SISTEMAS DISTRIBUÍDOS 1º EXAME

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1 SISTEMAS DISTRIBUÍDOS 1º EXAME Ano Lectivo: 2005/2006 Data: 12 de Junho de 2006 Ano Curricular: 4º Ano 2º Semestre Duração: 2h00 INFORMAÇÕES GERAIS 1. O exame encontra-se em Inglês devido à existência de alunos estrangeiros, mas as respostas podem ser obviamente dadas em Português. Consulte o docente para esclarecer qualquer dúvida de interpretação das questões. 2. Identifique todas as folhas do enunciado com nome e número antes de começar a resolver o exame. 3. Mantenha na secretária apenas a sua identificação e uma caneta (azul ou preta) Não são admitidas folhas de rascunho (utilize as costas do teste), calculadoras, telemóveis, PDAs ou qualquer tipo de consultas. 4. Responda no espaço delimitado a seguir a cada questão. 5. Utilize o verso da página onde se encontra a pergunta, para rascunho ou para espaço adicional. 6. Respostas incorrectas às perguntas de escolha múltipla descontam (1/n)*cotação_pergunta, sendo n o número possível de opções. A soma da cotação de todas as perguntas de escolha múltipla nunca pode ser menor que zero. 7. Existem várias versões não identificadas do exame. 1. Although the questions are in English, you can answer them in Portuguese. 2. ID all pages (name and number) before starting solving the exam. 3. Only a pen and your ID are allowed over the desk. You cannot use calculators, cellular phones, PDAs, textbooks, personal notes, etc. 4. Answer each question using the available space. 5. If you need additional space use the back of the pages. 6. In the multiple choice questions, wrong answers are penalized according to the formula (1/n)*question_value, where n is the number of possible answers. 7. The exam has several different non-identified versions. 1. [1.2 val.] On a public key encryption system private keys are usually used to encrypt messages, and public keys are used to decrypt private keys are usually used to decrypt messages, and public keys are used to encrypt 2. [1.2 val.] The Berkeley Algorithm can be considered a Centralized clock synchronization algorithm Distributed clock synchronization algorithm Centralized logical clock algorithm Distributed logical clock algorithm 1 de 5

2 3. [1.2 val.] What makes a fault-model Byzantine? Intermittent faults Arbitrary behaviour after the fault occurs Fault result is a severe crash Servers stop responding due to faults although clients keep working perfectly 4. [1.2 val.] Consider the following communication: a) A wants to send B a message b) B is not running, therefore A simply continues c) B is now running d) A sends B the message and continues running e) B receives the message What kind of communication is described in the above example? Transient asynchronous communication Transient synchronous communication Persistent asynchronous communication Persistent synchronous communication 5. [1.2 val.] When resolving a name on a given name space, iterative name resolution is worse than recursive name resolution because on long distance accesses communication costs are lower it cannot take advantage of caching efficiency on server side there is more burden on name servers client s name resolver is simpler 6. [1.2 val.] In which of the following consistency semantics is (a) valid, (b) valid and (c) invalid? Sequential consistency FIFO consistency Causal consistency Strict consistency 7. [1.2 val.] On a Distributed File System, it is better to offer an Upload/Download model because server is not a bottleneck it is easier to maintain consistency one can use an Automounter to improve consistency one can use stateful servers 2 de 5

3 8. [1.2 val.] Consider the following authentication protocol, where R A and R B are once-in-alifetime-only numbers: Which of the following sentences is true? The protocol is vulnerable to the reflection attack The protocol is vulnerable to old message replay attacks The protocol is based on symmetric key cryptography The protocol is vulnerable because the first message is not encrypted 9. [1.2 val.] Which of the following represents an usual application for an Hash function: Compute the hash of a message; send to the receiver the plaintext message, and the hash result encrypted using sender s private key Compute the hash of a message; send to the receiver the plaintext message, and the hash result encrypted using sender s public key Encrypt the message using sender s public key; compute the hash of the encrypted message; send both to the receiver Encrypt the message using sender s private key; compute the hash of the encrypted message; send both to the receiver 10. [1.2 val.]: On a Primary-based Remote-Write Protocol There is a single fixed server to which all read and write operations are forwarded Remote servers can be used on read operations, but all write operations are performed locally Local servers can be used on read operations, but all write operations are performed on a remote server Local servers are used on read operations, but all write operations are performed on a primary server (that can be local or remote) 3 de 5

4 11. [2 val.] What are the advantages and disadvantages of using a push-based approach vs. using a pull based approach regarding web-cache consistency? 12. [2 val.] What means at least once semantics on a RPC context? What is the main problem associated with this semantic? 4 de 5

5 13. [2 val.] Which election protocol did you use in the 3 rd laboratory assignment? Describe it briefly. How does it compare to the Bully algorithm? (in case you implemented the Bully algorithm, describe a way to improve leader election performance in your system) 14. [2 val.] Show that, even in the presence of a reliable channel, three processes can never reach agreement if at least one of them exhibits Byzantine fault behaviour 5 de 5