<sect1 id="configurations-dialup-usage">

    <title>Usage Convenctions</title>

    <para>

        The infrastructure described in this chapter uses the

        client/server model to provide a public mail service through

        the telephone line. In this configuration, we (the poeple

        building the infrastructure) provide the information you (the

        person using the infrastructure) need to know in order to

        establish a point-to-point connection from the client computer

        to the server computer through the telephone line.  </para>

    <para>

        The infrastructure described in this chapter is made available

        to you free of charge, however, you should know that

        maintaining it costs both money and time. For example, for

        each hour the server computer is on production there is an

        electrical consume that need to be paid every month.

        Likewise, each call that you establish from the client

        computer to the server computer will cost you money, based on

        the location you made the call from and the time you spend

        connected.

    </para>

    <para>

        In this section we discuss usage convenctions we all must be

        agree with, in order to achieve a practical and secure

        interchange system.

    </para>

    <sect2 id="configurations-dialup-usage-connlimits">

    <title>Administering Dial-Up Connections</title>

    <para>

        The lifetime of dial-up connections must be limitted based on

        the number of users you expect to establish connection and the

        kind of services you plan to provide. The mail service

        provided by the server computer is conceived as a public

        service so anyone with a modem attached to a computer would be

        able to have access to it.  However, due to hardware

        limitations, only 100 users will be allowed to be registered

        in the public mail service. Based on this information, the

        lifetime of established connections will be of 15 minutes from

        the established moment on. Once the connection has been

        established, if the link is idle for 1 minute, the server

        computer will 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.

    </para>

    <para>

        Only registered user profiles will be able to establish connections

        to the server computer.  This control can be implemented using

        the <option>allow-number</option> option in the

        <application>pppd</application>'s configuration file to define a

        list of all telephone numbers that are allowed to establish

        connection with the server computer, based on the list of

        registered user profiles.  By default, all telephone numbers

        are denied from establishing access with the server computer,

        except those ones explicitly set by

        <option>allow-number</option> option.  If the

        <option>allow-number</option> option is not present in

        <application>pppd</application>'s configuration file, all

        telephone numbers are allowed to establish connection with the

        server computer, so be sure to include the

        <option>allow-number</option> option in

        <application>pppd</application>'s configuration file if you

        want to control who can/cannot establish connection with the

        server computer.

    </para>

<screen>

##### centos-pppd-config will overwrite this part!!! (begin) #####

allow-number 12345

allow-number 21345

allow-number 34567

##### centos-pppd-config will overwrite this part!!! (end) #####

</screen>

    <para>

        The <application>centos-pppd-admin</application> application

        must be considered part of user profile registration process

        inside the server computer. The

        <application>centos-pppd-admin</application> application would

        be used to control the list of allowed telephone numbers

        inside the <application>pppd</application>'s configuration

        file, based on the list of user profiles. The

        <application>centos-pppd-admin</application> application

        should be executed after any registration/deletion action

        against the list of user profiles with 

        class="username">root</systemitem> privilages in order to be

        able of writing the settings on

        <application>pppd</application>'s configuration file.

    </para>

    <para>

        Redialing consecutive connections from the same telephone

        number without any dealy between call retries must be avoided

        from client computers. This would reduce the possibilities for

        other client computers to establish connection with the server

        computer. To prevent this issue from happening, it would be

        necessary to provide more telephone lines than users

        authorized to establish connection with the server computer.

        Nevertheless, there is only one telephone line available for

        the server computer to use.

    </para>

    </sect2>

    <sect2 id="configurations-dialup-usage-users">

    <title>Administering User Profiles</title>

    <para>

        In order for you to use any service provided by the server

        computer it is required that you register yourself inside the

        server computer creating a user profile.  The user profile

        provides the user information required by services inside the

        server computer (e.g., username, password, e-mail address,

        telephone number, etc.). To register new user profiles, you

        need to use the web application provided by the server

        computer. For example, assuming the domain name of the server

        computer is 

        class="domainname">example.com</systemitem>, the web

        application would be accessable through the following URL:

        <ulink url="https://example.com/people/?action=register" />.

    </para>

    <para>

        To reach the web interface, the first thing you need to do is

        establishing a dial-up connection to the server computer as

        described in 

        linkend="configurations-dialup-client-config-conn"/>. Once the

        dial-up connection has been established, you need to open a

        web browser (e.g., Firefox) and put the URL mentioned above in

        the address space, and press Enter to go. This will present

        you a web page with the instructions you need to follow in

        order to register your user profile.  Other actions like

        updating or deleting your own user profile should be also

        possible from this web interface.

    </para>

    <important>

    <para>

        The web interface used to manage user profiles inside the

        server computer must be presented over an encrypted session in

        order to protect all the information passing through.

    </para>

    </important>

    <para>

        Inside the server computer, all related subsystems in need of

        user information (e.g., Postix, Cyrus-Imapd and Saslauthd)

        retrive user information from one single (LDAP) source. The

        web application provided by the server computer manages all

        these subsystems' configuration files in order to provide a

        pleasant experience for end users.  The web interface must be

        as simple as possible in order to achieve all administration

        tasks in the range of time permitted by the server computer

        before it closes the connection established from the client

        computer.

    </para>

    <para>

        More information about the web interface you need to use to

        manage your user profile inside the server computer can be

        found in <xref linkend="administration-mail" />.

    </para>

    </sect2>

    <sect2 id="configurations-dialup-usage-scope">

    <title>Administering Services</title>

    <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

        registered inside the server computer. 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 

        url="http://www.wikipedia.org/">Wikipedia</ulink> either. For

        this to happen, it is required an established connection

        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.

    </para>

    <para>

        The implementation of services that required persistent

        connections (e.g., <application>chats</application>) will not

        be considered as a practical offer inside the server computer.

        Instead, only asynchronous services (e.g.,

        <application>e-mail</application>) will be supported. This

        restriction is required to reduce the amount of time demanded

        by services. For example, consider an environment where you

        connect to the server computer for sending/receiving e-mails

        messages and then quickly disconnect from it to free the

        telephone line for others to use.  In this environment, there

        is no need for you and other person 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). Morover, 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 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>

    </sect2>

    <sect2 id="configurations-dialup-usage-diskspace">

    <title>Administering Disk Space</title>

    <para>

        The maximum number of registered user profiles is limited

        inside the server computer, based on the maximum disk space

        the server computer confines to such purpose. For example,

        consider an environment where users can get registered

        themselves using a web interface. In this case the web

        interface must know how much disk space is available before

        proceeding to register new mail accounts inside the server

        computer and this way preventing any disk writing when there

        isn't enough free space on disk to perform a new user

        registration.  Considering the server computer has confined

        1GB of disk space to handle the mail service (e.g., mail

        queues, mailboxes, etc.) and each user mailbox is 10MB, it

        will be possible to provide self-registration through the web

        interface for 100 users in total.

    </para>

    <para>

        Another measure related to save disk space might be to remove

        unused user accounts and their related files (e.g., mailboxes)

        from the server computer. For example, consider an environment

        where user accounts are automatically removed from the server

        computer when they don't establish a connection with the

        server computer in a period greater than 7 days since the last

        valid connection established to the server computer.  Once the

        user account is removed, it is no longer functional of course,

        and the person whom lost the account will need to create a new

        one, assuming it want to have access to the mail service

        again.

    </para>

    </sect2>

</sect1>