Operating Systems CS 5523
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This 8 page Class Notes was uploaded by Mireya Heidenreich on Thursday October 29, 2015. The Class Notes belongs to CS 5523 at University of Texas at San Antonio taught by Staff in Fall. Since its upload, it has received 9 views. For similar materials see /class/231395/cs-5523-university-of-texas-at-san-antonio in ComputerScienence at University of Texas at San Antonio.
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Date Created: 10/29/15
CS 5523 Lecture 3 Fundamental Models I Answer questions on laboratory 1 I Why have system models I What should be captured in a model I lnteraction models I Failure models I Security models Why have system models I Make assumptions clear I Organize and classify different approaches I Make generalizations What should be captured in a model I Interaction How do components interact and coordinate to solve a problem I Failure What are the potential failures and how do they affect the results I Security What are the system s vulnerabilities and how should they be handled Interaction models I Processes interact through messagepassing to solve a ro em p I Interaction is in uenced by Performance of the communication channels Synchronization of the clocks Communication performance I Latency I Bandwidth I Jitter Latency a message by one CDK The delay between the sending of process and its receipt by another IDo a 100bit message and a 150bit message have the same latency I What are the components of latency fora typical message sent on a LA I What factors influence the va riability of message latency on a LAN Bandwidth CDK the total amount of information that can be transmitted overa network in a given time I What are the measurement units for bandwidth I What are typical numbers for Ethernet Cablemodem ADSL IHow is bandwidth shared Jitter CDK variation in time taken to delivera series of messages Iln what applications is jitter important I Is jitter bounded IHow do jittersensitive applications typically deal with jitter Clocks and event ordering IMessages are often timestamped to indicate ordering I Ea ch computer has at lea st one local clock I Timestamps are often based on local clock values ILocal clocks aren t synchronized How much do typical PC clocks drift How can local clocks be synchronized What other quot 39 39 be used for event ordering Synchronous distributed system Can bound I Time for each processing step I Time to transmit each message I Clock dri for each process Are synchronous systems useful What is required to build a synchronous system Asynchronous distributed system No bounds on I Time for each processing step I Time to transmit each message I Clock dri for each process Why are most real systems asynchronous Event ordering When does an event sending or receiving a message from one process occur relative to an event from another process Example What are the possible orderings of messages in the fo lowing scenario A mailing list contains users X Y Z and A 1 X sends an email to the list with subject Meeting m1 2 Y and 2 reply by sending each sending a message to the list with subject Re Meeting m2 and ms respectively thure 2 9 com Reamme ordermg or events send recewe receive Physiml ume recave recave Logical time Provide proper ordering of events without reference to physical clocks Lamport 19 78 Example In the email scenario X sends m before Y sends m2 Y sends m2 before X receives m2 Y receives m before Y sends m2 Failure models CDK define the ways in which failure may occur in order to provide and understanding of the effects of failures I Omission failures process or channel fails to do something I Arbitrary failures usually refer to sabotage I Timing failures required time bounds are exceeded Flgure 2 wow Processes and channels placessp process q Cammmcaumchannzl Oulgmng message buster Incmmngm essage humr Flgure 211CDK Omlsslon and arbltrary fallures Clam a Iallure AZZECtS DESCHEHOH Fallrswp Process Process halts and remarns halted OLherprocesses may de ect ths State Crash Process Process halts and remarns halted OLherprocesses may not be able w de ect ths State Omlsslon channel A message rnserted ln an outgorng message buffer never amv th buffa es atthe 0 er end s rncomrng message Smdromlsslon Process LLh ln rts outgorng message buffa buffa but that process does not recelve rt Arb m ary Process or Byzantlne channel sendtransmrt arbrtrary messages at arbrtrary trmes rssrons aprocess may stop ortake an rncorrect step Flgure 2 l2 CDK Tlmmg fallures class a Fame A eats Dem tron clock Proces Process s local clock exceeds the bounds on rts rate of dn from rea t Performance Process Process exceeds the bounds on the rntewal betweentwo steps Performance channel stated b ound Reliable communication I Validity any message in the outgoing message buffer is eventually delivered to the incoming message buffer I lntegrity the received message is identical to the sent message and is delivered exactly once How does a communication service provide validity and integrity Security model I Protect processes and channels from corruption I Protect resources encapsulated by objects from unauthorized access Figure 213mm Obiects arid pnricipais Access ngms Olen invucatiuri I Netwurk Principai server Principai user Figure 214mm Iii 1 Communication Channei Basic terminology I Cryptography science of keeping information secure I Encryption scrambling a message to hide its contents I Authentication method for assuring the true identity of an entit Figure 215CDL ecure channeis
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