UDP, defined in [RFC 768], does just about as little as a transport protocol can do. Aside from the multiplexing/demultiplexing function and some light error checking, it adds nothing to IP. In fact, if the application developer chooses UDP instead of TCP, then the application is almost directly talking with IP. UDP takes messages from the application process, attaches source and destination port number fields for the multiplexing/demultiplexing service, adds two other small fields, and passes the resulting segment to the network layer. The network layer encapsulates the transport-layer segment into an IP datagram and then makes a best-effort attempt to deliver the segment to the receiving host.

If the segment arrives at the receiving host, UDP uses the destination port number to deliver the segment’s data to the correct application process. Note that with UDP there is no handshaking between sending and receiving transport-layer entities before sending a segment. For this reason, UDP is said to be connectionless.

ONE is an example of an application-layer protocol that typically uses UDP. When the DNS application in a host wants to make a query, it constructs a DNS query message and passes the message to UDP. Without performing any handshaking with the UDP entity running on the destination end system, the host-side UDP adds header fields to the message and passes the resulting segment to the network layer. The network layer encapsulates the UDP segment into a datagram and sends the datagram to a name server. The DNS application at the querying host then waits for a reply to its query. If it doesn’t receive a reply (possibly because the underlying network lost the query or the reply), either it tries sending the query to another name server, or it informs the invoking application that it can’t get a reply.

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This item looks correct because most people refer to the port number (23) and think, “Yes, that’s the port used for Telnet.” The catch is that you can’t filter IP on port numbers (only TCP and UDP). Another indicator is the use of an extended access list number but no destination address or “any” for the destination.

Also, never forget that the right answer is the Cisco answer. In many cases, more than one appropriate answer is presented, but the correct answer is the one that Cisco recommends.

Here are some general tips for exam success:

• Arrive early at the exam center, so you can relax and review your study materials.
• Read the questions carefully. Don’t just jump to conclus½ns. Make sure that you’re clear about exactly what eAch question asks.
• Don’t leave any questions unanswered. They count against you.
• When answering multiple-choice questions that you’re not sure about, use a process of elimination to get rid of the obviously incorrect answers first. Doing this greatly improves your odds if you need to make an educated guess.
• As of this writing, the written exams still allow you to move forward and backward. However, it is best to always check the Cisco Web site before taking any exam to get the most up-to-date information.

After you complete an exam, you’ll get immediate, online notification of your pass or fail status, a printed Examination Score Report that indicates your pass or fail status, and your exam results by section. (The test administrator will give you the printed score report.) Test scores are automatically forwarded to Cisco within five working days after you take the test, so you don’t need to send your score to them. If you pass the exam, you’ll receive confirmation from Cisco, typically within two to four weeks.

Recall from many network applications consist of a pair of programs—a client program and a server program—residing in to different end systems. When these two programs are executed, .a client and a server process are created, and these processes communicate with each other by reading from and writing to sockets.

When creating a network application, the developer’s main task is to write the code for both the client and server programs, There are two sorts of network applications. One sort is an implementation of a protocol standard defined in, for example, an RFC. For such in implementation, the client and server programs must conform to the rules dictated by the RFC. For example, the client program could be an implementation of the client side of the FTP protocol the server program could be an implementation of the FTP server protocol, also explicitly defined in RFC 959. If one developer writes code for the client program and an independent developer writes code for the server program, and both develop- en carefully follow the rules of the RFC then the two programs will be able to interoperate. Indeed, many of today’s network applications involve communication between client and server programs that have been created by independent develop- en—for example, a firefox browser communicating with an Apache Web server, or an FI’P client on a PC uploading a file to a Linux FTP server. When a client or server program implements a protocol defined in an REC, it should use the port number associated with the protocol. The other sort of network application is a proprietary network application. In this case the application-layer protocols used by the client and server programs do not necessarily conform to any existing RFC.’A single developer (or development team) creates both the client and server programs, and the developer has complete control over what goes in the code. But because the code does not implement a public-domain protocol, other independent developers will not be able to develop code that interoperates with the application. When developing a proprietary application, the developer must be careful not to use one of the well-known port numbers defined in the RFC.

We examine the key issues in developing a proprietary client-server application. During the development phase, one of the first decisions the developer must make is whether the application is to run over TCP or over UDP. Recall that TCP is connection oriented and provides a reliable byte-stream channel through which data flows between two end systems. UDP is connectionless and sends independent packets of data from one end system to the other, without any guarantees about delivery.

We develop a simple client application that runs over TCP we develop a simple client application that runs over UDP. We present these simple TCP and UDP applications in Java. We could have written the code in C or C++, but we opted for Java mostly because the applications are more neatly and cleanly written in Java. With Java there are fewer lines of code, and each line can be explained to the novice programmer without much difficulty but there is no need to be frightened if you are not familiar with Java. You should be able to follow the code if you have experience programming in another language. For readers who are interested in client/server programming in C, there are several good references available [Donahoo 2001; Stevens 1997; Frost 1994; Kurose 1996].

Layered Architecture

Before attempting to organize our thoughts on Internet architecture, let’s look for a human analogy. Actually, we deal with complex systems all the time in our everyday life. Imagine if someone asked you to describe, for example, the airline system. How would you find the structure to describe this complex system that has ticketing agents, baggage checkers, gate personnel, pilots, airplanes, air traffic control, and a worldwide system for routing airplanes? One way to describe this system might be to describe the series of actions you take (or others take for you) when you fly on an airline. You purchase your ticket, check your bags, go to the gate, and eventually get loaded onto the plane. The plane takes off and is routed to its destination. After your plane lands, you deplane at the gate and claim your bags. If the trip was bad, you complain about the flight to the ticket agent (getting nothing for your effort). Source host to destination host in the Internet but this is not quite the analogy we are after. We are looking for some. We note that there is a ticketing function at each end; there is also a baggage function for already-ticketed passengers and a gate function for already-ticketed and already-baggage-checked passengers. For passengers who have made it through the gate (that is, passengers who are already ticketed, baggage-checked, and through the gate), there is a takeoff and landing function, and while in flight, there is an airplane- routing function. This suggests that we can look at the functionality in a horizontal manner has divided the airline functionality into layers, providing a framework in which we can discuss airline travel. Note that each layer, combined with the Layers below it Implements some functionality some service. At the ticketing layer and below, airline-counter-to-airline-counter transfer of a person is accomplished. At the baggage layer and below, baggage-check-to-baggage-claim transfer of a person and bags is accomplished. Note that the baggage layer provides this service only to an already-ticketed person. At the gate layer, departure-gate-to-arrival-gate transfer of a person and bags is accomplished. At the takeoff/landing layer, runway-to- runway transfer of people and their bags is accomplished. Each layer provides its service by (1) performing certain actions within that layer (for example, at the gate layer, loading and unloading people from an airplane) and by (2) using the services of the layer directly below it (for example, in the gate layer, using the runway-to- runway passenger transfer service of the takeoff/landing layer).

A layered architecture allows u to discuss a well-defined, specific part of a large and complex system. This simplification itself is of considerable value by providing modularity, making it much easier to change the implementation of the service provided by the layer. As long as the layer provides the same service to the layer above it, and uses the same services from the layer below it, the remainder of the system remains unchanged when a layer’s implementation is changed. (Note that changing the implementation of a service is very different from changing the service itself!) For example, if the gate functions were changed (for instance, to have people board and disembark by height), the remainder of the airline system would remain unchanged since the gate layer still provides the same function (loading and unloading people); it simply implements that function in a different manner after the change. For large and complex systems that are constantly being updated, the ability to change the implementation of a service without affecting other components of the system is another important advantage of layering.

• ALOHANet, a microwave network linking universities on the Hawaiian islands [Abramson 1970], as well as DARPA’s packet-satellite [RFC 829] and packet-radio networks [Kahn 1978]

• Telenet, a BBN commercial packet-switching network based on ARPAnet technology
• Cyclades, a French packet-switching network pioneered by Louis Pouzin [Think 2007]
• Time-sharing networks such as Tymnet and the GE Information Services network. among others, in the late 1960s and early 1970s [Schwartz 1977]

• IBM’s SNA (1969—1974), which paralleled the ARPAnet work [Schwartz 19771

The number of networks was growing. With perfect. hindsight we can see that the time was ripe for developing an encompassing architecture for connecting networks together. Pioneering work on interconnecting networks (under the sponsorship of the Defense Advanced Research Projects Agency (DARPA), in essence creating a network of networks, was done by Vinton Cerf and Robed Kahn [Cerf 1974]; the term internetting was coined to describe this work.

These architectural principles were embodied in TCP. The early versions of TCP, however, were quite different from today’s TCP. The early versions of TCP combined a reliable in-se4uence delivery of data via end-system retransmission (still part of today’s TCP) with forwarding functions (which today are performed by IP). Early experimentation with TCP, combined with the recognition of the importance of an unreliable, non-flow-controlled, end-to-end transport service for applications such as packetized voice, led to the separation of IP out of TCP and the development of the. UDP protocol. The three key Internet protocols that we see today—TCP, UDP, and IP—were conceptually in place by the end of the 1970s.

In addition to the DARPA Internet-related research, many other important networking activities were underway. In Hawaii, Norman Abramson was developing ALOHAnet, a packet-based radio network that allowed multiple remote sites on the Hawaiian islands to communicate with each other. The ALOHA protocol [Abramson 1970] was the first multiple-access protocol, allowing geographically distributed users to share a single broadcast communication medium (a radio frequency). Metcalfe and Boggs built on Abramson’s multiple-access protocol work wh5n they developed the Ethernet protocol [Metcalfe 1976] for wire-based shared broadcast networks. Interestingly, Metcalfe and Boggs’ Ethernet protocol was motivated by the need to connect multiple PCs, printers, and shared disks [Perkins 1994]. Twenty-five years ago, well before’the PC revolution and the explosion of networks, Metcalfe and Boggs were laying the foundation for today’s PC LANs. Ethernet technology represented an important step for internetworking as well. Each Ethernet local area network was itself a network, and as the number of LANs proliferated, the need to internetwork these LANs together became increasingly impor$ant. We’ll discuss Ethernet, ALOHA, and other LAN technologies.

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Processes on two different end systems communicate with each other by exchanging messages across the, computer network. A sending process creates and sends messages into the network; a receiving process receives these messages and possibly responds by sending messages back.

Client and Server Processes

A network application consists of pain of processes that send messages to each other over a network. For example, in the Web application a client browser process exchanges messages with a Web server process. In a P2P file-sharing system, a file is transferred from a process in one peer to a process in another peer. For each pair of communicating processes, we typically label one of the two processes as the client and

the other process as the server. With the Web, a browser is a client process and a Web server is a server process. With P2P file sharing, the peer that is downloading the file is labeled as the client, and the peer that is uploading the file is labeled as the server, You may have observed that in some applications, such as in P2P file sharing, a process can be both a client and a server Indeed, a process in a P2P file-sharing system can both upload and download files. Nevertheless, in the context of any given communication session between a pair of processes we can still label one process the client and the other process as the server. We define the client and server processes as follows:

In the context of a communication session between a pair of processes the process that initiates the communication (that is, initially contacts the other process at the beginning of the session) is labeled as the client. The process that waits to be contacted to begin the session is the server.

In the Web, a browser process initializes contact with a Web server process; hence the browser process is the client and the Web server process is the server. In P2P file sharing, when Peer A asks Peer B to send a specific file, Peer A is the client and Peer B is the server in the context of this specific communication session. When there’s no confusion, we’ll sometimes also use the terminology “client side and server side of an application.” we’ll step through simple code for both the client and server sides of network applications.

Cisco Certified Network Associate (CCNA) 2.0

The CCNA certification is the first in the new line of Cisco, certifications and is a precursor to all current Cisco certifications. With the new certification programs, Cisco has created a stepping stone approach to CCIE certification non Now you can become a Cisco Certified Network Associate for the mea get cost of  Sybex’s CCNA Cisco Certified Network associate Study Guide, plus $100 for the test And you don’t have to stop there—you can continue with your studies and achieve a higher certification called the Cisco Certified Network Professional (CCNP) Someone with a CCNP has all the skills and knowledge needed to attempt the CUE tab. However, because no textbook: can take the place of practical experience, we’ll discuss what else you need to be ready for the CCIE tab shortly.

Cisco Certified Network Professional (CCNP) 2.0

Cisco Certified Network Professional CCNP), Cisco’s new certification, has opened up many opportunities for those individuals wishing to become Osco certified but lacking the training, the expertise, or the bucks to pass the notorious and often failed two-day Cisco torture lab. The new Cisco certifications will truly provide exciting new opportunities for the CNE and MCSE who are unsure of how to advance to a higher level.

So, you’re thinking, Great, what do I do after passing the CCNA exam Well, if you want to become a CUE f Routing and Switching (the most popular certification), understand that there’s more than one path to that much- coveted CCIE certification The first way is to continue studying and become a Cisco Certified Network Professional (CCNP), which means four more tests, in addition to the CCNA certification. The CCNP program will prepare you to understand and comprehensively tackle the internetworking issues of today and beyond—and it is not limited to the Cisco world. You will undergo an immense metamorphosis, vastly increasing your knowledge and skills through the process of obtaining these certifications.

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