<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 achieve 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 

        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 

        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., 

        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 (

        class="ipaddress">192.168.1.1</systemitem> and 

        class="ipaddress">192.168.1.2</systemitem>) inside the same

        newtork (

        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 

        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 

        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 

        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?

        — 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? — 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 

        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 

        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 (

        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--------------|<~~~~~~~~~>|---v----------------|<~~~~~~~~~>|---v----------------|<~~~~~~~~~>|------------------|---

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

-------------------|<~~~~~~~~~>|--------------------|<~~~~~~~~~>|--------------------|<~~~~~~~~~>|----------------------

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---------------------|<~~~~~~~~~~>|-------------------|---

srv-1.a.domain.tld       | 225Km Call | srv-1.d.domain.tld

-------------------------|<~~~~~~~~~~>|-----------------------

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 

        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 ] && /bin/rm /etc/nologin.ttyACM0

# Deactivate Modem to prevent incoming calls from being attended. 

59 23 * * * [ ! -f /etc/nologin.ttyACM0 ] && /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 

        url="http://www.google.com/">Google</ulink> or 

        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>