Chapter 29 Structure of Computer Names Domain Names Within an Organization The DNS Client-Server...
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Transcript of Chapter 29 Structure of Computer Names Domain Names Within an Organization The DNS Client-Server...
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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 internet’s 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@computer– Allows each computer system to assign mailbox
identifiers independently– Sender’s computer uses the second part to select a
destination– Recipient’s 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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•
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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