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Next Few Classes

Networking basics Protection & Security

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Distributed Systems distributed system: set of physically separate

processors connected by one or more communication links

no shared clock or memory

Many systems today distributed in some way e-mail, p2p system, network printers, remote

backup, web...

P2

P1

P3P4

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Parallel vs. Distributed Systems

Tightly-coupled systems: “parallel processing” Processors share clock, memory, run one OS Frequent communication

Loosely-coupled systems: “distributed computing” Each processor has own memory, runs

independent OS Infrequent communication

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Advantages of Distributed Systems

Resource sharing Computational speedup Reliability Communication

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Networks Goal: provide efficient, correct, robust

message passing between two separate nodes

Local area network (LAN) – connects nodes in single building, fast & reliable (Ethernet, WLAN) Media: twisted-pair, coax, fiber, radio Bandwidth: 10Mbps – 1Gbps

Wide area network (WAN) – connects nodes across large geographic area (Internet) Media: fiber, microwave links, satellite channels,

radio Bandwidth: 1.544MB/s (T1), 45 MB/s (T3),…,

1Gbps

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LAN Topologies

Two basic topologies: Point-to-point Bus

Connection of nodes impacts: Speed: maximum & average

communication time Reliability: fault tolerance Expense

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Point-to-Point Topology: Fully-connected

Each message takes one “hop” Node failure – no effect on

communication with others Expensive – impractical for WANs

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Point-to-Point Topology: Partially connected

Links between some, but not all nodes Less expensive, less tolerant to failures

Single node failure can partition network Sending message takes several hops

Needs routing algorithms

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Point-to-Point Topology: Tree

Tree structure: network hierarchy Messages past between direct descendants

Max message cost? Not failure tolerant

Any interior node fails – network partitioned

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Point-to-Point Topology: Star

Star network: all nodes connect to central node Each message takes how many hops? Not failure tolerant Inexpensive – sometimes used for LANs

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Point-to-Point Topology: One-directional Ring

Given n nodes, max hops? Inexpensive Fault-tolerant?

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Bus Network Topologies

Bus nodes connect to common network

Linear bus – single shared link Nodes connect directly to each other via

bus Inexpensive (linear in # of nodes) Tolerant of node failures Traditional Ethernet LAN

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Bus Network Topologies

Ring bus – single shared circular link Same technology & tradeoffs as

linear bus

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WAN Topology: Internet Graph

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Principles ofNetwork Communication

Data broken into packets Basic unit of transfer

Packets sent through network Routers at switching points control packet

flow

Road analogy: Packets = cars Network = roads router = traffic lights (intersection) Too many packets on shared link/node =

traffic jam

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What’s a protocol?a human protocol and a computer network protocol:

Q: Other human protocols?

Hi

Hi

Got thetime?

2:00

TCP connection req

TCP connectionresponse

Get http://www.engr.uconn.edu/~bing

<file>

time

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What’s a protocol?

human protocols: “what’s the time?” “I have a question” introductions

… specific msgs sent… specific actions

taken when msgs received, or other events

network protocols: machines rather

than humans all communication

activity in Internet governed by protocols

protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission,

receipt, other events

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Internet Protocol Stack application: supporting network

applications FTP, SMTP, HTTP

transport: host-host data transfer TCP, UDP

network: routing of datagrams from source to destination

IP, routing protocols link: data transfer between

neighboring network elements PPP, Ethernet

physical: bits “on the wire”

application

transport

network

link

physical

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Layering: Physical Communication

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

data

data

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Protocol Layering and Data

Each layer takes data from above adds header information to create new data unit passes new data unit to layer below

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

source destination

M

M

M

M

Ht

HtHn

HtHnHl

M

M

M

M

Ht

HtHn

HtHnHl

message

segment

datagram

frame

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Summary

Virtually all computer systems contain distributed components

Networks connect them Key tradeoffs:

Speed Reliability Expense