Chapter 31 -32 Internet Applications DNS E-mail 1

Chapter 31 -32 Internet Applications (DNS, E-mail) 1

Internet Applications t. Domain Name System t. Electronic mail t. Remote login t. File transfer t. World Wide Web t. All use client-server model 2

Names t. Internet communication requires IP addresses t. Humans prefer to use computer names t. Automated system available to translate names to addresses t. Known as Domain Name System (DNS) 3

DNS Functionality t. Given t. Name of a computer t. Returns t. Computer’s internet address t. Method t. Distributed lookup t. Client contacts server(s) as necessary 4

Domain Name Syntax t. Alphanumeric segments separated by dots t. Examples www. netbook. cs. purdue. edu www. eg. bucknell. edu t. Most significant part on right 5

Obtaining a Domain Name t. Organization t. Chooses a desired name t. Must be unique t. Registers with central authority t. Placed under one top-level domain t. Names for subject to international law t. Trademarks t. Copyright 6

Some Top-Level Domains t. Meaning assigned to each 7

Within Organization t. Subdivision possible t. Arbitrary levels possible t. Not standardized t. Controlled locally by organization 8

Example Name Structure t. First level is. com t. Second level is company name t. Third level is division within company t. Fourth level either t. Company subdivision t. Individual computer 9

An Example t. Assume t. Company is Foobar t. Has two divisions t. Soap division t. Candy division has subdivisions t. Soap Division has no subdivisions 10

An Example (continued) t. Names in soap division have form computer. soap. foobar. com t. Names in candy division have form computer. subdivision. candy. foobar. com 11

Illustration of Foobar Naming Hierarchy 12

The Point About Names The number of segments in a domain name corresponds to the naming hierarchy. There is no universal standard; each organization can choose how to structure names in its hierarchy. Furthermore, names within an organization do not need to follow a uniform pattern; individual groups within the organization can choose a hierarchical structure that is appropriate for the group. 13

DNS Client-Server Interaction t. Client known as resolver t. Multiple DNS servers used t. Arranged in hierarchy t. Each server corresponds to contiguous part of naming hierarchy 14

Two Possible DNS Hierarchies t. Choice made by organization 15

Inter-Server Links All domain name servers are linked together to form a unified system. Each server knows how to reach a root server and how to reach servers that are authorities for names further down the hierarchy. 16

In Practice t. DNS uses backup server(s) t. ISPs and others t. Offer t. Small DNS service to subscribers organizations and individuals t. Only need domain names for computers running servers t. Contract with an ISP for domain service 17

DNS Lookup t Application t Becomes DNS client t Sends request to local DNS server t Local server t If answer known, returns response t If answer unknown t. Starts at top-level server t. Follows links t. Returns response t Called name resolution 18

Interative DNS Lookup 19

Root name servers t. The root domain contains all top-level domains of the Internet. t. As of June 2009, there are 20 generic top-level domains (g. TLDs) and 248 country code top-level domains (cc. TLDs) in the root domain. 20

Root name servers (cont. ) t There are currently 13 root name servers specified, with names in the form letter. rootservers. net, where letter ranges from A to M. t This does not mean there are 13 physical servers; each operator uses redundant computer equipment to provide reliable service even if failure of hardware or software occur. t Additionally, nine of the servers operate in multiple geographical locations using anycast, providing increased performance and even 21

Root name servers (cont. ) 22

Caching in DNS t. Server always caches answers t. Host can cache answers t. Caching t. Improves efficiency t. Eliminates unnecessary search t. Works well because high locality of reference 23

DNS Types t Each entry in server consists of t Domain name t DNS type for name t Value to which name corresponds t During lookup, client must supply t Name t Type t Server t Matches both name and type 24

The Point About Types The domain name system stores a type with each entry. When a resolver looks up a name, the resolver must specify the type that is desired; a DNS server returns only entries that match the specified type. 25

Example DNS Types t. Type A (Address) t. Value t. Type is IP address for named computer MX (Mail e. Xchanger) t. Value is IP address of computer with mail server for name t. Type CNAME (Computer NAME) t. Value is another domain name t. Used to establish alias (www) 26

Domain Name Abbreviation t. DNS lookup uses full names t. Users desire abbreviations t. Technique t. Configure resolver with list of suffixes t. Try suffixes one at a time 27

Example of DNS Abbreviations t. Suffixes are tfoobar. com tcandy. foobar. com t. User enters name walnut t. Resolver tries twalnut. foobar. com twalnut. candy. foobar. com 28

Other Internet Applications t. Invoked directly by user t. E-mail t. Remote login t. File transfer t. Web browsing 29

Electronic Mail t. Originally t. Memo sent from one user to another t. Now t. Memo sent to one or more mailboxes t. Mailbox t. Destination point for messages t. Can be storage or program t. Given unique address 30

E-mail Address t Text string t Specifies mail destination t General form mailbox@computer t Domain name of computer t Actually type MX t mailbox t Destination on the computer 31

Use of E-mail Address Each electronic mailbox has a unique address, which is divided into two parts: the first identifies a user’s mailbox, and the second identifies a computer on which the mailbox resides. E-mail software on the sender’s computer uses the second part to select a destination; e-mail software on the recipient’s computer uses the first part to select a particular mailbox. 32

Mail Message Format t. Header t. Identifies sender, recipient(s), memo contents t. Lines of form keyword: information t. Blank line t. Body t. Contains text of message 33

Example E-mail Header Fields t. Most header lines optional 34

Extending E-mail t. Original t. SMTP t. Users e-mail - message restricted to ASCII text desire to send t. Image files t. Audio clips t. Compiled (binary) programs t. Solution t. Multi-purpose (MIME) Internet Mail Extensions 35

MIME t. Allows transmission of t. Binary data t. Multimedia files (video/audio clips) t. Multiple types in single message t. Mixed formats t. Backward compatible 36

MIME Encoding t. Sender t. Inserts additional header lines t. Encodes binary data in (printable) ASCII t. Sent like standard message t. Receiver t. Interprets header lines t. Extracts and decodes parts t. Separate encoding standards for content and 37

Example of MIME t. Header lines added MIME-Version: 1. 0 Content-Type: Multipart/Mixed; Boundary=Mime_sep t. Specifies t. Using MIME version 1. 0 t. Line Mime_sep appears before each message part 38

MIME Although Internet e-mail only transfers text, MIME can be used to transport binary data by encoding it in printed characters. A MIME mail message includes additional information that a receiving application uses to decode the message. 39

Mail Transfer t Protocol is Simple Mail Transfer Protocol (SMTP) t Runs over TCP t Used between t Mail transfer program on sender’s computer t Mail server on recipient’s computer t Specifies how t Client interacts with server t Recipients specified t Message is transferred 40

Illustration of Mail Transfer t. Server t. Required to receive mail t. Places message in user’s mailbox 41

Terminology t Mail exploder t Program t Accepts incoming message t Delivers to multiple recipients t Mailing list t Database t Used t Mail by exploder gateway t Connects two mail systems 42

Illustration of a Mailing List t Separate permissions for t Mailing to list t Adding/deleting members t. Public – anyone can join t. Private – access restricted by owner 43

Illustration of a Mail Gateway t Can connect two t Heterogeneous systems t Internet to non-Internet 44

Automated Mailing Lists t. Automated program to handle routine chores of maintaining mailing list: list manager t. Used in conjunction with exploder t. Example expected command: add mailbox to list 45

Computers Without Mail Servers t. Typically t. Small, personal computer or mobile device t. Not continuously connected to Internet t. To receive e-mail, user must t. Establish mailbox on large computer t. Access mailbox as necessary t. Post used Office Protocol (POP) or IMAP 46

POP t. Current version named POP 3 47

IMAP Internet Message Access Protocol (IMAP 4 rev 1 1996) t A more recent protocol for accessing email messages t Leaves mail on server t Built in security features t 48

Summary t. Domain Name System t. Maps name to IP address t. Uses on-line servers t. Uses caching for efficiency t. Three e-mail transfer protocols t. SMTP t. POP 3 IMAP 49