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Chapter 3: Problem 10
In our rdt protocols, why did we need to introduce timers?
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Suppose the network layer provides the following service. The network layer in the source host accepts a segment of maximum size 1,200 bytes and a destination host address from the transport layer. The network layer then guarantees to deliver the segment to the transport layer at the destination host. Suppose many network application processes can be running at the destination host. a. Design the simplest possible transport-layer protocol that will get application data to the desired process at the destination host. Assume the operating system in the destination host has assigned a 4-byte port number to each running application process. b. Modify this protocol so that it provides a "return address" to the destination process. c. In your protocols, does the transport layer "have to do anything" in the core of the computer network?
In this problem we investigate whether either UDP or TCP provides a degree of end-point authentication. a. Consider a server that receives a request within a UDP packet and responds to that request within a UDP packet (for example, as done by a DNS server). If a client with IP address \(\mathrm{X}\) spoofs its address with address Y, where will the server send its response? b. Suppose a server receives a SYN with IP source address Y, and after responding with a SYNACK, receives an ACK with IP source address Y with the correct acknowledgment number. Assuming the server chooses a random initial sequence number and there is no "man-in-the-middle," can the server be certain that the client is indeed at \(Y\) (and not at some other address \(\mathrm{X}\) that is spoofing \(\mathrm{Y})\) ?
Consider that only a single TCP (Reno) connection uses one \(10 \mathrm{Mbps}\) link which does not buffer any data. Suppose that this link is the only congested link between the sending and receiving hosts. Assume that the TCP sender has a huge file to send to the receiver, and the receiver's receive buffer is much larger than the congestion window. We also make the following assumptions: each TCP segment size is 1,500 bytes; the two-way propagation delay of this connection is \(150 \mathrm{msec}\); and this TCP connection is always in congestion avoidance phase, that is, ignore slow start. a. What is the maximum window size (in segments) that this TCP connection can achieve? b. What is the average window size (in segments) and average throughput (in bps) of this TCP connection? c. How long would it take for this TCP connection to reach its maximum window again after recovering from a packet loss?
a. Suppose you have the following 2 bytes: 01011100 and 01100101 . What is the 1 s complement of the sum of these 2 bytes? b. Suppose you have the following 2 bytes: 11011010 and 01100101 . What is the \(1 \mathrm{~s}\) complement of the sum of these 2 bytes? c. For the bytes in part (a), give an example where one bit is flipped in each of the 2 bytes and yet the 1 s complement doesn't change.
In this problem, we consider the delay introduced by the TCP slow-start phase. Consider a client and a Web server directly connected by one link of rate \(R\). Suppose the client wants to retrieve an object whose size is exactly equal to \(15 S\), where \(S\) is the maximum segment size (MSS). Denote the round-trip time between client and server as RTT (assumed to be constant). Ignoring protocol headers, determine the time to retrieve the object (including TCP connection establishment) when a. \(4 S / R>S / R+R T T>2 S / R\) b. \(S / R+R T T>4 S / R\) c. \(S / R>R T T\).
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