eec-484/584 computer networks lecture 3 wenbing zhao [email protected]
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EEC-484/584EEC-484/584Computer NetworksComputer Networks
Lecture 3Lecture 3
Wenbing ZhaoWenbing [email protected]@ieee.org
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OutlineOutline
• Review – multilayer communication • Network Software
– Design issues for the layers– Connection-oriented and connectionless services– Circuit switched and packet switched networks
• Reference models• Networking Standards
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Multilayer CommunicationMultilayer Communication
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Design Issues for the LayersDesign Issues for the Layers
• Mechanisms for – Connection establishment and addressing– Connection termination or release
• Rules for data transfer– Simplex – data flows in one direction– Half duplex – data flows in either direction, but
not simultaneously– Full duplex – both directions simultaneously– Number of logical channels and their priority
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Design Issues for the LayersDesign Issues for the Layers
• Error control– Error detecting and correcting codes– Positive and negative acknowledgement– Re-sequencing packets that arrives out of order
• Flow control– Need to keep a fast sender from swamping a
slow receiver– Use feedback from receiver to sender
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Design Issues for the LayersDesign Issues for the Layers
• Finite buffers– Use mechanisms for disassembling,
transmitting, reassembling large messages
• Multiplexing– Several unrelated conversations between pair
of communicating processes
• Routing– Which route to use if there are several
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Connection-Oriented ServicesConnection-Oriented Services
• Modeled after telephone system – establish connection before communication
• Some service allows a negotiation among sender, receiver and subnet regarding the parameters to be used, such as max message size, etc.
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Connectionless ServicesConnectionless Services
• Modeled after postal system – a message carries full destination address, and each one is routed through the system independent of all the others
• Ordering of messages are not guaranteed
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Quality of ServicesQuality of Services
• Quality of service – characterize each service
• Reliable service – it does not lose data– Implemented by having the receiver
acknowledge the receipt of each message– Possible for both connection-oriented and
connectionless services• Connection-oriented service is not necessarily a
reliable service!
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Connection-Oriented and Connection-Oriented and Connectionless ServicesConnectionless Services
First class mail
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SwitchingSwitching
• Circuit switching
• Packet switching
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Circuit Switching and Packet SwitchingCircuit Switching and Packet Switching
(a) Circuit switching
(b) Packet switching
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Circuit SwitchingCircuit Switching
• Circuit switching – physical path set up from source to destination before any data transmitted, e.g., phone system– Adv: no congestion problem, only delay is
propagation time– Disadv: unused bandwidth on allocated circuit is
wasted
• Will go back on this in lecture 11 (network layer)
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Packet SwitchingPacket Switching
• Packet switching – store-and-forward, one hop at a time, uses pipelining, each packet has limited size– Adv: low overhead – no setup required, high
utilization– Disadv: packets may be received out of order,
packets may be lost due to buffer overflow• Will go back on this in lecture 11 (network layer)
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Reference ModelsReference Models
• The OSI Reference Model– ISO – International Standards Organization– OSI – Open Systems Interconnection
• Open: open for communication with other systems
• The TCP/IP Reference Model
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The OSI Reference ModelThe OSI Reference Model
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Principles Applied in Deriving LayersPrinciples Applied in Deriving Layers
• A layer should be created where a different level of abstraction is needed
• Each layer should perform a well-defined function
• The layer boundaries should be chosen to minimize info flow across interfaces
• The number of layers should be – large enough to separate distinct functions – small enough to have a manageable architecture
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TCP/IP Reference ModelTCP/IP Reference Model
• TCP – Transmission Control Protocol
• IP – Internet Protocol
• Used in Internet and its predecessor ARPANET
• TCP/IP invented by Cerf and Kahn in 1974, became official protocol of ARPANET in 1983
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TCP/IP Reference ModelTCP/IP Reference Model
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TCP/IP Reference ModelTCP/IP Reference Model
• Internet Layer– Connectionless (packet switched)– Injects packets into the network; delivers them
to the destination– May be delivered out-of-order– Packet routing and congestion control are key
issues– Uses Internet Protocol
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TCP/IP Reference ModelTCP/IP Reference Model
• Transport layer, two protocols• TCP – Transmission Control Protocol
– Point-to-point– Connection-oriented– Reliable (no message loss or corruption)– Source ordered (sequenced)– Flow control– Byte stream, does not maintain message
boundary
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TCP/IP Reference ModelTCP/IP Reference Model
• UDP – User Datagram Protocol– Point-to-point– Connectionless– Unreliable– Not source ordered– No flow control– Preserve message boundary
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TCP/IP Reference ModelTCP/IP Reference Model
• Application Layer – contains higher-level protocols– DNS – Domain Name Service
• Maps host names onto their network addresses
– HTTP – HyperText Transfer Protocol• Fetches pages on the World Wide Web
– FTP – File Transfer Protocol• Allows user to transfer files efficiently from one
machine to another
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TCP/IP Reference ModelTCP/IP Reference Model
• Host-to-Network Layer– Host has to connect to the network using
some protocol so it can send IP packets to it– No protocol is defined
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Network StandardizationNetwork Standardization
• Why standard?– Only way to achieve interoperability– Standards also increase the market for
products adhering to them– Two kinds of standards
• De facto – from the fact (standards that just happened)
• De jure – by law (formal, legal standards adopted by authorized organization)
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Treaty Organization between Treaty Organization between NationsNations
United Nations
ITU - International Telecommunications Union
CCITT/ITU-T – telephone and data communications
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Voluntary, Nontreaty OrganizationVoluntary, Nontreaty Organization
ISO (International Standards Organization)issues standards on wide range of topics
200 TC (Technical Committees)
TC97 – computers and info processing
SC (Subcommittees)
WG (Working Groups)
ANSI (American National Standards Institute)
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Internet Standard BodyInternet Standard Body• Internet Society (used to be Internet Architecture
Board)– Internet Research Task Force (IRTF)
• Concentrate on long term research
– Internet Engineering Task Force (IETF)• Deal with short term engineering issues
• Standardization process– Proposed standard: request for comments (RFCs)– Draft standard: > 4 month test– Internet standard: if convinced the idea is sound