Questions and Answers of Computer Network

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
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
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
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
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
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 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
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
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 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
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
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
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.
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
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
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
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
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
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
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.
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
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,
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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'
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
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
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 that in UDPClient.java we replace the line DatagramSocket clientSocket = new DatagramSocket(); With DatagramSocket clientSocket = new DatagramSocket(5432); Will it become necessary to change
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
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
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
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
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
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
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.
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
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
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
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
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
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.
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
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.
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
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
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
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
In the generic SR protocol that we studied in Section 3.4.4, the sender transmits a message as soon as it is available (if it is in the window) without waiting for an acknowledgment. Suppose now that
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
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
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
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
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
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
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
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
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
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
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
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
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

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