IRIX Base Documentation 2002 November/SGI IRIX Base Documentation 2002 November.iso/usr/share/catman/a_man/cat1/slip.z/slip

Text File | 2002-10-03 | 18.8 KB | 397 lines

SSSSLLLLIIIIPPPP((((1111MMMM))))                                                              SSSSLLLLIIIIPPPP((((1111MMMM))))



NNNNAAAAMMMMEEEE
     slip, if_sl, if_sl.o - Serial Line IP

SSSSYYYYNNNNOOOOPPPPSSSSIIIISSSS
     sssslllliiiipppp [----ddddccccIIII] [----mmmm _n_e_t_m_a_s_k] [----MMMM _m_e_t_r_i_c] [----ssss _M_T_U] [----AAAA _a_t_i_m_e]
         [----PPPP _p_o_r_t] [----TTTT _i_c_m_p__t_y_p_e] [----RRRR "_r_o_u_t_e_c_m_d"] [----SSSS _s_m_o_d]
         [----pppp _p_r_o_t_o] [----uuuu _u_u_c_p] [----llll _l_o_c_a_l] ----rrrr _r_e_m_o_t_e {----iiii|----oooo|----qqqq}

DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
     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 sssslllliiiipppp 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 iiiiffffccccoooonnnnffffiiiigggg(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 sssslllliiiipppp program.  When simply dialing a
     remote machine, the ----oooo or "output" argument should be used.  When simply
     answering the telephone or using an existing link, the ----iiii or "input" flag
     must be used.

     When starting a "demand-dialing" link, the ----qqqq 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 /_e_t_c/_s_e_r_v_i_c_e_s) and
     ICMP packets of types ICMP_UNREACH, ICMP_SOURCEQUENCH, ICMP_TSTAMP, and
     ICMP_TSTAMPREPLY are not counted as activity.  These lists can be changed
     with the ----PPPP and ----TTTT arguments.

     While a qqqquuuuiiiieeeetttt 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 sssslllliiiipppp program using demand-dialing can call another sssslllliiiipppp 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.




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SSSSLLLLIIIIPPPP((((1111MMMM))))                                                              SSSSLLLLIIIIPPPP((((1111MMMM))))



     The ----PPPP 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 ----TTTT argument is similar, except that it is given either a
     single dash (-) or a decimal number to clear or augment the list of ICMP
     types.

     ----AAAA 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
     default.

     The ----rrrr argument must be used to specify the host name or IP address of
     the remote end of the link.  If the ----llll argument is not used, the sssslllliiiipppp
     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 ----pppp argument selects the variety of SLIP protocol.  The common or
     standard variety is the default or specified with _s_t_d.  A protocol with
     adds a checksum to the standard variety is specified with _c_k_s_u_m.  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 _c_o_m_p.  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 _c_s_l_i_p.  This is what is
     commonly known as "Van Jacobson Compression," described in RFC 1144.

     The standard (----pppp _s_t_d) and "Van Jacobson Compression" ( ----pppp _c_s_l_i_p)
     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.




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SSSSLLLLIIIIPPPP((((1111MMMM))))                                                              SSSSLLLLIIIIPPPP((((1111MMMM))))



     The ----uuuu argument specifies a name in the /_e_t_c/_u_u_c_p/_S_y_s_t_e_m_s 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 ----dddd flag requests additional debugging information.  Additional
     instances of -d produce more information.  The debugging information is
     sent to the system log, /_v_a_r/_a_d_m/_S_Y_S_L_O_G, 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
     passwords.

     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 ----cccc 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 sssslllliiiipppp program must
     be killed to finally terminate a link that is "camping" or in "quiet"
     mode (see kkkkiiiillllllll(1) or kkkkiiiillllllllaaaallllllll(1M)).

     The ----RRRR 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 ----cccc is used, the string is given to the rrrroooouuuutttteeee(1M) command.
     The command can be "-", in which case "route add default remote" is
     assumed.

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

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






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SSSSLLLLIIIIPPPP((((1111MMMM))))                                                              SSSSLLLLIIIIPPPP((((1111MMMM))))



   IIIInnnnssssttttaaaallllllllaaaattttiiiioooonnnn NNNNooootttteeeessss
     The program uses the dialing information on each appropriate line of the
     /_e_t_c/_u_u_c_p/_S_y_s_t_e_m_s 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 /_e_t_c/_u_u_c_p/_S_y_s_t_e_m_s 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 sssslllliiiipppp 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 /_e_t_c/_h_o_s_t_s database, the following
     shell script can be used to start the sssslllliiiipppp program with the Systems file
     entry above:

          #!/bin/sh
          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 sssslllliiiipppp 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 /_e_t_c/_u_u_c_p/_D_i_a_l_e_r_s,
     /_e_t_c/_u_u_c_p/_D_e_v_i_c_e_s, and /_e_t_c/_u_u_c_p/_S_y_s_t_e_m_s files, and then "debugs" the
     connection with _c_u -_d _r_e_m_o_t_e_s_y_s_t_e_m.

     A server which other machines call to use SLIP should establish separate
     "user names" in /etc/passwd (see ppppaaaasssssssswwwwdddd(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 sssslllliiiipppp program.

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

     Routing demons can be used to exchange RIP packets (see rrrroooouuuutttteeeedddd(1M) or
     ggggaaaatttteeeedddd(1M)) over the link, as well as advertise the link to the rest of
     the IP network.  The -h option to rrrroooouuuutttteeeedddd can usefully reduce the
     resulting clutter of "host-routes."  The -F option to rrrroooouuuutttteeeedddd 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.




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SSSSLLLLIIIIPPPP((((1111MMMM))))                                                              SSSSLLLLIIIIPPPP((((1111MMMM))))



     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 ----RRRR argument described below or
     with rrrroooouuuutttteeee(1M) commands in a /etc/init.d/network.local files associated
     with the /_e_t_c/_i_n_i_t._d/_n_e_t_w_o_r_k 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
     /_v_a_r/_s_y_s_g_e_n/_m_a_s_t_e_r._d/_b_s_d, and then reconfiguring the kernel with
     aaaauuuuttttooooccccoooonnnnffffiiiigggg(1M).

     The network information service (NIS, see yyyyppppbbbbiiiinnnndddd(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 /_u_s_r/_e_t_c/_r_e_s_o_l_v._c_o_n_f  file (see rrrreeeessssoooollllvvvveeeerrrr(4)) similar to the
     following but with the addresses and domain name changed appropriately:

          domain your.dom.ain
          hostresorder local bind
          nameserver 192.26.61.24
          nameserver 192.26.61.21
          nameserver 192.26.51.194


     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 aaaauuuuttttoooommmmoooouuuunnnntttt(1M) and ffffssssttttaaaabbbb(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 _t_i_m_e_o
     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 ttttiiiimmmmeeeedddd(1M) can be used, but
     ttttiiiimmmmeeeessssllllaaaavvvveeee(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.

DDDDIIIIAAAAGGGGNNNNOOOOSSSSTTTTIIIICCCCSSSS
     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
     system.




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SSSSLLLLIIIIPPPP((((1111MMMM))))                                                              SSSSLLLLIIIIPPPP((((1111MMMM))))



FFFFIIIILLLLEEEESSSS
     /etc/init.d/network              network start-up script
     /etc/passwd
     /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.

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






































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