{small lecturenumber - hepage :} TCP/IP in 30 minutes{small lecturenumber - hepage :} Layering{small lecturenumber - hepage :} The OSI seven-layer model{small lecturenumber - hepage :} The OSI seven-layer model{small lecturenumber - hepage :} Message transmission across layers{small lecturenumber - hepage :} Layers and packets{small lecturenumber - hepage :} Physical Layer{small lecturenumber - hepage :} Data Link Layer{small lecturenumber - hepage :} Network Layer{small lecturenumber - hepage :} Network Layer{small lecturenumber - hepage :} Transport Layer{small lecturenumber - hepage :} Session Layer{small lecturenumber - hepage :} Presentation Layer{small lecturenumber - hepage :} Application Layer{small lecturenumber - hepage :} An example: HTTP{small lecturenumber - hepage :} HTTP requests{small lecturenumber - hepage :} HTTP{small lecturenumber - hepage :} Summary{small lecturenumber - hepage :} What is a Distributed System?{small lecturenumber - hepage :} Advantages of a distributed system{small lecturenumber - hepage :} Advantages of a distributed system{small lecturenumber - hepage :} Disadvantages of distributed systems{small lecturenumber - hepage :} Design Issues{small lecturenumber - hepage :} Transparency{small lecturenumber - hepage :} Transparency{small lecturenumber - hepage :} Flexibility{small lecturenumber - hepage :} Reliability{small lecturenumber - hepage :} Performance{small lecturenumber - hepage :} Types of process failure{small lecturenumber - hepage :} Types of Communication Failure{small lecturenumber - hepage :} Communication paradigms{small lecturenumber - hepage :} Dealing with time{small lecturenumber - hepage :} Global time servers{small lecturenumber - hepage :} Global time servers{small lecturenumber - hepage :} Logical time{small lecturenumber - hepage :} SummaryDistributed Software DevelopmentFundamentals of Distributed SystemsChris BrooksDepartment of Computer ScienceUniversity of San FranciscoDepartment of Computer Science — University of San Francisco – p. 1/??2-2: TCP/IP in 30 minutes•Goal: Understand how a network transmits messages atdifferent layers.•How is a network composed?•What really happens when Firefox opens a connection to a webserver?•Note: this will be an overview: For more details, you shouldtake Prof. Buckwalter’s class.Department of Computer Science — University of San Francisco – p. 2/??2-3: Layering•Modern network design takes advantage of the idea of layering•A particular service or module is constructed as a black box.•Users of that service do not need to know its internals, just itsinterface.•This makes it easy to later build new modules (or layers) thatuse the lower layers.•For example, HTTP is built on top of TCP.◦A web browser does not typically need to worry about theimplementation of TCP, just that it works.•Unlike modules in a typical OO system, the layers in anetworked system comprise protocols that span multiplemachines.Department of Computer Science — University of San Francisco2-4: The OSI seven-layer model•ISO (a standards body) developed a reference model calledOSI that defines the different layers needed for communication,and specifies which should do each job.•The goal is to produce an open protocol that allows forheterogeneous, extensible systems.•A protocol is a specification describing the order and format ofmessages.•An open protocol is one in which all of this information ispublicly available.Department of Computer Science — University of San Francisco – p. 4/??2-5: The OSI seven-layer model•Application•Presentation•Session•Transport•Network•Data Link•PhysicalDepartment of Computer Science — University of San Francisco – p. 5/??2-6: Message transmission across layers•An application (such as a web browser) wants to send amessage to another computer.•That application constructs a message and passes it to theapplication layer.•The application layer attaches a header to the message andpasses it to the presentation layer.•The presentation layer attaches a header and passes it to thesession layer, and so on.•On the other end, the message is received by the physicallayer, who strips off the appropriate header and passes themessage up to the data link layer.•This continues until the message reaches the application layerof the receiving machine.•High-level layers don’t need to worry about lower-level layers.•Lower-level layers treat everything from higher layers as data tobe sent.Department of Computer Science — University of San Francisco2-7: Layers and packets•Each layer constructs a packet containing a portion of the datato be transmitted.•This packet has a data section, and a header.◦The header contains origin and destination information,checksums, sequence numbers, and other identifyinginformation.•When a message is send by (for example) TCP, a packet isconstructed and passed down to the IP layer.•This entire packet then becomes the data portion of the IPpacket, which is passed down to the network layer, and so on.•On the other end, the lowest layer removes the header andchecks the data integrity, then passes the data portion up to thenext layer.Department of Computer Science — University of San Francisco – p. 7/??2-8: Physical Layer•This is the lowest-level layer, responsible for transmitting 0s and1s.•Governs transmission rates, full or half-duplex, etc.•A modem works at the physical layer.•Lots of interesting problems at this level that we won’t get into ...Department of Computer Science — University of San Francisco – p. 8/??2-9: Data Link Layer•The data link layer provides error handling for the physical layer.•Individual bits are grouped together into frames.•A checksum is then computed to detect transmission errors.•The data link layer can then request a retransmission of anerror is detected.•Messages are numbered; receiver can request re-transmissionof any message in a sequence.•Each frame is a separate, distinct message.•The Data link layer provides error-free transmission toupper-level layers.Department of Computer Science — University of San Francisco2-10: Network Layer•The network layer is responsible for routing and flow control.•The network layer removes the data link header and examinesthe resulting packet for a destination, and then forwards it asappropriate.•the Internet Protocol (IP) is one of the best-knownnetwork-layer protocols.•Primary role:
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