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 the client computers will be public and will rarely change. Credentials for PPP authentication ISP Name: projects.centos.org ISP Phone: +53043515094 Username: faith Password: mail4u.2k10 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). 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. When the client computer calls the server computer, the call is attended by mgetty and then passed to pppd 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 ) 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. 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 pppd 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., 255.255.255.255) which provokes the point-to-point connection to fail when someone tries to transfer data through it.

One PPP network of two computers Provice-A PPP Server Province-A PPP Client --------------------------\ /-------------------------- 192.168.1.1/24 | Modem ~~~ TelephoneLine ~~~ Modem | 192.168.1.2/24 --------------------------/ \--------------------------

The 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 (192.168.1.1 and 192.168.1.2) inside the same newtork (255.255.255.0). 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 ) 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). Based on , 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 . 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. 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.

One PPP network of several computers 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 --------------------------/ \--------------------------

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. 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. 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. 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. 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. 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 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 .

One PPP+Ethernet network of several computers 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

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 (192.168.1/24) 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. 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. 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. +------------------------+ +------------------------+ +------------------------+ +---------------------+ | 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 | 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. +------------------------+ +---------------------+ | 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 | 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. 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 maxconnect and idle options inside the pppd's configuration file. The server computer will attend incoming calls from client computers every night from 10:00PM to 12:00AM. Outside this range of time, the telephone could be answered by somebody, not the computer. This control can be implemented through a cron job and the /etc/nologin.ttyxx file; where ttyxx represents the device name of your Modem (e.g., /etc/nologin.ttyACM0 would prevent the Modem device installed in /dev/ttyACM0 from answering calls). The implementation of internet services which require persistent connections (e.g., chats) should not be considered as a practical offer for PPP client computers. Instead, only asynchronous services (e.g., e-mail) 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. 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. 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 Postfix 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. 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 Google or Wikipedia 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.