465/565 ECE Computer Networks and Protocols Fall 06 Dr. Thinh Nguyen Homework 3 Due 10/26/06 at the beginning of the class
1. Problem 16 from the problem section of chapter 3 from the text book (4 pts) a) Here we have a window size of N=3. Suppose the receiver has received packet k-1, and has ACKed that and all other preceeding packets. If all of these ACK's have been received by sender, then sender's window is [k, k+N-1]. Suppose next that none of the ACKs have been received at the sender. In this second case, the sender's window contains k-1 and the N packets up to and including k-1. The sender's window is thus [kN,k-1]. By these arguments, the senders window is of size 3 and begins somewhere in the range [k-N,k]. b) If the receiver is waiting for packet k, then it has received (and ACKed) packet k-1 and the N-1 packets before that. If none of those N ACKs have been yet received by the sender, then ACK messages with values of [k-N,k-1] may still be propagating back. Because the sender has sent packets [k-N, k-1], it must be the case that the sender has already received an ACK for k-N-1. Once the receiver has sent an ACK for k-N-1 it will never send an ACK that is less that k-N-1. Thus the range of in-flight ACK values can range from k-N-1 to k-1.
2. Problem 27 from the problem section of chapter 3 from the text book (9 pts)
a) TCP slowstart is operating in the intervals [1,6] and [23,26] b) TCP congestion advoidance is operating in the intervals [6,16] and [17,22] c) After the 16th transmission round, packet loss is recognized by a triple duplicate ACK. If there was a timeout, the congestion window size would have dropped to 1. d) After the 22nd transmission round, segment loss is detected due to timeout, and hence the congestion window size is set to 1. e) The threshold is initially 32, since it is at this window size that slowtart stops and congestion avoidance begins. f) The threshold is set to half the value of the congestion window when packet loss is detected. When loss is detected during transmission round 16, the congestion windows size is 42. Hence the threshold is 21 during the 18th transmission round.
g) The threshold is set to half the value of the congestion window when packet loss is detected. When loss is detected during transmission round 16, the congestion size windows is 42. Hence the threshold is 21 during the 18th transmission round. h) During the 1st transmission round, packet 1 is sent; packet 2-3 are sent in the 2nd transmission round; packets 4-7 are sent in the 3rd transmission round; packets 815 are sent in the 4th transmission round; packets15-31 are sent in the 5th transmission round; packets 32-63 are sent in the 6th transmission round; packets 64 96 are sent in the 7th transmission round. Thus packet 70 is sent in the 7th transmission round. i) The congestion window and threshold will be set to half the current value of the congestion window (8) when the loss occurred. Thus the new values of the threshold and window will be 4.
Programming Assignment (17pts)
import import import import import import java.io.BufferedReader; java.io.DataOutputStream; java.io.InputStreamReader; java.net.Socket; java.util.StringTokenizer; java.io.InterruptedIOException;
/** * @author Thinh Nguyen * * RDT Sender * */ public class RdtSender { final static String CRLF = "\r\n"; public static void main(String[] argv) throws Exception{ // Get the server name and port number from the command line. String server = argv[0]; int port = (new Integer(argv[1])).intValue(); // Connect to the server Socket socket = new Socket(server, port); socket.setSoTimeout(1000); DataOutputStream toServer = new DataOutputStream(socket.getOu...