[SOLVED] CS DHCP dns FTP Note: We will start at 12:53 pm ET

Note: We will start at 12:53 pm ET
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18-441/741: Computer Networks Lectures 3: Layers II & PHY I
Swarun Kumar
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Layer Encapsulation
User A
User B
Get index.html
Connection ID
Source/Destination Link Address
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Multiplexing and Demultiplexing
TCP TCP
IP IP
There may be multiple implementations of each layer.
How does the receiver know what version of a layer to use?
Each header includes a demultiplexing field that is used to identify the next layer.
Filled in by the sender Used by the receiver
Multiplexing occurs at multiple layers. E.g., IP, TCP,
V/HL
TOS
Length
ID
Flags/Offset
TTL
Prot.
H. Checksum
Source IP address
Destination IP address
Options..
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Protocol Demultiplexing
Multiple choices at each layer
FTP
TCP
HTTP
NV
UDP
TFTP
Network
IP
TCP/UDP
IPX
IP
Type Protocol Port Field Field Number
NET1

NET2
NETn
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Server and Client
Server and Client exchange messages over the network through a common Socket API
Server
ports
Socket API
user space
Clients
TCP/UDP
TCP/UDP
kernel space
IP
IP
Ethernet Adapter
Ethernet Adapter
hardware
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The Internet Protocol Suite
FTP
TCP
Applications UDP TCP
Data Link Physical
HTTP
NV
TFTP
UDP
IP
Narrow Waist
NET1

NET2
NETn
The Hourglass Model
The waist facilitates interoperability but evolution is hard
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IP based on a Minimalist Approach
Dumbnetwork
IP provide minimal functionalities to support connectivity
Addressing, forwarding, routing Smartendsystem
Transport layer or application performs more sophisticated functionalities
Flow control, error control, congestion control
Advantages
Accommodate heterogeneous technologies (Ethernet, modem, satellite, wireless)
Support diverse applications (telnet, ftp, Web, X windows)
Decentralized network administration
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Sample Quiz Question
Question: Which of these will be hardest launch at Internet-scale:
[Option A] a new version of TCP, [Option B] a new version of IP [Option C] or a new version of WiFi
Answer: New IP (why?)
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Todays Lecture
Network applications
Requirements
Latency and bandwidth
Internet architecture A layered design
Protocols
Life of a packet
Network utilities
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Protocol Stack (cotd.)
Network applications
Requirements
Latency and bandwidth
Internet architecture A layered design
Protocols
Life of a packet
Network utilities
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Network tools
ping
traceroute ipconfig
tcpdump
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ping
Application to determine if host is reachable
Based on Internet Control Message Protocol
ICMP informs source host about errors encountered in IP packet processing by routers or by destination host
ICMP Echo message requests reply from destination host
PING sends echo message & sequence #
Determines reachability & round-trip delay
Sometimes disabled for security reasons

traceroute
Findroutefromlocalhosttoaremotehost
Time-to-Live(TTL)
IP packets have TTL field that specifies maximum # hops traversed before packet discarded
Each router decrements TTL by 1
When TTL reaches 0 packet is discarded
Traceroute
Send UDP to remote host with TTL=1
First router will reply ICMP Time Exceeded Message Send UDP to remote host with TTL=2,
Each step reveals next router in path to remote host
tracert (windows), tracepath (linux)

ipconfig
Utility in Microsoft Windows to display TCP/IP information about a host
Many options
Simplest: IP address, subnet mask, default gateway for the host
Information about each IP interface of a host
DNS hostname, IP addresses of DNS servers, physical address of network card, IP address,
Renew IP address from DHCP server

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netstat
Queries a host about TCP/IP network status
Status of network drivers & their interface cards
#packets in, #packets out, errored packets,
State of routing table in host
TCP/IP active server processes
TCP active connections

tcpdump and Network Protocol Analyzers
tcpdump program captures IP packets on a network interface (usually Ethernet NIC)
Filtering used to select packets of interest
Packets & higher-layer messages can be displayed and analyzed
tcpdump basis for many network protocol analyzers for troubleshooting networks
We use the open source Ethereal analyzer to generate examples (or wireshark, etc.)
www.ethereal.com

How the layers work together: Network Analyzer Example
Internet
l User clicks on http://www.nytimes.com/
l Ethereal network analyzer captures all frames observed
by its Ethernet NIC (or Wireshark)
l Sequence of frames and contents of frame can be examined in detail down to individual bytes

Top Pane shows frame/packet
Middle Pane shows encapsulation for a
sequence
given frame
Ethereal windows
Bottom Pane shows hex & text

Top pane: frame sequence
DNS Query
TCP Connection Setup
HTTP Request & Response

Middle pane: Encapsulation
Ethernet Frame
Protocol Type
Ethernet Destination and Source Addresses

IP Source and Destination Addresses
Middle pane: Encapsulation
And a lot of other stuff!
IP Packet
Protocol Type

Source and Destination Port Numbers
GET
Middle pane: Encapsulation
TCP Segment
HTTP Request

Goals [Clark88] 0 Connect existing networks
initially ARPANET and ARPA packet radio network
1. Survivability
ensure communication service even in the presence of network and router failures
2. Support multiple types of services
3. Must accommodate a variety of networks 4. Allow distributed management
5. Allow host attachment with a low level of effort
6. Be cost effective
7. Allow resource accountability
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Principle: End-to-End Argument (Saltzer81)
Focusofthepaperissystem Not a pure networking paper
Dealswithwheretoplacefunctionality Inside the network (in switching elements) At the edges
Argument:Somefunctionscanonlybe correctly implemented by the endpoints do not try to implement these elsewhere
Not a law more of a best practices
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Example: Reliable File Transfer
Host A
Host B
Appl.
OS
Appl.
OS
OK
Solution 1: make each step reliable, and then concatenate them
Solution 2: end-to-end check and retry
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Sample Quiz Question
Question: A switch and a router both cost $100 and have similar specs and achieve similar performance in packet switching/routing. As a rational buyer, I would buy the router. [True/False]
Answer: True, the router (why?)
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Life of Packet
Application
Presentation Session Transport Network
Data Link Physical
Host Bridge/Switch
Router/Gateway Host
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Physical Layer (PHY) I
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Physical Layer: Outline
Digitalnetworking
Modulation
CharacterizationofCommunicationChannels
FundamentalLimitsinDigitalTransmission
ModemsandDigitalModulation
LineCoding
PropertiesofMediaandDigitalTransmission Systems
ErrorDetectionandCorrection
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Digital Networks
Digitaltransmissionenablesnetworksto support many services
TV
E-mail
Telephone
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Analog versus Digital Information
Analoginformationtakesoncontinuous values
Sound, images, etc.
Digitalinformationtakesondiscretevalues Text, banking data, etc.
Canconvertbetweenthetworepresentations of information
Sampling and interpolation
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Block vs. Stream Information
Block
Information that occurs in a single block
T ext message
Data file
JPEG image
MPEG file
Size = bits / block
or Bytes/block
1 KByte (KB) = 210 bytes
1 MByte (MB) = 220 bytes
1 GByte (GB) = 230 bytes
Stream
Information that is produced & transmitted continuously
Real-time voice
Streaming video
Bit rate = bits / second 1Kbps =103 bps
1Mbps=106 bps
1Gbps=109bps
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Many Types of Information
Stream Block
Analog Digital
Voice, video
Stock market
Images, radar map,
Spreadsheets, text file,
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Traditional Communication Options
Sendanaloginformationoveranalog networks
Voice over the telephone network Video using broadcast TV
Pictures using the USPS
Senddigitalinformationoverdigitalnetworks
Messages via telegraph: beacons electrical
Internet: many applications, e.g., http, (text) email, ssh, social networks,
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But Can Mix and Match
Analog information can be digitized and sent over digital network
Video becomes MPEG Image becomes JPEG
Digital networks use analog channels Bits are encoded on analog waveforms But switching is done based on the bits
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JPEG
Modem
JPEG
Modem
Example
IP Telephone Network Optical Backbone
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Why Use a Single Digital Network?
Economicallyadvantageoustohaveasingle network
Multimediaapplicationswanttomixdifferent types of data
More convenient if a single networks is used
Computersoperateonlyondigitaldata
Digitaltransmissioncanrecoverfromerrors (e.g. noise, distortion)
Not possible when transmitting analog information over an analog network
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Sent
Distortion Attenuation
Received
Analog Transmission
All details must be reproduced accurately
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Why digital? Problem with Analog Long- Distance Communications
Transmission segment
. . . Destination
Eachrepeaterattemptstorestoreanalogsignalto
its original form
Restorationisimperfect
Distortion is not completely eliminated
Noise & interference is only partially removed
Signalqualitydecreaseswith#ofrepeaters
Communicationsbecomesdistance-limited
StillusedinanalogcableTVsystems
Analogy: Copy a song using a cassette recorder
Source
Repeater
Repeater
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Sent
Distortion Attenuation
Received
Digital Transmission
Only discrete levels need to be reproduced
Simple Receiver: Was original pulse positive or negative?
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Digital Long-Distance Communications
Transmission segment
Regenerator
. . .
Destination
Source
Regenerator
Regenerator recovers original data sequence and retransmits on next segment
Can design so error probability is very small
Then each regeneration is like the first time!
Analogy: copy an MP3 file
Communications is possible over very long distances
Digital systems advantage over analog systems
Less power, longer distances, lower system cost
Monitoring, multiplexing, coding, encryption, protocols
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