COSC6377: Computer Networks

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COSC6377 Computer Networks
Computer Networking A Top Down Approach
Featuring the Internet, 3rd edition. Jim
Kurose, Keith RossAddison-Wesley, July 2004.

• Rong Zheng
• rzheng_at_cs.uh.edu

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Introduction

• Overview of the course
• Basic concepts and structures in computer
  networking
• Network architecture

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Whats the Internet nuts and bolts view

• End systems
• Host computer
• Network applications
• Access networks
• Local area networks
• communication links
• Network core
• routers
• network of networks

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Whats the Internet nuts and bolts view

• Protocols define format, order of msgs sent and
  received among network entities, and actions
  taken on msg transmission, receipt
• e.g., TCP, IP, HTTP, FTP, PPP
• Internet network of networks
• loosely hierarchical
• public Internet versus private intranet
• Internet standards
• RFC Request for comments
• IETF Internet Engineering Task Force

router
workstation
server
mobile
local ISP
regional ISP
company network
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Network Components (Examples)
Links
Interfaces
Switches/routers
Ethernet card
Large router
Fibers
Wireless card
Coaxial Cable
Switch
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Internet structure network of networks

• roughly hierarchical
• at center tier-1 ISPs (e.g., MCI, Sprint,
  ATT, Cable and Wireless), national/international
  coverage
• treat each other as equals

Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
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Tier-1 ISP e.g., Sprint
Sprint US backbone network
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Internet structure network of networks

• Tier-2 ISPs smaller (often regional) ISPs
• Connect to one or more tier-1 ISPs, possibly
  other tier-2 ISPs

Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
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Internet structure network of networks

• Tier-3 ISPs and local ISPs
• last hop (access) network (closest to end
  systems)

Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
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Internet structure network of networks

• a packet passes through many networks!

Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
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Protocol Layers

• Networks are complex!
• many pieces
• hosts
• routers
• links of various media
• applications
• protocols
• hardware, software

• Question
• Is there any hope of organizing structure of
  network?
• Or at least our discussion of networks?

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An Imaginary 2-tier
SSH
FTP
SMTP
Application
Coaxial cable
Fiber optic
Transmission Media

• New application has to interface to all existing
  media
• adding new application requires O(m) work, m
  number of media
• New media requires all existing applications be
  modified
• adding new media requires O(a) work, a number
  of applications
• Total work in system O(ma) ? eventually too much
  work to add apps/media
• Application end points may not be on the same
  media!

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Solution Indirection

• Solution introduce an intermediate layer that
  provides a single abstraction for various network
  technologies
• O(1) work to add app/media
• Indirection is an often used technique in
  computer science

SSH
NFS
SMTP
Application
Intermediate layer
Coaxial cable
Fiber optic
Transmission Media
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Network Architecture

• Architecture is not the implementation itself
• Architecture is how to organize implementations
• what interfaces are supported
• where functionality is implemented
• Architecture is the modular design of the network

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Software Modularity

• Break system into modules
• Well-defined interfaces gives flexibility
• can change implementation of modules
• can extend functionality of system by adding new
  modules
• Interfaces hide information
• allows for flexibility
• but can hurt performance

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Network Modularity

• Like software modularity, but with a twist
• Implementation distributed across routers and
  hosts
• Must decide both
• how to break system into modules
• where modules are implemented

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Layering

• Layering is a particular form of modularization
• The system is broken into a vertical hierarchy of
  logically distinct entities (layers)
• The service provided by one layer is based solely
  on the service provided by layer below
• Rigid structure easy reuse, performance suffers

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ISO OSI Reference Model

• ISO International Standard Organization
• OSI Open System Interconnection
• Seven layers
• Lower two layers are peer-to-peer
• Network layer involves multiple switches
• Next four layers are end-to-end

Host 1
Intermediate switch
Host 2
Application
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Network
Datalink
Datalink
Datalink
Physical
Physical
Physical
Physical medium A
Physical medium B
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Key Concepts

• Service says what a layer does
• Ethernet unreliable subnet unicast/multicast/broa
  dcast datagram service
• IP unreliable end-to-end unicast datagram
  service
• TCP reliable end-to-end bi-directional byte
  stream service
• Service Interface says how to access the
  service
• E.g. UNIX socket interface
• Protocol says how is the service implemented
• a set of rules and formats that govern the
  communication between two peers

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Internet Protocol Architecture

• The TCP/IP protocol suite is the basis for the
  networks that we call the Internet.
• The TCP/IP suite has four layers Application,
  Transport, Network, and (Data) Link Layer.
• Computers (hosts) implement all four layers.
  Routers (gateways) only have the bottom two
  layers.

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Functions of the Layers

• Service Handles details of application
  programs.
• Functions
• Service Controls delivery of data between hosts.
• Functions Connection establishment/termination,
• error control, flow control, congestion
  control, etc.
• Service Moves packets inside the network.
• Functions Routing, addressing, switching, etc.
• Service Reliable transfer of frames over a link.
• Functions Synchronization, error control, flow
  control, etc.

telnet, ftp, email www, NFS
TCP, UDP
IP, ICMP, OSPF RIP, BGP
Ethernet, WiFi T1
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Internet Protocol Architecture
IP protocol
IP protocol
Ethernet
ATM
protocol
protocol
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Internet Protocol Architecture
MPEG Servier
MPEG Player
RTP protocol
program
program
UDP
UDP
UDP protocol
IP
IP protocol
IP protocol
IP
IP
Ethernet
Ethernet
ATM
ATM
Ethernet
ATM
Driver
Driver
Driver
Driver
protocol
protocol
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Encapsulation
source
message
application transport network link physical
segment
datagram
frame
switch
destination
application transport network link physical
router
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Hourglass
Note Additional protocols like routing protocols
(RIP, OSPF) needed to make IP work
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Implications of Hourglass

• A single Internet layer module
• Allows all networks to interoperate
• all networks technologies that support IP can
  exchange packets
• Allows all applications to function on all
  networks
• all applications that can run on IP can use any
  network
• Simultaneous developments above and below IP

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Reality

• Layering is a convenient way to think about
  networks
• But layering is often violated
• Firewalls
• Transparent caches
• NAT boxes

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Thinking Exercise Interplanetary Internet

• Network entities
• Ground stations
• Satellites, space stations at near-Earth orbit
  600 kilometers ( 2s)
• Mars rovers take 10 to 30 min
• Planets farther away?
• Objective
• Interconnect with earth-bound networks?

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