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Computer Networking A Top-Down Approach 5th edition James F. Kurose, Keith W. Ross - Solutions

Assume that we know the bottleneck link along the path from the server to the client is the first link with rate R, bits/sec. Suppose we send a pair of packets back to back from the server to the client, and there is no other traffic on this path. Assume each packet of size L bits, and both links
Suppose you would like to urgently deliver 40 terabytes data from Boston to Los Angeles. You have available a 100 Mbps dedicated link for data transfer. Would you prefer to transmit the data via this link or instead use FedEx overnight delivery? Explain.
Suppose two hosts, A and B, are separated by 20,000 kilometers and are connected by a direct link of R = 2 Mbps. Suppose the propagation speed over the link is 2.5.108 meters/sec. a. Calculate the bandwidth-delay product, R • dprop b. Consider sending a file of 800,000 bits from Host A to Host
Referring to problem P24, suppose we can modify R. For what value of R is the width of a bit as long as the length of the link? Problem 24 Suppose two hosts, A and B, are separated by 20,000 kilometers and are connected by a direct link of R = 2 Mbps. Suppose the propagation speed over the link is
Consider problem P24 but now with a link of R =1 Gbps. a. Calculate the bandwidth-delay product, R • dprop b. Consider sending a file of 800,000 bits from Host A to Host B. Suppose the file is sent continuously as one big message. What is the maximum number of bits that will be in the link at any
Refer again to problem P24. a. How long does it take to send the file, assuming it is sent continuously? b. Suppose now the file is broken up into 20 packets with each packet containing 40,000 bits. Suppose that each packet is acknowledged by the receiver and the transmission time of an
Suppose there is a 10 Mbps microwave link between a geostationary satellite and its base station on Earth. Every minute the satellite takes a digital photo and sends it to the base station. Assume a propagation speed of 2.4•108 meters/sec. a. What is the propagation delay of the link? b. What is
Consider the airline travel analogy in our discussion of layering in Section 1.5, and the addition of headers to protocol data units as they flow down the protocol stack. Is there an equivalent notion of header information that is added to passengers and baggage as they move down the airline
Consider the circuit-switched network in Figure 1.12. Recall that there are n circuits on each link. a. What is the maximum number of simultaneous connections that can be in progress at anyone time in this network? b. Suppose that all connections are between the switch in the upper-left-hand corner
In modern packet-switched networks, the source host segments long, application-layer messages (for example, an image or a music file) into smaller packets and sends the packets into the network. The receiver then reassembles the packets back into the original message. We refer to this process as
Consider sending a large file of F bits from Host A to Host B. There are three links (and two switches) between A and B, and the links are uncongested (that is, no queuing delays). Host A segments the file into segments of S bits each and adds 80 bits of header to each segment, forming packets of L
Review the car-caravan analogy in Section lA. Assume a propagation speed of 100 kmJhour. a. Suppose the caravan travels 150 km, beginning in front of one tollbooth, passing through a second tollbooth, and finishing just after a third tollbooth. What is the end-to-end delay? b. Repeat (a), now
This elementary problem begins to explore propagation delay and transmission delay, two central concepts in data networking. Consider two hosts, A and B, connected by a single link of rate R bps. Suppose that the two hosts are separated by m meters, and suppose the propagation speed along the link
In this problem we consider sending real-time voice from Host A to Host B over a packet-switched network (VoIP). Host A converts analog voice to a digital 64 kbps bit stream on the fly. Host A then groups the bits into 56-byte packets. There is one link between Host A and B; its transmission rate
Suppose users share a 3 Mbps link. Also suppose each user requires I50 kbps when transmitting, but each user transmits only 10 percent of the time. (See the discussion of statistical multiplexing in Section 1.3.) a. When circuit switching is used, how many users can be supported? b. For the
Consider the discussion in Section 1.3 of statistical multiplexing in which an example is provided with a I Mbps link. Users are generating data at a rate of 100 kbps when busy, but are busy generating data only with probability p =0.1. Suppose that the I Mbps link is replaced by a I Gbps link. a.
Consider a packet of length L which begins at end system A and travels over three links to a destination end system. These three links are connected by two packet switches. Let dp, sp and Ri denote the length, propagation speed, and the transmission rate of link i, for i = I, 2, 3. The packet
Consider an e-commerce site that wants to keep a purchase record for each of its customers. Describe how this can be done with cookies.
Describe how Web caching can reduce the delay in receiving a requested object. Will Web caching reduce the delay for all objects requested by a user or for only some of the objects? Why?
Telnet into a Web server and send a multiline request message. Include in the request message the If-modified-since: header line to force a response message with the 304 Not Modified status code.
Why is it said that FTP sends control information "out-of-band"?
Suppose Alice, with a Web-based e-mail account (such as Hotmail or gmail), sends a message to Bob, who accesses his mail from his mail server using POP3. Discuss how the message gets from Alice's host to Bob's host. Be sure to list the series of application-layer protocols that are used to move the
Print out the header of an e-mail message you have recently received. How many Received: header lines are there? Analyze each of the header lines in the message.
From a user's perspective, what is the difference between the download-and delete mode and the download-and-keep mode in POP3?
Is it possible for an organization's Web server and mail server to have exactly the same alias for a hostname (for example, foo. com)? What would be the type for the RR that contains the host name of the mail server?
What is the difference between network architecture and application architecture?
Consider a new peer Alice that joins BitTorrent without possessing any chunks . Without any chunks, she cannot become a top-four uploader for any of the other peers, since she has nothing to upload. How then will Alice get her first chunk?
What is an overlay network? Does it include routers? What are the edges in the overlay network? How is the query-flooding overlay network created and maintained?
In what way is instant messaging with a centralized index a hybrid of c1ientserver and P2P architectures?
Consider a DHT with a mesh overlay topology (that is, every peer tracks all peers in the system). What are the advantages and disadvantages of such a design? What are the advantages and disadvantages of a circular DHT (with no shortcuts)?
List at least four different applications that are naturally suitable for P2P architecture.
The UDP server described in Section 2.8 needed only one socket, whereas the TCP server described in Section 2.7 needed two sockets. Why? If the TCP server were to Support n simultaneous connections, each from a different client host, how many sockets would the TCP server need?
For the client-server application over TCP described in Section 2.7, why must the server program be executed before the client program? For the clientserver application over UDP described in Section 2.8, why may the client program be executed before the server program?
For a P2P file-sharing application, do you agree with the statement, "There is no notion of client and server sides of a communication session"? Why or why not?
Suppose you wanted to do a transaction from a remote client to a server as fast as possible. Would you use UDP or TCP? Why?
List the four broad classes of services that a transport protocol can provide. For each of the service classes, indicate if either UDP or TCP (or both) provides such a service.
Recall that TCP can be enhanced with SSL to provide process-to-process security services, including encryption. Does SSL operate at the transport layer or the application layer? If the application developer wants TCP to be enhanced with SSL, what does the developer have to do?
True or false? a. A user requests a Web page that consists of some text and three images. For this page, the client will send one request message and receive four response messages. b. Two distinct Web pages (for example, www.mit.edu/research.html and www . mit . edu / students. html) can be sent
Consider a short, l0-meter link, over which a sender can transmit at a rate of 150 bits/sec in both directions. Suppose that packets containing data are 100,000 bits long, and packets containing only control (e.g., ACK or handshaking) are 200 bits long. Assume that N parallel connections each get
Consider the scenario introduced in the previous problem. Now suppose that the link is shared by Bob with four other users. Bob uses parallel instances of non-persistent HTTP, and the other four users use non-persistent HTTP without parallel downloads. a. Do Bob's parallel connections help him get
Write a simple TCP program for a server that accepts lines of input from a client and prints the lines onto the server's standard output. (You can do this by modifying the TCPServer java program in the text.) Compile and execute your program. On any other machine that contains a Web browser, set
What is the difference between MAIL FROM: in SMTP and From: in the mail message itself?
How does SMTP mark the end of a message body? How about HTTP? Can HTTP use the same method as SMTP to mark the end of a message body? Explain.
Read RFC 5321 for SMTP. What does MTA stand for? Consider the following received spam email (modified from a real spam email). Assuming only the originator of this spam email is malacious and all other hosts are honest, identify the malacious host that has generated this spam email. From - Fri Nov
Read the POP3 RFC, RFC 1939. What is the purpose of the UIDL POP3 command?
Consider accessing your e-mail with POP3. a. Suppose you have configured your POP mail client to operate in the download-and-deIete mode. Complete the following transaction: c: list S: 1 498 S: 2 912 S: • C: retr 1 S: blah blah • • • S: •......... blah S: • ? ? b. Suppose you have
a. What is a who is database? b. Use various who is databases on the Internet to obtain the names of two DNS servers. Indicate which who is databases you used. c. Use nslookup on your local host to send DNS queries to three DNS servers: your local DNS server and the two DNS servers you found in
In this problem, we use the useful tool dig tool available on Unix and Linux hosts to explore the hierarchy of DNS servers. Recall that in Figure 2.21 , a DNS server higher in the DNS hierarchy delegates a DNS query to a DNS server lower in the hierarchy, by sending back to the DNS client the name
Read RFC 959 for FTP. List all of the client commands that are supported by the RFC.
Suppose you can access the caches in the local DNS servers of your department. Can you propose a way to roughly determine the Web servers (outside your department) that are most popular among the users in your department? Explain.
Suppose that your department has a local DNS server for all computers in the department. You are an ordinary user (i.e ., not a network/system administrator). Can you come up a way to determine if an external Web site was very likely accessed from a computer in your department a couple of seconds
Consider distributing a file of F = 15 Gbits to N peers. The server has an upload rate of μs, = 30 Mbps, and each peer has a download rate of di =2 Mbps and an upload rate of μ. For N =10, 100, and 1,000 and u = 300 Kbps, 700 Kbps, and 2 Mbps, prepare a chart giving the minimum distribution time
Consider distributing a file of F bits to N peers using a client-server architecture. Assume a fluid model where the server can simultaneously transmit to multiple peers, transmitting to each peer at different rates, as long as the combined rate does not exceed μs. a. Suppose that us/N ≤ dmin.
Consider distrihuting a file of F bits to N peers using a P2P architecture. Assume a fluid model. For simplicity assume that dmin is very large, so that peer download bandwidth is never a bottleneck. a. Suppose that μs ≤ (us + μ1 + ... + uN)/N. Specify a distribution scheme that has a
Suppose Bob joins a BitTorrent torrent, but he does not want to upload any data to any other peers (so called free-riding). a. Bob claims that he can receive a complete copy of the file that is shared by the swarm. Is Bob's claim possible? Why or why not? b. Bob further claims that he can further
In this problem. we are interested in finding out the efficiency of a BitTorrent- like P2P file sharing system . Consider two peers Bob and Alice. They join a torrent with M peers in total (including Bob and Alice) that are sharing a file consisting of N chunks. Assume that at a particular time t,
In the circular DHT example in Section 2.6.2, suppose that peer 3 learns that peer 5 has left. How does peer 3 update its successor state information? Which peer is now its first successor? Its second successor?
In the circular DHT example in Section 2.6.2, suppose that a new peer 6 wants to join the DHT and peer 6 initially only knows peer I5's IP address. What steps are taken?
Consider a circular DHT with node and key identifiers in the range [0,63]. Suppose there are eight peers with identifiers 0, 8, 16, 24, 32,40,48, and 56.a. Suppose each peer can have one shortcut peer. For each of the eight peers, determine its shortcut peer so that the number of messages sent for
Because an integer in [0, 2/n - I] can be expressed as an n-bit binary number in a DHT, each key can be expressed as k =(k0' k1 . .. , k0_1,), and each peer idenn tifier can be expressed P =(p0' Pl' ... ,Pn-1 ,). Let's now define the XOR distance between a key k and peer P asDescribe how this
Consider a generalized version of the scheme described in the previous problem. Instead of using binary numbers, we now treat key and peer identifiers as base-b numbers where b > 2, and then use the metric in the previous problem to design a DHT (with 2 replace with b). Compare this DHT based on
As DHTs are overlay networks, they may not necessarily match the underlay physical network well in the sense that two neighboring peers might be physically very far away; for example, one peer could be in Asia and its neighbor could be in North America. If we randomly and uniformly assign
Install and compile the Java programs TCPClient and UDPClient on one host and TCPServer and UDPServer on another host. a. Suppose you run TCPClient before yon run TCPServer. What happens? Why? b. Suppose you run UDPClient before you nm UDPServer. What happens·) Why? c. What happens if you use
Suppose that in UDPClient.java we replace the line DatagramSocket clientSocket = new DatagramSocket(); With DatagramSocket clientSocket = new DatagramSocket(5432); Will it become necessary to change UDPServer.java? What are the port numbers for the sockets in UDPClient and UDPServer? What were they
Consider the foll owing string of ASCll characters that were captured by Wireshark when the browser sent an HTTP GET message (i .e., this is the actual content of an HTTPGET message). The characters are carriage return and line-feed characters (that is, the italized character string in the text
The text below shows the reply sent from the server in response to the HTTP GET message in the question above. Answer the following questions, indicating where in the message below you find the answer. HTTP/1.1 200 OKDate: Tue, 07 Mar 2008 12:39:45GMTServer: Apache/2.0.52 (Fedora) Last-Modified:
Obtain the HTTP/l.l specification (RFC 2616). Answer the following questions: a. Explain the mechanism used for signaling between the client and server to indicate that a persistent connection is being closed. Can the client, the server, or both signal the close of a connection? b. What encryption
Suppose within your Web browser you click on a link to obtain a Web page. The IP address for the associated URL is not cached in your local host, so a DNS lookup is necessary to obtain the IP address. Suppose that n DNS servers are visited before y ur host receives the IP address from DNS; the
Referring to Problem P7, suppose the HTML file references eight very small objects on the same server. Neglecting transmission times, how much time elapses with a. Non-persistent HTTP with no parallel TCP connections? b. Non-persistent HTTP with the browser configured for 5 parallel connections? c.
Consider Figure 2. 12, for which there is an institutional network connected to the Internet. Suppose that the average object size is 850,000 bits and that the average request rate from the institution's browsers to the origin servers is 16 requests per second. Also suppose that the amount of time
Suppose the network layer provides the following service. The network layer in the source host accepts a segment of maximum size l,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
In our rdt protocols, why did we need to introduce timers?
Suppose that the roundtrip delay between sender and receiver is constant and known to the sender. Would a timer still be necessary in protocol rdt 3.0, assuming that packets can be lost? Explain.
True or false? a. Host A is sending Host B a large file over a TCP connection. Assume Host B has no data to send Host A. Host B will not send acknowledgments to Host A because Host B cannot piggyback the acknowledgments on data. b. The size of the TCP rwnd never changes throughout the duration of
Consider the Telnet example discussed in Section 3.5. A few seconds after the user types the letter 'C,' the user types the letter 'R.' After typing the letter 'R,' how many segments are sent, and what is put in the sequence number and acknowledgment fields of the segments?
Consider a planet where everyone belongs to a family of six, every family lives in its own house, each house has a unique address, and each person in a given house has a unique name. Suppose this planet has a mail service that delivers letters from source house to destination house. The mail
Describe why an application developer might choose to run an application over UDP rather than TCP.
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
Suppose that a Web server runs in Host C on port 80. Suppose this Web server uses persistent connections, and is currently receiving requests from two different Hosts, A and B. Are all of the requests being sent through the same socket at Host C? If they are being passed through different sockets,
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 A to S. b. The segments sent from B to S. c. The segments sent from S to
Consider a channel that can lose packets but has a maximum delay that is known. Modify protocol rdt2. 1 to include sender timeout and retransmit. Informally argue why your protocol can communicate correctly over this channel.
The sender side of rdt3 • 0 simply ignores (that is, takes no action on) all received packets that are either in error or have the wrong value in the acknum field of an acknowledgment packet. Suppose that in such circumstances, rdt3. 0 were simply to retransmit the current data packet. Would the
Consider the rdt 3.0 protocol. Draw a diagram showing that if the network connection between the sender and receiver can reorder messages (that is, that two messages propagating in the medium between the sender and receiver can be reordered), then the alternating-bit protocol will not work
Consider a reliable data transfer protocol that uses only negative acknowledgments. Suppose the sender sends data only infrequently. Would a NAK-only protocol be preferable to a protocol that uses ACKs? Why? Now suppose the sender has a lot of data to send and the end-to-end connection experiences
Consider the cross-country example shown in Figure 3. 17. How big would the window size have to be for the channel utilization to be greater than 95 percent? Suppose that the size of a packet is 1,500 bytes, including both header fields and data.
Suppose an application uses rdt 3.0 as its transport layer protocol. As the stop-and-wait protocol has very low channel utilization (shown in the cross country example), the designers of this application let the receiver keep sending back a number (more than two) of alternating ACK 0 and ACK I even
Consider a scenario in which Host A wants to simultaneously send packets III Hosts Band C. A is connected to Band C via a broadcast channel-a packet sent by A is carried by the channel to both Band C. Suppose that the broadcast channel connecting A, B, and C can independently lose and corrupt
Consider a scenario in which Host A and Host B want to send messages to Host C. Hosts A and C are connected by a channel that can lose and corrupt (but not reorder) messages. Hosts Band C are connected by another channel (independent of the channel connecting A and C) with the same properties. The
Consider the GBN protocol with a sender window size of 3 and a sequence number range of 1,024. Suppose that at time t. the next in-order packet that the receiver is expecting has a sequence number of k. Assume that the medium does not reorder messages. Answer the following questions : a. What are
Consider Figure 3.5. What are the source and destination port values in the segments flowing from the server back to the clients' processes? What are the IP addresses in the network-layer data grams carrying the transport - layer segments?
Suppose we have two network entities, A and B. B has a supply of data messages that will be sent to A according to the following conventions. When A gets a request from the layer above to get the next data (D) message from B, A must send a request (R) message to B on the A-to-B channel. Only when B
Consider the GBN and SR protocols. Suppose the sequence number space is of size k. What is the largest allowable sender window that will avoid the occurrence of problems such as that in Figure 3.27 for each of these protocols?
Answer true or false se to the following questions and briefly justify your answer: a. With the SR protocol, it is possible for the sender to receive an ACK for a packet that falls outside of its current window. b. With GBN, it is possible for the sender to receive an ACK for a packet that falls
We have said that an application may choose UDP for a transport protocol because UDP offers finer application control (than TCP) of what data is sent in a segment and when. a. Why does an application have more control of what data is sent in a segment? b. Why does an application have more control
Consider transfelTing an enormous file of L bytes from Host A to Host B. Assume an MSS of 536 bytes. a. What is the maximum value of L such that TCP sequence numbers are not exhausted? Recall that the TCP sequence number field has 4 bytes. b. For the L you obtain in (a), find how long it takes to
Host A and B are communicating over a TCP connection, and Host B has already received from A all bytes up through byte 126. Suppose Host A then sends two segments to Host B back-to-back. The first and second segments contain 70 and 50 bytes of data, respectively. In the first segment, the sequence
Host A and B are directly connected with a 100 Mbps link. There is one TCP connection between the two hosts, and Host A is sending to Host B an enormous file over this connection. Host A can send its application data into its TCP socket at a rate as high as 120 Mbps but Host B can read out of its
SYN cookies were discussed in Section 3.5.6. a. Why is it necessary for the server to use a special initial sequence number in the SYNACK? b. Suppose an attacker knows that a target host uses SYN cookies. Can the attacker create half-open or fully open connections by simply sending an ACK packet to
Consider the network shown in Scenario 2 in Section 3.6. 1. Suppose both sending hosts A and B have some fixed timeout values. a. Argue that increasing the size of the finite buffer of the router might possibly decrease the throughput (λout)' b. Now suppose both hosts dynamically adjust their

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