Internet - Computing System - Lecture Slides, Slides of Computer Science

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The Internet

CT101 –Computing Systems

Contents

• The Internet
  • Architecture, Addressing, Protocols
  • DNS
  • E-Mail
  • WWW
  • Security

Internet Architecture

• Internet Service Provider (ISP)
  • Tier-
  • Tier-
  • Tire 1 and tier 2ISPs are networks of routers that collectively provide the Internet’s communication infrastructure
• Access ISP: Provides connectivity to the Internet
  • Traditional telephone (dial up connection)
  • Cable connections
  • DSL
  • Wireless

Internet Composition

Package-shipping example

Internet Software Layers

• Application: Constructs message

with address

• Transport: Chops message into

packets

• Network: Handles routing

through the Internet

• Link: Handles actual transmission

of packets

DNS

• IP addresses are difficult to remember
  • [email protected] would be difficult to remember
  • If JohnDoe’s mail server moves to another machine, then his e-mail address would not be valid anymore
  • Something like [email protected] would be appropriate
• Need some mechanisms to translate wuzwuz.ucg.ie to the IP address
  • To solve this problem DNS was invented
• It is a hierarchical, domain-based naming scheme and a distributed database system for implementing this naming scheme
• Usage:
  • Map name onto an IP address, an application program calls an library procedure, called resolver , passing it the name as a parameter (i.e. gethostbyname() is an resolver)
  • The resolver sends UDP packet to a local DNS server which looks up the name and returns the IP address to the resolver
  • The resolver returns the IP address to the application, which can establish an TCP/IP connection with the destination (or send UDP packets)

DNS Name Space

• Internet is divided into over 200 top level domains
  • Each domain is divided into sub-domains, which are further partitioned, etc..
  • All domains can be represented by a tree
    • The leaves of the tree represent domains that have no sub-domains (but contain machines)
    • A leaf domain may contain a single host or represent a company and contain thousands of hosts
• Top level domains could be generic and country domains

Name servers

• One DNS server could service all requests
  • In practice it will be overloaded
• To solve this, DNS name space is divided in non overlapping zones
  • Each zone contains some part of the tree and name servers holding zone info
    • A zone would have a primary DNS (gets info from disk)
    • One or more secondary DNS (get info from the primary DNS)

Name Servers – Lookup mechanism

• In the example, a resolver on flits.cs.vu.nl is looking for IP address of linda.cs.yale.edu (using recursive query ; some servers don’t implement recursive query and return the address of the next server to try) - The resolver sends a query containing the domain name sought - The query is forwarded by the local name server to the name server for domain edu , that is found in its database….etc
• Once the records get back to cs.vu.nl name server, they will be entered in a local cache, in case they are needed later;

Traditional Internet Applications

• Electronic Mail (email)

• Domain mail server collects incoming mail and transmits

outgoing mail

• Mail server delivers collected incoming mail to clients

via POP3 or IMAP

• File Transfer Protocol (FTP)

• Telnet and SSH

• WWW (World Wide Web)

More Recent Applications

• Voice Over IP (VoIP)

• Internet Radio

• N-unicast
• Multicast

E-Mail Architecture

• E-mail system consists of two parts

• User agents, which allow people to read and send email
  • Local programs that provide a command based or graphical method for interacting with e-mail system
• Message transfer agents, which move the messages from

source to destination

• Are typically system daemons or processes that run in background, having the job to move messages

E-Mail functions

• E-mail system functions

• Composition – refers to the process of creating messages

and answers; although any text editor can be used for the

text of the message, the system itself can provide

assistance with addressing and numerous header fields

attached to each message

• Transfer – refers to moving messages from the

originator to the recipient; this requires establishing a

connection to the destination or some intermediate

machine, outputting the message and releasing the

connection