Network BasicsNetwork Basics
염익준
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What is a computer network?What is a computer network?
• a collection of computing nodes and links connecting the nodes
• why we need it?
– to transfer data to others
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What is the Internet?What is the Internet?
• originally means “Inter-network”• a network of networks
• now means a network using IP (Internet Protocol) for inter-networking
• equivalent to IP networks
• deployed in the whole world
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What is a network protocol?What is a network protocol?
• a pre-defined rule for data transfer
• examples: IP, TCP, UDP, HTTP, RTP
• why do we need it?
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What we learn in this classWhat we learn in this class
• architecture of the current networks
• basic operations of network protocols
• how they are implemented
• several well-known network applications
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Network LayeringNetwork Layering
• the current network protocols are implemented based on layering concept
• currently, there are five layers:
– physical, data-link, network, transport and application
• why ?
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Physical LayerPhysical Layer
Computer A Computer B
NICNetwork Interface CardEthernet CardLAN card
Wire
Physical Layer
Data
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Physical LayerPhysical Layer
• Twisted pair
• Coaxial Cable
• Fiber Optics
• RF
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Physical MediaPhysical Media
• physical link:transmitted data bit propagates across link
• guided media:
– signals propagate in solid media: copper, fiber
• unguided media:
– signals propagate freelye.eg., radio
Twisted Pair (TP)
• two insulated copper wires
– Category 3: traditional phone wires, 10 Mbps ethernet
– Category 5 TP: 100Mbps ethernet
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Physical Media: coax, fiberPhysical Media: coax, fiber
Coaxial cable:
• wire (signal carrier) within a wire (shield)
– baseband: single channel on cable
– broadband: multiple channel on cable
• bidirectional
• common use in 10Mbs Ethernet
Fiber optic cable:
• glass fiber carrying light pulses
• high-speed operation:
– 100Mbps Ethernet
– high-speed point-to-point transmission (e.g., 5 Gps)
• low error rate
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Physical media: radioPhysical media: radio
• signal carried in electromagnetic spectrum
• no physical “wire”• bidirectional
• propagation environment effects:
– reflection
– obstruction by objects
– interference
Radio link types:
• microwave
– e.g. up to 45 Mbps channels
• LAN (e.g., waveLAN)
– 2Mbps, 11Mbps
• wide-area (e.g., cellular)
– e.g. CDPD, 10’s Kbps• satellite
– up to 50Mbps channel (or multiple smaller channels)
– 270 msec end-end delay
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Hardware Building BlocksHardware Building Blocks
• General purpose computers such as PC, workstation, Laptop and PDA connected to a network.
• Either end-hosts or intermediate nodes (switch or router)
CPU
Cache
Memory I/O bus
Networkadaptor (To network)
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Hardware Building Blocks Hardware Building Blocks --
LinksLinks• Implemented on a variety of different physical media, including twisted pair, coaxial cable, optical fiber and space.
• Used to propagate signals.
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Wireless LinksWireless Links
• Global links – PCS, GSM
• Wireless LAN – IEEE 802.11a, b, g
• Piconet – Bluetooth (upto 1 Mbps)
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Data Link LayerData Link Layer
Computer A Computer B Computer C
Data
DataMACData link layer Data link layer
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Data Link LayerData Link Layer
• Unique ID
• Medium Access Control (MAC)
– CSMA/CD (Carrier Sense Multiple Access/Collision Detection)
– CSMA/CA (Collision Avoidance)
– Token Ring
– TDMA (Time Division Multiple Access)
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Network LayerNetwork Layer
Data link layer
Router
IP
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Network LayerNetwork Layer
• Routing
– RIP – Routing Information Protocol
– OSPF – Open Shortest Path First
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IP Packet FormatIP Packet Format
Version HLen TOS Length
Ident Flags Offset
TTL Protocol Checksum
SourceAddr
DestinationAddr
Options (variable) Pad(variable)
0 4 8 16 19 31
Data
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telnet
netscape
ftptelnet
netscape
ftp
Computer B
Data link layerIP
Computer A
Data link layerIPTCP TCP
Date
IP
MAC IPTCP
DateTCP
DateTCP
Date
Date
IP
MAC IPTCP
DateTCP
DateTCP
Date
Transport LayerTransport Layer
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Transport LayerTransport Layer
• End-to-end delivery
• Transmission Control Protocol (TCP)
– Flow control – Advertised Window
– Error control – Checksum
– Reliable delivery – Loss detection and
retransmission
• User Datagram Protocol (UDP)
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Typical Limitations of the Typical Limitations of the
NetworkNetwork• Drop messages
• Reorder messages
• Deliver duplicate copies of a given message
• Limit messages to some finite size
• Deliver messages after an arbitrarily long delay
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UDP (User Datagram Protocol)UDP (User Datagram Protocol)
• Extending the host-to-host deliver service of the underlying network into a process-to-process communication service.
• Port number
– Well-known port number
– Port mapper
• Checksum – pseudoheader
• Used for delay-sensitive applications
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UDP Header FormatUDP Header Format
SrcPort DstPort
Checksum Length
Data
0 16 31
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Source Port Destination Port
Sequence Number
Acknowledgement Number
Checksum Urgent Pointer
Options Padding
0 4 10 16 24 31
URG
ACK
PSH
RST
SYN
FIN
HeaderLength Reserved Window Size
Data
TCP SegmentTCP Segment
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Internet Protocol GraphInternet Protocol Graph
…
FTP HTTP NV TFTP
TCP UDP
IP
NET1 NET2 NETn
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Alternative View of the Alternative View of the
Internet ArchitectureInternet Architecture
TCP UDP
IP
Network
Application