Blame Manuals/Tcpi-ug/Connectivity/Ppp/network.docbook

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<sect1 id="connectivity-ppp-network">
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    <title>The Network Of Computers</title>
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    <para>
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        This section describes how you could distribute server and
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        client computers to create a collaborative network.
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    </para>
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    <sect2 id="connectivity-ppp-policy-network">
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    <title>One PPP Network Of Two Computers</title>
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    <para>
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        The simpliest configuration we can achieve over the telephone
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        network involves two computers only, where one computer would
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        be acting as server and another as client. In this
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        configuration, the client computer establishes connection to
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        the server to make use of internet services provided therein.
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    </para>
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    <para>
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        When the client computer calls the server computer, the call
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        is attended by <application>mgetty</application> and then
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        passed to <application>pppd</application> for establishing a
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        PPP conversation between the two computers.  The first thing
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        in a PPP conversation is the user authentication and then
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        (after a sucessful athentication), the IPCP conversation takes
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        place to set IP addresses and start data transmission over the
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        link recently created. In this configuration, the client
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        computer can set its IP address when configuring the Modem
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        device (see 
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        linkend="connectivity-ppp-modem-config" />) or 
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        leave the server computer to assign one (assuming you are
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        calling a server computer). If you are configuring a server
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        computer, then it is necessary that you set the IP address and
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        netmask of the IP network you are planning to set, using the
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        Modem device configuration file.
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    </para>
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    <para>
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        Configuring the IP address and netmask information inside
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        Modem device configuration file is very important in order to
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        prevent errors when transmitting data across the link. When
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        the the netmask information isn't set in the Modem device
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        configuration file, the 
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        class="daemon">pppd</systemitem> daemon on the server computer
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        tries to retrive such information from the client computer and
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        if the client computer didn't specify one either, the network
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        recently created would end up having a wrong information
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        (e.g., 
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        class="netmask">255.255.255.255</systemitem>) which provokes
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        the point-to-point connection to fail when someone tries to
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        transfer data through it.
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    </para>
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    <figure id="connectivity-ppp-policy-network-basic">
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    <title>One PPP network of two computers</title>
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    <screenshot>
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    <screeninfo>One PPP network of two computers</screeninfo>
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    <mediaobject>
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    <textobject>
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<screen>
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Provice-A PPP Server                          Province-A PPP Client
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--------------------------\             /--------------------------
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192.168.1.1/24 | Modem ~~~ TelephoneLine ~~~ Modem | 192.168.1.2/24
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--------------------------/             \--------------------------
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</screen>
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    </textobject>
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    </mediaobject>
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    </screenshot>
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    </figure>
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    <para>
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        The <xref linkend="connectivity-ppp-policy-network-basic" />
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        describes the simpliest configuration we can implement for a
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        point-to-point connection. This configuration involves two
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        computers only, one acting as server (the server computer) and
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        other acting as client (the client computer). The client
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        computer calls the server computer to establish a PPP
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        connection in order to use whatever internet service the
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        server computer provides. In the figure we can see that there
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        are two IP addresses involved (
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        class="ipaddress">192.168.1.1</systemitem> and 
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        class="ipaddress">192.168.1.2</systemitem>) inside the same
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        newtork (
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        class="netmask">255.255.255.0</systemitem>).
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    </para>
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    <para>
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        This configuration might be convenient for people in the same
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        location, near one another. Here, the client computer
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        establishes connection by mean of a local telephone call and
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        can use whatever internet service the server computer
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        provides. Since the connection lifetime is limited (see 
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        linkend="connectivity-ppp-policy-lifetime" />) and only two
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        peers can be connected at the same time (assuming only one
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        Modem is attached to the server computer), the implementation
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        of some internet services like chat may be not a practical
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        offer for the server computer to provide.  However, internet
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        services like e-mail fit perfectly on this environment where
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        more than one client computer would be struggling among
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        themselves for establishing connection with the server
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        computer (e.g., people connect to send/receive their e-mail
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        messages to/from the server computer).
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    </para>
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    </sect2>
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    <sect2 id="connectivity-ppp-policy-network-extended">
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    <title>One PPP Network Of Several Computers</title>
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    <para>
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        Based on 
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        linkend="connectivity-ppp-policy-network" />, it is
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        possible to provide an extended version including several
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        server computers that may communicate between themselves to
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        distribute data collected from client computers they serve to.
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        For example, consider the telephone network of a country which
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        is organized in provinces and each province is divided in
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        several municipalities. In such organization, it would be
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        possible to set one or more server computers for each province
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        and let near people to dial-up on them to use whatever
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        internet service they provide.  Later, it could be possible
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        for each server computer to establish a dial-up connections
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        with other near server computers in order to share information
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        from one province to another, as it is illustrated in 
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        linkend="connectivity-ppp-policy-network-extended.fig-1"
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        />.
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    </para>
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    <para>
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        When setting the IP information, it is important that each
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        server computer sets both IP address and IP network mask
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        information in the Modem device configuration file so
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        different IP address can be use between different server
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        computers. It is also important that they all be configured to
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        use authentication between themselves before transmitting any
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        data across a PPP established connection so the information
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        being transmitted can be protected.
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    </para>
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    <para>
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        When making telephone calls, if someone in Province-A needs to
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        send a message to someone in Province-C (which is far away
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        from Province-A and making a telephone call there would imply
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        a considerable amount of money), there is no need (even it is
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        possible and sometimes prefered) for that person to realize a
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        direct telephone call from Province-A to Province-C. Instead,
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        that person in Province-A can send its messages to the server
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        computer on its province (the nearest server on its location)
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        making a local telephone call and then, such server computer
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        would take care of delivering the information using other
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        server computers, following the same concept of nearest
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        delivery.
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    </para>
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    <figure id="connectivity-ppp-policy-network-extended.fig-1">
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    <title>One PPP network of several computers</title>
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    <screenshot>
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    <screeninfo>One PPP network of several computers</screeninfo>
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    <mediaobject>
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    <textobject>
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<screen>
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Provice-A PPP Server                          Province-A PPP Client
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--------------------------\             /--------------------------
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192.168.1.1/24 | Modem ~~~ TelephoneLine ~~~ Modem | 192.168.1.2/24
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--------------------------/      |      \--------------------------
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                                 |
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Provice-B PPP Server             |            Province-B PPP Client
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--------------------------\      |      /--------------------------
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192.168.1.3/24 | Modem ~~~ TelephoneLine ~~~ Modem | 192.168.1.4/24
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--------------------------/      |      \--------------------------
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                                 |
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Provice-C PPP Server             |            Province-C PPP Client
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--------------------------\      |      /--------------------------
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192.168.1.5/24 | Modem ~~~ TelephoneLine ~~~ Modem | 192.168.1.6/24
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--------------------------/             \--------------------------
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</screen>
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    </textobject>
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    </mediaobject>
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    </screenshot>
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    </figure>
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    <para>
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        The more distant a telephone call is, the more expensive it
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        is. This way, to move information from one province to
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        another, each server computers must be configured to send
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        information to the nearest province until reaching its
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        destination. For example, if you are in Province-A and want to
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        send an e-mail message to Province-D, the server computer
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        configured in Province-A must sed the e-mail message to
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        Province-B, then server in Province-B must be configured to
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        send such message to Province-C, and finally C to D. This is
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        required because making a direct call from Province-A to
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        Province-D would be otherwise too much expensive to pay.
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    </para>
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    <para>
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        Since telephone calls are required to establish connections
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        between computers and each call costs money based on the
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        location and the destination, it is required to set a
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        convenction in how telephone calls are realized from one
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        server computer to another, specially if you plan to establish
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        connection between server computer placed on different
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        provices in order to exchange data between them.
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    </para>
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    <itemizedlist>
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    <listitem>
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    <para>
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        Do you make direct telephone calls to make direct data delivery?
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        — This configuration could be very expensive to maintain
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        (considering the telephone call distances), but data will be
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        delivered very fast to their destinations.
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    </para>
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    </listitem>
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    <listitem>
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    <para>
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        Do you call the nearest server computer and let it to deliver
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        your data to its destination? — This configuration could
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        be less expensive to maintain (considering the telephone call
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        distances), but data delivery will take much more time to
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        reach their destinations and there is no way to be sure it
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        will do.
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    </para>
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    </listitem>
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    </itemizedlist>
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    <para>
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        Whatever calling schema be chosen, the server computers will
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        always talk through UUCP to transfer data from one place to
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        another. The server computers will operate with two IP
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        addresses each, unless you plan to connect one of the server
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        computers to a different network (Internet, maybe?). One IP
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        address would identify the server computer itself and the
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        other would identify the client computer establishing PPP
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        connection to the server computer.  In this configuration it
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        is very importat that each server and client computer does
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        have one unique IP address. This way it would be possible to
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        move the information from one computer to another. Notice that
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        the number of PPP clients is directly related to the number of
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        telephone lines a server computer has configured to receive
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        incomming calls on. If there is only one telephone line
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        attached to the server computer then, only one client computer
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        will be able to establish connection to that server computer.
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        Other PPP clients will need to wait until the telephone line
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        gets free in order to establish connection with that server
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        computer.  On the other hand, if the server computer has two
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        (or more) attached telephone lines, it would be possible to
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        attend incoming calls from two (or more) PPP client at the
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        same time. As resume, we can say that: the more telephone
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        lines the server computer has attached in, the more
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        simultaneous connections that computer will be able to
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        attend/realize from/to other computers.
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    </para>
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    </sect2>
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    <sect2 id="connectivity-ppp-policy-network-eth">
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    <title>One PPP+Ethernet Network Of Several Computers</title>
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    <para>
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        Assuming all server computers with a Modem device have also
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        one (or more) Ethernet interface attached (which is very
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        common nowadays), it would be possible to extend the
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        configuration described in 
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        linkend="connectivity-ppp-policy-network-extended.fig-1" />
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        creating one Ethernet network for each server computer in the
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        configuration. For this configuration to be implemented it is
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        required one or more switch devices (based on the amount of
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        computers such network needs to have) for each ethernet
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        network interface a server computer has, as described in 
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        linkend="connectivity-ppp-policy-network-extended.fig-2"
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        />.
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    </para>
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    <figure id="connectivity-ppp-policy-network-extended.fig-2">
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    <title>One PPP+Ethernet network of several computers</title>
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    <screenshot>
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    <screeninfo>One PPP+Ethernet network of several computers</screeninfo>
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    <mediaobject>
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    <textobject>
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<screen>
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Province-A PPP/ETH Server                     Province-A PPP Client
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--------------------------\             /--------------------------
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192.168.1.1/24 | Modem ~~~ TelephoneLine ~~~ Modem | 192.168.1.2/24
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--------------------------/      |      \--------------------------
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192.168.0.1/24 | Ethernet        |
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---------------------|----       |
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                     |           |
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              +--------+         |
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              | Switch |         |
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              +--------+         |
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                     |           |
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---------------------|--         |
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LAN1: 192.168.0.2-254/24         |
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------------------------         |
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Province-A ETH Clients           |
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                                 |
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Province-B PPP/ETH Server        |            Province-B PPP Client
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--------------------------\      |      /--------------------------
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192.168.1.3/24 | Modem ~~~ TelephoneLine ~~~ Modem | 192.168.1.4/24
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--------------------------/      |      \--------------------------
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192.168.2.1/24 | Ethernet        |
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---------------------|----       |
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                     |           |
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              +--------+         |
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              | Switch |         |
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              +--------+         |
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                     |           |
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---------------------|--         |
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LAN2: 192.168.2.2-254/24         |
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------------------------         |
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Province-B ETH Clients           |
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                                 |
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Province-C PPP/ETH Server        |            Province-C PPP Client
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--------------------------\      |      /--------------------------
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192.168.1.5/24 | Modem ~~~ TelephoneLine ~~~ Modem | 192.168.1.6/24
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--------------------------/             \--------------------------
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192.168.3.1/24 | Ethernet
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---------------------|----
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                     |
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              +--------+
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              | Switch |
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              +--------+
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                     |  
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---------------------|--
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LAN3: 192.168.3.2-254/24
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------------------------
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Province-C ETH Clients
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</screen>
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    </textobject>
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    </mediaobject>
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    </screenshot>
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    </figure>
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    <para>
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        In this configuration, computers connected to the switch will
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        also be considered as client computers. It is necessary that a
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        coordination be implemented at time of setting IP addresses to
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        new server computers so no IP address be duplicated on the
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        computer network. The illustration above describes one main
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        network (
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        class="ipaddress">192.168.1/24</systemitem>) which connects
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        all the server computers using the telephone lines as medium
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        for data transmission. The Modem interface connects just one
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        computer at a time either client or server (assuming only one
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        Modem device is installed and configured in
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        the computer acting as server).  The telephone line is used by
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        client computers to establish PPP connections with the server
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        computer and by server computers to exchange data with other
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        server computers, as well. On the other hand, the ethernet
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        interface attached to each server computer let the
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        administrator of each server computer to connect up to 252
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        computers simultaneously, assuming a class C network as shown
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        above be used.<footnote>
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        <para>
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        There are also class A and class B network types which can be
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        used to connect much more computers than a class C network
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        allows to.
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        </para>
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        </footnote>
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    </para>
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    </sect2>
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    <sect2 id="connectivity-ppp-policy-bridgedcall">
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    <title>About Bridging Calls To Transfer Data</title>
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    <para>
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        When the server computers call other server computers to
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        bridge data delivery, the server computer in, let's say,
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        Province-A (srv-1.a.domain.tld) will never know that there is
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        a server computer on Province-C (srv-1.c.domain.tld) or
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        Province-D (srv-1.d.domain.tld), but in Province-B
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        (srv-1.b.domain.tld)
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        only, its nearest location.  So, when a message is sent from
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        srv-1.a.domain.tld to the server computer in
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        srv-1.d.domain.tld, the server computer in srv-1.a.domain.tld
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        contacts its nearest server computer (i.e.,
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        srv-1.b.domain.tld) and delivers to it all messages sent to
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        srv-1.d.domain.tld. Later, since srv-1.b.domain.tld doesn't
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        know about srv-1.d.domain.tld server either, it delivers all
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        messages directed to srv-1.d.domain.tld to its nearest server
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        computer (i.e., srv-1.c.domain.tld).  Later, the server
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        computer in srv-1.c.domain.tld, which knows about
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        srv-1.d.domain.tld, delivers to it all the messages it has for
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        it. Notice that, in order for this configuration to work,
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        system administrators attending the server computers must work
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        syncronized to garantee a well defined route for messages to
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        follow.  Otherwise, if one of the server computers in the path
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        creates a route for a server computer that doesn't exist
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        (or doesn't define a route at all), the information will never
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        reach its destination when such computer is acting as a bridge
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        between other two server computers.
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    </para>
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<screen>
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+------------------------+     +------------------------+       +------------------------+       +---------------------+
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| To: bob@d.domain.tld   |     | To: bob@d.domain.tld   |       | To: bob@d.domain.tld   |       |    Bob's mailbox    |
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| From: mat@a.domain.tld |     | From: ana@b.domain.tld |       | From: jef@c.domain.tld |       | (Final destination) |
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| Body: 500KB            |     | Body: 500KB            |       | Body: 500KB            |       |                     |
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+---|--------------------+     +---|--------------------+       +---|--------------------+       +------------------^--+
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    |                              |                                |                                               |
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----v--------------|<~~~~~~~~~>|---v----------------|<~~~~~~~~~>|---v----------------|<~~~~~~~~~>|------------------|---
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srv-1.a.domain.tld | 75Km Call | srv-1.b.domain.tld | 75Km Call | srv-1.c.domain.tld | 75Km Call | srv-1.d.domain.tld
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-------------------|<~~~~~~~~~>|--------------------|<~~~~~~~~~>|--------------------|<~~~~~~~~~>|----------------------
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relay to:          |   5 min   | relay to:          |   10 min  | relay to:          |  15 min   |
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srv-1.b.domain.tld |   500KB   | srv-1.c.domain.tld |   1.0MB   | srv-1.d.domain.tld |  1.5MB    |
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</screen>
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    </sect2>
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73737d
    <sect2 id="connectivity-ppp-policy-directcalls">
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    <title>About Directing Calls To Transfer Data</title>
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    <para>
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        When the server computers make direct telephone calls (no
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        bridge in-between is used to transfer data), the server
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        computer in Province-A (srv-1.a.domain.tld) contacts the
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        server computer in Province-D (srv-1.d.domain.tld) making a
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        direct telephone call up to it. In this configuration, the
f4d51e
        telephone call might cost more than those in a bridged
f4d51e
        configuration where several smaller telephone calls are dialed
f4d51e
        in-between the final server computer; or less, considering
f4d51e
        that when server computers in a bridged configuration exchange
f4d51e
        data they may move data accumulated from other server
f4d51e
        computers, while a direct telephone call would transmit data
f4d51e
        from one server computer to another without any accumulated
f4d51e
        data from other server computers.  There is no need to
f4d51e
        overload the server computers with foreign data when each
f4d51e
        server computer could call themselves to transfer data
f4d51e
        directly.
f4d51e
    </para>
f4d51e
f4d51e
<screen>
f4d51e
+------------------------+             +---------------------+
f4d51e
| To: bob@d.domain.tld   |             |    Bob's mailbox    |
f4d51e
| From: mat@a.domain.tld |             | (Final destination) |
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| Body: 500KB            |             |                     |
f4d51e
+--|---------------------+             +------------------^--+
f4d51e
   |                                                      | 
f4d51e
---v---------------------|<~~~~~~~~~~>|-------------------|---
f4d51e
srv-1.a.domain.tld       | 225Km Call | srv-1.d.domain.tld
f4d51e
-------------------------|<~~~~~~~~~~>|-----------------------
f4d51e
relay to:                |   5 min    |
f4d51e
srv-1.d.domain.tld       |   500KB    |
f4d51e
</screen>
f4d51e
f4d51e
    <para>
f4d51e
        The elapsed time in a server-to-server conversation is
f4d51e
        directly related to the amount of data that need to be moved
f4d51e
        from one server to another and the baud rate of the connection
f4d51e
        established between the two Modem devices. In a direct
f4d51e
        telephone call configuration, telephone calls could result to
f4d51e
        be less expensive than those in bridged configurations where
f4d51e
        server computers may accumulate traffic from other server
f4d51e
        computers in the path.  The accumulation of traffic between
f4d51e
        server computers increases the amount of time the last server
f4d51e
        computer in the path before the final destination needs, in
f4d51e
        order to transmit everything to the final destination. In a
f4d51e
        bridged telephone call configuration, server computers acting
f4d51e
        as bridges do act as servers as well and produce their own
f4d51e
        traffic which is added to that one already accumulated in
f4d51e
        them from other server computers. This may provoke a heugh
f4d51e
        traffic in a server-to-server conversation (remarkably on the
f4d51e
        last destination before the final destination), that could be
f4d51e
        potentially increased with each new server computer added to
f4d51e
        the string of server computers acting as bridges one another.
f4d51e
    </para>
f4d51e
f4d51e
    </sect2>
f4d51e
    
73737d
    <sect2 id="connectivity-ppp-policy-auth">
f4d51e
    <title>About Authenticating PPP Users</title>
f4d51e
f4d51e
    <para>
f4d51e
        The client computers will need to authenticate against the
f4d51e
        server computer each time they intend to establish a PPP
f4d51e
        connection. The username and password required by client
f4d51e
        computers will be public and will be rarely changed.
f4d51e
    </para>
f4d51e
73737d
    <example id="connectivity-ppp-policy-auth.fig-1">
f4d51e
    <title>Credentials for PPP authentication</title>
f4d51e
    <screenshot>
f4d51e
    <screeninfo>Credentials for PPP authentication</screeninfo>
f4d51e
    <mediaobject>
f4d51e
    <textobject>
f4d51e
<screen>
f4d51e
 ISP Name: projects.centos.org
f4d51e
ISP Phone: +53043515094
f4d51e
 Username: faith
f4d51e
 Password: mail4u.2k10
f4d51e
</screen>
f4d51e
    </textobject>
f4d51e
    </mediaobject>
f4d51e
    </screenshot>
f4d51e
    </example>
f4d51e
f4d51e
    <para>
f4d51e
        The server computer provides only one telephone line available
f4d51e
        (e.g., +53043515094) to receive incoming calls.  This affects
f4d51e
        directly the possibilities a client computer has to establish
f4d51e
        connection with the server computer in an environment where
f4d51e
        several client computers are struggling among themselves to
f4d51e
        establish a dial-up connection with the server computer.  To
f4d51e
        prevent this kind of issues from happening, it is innevitable
f4d51e
        for the server computer to provide more telephone lines for
f4d51e
        incoming calls (at least one for each user the server computer
f4d51e
        expects to receive incoming calls from).
f4d51e
    </para>
f4d51e
f4d51e
    </sect2>
f4d51e
73737d
    <sect2 id="connectivity-ppp-policy-lifetime">
f4d51e
    <title>About Restricting PPP Connections</title>
f4d51e
f4d51e
    <para>
f4d51e
        The server computer restricts the lifetime of established
f4d51e
        Modem connections to 15 minutes from the establishment moment
f4d51e
        on.  Once the connection has been established, if the link is
f4d51e
        idle for 1 minute, the server computer will also close the
f4d51e
        established connection to free the telephone line.  This
f4d51e
        control can be implemented through the
f4d51e
        <option>maxconnect</option> and <option>idle</option> options
f4d51e
        inside the <application>pppd</application>'s configuration
f4d51e
        file as described in 
73737d
        linkend="connectivity-ppp-server-pppd-options" />.
f4d51e
    </para>
f4d51e
f4d51e
    <para>
f4d51e
        The server computer restricts the incoming calls from client
f4d51e
        computers every night from 10:00PM to 12:00AM. Outside this
f4d51e
        range of time, the telephone could be answered by a person,
f4d51e
        not a computer. This control can be implemented through a cron
f4d51e
        job and the <filename>/etc/nologin.ttyxx</filename> file;
f4d51e
        where ttyxx represents the device name of your Modem (e.g.,
f4d51e
        <filename>/etc/nologin.ttyACM0</filename> would prevent the
f4d51e
        Modem device installed in <filename>/dev/ttyACM0</filename>
f4d51e
        from answering calls).
f4d51e
    </para>
f4d51e
f4d51e
<screen>
f4d51e
# Activate Modem to attend incoming calls.
f4d51e
59 21 * * * [ -f /etc/nologin.ttyACM0 ] && /bin/rm /etc/nologin.ttyACM0
f4d51e
# Deactivate Modem to prevent incoming calls from being attended. 
f4d51e
59 23 * * * [ ! -f /etc/nologin.ttyACM0 ] && /bin/touch /etc/nologin.ttyACM0
f4d51e
</screen>
f4d51e
f4d51e
    </sect2>
f4d51e
73737d
    <sect2 id="connectivity-ppp-services">
f4d51e
    <title>About Providing Internet Services</title>
f4d51e
f4d51e
    <para>
f4d51e
        The implementation of internet services which require
f4d51e
        persistent connections (e.g.,
f4d51e
        <application>chats</application>) should not be considered as
f4d51e
        a practical offer for PPP client computers.  Instead, only
f4d51e
        asynchronous services (e.g.,
f4d51e
        <application>e-mail</application>) should be supported for
f4d51e
        them. This restriction is required to reduce the connection
f4d51e
        times demanded such services. For example, consider an
f4d51e
        environment where you establish connection with a server
f4d51e
        computer to send/receive e-mails messages and then quickly
f4d51e
        disconnect from it to free the telephone line so others be
f4d51e
        able of using it.  In this environment, there is no need for
f4d51e
        you and others to be both connected at the same time to
f4d51e
        send/receive e-mail messages to/from each other.  The e-mails
f4d51e
        sent from other person to you will be available in your
f4d51e
        mailbox the next time you get connected to the server computer
f4d51e
        and use your e-mail client to send/receive e-mail messages.
f4d51e
        Likewise, you don't need to be connected to the server
f4d51e
        computer in order to write your e-mail messages.  You can
f4d51e
        write down your messages off-line and then establish
f4d51e
        connection once you've finished writing, just to send them out
f4d51e
        and receive new messages that could have been probably sent to
f4d51e
        you.
f4d51e
    </para>
f4d51e
f4d51e
    <para>
f4d51e
        Another issue related to e-mail exchange is the protocol used
f4d51e
        to receive messages. Presently, there are two popular ways to
f4d51e
        do this, one is through IMAP and another through POP3.  When
f4d51e
        you use IMAP protocol, e-mail messages are retained in the
f4d51e
        server computer and aren't downloaded to client computer.
f4d51e
        Otherwise, when you use POP3 protocol, e-mail messages are
f4d51e
        downloaded to the client computer and removed from server
f4d51e
        computer. Based on the resources we have and the kind of link
f4d51e
        used by the client computer to connect the server computer,
f4d51e
        using POP3 is rather prefered than IMAP. However both are made
f4d51e
        available.
f4d51e
    </para>
f4d51e
f4d51e
    <para>
f4d51e
        Assuming you use IMAP protocol to read your mailbox, be aware
f4d51e
        that you need to be connected to the server computer.  Once
f4d51e
        the connection is lost you won't be able to read your messages
f4d51e
        (unless your e-mail client possesses a feature that let you
f4d51e
        reading messages off-line). Moreover, you run the risk of
f4d51e
        getting your mailbox out of space. If your mailbox gets out of
f4d51e
        space, new messages sent to you will not be deliver to your
f4d51e
        mailbox.  Instead, they will be deferred for a period of time
f4d51e
        (e.g., about 5 days when using
f4d51e
        <application>Postfix</application> defaults) hoping you to
f4d51e
        free the space in your mailbox to deliver them.  If you don't
f4d51e
        free space on your mailbox within this period of time, the
f4d51e
        deferred e-mails will be bounced back to their senders and you
f4d51e
        will never see them.  On the other hand, assuming you are
f4d51e
        using POP3 protocol to read your mailbox, you always keep your
f4d51e
        mailbox free to receive new e-mails messages and keep them for
f4d51e
        you until the next time you establish connection with the
f4d51e
        server computer and download them to your client computer
f4d51e
        using your e-mail client.
f4d51e
    </para>
f4d51e
f4d51e
    <para>
f4d51e
        The information generated inside the server computer is
f4d51e
        isolated from Internet. This way, any information generated
f4d51e
        inside the server computer will be available only to people
f4d51e
        connected to the same network the server computer is connected
f4d51e
        to. For example, don't ever expect to send/receive e-mails
f4d51e
        to/from Internet e-mail accounts like Gmail or Yahoo, nor
f4d51e
        visiting web sites like 
f4d51e
        url="http://www.google.com/">Google</ulink> or 
f4d51e
        url="http://www.wikipedia.org/">Wikipedia</ulink> either. For
f4d51e
        this to happen, an established connection must exist first
f4d51e
        between the server computer you are establishing connection
f4d51e
        through and the Internet network those services are available
f4d51e
        in. Without that link, it is not possible to direct your
f4d51e
        requests to those sites, nor receive any response from them.
f4d51e
    </para>
f4d51e
f4d51e
    </sect2>
f4d51e
f4d51e
</sect1>