~kim0/serverguide/serverguide-review-ch19

<div class="breadcrumbs"><a href="https://help.ubuntu.com/">Ubuntu Documentation</a> > <a href="https://help.ubuntu.com/11.04">Ubuntu 11.04</a> > <span class="breadcrumb-link"><a href="index.html">Ubuntu Server Guide</a></span> > <span class="breadcrumb-link"><a href="networking.html">Networking</a></span> > <span class="breadcrumb-node">TCP/IP</span></div>

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<p>

The Transmission Control Protocol and Internet Protocol (TCP/IP) is a standard

set of protocols developed in the late 1970s by the Defense Advanced Research

Projects Agency (DARPA) as a means of communication between different types of

computers and computer networks. TCP/IP is the driving force of the Internet,

and thus it is the most popular set of network protocols on Earth.

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<h3 class="title"><a id="tcpip-introduction"></a>TCP/IP Introduction</h3>

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<p>

The two protocol components of TCP/IP deal with different aspects of computer networking.

<span class="emphasis"><em>Internet Protocol</em></span>, the "IP" of TCP/IP is a

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

IP Datagram consists of a header followed by a message. The <span class="emphasis"><em>

Transmission Control Protocol</em></span> is the "TCP" of TCP/IP and enables network hosts

to establish connections which may be used to exchange data streams. TCP also guarantees

that the data between connections is delivered and that it arrives at one network host in

the same order as sent from another network host.

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<h3 class="title"><a id="tcpip-configuration"></a>TCP/IP Configuration</h3>

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<p>

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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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<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, where twelve (12)

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represents 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. A network

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host using the private IP address 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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network.

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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 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 private Class C IP network,

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192.168.1.0, the broadcast address is 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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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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<p>

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Access the system manual page for <code class="filename">interfaces</code> with the following command:

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<span class="command"><strong>man interfaces</strong></span>

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</pre>

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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 class="command"><strong>man resolv.conf</strong></span>

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</pre>

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<h3 class="title"><a id="ip-routing"></a>IP Routing</h3>

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<p>

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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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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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<p>

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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 class="application"><strong>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 is 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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<p>

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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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</p>

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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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<p>

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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 class="application"><strong>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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<h3 class="title"><a id="daemons"></a>Daemons</h3>

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<p>

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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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<h3 class="title"><a id="tcpip-resources"></a>Resources</h3>

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There are man pages for <a class="ulink" href="http://manpages.ubuntu.com/manpages/natty/en/man7/tcp.7.html" target="_top">TCP</a> and

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<a class="ulink" href="http://manpages.ubuntu.com/manpages/natty/man7/ip.7.html" target="_top">IP</a> that contain more useful information.

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Also, see the <a class="ulink" href="http://www.redbooks.ibm.com/abstracts/gg243376.html" target="_top">TCP/IP Tutorial and Technical Overview</a>

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IBM Redbook.

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Another resource is O'Reilly's <a class="ulink" href="http://oreilly.com/catalog/9780596002978/" target="_top">TCP/IP Network Administration</a>.

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<td width="40%" align="left" valign="top">Network Configuration </td>

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</a>

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<td width="40%" align="right" valign="top"> Dynamic Host Configuration Protocol (DHCP)</td>

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<p>The material in this document is available under a free license, see <a href="/legal.html">Legal</a> for details<br />

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For information on contributing see the <a href="https://wiki.ubuntu.com/DocumentationTeam">Ubuntu Documentation Team wiki page</a>. To report a problem, visit the <a href="https://bugs.launchpad.net/ubuntu/+source/ubuntu-docs">bug page for Ubuntu Documentation</a></p>

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