Test 2

Problem X2.1.

[12 pts] List all possible outputs for the below program, and explain how each could occur.

public class ThreadExample extends Thread {
    private int x = 0;
    private int y = 2;

    public void run() {
        x = 1;
        y = 4;
    }

    public void doMain() {
        x += y;
    }

    public static void main(String[] args) throws InterruptedException {
        ThreadExample sub = new ThreadExample();
        sub.start();   // kick off 'run'
        sub.doMain();  // simultaneously do 'doMain'
        sub.join();    // 'doMain' is complete; wait for 'run' to complete
        System.out.println(sub.x);
    }
}

Problem X2.2.

[10 pts] UCA's CIDR address block is 69.196.224.0/19.

a. What is the network mask for this address block?

b. How many valid IP addresses exist for this block?

Problem X2.3.

[14 pts] Using shared-memory concurrency, implement a Java class that supports a reading/writing lock, with three methods: getRead, getWrite, and release. At any time, this lock will either be in a reading or a writing state; when in reading state, multiple threads can have a read lock, but when in writing state, only one thread at a time can have a write lock. It is eligible to switch between reading and writing states once nobody holds the lock.

A thread will call getRead, and the method will return once the lock is in reading state (and the thread calling the method is allocated a reading lock). A thread will call getWrite, and the method will return once the calling thread has reserved the lock while it is in writing state. Any thread calling release will lead to one less reservation on the lock.

Do not worry about starvation. Of course, your solution should not busy-wait.

public class ReadWriteLock {
    // suggested instance variables - feel free to ignore or add more
    private Object monitor = new Object();
    private int numReading = 0; // number of read locks acquired
    private int numWriting = 0; // number of write locks (0 or 1)

    public void getRead() {


    }

    public void getWrite() {


    }

    public void release() {


    }
}

Problem X2.4.

[12 pts] Suppose we want a D program using message passing for concurrency, where the user can type “begin” to tell a different thread to start displaying the integers starting from 1, with a one-second delay between each number. When the user types “end,” though, the thread should immediately halt its counting.

Without actually writing any code, describe what threads should exist in the D program, when they should be spawned, and what messages should be passed between them to implement the desired behavior.

Problem X2.5.

[14 pts] Complete the following D function so that when spawned in a separate thread it implements semaphore-like behavior. When another thread wants to execute a DOWN request, it will send a message containing its tid (thread ID) to semaphore, and it should send a message containing true once the DOWN request is satisfied. When another thread wants to execute a UP request, it will send an integer (assumed 1, but semaphore can ignore the value).

You will likely want to maintain a queue of waiting threads. The starter code already creates an empty queue. To insert, use queue.insertFront(value); to remove from the back of the queue, use queue.removeAny(), which returns the value removed; and to test whether the queue is empty, use queue.empty(), which returns true or false.

void semaphore(int initValue) {
    DList!Tid queue = DList!int(cast(Tid[]) []);

Problem X2.6.

[8 pts] Layer 4 of the OSI model is the “transport layer,” sandwiched between layer 3 (network) and layer 5 (session). What is the purpose of the transport layer?

Problem X2.7.

[10 pts] Modern Ethernet networks typically use two different kinds of cabling. Name each, and identify an advantage of each.

Problem X2.8.

[8 pts] Describe what an ARP proxy does.

Problem X2.9.

[12 pts] The below diagram represents an internetwork; each thick solid line represents a single network, with the IP address block as shown (e.g., 10.0.0.0/16). The squares represent routers, each containing two or three Ethernet cards, identified as eth0, eth1, or eth2, configured with the IP address shown.

For the router named gummo, in the upper right corner, what should the routing table be? Note that each row of the routing table should contain a network specification (or “default”), an Ethernet card identifier, and either the gateway IP address or the word “direct.”