Transcript Lecture 1 Internet

Lecture 1
Introduction
Lecture 1
Internet
CPE 401 / 601
Computer Network Systems
slides are modified from Dave Hollinger and Daniel Zappala
Network
“ ... communication system for connecting endsystems”
End-systems a.k.a. “hosts”
• PCs, workstations
• dedicated computers
• network components
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Multiaccess vs. Point-to-point
• Multi-access means shared medium
– many end-systems share the same physical
communication resources (wire, frequency, ...)
– There must be some arbitration mechanism.
• Point-to-point
– only 2 systems involved
– no doubt about where data came from !
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Internetwork
• Connection of 2 or more distinct (possibly dissimilar)
networks.
• Requires some kind of network device to facilitate the
connection.
Net A
Net B
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The Internet
PC
server
wireless
laptop
cellular
handheld
• millions of connected
computing devices:
hosts = end systems
– running network apps
• communication links
access
points
wired
links
router
– fiber, copper, radio,
satellite
Mobile network
Global ISP
Home network
Regional ISP
Institutional network
• routers:
– forward packets
(chunks of data)
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The network edge:
• end systems (hosts):
– run application programs
– e.g. Web, email
– at “edge of network”
peer-peer
 client/server model


client host requests, receives
service from always-on server
e.g. Web browser/server;
email client/server
 peer-peer model:


minimal (or no) use of
dedicated servers
e.g. Skype, BitTorrent
client/server
The Internet
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Internet Mapping Project, Bill Cheswick
A Network of Networks
• roughly hierarchical
– Tier-1 ISPs provide national,
international coverage
– Tier-2 ISPs provide
regional coverage
– Tier-3 and lower levels
provide local coverage
• any tier may sell to
business and residential
customers
• any ISP may have a
link to any other ISP
– not strictly hierarchical
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Inernet2 Backbone
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Level3 Backbone
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Many Different Internet Service Providers
• Each network is
independent
• Interoperability requires using
Internet standards: IP, TCP
– the Internet is global and must
run these standards
– your private intranet can do
whatever you want it to do
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Internet Design Goals
• primary goal: interoperability among existing networks
– a network of networks
– obey administrative boundaries
• secondary goals
–
–
–
–
–
fault tolerance
multiple transport protocols
support a variety of networks
distributed management
cost effective, low effort for host attachment, accountability
• first three were more important, so remaining four did not
receive as much attention
• no mention of security
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Internet Design Principles
• minimal assumptions about services network
should support
– ability to send packets
– no reliability or security
• end-to-end principle
– keep the core of the network as simple as possible
– put complex functionality at the edges
– exception: significant performance improvement
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The Internet Hourglass
Internet
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Layering
• Divide a task into pieces and then solve each
piece independently (or nearly so)
• Establishing a well defined interface between
layers makes porting easier
• Major Advantages:
Code Reuse
Extensibility
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The Internet at each Hop
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Interface and Peer-to-peer Protocols
• Interface protocols describe communication between layers
on the same endpoint.
• Peer-to-peer protocols describe communication between
peers at the same layer.
Process
Process
Interface Protocols
Transport
Transport
Peer-to-peer Protocols
Network
Network
Data Link
Data Link
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Protocol
• An agreed upon convention for communication.
– both endpoints need to understand the protocol.
• Protocols must be formally defined and unambiguous!
• Protocols define
– format,
– order of msgs sent and received among network entities,
– actions taken on msg transmission, receipt
• We will study lots of existing protocols and perhaps develop a
few of our own.
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The Physical Layer
• Responsibility:
– transmission of raw bits over a communication
channel
• Issues:
– mechanical and electrical interfaces
– time per bit
– distances
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The Data Link Layer
• Responsibility:
– provide an error-free communication link
– Data Link Control sublayer
– Medium Access Control sublayer
• Issues:
– framing (dividing data into chunks)
• header & trailer bits
– addressing
10110110101
01100010011
10110000001
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The Network Layer
• Responsibilities:
– path selection between end-systems (routing).
– flow control.
– fragmentation & reassembly
– translation between different network types.
• Issues:
– packet headers
– virtual circuits
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The Transport Layer
• Responsibilities:
– provides virtual end-to-end links between peer
processes.
– end-to-end flow control
• Issues:
– headers
– error detection
– reliable communication
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The Application Layer
• Responsibilities:
– anything not provided by any of the other layers
– TCP/IP model
• Session and Presentation Layer functions
• Issues:
– application level protocols
– appropriate selection of “type of service”
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Layering & Headers
• Each layer needs to add some control information to
the data in order to do it’s job.
• This information is typically prepended to the data
before being given to the lower layer.
• Once the lower layers deliver the data and control
information - the peer layer uses the control
information.
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