Structure of Computer Names

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Chapter 29

• Structure of Computer Names
• Domain Names Within an Organization
• The DNS Client-Server Model
• The DNS Server Hierarchy
• Resolving a Name
• Optimization of DNS Performance

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Introduction

• Computers are assigned symbolic names
• Convenient for humans
• Inconvenient for computers
• IP addresses require less computation
• IP addresses require less memory and time to
  transmit
• Symbolic names need to be translated into
  equivalent IP addresses

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Introduction

• Domain names are used in the Web and for email
• The URL http//www.calpoly.edu contains the
  domain name calpoly.edu
• Everytime you use a domain name, you use the
  internets domain name servers to translate it
  into an IP address

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Structure of Computer Names

• Naming scheme used in the Internet is called
  DNS(Domain Name System)
• Each computer name consists of a sequence of
  alpha-numeric segments separated by periods
• Ex. Mordred.cs.purdue.edu
• Domain names are arranged in a hierarchy
• Most significant is on the right
• Left most segment is the name of the computer
• Other segments identify the group that owns the
  name

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• Each organization chooses structure of computer
  name
• Most significant segment is called the top level
  of the DNS

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Structure of Computer Names Continued

• ICANN(Internet Corporation for Assigned Names and
  Numbers) is the organization that is responsible
  for domain names
• To obtain a domain name, each organization has to
  apply for a name under one of the top-level
  domains
• Once an organization has been assigned a domain,
  the suffix is reserved for that organization
• DNS allows organizations to use geographic
  registration
• Ex., cnri.reston.va.us
• Ex., ac.uk

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Domain Names Within an Organization

• Once an organization owns a domain, the
  organization can decide whether to introduce
  hierarchical structure

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DNS Server Client Model

• DNS is autonomous
• Organizations that have an Internet connection
  runs a domain server
• Software that translates computer names into
  addresses demonstrates the use of client-server
  interaction
• Whenever an application program needs to
  translate a name, the application become a client
  of the naming system
• Client places the name to be translated in a DNS
  request message and sends it to the DNS server
• Server extracts the names, translates it, and
  returns the resulting address in a reply message

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DNS Server Hierarchy

• DNS Servers are arranged in a hierarchy that
  matches the naming hierarchy
• Root server occupies the top of the hierarchy and
  is an authority for the top-level domains
• Root server contains info about how to reach
  other servers
• Corporation can choose to place all its domain
  names in a single server, or it can choose to run
  several servers

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Resolving a Name

• Software that performs the translation is called
  resolver software
• Each resolver is configured with the address of a
  local domain name server
• When a request specifies a name for which a
  server is an authority, the server answers
  directly, otherwise the server becomes a client
  of another server

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Optimization of DNS Performance

• DNS that was just described is inefficient
• 2 primary optimizations
• Replication
• each server is replicated
• Caching
• each server maintains a cache of names

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Summary

• DNS provides an automated mapping b/w computer
  names and equivalent IP addresses
• Names are allocated hierarchically, and segments
  in the name correspond to levels in the hierarchy
• The client sends a request to its local server,
  which either answers the request directly or
  contacts other servers.
• DNS servers use two performance optimization
  techniques replication and caching

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Chapter 30

• Electronic Mail Paradigm
• Electronic Mailboxes and Addresses
• Electronic Mail Message Format
• Multipurpose Internet Mail Extensions
• Mail Transfer
• SMTP and POP Protocols

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Chapter 30

• Mail Exploders,lists, and forwarders
• Mailbox Gateways
• Mailbox Access

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The Electronic Mail Paradigm

• Email was designed from the traditional office
  memo
• It was built to allow a person to communicate
  with other people
• Electronic mail systems have evolved from the
  original design and are now permitted to more
  complex interactions

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Electronic Mailboxes and Addresses

• Before email can be sent, the person must be
  assigned a mailbox
• Email address contains 2 parts
• Mailbox_at_computer
• Allows each computer system to assign mailbox
  identifiers independently
• Senders computer uses the second part to select
  a destination
• Recipients computer uses the first part to
  select a mailbox

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Electronic Mail Message Format

• Message consists of ASCII text that is separated
  into 2 parts by a blank line
• The first part is called the header
• The second part is the body

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Multipurpose Internet Mail Extensions

• Email was designed to handle only text
• MIME standard allows sender to encode non-text
  data for transmission
• MIME software decodes the attached data
  automatically
• For transparent encoding and decoding, MIME adds
  two lines to an email header
• MIME-Version 1.0
• Content-Type multipart/mixed

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Mail Transfer

• A user interacts with an email interface
• Composing or reading
• Email system contains a mail transfer program
• Handles the detail of sending message

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Simple Mail Transfer Protocol

• When a transfer mail program contacts a server on
  a remote machine, it forms a TCP connection over
  which it communicates
• Once the connection is in place, SMTP allows the
  sender to identify itself, specify a recipient,
  and transfer an email

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Optimizing For Multiple Recipients

• Most mail transfer programs are optimized to
  handle all recipients on a given remote computer
  at the same time
• Optimizing reduces
• Network bandwidth
• Delay required for all users to receive a copy of
  the message

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Mail Exploders,Lists,Forwarders

• Mail exploder/forwarder-program that forwards
  copies of a message
• When an email message arrives, the mail exploder
  examines the destination address

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Mail Gateways

• Mail gateway is a computer dedicated to
  forwarding email

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Mailbox Access

• TCP/IP protocol called POP(Post Office Protocol)
  allows remote access to an electronic mailbox
• The server with the mailbox uses the POP protocol
• User runs email software that becomes a client of
  the POP server to access the mailbox

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Mailbox Access
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Email Client Example

• Email client connects to the SMTP server at
  openmail2.calpoly.edu
• Email client gets the address of the sender and
  recipient as well as the body of the message from
  the SMTP server
• SMTP server at recipient connects with the SMTP
  server at the sender

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Summary

• Electronic mail uses the office memo paradigm
• Email address is separated into 2 parts
• MIME standard allows a sender to encode non-text
  data
• SMTP protocol is used to transfer a message
• Optimizing for multiple recipients reduces
  bandwidth and delay
• Mail exploders make it possible for a large group
  to communicate via email

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Summary

• Mail gateways forward email
• POP protocol used to access a remote mailbox