Tufts COMP 150-IDS Test #1

Transcription

1 October 3, 2012 Student name: Login: Tufts COMP 150-IDS Test #1 This test is closed book. In most cases, answers of no more than 2-3 sentences should be sufficient. If you need more space or need to cross out and re-do, you may attach additional sheets of paper. Answers are shown like this. In some cases, these provide more detail than is necessary for full credit. In cases where there s likely to be confusion about what s most important, some key points we re looking for are highlighted like this. TCP/IP 1a. (5 points) Briefly explain the difference between TCP and UDP. What service does each provide? What is similar about them and what is different? TCP and UDP are both user-level protocols intended for communicating between programs (as opposed to hosts). Both use (IP-Address:port) pairs for addressing. UDP provides an unreliable datagram service. Individual packets are checksummed and their length is preserved, but packets can be lost or reordered. TCP provides a reliable stream service transmitting bi-directional ordered octet (byte) streams. Message boundaries are not preserved. 1b. (5 points) In most cases, queries for DNS name lookup are implemented using UDP. Explain why this is a better choice than TCP. DNS host name lookup messages are short and idempotent, and performance is important. If TCP were used, then extra IP packets would be sent to set up and maintain the connections. With UDP, the typical lookup is one packet outbound, and one packet for the response. If no response is received, the entire request is resent. 1

2 1c. (3 points) Comparing IP and UDP: what is the relationship between them, what is similar about them, and what is different? IP is the packet service provided by the Internet. IP packets are addressed to a host (IP address). Headers of IP packets are checksummed but contents are not. UDP is a user-level datagram built using IP, and addressed to a host/port (IP address/port) pair, and designed for program-to-program communication. See 1a for more details. Idempotence 2a. (5 points) What is the difference between an operation that is idempotent and one that is not? Idempotent operations yield the same results when executed multiple times. Non-idempotent operations need not. Usually, we slightly loosen this to include cases where reads are attempted against data that varies with time, as long as we are happy to get the latest answer. Informally, idempotent operations are safe to retry; non-idempotent operations typically are not. 2

3 2. (5 points total) Sometimes designers have the opportunity to choose between making an operation idempotent and designing it in some other way. Briefly explain two advantages of using idempotent operations: 2b. (3 points) Advantage #1: As discussed above in the answer to 1b, idempotent operations can usually be implemented with simple, high-performance datagram protocols. 2c. (2 points) Advantage #2: Idempotent operations can be easier to reason about. Proving the correctness of a system with idempotent operations is often much easier than proving correctness of a more complex system. Some other answers may be accepted too. 3

4 Metcalfe s Law 3. (5 points) Metcalfe s law suggest that, for a given number of nodes, one single network is more valuable than two smaller ones. Why? (I.e. where does the additional value come from?) Metcalfe s Law posits that the value of a network grows in proportion to the number of point-to-point connections that can be made, and that number grows as the square of the number of nodes. Since the growth is much faster than linear, a single large network has more value. To justify this with an example: if we had separate Webs for Europe and for the US, we would lose the value of all the links that could have been made between resources on the two continents. Text formats, Binary Formats, HTML & Metcalfe's law 4. (17 points total) Background: When Tim Berners-Lee designed the HTML document format he chose to make it a text file rather than binary and he allowed for extensible structured markup. That is, he defined an initial set of tags such as <p> and he also provided rules that would allow for additional tags to be added in the future. These four questions ask you to describe how these design decisions were motivated in part by Metcalfe's law and to explore some related trade-offs: 4a. (4 points 2 points for 1 st reason, one each for two more) Briefly explain how the decision to use text rather than binary for HTML relates to Metcalfe's law (give at least 3 reasons) If we believe Metcalfe s Law, then the Web grows in value as more documents are added and linked. Using a text based format encourages such growth in a number of ways: A very wide range of tools can edit and create HTML (e.g. Emacs, Vim, Notepad) 4

5 View source allows users to learn how to write HTML Text-based formats are platform independent, so HTML is easy to share across platforms It is relatively easy to use a wide range of programming languages and tools to write and read HTML (cout, printf, etc.) Etc., etc. (Any three of these earns full credit) 4b (4 points 2 for each) List two disadvantages of using text-based formats: 1. Text formats tend to waste space 2. Text formats can be slow to parse 3. It can be difficult to host more complex datatypes in HTML (imagine trying to include a photograph directly in an HTML file as opposed to linking to a JPEG) (Any two of these earns full credit) 4c. (5 points) The choice to use a text vs. a binary format sometimes depends on whether all the communicating computers use the same machine architecture. Describe an optimization that s available when transmitting structured binary information between between compatible computers, and explain why this technique may be more difficult to use when sending messages between machines with different architectures. Define your message format as a C struct and send the bytes through the wire. The language will help you to set fields very efficiently, and no parsing is needed at the receiving end. 5

6 On incompatible machines, byte order for types like integers may be different, and structure packing rules also vary from one compiler or target machine to another. 4d. (4 points) Briefly explain how the decision provide extensible structured markup in HTML relates to Metcalfe's law: The use of explicit markup allows HTML to deal well with many real-world documents such as those typically created in applications like MS Word. The extensibility of the tag space means that HTML itself can grow as needs change. Many current users of the Web miss the fact that the original HTML had no <img> tag, and that several proposals for how to do images competed before the <img> tag we all use emerged as the consensus solution. 6

7 Mixed Content 5a. (8 points) All of the following are valid fragments of HTML. Which of the following is an example of mixed content (put a check mark near each one that is mixed, leave others unchecked): <div><p>some divs </p><p> have lots of paragraphs!</p></div> <ul><li>i ve got</li><li>a little list</li></li> <p>this one looks simple, and it is!</p> _X_ <h1><style= color:red >Very<style> interesting heading</h1> _ X_ <style= font-weight:bold >Very<style> interesting but not a heading <h2 style= font-style:italic >Looks like we didn t like the default heading style</h2> <em>nice weather we re having</em> Nice weather we re having 5b (2 points) Describe one disadvantage of supporting mixed content in a document format : Mixed content strings tend to be difficult to process in traditional programming languages 7

8 The End-to-End Principle 6. (15 points) Like many graduates of COMP 150 IDS, Mary got a job working on an exciting distributed system and was soon promoted from programmer to lead architect. Two members of her team, Bob and Ted, have come to her office with a design question. Their system requires that data be transmitted through a network, and the typical path involves multiple hops. Bob and Ted have a disagreement about the implications of the end-to-end principle: Bob says that the end-to-end principle means that checks have to be put at the endpoints to ensure correctness, and that any other checks done within the network will make the system run slower. Ted agrees that checking must be done at the endpoints to ensure success, but he says that additional checks on individual links will improve the performance of the system. Based on what she has learned in comp 150 IDS, Mary is able to tell Bob and Ted either that one of them is always right, or that who is right depends on getting additional information about the network and the application. If either Bob or Ted is always right, then identify who is and carefully explain why. If the answer depends on having additional information, then explain exactly what additional information is needed, and as specifically as you can predict based on that additional information when the additional checks will lead to faster performance. (Correctly telling us Bob/Ted/need-more-info will earn you 3 points; giving a good explanation of why will earn you the additional 12 points.) Mary should tell her team that the answer depends on several factors: How long it takes to do an end-to-end recovery How frequently errors occur on individual links How much overhead is added to the protocol by implementing the link-level recover (presumably, extra acknowledgement packets are needed, and senders may need to store sent packets until it s confirmed that they have indeed made it to the next node) If failures are very rare and the time to recover end-to-end is small, then link-level checking may not be worthwhile; if link-level errors are common 8

9 and the time for end-to-end recovery large, then retrying at the link level is almost surely worthwhile. Model/View Separation 7. (12 points total) BACKGROUND FOR QUESTIONS 7a and 7b: In the early versions of HTML, information about styling was often included directly on element attributes. For example, this is a slightly simplified version of a sample element included in the early HTML 3.2 specification : <body bgcolor=white text=black> As we discussed in class, HTML uses are now encouraged to specify such styling information separately, typically using CSS. 7a. points 4 points for first reason, 2 for second) List two ways in which this change has made HTML a more powerful system for publishing information on the web. 1. It is easy to share a common style or theme across multiple documents. Indeed, this technique is commonly used to achieve a consistent look and feel for a whole Web site or family of documents. 2. It is relatively easy to update the style of multiple documents that share common style sheets 3. Sometimes, but not always, mixing style information with data can make it harder to extract and manipulate the data ( Two of the three are needed for full credit. The third is less clear cut than the other two, but will be accepted as an answer for full credit. 9

10 7b. (6 points 4 for one correct, 6 for both) List two disadvantages of separating content from style in this way? 1. Very often one wants to tune the look of a document at the same time that one updates the content. Separating the styling means that stylesheets and content must both be edited. 2. The document is no longer self-contained. If you want to send it to someone else and have it look the same, you need both the stylesheet and the content. 3. As we discussed in class, there are cases in which changing the stylesheet can effectively alter the content of a document. For example, if a document had the string: I hereby agree to this contract and a stylesheet changes the font color to white (the background color), it may not be clear that the contract has been accepted. (Any two answers gets full credit) 10

11 Exception Handling #include <string> #include <iostream> using namespace std; class Except { public: void printmsg() { cout << "Except::printMsg() called" << endl; } }; class OtherExcept { public: void printmsg() { cout << "OtherExcept::printMsg() called" << endl; } }; void function2() { cout << "Function 2 entered" << endl; throw OtherExcept(); cout << "Leaving function 2" << endl; } void function1() { cout << "Function 1 entered" << endl; function2(); throw Except(); cout << "Leaving function 1" << endl; } // // Main program // int main (int argc, char *argv[]) { try { function1(); } catch (OtherExcept e) { e.printmsg(); }catch (Except e) { e.printmsg(); } cout << "All done!" << endl; } 11

12 8a (5 points). What will the program output? (Hint: the two catch blocks work as you d expect: each catches the corresponding named error) Function 1 entered Function 2 entered OtherExcept::printMsg() called All done! 8b. (6 points) Unix I/O system calls use return values to reflect error conditions to the calling program; some other systems use exceptions, such as the ones shown above. Why are exceptions particularly useful for networking code? Errors are relatively common when attempting network operations, and very often the appropriate recovery strategy is common across multiple operations. For example, one might have many different lines of code sending packets to attempt to copy a file. It might be that the recovery strategy for any network error is to retry the entire transfer. With return codes, checking must be done after each operation; that checking code is repetitive and may need to be updated in many places if strategies change. Exceptions let us keep most of the code clean, by putting the recovery in one place. In languages like C++, exception handling automatically calls the destructors of variables that go out of scope (automatic variables declared within methods or functions that we re blasting out of). Overall, exceptions tend support organized strategies for recovering at a higher level. 12

13 Open Source and Open Standards 9a. (3 points) What is the difference between open source and open standards? An open standard is typically a specification that is offered with few if any legal or financial impediments to implementation. Many of the most important open standards are designed to be usable on a wide variety of systems, and possibly to support interoperation across diverse systems. Open source is an implementation made available in source form (possibly along with binaries), To be open source, the system must be provided for free, and with minimal restrictions on adaptation and reuse (the details of just how minimal these restrictions must be have been debated for years). 9b. (5 points) Explain why open source and open standards are important to the Web. Which approach is used for what purposes? The Web is fundamentally set down as open standards for URIs, HTTP, HTML, and many related technologies. If you can implement these in your own code, you can be part of the Web! Open source implementations like Firefox and Apache are very useful in saving everyone from writing their own implementations, and also in making widely available implementations that conform to the standards. Since other implementations, including closed source ones will need to interoperate with these, the open source tends to support and protect the open standards! 13