<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?
— 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 <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--------------|<~~~~~~~~~>|---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 <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 ] && /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 <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>