A protocol is, simply put, a set of rules for communication.

In order to get data over the network, for instance an E-mail from your computer to some computer at the other end of the world, lots of different hard- and software needs to work together.

All these pieces of hardware and the different software programs speak different languages. Imagine your E-mail program: it is able to talk to the computer operating system, through a specific protocol, but it is not able to talk to the computer hardware. We need a special program in the operating system that performs this function. In turn, the computer needs to be able to communicate with the telephone line or other Internet hookup method. And behind the scenes, network connection hardware needs to be able to communicate in order to pass your E-mail from one appliance to the other, all the way to the destination computer.

All these different types of communication protocols are classified in 7 layers, which are known as the Open Systems Interconnection Reference Model, the OSI Model for short. For easy understanding, this model is reduced to a 4-layer protocol description, as described in the table below:

Table 10-1. The simplified OSI Model

Each layer can only use the functionality of the layer below; each layer can only export functionality to the layer above. In other words: layers communicate only with adjacent layers. Let’s take the example of your E-mail message again: you enter it through the application layer. In your computer, it travels down the transport and network layer. Your computer puts it on the network through the network access layer. That is also the layer that will move the message around the world. At the destination, the receiving computer will accept the message through it’s own network layer, and will display it to the recepient using the transport and application layer.

• It’s really much more complicated
  The above and following sections are included because you will come across some networking terms sooner or later; they will give you some starting points, should you want to find out about the details.

Linux supports many different networking protocols. We list only the most important:

The Transport Control Protocol and the Internet Protocol are the two most popular ways of communicating on the Internet. A lot of applications, such as your browser and E-mail program, are built on top of this protocol suite.

Very simply put, IP provides a solution for sending packets of information from one machine to another, while TCP ensures that the packets are arranged in streams, so that packets from different applications don’t get mixed up, and that the packets are sent and received in the correct order.

The Internet was originally developed in 1969 for the United States Department of Defense (DoD), mainly for the purpose of interconnecting different-brand computers. Another reason for the development of TCP/IP was to provide a reliable data transport system over an unreliable network.

TCP/IP networking has been present in Linux since its beginnings. It has been implemented from scratch. It is one of the most robust, fast and reliable implementations and is one of the key factors of the success of Linux. Linux and networking are made for each other, in so much that not connecting your Linux system to the network may result in slow startup and other troubles. Even if you don’t use any network connections to other computers, networking protocols are used for internal system and application communications. Linux expects to be networked.

A good starting point for learning more about TCP and IP is in the following documents:

• man 7 ip: Describes the IPv4 protocol implementation on Linux (version 4 currently being the most wide-spread edition of the IP protocol).
• man 7 tcp: Implementation of the TCP protocol.
• RFC793, RFC1122, RFC2001 for TCP, and RFC791, RFC1122 and RFC1112 for IP.
  • The Request For Comments documents contain the descriptions of networking standards, protocols, applications and implementation. These documents are managed by the Internet Engineering Task Force, an international community concerned with the smooth operation of the Internet and the evolution and development of the Internet architecture.
  • Your ISP usually has an RFC archive available, or you can browse the RFCs via http://www.ietf.org/rfc.html.

Nobody expected the Internet to grow as fast as it does. IP proved to have quite some disadvantages when a really large number of computers is in a network, the most important being the availability of unique addresses to assign to each machine participating. Thus, IP version 6 was devised to meet the needs of today’s Internet.

Unfortunately, not all applications and services support IPv6, yet. A migration is currently being set in motion in many environments that can benefit from an upgrade to IPv6. For some applications, the old protocol is still used, for applications that have been reworked the new version is already active. So when checking your network configuration, sometimes it might be a bit confusing since all kinds of measures can be taken to hide one protocol from the other so the two don’t mix up connections.

More information can be found in the following documents:

• man 7 ipv6: the Linux IPv6 protocol implementation.
• RFC1883 describing the IPv6 protocol.

The Linux kernel has built-in support for PPP (Point-to-Point-Protocol), SLIP (Serial Line IP) and PLIP (Parallel Line IP). PPP is the most popular way individual users access their ISP (Internet Service Provider), although in densely populated areas it is often being replaced by PPPOE, PPP over Ethernet, the protocol used in cable modem connections.

Most Linux distributions provide easy-to-use tools for setting up an Internet connection. The only thing you basically need is a username and password to connect to your Internet Service Provider (ISP), and a telephone number in the case of PPP. These data are entered in the graphical configuration tool, which will likely also allow for starting and stopping the connection to your provider.

The Linux kernel has built-in ISDN capabilities. Isdn4linux controls ISDN PC cards and can emulate a modem with the Hayes command set (AT commands). The possibilities range from simply using a terminal program to full connection to the Internet.

Check your system documentation.

Appletalk is the name of Apple’s internetworking stack. It allows a peer-to-peer network model which provides basic functionality such as file and printer sharing. Each machine can simultaneously act as a client and a server, and the software and hardware necessary are included with every Apple computer.

Linux provides full AppleTalk networking. Netatalk is a kernel-level implementation of the AppleTalk Protocol Suite, originally for BSD-derived systems. It includes support for routing AppleTalk, serving UNIX and AFS file systems using AppleShare and serving UNIX printers and accessing AppleTalk printers.

For compatibility with MS Windows environments, the Samba suite, including support for the NMB and SMB protocols, can be installed on any UNIX-like system. The Server Message Block protocol (also called Session Message Block, NetBIOS or LanManager protocol) is used on MS Windows 3.11, NT, 95/98, 2K and XP to share disks and printers.

The basic functions of the Samba suite are: sharing Linux drives with Windows machines, accessing SMB shares from Linux machines, sharing Linux printers with Windows machines and sharing Windows printers with Linux machines.

Most Linux distributions provide a samba package, which does most of the server setup and starts up smbd, the Samba server, and nmbd, the netbios name server, at boot time by default. Samba can be configured graphically, via a web interface or via the command line and text configuration files. The daemons make a Linux machine appear as an MS Windows host in an MS Windows My Network Places/Network Neighbourhood window; a share from a Linux machine will be indistinguishable from a share on any other host in an MS Windows environment.

More information can be found at the following locations:

• man smb.conf: describes the format of the main Samba configuration file.
• Samba Tutorial on LinuxBasics.org.
• Setting up Samba on the Ubuntu wiki.
• The Samba Project Documentation contains an easy to read installation and testing guide, which also explains how to configure your Samba server as a Primary Domain Controller.
• The Samba website contains more documentation, including a troubleshooting guide.

Linux also has support for Amateur Radio, WAN internetworking (X25, Frame Relay, ATM), InfraRed and other wireless connections, but since these protocols usually require special hardware, we won’t discuss them in this document.

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