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Linux. Networking. Clearly. Explained. Bryan Pfaffenberger. University of Virginia. Morgan. Kaufmann. AN IMPRINT OF ACADEMIC PRESS. A HARCOURT.
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This introductory chapter forms the foundation on which the following network configuration and troubleshooting chapters will be built. Familiarity with the concepts explained in the following sections will help answer many of the daily questions often posed by coworkers, friends, and even yourself. The Open System Interconnection OSI model, developed by the International Organization for Standardization, defines how the various hardware and software components involved in data communication should interact with each other.

A good analogy would be a traveler who prepares herself to return home through many dangerous kingdoms by obtaining permits to enter each country at the very beginning of the trip. At each frontier our friend has to hand over a permit to enter the country. Once inside, she asks the border guards for directions to reach the next frontier and displays the permit for that new kingdom as proof that she has a legitimate reason for wanting to go there.

In the OSI model each component along the data communications path is assigned a layer of responsibility, in other words, a kingdom over which it rules. Each layer extracts the permit, or header information, it needs from the data and uses this information to correctly forward what's left to the next layer. This layer also strips away its permit and forwards the data to the next layer, and so the cycle continues for seven layers.

The very first layer of the OSI model describes the transmission attributes of the cabling or wireless frequencies used at each "link" or step along the way. Layer 2 describes the error correction methodologies to be used on the link; layer 3 ensures that the data can hop from link to link on the way to the final destination described in its header. When the data finally arrives, the layer 4 header is used to determine which locally installed software application should receive it. The application uses the guidelines of layer 5 to keep track of the various communications sessions it has with remote computers and uses layer 6 to verify that the communication or file format is correct.

Computer Networking Tutorial - 39 - Routing Tables Explained

Finally, layer 7 defines what the end user will see in the form of an interface, be it graphical on a screen or otherwise. A description of the functions of each layer in the model can be seen in Table It is part of the larger OSI model upon which most data communications is based.

For manageability, the data is usually split into multiple pieces or packets each with its own error detection bytes in the control section or header of the packet.


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The remote computer then receives the packets and reassembles the data and checks for errors. It then passes the data to the program that expects to receive it. How does the computer know what program needs the data? Each IP packet also contains a piece of information in its header called the type field. This informs the computer receiving the data about the type of layer 4 transportation mechanism being used.

This is explained in more detail later.

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TCP keeps track of the packets sent by giving each one a sequence number with the remote server sending back acknowledgment packets confirming correct delivery. Programs that use TCP therefore have a means of detecting connection failures and requesting the retransmission of missing packets. TCP is a good example of a connection-oriented protocol.

Any form of communication requires some form of acknowledgement for it to become meaningful. Someone knocks on the door to a house, the person inside asks "Who is it? Both persons knew who was on the other side of the door before it opened and now a conversation can now begin. TCP acts in a similar way. The communication then continues with a series of segment exchanges, each with the ACK bit set.

The communication terminates with a final ACK from the server that wanted to end the session. This is the equivalent of ending a conversation by saying "I really have to go now, I have to go for lunch", to which the reply is "I think I'm finished here too, see you tomorrow Here is a modified packet trace obtained from the tethereal program discussed in Chapter 4, "Simple Network Troubleshooting". You can clearly see the three way handshake to connect and disconnect the session. In this trace, the sequence number represents the serial number of the first byte of data in the segment.

So in the first line, a random value of was assigned to the first byte and all subsequent bytes for the connection from this host will be sequentially tracked. This makes the second byte in the segment number , the third number etc. The acknowledgment number or Ack, not to be confused with the ACK bit, is the byte serial number of the next segment it expects to receive from the other end, and the total number of bytes cannot exceed the Win or window value that follows it.

If data isn't received correctly, the receiver will re-send the requesting segment asking for the information to be sent again. The TCP code keeps track of all this along with the source and destination ports and IP addresses to ensure that each unique connection is serviced correctly. UDP is a connectionless protocol. Data is sent on a "best effort" basis with the machine that sends the data having no means of verifying whether the data was correctly received by the remote machine.

UDP is usually used for applications in which the data sent is not mission-critical. It is also used when data needs to be broadcast to all available servers on a locally attached network where the creation of dozens of TCP connections for a short burst of data is considered resource-hungry. Certain programs are assigned specific ports that are internationally recognized. Ports below are reserved for privileged system functions, and those above are generally reserved for non-system third-party applications. Usually when a connection is made from a client computer requesting data to the server that contains the data:.

Each IP packet has a Time to Live TTL section that keeps track of the number of network devices the packet has passed through to reach its destination. The server sending the packet sets the initial TTL value, and each network device that the packet passes through then reduces this value by 1. If the TTL value reaches 0, the network device will discard the packet. This mechanism helps to ensure that bad routing on the Internet won't cause packets to aimlessly loop around the network without being removed.

TTLs therefore help to reduce the clogging of data circuits with unnecessary traffic. Remember this concept as it will be helpful in understanding the traceroute troubleshooting technique outlined in Chapter 4, " Simple Network Troubleshooting ", that covers Network Troubleshooting. ICMP provides a suite of error, control, and informational messages for use by the operating system. For example, IP packets will occasionally arrive at a server with corrupted data due to any number of reasons including a bad connection; electrical interference, or even misconfiguration. The server will usually detect this by examining the packet and correlating the contents to what it finds in the IP header's error control section.

It will then issue an ICMP reject message to the original sending machine saying that the data should be re-sent because the original transmission was corrupted. ICMP also includes echo and echo reply messages used by the Linux ping command to confirm network connectivity. Just like a telephone number, it helps to uniquely identify a user of the system.

IP addresses are in reality a string of 32 binary digits or bits.

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For ease of use, network engineers often divide these 32 bits into four sets of 8 bits or octets , each representing a number from 0 to Each number is then separated by a period. An example of an IP address that follows these rules is Note: Chapter 3, " Linux Networking ", which covers Linux specific networking topics, explains how to configure the IP address of your Linux box. Some groups of IP addresses are reserved for use only in private networks and are not routed over the Internet. These are called private IP addresses and have the following ranges:.

You may be wondering how devices using private addresses could ever access the Internet if the use of private addresses on the Internet is illegal. The situation gets even more confusing if you consider the fact that hundreds of thousands of office and home networks use these same addresses. This must cause networking confusion. Don't worry, this problem is overcome by NAT. Whether or not your computer has a network interface card it will have a built-in IP address with which network-aware applications can communicate with one another. This IP address is defined as This concept is important to understand, and will be revisited in many later chapters.

There are many good reasons for this, the two most commonly stated are:. You can configure NAT to be one to one in which you request your ISP to assign you a number of public IP addresses to be used by the Internet-facing interface of your firewall and then you pair each of these addresses to a corresponding server on your protected private IP network. You can also use many to one NAT, in which the firewall maps a single IP address to multiple servers on the network.


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As a general rule, you won't be able to access the public NAT IP addresses from servers on your home network. Basic NAT testing requires you to ask a friend to try to connect to your home network from the Internet. Some of the terms mentioned here may be unfamiliar to you but they will be explained in later sections of this chapter. The reverse isn't true. In this case the data is usually discarded. Port forwarding is a method of counteracting this. The assignment usually occurs when the DHCP configured machine boots up, or regains connectivity to the network.

Linux Network Configuration

The assignment of the IP address usually expires after a predetermined period of time, at which point the DHCP client and server renegotiate a new IP address from the server's predefined pool of addresses. Configuring firewall rules to accommodate access from machines who receive their IP addresses via DHCP is therefore more difficult because the remote IP address will vary from time to time.


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  4. The domain name system DNS is a worldwide server network used to help translate easy to remember domain names like www. Here step by step description of what happens with a DNS lookup. As you can imagine, this process can cause a noticeable delay when you are browsing the Web.

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    Each server in the chain will store the most frequent DNS name to IP address lookups in a memory cache which helps to speed up the response. You should also be aware that there is now a version 6 IPv6 that has recently been developed as a replacement. With only 32 bits, the allocation of version 4 addresses will soon be exhausted between all the world's ISPs. Version 6, which uses a much larger bit address offers eighty billion, billion, billion times more IP addresses which it is hoped should last for most of the 21st century.

    IPv6 packets are also labeled to provide quality-of-service information that can be used in prioritizing real-time applications, such as video and voice, over less time-sensitive ones such as regular Web surfing and chat. Most current operating systems support IPv6 even though it isn't currently being used extensively within corporate or home environments.

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    Expect it to become an increasingly bigger part of your network planning in years to come. Subnet masks are used to tell which part of the IP address represents the network on which the computer is connected network portion and the computer's unique identifier on that network host portion. The term netmasks is often used interchangeably with the term subnet masks , this book will use the latter term for the sake of consistency.

    A simple analogy would be a phone number, such as The represents the area code, and the represents the telephone within that area code. Subnet masks allow you to specify how long you want the area code to be network portion at the expense of the number of telephones in that are in the area code host portion.

    Most home networks use a subnet mask of Each means this octet is for the area code network portion. So if your server has an IP address of In all cases, the first IP address in a network is reserved as the network's base address and the last one is reserved for broadcast traffic that is intended to be received by all devices on the network.