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<div style="position: absolute; top: 25px; right: 100px; text-align: right; font-size: xx-large; font-weight: bold; text-shadow: #fff 0px 0px 5px; color: #444">TCP/IP</div>
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<div class="navCenter">Networking</div>
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<div class="sect1" lang="C" xml:lang="C">
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<div class="titlepage">
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<h2 class="title" style="clear: both"><a id="tcpip"></a>TCP/IP</h2>
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The Transmission Control Protocol and Internet Protocol (TCP/IP) is a standard
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set of protocols developed in the late 1970s by the Defense Advanced Research
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Projects Agency (DARPA) as a means of communication between different types of
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computers and computer networks. TCP/IP is the driving force of the Internet,
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and thus it is the most popular set of network protocols on Earth.
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<div class="sect2" lang="C" xml:lang="C">
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<div class="titlepage">
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<h3 class="title"><a id="tcpip-introduction"></a>TCP/IP Introduction</h3>
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The two protocol components of TCP/IP deal with different aspects of computer networking.
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<span class="emphasis"><em>Internet Protocol</em></span>, the "IP" of TCP/IP is a
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connectionless protocol which deals only with network packet routing using the <span class="italics">IP datagram</span> as the basic unit of networking information. The
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IP datagram consists of a header followed by a message. The <span class="emphasis"><em>
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Transmission Control Protocol</em></span> is the "TCP" of TCP/IP and enables network hosts
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to establish connections which may be used to exchange data streams. TCP also guarantees
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that the data between connections is delivered and that it arrives at one network host in
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the same order as sent from another network host.
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<div class="sect2" lang="C" xml:lang="C">
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<div class="titlepage">
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<h3 class="title"><a id="tcpip-configuration"></a>TCP/IP Configuration</h3>
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The TCP/IP protocol configuration consists of several elements which must be set by
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editing the appropriate configuration files, or deploying solutions such as the Dynamic
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Host Configuration Protocol (DHCP) server which in turn, can be configured to provide the
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proper TCP/IP configuration settings to network clients automatically. These
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configuration values must be set correctly in order to facilitate the proper network
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operation of your Ubuntu system.
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The common configuration elements of TCP/IP and their purposes are as follows:
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<div class="itemizedlist">
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<span class="bold"><strong>IP address</strong></span> The IP address is a unique
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identifying string expressed as four decimal numbers ranging from zero (0)
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to two-hundred and fifty-five (255), separated by periods,
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with each of the four numbers representing eight (8) bits of the address for a
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total length of thirty-two (32) bits for the whole address. This format is called
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<span class="emphasis"><em>dotted quad
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notation</em></span>.</p>
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<span class="bold"><strong>Netmask</strong></span> The Subnet Mask (or simply,
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<span class="emphasis"><em>netmask</em></span>) is a local bit mask, or set of flags
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which separate the portions of an IP address significant to the network from the
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bits significant to the <span class="emphasis"><em>subnetwork</em></span>. For example,
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in a Class C network, the standard netmask is 255.255.255.0 which masks the first
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three bytes of the IP address and allows the last byte of the IP address to
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remain available for specifying hosts on the subnetwork.
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<span class="bold"><strong>Network Address</strong></span> The Network Address represents the
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bytes comprising the network portion of an IP address. For example, the host 12.128.1.2
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in a Class A network would use 12.0.0.0 as the network address, which uses twelve (12)
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to represent the first byte of the IP address, (the network part) and zeroes (0)
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in all of the remaining three bytes to represent the potential host values. Network
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hosts using the very common private and non-routable IP addresses such as 192.168.1.100
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would in turn use a Network Address of 192.168.1.0, which specifies the first three
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bytes of the Class C 192.168.1 network and a zero (0) for all the possible hosts on the
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<span class="bold"><strong>Broadcast Address</strong></span> The Broadcast Address is an IP address
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which allows network data to be sent simultaneously to all hosts on a given subnetwork rather
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than specifying a particular network host. The standard general broadcast address for IP networks is
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255.255.255.255, but this broadcast address cannot be used to send a broadcast message to
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every host on the Internet because routers block it. A more appropriate broadcast address
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is set to match a specific subnetwork. For example, on the popular private Class C IP network,
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192.168.1.0, the broadcast address should be configured as 192.168.1.255. Broadcast messages are
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typically produced by network protocols such as the Address Resolution Protocol (ARP) and the
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Routing Information Protocol (RIP).
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<span class="bold"><strong>Gateway Address</strong></span> A Gateway Address is the IP address through which
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a particular network, or host on a network, may be reached. If one network host wishes to communicate
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with another network host, and that host is not located on the same network, then a
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<span class="emphasis"><em>gateway</em></span> must be used. In many cases, the Gateway Address will be that
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of a router on the same network, which will in turn pass traffic on to other networks or hosts, such as
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Internet hosts. The value of the Gateway Address setting must be correct, or your system will not be able
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to reach any hosts beyond those on the same network.
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<span class="bold"><strong>Nameserver Address</strong></span> Nameserver Addresses represent the IP addresses of
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Domain Name Service (DNS) systems, which resolve network hostnames into IP addresses. There are three levels of
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Nameserver Addresses, which may be specified in order of precedence: The
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<span class="emphasis"><em>Primary</em></span>
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Nameserver, the <span class="emphasis"><em>Secondary</em></span> Nameserver, and the
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<span class="emphasis"><em>Tertiary</em></span>
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Nameserver. In order for your system to be able to resolve network hostnames into their
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corresponding IP addresses, you must specify valid Nameserver Addresses which you are authorized to use
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in your system's TCP/IP configuration. In many cases these addresses can and will be provided by your
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network service provider, but many free and publicly accessible Nameservers are available for use, such as
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the Level3 (Verizon) servers with IP addresses from
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4.2.2.1 to 4.2.2.6. </p>
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<div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;">
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<h3 class="title">Tip</h3>
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The IP address, Netmask, Network Address, Broadcast Address, and Gateway Address
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are typically specified via the appropriate directives in the file <code class="filename">/etc/network/interfaces</code>. The Nameserver Addresses are typically specified via <span class="emphasis"><em>nameserver</em></span>
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directives in the file <code class="filename">/etc/resolv.conf</code>. For more information,
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view the system manual page for <code class="filename">interfaces</code> or <code class="filename">resolv.conf</code> respectively, with the following commands typed at a terminal prompt:
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Access the system manual page for <code class="filename">interfaces</code> with the following command:
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<span xmlns="" xmlns:doc="http://nwalsh.com/xsl/documentation/1.0" class="command"><span xmlns="http://www.w3.org/1999/xhtml"><strong class="command">man interfaces</strong></span></span>
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Access the system manual page for <code class="filename">resolv.conf</code> with the following command:
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<span xmlns="" xmlns:doc="http://nwalsh.com/xsl/documentation/1.0" class="command">
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<span xmlns="http://www.w3.org/1999/xhtml">
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<strong class="command">man resolv.conf</strong>
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<div class="sect2" lang="C" xml:lang="C">
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<div class="titlepage">
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<h3 class="title"><a id="ip-routing"></a>IP Routing</h3>
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IP routing is a means of specifying and discovering paths in a TCP/IP network along which
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network data may be sent. Routing uses a set of <span class="emphasis"><em>routing tables</em></span>
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to direct the forwarding of network data packets from their source to the destination, often
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via many intermediary network nodes known as <span class="emphasis"><em>routers</em></span>.
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IP Routing is the principal mode of path discovery on the Internet. There are two primary forms of
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IP Routing: <span class="emphasis"><em>Static Routing</em></span> and
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<span class="emphasis"><em>Dynamic Routing.</em></span>
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Static routing involves manually adding IP routes to the system's routing table, and this is usually
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done by manipulating the routing table with the <span><strong class="application">route</strong></span> command. Static routing enjoys
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many advantages over dynamic routing, such as simplicity of implementation on smaller networks,
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predictability (the routing table is always computed in advance, and thus the route is precisely the
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same each time it used), and low overhead on other routers and network links due to the lack of a
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dynamic routing protocol. However, static routing does present some disadvantages as well. For example,
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static routing is limited to small networks and does not scale well. Static routing also fails completely
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to adapt to network outages and failures along the route due to the fixed nature of the route.
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Dynamic Routing depends on large networks with multiple possible IP routes from a source to a destination
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and makes use of special routing protocols, such as the Router Information Protocol (RIP), which handle
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the automatic adjustments in routing tables that make dynamic routing possible. Dynamic routing
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has several advantages over static routing, such as superior scalability and the ability to adapt
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to failures and outages along network routes. Additionally, there is less manual configuration of the
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routing tables, since routers learn from one another about their existence and available routes. This trait
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also eliminates the possibility of introducing mistakes in the routing tables via human error.
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Dynamic routing is not perfect, however, and presents disadvantages such as heightened complexity and
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additional network overhead from router communications, which does not immediately benefit the end users,
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but still consumes network bandwidth.
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<div class="sect2" lang="C" xml:lang="C">
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<div class="titlepage">
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<h3 class="title"><a id="tcp-and-udp"></a>TCP and UDP</h3>
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TCP is a connection-based protocol, offering error correction and guaranteed delivery of
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data via what is known as <span class="emphasis"><em>flow control</em></span>. Flow control
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determines when the flow of a data stream needs to be stopped, and previously sent data
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packets should to be re-sent due to problems such as <span class="emphasis"><em>collisions</em></span>,
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for example, thus ensuring complete and accurate delivery of the data. TCP is typically
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used in the exchange of important information such as database transactions.
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The User Datagram Protocol (UDP), on the other hand, is a <span class="emphasis"><em>connectionless</em></span>
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protocol which seldom deals with the transmission of important data because it lacks flow
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control or any other method to ensure reliable delivery of the data. UDP is commonly used
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in such applications as audio and video streaming, where it is considerably faster than
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TCP due to the lack of error correction and flow control, and where the loss of a few packets
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is not generally catastrophic.
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<div class="sect2" lang="C" xml:lang="C">
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<div class="titlepage">
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<h3 class="title"><a id="icmp"></a>ICMP</h3>
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The Internet Control Messaging Protocol (ICMP) is an extension to the Internet Protocol (IP) as defined
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in the Request For Comments (RFC) #792 and supports network packets containing control, error, and
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informational messages. ICMP is used by such network applications as the <span><strong class="application">ping</strong></span>
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utility, which can determine the availability of a network host or device. Examples of some error messages
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returned by ICMP which are useful to both network hosts and devices such as routers, include
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<span class="emphasis"><em>Destination Unreachable</em></span> and <span class="emphasis"><em>Time Exceeded</em></span>.
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<div class="sect2" lang="C" xml:lang="C">
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<div class="titlepage">
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<h3 class="title"><a id="daemons"></a>Daemons</h3>
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Daemons are special system applications which typically execute continuously in the background and
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await requests for the functions they provide from other applications. Many daemons are network-centric; that is,
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a large number of daemons executing in the background on an Ubuntu system may provide network-related functionality.
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Some examples of such network daemons include the <span class="emphasis"><em>Hyper Text Transport Protocol Daemon</em></span>
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(httpd), which provides web server functionality; the <span class="emphasis"><em>Secure SHell Daemon</em></span> (sshd), which
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provides secure remote login shell and file transfer capabilities; and the <span class="emphasis"><em>Internet Message Access
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Protocol Daemon</em></span> (imapd), which provides E-Mail services.
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