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<sect1 id="connectivity-ppp-network">

    <title>The Network Of Computers</title>

    <para>
        This section describes how you could distribute server and
        client computers to create a collaborative network.
    </para>
    
    <sect2 id="connectivity-ppp-policy-network">
    <title>One PPP Network Of Two Computers</title>

    <para>
        The simpliest configuration we can achive over the telephone
        network involves two computers only, where one computer would
        be acting as server and another as client. In this
        configuration, the client computer establishes connection to
        the server to make use of internet services provided therein.
    </para>

    <para>
        When the client computer calls the server computer, the call
        is attended by <application>mgetty</application> and then
        passed to <application>pppd</application> for establishing a
        PPP conversation between the two computers.  The first thing
        in a PPP conversation is the user authentication and then
        (after a sucessful athentication), the IPCP conversation takes
        place to set IP addresses and start data transmission over the
        link recently created. In this configuration, the client
        computer can set its IP address when configuring the Modem
        device (see <xref
        linkend="connectivity-ppp-modem-config" />) or 
        leave the server computer to assign one (assuming you are
        calling a server computer). If you are configuring a server
        computer, then it is necessary that you set the IP address and
        netmask of the IP network you are planning to set, using the
        Modem device configuration file.
    </para>
    
    <para>
        Configuring the IP address and netmask information inside
        Modem device configuration file is very important in order to
        prevent errors when transmitting data across the link. When
        the the netmask information isn't set in the Modem device
        configuration file, the <systemitem
        class="daemon">pppd</systemitem> daemon on the server computer
        tries to retrive such information from the client computer and
        if the client computer didn't specify one either, the network
        recently created would end up having a wrong information
        (e.g., <systemitem
        class="netmask">255.255.255.255</systemitem>) which provokes
        the point-to-point connection to fail when someone tries to
        transfer data through it.
    </para>

    <figure id="connectivity-ppp-policy-network-basic">
    <title>One PPP network of two computers</title>
    <screenshot>
    <screeninfo>One PPP network of two computers</screeninfo>
    <mediaobject>
    <textobject>
<screen>
Provice-A PPP Server                          Province-A PPP Client
--------------------------\             /--------------------------
192.168.1.1/24 | Modem ~~~ TelephoneLine ~~~ Modem | 192.168.1.2/24
--------------------------/             \--------------------------
</screen>
    </textobject>
    </mediaobject>
    </screenshot>
    </figure>

    <para>
        The <xref linkend="connectivity-ppp-policy-network-basic" />
        describes the simpliest configuration we can implement for a
        point-to-point connection. This configuration involves two
        computers only, one acting as server (the server computer) and
        other acting as client (the client computer). The client
        computer calls the server computer to establish a PPP
        connection in order to use whatever internet service the
        server computer provides. In the figure we can see that there
        are two IP addresses involved (<systemitem
        class="ipaddress">192.168.1.1</systemitem> and <systemitem
        class="ipaddress">192.168.1.2</systemitem>) inside the same
        newtork (<systemitem
        class="netmask">255.255.255.0</systemitem>).
    </para>

    <para>
        This configuration might be convenient for people in the same
        location, near one another. Here, the client computer
        establishes connection by mean of a local telephone call and
        can use whatever internet service the server computer
        provides. Since the connection lifetime is limited (see <xref
        linkend="connectivity-ppp-policy-lifetime" />) and only two
        peers can be connected at the same time (assuming only one
        Modem is attached to the server computer), the implementation
        of some internet services like chat may be not a practical
        offer for the server computer to provide.  However, internet
        services like e-mail fit perfectly on this environment where
        more than one client computer would be struggling among
        themselves for establishing connection with the server
        computer (e.g., people connect to send/receive their e-mail
        messages to/from the server computer).
    </para>

    </sect2>

    <sect2 id="connectivity-ppp-policy-network-extended">
    <title>One PPP Network Of Several Computers</title>

    <para>
        Based on <xref
        linkend="connectivity-ppp-policy-network" />, it is
        possible to provide an extended version including several
        server computers that may communicate between themselves to
        distribute data collected from client computers they serve to.
        For example, consider the telephone network of a country which
        is organized in provinces and each province is divided in
        several municipalities. In such organization, it would be
        possible to set one or more server computers for each province
        and let near people to dial-up on them to use whatever
        internet service they provide.  Later, it could be possible
        for each server computer to establish a dial-up connections
        with other near server computers in order to share information
        from one province to another, as it is illustrated in <xref
        linkend="connectivity-ppp-policy-network-extended.fig-1"
        />.
    </para>

    <para>
        When setting the IP information, it is important that each
        server computer sets both IP address and IP network mask
        information in the Modem device configuration file so
        different IP address can be use between different server
        computers. It is also important that they all be configured to
        use authentication between themselves before transmitting any
        data across a PPP established connection so the information
        being transmitted can be protected.
    </para>

    <para>
        When making telephone calls, if someone in Province-A needs to
        send a message to someone in Province-C (which is far away
        from Province-A and making a telephone call there would imply
        a considerable amount of money), there is no need (even it is
        possible and sometimes prefered) for that person to realize a
        direct telephone call from Province-A to Province-C. Instead,
        that person in Province-A can send its messages to the server
        computer on its province (the nearest server on its location)
        making a local telephone call and then, such server computer
        would take care of delivering the information using other
        server computers, following the same concept of nearest
        delivery.
    </para>

    <figure id="connectivity-ppp-policy-network-extended.fig-1">
    <title>One PPP network of several computers</title>
    <screenshot>
    <screeninfo>One PPP network of several computers</screeninfo>
    <mediaobject>
    <textobject>
<screen>
Provice-A PPP Server                          Province-A PPP Client
--------------------------\             /--------------------------
192.168.1.1/24 | Modem ~~~ TelephoneLine ~~~ Modem | 192.168.1.2/24
--------------------------/      |      \--------------------------
                                 |
Provice-B PPP Server             |            Province-B PPP Client
--------------------------\      |      /--------------------------
192.168.1.3/24 | Modem ~~~ TelephoneLine ~~~ Modem | 192.168.1.4/24
--------------------------/      |      \--------------------------
                                 |
Provice-C PPP Server             |            Province-C PPP Client
--------------------------\      |      /--------------------------
192.168.1.5/24 | Modem ~~~ TelephoneLine ~~~ Modem | 192.168.1.6/24
--------------------------/             \--------------------------
</screen>
    </textobject>
    </mediaobject>
    </screenshot>
    </figure>

    <para>
        The more distant a telephone call is, the more expensive it
        is. This way, to move information from one province to
        another, each server computers must be configured to send
        information to the nearest province until reaching its
        destination. For example, if you are in Province-A and want to
        send an e-mail message to Province-D, the server computer
        configured in Province-A must sed the e-mail message to
        Province-B, then server in Province-B must be configured to
        send such message to Province-C, and finally C to D. This is
        required because making a direct call from Province-A to
        Province-D would be otherwise too much expensive to pay.
    </para>

    <para>
        Since telephone calls are required to establish connections
        between computers and each call costs money based on the
        location and the destination, it is required to set a
        convenction in how telephone calls are realized from one
        server computer to another, specially if you plan to establish
        connection between server computer placed on different
        provices in order to exchange data between them.
    </para>
    
    <itemizedlist>
    <listitem>
    <para>
        Do you make direct telephone calls to make direct data delivery?
        &mdash; This configuration could be very expensive to maintain
        (considering the telephone call distances), but data will be
        delivered very fast to their destinations.
    </para>
    </listitem>
    <listitem>
    <para>
        Do you call the nearest server computer and let it to deliver
        your data to its destination? &mdash; This configuration could
        be less expensive to maintain (considering the telephone call
        distances), but data delivery will take much more time to
        reach their destinations and there is no way to be sure it
        will do.
    </para>

    </listitem>
    </itemizedlist>

    <para>
        Whatever calling schema be chosen, the server computers will
        always talk through UUCP to transfer data from one place to
        another. The server computers will operate with two IP
        addresses each, unless you plan to connect one of the server
        computers to a different network (Internet, maybe?). One IP
        address would identify the server computer itself and the
        other would identify the client computer establishing PPP
        connection to the server computer.  In this configuration it
        is very importat that each server and client computer does
        have one unique IP address. This way it would be possible to
        move the information from one computer to another. Notice that
        the number of PPP clients is directly related to the number of
        telephone lines a server computer has configured to receive
        incomming calls on. If there is only one telephone line
        attached to the server computer then, only one client computer
        will be able to establish connection to that server computer.
        Other PPP clients will need to wait until the telephone line
        gets free in order to establish connection with that server
        computer.  On the other hand, if the server computer has two
        (or more) attached telephone lines, it would be possible to
        attend incoming calls from two (or more) PPP client at the
        same time. As resume, we can say that: the more telephone
        lines the server computer has attached in, the more
        simultaneous connections that computer will be able to
        attend/realize from/to other computers.
    </para>

    </sect2>

    <sect2 id="connectivity-ppp-policy-network-eth">
    <title>One PPP+Ethernet Network Of Several Computers</title>

    <para>
        Assuming all server computers with a Modem device have also
        one (or more) Ethernet interface attached (which is very
        common nowadays), it would be possible to extend the
        configuration described in <xref
        linkend="connectivity-ppp-policy-network-extended.fig-1" />
        creating one Ethernet network for each server computer in the
        configuration. For this configuration to be implemented it is
        required one or more switch devices (based on the amount of
        computers such network needs to have) for each ethernet
        network interface a server computer has, as described in <xref
        linkend="connectivity-ppp-policy-network-extended.fig-2"
        />.
    </para>

    <figure id="connectivity-ppp-policy-network-extended.fig-2">
    <title>One PPP+Ethernet network of several computers</title>
    <screenshot>
    <screeninfo>One PPP+Ethernet network of several computers</screeninfo>
    <mediaobject>
    <textobject>
<screen>
Province-A PPP/ETH Server                     Province-A PPP Client
--------------------------\             /--------------------------
192.168.1.1/24 | Modem ~~~ TelephoneLine ~~~ Modem | 192.168.1.2/24
--------------------------/      |      \--------------------------
192.168.0.1/24 | Ethernet        |
---------------------|----       |
                     |           |
              +--------+         |
              | Switch |         |
              +--------+         |
                     |           |
---------------------|--         |
LAN1: 192.168.0.2-254/24         |
------------------------         |
Province-A ETH Clients           |
                                 |
Province-B PPP/ETH Server        |            Province-B PPP Client
--------------------------\      |      /--------------------------
192.168.1.3/24 | Modem ~~~ TelephoneLine ~~~ Modem | 192.168.1.4/24
--------------------------/      |      \--------------------------
192.168.2.1/24 | Ethernet        |
---------------------|----       |
                     |           |
              +--------+         |
              | Switch |         |
              +--------+         |
                     |           |
---------------------|--         |
LAN2: 192.168.2.2-254/24         |
------------------------         |
Province-B ETH Clients           |
                                 |
Province-C PPP/ETH Server        |            Province-C PPP Client
--------------------------\      |      /--------------------------
192.168.1.5/24 | Modem ~~~ TelephoneLine ~~~ Modem | 192.168.1.6/24
--------------------------/             \--------------------------
192.168.3.1/24 | Ethernet
---------------------|----
                     |
              +--------+
              | Switch |
              +--------+
                     |  
---------------------|--
LAN3: 192.168.3.2-254/24
------------------------
Province-C ETH Clients
</screen>
    </textobject>
    </mediaobject>
    </screenshot>
    </figure>

    <para>
        In this configuration, computers connected to the switch will
        also be considered as client computers. It is necessary that a
        coordination be implemented at time of setting IP addresses to
        new server computers so no IP address be duplicated on the
        computer network. The illustration above describes one main
        network (<systemitem
        class="ipaddress">192.168.1/24</systemitem>) which connects
        all the server computers using the telephone lines as medium
        for data transmission. The Modem interface connects just one
        computer at a time either client or server (assuming only one
        Modem device is installed and configured in
        the computer acting as server).  The telephone line is used by
        client computers to establish PPP connections with the server
        computer and by server computers to exchange data with other
        server computers, as well. On the other hand, the ethernet
        interface attached to each server computer let the
        administrator of each server computer to connect up to 252
        computers simultaneously, assuming a class C network as shown
        above be used.<footnote>
        <para>
        There are also class A and class B network types which can be
        used to connect much more computers than a class C network
        allows to.
        </para>
        </footnote>
    </para>

    </sect2>

    <sect2 id="connectivity-ppp-policy-bridgedcall">
    <title>About Bridging Calls To Transfer Data</title>
 
    <para>
        When the server computers call other server computers to
        bridge data delivery, the server computer in, let's say,
        Province-A (srv-1.a.domain.tld) will never know that there is
        a server computer on Province-C (srv-1.c.domain.tld) or
        Province-D (srv-1.d.domain.tld), but in Province-B
        (srv-1.b.domain.tld)
        only, its nearest location.  So, when a message is sent from
        srv-1.a.domain.tld to the server computer in
        srv-1.d.domain.tld, the server computer in srv-1.a.domain.tld
        contacts its nearest server computer (i.e.,
        srv-1.b.domain.tld) and delivers to it all messages sent to
        srv-1.d.domain.tld. Later, since srv-1.b.domain.tld doesn't
        know about srv-1.d.domain.tld server either, it delivers all
        messages directed to srv-1.d.domain.tld to its nearest server
        computer (i.e., srv-1.c.domain.tld).  Later, the server
        computer in srv-1.c.domain.tld, which knows about
        srv-1.d.domain.tld, delivers to it all the messages it has for
        it. Notice that, in order for this configuration to work,
        system administrators attending the server computers must work
        syncronized to garantee a well defined route for messages to
        follow.  Otherwise, if one of the server computers in the path
        creates a route for a server computer that doesn't exist
        (or doesn't define a route at all), the information will never
        reach its destination when such computer is acting as a bridge
        between other two server computers.
    </para>

<screen>
+------------------------+     +------------------------+       +------------------------+       +---------------------+
| To: bob@d.domain.tld   |     | To: bob@d.domain.tld   |       | To: bob@d.domain.tld   |       |    Bob's mailbox    |
| From: mat@a.domain.tld |     | From: ana@b.domain.tld |       | From: jef@c.domain.tld |       | (Final destination) |
| Body: 500KB            |     | Body: 500KB            |       | Body: 500KB            |       |                     |
+---|--------------------+     +---|--------------------+       +---|--------------------+       +------------------^--+
    |                              |                                |                                               |
----v--------------|&lt;~~~~~~~~~&gt;|---v----------------|&lt;~~~~~~~~~&gt;|---v----------------|&lt;~~~~~~~~~&gt;|------------------|---
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
-------------------|&lt;~~~~~~~~~&gt;|--------------------|&lt;~~~~~~~~~&gt;|--------------------|&lt;~~~~~~~~~&gt;|----------------------
relay to:          |   5 min   | relay to:          |   10 min  | relay to:          |  15 min   |
srv-1.b.domain.tld |   500KB   | srv-1.c.domain.tld |   1.0MB   | srv-1.d.domain.tld |  1.5MB    |
</screen>
    </sect2>

    <sect2 id="connectivity-ppp-policy-directcalls">
    <title>About Directing Calls To Transfer Data</title>

    <para>
        When the server computers make direct telephone calls (no
        bridge in-between is used to transfer data), the server
        computer in Province-A (srv-1.a.domain.tld) contacts the
        server computer in Province-D (srv-1.d.domain.tld) making a
        direct telephone call up to it. In this configuration, the
        telephone call might cost more than those in a bridged
        configuration where several smaller telephone calls are dialed
        in-between the final server computer; or less, considering
        that when server computers in a bridged configuration exchange
        data they may move data accumulated from other server
        computers, while a direct telephone call would transmit data
        from one server computer to another without any accumulated
        data from other server computers.  There is no need to
        overload the server computers with foreign data when each
        server computer could call themselves to transfer data
        directly.
    </para>

<screen>
+------------------------+             +---------------------+
| To: bob@d.domain.tld   |             |    Bob's mailbox    |
| From: mat@a.domain.tld |             | (Final destination) |
| Body: 500KB            |             |                     |
+--|---------------------+             +------------------^--+
   |                                                      | 
---v---------------------|&lt;~~~~~~~~~~&gt;|-------------------|---
srv-1.a.domain.tld       | 225Km Call | srv-1.d.domain.tld
-------------------------|&lt;~~~~~~~~~~&gt;|-----------------------
relay to:                |   5 min    |
srv-1.d.domain.tld       |   500KB    |
</screen>

    <para>
        The elapsed time in a server-to-server conversation is
        directly related to the amount of data that need to be moved
        from one server to another and the baud rate of the connection
        established between the two Modem devices. In a direct
        telephone call configuration, telephone calls could result to
        be less expensive than those in bridged configurations where
        server computers may accumulate traffic from other server
        computers in the path.  The accumulation of traffic between
        server computers increases the amount of time the last server
        computer in the path before the final destination needs, in
        order to transmit everything to the final destination. In a
        bridged telephone call configuration, server computers acting
        as bridges do act as servers as well and produce their own
        traffic which is added to that one already accumulated in
        them from other server computers. This may provoke a heugh
        traffic in a server-to-server conversation (remarkably on the
        last destination before the final destination), that could be
        potentially increased with each new server computer added to
        the string of server computers acting as bridges one another.
    </para>

    </sect2>
    
    <sect2 id="connectivity-ppp-policy-auth">
    <title>About Authenticating PPP Users</title>

    <para>
        The client computers will need to authenticate against the
        server computer each time they intend to establish a PPP
        connection. The username and password required by client
        computers will be public and will be rarely changed.
    </para>

    <example id="connectivity-ppp-policy-auth.fig-1">
    <title>Credentials for PPP authentication</title>
    <screenshot>
    <screeninfo>Credentials for PPP authentication</screeninfo>
    <mediaobject>
    <textobject>
<screen>
 ISP Name: projects.centos.org
ISP Phone: +53043515094
 Username: faith
 Password: mail4u.2k10
</screen>
    </textobject>
    </mediaobject>
    </screenshot>
    </example>

    <para>
        The server computer provides only one telephone line available
        (e.g., +53043515094) to receive incoming calls.  This affects
        directly the possibilities a client computer has to establish
        connection with the server computer in an environment where
        several client computers are struggling among themselves to
        establish a dial-up connection with the server computer.  To
        prevent this kind of issues from happening, it is innevitable
        for the server computer to provide more telephone lines for
        incoming calls (at least one for each user the server computer
        expects to receive incoming calls from).
    </para>

    </sect2>

    <sect2 id="connectivity-ppp-policy-lifetime">
    <title>About Restricting PPP Connections</title>

    <para>
        The server computer restricts the lifetime of established
        Modem connections to 15 minutes from the establishment moment
        on.  Once the connection has been established, if the link is
        idle for 1 minute, the server computer will also close the
        established connection to free the telephone line.  This
        control can be implemented through the
        <option>maxconnect</option> and <option>idle</option> options
        inside the <application>pppd</application>'s configuration
        file as described in <xref
        linkend="connectivity-ppp-server-pppd-options" />.
    </para>

    <para>
        The server computer restricts the incoming calls from client
        computers every night from 10:00PM to 12:00AM. Outside this
        range of time, the telephone could be answered by a person,
        not a computer. This control can be implemented through a cron
        job and the <filename>/etc/nologin.ttyxx</filename> file;
        where ttyxx represents the device name of your Modem (e.g.,
        <filename>/etc/nologin.ttyACM0</filename> would prevent the
        Modem device installed in <filename>/dev/ttyACM0</filename>
        from answering calls).
    </para>

<screen>
# Activate Modem to attend incoming calls.
59 21 * * * [ -f /etc/nologin.ttyACM0 ] &amp;&amp; /bin/rm /etc/nologin.ttyACM0
# Deactivate Modem to prevent incoming calls from being attended. 
59 23 * * * [ ! -f /etc/nologin.ttyACM0 ] &amp;&amp; /bin/touch /etc/nologin.ttyACM0
</screen>

    </sect2>

    <sect2 id="connectivity-ppp-services">
    <title>About Providing Internet Services</title>

    <para>
        The implementation of internet services which require
        persistent connections (e.g.,
        <application>chats</application>) should not be considered as
        a practical offer for PPP client computers.  Instead, only
        asynchronous services (e.g.,
        <application>e-mail</application>) should be supported for
        them. This restriction is required to reduce the connection
        times demanded such services. For example, consider an
        environment where you establish connection with a server
        computer to send/receive e-mails messages and then quickly
        disconnect from it to free the telephone line so others be
        able of using it.  In this environment, there is no need for
        you and others to be both connected at the same time to
        send/receive e-mail messages to/from each other.  The e-mails
        sent from other person to you will be available in your
        mailbox the next time you get connected to the server computer
        and use your e-mail client to send/receive e-mail messages.
        Likewise, you don't need to be connected to the server
        computer in order to write your e-mail messages.  You can
        write down your messages off-line and then establish
        connection once you've finished writing, just to send them out
        and receive new messages that could have been probably sent to
        you.
    </para>

    <para>
        Another issue related to e-mail exchange is the protocol used
        to receive messages. Presently, there are two popular ways to
        do this, one is through IMAP and another through POP3.  When
        you use IMAP protocol, e-mail messages are retained in the
        server computer and aren't downloaded to client computer.
        Otherwise, when you use POP3 protocol, e-mail messages are
        downloaded to the client computer and removed from server
        computer. Based on the resources we have and the kind of link
        used by the client computer to connect the server computer,
        using POP3 is rather prefered than IMAP. However both are made
        available.
    </para>

    <para>
        Assuming you use IMAP protocol to read your mailbox, be aware
        that you need to be connected to the server computer.  Once
        the connection is lost you won't be able to read your messages
        (unless your e-mail client possesses a feature that let you
        reading messages off-line). Moreover, you run the risk of
        getting your mailbox out of space. If your mailbox gets out of
        space, new messages sent to you will not be deliver to your
        mailbox.  Instead, they will be deferred for a period of time
        (e.g., about 5 days when using
        <application>Postfix</application> defaults) hoping you to
        free the space in your mailbox to deliver them.  If you don't
        free space on your mailbox within this period of time, the
        deferred e-mails will be bounced back to their senders and you
        will never see them.  On the other hand, assuming you are
        using POP3 protocol to read your mailbox, you always keep your
        mailbox free to receive new e-mails messages and keep them for
        you until the next time you establish connection with the
        server computer and download them to your client computer
        using your e-mail client.
    </para>

    <para>
        The information generated inside the server computer is
        isolated from Internet. This way, any information generated
        inside the server computer will be available only to people
        connected to the same network the server computer is connected
        to. For example, don't ever expect to send/receive e-mails
        to/from Internet e-mail accounts like Gmail or Yahoo, nor
        visiting web sites like <ulink
        url="http://www.google.com/">Google</ulink> or <ulink
        url="http://www.wikipedia.org/">Wikipedia</ulink> either. For
        this to happen, an established connection must exist first
        between the server computer you are establishing connection
        through and the Internet network those services are available
        in. Without that link, it is not possible to direct your
        requests to those sites, nor receive any response from them.
    </para>

    </sect2>

</sect1>