Midterm1 Solucionario Cap 1 4 e 5

Coulouris Chapter 1 1.2 How

might the clocks in two computers that are linked by a local network be synchronized without reference to an external time source? What factors limit the accuracy of the procedure you have described? How could the clocks in a large number  of computers connected by the Internet be synchronized? iscuss the accuracy of that  procedure! " #o global clock$ When programs need to cooperate they coordinate their actions by exchanging messages! Close coordination often depends on a shared idea of the time at which the programs% actions occur! &ut it turns out that there are limits to the accuracy with which the computers in a network can synchronize their clocks ' there is no single global notion of the correct time! (his is a direct conse)uence of the fact that the only communication is by sending messages through a network! * user arrives at a railway station that they has never visited before+ carrying a ,* that is capable of wireless networking! -uggest how the user could be provided with information about the local services and amenities at that station+ without entering the station%s name or attributes! What technical challenges must be overcome?  page 1. 1.4

" (he user must be able to ac)uire ac)u ire the address of locally relevant information as automatically as  possible! /ne method is for the local wireless network network to provide the 02 of web pages about the locality over a local wireless network! 3or this to work$ 415 the user must run a program on her device that listens for these 02s+ and which gives the user sufficient control that she is not swamped by unwanted 02s of the places she passes through6 and 475 the means of propagating the 02 4e!g! infrared or an 897!11 wireless 2*#5 should have a reach that corresponds to the  physical spread of the place itself! 1.6 0se

the World Wide Wide Web Web as an example to illustrate the concept co ncept of resource sharing+ client and server! What are the advantages and disadvantages of H(:2+ 02s and H((, as core technologies for information browsing? *re any of these technologies suitable as a basis for client"server computing in general?

" Web Web ,ages are examples of resources that are shared! (hese resources are managed by b y Web Web servers! Client"server architecture! (he Web Web &rowser is a client program 4e!g! #etscape5 that runs on the user;s computer! co mputer! (he Web Web server accesses local files containing the Web Web pages and a nd then supplies them to client browser processes! H(:2 is a relatively straightforward straightforward language to parse and render but it confuses presentation with the underlying data that is being presented!

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order to print this document from Scribd, you'll 02s are efficient resourceInlocators but they are not sufficiently rich as resource links! 3or first need to example+ they may point at a resourcedownload that has it. been relocated or destroyed6 their granularity 4a whole resource5 is too coarse"grained for many purposes! Cancel Download And Print H((, is a simple protocol that can be implemented with a s mall footprint+ and which can be put to use in many types of content transfer and other types of service! Its verbosity 4H(:2 messages tend to contain many strings5 makes it inefficient for passing small amounts of data!

H((, and 02s are acceptable as a basis for client"server computing exc ept that 4a5 there is no strong type"checking 4web services operate by"value t ype checking without compiler support5+ 4b5 there is the inefficiency that we have mentioned! 1.8 *n

open distributed system allows new resource"sharing services such as the &2/& ob to be added and accessed by a variety of client programs! iscuss in the context of this example+ to what extent the needs of openness differ from those of  heterogeneity! " (o add the &2/& ob must already have been agreed for the distributed system (o list them again$  the distributed system uses a common set of communication protocols 4probably Internet  protocols5!  it uses an defined standard for representing data items 4to deal with heterogeneity of hardware5!  It uses a common standard for message passing operations 4or for invocations5!  It uses a language independent standard for representing data structures! &ut for the open distributed system the standards must have been agreed and documented before the &2/& ob
the three main software components that may fail wh en a client process invokes a method in a server ob
(he three main software components that may fail are$ the client process e!g! it may crash the server process e!g! the process may crash the communication software e!g! a message may fail to arrive (he failures are generally caused independently of one another! =xamples of dependent failures$  if the loss of a message causes the client or server process to crash! 4(he crashing of a server would cause a client to perceive that a reply message is missing and might indirectly cause it to fail5!  if clients crashing cause servers problems!  if the crash of a process causes a failures in the communication software!

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In order to print this document from Scribd, you'll &oth processes should be able to tolerate missing messages! (he client must tolerate a missing first need to download it. reply message after it has sent an invocation re)uest message! Instead of making the user wait forever for the reply+ a client process could use a timeout and then tell the user it has not been able to contact the server! * simpleCancel server
Chapter @ 4.1 Is

it conceivably useful for a port to have several receivers?

If several processes share a port+ then it must be possible for all of the messages that arrive on that port to be received and processed independently by those processes! ,rocesses do not usually share data+ but sharing a port would re)uires access to common data representing the messages in the )ueue at the port! In addition+ the )ueue structure would be complicated by the fact that each process has its own idea of the front of the )ueue and when the )ueue is empty! #ote that a port group may be used to allow several processes to receive the same message! 4.2 *

server creates a port which it uses to receive re)uests from clients! iscuss the design issues concerning the relationship between the name of this port and the names used by clients! (he main design issues for locating server ports are$ 4i5 How does a client know what port and I, address to use to reach a service? (he options are$  use a name serverAbinder to map the textual name of each service to its port6  each service uses well"known location"independent port id+ which avoids a lookup at a name server! (he operating system still has to look up the whereabouts of the server+ but the answer may be cached locally! 4ii5 How can different servers offer the service at different times? 2ocation"independent port identifiers allow the service to have the same port at different locations! If a binder  is used+ the client needs to reconsult the client to find the new location! 4iii5 =fficiency of access to ports and local identifiers! -ometimes operating systems allow processes to use efficient local names to refer to ports! (his  becomes an issue when a server creates a non"public port for a particular client to send messages to+ because the local name is meaningless to the client and must be translated to a global identifier for use by the client!

4.7 -un

 marshals data by converting it into a standard big"endian form before transmission!

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order to print this document from Scribd, you'll iscuss the advantages andIndisadvantages of this method when compared with C/&*%s C! first need to download it.

(he  method which uses a standard form is inefficient when communication takes place  between pairs of  Cancel Download And Print similar computers whose byte orderings differ from the standard! It is efficient in networks in which the byte"ordering used by the ma
:2 is used to enable clients to communicate with web services and for defining the interfaces and other properties of web services! However+ :2 is also used in many other ways+ including in archiving and retrieval systems ' although an :2 archive may be larger than a binary one+ it has the advantage of being readable on any computer! /ther examples of uses of :2 include for the specification of user interfaces and the encoding of configuration files in operating systems! :2 documents+ being textual+ can be read b y humans! In practice+ most :2 documents are generated and read by :2 processing software+ but the ability to read :2 can be useful when things go wrong! (he use of a textual rather than a binary representation+ together with the use of tags+ makes the messages large+ so they re)uire longer processing and transmission times+ as well as more space to store!

4.26 evise

a scenario in which multicasts sent by different clients are delivered in different orders at two group members! *ssume that some form of message retransmissions are in use+ but that messages that are not dropped arrive in sender ordering! -uggest how recipients might remedy this situation! -ender1 sends re)uest r1 to members m1 and m7 but the message to m7 is dropped -ender7 sends re)uest r7 to members m1 and m7 4both arrive safely5 -ender1 re"transmits re)uest r1 to member m7 4it arrives safely5! :ember m1 receives the messages in the order r16r7! However m7 receives them in the order r76r1! (o remedy the situation! =ach recipient delivers messages to its application in sender order! When it receives a message that is ahead of the next one expected+ it hold it back until it has received and delivered the earlier re"transmitted messages!

Chapter B

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In order to print this document from Scribd, you'll first need to download it. 5.8 =xplain the design choices that are relevant to minimizing the amount of reply data held

at a server! Compare the storage re)uirements when the  and * protocols are used!  page 1E1 Cancel Download And Print (o enable reply messages to be re"transmitted without re"executing operations+ a server must retain the last reply to each client! When  is used+ it is assumed that a re)uest message is an acknowledgement of the last reply message! (herefore a rep ly message must be held until a subse)uent re)uest message arrives from the same client! (he use of storage can be reduced by applying a timeout to the period during which a reply is stored! (he storage re)uirement for  F average message size x number of clients that have made re)uests since timeout period! When * is used+ a reply message is held only until an acknowledgement arrives! When an acknowledgment is lost+ the reply message will be h eld as for the  protocol!

5.9 *ssume

the * protocol is in use! How long should servers retain un acknowledged reply data? -hould servers repeatedly send the reply in an attempt to receive an acknowledgement? "(he timeout period for storing a reply message is the max imum time that it is likely for any client to a retransmit a re)uest message! (here is no definite value for this+ and there is a trade" off between safety and buffer space! In the case of *+ reply messages are generally discarded  before the timeout period has expired because an acknowledgement is received! -uppose that a server using * re"transmits the reply message after a delay and consider the case where the client has sent an acknowledgement which was late or lost! (his re)uires 4i5 the client to recognise duplicate reply messages and send corresponding extra acknowledgements and 4ii5 the server to handle delayed acknowledgments after it has re"transmitted reply messages! (his possible improvement gives little reduction in storage re)uirements 4corresponding to the occasional lost acknowledgement message5 and is not convenient for the single threaded client which may be otherwise occupied and not be in a position to send further acknowledgements! 5.12 iscuss

the invocation semantics that can be achieved when the re)uest"reply protocol is implemented over a (C,AI, connection+ which guarantees that data is delivered in the order sent+ without loss or duplication! (ake into account all of the conditions causing a connection to be broken! " * process is informed that a connection is broken$  when one of the processes exits or closes the connection!  when the network is congested or fails altogether  (herefore a client process cannot distinguish between network failure and failure of the server! ,rovided that the connection continues to exist+ no messages are lost+ therefore+ every re)uest will receive a corresponding reply+ in which case the client knows that the method was executed exactly once! However+ if the server process crashes+ the client will be informed that the connection is broken and the client will know that the method was executed either once 4if the server crashed after executing it5 or not at all 4if the server crashed before executing it5!

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order to print this document from Scribd, you'll &ut+ if the network fails theInclient will also be informed that the connection is broken! (his may first need to downloadof it. the re)uest message or during the transmission have happened either during the transmission of the reply message! *s before the method was executed either on ce or not at all! (herefore we have at"most"once call semantics! Cancel Download And Print 5.14

(he =lection service must ensure that a vote is recorded whenever any user thinks they have cast a vote! iscuss the effect of maybe call semantics on the =lection service! Would at"least"once call semantics be acceptable for the =lection service or would you recommend at"most"once call semantics? " :aybe call semantics is obviously inade)uate for voteG =x B!1 specifies that the voter%s number  is used to ensure that the user only votes once! (his means that the server keeps a record of who has voted! (herefore at"least"once semantics is alright+ because any repeated attempts to vote are foiled by the server! 5.15

* re)uest"reply protocol is implemented over a communication service with omission failures to  provide at"least"once :I invocation semantics! In the first case the implementor assumes an asynchronous distributed system! In the second case the implementor assumes that the maximum time for the communication and the execution of a remote method is (! In what way does the latter assumption simplify the implementation? " In the first case+ the implementor assumes that if the client observes an omission failure it cannot tell whether it is due to loss of the re)uest or reply message+ to the server having crashed or having taken longer than usual! (herefore when the re)uest is re"transmitted the client may receive late replies to the original re)uest! (he implementation must deal with this! In the second case+ an omission failure observed by the client cannot be due to the server taking too long! (herefore when the re)uest is re"transmitted after time (+ it is certain that a late reply will not come from the server! (here is no need to deal with late replies!