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Application layer Lecture 7
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Introduction Having finished all the previous layers , we now come to the layer where all the applications are found. The layers below the application layer are there to provide transport services, but they do not do real work for users. However, even in the application layer there is a need for support protocols, to allow the applications to function. Accordingly, we will look at an important one of these protocols. The item in question is DNS, which handles naming within the Internet. Note that there is three real applications: electronic mail, the World Wide Web, and multimedia.
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Domain Name System https://www.youtube.com/watch?v=sABA3S1NtlE
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Introduction There are several applications in the application layer of the Internet model that follow the client/server paradigm. The client/server programs can be divided into two categories: Application that directly used by the user, such as Application that support other application programs. The Domain Name System (DNS) is a supporting program that is used by other programs such as .
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Figure Example of using the DNS service
Figure shows an example of how a DNS client/server program can support an program to find the IP address of an recipient A user of an program may know the address of the recipient The DNS client program sends a request to a DNS server to map the address to the corresponding IP address After that DNS will response to the client with the IP address
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Introduction To identify an entity, TCP\IP protocols use the IP address, which uniquely identifies the connection of a host to the Internet. However, people prefer to use names instead of numeric addresses. Therefore, we need a system that can map a name to an address or an address to a name, which is the DNS server
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DOMAIN NAME SPACE In this design the names are defined in an inverted-tree structure with the root at the top. The tree can have only 128 levels: level 0 (root) to level 127.
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DOMAIN NAME SPACE Label : Domain Name:
Each node in the tree has a label, which is a string with a maximum of 63 characters. The root label is a null string (empty string). Domain Name: Each node in the tree has a domain name. A full domain name is a sequence of labels separated by dots Fully Qualified Domain Name Partially Qualified Domain Name
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Figure 25.3 Domain names and labels
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Hierarchy of Name Servers:
With huge information, there is a need to distribute the information among many computers called DNS servers. One way to do this is to divide the whole space into many domains based on the first level. we let the root stand alone and create as many domains (subtrees) as there are first-level nodes
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DNS IN THE INTERNET DNS is a protocol that can be used in different platforms. In the Internet, the domain name space (tree) is divided into three different sections: generic domains, country domains, and the inverse domain. Topics Generic Domains Country Domains Inverse Domain
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Remote logging & FTP https://www.youtube.com/watch?v=sABA3S1NtlE
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Introduction The main task of the Internet is to provide services for users. Among the most popular applications are: remote logging electronic mail file transfer
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1. REMOTE LOGGING In the Internet, users may want to run application programs at a remote site and create results that can be transferred to their local site. One way to satisfy that demand is to create a client/server application program for each desired service. The better solution is a general-purpose client/server program that lets a user access any application program on a remote computer;
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Logging users are part of the system with some right to access resources. Each authorized user has an identification and password. The user identification defines the user as part of the system. To access the system, the user logs into the system with a user id or log-in name and password. to prevent an unauthorized user from accessing the resources.
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2. FILE TRANSFER File Transfer Protocol (FTP) is the standard mechanism provided by TCP/IP for copying a file from one host to another. Although transferring files from one system to another seems simple and straightforward. Before transferring, some problems must be dealt with first, such as: Two systems may use different file name conventions. Two systems may have different ways to represent text and data. Two systems may have different directory structures. All of these problems have been solved by FTP in a very simple and elegant approach.
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FILE TRANSFER FTP differs from other client-server applications in that it establishes two connections between the hosts. One connection is used for data transfer, the other for control information (commands and responses). Separation of commands and data transfer makes FTP more efficient. We need to transfer only a line of command or a line of response at a time.
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FTP
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Command Processing FTP uses the control connection to establish a communication between the client control process and the server control process. During this communication, the commands are sent from the client to the server and the responses are sent from the server to the client.
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File Transfer File transfer in FTP means one of three things:
Retrieving: A file is to be copied from the server to the client. It is done under the supervision of the RETR command. Storing: A file is to be copied from the client to the server. It is done under the supervision of the STOR command. List: A list of directory or file names is to be sent from the server to the client. This is done under the supervision of the LIST command. FTP treats a list of directory or file names as a file.
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File Transfer
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Another common protocol used by the Internet HTTP: Hyper Transfer Protocol
What is HTTP? Hypertext Transfer Protocol – the Web’s application layer protocol is perhaps the most popular application protocol used in the Internet (or The WEB). HTTP is an request-response client-server protocol . An HTTP client sends a request message to an HTTP server. The server, in turn, returns a response message Prosperities: Pipelined – requests sent as soon as it is encountered Persistent – multiple objects can be sent over a single TCP connection between the server and the client
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There are 4 parts in a URL (Uniform Resource Locator )
Whenever you issue a URL from your browser to get a web resource using HTTP, the browser turns the URL into a request message and sends it to the HTTP server. The HTTP server interprets the request message, and returns you an appropriate response message, which is either the resource you requested or an error message There are 4 parts in a URL (Uniform Resource Locator ) Protocol: The application-level protocol used by the client and server, e.g., HTTP, FTP, and telnet. Hostname: The DNS domain name or IP address of the server. Port: The TCP port number that the server is listening for incoming requests from the clients. Path-and-file-name: The name and location of the requested resource, under the server document base directory.
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The end The end of this course
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