Problem 1
Suppose Client A initiates a Telnet session with Server S. At about the same time, Client B also initiates a Telnet session with Server S. Provide possible source and destination port numbers for a. The segments sent from \(\mathrm{A}\) to \(\mathrm{S}\). b. The segments sent from \(B\) to \(S\). c. The segments sent from \(S\) to \(A\). d. The segments sent from \(S\) to \(B\). e. If \(A\) and \(B\) are different hosts, is it possible that the source port number in the segments from \(\mathrm{A}\) to \(\mathrm{S}\) is the same as that from \(\mathrm{B}\) to \(\mathrm{S}\) ? f. How about if they are the same host?
Problem 1
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?
Problem 3
UDP and TCP use 1s complement for their checksums. Suppose you have the following three 8-bit bytes: \(01010011,01100110,01110100\). What is the \(1 \mathrm{~s}\) complement of the sum of these 8-bit bytes? (Note that although UDP and TCP use 16-bit words in computing the checksum, for this problem you are being asked to consider 8-bit sums.) Show all work. Why is it that UDP takes the Is complement of the sum; that is, why not just use the sum? With the 1 s complement scheme, how does the receiver detect errors? Is it possible that a 1-bit error will go undetected? How about a 2-bit error?
Problem 3
Consider a TCP connection between Host A and Host B. Suppose that the TCP segments traveling from Host A to Host B have source port number \(x\) and destination port number \(y\). What are the source and destination port numbers for the segments traveling from Host B to Host A?
Problem 4
Describe why an application developer might choose to run an application over UDP rather than TCP.
Problem 4
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.
Problem 5
Why is it that voice and video traffic is often sent over TCP rather than UDP in today's Internet? (Hint: The answer we are looking for has nothing to do with TCP's congestion-control mechanism.)
Problem 5
Suppose that the UDP receiver computes the Internet checksum for the received UDP segment and finds that it matches the value carried in the checksum field. Can the receiver be absolutely certain that no bit errors have occurred? Explain.
Problem 6
Is it possible for an application to enjoy reliable data transfer even when the application runs over UDP? If so, how?
Problem 7
Suppose a process in Host C has a UDP socket with port number 6789. Suppose both Host A and Host B each send a UDP segment to Host C with destination port number 6789 . Will both of these segments be directed to the same socket at Host C? If so, how will the process at Host C know that these two segments originated from two different hosts?
