The Application Layer

Transcription

1 The Application Layer Subject Name: Fundamentals of Networking (FON) Subject Code: Domain Name System, Electronic Mail 21-Apr-12 Having finished all the preliminaries, we now come to the layer where all the applications are found. The layers below the application layer are there to provide reliable transport, but they do not do real work for users. Processes running in different hosts communicate with an application-layer protocol Popular application-level protocols: DNS, HTTP, FTP, SMTP, POP3, IMAP In this chapter we will study some real network applications. Programs could refer to hosts, mailboxes and other resources by their network IP. These Numbers are difficult to remember by people? Also, if moving the servers to different machine? Use names instead of IP numbers. Ex. ce.sharif.edu, Need a mechanism to convert names to IP Number Use a [ hosts ] file: As the number of PCs increase? Size increase, Conflict of host names Need a central manager: DNS: Domain Name System 3 4 DNS Name Space For managing large and changing set of names, need a postal like management, JVIMS-MCA, Shree B.K.Edu. Complex, Jamnagar, Gujarat, India Hierarchical addressing Top-level domain: 200 top-level domains Every domain, whether it is a single host or a top-level domain, can have a set of resource records associated with it. For a single host, the most common resource record is just its IP address, but many other kinds of resource records also exist. When a resolver gives a domain name to DNS, what it gets back are the resource records associated with that name. Thus, the primary function of DNS is to map domain names onto resource records. A resource record is a five-tuple. Although they are encoded in binary for efficiency, in most expositions, resource records are presented as ASCII text, one line per resource record. The format we will use is as follows: Domain_name Time_to_live Class Type Value

2 The Domain_name tells the domain to which this record applies. The Time_to_live field gives an indication of how stable the record is. Information that is highly stable is assigned a large value, such as (the number of seconds in 1 day). Class : For Internet information, it is always IN. For non-internet information, other codes can be used, but in practice, these are rarely seen The Type field tells what kind of record this is. The most important types are listed in below table The principal DNS resource records types for IPv In theory at least, a single name server could contain the entire DNS database and respond to all queries about it. In practice, this server would be so overloaded as to be useless. Furthermore, if it ever went down, the entire Internet would be crippled. To avoid the problems associated with having only a single source of information, the DNS name space is divided into nonoverlapping zones. One possible way to divide the name space is shown in below figure. Each zone contains some part of the tree and also contains name servers holding the information about that zone Part of the DNS name space showing the division into zones. 11 Where the zone boundaries are placed within a zone is up to that zone's administrator. This decision is made in large part based on how many name servers are desired, and where. For example, in above figure, Yale has a server for yale.edu that handles eng.yale.edu but not cs.yale.edu, which is a separate zone with its own name servers. Such a decision might be made when a department such as English does not wish to run its own name server, but a department such as computer science does. Consequently, cs.yale.edu is a separate zone but eng.yale.edu is not. When a resolver has a query about a domain name, it passes the query to one of the local name servers. If the domain being sought falls under the jurisdiction of the name server, such as ai.cs.yale.edu falling under cs.yale.edu, it returns the authoritative resource records. An authoritative record is one that comes from the authority that manages the record and is thus always correct

3 They normally consist of two subsystems: The user agents, which allow people to read and send , and the message transfer agents, which move the messages from the source to the destination. The user agents are local programs that provide a command-based, menu-based, or graphical method for interacting with the system. The message transfer agents are typically system daemons, that is, processes that run in the background. Their job is to move through the system Reporting Reporting has to do with telling the originator what happened 1. Composition to the message. Composition refers to the process of creating messages and answers. Was it delivered? Was it rejected? Was it lost? Although any text editor can be used for the body of the message, the 4. Displaying system itself can provide assistance with addressing and the numerous header fields attached to each message. Displaying incoming messages is needed, so, people can read For example, when answering a message, the system can extract the their . sender/originator's address from the incoming and automatically insert it into the proper place in the reply. 5. Disposition 2. Transfer Disposition is the final step and concerns what the recipient does with the message after receiving it. Transfer refers to moving messages from the originator to the recipient. In large part, this requires establishing a connection to the destination or Possibilities include throwing it away before reading, throwing some intermediate machine, outputting the message, and releasing the it away after reading, saving it, and so on. connection. It should also be possible to retrieve and reread saved The system should do this automatically, without bothering the user. messages, forward them, or process them in other ways (a) Paper mail. (b) Electronic mail. A user agent is normally a program (sometimes called a mail reader) that accepts a variety of commands for composing, receiving, and replying to messages, as well as for manipulating mailboxes Sending To send an message, a user must provide the message, the destination address, and possibly some other parameters. The message can be produced with a free-standing text editor, a word processing program, or possibly with a specialized text editor built into the user agent. The destination address must be in a format that the user agent can deal with. Many user agents expect addresses of the form user@dns-address

4 Reading Typically, when a user agent is started up, it looks at the user's mailbox for incoming before displaying anything on the screen. Then it may announce the number of messages in the mailbox or display a one-line summary of each one and wait for a command. The message transfer system is concerned with relaying messages from the originator to the recipient. The simplest way to do this is to establish a transport connection from the source machine to the destination machine and then just transfer the message. An example display of the contents of a mailbox Within the Internet, is delivered by having the source machine establish a TCP connection to port 25 of the destination machine. Listening to this port is an daemon that speaks SMTP It accepts incoming connections and copies messages from them into the appropriate mailboxes. If a message cannot be delivered, an error report containing the first part of the undeliverable message is returned to the sender. SMTP is a simple ASCII protocol. Up until now, we have assumed that all users work on machines that are capable of sending and receiving . As we saw, is delivered by having the sender establish a TCP connection to the receiver and then ship the over it. However, with the advent of people who access the Internet by calling their ISP over a modem, it breaks down. Then it cannot run the SMTP protocol. One solution is to have a message transfer agent on an ISP machine accept for its customers and store it in their mailboxes on an ISP machine. Since this agent can be on-line all the time, can be sent to it 24 hours a day The solution to this problem is to create another protocol that allows user transfer agents (on client PCs) to contact the message transfer agent (on the ISP's machine) and allow e- mail to be copied from the ISP to the user. One such protocol is POP3 (Post Office Protocol Version 3) The situation that used to hold (both sender and receiver having a permanent connection to the Internet) is illustrated in Fig.(a). A situation in which the sender is (currently) on-line but the receiver is not is illustrated in Fig.(b). (a) Sending and reading mail when the receiver has a permanent Internet connection and the user agent runs on the same machine as the message transfer agent. (b) Reading when the receiver has a dial-up connection to an ISP

5 POP3 begins when the user starts the mail reader. The mail reader calls up the ISP (unless there is already a connection) and establishes a TCP connection with the message transfer agent at port 110. Once the connection has been established, the POP3 protocol goes through three states in sequence: Authorization. The authorization state deals with having the user log in. Transactions. The transaction state deals with the user collecting the s and marking them for deletion from the mailbox. Update. The update state actually causes the s to be deleted. The lines marked C: are from the client (user) and S: are from the server (message transfer agent on the ISP's machine). RETR : Retrieve DELE : Delete For a user with one account at one ISP that is always accessed from one PC, POP3 works fine and is widely used due to its simplicity and robustness But, what happen if user wish to use a single account from multiple PC Then POP3 face some problem For the solution of this problem new protocol introduce that is IMAP IMAP assumes that all the will remain on the server indefinitely in multiple mailboxes. IMAP provides extensive mechanisms for reading messages or even parts of messages, a feature useful when using a slow modem to read the text part of a multipart message with large audio and video attachments. Since the working assumption is that messages will not be transferred to the user's computer for permanent storage, IMAP provides mechanisms for creating, destroying, and manipulating multiple mailboxes on the server. In this way a user can maintain a mailbox for each correspondent and move messages there from the inbox after they have been read Independently of whether POP3 or IMAP is used, many systems provide One final topic worth mentioning is Webmail. Some Web sites, for example, hooks for additional processing of incoming . Hotmail and Yahoo, provide service to anyone who wants it. They work An especially valuable feature for many users is the ability to set up as follows. They have normal message transfer agents listening to port 25 for filters. incoming SMTP connections These are rules that are checked when comes in or when the user agent you can establish a TCP connection over which SMTP commands can be sent is started. in the usual way. So far, nothing unusual, except that these big servers are Each rule specifies a condition and an action. often busy, so it may take several attempts to get a TCP connection accepted. For example, a rule could say that any message received from the boss goes to The interesting part is how is delivered. Basically, when the user goes mailbox number 1, any message from a select group of friends goes to mailbox to the Web page, a form is presented in which the user is asked for a number 2, and any message containing certain objectionable words in the login name and password. When the user clicks on Sign In, the login name and Subject line is discarded without comment. password are sent to the server, which then validates them. If the login is successful, the server finds the user's mailbox and builds a listing similar to Still another common feature of final delivery is the ability to install a vacation that of Fig. 7-8, only formatted as a Web page in HTML. The Web page is daemon. This is a program that examines each incoming message and sends then sent to the browser for display. Many of the items on the page are the sender an insipid reply such as clickable, so messages can be read, deleted, and so on. Hi. I'm on vacation. I'll be back on the 24th of August. Have a nice summer

6 Bipin S. Rupadiya (MCA, PGDCA, BCA) Assistant Professor, JVIMS Mo. : rupadiyabipin@gmail.com Blog :