1 cs 164 : computer networks my name : srikanth krishnamurthy tas : jianxia ning ([email protected])...
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CS 164 : Computer Networks
My name : Srikanth Krishnamurthy
TAs : Jianxia Ning
Office hrs: W 10-11 a.m.
Web Site:TBA
Please make sure you subscribe to mailing list of CS 164
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• Attendance to Labs are Compulsory
• Written homework assigned and collected one week later.
• If you miss more than 2 homework assignments, then there will be a penalty assessed.
• In Lab assignments need to be completed in the lab.
• One project -- individual effort.
• No cooperation on anything in this course.
• Very Important : Please ask questions in class - if you don’t understand something please let me know.
• Otherwise I have no way of knowing!
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• Three quizzes -- approximately 3 weeks apart.
• Each Quiz will be for 15 points
• We will only take your best two quizzes.
• Multiple choice
• Final Exam -- about 35 %
• Project -- 15 %
• Labs -- 10 %
• Homeworks 10 %
• More specific details will be on website in a day or two.
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Roadmap for this lecture
o Introduction -- Chapter 1. Whatever I don’t cover is for self-study.
o The Internet and networks in general
o Basic types of connections between computers
o How is information sent from one point to the other ?
o What are protocols and what role do they play ?
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Internet and Stuff:The Basicso What is the Internet?o What does it consist of?o Main terminology and lingoo Fundamental Concepts
Re-using some slides from Kurose and Ross book -- a reference book that you may want to refer to if you are interested.
Contents still reflect Chapter 1 from book.
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Visualizing the Internet
o millions of connected computing devices: hosts, end-systems pc’s workstations,
servers PDA’s phones, toasters
running network appso communication links
fiber, copper, radio, satellite
o routers: forward packets (chunks) of data thru network
local ISP
companynetwork
regional ISP
router workstation
servermobile
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“Cool” Internet Appliances
World’s smallest web serverhttp://www-ccs.cs.umass.edu/~shri/iPic.html
IP picture framehttp://www.ceiva.com/
Web-enabled toaster+weather forecasterhttp://dancing-man.com/robin/toasty/
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Internet structure: network of networks
o roughly hierarchicalo national/international
backbone providers (NBPs) e.g. BBN/GTE, Sprint,
AT&T, IBM, UUNet interconnect (peer) with
each other privately, or at public Network Access Point (NAPs)
o regional ISPs connect into NBPs
o local ISP, company connect into regional
ISPs
NBP A
NBP B
NAP NAP
regional ISP
regional ISP
localISP
localISP
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National Backbone Provider
e.g. Sprint US backbone network
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Links between computers
o Point to point link -- a direct cable or wire that is dedicated for use between the computers.
o Multiple access link -- shared between many nodes -- example a bus, wireless medium etc.
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Why structure ?
o Direct connectivity does not scale -- all computers cannot be directly connected to each other.
o Need organization End hosts -- clients and servers -- usually house information
Routers and switches -- nodes that are primarily used for relaying information -- sending information where it needs to go.
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Component networks of the Interneto Local Area Network -- LAN -- Extends up to 1 Km or so -- example -- Ethernet, Wireless LAN
o Metropolitan Area Network -- larger scale -- tens of km.
o Wide Area Networks --- Largest -- could consist of many component networks.
o System area networks -- single room.
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Examples of networks
o Ad hoc and sensor networks -- specific applications.
o Wireless hotspots Wi-Fio Of course, satellite networks have existed for some time now.
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End-hosts and applicationso Client -- requests information services.
o Server -- provides information services.
o Examples of applications -- Web (http), File transfer (FTP), E-mail.
o Typically clients and server “processes” run on different computers.
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The IETF
o Internet Engineering Task Force Provides standards/ documents
• www.ietf.org
o Contains Internet Drafts and what are called RFCs -- Request for Comments. Quite detailed descriptions of what are called protocols.
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What’s a protocol?The definition of a behavior --set of rulesHere: the format of a communication exchange: Sequence of actions, format of information, predefined interpretation
Hi
Hi
Got thetime?
2:00
connection req.
connectionreply.
Get http://gaia.cs.umass.edu/index.htm
<file>time
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Centralized versus Distributed Protocolso Centralized: all information is collected in one place and then processed
o Distributed: decisions are taken locally with partial or summary of the information Most of the protocols distributed for scalability considerations.
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Packets
o Packets are similar to postal letters -basic units of information From, to, content Postman handles all packets similarly Addressing is hierarchical.
o The protocol that defines how packets are to be “routed” is the Internet Protocol or IP. Hierarchical addresses like in the postal world.
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Types of Communications
• Circuit Switching (reserve a slice of resources)• Frequency division multiplexing• Time division multiplexing
• Packet switching • Virtual Circuits: routers keep per connection
info• Datagrams: no per connection information
• Connection oriented (state at end points, handshake - TCP)• TCP -- Transmission Control Protocol
• Connectionless (no state at end points - UDP)• UDP -- User Datagram protocol.
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Types of Communications
o Advantage of packet switching: Resource sharing No need for reservations Easier to implement in a distributed way.
o Advantage of circuit switching: Can guarantee performance (Quality of Service) -- reserved resource.
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The Principles of the Interneto Goal: Interconnect existing net technologies ARPA packet radio, and ARPANET Packet switching? Flexibility Trade-off: poorer non-guaranteed performance Higher layer protocols try to provide reliability.
o “The Design Philosophy of The DARPA Internet Protocols”, David Clark, MIT.
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Secondary Internet Principles1. Fault-tolerance to component
failures2. Support multiple types of services3. Interoperate with different
technologies4. Allow distributed management5. Be cost effective (i.e. sharing)6. Be easily extendible7. Resources and entities must be
accountable (for security purpose)
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Internet Architecture Characteristicso Scalability to millions of users
Stateless routing: Routers cannot keep detailed info per connection
o Best-effort service: no guarantees
o Decentralized controlo Self-configuration
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Organization of air travel
o a series of steps
ticket (purchase)
baggage (check)
gates (load)
runway takeoff
airplane routing
ticket (complain)
baggage (claim)
gates (unload)
runway landing
airplane routing
airplane routing
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Organization of air travel: a different view
Layers: each layer implements a service via its own internal-layer actions relying on services provided by layer below
ticket (purchase)
baggage (check)
gates (load)
runway takeoff
airplane routing
ticket (complain)
baggage (claim)
gates (unload)
runway landing
airplane routing
airplane routing
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Internet protocol stacko application: supporting network
applications ftp, smtp, http
o transport: host-host data transfer tcp, udp
o network: routing of datagrams from source to destination ip, routing protocols
o link: data transfer between neighboring network elements ppp, ethernet
o physical: bits “on the wire” -- representation, signal modulation.
application
transport
network
link
physical
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Other OSI layers
o OSI -- open systems interconnection had two other layers. Session layer -- bundles multiple transport layer streams into a single session -- audio and video.
Presentation layer -- representation of data to the application
o Not much used.
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Why layering?Dealing with complex systems:o modularization eases maintenance, updating of system change of implementation of layer’s service transparent to rest of system
e.g., change in gate procedure doesn’t affect rest of system
o Isolating “functions” and interactions components layered reference model for discussion
o layering considered harmful?
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Layering: logical communication
applicationtransportnetworklink
physical
applicationtransportnetworklink
physicalapplicationtransportnetworklink
physical
applicationtransportnetworklink
physical
networklink
physical
Each layer:o distributedo “entities”
implement layer functions at each node
o entities perform actions, exchange messages with peers
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Layering: logical communication
applicationtransportnetworklink
physical
applicationtransportnetworklink
physicalapplicationtransportnetworklink
physical
applicationtransportnetworklink
physical
networklink
physical
data
dataE.g.: transport
o take data from app
o add addressing, reliability check info to form “datagram”
o send datagram to peer
o wait for peer to ack receipt
o analogy: post office
data
transport
transport
ack
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Layering: physical communication
applicationtransportnetworklink
physical
applicationtransportnetworklink
physicalapplicationtransportnetworklink
physical
applicationtransportnetworklink
physical
networklink
physical
data
data
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Message flow through stack
M
M1 M2H H
M1H M2HH H
• At each layer, headers added.
• A protocol defines maximum packet size -- might require higher layer packet to be fragmented.
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Protocol layering and data
Each layer takes data from aboveo adds header information to create new data unito passes new data unit to layer below
applicationtransportnetworklink
physical
applicationtransportNetworklink
physical
source destination
M
M
M
M
H tH tH nH tH nH l
M
M
M
M
H tH tH nH tH nH l
message
segment
datagram
frame
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Basic Routing Concepts
o Packet = postal lettero Router receives packeto Needs to decide which link to send it to
o Scalability: decide on local information
o Routers keep “summary” of information
o Exploit the hierarchy in the IP address
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The network edge:o end systems (hosts):
run application programs e.g., WWW, email at “edge of network”
o client/server model client host requests,
receives service from server
e.g., WWW client (browser)/ server; email client/server
o peer-peer model: host interaction
symmetric e.g.: Gnutella, KaZaA
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Network edge: connection-oriented service
Goal: data transfer between end sys.
o handshaking: setup (prepare for) data transfer ahead of time Hello, hello back
human protocol set up “state” in two
communicating hostso TCP - Transmission
Control Protocol Internet’s
connection-oriented service
TCP service [RFC 793]o reliable, in-order
byte-stream data transfer loss: acknowledgements
and retransmissions
o flow control: sender won’t overwhelm
receiver
o congestion control: senders “slow down
sending rate” when network congested
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Network edge: connectionless service
Goal: data transfer between end systems same as before!
o UDP - User Datagram Protocol [RFC 768]: Internet’s connectionless service unreliable data transfer no flow control no congestion control
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The Network Core
o mesh of interconnected routers
o the fundamental question: how is data transferred through net? circuit switching: dedicated circuit per call: telephone net
packet-switching: data sent thru net in discrete “chunks”
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Network Core: Circuit Switching
End-end resources reserved for “call”
o link bandwidth, switch capacity
o dedicated resources: no sharing
o circuit-like (guaranteed) performance
o call setup required
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Network Core: Circuit Switchingnetwork resources (e.g., bandwidth) divided into “pieces”
o pieces allocated to callso resource piece idle if not used by owning call (no sharing)
o dividing link bandwidth into “pieces” frequency division time division
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Circuit Switching: TDMA and TDMA
FDMA
frequency
time
TDMA
frequency
time
4 users
Example:
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Network Core: Packet Switchingeach end-end data stream
divided into packetso user A, B packets share
network resources o each packet uses full
link bandwidth o resources used as
needed,
resource contention: o aggregate resource
demand can exceed amount available
o congestion: packets queue, wait for link use
o store and forward: packets move one hop at a time transmit over link wait turn at next link
Bandwidth division into “pieces”
Dedicated allocation
Resource reservation
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Network Core: Packet Switching
A
B
C10 MbsEthernet
1.5 Mbs
45 Mbs
D E
statistical multiplexing
queue of packetswaiting for output
link
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Packet switching versus circuit switching -- An example
o 1 Mbps linko each user:
100 Kbps when “active”
active 10% of time
o circuit-switching: 10 users
o packet switching: with 35 users,
probability > 10 active less than .0004
Packet switching allows more users to use network!
N users
1 Mbps link
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Packet switching versus circuit switching
o Great for bursty data resource sharing no call setup
o Excessive congestion: packet delay and loss protocols needed for reliable data transfer, congestion control
o Q: How to provide circuit-like behavior? bandwidth guarantees needed for audio/video apps
still an unsolved problem (see QoS, multimedia)
Is packet switching a “slam dunk winner?”
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Classification of the Types of Communication
Circuit Switching Packet Switching
Datagrams(IP)
Virtual Circuits (ATM)
Connectionless(UDP, endpointsdon’t -much- keep state)
Connection-Oriented(TCP, end pointsKeep state)
(no per flowState in routers)
(per flow state in routers - resourcereservation)
Dedicatedhardware
DedicatedShared hardware
TimeDivisionMultiplexing
FrequencyDivisionMultiplexing
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Packet-switched networks: routingo Goal: move packets among routers from source to
destination we’ll study several path selection algorithms
(chapter 4)
o datagram network: destination address determines next hop routes may change during session analogy: driving, asking directions
o virtual circuit network: each packet carries tag (virtual circuit ID), tag
determines next hop fixed path determined at call setup time, remains
fixed thru call routers maintain per-call state
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Access networks and physical mediaQ: How to connect end
systems to edge router?
o residential access nets
o institutional access networks (school, company)
o mobile access networks
Keep in mind: o bandwidth (bits per
second) of access network?
o shared or dedicated?
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Residential access: point to point accesso Dialup via modem
up to 56Kbps direct access to router (conceptually)
o ISDN: integrated services digital network: 128Kbps all-digital connect to router
o ADSL: asymmetric digital subscriber line up to 1 Mbps home-to-router
up to 8 Mbps router-to-home
ADSL deployment: happening
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Residential access: cable modems
o HFC: hybrid fiber coax asymmetric: up to 10Mbps upstream, 1 Mbps downstream
o network of cable and fiber attaches homes to ISP router shared access to router among home issues: congestion, dimensioning
o deployment: available via cable companies, e.g., MediaOne
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Residential access: cable modems
Diagram: http://www.cabledatacomnews.com/cmic/diagram.html
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Physical Media
o physical link: transmitted data bit propagates across link
o guided media: signals propagate in solid media: copper, fiber
o unguided media: signals propagate freely, e.g., radio
Twisted Pair (TP)o two insulated copper wires Category 3: traditional phone wires, 10 Mbps Ethernet
Category 5: 100Mbps Ethernet
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Physical Media: coax, fiber
Coaxial cable:o wire (signal carrier) within a wire (shield)
o bidirectionalo common use in 10Mbs Ethernet
Fiber optic cable:o glass fiber carrying light pulses
o high-speed operation: 100Mbps Ethernet high-speed point-to-point transmission (e.g., 5 Gps)
o low error rate
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Physical media: radio
o signal carried in electromagnetic spectrum
o no physical “wire”o bidirectionalo propagation environment effects: reflection obstruction by objects
interference
Radio link types:o microwave
e.g. up to 45 Mbps channels
o LAN (e.g., WaveLAN) 2Mbps, 11Mbps
o wide-area (e.g., cellular) e.g. CDPD, 10’s
Kbpso satellite
up to 50Mbps channel (or multiple smaller channels)
270 Msec end-end delay
geosynchronous versus LEOS
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Delay in packet-switched networkspackets experience delay on end-to-end path
o four sources of delay at each hop
o nodal processing: check bit errors determine output link
o queueing time waiting at
output link for transmission
depends on congestion level of router
A
B
propagation
transmission
nodalprocessing queueing
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Delay in packet-switched networksTransmission delay:o R=link bandwidth (bps)
o L=packet length (bits)
o time to send bits into link = L/R
Propagation delay:o d = length of physical link
o s = propagation speed in medium (~3x108 m/sec)
o propagation delay = d/s
A
B
propagation
transmission
nodalprocessing queueing
Note: s and R are very different quantities!
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Queueing delay
o R=link bandwidth (bps)o L=packet length (bits)o a=average packet
arrival rate
traffic intensity = La/R
o La/R ~ 0: average queueing delay smallo La/R -> 1: delays become largeo La/R > 1: more “work” arriving than can be serviced, average delay infinite!
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Summary
Covered a “ton” of material!
o Internet introductiono what’s a protocol?o network edge, core, access network packet-switching versus circuit-switching
o performance: loss, delay
o layering and service models
You now have: o context, overview, “feel” of networking
o more depth, detail later in course
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What is to follow ?
o Complete Chapter 1: Concepts of throughput and end to end latency
Bandwidth delay product -- network capacity
Brief intro to sockets
o Chapter 2: PHY Layer and Link Layer