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

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

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

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

•Each organization chooses structure of computer name

•Most significant segment is called the top level of the DNS

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

Domain Names Within an Organization

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

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

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

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

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

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

Chapter 30

• Electronic Mail Paradigm

• Electronic Mailboxes and Addresses

• Electronic Mail Message Format

• Multipurpose Internet Mail Extensions

• Mail Transfer

• SMTP and POP Protocols

Chapter 30

• Mail Exploders,lists, and forwarders

• Mailbox Gateways

• Mailbox Access

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

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

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

•

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;

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

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

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

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

Mail Gateways

• Mail gateway is a computer dedicated to forwarding email

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

Mailbox Access

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

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

Summary

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