tyt02fi.htm

tcpip 协议学习电子书籍 第一次上传东西

<IMG SRC="note.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/note.gif" WIDTH = 75 HEIGHT = 46>It is usually easy to tell which type of Ethernet network is being used by checking the connector to a network card. If it has a telephone-style plug, it is 10BaseT. The cable for 10BaseT looks the same as telephone cable. If the network has a D-shaped connector with many pins in it, it is 10Base5. A 10Base2 network has a connector similar to a cable TV coaxial connector, except it locks into place. The 10Base2 connector is always circular.

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<BR>The size of a network is also a good indicator. 10Base5 is used in large networks with many devices and long transmission runs. 10Base2 is used in smaller networks, usually with all the network devices in fairly close proximity. Twisted-pair (10BaseT) networks are often used for very small networks with a maximum of a few dozen devices in close proximity.</NOTE>

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<P>Ethernet and TCP/IP work well together, with Ethernet providing the physical cabling (layers one and two) and TCP/IP the communications protocol (layers three and four) that is broadcast over the cable. The two have their own processes for packaging information: TCP/IP uses 32-bit addresses, whereas Ethernet uses a 48-bit scheme. The two work together, however, because of one component of TCP/IP called the Address Resolution Protocol (ARP), which converts between the two schemes. (I discuss ARP in more detail later, in the section titled &quot;Address Resolution Protocol.&quot;)

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<P>Ethernet relies on a protocol called Carrier Sense Multiple Access with Collision Detect (CSMA/CD). To simplify the process, a device checks the network cable to see if anything is currently being sent. If it is clear, the device sends its data. If the cable is busy (carrier detect), the device waits for it to clear. If two devices transmit at the same time (a collision), the devices know because of their constant comparison of the cable traffic to the data in the sending buffer. If a collision occurs, the devices wait a random amount of time before trying again.

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<FONT SIZE=5 COLOR="#FF0000"><B>The Internet</B></FONT></CENTER></H3>

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<P>As ARPANET grew out of a military-only network to add subnetworks in universities, corporations, and user communities, it became known as the Internet. There is no single network called the Internet, however. The term refers to the collective network of subnetworks. The one thing they all have in common is TCP/IP as a communications protocol.

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<P>As described in the first chapter, the organization of the Internet and adoption of new standards is controlled by the Internet Advisory Board (IAB). Among other things, the IAB coordinates several task forces, including the Internet Engineering Task Force (IETF) and Internet Research Task Force (IRTF). In a nutshell, the IRTF is concerned with ongoing research, whereas the IETF handles the implementation and engineering aspects associated with the Internet.

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<P>A body that has some bearing on the IAB is the Federal Networking Council (FNC), which serves as an intermediary between the IAB and the government. The FNC has an advisory capacity to the IAB and its task forces, as well as the responsibility for managing the government's use of the Internet and other networks. Because the government was responsible for funding the development of the Internet, it retains a considerable amount of control, as well as sponsoring some research and expansion of the Internet.

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<FONT SIZE=4 COLOR="#FF0000"><B>The Structure of the Internet</B></FONT></CENTER></H4>

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<P>As mentioned earlier, the Internet is not a single network but a collection of networks that communicate with each other through gateways. For the purposes of this chapter, a <I>gateway</I> (sometimes called a <I>router</I>) is defined as a system that performs relay functions between networks, as shown in Figure 2.3. The different networks connected to each other through gateways are often called subnetworks, because they are a smaller part of the larger overall network. This does not imply that a subnetwork is small or dependent on the larger network. Subnetworks are complete networks, but they are connected through a gateway as a part of a larger internetwork, or in this case the Internet.

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<P><B><A HREF="02tyt03.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/02tyt03.gif">Figure 2.3. Gateways act as relays between </B><B>subnetworks.</A></B>

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<P>With TCP/IP, all interconnections between physical networks are through gateways. An important point to remember for use later is that gateways route information packets based on their destination network name, not the destination machine. Gateways are supposed to be completely transparent to the user, which alleviates the gateway from handling user applications (unless the machine that is acting as a gateway is also someone's work machine or a local network server, as is often the case w<A ID="I2" NAME="I2"></A>ith small networks). Put simply, the gateway's sole task is to receive a Protocol Data Unit (PDU) from either the internetwork or the local network and either route it on to the next gateway or pass it into the local network for routing to the proper user.

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<P>Gateways work with any kind of hardware and operating system, as long as they are designed to communicate with the other gateways they are attached to (which in this case means that it uses TCP/IP). Whether the gateway is leading to a Macintosh network, a set of IBM PCs, or mainframes from a dozen different companies doesn't matter to the gateway or the PDUs it handles.

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<IMG SRC="note.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/note.gif" WIDTH = 75 HEIGHT = 46>There are actually several types of gateways, each performing a difference type of task. I look at the different gateways in more detail on Day 5, &quot;Gateway and Routing Protocols.&quot;</NOTE>

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<P>In the United States, the Internet has the NFSNET as its backbone, as shown in Figure 2.4. Among the primary networks connected to the NFSNET are NASA's Space Physics Analysis Network (SPAN), the Computer Science Network (CSNET), and several other networks such as WESTNET and the San Diego Supercomputer Network (SDSCNET), not shown in Figure 2.4. There are also other smaller user-oriented networks such as the Because It's Time Network (BITNET) and UUNET, which provide connectivity through gateways for smaller sites that can't or don't want to establish a direct gateway to the Internet.

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<P><B><A HREF="02tyt04.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/02tyt04.gif">Figure 2.4. The US Internet network.</A></B>

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<P>The NFSNET backbone is comprised of approximately 3,000 research sites, connected by T-3 leased lines running at 44.736 Megabits per second. Tests are currently underway to increase the operational speed of the backbone to enable more throughput and accommodate the rapidly increasing number of users. Several technologies are being field-tested, including Synchronous Optical Network (SONET), Asynchronous Transfer Mode (ATM), and ANSI's proposed High-Performance Parallel Interface (HPPI). These new systems can produce speeds approaching 1 Gigabit per second.

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<FONT SIZE=4 COLOR="#FF0000"><B>The Internet Layers</B></FONT></CENTER></H4>

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<P>Most internetworks, including the Internet, can be thought of as a layered architecture (yes, even more layers!) to simplify understanding. The layer concept helps in the task of developing applications for internetworks. The layering also shows how the different parts of TCP/IP work together. The more logical structure brought about by using a layering process has already been seen in the first chapter for the OSI model, so applying it to the Internet makes sense. Be careful to think of these layers as conceptual only; they are not really physical or software layers as such (unlike the OSI or TCP/IP layers).

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<P>It is convenient to think of the Internet as having four layers. This layered Internet architecture is shown in Figure 2.5. These layers should not be confused with the architecture of each machine, as described in the OSI seven-layer model. Instead, they are a method of seeing how the internetwork, network, TCP/IP, and the individual machines work together. Independent machines reside in the subnetwork layer at the bottom of the architecture, connected together in a local area network (LAN) and referred to as the subnetwork, a term you saw in the last section.

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<P><B><A HREF="02tyt05.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/02tyt05.gif">Figure 2.5. The Internet architecture.</A></B>

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<P>On top of the subnetwork layer is the internetwork layer, which provides the functionality for communications between networks through gateways. Each subnetwork uses gateways to connect to the other subnetworks in the internetwork. The internetwork layer is where data gets transferred from gateway to gateway until it reaches its destination and then passes into the subnetwork layer. The internetwork layer runs the Internet Protocol (IP).

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<P>The service provider protocol layer is responsible for the overall end-to-end communications of the network. This is the layer that runs the Transmission Control Protocol (TCP) and other protocols. It handles the data traffic flow itself and ensures reliability for the message transfer.

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<P>The top layer is the application services layer, which supports the interfaces to the user applications. This layer interfaces to electronic mail, remote file transfers, and remote access. Several protocols are used in this layer, many of which you will read about later.

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<P>To see how the Internet architecture model works, a simple example is useful. Assume that an application on one machine wants to transfer a datagram to an application on another machine in a different subnetwork. Without all the signals between layers, and simplifying the architecture a little, the process is shown in Figure 2.6. The layers in the sending and receiving machines are the OSI layers, with the equivalent Internet architecture layers indicated.

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<P><B><A HREF="02tyt06.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/02tyt06.gif">Figure 2.6. Transfer of a datagram over an </B><B>internetwork.</A></B>

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<P>The data is sent down the layers of the sending machine, assembling the datagram with the Protocol Control Information (PCI) as it goes. From the physical layer, the datagram (which is sometimes called a <I>frame</I> after the data link layer has added its header and trailing information) is sent out to the local area network. The LAN routes the information to the gateway out to the internetwork. During this process, the LAN has no concern about the message contained in the datagram. Some networks, however, alter the header information to show, among other things, the machines it has passed through.

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<P>From the gateway, the frame passes from gateway to gateway along the internetwork until it arrives at the destination subnetwork. At each step, the gateway analyzes the datagram's header to determine if it is for the subnetwork the gateway leads to. If not, it routes the datagram back out over the internetwork. This analysis is performed in the physical layer, eliminating the need to pass the frame up and down through different layers on each gateway. The header can be altered at each gateway to reflect its routing path.

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<P>When the datagram is finally received at the destination subnetwork's gateway, the gateway recognizes that the datagram is at its correct subnetwork and routes it into the LAN and eventually to the target machine. The routing is accomplished by reading the header information. When the datagram reaches the destination machine, it passes up through the layers, with each layer stripping off its PCI header and then passing the result on up. At long last, the application layer on the destination machine processes the final header and passes the message to the correct application.

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<P>If the datagram was not data to be processed but a request for a service, such as a remote file transfer, the correct layer on the destination machine would decode the request and route the file back over the internetwork to the original machine. Quite a process!

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<FONT SIZE=4 COLOR="#FF0000"><B>Internetwork Problems</B></FONT></CENTER></H4>

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<P>Not everything goes smoothly when transferring data from one subnetwork to another. All manner of problems can occur, despite the fact that the entire network is using one protocol. A typical problem is a limitation on the size of the datagram. The sending network might support datagrams of 1,024 bytes, but the receiving network might use only 512-byte datagrams (because of a different hardware protocol, for example). This is where the processes of segmentation, separation, reassembly, and concatenation (explained in the last chapter) become important.

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<P>The actual addressing methods used by the different subnetworks can cause conflicts when routing datagrams. Because communicating subnetworks might not have the same network control software, the network-based header information might differ, despite the fact that the communications methods are based on TCP/IP. An associated problem occurs when dealing with the differences between physical and logical machine names. In the same manner, a network that requires encryption instead of clear-text datagrams can affect the decoding of header information. Therefore, differences in the security implemented on the subnetworks can affect datagram traffic. These differences can all be resolved with software, but the problems associated with addressing methods can become considerable.

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<P>Another common problem is the different networks' tolerance for timing problems. Time-out and retry values might differ, so when two subnetworks are trying to establish communication, one might have given up and moved on to another task while the second is still waiting patiently for an acknowledgment signal. Also, if two subnetworks are communicating properly and one gets busy and has to pause the communications process for a short while, the amount of time before the other network assumes a disconnection and gives up might be important. Coordinating the timing over the internetwork can become very complicated.

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<P>Routing methods and the speed of the machines on the network can also affect the internetwork's performance. If a gateway is managed by a particularly slow machine, the traffic coming through the gateway can back up, causing delays and incomplete transmissions for the entire internetwork. Developing an internetwork system that can dynamically adapt to loads and reroute datagrams when a bottleneck occurs is very important.

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<P>There are other factors to consider, such as network management and troubleshooting information, but you should begin to see that simply connecting networks together without due thought does not work. The many different network operating systems and hardware platforms require a logical, well-developed approach to the internetwork. This is outside the scope of TCP/IP, which is simply concerned with the transmission of the datagrams. The TCP/IP implementations on each platform, however, must be able to handle the problems mentioned.

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<FONT SIZE=5 COLOR="#FF0000"><B>Internet Addresses</B></FONT></CENTER></H3>

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<P>Network addresses are analogous to mailing addresses in that they tell a system where to deliver a datagram. Three terms commonly used in the Internet relate to addressing: name, address, and route.

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<IMG SRC="note.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/note.gif" WIDTH = 75 HEIGHT = 46>The term <I>address</I> is often generically used with communications protocols to refer to many different things. It can mean the destination, a port of a machine, a memory location, an application, and more. Take care when you encounter the term to make sure you know what it is really referring to.</NOTE>

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<P>A <I>name</I> is a specific identification of a machine, a user, or an application. It is usually unique and provides an absolute target for the datagram. An <I>address</I> typically identifies where the target is located, usually its physical or logical location in a network. A <I>route</I> tells the system how to get a datagram to the address.

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<P>You use the recipient's name often, either specifying a user name or a machine name, and an application does the same thing transparently to you. From the name, a network software package called the <I>name server</I> tries to resolve the address and the route, making that aspect unimportant to you. When you send electronic mail, you simply indicate the recipient's name, relying on the name server to figure out how to get the mail message to them.

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<P>Using a name server has one other primary advantage besides making the addressing and routing unimportant to the end user: It gives the system or network administrator a lot of freedom to change the network as required, without having to tell each user's machine about any changes. As long as an application can access the name server, any routing changes can be ignored by the application and users.

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<P>Naming conventions differ depending on the platform, the network, and the software release, but following is a typical Ethernet-based Internet subnetwork as an example. There are several types of addressing you need to look at, including the LAN system, as well as the wider internetwork addressing conventions.

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<FONT SIZE=4 COLOR="#FF0000"><B>Subnetwork Addressing</B></FONT></CENTER></H4>