Computer Networks Unit I Application layer (2012 pattern)

Transcription

1 Computer Networks Unit I Application layer (2012 pattern) By Prof. B.A.Khivsara Assistant Prof. Department of Computer Engg. SNJB s KBJ COE, Chandwad Introduction 1-1

2 Chapter 1: Introduction What is Internet? network edge; hosts, access net, physical media OSI Layer Introduction to application layer services HTTP FTP SMTP DNS DHCP TELNET Introduction 1-2

3 Chapter 1: ROAD MAP What is Internet? network edge; hosts, access net, physical media OSI Layer Introduction to application layer services HTTP FTP SMTP DNS DHCP TELNET Introduction 1-3

4 What s the Internet: nuts and bolts view PC server wireless laptop cellular handheld router access points wired links millions of connected computing devices: hosts = end systems running network apps communication links fiber, copper, radio, satellite transmission rate = bandwidth routers: forward packets (chunks of data) Mobile network Global ISP Home network Regional ISP Institutional network Introduction 1-4

5 What s the Internet: nuts and bolts view protocols control sending, receiving of msgs e.g., TCP, IP, HTTP, Skype, Ethernet Internet: network of networks loosely hierarchical public Internet versus private intranet Internet standards RFC: Request for comments IETF: Internet Engineering Task Force Mobile network Global ISP Home network Regional ISP Institutional network Introduction 1-5

6 What s the Internet: a service view communication infrastructure enables distributed applications: Web, VoIP, , games, e-commerce, file sharing communication services provided to apps: reliable data delivery from source to destination best effort (unreliable) data delivery Introduction 1-6

7 Chapter 1: ROAD MAP What is Internet? network edge; hosts, access net, physical media OSI Layer Introduction to application layer services HTTP FTP SMTP DNS DHCP TELNET Introduction 1-7

8 The network edge: end systems (hosts): run application programs e.g. Web, at edge of network client/server model peer-peer client host requests, receives service from always-on server client/server e.g. Web browser/server; client/server peer-peer model: minimal (or no) use of dedicated servers e.g. Skype, BitTorrent Introduction 1-8

9 Connection oriented and conn.less services Connection oriented Connectionless 1> Connection Establishment use it and Release it. 1>No connection establishment 2> Reliable 2>Not Reliable 3>Information can be resent to receiver if data is missed or with error 4>Packets come in sequence 5>Each packet uses same route 6>If connection fails cannot sent data 3>It is not Possible 4>No sequencing 5>Different route 6>Can sent data using diff route

10 Chapter 1: ROAD MAP What is Internet? network edge; hosts, access net, physical media OSI Layer Introduction to application layer services HTTP FTP SMTP DNS DHCP TELNET Introduction 1-10

11 OSI Open Systems Interconnection (OSI) is a set of internationally recognized, non-proprietary standards for networking and for operating system involved in networking functions.

12 OSI model block diagram

13 7 Layers 7. Application Layer 6. Presentation Layer 5. Session Layer 4. Transport Layer 3. Network Layer 2. Data Link Layer 1. Physical Layer All People Seem To Need Data Processing

14 LAYER 7 The APPLICATION Layer The application layer is responsible for providing services to the user. Resource sharing Provides a set of interfaces for sending and receiving applications to gain access to and use network services, such as: networked file transfer, message handling and database query processing , File transfer

15 LAYER 6 The PRESENTATION Layer Manages data-format information for networked communications (the network s translator) For outgoing messages, it converts data into a generic format for network transmission; for incoming messages, it converts data from the generic network format to a format that the receiving application can understand This layer is also responsible for certain protocol conversions, data encryption/decryption, or data compression/decompression

16 LAYER 5 The SESSION Layer Enables two networked resources to hold ongoing communications (called a session) across a network Applications on either end of the session are able to exchange data for the duration of the session Responsible for initiating, maintaining and terminating sessions Responsible for security and access control to session information (via session participant identification) Responsible for synchronization services, and for checkpoint services

17 LAYER 4 The TRANSPORT Layer Manages the flow of data between parties by segmenting long data streams into smaller data chunks (based on allowed packet size for a given transmission medium) Reassembles chunks into their original sequence at the receiving end Reliable transmission (Provides acks of successful transmissions and requests resends for packets which arrive with errors) Provides Congestion control and flow control Multiplexing and demultiplexing End-to-end data delivery

18 LAYER 3 The NETWORK Layer Handles addressing messages for delivery, as well as translating logical network addresses and names into their physical counterparts Responsible for deciding how to route transmissions between computers Find best route to route the packet This layer also handles packet switching and network congestion control

19 LAYER 2 The DATA LINK Layer Handles special data frames (packets) between the Network layer and the Physical layer At the receiving end, this layer packages raw data from the physical layer into data frames for delivery to the Network layer At the sending end this layer handles conversion of data into raw formats that can be handled by the Physical Layer Error control Flow control Framing

20 LAYER 1 The PHYSICAL Layer Converts bits into electronic signals for outgoing messages Converts electronic signals into bits for incoming messages This layer manages the interface between the computer and the network medium (coax, twisted pair, etc.) This layer tells the driver software for the MAU (media attachment unit, ex. network interface cards (NICs, modems, etc.)) what needs to be sent across the medium The bottom layer of the OSI model

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22 Chapter 1: ROAD MAP What is Internet? network edge; hosts, access net, physical media OSI Layer Introduction to application layer services HTTP FTP SMTP DNS DHCP TELNET Introduction 1-22

23 Some network apps web instant messaging remote login P2P file sharing multi-user network games streaming stored video clips voice over IP real-time video conferencing grid computing 2: Application Layer 23

24 Application architectures Client-server Peer-to-peer (P2P) Hybrid of client-server and P2P 2: Application Layer 24

25 Client-server architecture client/server server: always-on host permanent IP address server farms for scaling clients: communicate with server may be intermittently connected may have dynamic IP addresses do not communicate directly with each other 2: Application Layer 25

26 Pure P2P architecture no always-on server arbitrary end systems directly communicate peers are intermittently connected and change IP addresses peer-peer Highly scalable but difficult to manage 2: Application Layer 26

27 Processes communicating Process: program running within a host. within same host, two processes communicate using inter-process communication (defined by OS). processes in different hosts communicate by exchanging messages Client process: process that initiates communication Server process: process that waits to be contacted Note: applications with P2P architectures have client processes & server processes 2: Application Layer 27

28 Sockets process sends/receives messages to/from its socket socket analogous to door sending process shoves message out door sending process relies on transport infrastructure on other side of door which brings message to socket at receiving process host or server process socket TCP with buffers, variables controlled by app developer controlled by OS Internet host or server process socket TCP with buffers, variables API: (1) choice of transport protocol; (2) ability to fix a few parameters (lots more on this later) 2: Application Layer 28

29 Addressing processes to receive messages, process must have identifier host device has unique 32-bit IP address Q: does IP address of host on which process runs suffice for identifying the process? A: No, many processes can be running on same host identifier includes both IP address and port numbers associated with process on host. Example port numbers: HTTP server: 80 Mail server: 25 to send HTTP message to gaia.cs.umass.edu web server: IP address: Port number: 80 more shortly 2: Application Layer 29

30 App-layer protocol defines Types of messages exchanged, e.g., request, response Message syntax: what fields in messages & how fields are delineated Message semantics meaning of information in fields Rules for when and how processes send & respond to messages Public-domain protocols: defined in RFCs allows for interoperability e.g., HTTP, SMTP Proprietary(Private) protocols: e.g., Skype 2: Application Layer 30

31 Chapter 1: ROAD MAP What is Internet? network edge; hosts, access net, physical media OSI Layer Introduction to application layer services HTTP FTP SMTP DNS DHCP TELNET Introduction 1-31

32 Web and HTTP First some jargon Web page consists of objects Object can be HTML file, JPEG image, Java applet, audio file, Web page consists of base HTML-file which includes several referenced objects Each object is addressable by a URL Example URL: host name path name 2: Application Layer 32

33 HTTP overview HTTP: hypertext transfer protocol Web s application layer protocol client/server model client: browser that requests, receives, displays Web objects server: Web server sends objects in response to requests HTTP uses port no 80 PC running Explorer Mac running Navigator Server running Apache Web server 2: Application Layer 33

34 HTTP overview (continued) Uses TCP: client initiates TCP connection (creates socket) to server, port 80 server accepts TCP connection from client HTTP messages (applicationlayer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server) TCP connection closed HTTP is stateless server maintains no information about past client requests 2: Application Layer 34

35 HTTP connections Nonpersistent HTTP At most one object is sent over a TCP connection. Persistent HTTP Multiple objects can be sent over single TCP connection between client and server. 2: Application Layer 35

36 Nonpersistent HTTP Suppose user enters URL(contains text, references to 10 jpeg images) 1a. HTTP client initiates TCP connection to HTTP server 1b. HTTP server waiting for TCP connection at port 80. accepts connection, notifying client 2. HTTP client sends HTTP request message into TCP connection socket. Message indicates that client wants object. 3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket time 2: Application Layer 36

37 Nonpersistent HTTP (cont.) time 5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects 6. Steps 1-4 repeated for each of 10 jpeg objects 4. HTTP server closes TCP connection. 2: Application Layer 37

38 Non-Persistent HTTP: Response time Definition of RTT(Round Trip Time): time for a small packet to travel from client to server and back. Response time: one RTT to initiate TCP connection one RTT for HTTP request file transmission time total = 2RTT+transmit time initiate TCP connection RTT request file RTT file received time time time to transmit file 2: Application Layer 38

39 Persistent HTTP Nonpersistent HTTP Shortcomings requires 2 RTTs per object OS overhead for each TCP connection New connection must be established and maintained for each requested object Persistent HTTP server leaves connection open after sending response subsequent HTTP messages between same client/server sent over open connection client sends requests as soon as it encounters a referenced object as little as one RTT for all the referenced objects 2: Application Layer 39

40 HTTP Message Formats Two types of message 1. HTTP Request message 2. HTTP Response Message Introduction 1-40

41 HTTP request message: general format 2: Application Layer 41

42 HTTP request message Example HTTP request message: ASCII (human-readable format) request line (GET, POST, HEAD commands) Carriage return, line feed indicates end of message header lines GET /somedir/page.html HTTP/1.1 Host: User-agent: Mozilla/4.0 Connection: close Accept-language:fr (extra carriage return, line feed) 2: Application Layer 42

43 Uploading form input Post method: Web page often includes form input Input is uploaded to server in entity body URL method: Uses GET method Input is uploaded in URL field of request line: Example rch?monkeys&banana 2: Application Layer 43

44 HTTP Methods The POST DELETE TRACE GET HEAD PUT is method similar does method what to is is requests PUT. used just the you reverse asks for It expect: the too, debugging. for server bears of the deletes GET. message to an It send Instead URL, a instructs page the header, but from of page instead reading the a without. server remote The of a page, replacing to the web send is actual it server; suitably writes back the page. existing authentication a encoded page. This request. method data, This in This it MIME. can method appends and is useful be permissions used The makes it when to to vast any get requests it play possible majority existing a page s a major are to data. of time not build the role processed Posting of requests a here. last collection modification, a There message on correctly the is of no web to and guarantee are to pages newsgroup the collect GETs. client on that information a remote wants a DELETE adding to web know for a server. file operation indexing it to sent The a the is GET purposes, body bulletin successful proper filename of request. board the or since request just are HTTP/1.1 even to example test contains if an the URL of remote the the for page. usage. web validity It server may be is encoded willing to using delete MIME, a page, in the which file case, the lines permissions following may the be PUT set in might such include a way that Content-Type the server process and authentication doesn t have headers rights to do prove it. Where CONNECT that the filename is not caller indeed is the used has resource at the moment permission name to to perform be fetched the and operation 1.1 is the protocol version being used OPTIONS provides a way for the client to query the server about its properties or those of a specified file

45 HTTP response message: general format Introduction 1-45

46 HTTP response message example status line (protocol status code status phrase) data, e.g., requested HTML file header lines HTTP/ OK Connection close Date: Thu, 06 Aug :00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun Content-Length: 6821 Content-Type: text/html data data data data data... 2: Application Layer 46

47 HTTP response status codes In first line in server->client response message. A few sample codes: 200 OK request succeeded, requested object later in this message 301 Moved Permanently requested object moved, new location specified later in this message (Location:) 400 Bad Request request message not understood by server 404 Not Found requested document not found on this server 505 HTTP Version Not Supported 2: Application Layer 47

48 HTTP Version differences HTTP/1.0 Have only 16 status codes Provides only basic authentication Uses Non persistent connection RTT is more so bandwidth waste is vast Stateless Supports only GET POST and HEAD method HTTP/1.1 Have introduced new 24 status code Provides strong authentication Uses Persistent connection RTT is less so bandwidth utilization is good Uses cookies as state management mechanism Supports GET, POST, HEAD,PUT and DELETE 2: Application Layer 48

49 User-server state: cookies Many major Web sites use cookies Four components: 1) cookie header line of HTTP response message 2) cookie header line in HTTP request message 3) cookie file kept on user s host, managed by user s browser 4) back-end database at Web site Example: Susan always access Internet always from PC visits specific e- commerce site for first time when initial HTTP requests arrives at site, site creates: unique ID entry in backend database for ID 2: Application Layer 49

50 Cookies: keeping state (cont.) client server ebay 8734 cookie file ebay 8734 amazon 1678 one week later: ebay 8734 amazon 1678 usual http request msg usual http response Set-cookie: 1678 usual http request msg cookie: 1678 usual http response msg usual http request msg cookie: 1678 usual http response msg Amazon server creates ID 1678 for user create entry cookiespecific action cookiespectific action access access backend database 2: Application Layer 50

51 Cookies (continued) What cookies can bring: authorization shopping carts recommendations user session state (Web ) aside Cookies and privacy: cookies permit sites to learn a lot about you you may supply name and to sites How to keep state : protocol endpoints: maintain state at sender/receiver over multiple transactions cookies: http messages carry state 2: Application Layer 51

52 Web caches (proxy server) Goal: satisfy client request without involving origin server user sets browser: Web accesses via cache browser sends all HTTP requests to cache client Proxy server origin server object in cache: cache returns object else cache requests object from origin server, then returns object to client client origin server 2: Application Layer 52

53 More about Web caching Why Web caching? reduce response time for client request reduce traffic on an institution s access link. Internet dense with caches: enables poor content providers to effectively deliver content (but so does P2P file sharing) 2: Application Layer 53

54 Chapter 1: ROAD MAP What is Internet? network edge; hosts, access net, physical media OSI Layer Introduction to application layer services HTTP FTP ( File Transfer Protocol) SMTP DNS DHCP TELNET Introduction 1-54

55 FTP (File Transfer Protocol) Objective 1. Allow file sharing between remote machine 2. Transfer data reliably and efficiently FTP Protocol falls within client server model Both client & server have 2 process allowing information (Data & command) to be managed, they are 1. DTP(Data transfer Process) 2. PI(Protocol Interpreter) Introduction 1-55

56 FTP model/connection Introduction 1-56

57 FTP: the file transfer protocol user at host FTP user interface FTP client local file system file transfer FTP server remote file system During FTP Connection 2 transmission channels are open 1. Control Channel (For command) 2. Data Channel (For data) Control uses port no 21 Data connection uses Port no 20 2: Application Layer 57

58 FTP: separate control, data connections FTP client contacts FTP server at port 21, TCP is transport protocol client authorized over control connection client browses remote directory by sending commands over control connection. when server receives file transfer command, server opens 2 nd TCP connection (for file) to client after transferring one file, server closes data connection. FTP client TCP control connection port 21 TCP data connection port 20 FTP server server opens another TCP data connection to transfer another file. control connection: out of band FTP server maintains state : current directory, earlier authentication 2: Application Layer 58

59 FTP commands, responses Sample commands: sent as ASCII text over control channel USER username PASS password LIST return list of file in current directory RETR filename retrieves (gets) file STOR filename stores (puts) file onto remote host Sample return codes status code and phrase (as in HTTP) 331 Username OK, password required 125 data connection already open; transfer starting 425 Can t open data connection 452 Error writing file 2: Application Layer 59

60 Comparison between HTTP and FTP HTTP uses TCP Only one TCP connection between client and server Send its control information in-band HTTP is stateless FTP uses TCP Two parallel TCP connection, one for data and one for control Send its control information out-oforder FTP maintains State Introduction 1-60

61 Chapter 1: ROAD MAP What is Internet? network edge; hosts, access net, physical media OSI Layer Introduction to application layer services HTTP FTP ( File Transfer Protocol) SMTP (Simple Mail Transfer Protocol) DNS DHCP TELNET Introduction 1-61

62 Functions Architecture and services Message formats Message transfer agents SMTP POP3 and IMAP

63 functions system functions Composition Transfer Reporting Displaying Disposition

64 Electronic Mail System Architecture Three major components: user agents mail servers simple mail transfer protocol: SMTP User Agent mail reader composing, editing, reading mail messages e.g., Eudora, Outlook, elm, Mozilla Thunderbird outgoing, incoming messages stored on server mail server SMTP mail server user agent user agent SMTP SMTP user agent mail server outgoing message queue user mailbox user agent user agent user agent 2: Application Layer 64

65 Electronic Mail: mail servers Mail Servers mailbox contains incoming messages for user message queue of outgoing (to be sent) mail messages SMTP protocol between mail servers to send messages client: sending mail server mail server SMTP mail server user agent SMTP SMTP mail server user agent user agent user agent server : receiving mail server user agent user agent 2: Application Layer 65

66 message format RFC 822 header fields related to message transport. Some fields used in the RFC 822 message header.

67 SMTP Simple Mail Transfer Protocol Source machine establishes a TCP connection on port 25 on destination machine, where SMTP daemon listens. This daemon accepts the incoming connections and copies messages from them into the appropriate mailboxes If a message can t be delivered, an error report containing the first part of the undeliverable message is returned to the sender It is a simple ASCII protocol

68 Electronic Mail: SMTP [RFC 2821] uses TCP to reliably transfer message from client to server, port 25 direct transfer: sending server to receiving server three phases of transfer handshaking (greeting) transfer of messages closure command/response interaction commands: ASCII text response: status code and phrase messages must be in 7-bit ASCII 2: Application Layer 68

69 Scenario: Alice sends message to abc 1) Alice uses UA to compose message and to 2) Alice s UA sends message to her mail server; message placed in message queue 3) Client side of SMTP opens TCP connection with abc s mail server 4) SMTP client sends Alice s message over the TCP connection 5) abc s mail server places the message in abc s mailbox 6) abc invokes his user agent to read message 1 user agent mail server mail server 5 6 user agent 2: Application Layer 69

70 Try SMTP interaction for yourself: telnet servername 25 see 220 reply from server enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands above lets you send without using client (reader) 2: Application Layer 70

71 SMTP: final words SMTP uses persistent connections SMTP requires message (header & body) to be in 7- bit ASCII SMTP server uses CRLF.CRLF to determine end of message Comparison with HTTP: HTTP: pull SMTP: push both have ASCII command/response interaction, status codes HTTP: each object encapsulated in its own response msg SMTP: multiple objects sent in multipart msg 2: Application Layer 71

72 Mail message format SMTP: protocol for exchanging msgs RFC 822: standard for text message format: header lines, e.g., To: From: Subject: different from SMTP commands! body the message, ASCII characters only header body blank line 2: Application Layer 72

73 Message format: multimedia extensions MIME: multimedia mail extension, RFC 2045, 2056 additional lines in msg header declare MIME content type MIME version method used to encode data multimedia data type, subtype, parameter declaration encoded data From: To: Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data base64 encoded data 2: Application Layer 73

74 Mail access protocols user agent SMTP SMTP access protocol user agent sender s mail server receiver s mail server SMTP: delivery/storage to receiver s server Mail access protocol: retrieval from server POP: Post Office Protocol [RFC 1939] authorization (agent <-->server) and download IMAP: Internet Mail Access Protocol [RFC 1730] more features (more complex) manipulation of stored msgs on server HTTP: gmail, Hotmail, Yahoo! Mail, etc. 2: Application Layer 74

75 POP3 Starts when the user starts the mail reader Mail reader calls up the ISP (if there is no connection) and establishes a TCP connection with the message transfer agent on port 110; Authorization Having user logged in by sending its username and password Transactions User collecting the s and marking them for deletion Update Causes the s to be deleted

76 IMAP POP3 works fine for users with one account with one ISP, accessed from one PC If mail was accessed from different locations, user may loose s, security issues may appear, etc An alternative final delivery protocol, IMAP (Internet Message Access Protocol), defined in RFC2060 Instead assuming that all messages will be downloaded and work offline after that (like POP3), IMAP assumes that all will remain on the server indefinite in multiple mailboxes Provides extensive mechanisms to read messages or parts of messages, mechanisms to create, destroy and manipulate multiple mailboxes.

77 POP3 vs. IMAP

78 Chapter 1: ROAD MAP What is Internet? network edge; hosts, access net, physical media OSI Layer Introduction to application layer services HTTP (Hyper text transfer Protocol) FTP ( File Transfer Protocol) SMTP (Simple Mail Transfer Protocol) DNS (Domain Name System) DHCP TELNET Introduction 1-78

79 DNS (Domain Name System)Basic IP addresses are difficult to remember would be difficult to remember If JohnDoe s mail server moves to another machine, then his address would not be valid anymore Something like JohnDoe@gmail.com would be appropriate Need some mechanisms to translate gmail.com to the IP address. To solve this problem DNS was invented DNS is a hierarchical, domain-based naming scheme and a distributed database system for implementing this naming scheme Usage: Map URL name into an IP address

80 DNS: Domain Name System Basic: Maps URL to IP address and vice versa Runs over UDP and TCP Uses port No 53 Application layer protocol 2: Application Layer 80

81 DNS : DNS services hostname to IP address translation host aliasing Canonical, alias names mail server aliasing load distribution replicated Web servers: set of IP addresses for one canonical name 2: Application Layer 81

82 DNS components Name space Name Servers Resolver Introduction 1-82

83 The Name Space The name space is the structure of the DNS database An inverted tree with the root node at the top Each node has a label The root node has a null label, written as The root node "" top-level node top-level node top-level node second-level node second-level node second-level node second-level node second-level node third-level node third-level node third-level node

84 DNS Name Space (Top level Domain) Internet is divided into over 200 top level domains Top level domains could be generic and country domains Each domain is divided into sub-domains, which are further partitioned, etc..

85 DNS Name Space (Domain Names) A domain name is the sequence of labels from a node to the root, separated by dots (. s), read left to right A node s domain name identifies its position in the name space "" com edu gov int mil net org nominum metainfo berkeley nwu nato army uu west east www dakota tornado

86 DNS Name servers One DNS server could service all requests But in practice it will be overloaded To solve this, DNS name space is divided in non overlapping zones Each zone contains some part of the tree and name servers holding zone info

87 DNS Name Resolver Resolver Query Response Name Server Resolver maps a name to an IP address and vice versa DNS uses large no of servers organized in hierarchical and distributed around the world. There are 3 classes of DNS servers Root DNS Server Top-Level Domain(TLD) Server Authoritative DNS Server 2: Application Layer 87

88 DNS Resolver (Types of Servers) Root DNS Server 13 root DNS server Labeled A to M Top-Level Domain(TLD) Servers Responsible for top level domains Ex. Com,org,edu,gov and all country domains Authoritative DNS Servers Contains DNS record for every publically accessible organization on Internet DNS record maps names to IP address 1-88

89 DNS Resolver ways Resolving can be done in two ways Iterative resolution Recursive resolution Introduction 1-89

90 DNS name Iterative resolution example root DNS server Host at cis.poly.edu wants IP address for gaia.cs.umass.edu TLD DNS server (for edu) iterated query: contacted server replies with name of server to contact I don t know this name, but ask this server local DNS server dns.poly.edu 1 8 requesting host cis.poly.edu 7 6 authoritative DNS server dns.cs.umass.edu gaia.cs.umass.edu 2: Application Layer 90

91 DNS name Recursive resolution example root DNS server recursive query: puts burden of name resolution on contacted name server heavy load? TLD DNS server local DNS server dns.poly.edu requesting host cis.poly.edu authoritative DNS server dns.cs.umass.edu gaia.cs.umass.edu 2: Application Layer 91

92 Resource Records 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 DNS zone database is made up of a collection of resource records. Each resource record specifies information about a particular object. For example, address mapping (A) records map a host name to an IP address, and reverse-lookup pointer (PTR) records map an IP address to a host name. The server uses these records to answer queries for hosts in its zone

93 Resource Records A resource record has five parts: Domain name Time to Live Class Type Value The Domain_name tells the domain to which this record applies Time_to_live field gives an indication of how stable the record is Class field is always IN for Internet information

94 Chapter 1: ROAD MAP What is Internet? network edge; hosts, access net, physical media OSI Layer Introduction to application layer services HTTP FTP ( File Transfer Protocol) SMTP (Simple Mail Transfer Protocol) DNS DHCP (Dynamic Host Configuration Protocol) TELNET Introduction 1-94

95 Need of Dynamic Assignment of IP addresses Need of Dynamic assignment of IP addresses IP addresses are assigned on-demand Avoid manual IP configuration Support mobility of laptops Support temporary allocation of IP addresses Three Protocols: RARP (until 1985, no longer used) BOOTP ( ) DHCP (since 1993) Only DHCP is widely used today. 95

96 DHCP Designed in 1993 Uses UDP An extension of BOOTP (Many similarities to BOOTP) IP addresses are assigned dynamically on demand Supports temporary allocation ( leases ) of IP addresses DHCP client can acquire all IP configuration parameters DHCP can interoperate with BOOTP clients. 96

97 DHCP Interaction (simplified) Argon :a0:24:71:e4:44 DHCP Response: IP address: Default gateway: Netmask: DHCP Server 97

98 BOOTP/DHCP Message Format OpCode Hardware Type Number of Seconds Transaction ID Client IP address Your IP address Server IP address Gateway IP address Client hardware address (16 bytes) Server host name (64 bytes) Boot file name (128 bytes) Options Hardware Address Hop Count Length Unused (in BOOTP) Flags (in DHCP) 98 (There are >100 different options)

99 BOOTP/DHCP OpCode: 1 (Request), 2(Reply) Note: DHCP message type is sent in an option Hardware Type: 1 (for Ethernet) Hardware address length: 6 (for Ethernet) Hop count: set to 0 by client Transaction ID: Integer (used to match reply to response) Seconds: number of seconds since the client started to boot Client IP address, Your IP address, server IP address, Gateway IP address, client hardware address, server host name, boot file name: client fills in the information that it has, leaves rest blank 99

100 DHCP Message Type Value Message Type 1 DHCPDISCOVER 2 DHCPOFFER 3 DHCPREQUEST 4 DHCPDECLINE 5 DHCPACK 6 DHCPNAK 7 DHCPRELEASE 8 DHCPINFORM 100

101 DHCP Operation DHCP Client 00:a0:24:71:e4:44 DHCP Server DCHP DISCOVER DHCPDISCOVER Sent to DHCP Server DHCP Client 00:a0:24:71:e4:44 DHCPOFFER DHCP Server DCHP OFFER DHCPOFFER DHCP Server 102

102 DHCP Operation DHCP Client 00:a0:24:71:e4:44 DHCP Server DHCP RELEASE DHCPRELEASE At this time, the DHCP client has released the IP address DHCP Server 104

103 Chapter 1: ROAD MAP What is Internet? network edge; hosts, access net, physical media OSI Layer Introduction to application layer services HTTP FTP ( File Transfer Protocol) SMTP (Simple Mail Transfer Protocol) DNS DHCP (Dynamic Host Configuration Protocol) TELNET Introduction 1-105

104 TELNET vs. telnet TELNET is a protocol that provides a general, bi-directional, eight-bit byte oriented communications facility. telnet is a program that supports the TELNET protocol over TCP. Many application protocols are built upon the TELNET protocol. Netprog: TELNET 106

105 TELNET Protocol TELNET is a general-purpose client-server application program. TELNET is an abbreviation for TErminaL NETwork Uses Port No 23 and TCP Protocol Used for Virtual terminal service TELNET enables the establishment of a connection to a remote system in such a way that the local terminal appears to be a terminal at the remote system

106 Local login TCP/IP Protocol Suite 108

107 Remote login TCP/IP Protocol Suite 109

108 Concept of NVT TCP/IP Protocol Suite 110

109 Network Virtual Terminal Via a universal interface called the Network Virtual Terminal (NVT) character set, the TELNET client translates characters (data or commands) that come from the local terminal into NVT form and delivers them to the network. The TELNET server translates data and commands from NVT form into the form acceptable by the remote computer. Introduction 1-111

110 Network Virtual Terminal intermediate representation of a generic terminal. provides a standard language for communication of terminal control functions. NVT uses two sets of characters, one for data(0) and other for control(1) Netprog: TELNET 112

111 TELNET Command Structure Introduction 1-113

112 TELNET Basic Commands AYT EC AO QUIT Are you There Erase Character Abort Output Quit Session Introduction 1-114