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Computer Networking and the Internet:
Design Issues
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MotivationMotivationWhat is the importance of the computer
networking and the Internet? Applications: does it really make our lives easier?
Potential uses: where is it heading to?
As computer networking engineers:
What do we need to know about networks issues?
What do we do with these issues?
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Applications: Efficient WorkflowApplications: Efficient Workflow
Access to multiple sources Minimize travel No 2nd party involvement
Paperless information entry Status update Auditing Alerts
Engineering
Server
Machine
Automation
ManufacturerInternet
Process
Server
Office Plant
IT
Engineering
Server
Machine
Automation
ManufacturerInternet
Process
ServerEngineering
Server
MachineMachine
Automation
ManufacturerInternet
Process
Server
Office Plant
IT
Security Server
Machine
Controller
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Computer Networking ComponentsComputer Networking Components
and Issuesand Issues
(A Network Design Perspective)(A Network Design Perspective)
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Information, Computers, NetworksInformation, Computers, Networks
Information:
anything that can be represented in bits
Properties:
Information can be infinitely replicable
Computers can manipulate information
Networks create access to information
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NetworksNetworks
Potential of networking:
Move bits everywhere, cheaply, and withdesiredperformance characteristics
Break the space barrierfor information
Networks provide connectivity
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ConnectivityConnectivity
Building Blocks
Links: coax cable, optical fiber, wireless, etc.
Nodes: general-purpose workstations or devices
Direct Connectivity: Point-to-point
Multiple access
Point-to-Point
Bus
Node 1 Node 2 Node n
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ConnectivityConnectivity
Indirect Connectivity
Switched networksSwitches create temporary
physical connections E.g. Circuit-switched
Telephone Network
Inter-networks
Routers create temporarylogical connections E.g. Packet-switched
Internet
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ConnectivityConnectivity
The Internet:
Best-effort (i.e.; noperformance guarantees)
Packet-by-packet delivery
A point-to-point physical
link: Always-connected
Fixed bandwidth
Fixed delay
Zero-jitterPoint-to-Point
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PointPoint--toto--Point ConnectivityPoint Connectivity
Physical layer is, at least, needed for coding,modulation, etc.
Link layer is needed only if the link is: Shared: needs framing, medium access control,
multiplexing, etc.
Unreliable: needs reliability techniques
used sporadically and traffic can flood receivers:needs flow control mechanisms
No need for protocol concepts like addressing,names, routers, hubs, forwarding, or filtering in
point-to-point connections
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Connecting N usersConnecting N usersDirectlyDirectly
Bus:broadcast, collisions, media access control
Full mesh: Cost vs simplicity
. . .
Full meshBus
Address concept is needed if we want a specificreceiver (or receivers) to consume the packet!
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List of Design Problems (so far)List of Design Problems (so far)
Topologies
Framing
Error control
Flow control
Multiple access
How to share a link?
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How to build Scalable Networks?How to build Scalable Networks?
Scaling: the system allows for an increase of a
key parameter (e.g., the number of nodes),
without severely compromising the resources. Inefficiency limits scaling
Direct connectivity is inefficient & hence does
not scale
Mesh: inefficient in terms of the number of links Bus: inefficient in terms of the bandwidth use
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Filtering and ForwardingFiltering and Forwarding
Filtering: choosing a subset of elements from a set
Filtering is the key to efficiency & scaling
Forwarding: sending packets to a filtered subset of thelinks or nodes
Packet sent to one link or node improves efficiency
Solution: Build nodes that filter/forward and connectindirectly such as switches & routers
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ConnectingConnectingNNNodesNodesIndirectlyIndirectly
Star: One-hop path from a node to any other node
Switch S can filter and forward!
Reliability problem (single point of failure) Bandwidth problem (bottleneck at the switch)
It may forward multiple packets in parallel for
additional efficiency!
Star
S
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ConnectingConnectingNNNodesNodesIndirectlyIndirectly
Ring: each node is connected to two neighboringnodes forming a ring.
All nodes do forwarding and filtering
Near-minimal number of links
Reliability to link failure
Ring
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Connecting Nodes Indirectly:Connecting Nodes Indirectly:InterInter--NetworksNetworks
= Internet
The goal is to design the black box on the right
Inter-Network is a network of networks
E.g., the Internet
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InterInter--Networks: Networks of NetworksNetworks: Networks of Networks
Internetworking involves two fundamental
design problems: heterogeneity andscaleConcepts: To handle heterogeneity: use translation, address & name
resolution, fragmentation, etc.
To handle scaling: use hierarchical addressing, routing,naming, address allocation, congestion control, etc.
Covered in more detail in Internetworking course
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Additions to the Design Problem ListAdditions to the Design Problem List
Switching, bridging,
routingReliability
Fragmentation
Naming, addressing
Congestion control,
traffic management
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How to do system design? (Design Ideas)How to do system design? (Design Ideas)
Example Goal: Design an Inter-network
Resources:
Computation: planning, analysis, etc. Time: project management, due dates, etc.
Labor: team sizing, organizing, managing, etc.
Space: location, size, dimensions, etc. Money: cost, budget analysis, investment, etc.
Design Rule: tradeoff cheaper resourcesagainst expensive ones to meet design goals Life is all about compromising and optimizing
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Statistical MultiplexingStatistical Multiplexing
Reduce resource requirements by exploitingstatistical knowledge of the system.
E.g., average rate service rate peak rate
If the service rate < the average rate, then thesystem becomes unstable!!
First, design to ensure system stability!!
Then, for a stable multiplexed system:
Gain = peak rate/service rate.
Cost: buffering, queuing delays, and data loss.
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Stability of a Multiplexed SystemStability of a Multiplexed System
Average Input Rate >Average Output Rate
System will become unstable!
How to ensure stability ?
1. Reserve enough capacity so that the average
demand is less than the reserved capacity2. Dynamically detect overload and adapt either the
demand or the capacity to resolve the overload
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WhatWhats a performances a performancetradeofftradeoff??
R = link bandwidth (bps)
L = packet length (bits)a = averagepacket arrival rate
Traffic Intensity (or Link Utilization) = La/R
(Average traffic divided by the link capacity)
A situation where you cannot get something for nothing!
Also known as a zero-sum game.
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WhatWhats a performances a performancetradeofftradeoff??
La/R ~ 0: average queuingdelay small (Wasteful Design)
La/R 1: delay become large
La/R > 1: infinite average delay(service degrades unboundedly instability)!
Statistics suggest that the besttradeoffis when La/R ~ 0.8
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Example Design:Example Design: CircuitCircuit--SwitchingSwitching
Divide link bandwidth intopieces
Reserve pieces onsuccessive links and tiethem together to form acircuit
Map traffic into thereserved circuits
Resources are wasted ifunused: expensive design.
Circuit-switching: a form of multiplexing
Mapping can be done without headers.
Everything inferred from timing (i.e. connection is
immediately available).
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Example Design:Example Design:PacketPacket--SwitchingSwitching
Chop up data (not links!)
into packetsPackets: data + meta-data
(e.g., header)
Switch packets atintermediate nodes
Store-and-forwardifbandwidth is not
immediately available.
Bandwidth division intopieces
Dedicated allocation
Resource reservation
Packet-switching: another form of multiplexing:
Cost: self-descriptive header per-packet (i.e.; overhead),
buffering, and delays for applications.
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Summary of System Design IdeasSummary of System Design Ideas
Multiplexing
Statistical Multiplexing
Stability and
performance tradeoffsCircuit switching
Packet switching
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