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Computer Networks
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Chapter 1:Introduction Computer
Networking: A Top Down Approach ,
4th edition. Jim Kurose, Keith
Ross
Computer Networks
Chapter 1: IntroductionOur goal: get “feel” and
terminology more depth, detail
later in course approach:
use Internet as example
Overview: what’s the Internet? what’s a protocol? network edge; hosts, access
net, physical media network core: packet/circuit
switching, Internet structure performance: loss, delay,
throughput Protocol layers, service
models History of Internet
What’s the Internet: “nuts and bolts” view
Interconnects millions of computing devices: hosts = end systems running network
apps Home network
Institutional network
Mobile network
Global ISP
Regional ISP
router
PC
server
wirelesslaptop
cellular handheld
wiredlinks
access points
communication links fiber, copper, radio,
satellite transmission rate =
bandwidth (bits/sec) routers: forward
packets (chunks of data)
What’s the Internet: “nuts and bolts” view protocols control sending,
receiving of msgs e.g., TCP, IP, HTTP, Skype
Internet: “network of networks” public Internet versus
private intranet
Internet standards IETF: Internet Engineering
Task Force RFC: Request For Comments
IETF standard DocumentsMore than 5000 RFCs
Home network
Institutional network
Mobile network
Global ISP
Regional ISP
What’s the Internet: A Service View
Communication infrastructure that provides services to applications: Involve multiple end system
that exchange data with each other.
Web, VoIP, email, games, e-commerce, file sharing
Applications do not run on the routers
Communication services provided to applications: reliable data delivery from
source to destination (connection oriented)
“best effort” (unreliable) data delivery (connection less)
What’s a protocol?Human Protocols: “what’s the time?” “I have a question” Interview
… 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
What’s a protocol?a human protocol and a computer network protocol:
Hi
Hi
Got thetime?
2:00
TCP connection request
TCP connectionresponseGet http://www.awl.com/kurose-ross
<file>time
The Network Edge: end systems (hosts):
run application programs e.g. Web, email at “edge of network”
client/server
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, Kazaa
More in Chapter 2
Network AccessAccess Networks:
The physical link that connects an end system to its “edge router”.
Q: How to connect end systems to
edge router?Access Networks can be loosely
classified into three categories Residential access networks
Dial up, DSL etc Institutional access networks (school,
company) Ethernet
Mobile access networks
Wireless LAN (WiFi)
Wider-area wireless access (WiMAX)
Reading Assignment
The Network Core
Mesh of interconnected routers
the fundamental question: how is data transferred through net? circuit switching:
dedicated circuit per call: telephone networks
packet-switching: data sent through network in discrete “chunks” called packets
Hotel reservation analogy
Network Core: Circuit SwitchingEnd-end resources reserved for
theduration of the call Three phases
Establish, Transfer, Disconnect
dedicated resources: no sharing
Guaranteed performance Very Reliable Developed for Telephone
networks Inefficient
Channel capacity dedicated for duration of connection
If no data, capacity wasted Set up (connection) takes time
Network Core: Packet SwitchingToday’s internet is a packet switched network Each end-end data stream divided into packets
Each packet contains a portion of user data plus some control info.
Each packet has to find its own route to the destination
No predetermined path Decision as to which node to hop to in the next step is
taken only when a node is reached. Resources used as needed Congestion : packets queue, wait for link use Reliability Less reliable Store and forward switching Routers receives complete packet before forwarding Question: What is Cut through Switching?
Protocol Layers
Networks are complex!
• many “pieces”:– hosts– routers– links of various
media– applications– protocols– hardware,
software
Question: Is there any way of organizing network
architecture?
Answer:
Yes possible with a layered architecture
Organization of air travel
• A series of steps(actions)
ticket (purchase)
baggage (check)
gates (load)
runway takeoff
airplane routing
ticket (complain)
baggage (claim)
gates (unload)
runway landing
airplane routing
airplane routing
Layering of Airline Functionality
• Airline functionality can be divided into layers, providing a frame work in which we can discuss air travel.
• At the ticketing layer and below– Airline-counter-to-airline-counter transfer of a person.
• At the gate layer– Departure-gate –to-arrival-gate transfer of a person is
accomplishedLayers: each layer implements a service
– via its own internal-layer actions– Combined with the services directly below it
ticket (purchase)
baggage (check)
gates (load)
runway (takeoff)
airplane routing
departureairport
arrivalairport
intermediate air-trafficcontrol centers
airplane routing airplane routing
ticket (complain)
baggage (claim
gates (unload)
runway (land)
airplane routing
ticket
baggage
gate
takeoff/landing
airplane routing
Why layering?Dealing with complex systems:Discuss a well defined, specific part of a
large and complex systemModularization eases maintenance,
updating of systemChange of implementation of layer’s
service transparent to rest of systeme.g. change in gate procedure doesn’t
affect rest of system
Internet Protocol Stack To provide structure to design
of network protocols, network designers organize protocols in layers
Service – says what a layer does
Protocol – says how the service is implemented
Advantages Drawbacks When taken together the
protocols of various layers are called the Protocol Stack.
Internet Protocol Stack consists of Five layers
Physical, Link, Network, Transport and Application layers .
Organization of Book
Internet Protocol Stack
• Application Layer:
Network applications and their application layer protocols reside.
Provides user interfaces and support for services such as e-mail, file transfer etc.
Hyper Text Transfer Protocol (HTTP)File Transfer Protocol (FTP)Session Initiation Protocol (SIP)
An application layer protocol is distributed over multiple end systems
The packets of information at the application layer is called as a message.
Internet Protocol Stack• Transport Layer:
Transports application-layer messages between application end points.
Transport layer packet is called as a segment Breaks long messages into shorter segments There are two Transport Layer Protocols Transmission Control Protocol (TCP)
Connection Oriented serviceGuaranteed delivery of application layer messagesFlow controlCongestion Control
User Datagram Protocol (UDP)Connectionless serviceNo reliability, flow control and congestion control