CE80N Introduction to Networks & The Internet Dr. Chane L. Fullmer UCSC Winter 2002

February 5, 2002CE80N -- Lecture #102 General Information TA Office… –Trailer #15 is no more –Moved to the ISB –The Interdisciplinary Sciences Building (ISB) is across McLaughlin Drive and to the southeast of Baskin Engineering. It's about a 5 minute walk from BE. URL below is campus map with ISB shown. campusBW-lg.gif campusBW-lg.gif

Chapter 20

February 5, 2002CE80N -- Lecture #104 Electronic Mail The first “Killer App”… Allowed users to communicate via computer – asynchronously Modern day GUIs hide original “command line” operation Uses client-server architecture

February 5, 2002CE80N -- Lecture #105 Description Of Functionality Electronic mail systems permit complex communications and interactions. –Send a single message to many recipients –Send a message that includes text, audio, video, or graphics –Send a message to a user on a network outside the Internet –Send a message to which a computer program responds

February 5, 2002CE80N -- Lecture #106 The Best Of All Worlds Networks provide the speed of telephone communication and permanence of postal mail. –Can transfer small notes or large documents Caveat: there are size limitations on attachments at some sites –Have become extremely popular

February 5, 2002CE80N -- Lecture #107 Each User Has A Mailbox For E- mail Like a post office mailbox, each mailbox has an address. Any user can send to another user if they know the mailbox address. Only the owner of the box can open the mailbox – but others can snoop the mail while enroute.

February 5, 2002CE80N -- Lecture #108 Sending An Message To send across the Internet, the user: –Runs an application –Composes and edits a message Adds attachments –Specifies a recipient –Finishes entering the message –Sends the message Hey, where’s the send button?

February 5, 2002CE80N -- Lecture #109 Attachments Attachments are not added strictly “as- is” –Must be converted to text only characters for proper operation in mail servers. –Attachments are encoded using a well known method: MIME: (Multipurpose Internet Mail Extensions) BinHex: Macintosh systems UUENCODE: Unix systems and PCs

February 5, 2002CE80N -- Lecture #1010 Notification That Has Arrived A user can configure the software to: –Print text messages when mail arrives “You’ve got mail” –Play a recording or tone when mail arrives “You’ve got mail!” –Set an icon on the screen –Suppress notification altogether

February 5, 2002CE80N -- Lecture #1011 Reading An Message When the application begins, it: –Tells the user about waiting messages –Gives an initial summary of the mail –Displays the message contents –Allows the user to: Send a reply Leave the message in the inbox Save the message Delete the message

February 5, 2002CE80N -- Lecture #1012 Messages Look Like Interoffice Memos An message begins with a header: From: To: Date: Subject :

February 5, 2002CE80N -- Lecture #1013 Software Fills In Header Information User-friendly software hides unnecessary header lines when displaying an message. See the example below: Received: from amazon.com ([ ]) by company1.com with Microsoft SMTPSVC( ); Sat, 3 Jun :14: Received: by amazon.com id OAA04950; Sat, 3 Jun :28: (PDT) Date: Sat, 3 Jun :28: (PDT) Message-Id: X-AMAZON-TRACK: X-AMAZON-TRACK-2: fathers-day-4 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary=amazon From: Amazon.com Subject: Save $25 at Amazon.com's New Tools & Hardware Store To: Return-Path:

February 5, 2002CE80N -- Lecture #1014 How Works systems follow the client server approach. –Cooperate to send an message From sender to recipient’s mailbox Sender’s computer is the client –Contacts an server program on the recipient’s computer –Stores the message in the recipient’s mailbox

Figure 20.1 Figure 20.1 An transfer across the Internet requires two programs: a client on the sender’s computer and a server on the recipient’s computer.

February 5, 2002CE80N -- Lecture #1016 Using From A Personal Computer Most personal computers do not receive directly. –Arrange to have a mailbox on a large computer (I.e, at the ISP) –Contacts the main computer system Mail program: Eudora, Netscape, etc… –Obtains a copy of their mailbox

February 5, 2002CE80N -- Lecture #1017 remote operation w/PCs Mail Server A Mail Server B Sender A Recipient B

February 5, 2002CE80N -- Lecture #1018 Mailbox Address Format Addresses consist of a string of characters separated by the The prefix: –identifies the user. The suffix: – gives the domain name of the computer on which the user’s mailbox resides.

February 5, 2002CE80N -- Lecture #1019 Abbreviations Make Friendly Most systems allow a user to define abbreviations for the mailbox address. For example: venus.engineering.somecompany.com Becomes eng Allowing the address of (Instead of

February 5, 2002CE80N -- Lecture #1020 Aliases Permit Arbitrary Abbreviations Most commercially available software supports an alias. –Requires the user to prepare a list of aliases –Translates the alias to a longer address ‘mary’ can be used in place of:

February 5, 2002CE80N -- Lecture #1021 Aliases Shared By All Users Of A Computer System System-wide aliases make it possible for all users to share abbreviations. –Consider these: Webmaster Listmanager Help Printers

February 5, 2002CE80N -- Lecture #1022 Sending To Multiple Recipients systems allow users to send messages to multiple recipients. –Specify multiple mailbox addresses on the ‘To’ line of message For example: –To:

February 5, 2002CE80N -- Lecture #1023 Mailing List: An Alias for Multiple Recipients A mailing list is an alias that specifies multiple recipients The system: –Sends a message to the alias –Delivers a copy to each recipient on the list

February 5, 2002CE80N -- Lecture #1024 Public Mailing Lists And Mail Exploders A public list permits a user on any computer connected to the Internet to send a message to a list of recipients. When the message reaches the destination computer, an exploder: –Finds the name –Expands the abbreviation –Forwards a copy to each recipient

Figure 20.2 Figure 20.2 The path of a mail message sent to a public mailing list that contains three recipients. A Mail exploder retrieves the message, and forwards a copy to each recipient on the list.

February 5, 2002CE80N -- Lecture #1026 To And From Non- Internet Sites can be forwarded to other networks. For example, Compuserve: –Does not use the same Internet protocols –Uses software on an intermediate computer to send and receive Internet

February 5, 2002CE80N -- Lecture #1027 Access To Services Via A computer program can: –Be used to answer and reply to an e- mail message. An message can: –Be used to provide access to a variety of remote services.

February 5, 2002CE80N -- Lecture #1028 Speed, Reliability, And Expectations systems are more reliable than postal mail systems. –Delivery is usually within minutes –Sender is notified if a message cannot be delivered Not all homes have computers connected to the Internet. Differences in expectations can make e- mail frustrating. –Not all users read their at the same rate

February 5, 2002CE80N -- Lecture #1029 Impact And Significance Of Electronic Mail After using it, benefits become apparent. –Combines benefits of instantaneous communication with freedom from interruption –Provides a way for groups to share common interests –Can communicate with more people

February 5, 2002CE80N -- Lecture #1030 Joining A Mailing List To join a list, the user must send a request via . The request is not sent to the list but to a second alias used for joining or leaving a list. –To join: – to:

February 5, 2002CE80N -- Lecture #1032 Glossary Electronic Mail –( ) A service that permits one to send a memo to another person, a group, or a computer program. Mail Alias –A synonym for alias.

February 5, 2002CE80N -- Lecture #1033 Glossary Mailbox –A storage area, usually on disk, that holds incoming messages until a user reads the mail. Mailbox Address –A synonym for address.

February 5, 2002CE80N -- Lecture #1034 Glossary Mailing List –An electronic mail address that includes a list of recipients. Postmaster –By convention, an alias for the person who manage the electronic mail software on a given computer.

February 5, 2002CE80N -- Lecture #1035 Glossary Smiley –A sequence of characters, usually found in an message, that indicates humorous intent. The three character sequence :-) resembles a smiling face turned sideways. POP –Abbreviation for Post Office Protocol.

Midterm Results

February 5, 2002CE80N -- Lecture #1037 Midterm Results 50 questions –High score: 48 = 96% –Low score: 23 = 46% –Median: 38.4 = 76.8% –Passing grade >= 30

February 5, 2002CE80N -- Lecture #1038 Most missed questions #11, (77): Prefix bits #38, (73): Routing protocol #30, (60): LAN technologies #19, (56): IP layer #27, (55): Distributed computing #10, (54): CIDR network bits #32, (54): Packet switching

February 5, 2002CE80N -- Lecture #1039 Question #11 11) How many prefix bits (network bits in CIDR) are contained in the Class A address a. 32 b. 24 c. 16 d. 8  Class A has a fixed prefix of 8 bits e. 10

February 5, 2002CE80N -- Lecture #1040 Question #38 38) The Routing Information Protocol (RIP) only communicates routing information with its directly attached neighbors. RIP is an example of what type of routing protocol? a. Link state b. Round-robin c. Distance vector  RIP is distance vector d. Service oriented

February 5, 2002CE80N -- Lecture #1041 Question #30 30) LAN Technologies, in general are: a. Not Compatible  LANs are not compatible b. Highly interoperable c. Proprietary d. Plug and Play

February 5, 2002CE80N -- Lecture #1042 Question #19 19) IP resides in what layer of the protocol stack (Berkeley or OSI) a. Routing layer b. Transport layer c. Link layer d. Network layer  IP is in the network layer e. Physical layer

February 5, 2002CE80N -- Lecture #1043 Question #27 27) Any interaction that involves two or more computers over a network is called: a. Local communication b. Distributed computing  c. True networking d. Universal service

February 5, 2002CE80N -- Lecture #1044 Question #10 10) Based on CIDR addressing, how many network bits are contained in the address range /20 a. 128 b. 0 c. 100 d. 20  CIDR network bits

February 5, 2002CE80N -- Lecture #1045 Question #32 32) Packet switching is more efficient than circuit switching because of a. Faster router speeds b. Resource sharing  c. Short packets are always used d. Ethernet LANs e. All of the above

February 5, 2002CE80N -- Lecture #1046