Explain how the functions of the application layer, session layer, and presentation layer Explain how common application layer protocols interact with end user. Functionality of the TCP/IP Application Layer protocols fit roughly into the . Chapter 3 # SPENGERGASSE. Protocol examples. medical-site.info medical-site.info medical-site.info end-user applications? □ How do the TCP/IP application layer protocols provide the services specified by the upper layers of the OSI model? □ How do people.
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(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. write programs that: • run on (different) end systems. • communicate over network. • e.g., web server software communicates with browser software no need to. CS Computer Networks. 2: Application Layer. 2. Chapter 2: Application Layer. Goals: • conceptual, implementation aspects of network application protocols.
Hypertext is well organized documentation system which uses hyperlinks to link the pages in the text documents. HTTP works on client server model.
When a user wants to access any HTTP page on the internet, the client machine at user end initiates a TCP connection to server on port When the server accepts the client request, the client is authorized to access web pages.
To access the web pages, a client normally uses web browsers, who are responsible for initiating, maintaining, and closing TCP connections.
HTTP is a stateless protocol, which means the Server maintains no information about earlier requests by clients. HTTP versions: HTTP 1. At most one object can be sent over a single TCP connection.
In this version, multiple objects can be sent over a single TCP connection. It uses UDP protocol for transport layer communication. DNS uses hierarchical domain based naming scheme. The Domain Name System DNS is a hierarchical decentralized naming system for computers, services, or any resource connected to the Internet or a private network.
It associates various information with domain names assigned to each of the participating entities. Most prominently, it translates more readily memorized domain names to the numerical IP addresses needed for the purpose of locating and identifying computer services and devices with the underlying network protocols. By providing a worldwide, distributed directory service, the Domain Name System is an essential component of the functionality of the Internet.
The Domain Name System delegates the responsibility of assigning domain names and mapping those names to Internet resources by designating authoritative name servers for each domain.
Network administrators may delegate authority over sub-domains of their allocated name space to other name servers. This mechanism provides distributed and fault tolerant service and was designed to avoid a single large central database. The Domain Name System also specifies the technical functionality of the database service which is at its core. Historically, other directory services preceding DNS were not scalable to large or global directories as they were originally based on text files, prominently the HOSTS.
TXT resolver. The Domain Name System has been in use since the s. Internet name servers and a communication protocol implement the Domain Name System. Although not intended to be a general purpose database, DNS can store records for other types of data for either automatic lookups, such as DNSSEC records, or for human queries such as responsible person RP records. As a general purpose database, the DNS has also been used in combatingunsolicited email spam by storing a real-time blackhole list.
The DNS database is traditionally stored in a structured zone file. Each node or leaf in the tree has a label and zero or more resource records RR , which hold information associated with the domain name. The domain name itself consists of the label, possibly concatenated with the name of its parent node on the right, separated by a dot. A DNS zone may consist of only one domain, or may consist of many domains and sub-domains, depending on the administrative choices of the zone manager.
DNS can also be partitioned according to class; the separate classes can be thought of as an array of parallel namespace trees. Authority over the new zone is said to be delegated to a designated name server.
The parent zone ceases to be authoritative for the new zone. Domain Name Syntax: A domain name consists of one or more parts, technically called labels, that are conventionally concatenated, and delimited by dots, such as example.
The right-most label conveys the top-level domain; for example, the domain name www.
For example: This tree of subdivisions may have up to levels. A label may contain zero to 63 characters. The null label, of length zero, is reserved for the root zone. The full domain name may not exceed the length of characters in its textual representation. The characters allowed in their labels are a subset of the ASCII character set, consisting of characters a through z, A through Z, digits 0 through 9, and hyphen.
This rule is known as the LDH rule letters, digits, hyphen. Domain names are interpreted in case-independent manner. Name servers: The nodes of this database are the name servers. Each domain has at least one authoritative DNS server that publishes information about that domain and the name servers of any domains subordinate to it.
The top of the hierarchy is served by the root name servers, the servers to query when looking up resolving a TLD. Authoritative name server: An authoritative-only name server only returns answers to queries about domain names that have been specifically configured by the administrator.
As just one example, the authoritative name server for "example. An authoritative name server can either be a master server or a slave server. A master server is a server that stores the original master copies of all zone records. A slave server uses an automatic updating mechanism of the DNS protocol in communication with its master to maintain an identical copy of the master records.
A set of authoritative name servers has to be assigned for every DNS zone. An NS record about addresses of that set must be stored in the parent zone and servers themselves as self-reference.
When domain names are registered with a domain name registrar, their installation at the domain registry of a top level domain requires the assignment of a primary name server and at least one secondary name server.
The requirement of multiple name servers aims to make the domain still functional even if one name server becomes inaccessible or inoperable. For this purpose, generally only the fully qualified domain name of the name server is required, unless the servers are contained in the registered domain, in which case the corresponding IP address is needed as well.
An authoritative server indicates its status of supplying definitive answers, deemed authoritative, by setting a software flag a protocol structure bit , called the Authoritative Answer AA bit in its responses. This task is done by means of email client software User Agents the user is using.
User Agents help the user to type and format the email and store it until internet is available. When an email is submitted to send, the sending process is handled by Message Transfer Agent which is normally comes inbuilt in email client software.
The protocol for mail submission is the same, but uses port Although electronic mail servers and other mail transfer agents use SMTP to send and receive mail messages, user- level client mail applications typically use SMTP only for sending messages to a mail server for relaying. Mail use their own non-standard protocols to access mail box accounts on their own mail servers, all use SMTP when sending or receiving email from outside their own systems.
Mail processing model: Most mailbox providers still allow submission on traditional port Often, these two agents are just different instances of the same software launched with different options on the same machine. Local processing can be done either on a single machine, or split among various appliances; in the former case, involved processes can share files; in the latter case, SMTP is used to transfer the message internally, with each host configured to use the next appliance as a smart host.
The boundary MTA has to locate the target host. It uses the Domain name system DNS to look up the mail exchanger record MX record for the recipient's domain the part of the email address on the right of. The returned MX record contains the name of the target host. The article on MX record discusses many factors in determining which server the sending MTA connects to.
Message transfer can occur in a single connection between two MTAs, or in a series of hops through intermediary systems. Each hop implies a formal handoff of responsibility for the message, whereby the receiving server must either deliver the message or properly report the failure to do so. An MDA is able to save messages in the relevant mailbox format.
Again, mail reception can be done using many computers or just one —the picture displays two nearby boxes in either case. Once delivered to the local mail server, the mail is stored for batch retrieval by authenticated mail clients MUAs. Webmail clients may use either method, but the retrieval protocol is often not a formal standard. SMTP defines message transport, not the message content. Thus, it defines the mail envelope and its parameters, such as the envelope sender, but not the header except trace information nor the body of the message itself.
Protocol overview: An SMTP session consists of commands originated by an SMTP client the initiating agent, sender, or transmitter and corresponding responses from the SMTP server the listening agent, or receiver so that the session is opened, and session parameters are exchanged.
A session may include zero or more SMTP transactions. They are: MAIL command, to establish the return address, a. Return-Path, reverse-path, bounce address, m from, or envelope sender.
RCPT command, to establish a recipient of this message. This command can be issued multiple times, one for each recipient. These addresses are also part of the envelope. DATA to signal the beginning of the message text; the content of the message, as opposed to its envelope. It consists of a message header and a message body separated by an empty line. DATA is actually a group of commands, and the server replies twice: Besides the intermediate reply for DATA, each server's reply can be either positive 2xx reply codes or negative.
Negative replies can be permanent 5xx codes or transient 4xx codes. A drop is a positive response followed by message discard rather than delivery. Fully capable SMTP servers maintain queues of messages for retrying message transmissions that resulted in transient failures.
An SMTP server acting as client, i.
Relaying servers can also be configured to use a smart host. Smtp transport example: In this example, the conversation parts are prefixed with S: After the message sender SMTP client establishes a reliable communications channel to the message receiver SMTP server , the session is opened with a greeting by the server, usually containing its fully qualified domain name FQDN , in this case smtp.
The client initiates its dialog by responding with a HELO command identifying itself in the command's parameter with its FQDN or an address literal if none is available S: HELO relay. DATA S: Tue, 15 January Test message C: Hello Alice. This is a test message with 5 header fields and 4 lines in the message body. Your friend, C: Bob C: QUIT S: In this example, the email message is sent to two mailboxes on the same SMTP server: This enables system administrators to consolidate resources onto centralized servers on the network.
Command service nfs start 5. It works closely with something called the Mail Transfer Agent MTA to send your communication to the right computer and email inbox. Port number for SMTP is It is designed for printer sharing. It is the part that receives and processes the request. Command lpd [ -d ] [ -l ] [ -D DebugOutputFile] 7. The idea is to allow a program, called a client, to run on one computer. It is primarily used in networks of interconnected mainframes.
Command Run xdm in runlevel 5 8. It gathers data by polling the devices on the network from a management station at fixed or random intervals, requiring them to disclose certain information. It is a way that servers can share information about their current state, and also a channel through which an administrate can modify pre-defined values.
Every time you use a domain name, therefore, a DNS service must translate the name into the corresponding IP address. For example, the domain name www. Port number for DNS is It gives IP addresses to hosts. Port number for DHCP is 67, If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.