Introduction to Distributed
Systems and Networking
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
Server
Client/Server Model
Peer-to-Peer Model
• 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 can’t
get my work done because a computer that I’ve never
heard of has failed”
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Why use distributed systems?
• These are now a requirement:
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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 can’t do work because some
machine I’ve never heard of isn’t 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
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Location: Can’t tell where resources are located
Migration: Resources may move without the user knowing
Replication: Can’t tell how many copies of resource exist
Concurrency: Can’t 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
Machine C
Machine B
Machine A
Distributed Applications
Middleware
Local OS
Local OS
Local OS
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Network
Some Applications
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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
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No shared memory
– Inconsistent system state
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Scalability
Fault tolerance
– Availability, recoverability
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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 -> 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
com
local
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
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
end-to-end pacing and
error control
CNN.COM
routing
HOST
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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
A | B | # , CRC | bytes
CNN.COM: A
HOST: B
C
B: to
C
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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
01011...011...110
01011...011...110
Transmitter
Physical Medium
Receiver
Optical
Copper
Wireless
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Internet: Points to remember
• Separation of tasks
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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 don’t know full path
– names and addresses are hierarchical
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Internet : Challenges
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Addressing ?
Routing ?
Reliable transmission ?
Interoperability ?
Resource management ?
Quality of service ?
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Concepts at heart of the Internet
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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
network
users
Applications
Web, e-mail, file transfer, ...
Middleware
Reliable/ordered transmission, QOS,
security, compression, ...
Routing
Physical Links
End-to-end transmission,
resource allocation, routing, ...
Point-to-point links,
LANs, radios, ...
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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
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Data Link
Data Link
Physical
Physical
Network
Node B
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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
network
users
Application
HTTP, SMTP, FTP, TELNET, DNS, …
Transport
TCP, UDP.
Network
IP
Physical
Point-to-point links,
LANs, radios, ...
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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,
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