week 2 lec 1-bit
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Computer Networks IntroductionTRANSCRIPT
Chapter 1:Introduction Computer
Networking: A Top Down Approach ,
4th edition. Jim Kurose, Keith Ross
Computer Networks
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
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)
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?
Throughput The rate (bits/sec) at which bits are transferred
between sender/receiver Difference between Bandwidth and Throughput?
ISPs sell bandwidth In computer networks, the throughput is less
than the bandwidth for several reasons • The channel may be shared by other users• Packet loss due to congestion• Packet loss due to bit errors• Noise in the channel• Transmission rates of the link over which
the data flows.What is Goodput?
Throughput
Rs < Rc What is average end-end throughput?
Rs bits/sec Rc bits/sec
Rs > Rc What is average end-end throughput?
Rs bits/sec Rc bits/sec
Throughput is min {Rs,Rc} Transmission Rate of the bottleneck link
Throughput
10 connections share bottleneck link R
Rs
Rs
Rs
Rc
Rc
Rc
R
10 clients/servers pairs, Common link R traversed
by all 10. Rate of the link R is very
large then the throughput is min {Rs , Rc}
Rs=2Mbps, Rc=1Mbps,
R=5Mbps Common link divides
transmission rate equally among the 10 downloads
500kbps to each download
Shared Link R is now the bottleneck.
How do loss and delay occur?
packets queue in router buffers packets queue, wait for turn queue (aka buffer) has finite capacity packet arriving to full queue dropped (aka lost) lost packet may be retransmitted
A
B
packet being transmitted (delay)
packets queueing (delay)
free (available) buffers: arriving packets dropped (loss) if no free buffers
Four sources of packet delay 1. Processing Delay:
Time required to examine packet header and determine output link
check bit errors High Speed Routers
Microseconds or less
A
B
propagation
transmission
processing queueing
2. Queuing Delay: Time waiting at output
link for transmission depends on
congestion level of router
If queue empty no delay
Microseconds to milliseconds
Delay in packet-switched networks3. Transmission Delay: R=link bandwidth
(bps) L=packet length
(bits) time to push all of
packets bits into the link = L/R
Microseconds to milliseconds
4. Propagation Delay: Time to propagate from
the beginning of the link to the other router (node)
propagation delay = d/s
d = length of physical link
s = propagation speed in medium (~3x108 m/sec)
A
B
propagation
transmission
nodalprocessing queueing
Nodal delay
dproc = processing delay typically a few microsecs or less
dqueue = queuing delay depends on congestion
dtrans = transmission delay = L/R
dprop = propagation delay microsecs or msecs
End to End Delay?
proptransqueueprocnodal ddddd
Queuing Delay
R=Transmission Rate (bps) L=packet length(bits) a=average packet arrival
rate(packets/sec)Traffic Intensity = (Average rate at which bits arrive at the queue) =La
Transmission Rate R La/R ~ 0: average queuing delay small
La/R -> 1: delays become large (queue begins to get larger) La/R > 1: average rate at which bits arrive at the queue
exceeds the rate at which the bits can be transmitted from the queue.
more “work” arriving than can be serviced Packet Loss will occur
When is Queuing Delay large and when it is insignificant?
Rate at which traffic arrives at the queue
Transmission rate of the link Nature of the arriving traffic
Queuing DelayConsider the case La/R≤1Nature of arriving traffic impacts queuing delay in
such a case
(Find about Queuing Theory?)
In reality the arrival process to a queue is random and arrivals do not follow any pattern.
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 doesProtocol – says how the service is
implementedAdvantages DrawbacksWhen 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
Internet Protocol Stack• Network Layer:
Responsible for moving network layer packets known as datagrams from one host to another.
Transport layer passes a transport layer segment and a destination address to the network layer.
Network layer includes IP ProtocolDefines the fields in the datagram as well
as how end systems and routers act on these fields
Different routing protocols.Determine the route that datagrams take
between sources and destinations