diff --git a/Manuals/Tcpi-ug/Connectivity/Dialup/network.docbook b/Manuals/Tcpi-ug/Connectivity/Dialup/network.docbook new file mode 100644 index 0000000..ce55b99 --- /dev/null +++ b/Manuals/Tcpi-ug/Connectivity/Dialup/network.docbook @@ -0,0 +1,637 @@ + + + The Network Of Computers + + + This section describes how you could distribute server and + client computers to create a collaborative network. + + + + One PPP Network Of Two Computers + + + 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 + + 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). + + + + + + One PPP Network Of Several Computers + + + 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 + + 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. + + + + + + One PPP+Ethernet Network Of Several 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 + + 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. + + + + + + + + About Bridging Calls To Transfer Data + + + 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 | + + + + + About Directing Calls To Transfer Data + + + 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. + + + + + + About Authenticating PPP Users + + + 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. + + + + Credentials for PPP authentication + + 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). + + + + + + About Restricting PPP Connections + + + 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 + and options + inside the pppd's configuration + file as described in . + + + + 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 /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). + + + +# 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 + + + + + + About Providing Internet Services + + + 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, 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. + + + + +