Introduction to Distributed Systems and Networking

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Introduction to Distributed Systems and Networking
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Announcements

• Homework 4 due today
• Attempting to schedule Prelim II for Thursday,
  April 26th

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Goals for today

• Introduction to Distributed Systems
• Introduction to Networking

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Centralized vs Distributed Systems

• Centralized System System in which major
  functions are performed by a single physical
  computer
• Originally, everything on single computer
• Later client/server model
• Distributed System physically separate computers
  working together on some task
• Early model multiple servers working together
• Probably in the same room or building
• Often called a cluster
• Later models peer-to-peer/wide-spread
  collaboration

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Distributed Systems

• Definition
• Loosely coupled processors interconnected by
  network
• Distributed system is a piece of software that
  ensures
• Independent computers appear as a single coherent
  system
• Lamport A distributed system is a system where
  I cant get my work done because a computer that
  Ive never heard of has failed

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Why use distributed systems?

• These are now a requirement
• Economics dictate that we buy small computers
• Cheap way to provide reliability
• We all need to communicate
• It is much easier to share resources
• Allows a whole set of distributed applications
• A whole set of future problems need machine
  communication
• Collaboration Much easier for users to
  collaborate through network resources (such as
  network file systems)

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Distributed Systems Issues

• The promise of distributed systems
• Higher availability one machine goes down, use
  another
• Better durability store data in multiple
  locations
• More security each piece easier to make secure
• Reality has been disappointing
• Worse availability depend on every machine being
  up
• Lamport a distributed system is one where I
  cant do work because some machine Ive never
  heard of isnt working!
• Worse reliability can lose data if any machine
  crashes
• Worse security anyone in world can break into
  system
• Coordination is more difficult
• Must coordinate multiple copies of shared state
  information (using only a network)
• What would be easy in a centralized system
  becomes a lot more difficult

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Distributed Systems Goals

• Connecting resources and users
• Transparency the ability of the system to mask
  its complexity behind a simple interface
• Location Cant tell where resources are located
• Migration Resources may move without the user
  knowing
• Replication Cant tell how many copies of
  resource exist
• Concurrency Cant tell how many users there are
• Parallelism System may speed up large jobs by
  splitting them into smaller pieces
• Fault Tolerance System may hide various things
  that go wrong in the system
• Openness portability, interoperability
• Scalability size, geography, administrative
• Transparency and collaboration require some way
  for different processors to communicate with one
  another

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Software Concepts
System Description Main Goal
Distributed OS Tightly coupled OS for multiprocessors and homogeneous m/cs Hide and manage hardware resources
Networked OS Loosely coupled OS for heterogeneous computers, LAN/WAN Offer local services to remote clients
Middleware Additional layer atop NOS implementing general-purpose services Provide distribution transparency
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Some Applications

• Air traffic control
• Banking, stock markets
• Military applications
• Health care, hospital automation
• Telecommunications infrastructure
• E-commerce, e-cash

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Few Challenges

• No shared clocks
• How to order events
• No shared memory
• Inconsistent system state
• Scalability
• Fault tolerance
• Availability, recoverability
• Consensus
• Self management
• Security

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Networking

• Middleware gives guarantees not provided by
  networking
• How do you connect computers?
• Local area network (LAN)
• Wide area network (WAN)
• Let us consider the example of the Internet

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Internet Example

• Click -gt get page
• specifies - protocol (http) - location
• (www.cnn.com)

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Internet Locating Resource

• www.cnn.com
• name of a computer
• Implicitly also a file (index.html)
• Map name to internet protocol (IP) address
• Domain name system (DNS)

cnn.com?
cnn.com?
host
local
com
a.b.c.d
a.b.c.d
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Internet Connection

• Http (hyper-text transport protocol) sets up a
  connection
• TCP connection (transmission control protocol)
• between the host and cnn.com to transfer the page
• The connection transfers page as a byte stream
• without errors flow control error control

Host
www.cnn.com
Connect
OK
Get page
Page close
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Internet End-to-end

• Byte stream flows end to end across many
  links/switches
• routing ( addressing)
• That stream is regulated and controlled by both
  ends
• retransmission of erroneous or missing bytes
  flow control

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Internet Packets

• The network transports bytes grouped into packets
• Packets are self-contained routers handle them
  1 by 1
• The end hosts worry about errors and pacing
• Destination sends ACKs Source checks losses

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Internet Bits

• Equipment in each node sends packets as string of
  bits
• That equipment is not aware of the meaning of the
  bits
• Frames (packetizing) vs. streams

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Internet Points to remember

• Separation of tasks
• send bits on a link transmitter/receiver clock,
  modulation,
• send packet on each hop framing, error
  detection,
• send packet end to end addressing, routing
• pace transmissions detect congestion
• retransmit erroneous or missing packets acks,
  timeout
• find destination address from name DNS
• Scalability
• routers dont know full path
• names and addresses are hierarchical

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Internet Challenges

• Addressing ?
• Routing ?
• Reliable transmission ?
• Interoperability ?
• Resource management ?
• Quality of service ?

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Concepts at heart of the Internet

• Protocol
• Layered Architecture
• Packet Switching
• Distributed Control
• Open System

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Protocol

• Two communicating entities must agree on
• Expected order and meaning of messages they
  exchange
• The action to perform on sending/receiving a
  message
• Asking the time

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Layered Architectures

• Human beings can handle lots of complexity in
  their protocol processing.
• Ambiguously defined protocols
• Many protocols all at once
• How computers manage complex protocol processing?
• Specify well defined protocols to enact.
• Decompose complicated jobs into layers
• each has a well defined task

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Layered Architectures

• Break-up design problem into smaller problems
• More manageable
• Modular design easy to extend/modify.
• Difficult to implement
• careful with interaction of layers for efficiency

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Layered Architecture
users
network
Applications
Web, e-mail, file transfer, ...
Reliable/ordered transmission, QOS, security,
compression, ...
Middleware
End-to-end transmission, resource allocation,
routing, ...
Routing
Point-to-point links, LANs, radios, ...
Physical Links
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The OSI Model

• Open Systems Interconnect (OSI)
• standard way of understanding conceptual layers
  of network comm.
• This is a model, nobody builds systems like this.
• Each level
• provides certain functions and guarantees
• communicates with the same level on remote notes.
• A message
• generated at the highest level
• is passed down the levels, encapsulated by lower
  levels
• until it is sent over the wire.
• On the destination
• Encapsulated message makes its way up the layers
• until the high-level message reaches its
  high-level destination.

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OSI Levels
Node A
Application
Node B
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Data Link
Data Link
Physical
Physical
Network
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OSI Levels

• Physical Layer
• electrical details of bits on the wire
• Data Link Layer
• sending frames of bits and error detection
• Network Layer
• routing packets to the destination
• Transport Layer
• reliable transmission of messages,
  disassembly/assembly, ordering, retransmission of
  lost packets
• Session Layer
• really part of transport, typ. Not impl.
• Presentation Layer
• data representation in the message
• Application
• high-level protocols (mail, ftp, etc.)

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Internet protocol stack
users
network
Application
HTTP, SMTP, FTP, TELNET, DNS,
Transport
TCP, UDP.
Network
IP
Point-to-point links, LANs, radios, ...
Physical
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Air travel
Passenger Origin
Passenger Destination
Ticket (purchase)
Ticket (complain)
Baggage (check)
Baggage (claim)
Gates (load)
Gates (unload)
Runway (take off)
Runway (landing)
Airplane routing
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Summary

• Network physical connection that allows two
  computers to communicate
• Packet unit of transfer, sequence of bits
  carried over the network
• Protocol Agreement between two parties as to how
  information is to be transmitted
• Internet Protocol (IP)
• Used to route messages through routes across
  globe
• 32-bit addresses, 16-bit ports
• Reliable, Ordered, Arbitrary-sized Messaging
• Built through protocol layering on top of
  unreliable,