running this SLIP software:

	  rmt Any ACUSLIP 19200	5551234	"" @\r\c ogin: mynam ssword: xxx SLIP

     The last check for	"SLIP,"	output by the slip program by the remote IRIS
     just before it starts the IP protocol, gets past any banners or messages
     of	the day.  It ensures the remote	machine	is not waiting for an
     additional	password.  The check for "SLIP"	may not	be appropriate with
     other brands of computer.

     If	the hostname "rmt" appears in the /etc/hosts database, the following
     shell script can be used to start the slip	program	with the Systems file
     entry above:

	  exec </dev/null >/dev/null 2>&1
	  /usr/etc/slip	-cp cslip -r rmt $* &

     A machine which has no network connection other than SLIP should use a
     terminator	on its ethernet	port, and so act as if it has a	valid although
     very small	local area network.

     Because the slip program can use the UUCP control files, the best way to
     install a SLIP connection is to first install a UUCP connection.  So, one
     first creates appropriate entries in the /etc/uucp/Dialers,
     /etc/uucp/Devices,	and /etc/uucp/Systems files, and then "debugs" the
     connection	with cu	-d remotesystem.

     A server which other machines call	to use SLIP should establish separate
     "user names" in /etc/passwd (see passwd(4)), all using the
     /usr/etc/remoteslip script	as their "login	shell."	 The script should be
     modified to recognize the remote machine by its "username," and to	choose
     appropriate parameters with which to execute the slip program.

     Since the slip program configures network interfaces, it must be executed
     with UID 0, and so	the password entry on the remote system	should use UID

     Routing demons can	be used	to exchange RIP	packets	(see routed(1M)	or
     gated(1M))	over the link, as well as advertise the	link to	the rest of
     the IP network.  The -h option to routed can usefully reduce the
     resulting clutter of "host-routes."  The -F option	to routed on the
     machine gatewaying	a SLIP link to an ethernet causes a synthetic "default
     route" to be sent over the	SLIP link instead of the full routing tables,
     making the	cost of	running	RIP over the SLIP link negligible.

									Page 4

SLIP(1M)							      SLIP(1M)

     Each time the link	is (re)established, the	program	sends a	SIGHUP signal
     to	the gated and routed daemons, if they are running.  This causes	the
     routing daemons to	more quickly notice the	(probably) new network
     interface and to start advertising	adjusted routes.

     One can also use "static routing" with the	-R argument described below or
     with route(1M) commands in	a /etc/init.d/network.local files associated
     with the /etc/init.d/network file.

     The default TCP/IP	window on SGI machines is 61440, which makes data move
     faster over fast networks such as FDDI.  On slow networks such as PPP or
     SLIP, both	latency	and throughput can often be significantly improved by
     reducing the window to 4096 or even 2048.	The window can be change by
     modifying the settings of tcp_sendspace and tcp_recvspace in
     /var/sysgen/master.d/bsd, and then	reconfiguring the kernel with

     The network information service (NIS, see ypbind(1M)) is not often	useful
     over a SLIP link.	It is usually necessary	to use local copies of mail
     aliases.  However,	the Internet domain name server	can be useful, by
     creating a	/usr/etc/resolv.conf  file (see	resolver(4)) similar to	the
     following but with	the addresses and domain name changed appropriately:

	  domain your.dom.ain
	  hostresorder local bind

     It	is possible to use NFS over a SLIP link.  It is	usually	necessary to
     adjust the	mount options for the relatively long latencies	and low
     bandwidth (see automount(1M) and fstab(4)).  Timeouts should be set long
     enough to allow a complete	transaction to pass the	link before becoming
     too late, and having to be	retransmitted.	A plausible value for timeo
     with default 8KByte block sizes over a 19.2Kbit/s link is 90, for 9
     seconds.  It can be useful	to increase the	attribute timeouts
     substantially, to minutes.

     To	synchronize clocks over	a SLIP link timed(1M) can be used, but
     timeslave(1M) is generally	better.

     Once each day at about midnight, if the slip program has been running for
     at	least several hours, it	logs some statistics concerning	its work for
     the previous 24 hours.

DIAGNOSTICS    [Toc]    [Back]

     Error messages complaining	"open()/dev/if_sl: No such device or address"
     usually means that	the kernel does	not contain the	required SLIP STREAMS
     module, if_sl.  Ensure that the SLIP software has been installed on the

									Page 5

SLIP(1M)							      SLIP(1M)

FILES    [Toc]    [Back]

     /etc/init.d/network	      network start-up script
     /var/adm/SYSLOG		      system log for debugging messages
     /usr/etc/remoteslip	      incoming configuration script
     /etc/uucp/Systems		      "modem chat scripts"
     /etc/uucp/Dialers		      "chat scripts" to	control	modems
     /etc/uucp/Devices		      tty port/modem configurations
     /var/sysgen/master.d/if_sl	      configuration file for kernel module
     /var/sysgen/boot/if_sl.o	      if_sl kernel module
     /etc/hosts			      hostname database
     /tmp/.slip.*		      rendezvous for demand dialing
     /dev/tty[dmf]x		      tty port attached	to modem.

SEE ALSO    [Toc]    [Back]

     automount(1M), cu(1), fstab(4)), ifconfig(1M), icmp(7P), master(4),
     passwd(4),	ppp(1M), resolver(4), syslog(1M), uucico(1M)

									PPPPaaaaggggeeee 6666
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