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CP372

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Twisted-pair cooper wire is no longer present in computer networks.

False

With ADSL, each subscriber gets more downstream bandwidth than upstream bandwidth.

True

With SMTP, it is possible to send multiple mail messages over the same TCP connection.

True

01010110 01011000, what is the checksum?

10101001 10100111

What well-known port does DNS use?

What information does the type "A" resource record hold in the DNS database? Check all that apply.

A hostname and an IP address.

Check all of the services below that are provided by the UDP protocol.

A message abstraction, that preserves boundaries between message data sent in different socket send calls at the sender.

"services" view of the Internet

A platform for building network applications A place I go for information entertainment etc

3.3-3 UDP segment length field. Why is the UDP header length field needed?

Because the payload section can be of variable length, and this lets UDP know where the segment ends.

What transport protocol(s) does DNS use: TCP, UDP, or Both?

Both

Physical layer technologies that has the highest transmission rate and lowest bit error rate in practice

Fiber optic cable

Authoratative DNS

Provides authoritative hostname to IP mappings for organization's named hosts

Firewall

Specialized "middleboxes" filtering or blocking traffic, inspecting packet contents inspections

What approach is taken by a CDN to stream content to hundreds of thousands of simultaneous users?

Store/serve multiple copies of videos at multiple geographically distributed sites.

What is the purpose of the HTTP GET message?

The HTTP GET request message is used by a web client to request a web server to send the requested object from the server to the client.

True or False: When computing the Internet checksum for two numbers, a single flipped bit in each of the two numbers will always result in a changed checksum.

False

True or False: On the sending side, the TCP sender will take each application-layer chunk of data written into a TCP socket and send it in a distinct TCP segment. And then on the receiving side, TCP will deliver a segment's payload into the appropriate socket, preserving the application-defined message boundary.

False.

Is it possible for an organization's web server and mail server to have exactly the same alias for a hostname? What would be the type for the RR that contains the hostname of the mail server?

yes, MX

Which of the following physical layer technologies has the highest transmission rate and lowest bit error rate in practice?

Fiber optic cable

Choose one the following two definitions that makes the correct distinction between routing versus forwarding.

Forwarding is the local action of moving arriving packets from router's input link to appropriate router output link, while routing is the global action of determining the source-destination paths taken by packets.

Where is transport-layer functionality primarily implemented?

Transport layer functions are implemented primarily at the hosts at the "edge" of the network.

If a POP3 client does not send the dele command, copies of the messages that the client has retrieved remain in the mail server.

True

In TCP, the acknowledgement number that a host puts in a segment is the sequence number of the next byte the host is expecting from the sender.

True

In a virtual-circuit packet-switched network, a packet switch maintains state information for each connection passing through the router.

True

It is possible for a mail client to send e-mail messages over HTTP rather than SMTP.

True

Layers four and five of the Internet protocol stack are implemented in the end systems but not in the routers in the network core.

True

Network News was present in the Internet before the emergence of the World Wide Web?

True

Skype relay nodes are typically peers under the control of Skype users.

True

The TCP segment has a field in its header for RcvWindow .

True

The network portion of an IP address is the same for all the hosts on the same IP network.

True

True or False: It is possible for two TCP segments with source port 80 to be sent by the sending host to different clients.

True

Internet telephony typically runs over

UDP

Streams are not needed in which of the following programs:

UDPserver

What is meant by transport-layer multiplexing?

Taking data from one socket (one of possibly many sockets), encapsulating a data chuck with header information - thereby creating a transport layer segment - and eventually passing this segment to the network layer.

What is the purpose of a cookie value in the HTTP GET request?

The cookie value itself doesn't mean anything. It is just a value that was returned by a web server to this client during an earlier interaction.

RTT (round trip time)

The length of time it takes for a packet to go from sender to receiver, then back from receiver to sender. RTT is usually measured in milliseconds.

Which of the following statements about pipelining are true? One or more statements may be true.

With a pipelined sender, there may be transmitted packets "in flight" - propagating through the channel - packets that the sender has sent but that the receiver has not yet received. A pipelined sender can have transmitted multiple packets for which the sender has yet to receive an ACK from the receiver.

Can you send multiple DNS questions and get multiple RR answers in one message?

Yes

Does your local DNS server take advantage of caching similar to web requests? Answer with Yes or No

Yes

Transport Layer

handles the delivery of segments, may be reliable or unreliable

Which of the following characteristics below are associated with a TCP socket? Check one or more that apply.

provides reliable, in-order byte-stream transfer (a "pipe"), from client to server socket(AF_INET, SOCK_STREAM) creates this type of socket when contacted, the server will create a new server-side socket to communicate with that client a server can perform an accept() on this type of socket

When a UDP segment arrives to a host, in order to direct the segment to the appropriate socket, the OS uses

the destination port number

SMTP is used to

to transfer messages from one mail server to another

The Date: header in the HTTP response message indicates when the object in the response was last modified.

False

The Internet provides its applications two types of services, a TDM service and a FDM service.

False

This question requires a little bit of background in probability (but we'll try to help you though it in the solutions). Consider the two scenarios below: A circuit-switching scenario in which Ncs users, each requiring a bandwidth of 20 Mbps, must share a link of capacity 150 Mbps. A packet-switching scenario with Nps users sharing a 150 Mbps link, where each user again requires 20 Mbps when transmitting, but only needs to transmit 20 percent of the time. What is the probability that any 4 users (of the total 13 users) are transmitting and the remaining users are not transmitting?

0.15: The probability that 4 specific users of the total 13 users are transmitting and the other 9 users are idle is p4(1-p)9. Thus the probability that any 4 of the 7 users are busy is choose(13, 4) * p4(1-p)9, where choose(13, 4) is the (13, 4) coefficient of the binomial distribution). The numerical value of this probability is 0.15.

What is the sum of 00101100 11101101 and 00000010 11010101?

00101111 11000010

Suppose you want to send the following 2 16-bits numbers: 00101001 01101111 and 10011101 11111111 . What would be the checksum? The spaces are used to make the sequence of bits more readable.

00111000 10010001

Compute the Internet checksum value for these two 16-bit words: 11110101 11010011 and 10110011 01000100

01010110 11100111

Suppose there are two links between a source and a destination. The first link has transmission rate 100 Mbps and the second link has transmission rate 10 Mbps. Assuming that the only traffic in the network comes from the source, what is the throughput for a large file transfer?

10 Mbps

Suppose a packet is 10K bits long, the channel transmission rate connecting a sender and receiver is 10 Mbps, and the round-trip propagation delay is 10 ms. What is the channel utilization of a pipelined protocol with an arbitrarily high level of pipelining for this channel?

1.0

Consider the circuit-switched network shown in the figure below, with circuit switches A, B, C, and D. Suppose there are 14 circuits between A and B, 20 circuits between B and C, 16 circuits between C and D, and 15 circuits between D and A. Suppose that every connection requires 2 consecutive hops, and calls are connected clockwise. For example, a connection can go from A to C, from B to D, from C to A, and from D to B. With these constraints, what is the is the maximum number of connections that can be ongoing in the network at any one time?

30: There can be a maximum of 30 connections. Consider routes A->C and C->A, sum the bottleneck links, consider any leftover capacity that would allow for B->D and D->B connections, and compare that value to the equivalent of B->D and D->B.

Which of the following descriptions below correspond to a "nuts-and-bolts" view of the Internet?

A collection of billions of computing devices, and packet switches interconnected by links. A collection of hardware and software components executing protocols that define the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event. A "network of networks".

What is an HTTP cookie used for?

A cookie is a code used by a server, carried on a client's HTTP request, to access information the server had earlier stored about an earlier interaction with this Web browser. [Think about the distinction between a browser and a person.]

What is meant by a cumulative acknowledgment, ACK(n)?

A cumulative ACK(n) acks all packets with a sequence number up to and including n as being received.

The Web typically sends multiple objects in a Web page within a multipart MIME message.

False

Which of the following actions are associated with end-end congestion control (say versus network-assisted congestion control). Check all that apply.

A router drops a packet at a congested router, which causes the transport-layer sender to infer that there is congestion due to the missing ACK for the lost packet. A sender decreases its sending rate in response to packet loss detected via its transport-layer ACKing. The transport-layer sender decreases its sending rate in response to a measured increase in the RTT. A datagram experiences delay at a congested network router, which is then measured by the sender and used to decrease the sending rate.

Which of the following actions are used in network-assisted congestion control (say versus end-end congestion control) to signal congestion. Check all that apply.

A router sends an ICMP message to a host telling it to slow down its sending rate. A router marks a field in the datagram header at a congested router.

Which of the following pieces of information will appear in a server's application-level HTTP reply message? (Check all that apply.)

A response code A response phrase associated with a response code

The acronym API in this textbook stands for "Advanced Performance Internet".

False

TLD Server

Responsible for a domain (e.g., *.com, *.edu); knows how to contact authoritative name servers.

Types of RR (Resource Records)

A, CNAME, NS, and MX

For BitTorrent, which of the following is true:

Among the chunks that a peer does not have, the peer will request chunks that appear to be relatively rare in the torrent. Although a peer may have 50 concurrent TCP connections with other peers, at any given instant of time it sends data to less than 10 other peers. At a given instant of time, a peer A may upload to a peer B, even if peer B is not sending anything to A.

HTTP is stateless

An HTTP server does not remember anything about what happened during earlier steps in interacting with this HTTP client.

What do we mean when we say "HTTP is stateless"? In answering this question, assume that cookies are not used. Check all answers that apply.

An HTTP server does not remember anything about what happened during earlier steps in interacting with this HTTP client.

Which type of DNS server holds a company's DNS records? Answer with the full name

Authoritative DNS server

Why is the UDP header length field needed?

Because the payload section can be of variable length, and this lets UDP know where the segment ends.

Consider the TCP Telnet scenario below (from Fig. 3.31 in text). Why is it that the receiver sends an ACK that is one larger than the sequence number in the received datagram?

Because the send-to receiver segment carries only one byte of data, and after that segment is received, the next expected byte of data is just the next byte (i.e., has an index that is one larger) in the data stream.

In SR, why does the receiver have to acknowledge packets with sequence numbers that are less than (and to the left of) those in its window, which starts at rcv_base.

Because the sender may not have received an ACK for that packet yet.

When an application uses a UDP socket, what transport services are provided to the application by UDP? Check all that apply.

Best effort service. The service will make a best effort to deliver data to the destination but makes no guarantees that any particular segment of data will actually get there.

Which of the following are advantages of using a web cache? Sselect one or more answers.

Caching uses less bandwidth coming into an institutional network where the client is located, if the cache is also located in that institutional network. Caching generally provides for a faster page load time at the client, if the web cache is in the client's institutional network, because the page is loaded from the nearby cache rather than from the distant server.

Consider the TCP Telnet scenario below (from Fig. 3.36 in text). What timer-related action does the sender take on the receipt of ACK 120?

Cancels any running timers.

True or False: It is possible for two UDP segments to be sent from the same socket with source port 5723 at a server to two different clients.

True

3.1-5 Network-layer functionality. The transport layer sits on top of the network layer, and provides its services using the services provided to it by the network layer. Thus it's important that we know what is meant by the network layer's "best effort" delivery service. True or False:

Correct! The network layer's best effort service doesn't really provide much service at all, does it?

A server can use cookies to determine a user's postal address without the user's consent.

False

Before sending a packet into a datagram network, the source must determine all of the links that packet will traverse between source and destination.

False

Technique of packet switching

Resources are used on demand, not reserved in advance. Data may be queued before being transmitted due to other user's data that's also queueing for transmission. Congestion loss and variable end-end delays are possible with this technique. This technique is used in the Internet.

3.2-1 Transport-layer demultiplexing. What is meant by transport-layer demultiplexing?

Receiving a transport-layer segment from the network layer, extracting the payload (data) and delivering the data to the correct socket.

Local DNS name servers

cache resource records, but discard them after a period of time that is on the order of a few days

Encapsulation

Taking data from the layer above, adding header fields appropriate for this layer, and then placing the data in the payload field of the "packet" for that layer.

network of networks

The Internet is made up of a lot of different networks that are interconnected to each other. The Internet is made up of access networks at the edge, tier-1 networks at the core, and interconnected regional and content provider networks as well.

When we say that the Internet is a "network of networks," we mean? Check all that apply (hint: check two or more).

The Internet is made up of access networks at the edge, tier-1 networks at the core, and interconnected regional and content provider networks as well. The Internet is made up of a lot of different networks that are interconnected to each other.

Over what set of bytes is the checksum field in the UDP header computed over?

The entire UDP segment, except the checksum field itself, and the IP sender and receive address fields

In the SR receiver window (see diagram below, taken from PPT slides and video), why haven't the red packets been delivered yet? Check the one or more reasons below that apply.

There is a packet with a lower sequence number than any of the red packets that has yet to be received, so in-order delivery of data in the red packets up to the application layer is not possible.

What happens when a socket connect() procedure is called/invoked?

This procedure creates a new socket at the client, and connects that socket to the specified server.

True or False: When computing the Internet checksum for two numbers, a single flipped bit (i.e., in just one of the two numbers) will always result in a changed checksum.

True

Suppose now the server sends the following HTTP response message the client:HTTP/1.0 200 OKDate: Wed, 09 Sep 2020 23:46:21 +0000Server: Apache/2.2.3 (CentOS)Last-Modified: Wed, 09 Sep 2020 23:51:41 +0000ETag:17dc6-a5c-bf716880.Content-Length: 418Connection: CloseContent-type: image/htmlWill the web server close the TCP connection after sending this message?

Yes, the server will close this connection because version 1.0 of HTTP is being used, and TCP connections do not stay open persistently.

Consider the minimum time for P2P file distribution, as discussed in Section 2.6. Suppose the upload rate of the server is10 Mbps, the upload rates of all peers is 1 Mbps, and the download rates of all peers is 10 Mbps. Further suppose that there are 100 peers and the file is 100 million bits. The minimum distribution time is

approximately 100 seconds

Protocols

define the format, order of messages sent and received among network entities, and actions taken on message transmission, receipt

Consider the latency model for dynamic congestion windows. Which of the following components contribute to latency

round trip time, RTT the transmission time of the object, O/R the sum of all the stalled times

In this book we refer to a layer-4 PDU as a

segment

Pipelining requires which of the following

transmitting many packets before receiving acknowledgements unique sequence numbers for each in-transit packet sender-side buffering of unacknowledged packets

Consider the latency model for dynamic congestion windows. The value P is equal to

the number of times the server stalls

In DASH (Dynamic, Adaptive Streaming over HTTP), a server divides a video file into chunks that ... (pick best completion from below)

... are stored, each encoded at multiple rates (video quality). The client plays the video chunk-by-chunk, with each chunk requested at encoding rate that fits the available bandwidth at the time.

Suppose a packet is L = 1500 bytes long (one byte = 8 bits), and link transmits at R = 1 Gbps (i.e., a link can transmit bits 1,000,000,000 bits per second). What is the transmission delay for this packet?

.000012 secs

Suppose a packet is L = 1200 bytes long (one byte = 8 bits), and link transmits at R = 100 Mbps (i.e., a link can transmit bits 100,000,000 bits per second). What is the transmission delay for this packet?

.000096 secs

Consider the network shown in the figure below, with three links, each with the specified transmission rate and link length. Assume the length of a packet is 8000 bits. The speed of light propagation delay on each link is 3x10^8 m/sec. What is the propagation delay at (along) link 2?

.0033 secs

Suppose a packet is 10K bits long, the channel transmission rate connecting a sender and receiver is 10 Mbps, and the round-trip propagation delay is 10 ms. What is the maximum channel utilization of a stop-and-wait protocol for this channel?

This question requires a little bit of background in probability (but we'll try to help you though it in the solutions). Consider the two scenarios below: A circuit-switching scenario in which Ncs users, each requiring a bandwidth of 20 Mbps, must share a link of capacity 150 Mbps. A packet-switching scenario with Nps users sharing a 150 Mbps link, where each user again requires 20 Mbps when transmitting, but only needs to transmit 20 percent of the time. Suppose packet switching is used. What is the probability that a given (specific) user is transmitting, and the remaining users are not transmitting?

0.014 The probability that a given (specific) user is busy transmitting, which we'll denote p, is just the fraction of time it is transmitting, i.e. 0.2. The probability that one specific other user is not busy is (1-p), and so the probability that all of the other Nps-1 users are not transmitting is (1-p)Nps-1. Thus the probability that one specific user is transmitting and the remaining users are not transmitting is p*(1-p)Nps-1, which has the numerical value of 0.014.

This question requires a little bit of background in probability (but we'll try to help you though it in the solutions). Consider the two scenarios below: A circuit-switching scenario in which Ncs users, each requiring a bandwidth of 20 Mbps, must share a link of capacity 150 Mbps. A packet-switching scenario with Nps users sharing a 150 Mbps link, where each user again requires 20 Mbps when transmitting, but only needs to transmit 20 percent of the time. When one user is transmitting, what fraction of the link capacity will be used by this user? Write your answer as a decimal.

0.13: This user will be transmitting at a rate of 20 Mbps over the 150 Mbps link, using a fraction 0.13 of the link's capacity when busy.

Consider the queuing delay in a router buffer, where the packet experiences a delay as it waits to be transmitted onto the link. The length of the queuing delay of a specific packet will depend on the number of earlier-arriving packets that are queued and waiting for transmission onto the link. If the queue is empty and no other packet is currently being transmitted, then our packet's queuing delay will be zero. On the other hand, if the traffic is heavy and many other packets are also waiting to be transmitted, the queuing delay will be long. Assume a constant transmission rate of R = 1100000 bps, a constant packet-length L = 3700 bits, and a is the average rate of packets/second. Traffic intensity I = La/R, and the queuing delay is calculated as I(L/R)(1 - I) for I < 1. Assuming the router's buffer is infinite, the queuing delay is 0.5617 ms, and 1731 packets arrive. How many packets will be in the buffer 1 second later?

0: Packets left in buffer = a - floor(1000/delay) = 1731 - floor(1000/0.5617) = 0 packets.

Consider the figure below, in which a single router is transmitting packets, each of length L bits, over a single link with transmission rate R Mbps to another router at the other end of the link. Suppose that the packet length isL= 16000 bits, and that the link transmission rate along the link to router on the right isR= 1000 Mbps. What is the transmission delay?

1.60E-5: The transmission delay = L/R = 16000 bits / 1000000000 bps = 1.60E-5 seconds

In the following 7 problems, we are sending a 30 Mbit MP3 file from a source host to a destination host. All links in the path between source and destination have a transmission rate of 10 Mbps. Assume that the propagation speed is 2 * 108 meters/sec, and the distance between source and destination is 10,000 km. Initially suppose there is only one link between source and destination. Also suppose that the entire MP3 file is sent as one packet. The transmission delay is:

3 seconds

3.05 seconds

500,000 bits

Suppose a Web server has five ongoing connections that use TCP receiver port 80, and assume there are no other TCP connections (open or being opened or closed) at that server. How many TCP sockets are in use at this server?

In the following 7 problems, we are sending a 30 Mbit MP3 file from a source host to a destination host. All links in the path between source and destination have a transmission rate of 10 Mbps. Assume that the propagation speed is 2 * 108 meters/sec, and the distance between source and destination is 10,000 km. Now suppose there are two links between source and destination, with one router connecting the two links. Each link is 5,000 km long. Again suppose the MP3 file is sent as one packet. Suppose there is no congestion, so that the packet is transmitted onto the second link as soon as the router receives the entire packet. The end-to-end delay is

6.05 seconds

Consider the circuit-switched network shown in the figure below, with four circuit switches A, B, C, and D. Suppose there are 20 circuits between A and B, 19 circuits between B and C, 15 circuits between C and D, and 16 circuits between D and A. What is the maximum number of connections that can be ongoing in the network at any one time?

70 = (20 + 19 + 15 + 16)

Use the pulldown menu to match a congestion control approach to how the sender detects congestion. _____ The sender infers segment loss from the absence of an ACK from the receiver. _____ Bits are set at a congested router in a sender-to-receiver datagram, and bits are in the returned to the sender in a receiver-to sender ACK, to indicate congestion to the sender. _____ The sender measures RTTs and uses the current RTT measurement to infer the level of congestion. A. end-end B. network-assisted C. delay-based

A B C

HTTP reply

A response phrase associated with a response code A response code

Consider the rdt2.1 sender and receiver FSMs shown below, with labeled transitions S1 through S6 at the sender, and transitions R1 through R6 at the receiver. The sender and receiver start in the "Wait for call 0 from above" and "Wait for 0 from below" states, respectively. Suppose that the first packet from the sender is correctly received at the receiver but that ACK message sent from receiver-to-sender is corrupted; all other messages (before or after that ACK) are transmitted error-free. Match the unlabeled transitions x1, x2, x3, x4, x5 in the time-ordered sequence of transitions below (interleaved sender and receiver transitions) that will occur following the initial S1 transition, which is followed by a corrupted ACK transmission, that will result in a message being delivered at the receiver, with the sender and receiver returning to their initial states. Note that some transitions are already provided below.S1, x1 (ACK corrupted), x2, x3, x4, S4, x5, S6.

C A B D E

Nmap is often used to:

Do port scans over a range of ports at a target host.

Ethernet is a popular residential Internet access technology.

False

Which of the following are changes between HTTP 1.1 and HTTP/2? Note: select one or more answers.

HTTP/2 allows objects in a persistent connection to be sent in a client-specified priority order. HTTP/2 allows a large object to be broken down into smaller pieces, and the transmission of those pieces to be interleaved with transmission other smaller objects, thus preventing a large object from forcing many smaller objects to wait their turn for transmission.

DNS Root

Highest level of the DNS hierarchy, knows how to reach servers responsible for a given domain (e.g., *.com, *.edu).

Over a TCP connection, suppose host A sends two segments to host B, host B sends an acknowledgement for each segment, the first acknowledgement is lost, but the second acknowledgement arrives before the timer for the first segment expires.

Host A will retransmit neither segments

For the client-server application over UDP, does the server program have to be executed before the client program?

Consider the circuit-switched network shown in the figure below, with circuit switches A, B, C, and D. Suppose there are 14 circuits between A and B, 20 circuits between B and C, 16 circuits between C and D, and 15 circuits between D and A. Suppose that these maximum number of connections are all ongoing. What happens when another call connection request arrives to the network, will it be accepted? Answer Yes or No

No, it will be blocked because there are no free circuits.

Which of the characteristics below are associated with the technique of circuit switching?

This technique was the basis for the telephone call switching during the 20th century and into the beginning of this current century. Reserves resources needed for a call from source to destination. Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM) are two approaches for implementing this technique.

Suppose a DNS resource record has Type=MX. Then

Value is the hostname of the mail server that has the alias hostname Name.

Suppose a client is sending an HTTP GET message to a web server, gaia.cs.umass.edu. Suppose the client-to-server HTTP GET message is the following: GET /kurose_ross_sandbox/interactive/quotation2.htm HTTP/1.1Host: gaia.cs.umass.eduAccept: text/plain, text/html, text/xml, image/jpeg, image/gif, audio/mpeg, audio/mp4, video/wmv, video/mp4,Accept-Language: en-us, en-gb;q=0.1, en;q=0.7, fr, fr-ch, da, de, fiIf-Modified-Since: Wed, 09 Sep 2020 16:06:01 -0700User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/535.11 (KHTML, like Gecko) Chrome/17.0.963.56 Safari/535.11Does the client have a cached copy of the object being requested?

Yes, because this is a conditional GET.

In a router, queuing can occur

in the input ports in the output ports in an interconnection network

The stop-and-wait protocol is highly inefficient when

when there is a large distance between source and destination and the transmission rate is high

What is a protocol?

- One person asking, and getting, the time to/from another person. - Two people introducing themselves to each other. - A student raising her/his hand to ask a really insightful question, followed by the teaching acknowledging the student, listening carefully to the question, and responding with a clear, insightful answer. And then thanking the student for the question, since teachers love to get questions.

What is the maximum throughput achievable between sender and receiver in the scenario shown below? sender --1.5--> X --10--> receiver

1.5 Mbps

Consider the network shown in the figure below, with three links, each with the specified transmission rate and link length. Assume the length of a packet is 8000 bits. What is the transmission delay at link 2?

8 x 10^(-5) secs

Nuts and Bolts

A collection of billions of computing devices, and packet switches interconnected by links. A "network of networks". A collection of hardware and software components executing protocols that define the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event.

3.1-3 Transport layer services using TCP. Check all of the services below that are provided by the TCP protocol.

A flow-control service that ensures that a sender will not send at such a high rate so as to overflow receiving host buffers. Reliable data delivery. A congestion control service to ensure that multiple senders do not overload network links. In-order data delivery A byte stream abstraction, that does not preserve boundaries between message data sent in different socket send calls at the sender.

Conditional HTTP GET request message

Allows a server to only send the requested object to the client if this object has changed since the server last sent this object to the client

Consider the figure below, showing a link-layer frame heading from a host to a router. There are three header fields shown. Match the name of a header with a header label shown in the figure. _____ Header H1 _____ Header H2 _____ Header H3 A. Transport layer B. Link layer C. Network Layer

B C A

Consider a scenario in which 5 users are being multiplexed over a channel of 10 Mbps. Under the various scenarios below, match the scenario to whether circuit switching or packet switching is better. _____ Each user generates traffic at an average rate of 2.1 Mbps, generating traffic at a rate of 15 Mbps when transmitting _____ Each user generates traffic at an average rate of 2 Mbps, generating traffic at a rate of 2 Mbps when transmitting _____ Each user generates traffic at an average rate of 0.21 Mbps, generating traffic at a rate of 15 Mbps when transmitting A. Circuit switching B. Packet switching C. Neither works well in this overload scenario

C A B

ADSL bandwidth is shared.

False

Encryption

Provides confidentiality by encoding contents

Authentication

Proving you are who you say you are.

Which of the definitions below describe what is meant by the term "encapsulation"?

Taking data from the layer above, adding header fields appropriate for this layer, and then placing the data in the payload field of the "packet" for that layer.

True or False: with TCP's flow control mechanism, where the receiver tells the sender how much free buffer space it has (and the sender always limits the amount of outstanding, unACKed, in-flight data to less than this amount), it is not possible for the sender to send more data than the receiver has room to buffer.

True

With ADSL, each subscriber gets more downstream bandwidth than upstream bandwidth

True

Digital Signatures

Used to detect tampering/changing of message contents, and to identify the originator of a message.

Link Layer

passes frames from one node to another across some medium

Consider the figure below, which shows the application-to-application throughput achieved when two senders are competing at a shared bottleneck link. Suppose that when the overall arrival rate, lambdain' (for each sender) is close to R/2, the throughput to the application layer (at each receiver), lambdaout, is equal to 0.8 * lambdain'. What fraction of the packets transmitted at the sender are retransmissions?

.20

In a TCP connection, suppose that loss occurs whenever the window size of a connection hits W. Also suppose that the RTT is constant. Then the approximate average transmission rate is

.75 W*MSS/RTT

0.0012 The probability that more than 7 users of the total 13 users are transmitting is Σ i=8,13 choose(13, i) * pi(1-p)13 - i. The numerical value of this probability is 0.0012. Note that 7 is the maximum number of users that can be supported using circuit switching. With packet switching, nearly twice as many users (13) are supported with a small probability that more than 7 of these packet-switching users are busy at the same time.

Consider TCP use of an exponentially weighted moving average (EWMA) to compute the nth value of the estimated RTT:EstimatedRTTn = (1- a)*EstimatedRTTn-1 + a*SampleRTTnTrue or False: with this EWMA algorithm the value of EstimatedRTTn has no dependence on the earlier sample, SampleRTTn-1

False

Consider an HTTP Web server using persistent connections. Suppose the server spawns a separate process for each client that connects to the server. Then each of these spawned processes will have different server port numbers.

False

DNS is an application level protocol that runs over TCP

False

Host A is sending host B a large file over a TCP connection. Assume host B has no data to send A. Host B will not send acknowledgements to host A because host B cannot piggyback the acknowledgements on data.

False

If an HTTP request message uses the Accept-language: fr header, and the server only has an English version of the object, then the server will return the 404 Document Not Found error message.

False

In BitTorrent, a peer's instantaneous download rate can never exceed its instantaneous upload rate.

False

In the connection flooding attack, the attacker sends a deluge of packets to the targeted host, clogging the target's access link with packets.

False

Question 3:The size of the TCP RcvWindow never changes throughout the duration of the connection.

False

Suppose 10 connections traverse the same link of rate 1 Gbps. Suppose that the client access links all have rate 5 Mbps. Then the maximum throughput for each connection is 100 Mbps.

False

Suppose host A is sending a large file to host B over a TCP connection. If the sequence number for a segment of this connection is m, then the sequence number for the subsequent segment will necessarily be m+1.

False

Suppose host A sends host B one segment with sequence number 38 and 4 bytes of data. Then in this same segment the acknowledgement number is necessarily 42.

False

Suppose that host A wants to send data over TCP to host B, and host B wants to send data to host A over TCP. Two separate TCP connections - one for each direction - are needed.

False

Suppose that the last SampleRTT in a TCP connection is equal to 1 sec. Then Timeout for the connection will necessarily be set to a value >= 1 sec.

False

The HTTP protocol was invented at Netscape in the early 1990s.

False

The MSS is the maximum size of a TCP segment including headers.

False

True or False: When multiple TCP clients send TCP segments to the same destination port number at a receiving host, those segments (from different senders) will always be directed to the same socket at the receiving host.

False

With POP3 client, user folder information is kept on the mail server.

False

With non-persistent connections between browser and origin server, it is possible for a single TCP segment to carry two distinct HTTP request messages.

False

HTTP/2 versus HTTP/1.1

Which of the following characteristics apply to both HTTP and SMTP? Note: check one or more of the characteristics below.

Has ASCII command/response interaction, status codes. Is able to use a persistent TCP connection to transfer multiple objects.

HTTP and SMTP

Has ASCII command/response interaction, status codes. Is able to use a persistent TCP connection to transfer multiple objects

Consider TCP's Fast Retransmit optimization (see Figure 3.37 from the text, below). Of course, the sender doesn't know for sure that the segment with sequence # 100 is actually lost (it can't see into the channel). Can a sender get three duplicate ACKs for a segment that in fact has not been lost? Which of the following statements are true? Suppose a channel can lose, but will not corrupt, messages.

If the channel cannot reorder messages, a triple duplicate ACK indicates to the sender that a segment loss has happened for sure. Actually (again assuming the channel cannot corrupt or reorder messages), even a single duplicate ACK would indicate that a segment loss has happed for sure. If the channel can reorder messages, a triple duplicate ACK can occur even though a message is not lost; since it's possible that a message has just been reordered and has not yet arrived when the three duplicate ACKs were generated.

3.3-4 Internet checksum and UDP. Over what set of bytes is the checksum field in the UDP header computed over?

The entire UDP segment, except the checksum field itself, and the IP sender and receive address fields

Check all of the services below that are provided by the TCP protocol.

Reliable data delivery. In-order data delivery A byte stream abstraction, that does not preserve boundaries between message data sent in different socket send calls at the sender. A congestion control service to ensure that multiple senders do not overload network links. A flow-control service that ensures that a sender will not send at such a high rate so as to overflow receiving host buffers.

Local DNS

Replies to DNS query by local host, by contacting other DNS servers to answer the query.

HTTP GET

The HTTP GET request message is used by a web client to request a web server to send the requested object from the server to the client.

How does the networked application running on a server know the client IP address and the port number to reply to in response to a received datagram?

The application code at the server determines client IP address and port # from the initial segment sent by client, and must explicitly specify these values when sending into a socket back to that client.

Check all of the phrases below that state a true property of a local DNS server.

The local DNS server can decrease the name-to-IP-address resolution time experienced by a querying local host over the case when a DNS is resolved via querying into the DNS hierarchy. The local DNS server record for a remote host is sometimes different from that of the authoritative server for that host.

This question requires a little bit of background in probability (but we'll try to help you though it in the solutions). Consider the two scenarios below: A circuit-switching scenario in which Ncs users, each requiring a bandwidth of 20 Mbps, must share a link of capacity 150 Mbps. A packet-switching scenario with Nps users sharing a 150 Mbps link, where each user again requires 20 Mbps when transmitting, but only needs to transmit 20 percent of the time. Suppose packet switching is used. What is the probability that one user (any one among the 13 users) is transmitting, and the remaining users are not transmitting?

The probability that exactly one (any one) of the Nps users is transmitting is Nps times the probability that a given specific user is transmitting and the remaining users are not transmitting. The answer is thus Nps * p * (1-p)Nps-1, which has the numerical value of 0.18.

Suppose that the local DNS server caches all information coming in from all root, TLD, and authoritative DNS servers for 20 time units. (Thus, for example, when a root server returns the name and address of a TLD server for .com, the cache remembers that this is the TLD server to use to resolve a .com name). Assume also that the local cache is initially empty, that iterative DNS queries are always used, that DNS requests are just for name-to-IP-address translation, that 1 time unit is needed for each server-to-server or host-to-server (one way) request/response, and that there is only one authoritative name server (each) for any .edu or .com domain. Consider the following DNS requests, made by the local host at the given times: t=0, the local host requests that the name gaia.cs.umass.edu be resolved to an IP address. t=1, the local host requests that the name icann.org be resolved to an IP address. t=5, the local host requests that the name cs.umd.edu be resolved to an IP address. (Hint: be careful!) t=10, the local host again requests that the name gaia.cs.umass.edu be resolved to an IP address. t=12, the local host requests that the name cs.mit.edu be resolved to an IP address. t=30, the local host again requests that the name gaia.cs.umass.edu be resolved to an IP address. (Hint: be careful!) Which of the requests require 8 time units to be resolved?

The request at t=0. The request at t=1. The request at t=30.

The request at t=10.

Which of the characteristics below are associated with the technique of packet switching?

This technique is used in the Internet. Data may be queued before being transmitted due to other user's data that's also queueing for transmission. Congestion loss and variable end-end delays are possible with this technique. Resources are used on demand, not reserved in advance.

Suppose the computer in your office has been configured with an IP address, and you move (along with your computer) to an office down the hall. If after the move your computer is connected to the same IP network, then it is not necessary to reconfigure the IP address in your computer.

True

True or False: On the sending side, the UDP sender will take each application-layer chunk of data written into a UDP socket and send it in a distinct UDP datagram. And then on the receiving side, UDP will deliver a segment's payload into the appropriate socket, preserving the application-defined message boundary.

True

True or False: When multiple UDP clients send UDP segments to the same destination port number at a receiving host, those segments (from different senders) will always be directed to the same socket at the receiving host.

True

With a datagram network layer, each packet carries the address of the destination host.

True

True or False: The transport layer provides for host-to-host delivery service?

True.

Consider sending a 1300 byte datagram into a link that has an MTU of 500 bytes.

Three fragments are created with offsets 0, 480 and 960.

Again, suppose a client is sending an HTTP GET request message to a web server, gaia.cs.umass.edu. Suppose the client-to-server HTTP GET message is the following (same as in previous problem): GET /kurose_ross_sandbox/interactive/quotation2.htm HTTP/1.1Host: gaia.cs.umass.eduAccept: text/plain, text/html, text/xml, image/jpeg, image/gif, audio/mpeg, audio/mp4, video/wmv, video/mp4,Accept-Language: en-us, en-gb;q=0.1, en;q=0.7, fr, fr-ch, da, de, fiIf-Modified-Since: Wed, 09 Sep 2020 16:06:01 -0700User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/535.11 (KHTML, like Gecko) Chrome/17.0.963.56 Safari/535.11Does the client have a cached copy of the object being requested?

Yes, because this is a conditional GET, as evidenced by the If-Modified-Since field.

Suppose a router has n input ports each with identical line speeds, n output ports each with identical line speeds, and the line speed of an output port is at least n times as that of an input port. Further suppose that the switching fabric speed is at least n times as fast as an input line speed. Then

there is no queuing delay in the router

Single-homed hosts have one interface and routers typically have two or more interfaces.

True

In a high-performance router, shadow copies of the routing table are kept in

the input ports

Forwarding

the local action of moving arriving packets from router's input link to appropriate router output link

Consider the queuing delay in a router buffer, where the packet experiences a delay as it waits to be transmitted onto the link. The length of the queuing delay of a specific packet will depend on the number of earlier-arriving packets that are queued and waiting for transmission onto the link. If the queue is empty and no other packet is currently being transmitted, then our packet's queuing delay will be zero. On the other hand, if the traffic is heavy and many other packets are also waiting to be transmitted, the queuing delay will be long. Assume a constant transmission rate of R = 1100000 bps, a constant packet-length L = 3700 bits, and a is the average rate of packets/second. Traffic intensity I = La/R, and the queuing delay is calculated as I(L/R)(1 - I) for I < 1. Assuming that a = 33, what is the queuing delay? Give your answer in milliseconds (ms)

0.3319 : Queuing Delay = I(L/R)(1 - I) * 1000 = 0.111*(3700/1100000)*(1-0.111) * 1000 = 0.3319 ms.

Consider the scenario shown below, with four different servers connected to four different clients over four three-hop paths. The four pairs share a common middle hop with a transmission capacity of R = 300 Mbps. The four links from the servers to the shared link have a transmission capacity of RS = 50 Mbps. Each of the four links from the shared middle link to a client has a transmission capacity of RC = 90 Mbps. Assuming that the servers are all sending at their maximum rate possible, what are the link utilizations of the client links (with transmission capacity RC)? Enter your answer in a decimal form of 1.00 (if the utilization is 1) or 0.xx (if the utilization is less than 1, rounded to the closest xx).

0.56

Consider the queuing delay in a router buffer, where the packet experiences a delay as it waits to be transmitted onto the link. The length of the queuing delay of a specific packet will depend on the number of earlier-arriving packets that are queued and waiting for transmission onto the link. If the queue is empty and no other packet is currently being transmitted, then our packet's queuing delay will be zero. On the other hand, if the traffic is heavy and many other packets are also waiting to be transmitted, the queuing delay will be long. Assume a constant transmission rate of R = 1100000 bps, a constant packet-length L = 3700 bits, and a is the average rate of packets/second. Traffic intensity I = La/R, and the queuing delay is calculated as I(L/R)(1 - I) for I < 1. Assuming that a = 63, what is the queuing delay? Give your answer in milliseconds (ms)

0.5617: Queuing Delay = I(L/R)(1 - I) * 1000 = 0.2119*(3700/1100000)*(1-0.2119) * 1000 = 0.5617 ms.

Consider the scenario shown below, with four different servers connected to four different clients over four three-hop paths. The four pairs share a common middle hop with a transmission capacity of R = 300 Mbps. The four links from the servers to the shared link have a transmission capacity of RS = 50 Mbps. Each of the four links from the shared middle link to a client has a transmission capacity of RC = 90 Mbps.Assuming that the servers are all sending at their maximum rate possible, what are the link utilizations of the shared link (with transmission capacity R)? Enter your answer in a decimal form of 1.00 (if the utilization is 1) or 0.xx (if the utilization is less than 1, rounded to the closest xx).

0.67

Which of the characteristics below are associated with a P2P approach to structuring network applications (as opposed to a client-server approach)?

A process requests service from those it contacts and will provide service to processes that contact it. There is not a server that is always on.

Consider the scenario shown below, with four different servers connected to four different clients over four three-hop paths. The four pairs share a common middle hop with a transmission capacity of R = 300 Mbps. The four links from the servers to the shared link have a transmission capacity of RS = 50 Mbps. Each of the four links from the shared middle link to a client has a transmission capacity of RC = 90 Mbps.Assuming that the servers are all sending at their maximum rate possible, what are the link utilizations for the server links (with transmission capacity RS)? Enter your answer in a decimal form of 1.00 (if the utilization is 1) or 0.xx (if the utilization is less than 1, rounded to the closest xx).

1.00

Suppose a client is sending an HTTP GET request message to a web server, gaia.cs.umass.edu. Suppose the client-to-server HTTP GET message is the following:GET /kurose_ross_sandbox/interactive/quotation2.htm HTTP/1.1Host: gaia.cs.umass.eduAccept: text/plain, text/html, text/xml, image/jpeg, image/gif, audio/mpeg, audio/mp4, video/wmv, video/mp4,Accept-Language: en-us, en-gb;q=0.1, en;q=0.7, fr, fr-ch, da, de, fiIf-Modified-Since: Wed, 09 Sep 2020 16:06:01 -0700User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/535.11 (KHTML, like Gecko) Chrome/17.0.963.56 Safari/535.11What version of HTTP is the client using?

1.1

Suppose a packet is 10K bits long, the channel transmission rate connecting a sender and receiver is 10 Mbps, and the round-trip propagation delay is 10 ms. How many packets can the sender transmit before it starts receiving acknowledgments back?

Compute the Internet checksum value for these two 16-bit words: 01000001 11000100 and 00100000 00101011

10011110 00010000

Consider the figure below, adapted from Figure 1.17 in the text, which draws the analogy between store-and-forward link transmission and propagation of bits in packet along a link, and cars in a caravan being serviced at a toll booth and then driving along a road to the next tollbooth. Suppose the caravan has 20 cars, and that the tollbooth services (that is, transmits) a car at a rate of one car per 1 seconds. Once receiving serving a car proceeds to the next tool both, which is 200 kilometers away at a rate of 20 kilometers per second. Also assume that whenever the first car of the caravan arrives at a tollbooth, it must wait at the entrance to the tollbooth until all of the other cars in its caravan have arrived, and lined up behind it before being serviced at the toll booth. (That is, the entire caravan must be stored at the tollbooth before the first car in the caravan can pay its toll and begin driving towards the next tollbooth). Once the last car leaves the tollbooth, how long does it take until it arrives at the next tollbooth?

10: Just like in the previous question, it takes 10 seconds, regardless of the car

Consider the figure below, adapted from Figure 1.17 in the text, which draws the analogy between store-and-forward link transmission and propagation of bits in packet along a link, and cars in a caravan being serviced at a toll booth and then driving along a road to the next tollbooth. Suppose the caravan has 20 cars, and that the tollbooth services (that is, transmits) a car at a rate of one car per 1 seconds. Once receiving serving a car proceeds to the next tool both, which is 200 kilometers away at a rate of 20 kilometers per second. Also assume that whenever the first car of the caravan arrives at a tollbooth, it must wait at the entrance to the tollbooth until all of the other cars in its caravan have arrived, and lined up behind it before being serviced at the toll booth. (That is, the entire caravan must be stored at the tollbooth before the first car in the caravan can pay its toll and begin driving towards the next tollbooth). Once the first car leaves the tollbooth, how long does it take until it arrives at the next tollbooth?

10: It takes 10 seconds to travel to the next toll booth (200 km / 20 km/s)

Referring to the above problem, suppose now that when a car arrives at the second tollbooth, it proceeds through the tollbooth without waiting for the cars behind it. What is the end-to-end delay?

122 minutes and 12 seconds

Review the car-caravan example in Section 1.6. Again assume a propagation speed of 100 km/hour. Suppose the caravan travels 200 km, beginning in front of one tollbooth, passing through a second tollbooth, and finishing just before a third tollbooth. What is the end-to-end delay?

124 minutes

Consider the figure below, adapted from Figure 1.17 in the text, which draws the analogy between store-and-forward link transmission and propagation of bits in packet along a link, and cars in a caravan being serviced at a toll booth and then driving along a road to the next tollbooth. Suppose the caravan has 20 cars, and that the tollbooth services (that is, transmits) a car at a rate of one car per 1 seconds. Once receiving serving a car proceeds to the next tool both, which is 200 kilometers away at a rate of 20 kilometers per second. Also assume that whenever the first car of the caravan arrives at a tollbooth, it must wait at the entrance to the tollbooth until all of the other cars in its caravan have arrived, and lined up behind it before being serviced at the toll booth. (That is, the entire caravan must be stored at the tollbooth before the first car in the caravan can pay its toll and begin driving towards the next tollbooth). Once a car enters service at the tollbooth, how long does it take until it leaves service?

1: Service time is 1 seconds

Consider the figure below, adapted from Figure 1.17 in the text, which draws the analogy between store-and-forward link transmission and propagation of bits in packet along a link, and cars in a caravan being serviced at a toll booth and then driving along a road to the next tollbooth. Suppose the caravan has 20 cars, and that the tollbooth services (that is, transmits) a car at a rate of one car per 1 seconds. Once receiving serving a car proceeds to the next tool both, which is 200 kilometers away at a rate of 20 kilometers per second. Also assume that whenever the first car of the caravan arrives at a tollbooth, it must wait at the entrance to the tollbooth until all of the other cars in its caravan have arrived, and lined up behind it before being serviced at the toll booth. (That is, the entire caravan must be stored at the tollbooth before the first car in the caravan can pay its toll and begin driving towards the next tollbooth). How long does it take for the entire caravan to receive service at the tollbooth (that is the time from when the first car enters service until the last car leaves the tollbooth)?

20: It takes 20 seconds to service every car, (20 cars * 1 seconds per car)

Consider the figure below, adapted from Figure 1.17 in the text, which draws the analogy between store-and-forward link transmission and propagation of bits in packet along a link, and cars in a caravan being serviced at a toll booth and then driving along a road to the next tollbooth. Suppose the caravan has 20 cars, and that the tollbooth services (that is, transmits) a car at a rate of one car per 1 seconds. Once receiving serving a car proceeds to the next tool both, which is 200 kilometers away at a rate of 20 kilometers per second. Also assume that whenever the first car of the caravan arrives at a tollbooth, it must wait at the entrance to the tollbooth until all of the other cars in its caravan have arrived, and lined up behind it before being serviced at the toll booth. (That is, the entire caravan must be stored at the tollbooth before the first car in the caravan can pay its toll and begin driving towards the next tollbooth). Once the first car leaves the tollbooth, how long does it take until it enters service at the next tollbooth?

29: It takes 29 seconds until the first car gets serviced at the next toll booth (20-1 cars * 1 seconds per car + 200 km / 20 km/s)

How many RTTs are there from when a client first contacts an email server (by initiating a TCP session) to when the client can begin sending the email message itself - that is following all initial TCP or SMTP handshaking required? Recall the figure below from our class notes:

In the following 7 problems, we are sending a 30 Mbit MP3 file from a source host to a destination host. All links in the path between source and destination have a transmission rate of 10 Mbps. Assume that the propagation speed is 2 * 108 meters/sec, and the distance between source and destination is 10,000 km. Now suppose there is only one link between source and destination, and there are 10 FDM channels in the link. The MP3 file is sent over one of the channels. The end-to-end delay is

30.05 seconds

In the following 7 problems, we are sending a 30 Mbit MP3 file from a source host to a destination host. All links in the path between source and destination have a transmission rate of 10 Mbps. Assume that the propagation speed is 2 * 108 meters/sec, and the distance between source and destination is 10,000 km. Now suppose there is only one link between source and destination, and there are 10 TDM channels in the link. The MP3 file is sent over one of the channels. The end-to-end delay is

30.05 seconds

Suppose a client sends an HTTP request message with the If-modified-since: header. Suppose the object in a server has not changed since the last time a client retrieved the object. Then the server will send a response message with the status code:

304 Not Modified

In the following 7 problems, we are sending a 30 Mbit MP3 file from a source host to a destination host. All links in the path between source and destination have a transmission rate of 10 Mbps. Assume that the propagation speed is 2 * 108 meters/sec, and the distance between source and destination is 10,000 km. Now suppose that the MP3 file is broken into 3 packets, each of 10 Mbits. Ignore headers that may be added to these packets. Also ignore router processing delays. Assuming store and forward packet switching at the router, the total delay is

4.05 seconds

Consider the scenario shown below, with four different servers connected to four different clients over four three-hop paths. The four pairs share a common middle hop with a transmission capacity of R = 300 Mbps. The four links from the servers to the shared link have a transmission capacity of RS = 50 Mbps. Each of the four links from the shared middle link to a client has a transmission capacity of RC = 90 Mbps.What is the maximum achievable end-end throughput (an integer value, in Mbps) for each of four client-to-server pairs, assuming that the middle link is fairly shared (divides its transmission rate equally) and all servers are trying to send at their maximum rate?

Suppose an application generates chunks of 60 bytes of data every second, and each chunk gets encapsulated in a TCP segment and then an IP datagram. What percentage of each datagram will contain application data.

60%

62500: The number of packets that can be transmitted in a second into the link = R / L = 1000000000 bps / 16000 bits = 62500 packets

65: 1. The maximum number of connections that can be ongoing at any one time is the sum of all circuits, which happens when 14 connections go from A to B, 20 connections go from B to C, 16 connections go from C to D, and 15 connections go from D to A. This sum is 65.

This question requires a little bit of background in probability (but we'll try to help you though it in the solutions). Consider the two scenarios below: A circuit-switching scenario in which Ncs users, each requiring a bandwidth of 20 Mbps, must share a link of capacity 150 Mbps. A packet-switching scenario with Nps users sharing a 150 Mbps link, where each user again requires 20 Mbps when transmitting, but only needs to transmit 20 percent of the time. When circuit switching is used, what is the maximum number of users that can be supported?

7: When circuit switching is used, at most 7 users can be supported. This is because each circuit-switched user must be allocated its 20 Mbps bandwidth, and there is 150 Mbps of link capacity that can be allocated.

Consider the queuing delay in a router buffer, where the packet experiences a delay as it waits to be transmitted onto the link. The length of the queuing delay of a specific packet will depend on the number of earlier-arriving packets that are queued and waiting for transmission onto the link. If the queue is empty and no other packet is currently being transmitted, then our packet's queuing delay will be zero. On the other hand, if the traffic is heavy and many other packets are also waiting to be transmitted, the queuing delay will be long. Assume a constant transmission rate of R = 1100000 bps, a constant packet-length L = 3700 bits, and a is the average rate of packets/second. Traffic intensity I = La/R, and the queuing delay is calculated as I(L/R)(1 - I) for I < 1. If the buffer has a maximum size of 913 packets, how many of the 1731 packets would be dropped upon arrival from the previous question?

818: Packets dropped = packets - buffer size = 1731 - 913 = 818 dropped packets.

Match the function of a server to a given type of DNS server in the DNS server hierarchy. _____ Provides authoritative hostname to IP mappings for organization's named hosts. _____ Replies to DNS query by local host, by contacting other DNS servers to answer the query. _____ Responsible for a domain (e.g., *.com, *.edu); knows how to contact authoritative name servers. _____ Highest level of the DNS hierarchy, knows how to reach servers responsible for a given domain (e.g., *.com, *.edu). A. Authoritative DNS server B. Top Level Domain (TLD) servers C. DNS root servers D. Local DNS server

A D B C

Consider the rdt2.1 sender and receiver FSMs shown below, with labeled transitions S1 through S6 at the sender, and transitions R1 through R6 at the receiver. The sender and receiver start in the "Wait for call 0 from above" and "Wait for 0 from below" states, respectively. Suppose that no channel errors occur. A sequence of interleaved sender and receiver transitions is given below. Transitions S1 and S4 are already provided. Choose the sender or receiver transition for the unlabeled transitions x1, x2, x3, and x4 below to indicate the time-ordered sequence of transitions (interleaved sender and receiver transitions) that will result in two messages being delivered at the receiver, with the sender and receiver returning to their initial states (again, given that no channel errors occur). S1, x1, x2, S4, x3, x4 _____ transition x1 _____ transition x2 _____ transition x3 _____ transition x4 A. R1 B. S6 C. R4 D. S3

A D C B

Consider the five images below. Indicate which of these images suggest the need for flow control (the others would suggest the need for congestion control).

A glass overflowing A talking head

3.1-4 Transport-layer services using UDP. Check all of the services below that are provided by the UDP protocol.

A message abstraction, that preserves boundaries between message data sent in different socket send calls at the sender.

Which of the following descriptions below correspond to a "services" view of the Internet?

A place I go for information, entertainment, and to communicate with people. A platform for building network applications.

Suppose that as shown in the figure below, a TCP sender is sending segments with 100 bytes of payload. The TCP sender sends five segments with sequence numbers 100, 200, 300, 400, and 500. Suppose that the segment with sequence number 300 is lost. The TCP receiver will buffer correctly-received but not-yet-in-order segments for later delivery to the application layer (once missing segments are later received). Complete the sentences below .... _____ After receiving segment 100, the receiver responds with an ACK with value: _____ After receiving segment 200, the receiver responds with an ACK with value: _____ After receiving segment 500, the receiver responds with an ACK with value: _____ After receiving the retransmitted segment 300, the receiver responds with an ACK with value: _____ The TCP receiver does not respond in the example, with an ACK with value: A. 300 B. 200 C. 400 D. 600 E. 300, a duplicate ACK

B A E D C

Match the access network with the approximate speeds that a subscriber might experience. (Note: if you look these up, do so in the 8E textbook, slides,or video -- not in the 7E or earlier versions, since link access speeds are always increasing over the years). _____ Ethernet _____ 802.11 WiFi _____ Cable access network _____ Digital Subscriber Line _____ 4G cellular LTE A. Wired. Up to 10's to 100's of Mbps downstream per user. B. Wired. Up to 100's Gbps per link. C. Wired. Up to 10's of Mbps downstream per user. D. Wireless. 10's to 100's of Mbps per device. E. Wireless. Up to 10's Mbps per device.

B D A C E

Match the name of an Internet layer with unit of data that is exchanged among protocol entities at that layer, using the pulldown menu. _____ Application layer _____ Transport layer _____ Network layer _____ Link layer _____ Physical layer A. Frame B. Message C. Bit D. Segment E. Datagram

B D E A C

Consider the purposes/goals/use of different reliable data transfer protocol mechanisms. For the given purpose/goal/use match it to the RDT mechanism that is used to implement the given purpose/goal/use. _____ Lets the sender know that a packet was NOT received correctly at the receiver. _____ Used by sender or receiver to detect bits flipped during a packet's transmission. _____ Allows for duplicate detection at receiver. _____ Lets the sender know that a packet was received correctly at the receiver. _____ Allows the receiver to eventually receive a packet that was corrupted or lost in an earlier transmission. A. Sequence numbers B. NAK C. ACK D. Retransmission E. Checksum

B E A C D

Consider the rdt2.1 sender and receiver FSMs shown below, with labeled transitions S1 through S6 at the sender, and transitions R1 through R6 at the receiver. The sender and receiver start in the "Wait for call 0 from above" and "Wait for 0 from below" states, respectively. Suppose that the initial message transmission by the sender is corrupted, but that no other message transmissions are corrupted. Match the unlabeled transitions x1, x2, x3, x4, x5 in the time-ordered sequence of transitions below (interleaved sender and receiver transitions) that will occur following the initial S1 transition (which is corrupted), that will result in two messages being delivered at the receiver, with the sender and receiver returning to their initial states (again, given that the initial message transmission by the sender is corrupted). Note that transitions S1, S4, and S6 are already provided below. S1 (message corrupted), x1, x2, x3, x4, S4, x5, S6. _____ transition x1 _____ transition x2 _____ transition x3 _____ transition x4 _____ transition x5 A. S2 B. R4 C. R6 D. R1 E. S3

C A D E B

Match the functionality of a protocol with the name of a the email protocol (if any) that implements that functionality. _____ Pushes email from a mail client to a mail server. _____ Pulls mail from one mail server to another mail server. _____ Pulls email to a mail client from a mail server. A. IMAP B. Neither SMTP nor IMAP does this. C. SMTP

C B A

Suppose a UDP segment (A in the figure below) arrives at a host with an IP address of 128.119.40.186. The source port in the UDP segment is 4829 and the destination port is 3546. The IP address of the sending host is 60.54.75.24. Now consider the UDP datagram (and the IP datagram that will encapsulate it) sent in reply by the application on host 128.119.40.186 to the original sender host, labeled B in the figure above. Complete the sentences below ... What are the source and destination port numbers and IP addresses? (Enter the integer port number or the 4-part dotted decimal IP address, included the period) The source port number of the UDP segment (B) sent in reply is: The source IP address of the IP datagram containing the UDP segment (B) sent in reply is: The destination port number of the UDP segment (B) sent in reply is: The destination IP address of the IP datagram containing the UDP segment (B) sent in reply is: _____ The source port number of the UDP segment (B) sent in reply is: _____ The source IP address of the IP datagram containing the UDP segment (B) sent in reply is: _____ The destination port number of the UDP segment (B) sent in reply is: _____ The destination IP address of the IP datagram containing the UDP segment (B) sent in reply is: A. 4829 B. 128.119.40.186 C. 3546 D. 60.54.75.24

C B A D

Match the function of a layer in the Internet protocol stack to its its name in the pulldown menu. _____ Protocols that are part of a distributed network application. _____ Transfer of data between one process and another process (typically on different hosts). _____ Delivery of datagrams from a source host to a destination host (typically). _____ Transfer of data between neighboring network devices. _____ Transfer of a bit into and out of a transmission media. A. Network layer B. Transport layer C. Application Layer D. Physical layer E. Link layer

C B A E D

Advantages of using a web cache

The transport layer sits on top of the network layer, and provides its services using the services provided to it by the network layer. Thus it's important that we know what is meant by the network layer's "best effort" delivery service. True or False: The network layer's best-effort delivery service means that IP makes its "best effort" to deliver segments between communicating hosts, but it makes no guarantees. In particular, it does not guarantee segment delivery, it does not guarantee orderly delivery of segments, and it does not guarantee the integrity of the data in the segments.

Correct! The network layer's best effort service doesn't really provide much service at all, does it?

What are some reasons for not discarding received-but- out-of-sequence packets at the receiver in GBN? Indicate one or more of the following statements that are correct.

Even though that packet will be retransmitted, its next retransmission could be corrupted, so don't discard a perfectly well-received packet, silly!

Match the description of each component of packet delay to its name in the pull down list. _____ Time needed to perform an integrity check, lookup packet information in a local table and move the packet from an input link to an output link in a router. _____ Time spent waiting in packet buffers for link transmission. _____ Time spent transmitting packets bits into the link. _____ Time need for bits to physically propagate through the transmission medium from end one of a link to the other. A. Queueing delay B. Transmission delay C. Propagation delay D. Processing delay

D A B C

Match the description of a TCP connection management message with the name of the message used to accomplish that function. _____ A message from client to server initiating a connection request. _____ A message from server to client ACKing receipt of a SYN message and indicating the willingness of the server to establish a TCP connection with the client. _____ A message indicating that the sending side is initiating the protocol to terminate a connection. _____ A message sent in response to a request to terminate a connection, ACKing that the side receiving this message is also willing to terminate the connection _____ A general purpose error message used during connection set up or tear down to let the other side know that an error has occurred, and that the referenced connection should be shut down. A. FINACK message B. SYNACK message C. FIN message D. SYN mesage E. RESET message

D B C A E

Match the definition/function of an element or approach in a networked streaming video system, with its name. _____ A unit of video, each of which may be encoded at multiple different rates, stored in different files. _____ A file containing the location and encoding rate of files corresponding to video segments in a video. _____ An approach that allows a client to adapt the encoding rate of retrieved video to network congestion conditions. _____ A CDN approach that stores content in access networks, close to clients. A. DASH B. Enter deep C. Manifest D. Chunk

D C A B

What is the value of caching in the local DNS name server? Check all that apply.

DNS caching provides for faster replies, if the reply to the query is found in the cache. DNS caching results in less load elsewhere in DNS, when the reply to a query is found in the local cache.

For the given function of a field in the TCP segment, select the name of that field from the pull-down list. _____ This field contains the port number associated with the sending socket for this TCP segment. _____ This field contains application data that was written into a socket by the sender of this TCP segment. _____ This field contains the index in the sender-to-receiver byte stream of the first byte of that data in the payload carried in this segment. _____ This field contains the index in the byte stream of the next in-order byte expected at the receiver _____ If set, this segment cumulatively ACKs all data bytes up to, but not including, the byte index in the ACK value field of this segment. _____ This field contains the number of available bytes in the TCP receiver's buffer. _____ This field contains the Internet checksum of the TCP segment and selected fields in the IP datagram header. _____ This field contains the number of bytes in the TCP header. A. Data (or payload). B. Checksum C. Header length field D. Receiver advertised window E. Source port number F. ACK number field G. Sequence number H. ACK bit

E A G F H D B C

Match the description of a security defense with its name. _____ Specialized "middleboxes" filtering or blocking traffic, inspecting packet contents inspections _____ Provides confidentiality by encoding contents _____ Used to detect tampering/changing of message contents, and to identify the originator of a message. _____ Limiting use of resources or capabilities to given users. _____ Proving you are who you say you are. A. Access control B. Digital signatures C. Encryption D. Authentication E. Firewall

E C B A D

3.3-2 UDP header fields. Which of the fields below are in a UDP segment header? [Hint: note the use of the word "header" in this question statement.]

Internet checksum Source port number Destination port number Length (of UDP header plus payload)

What is the role of an authoritative name server in the DNS? (Check all that apply)

It provides the definitive answer to the query with respect to a name in the authoritative name server's domain.

Consider the latency model for static congestion windows. If the server receives an acknowledgement for the first data segment in the first window before the server completes the transmission of the first window, then the latency for an object of size O is:

Latency = 2 RTT + O/R

UDP header fields. Which of the fields below are in a UDP segment header? [Hint: note the use of the word "header" in this question statement.]

Length (of UDP header plus payload) Internet checksum Destination port number Source port number

Access Control

Limiting use of resources or capabilities to given users.

To which DNS server does a host send their requests to?

Local DNS server

When an application uses a TCP socket, what transport services are provided to the application by TCP? Check all that apply.

Loss-free data transfer. The service will reliably transfer all data to the receiver, recovering from packets dropped in the network due to router buffer overflow. Flow Control. The provided service will ensure that the sender does not send so fast as to overflow receiver buffers. Congestion control. The service will control senders so that the senders do not collectively send more data than links in the network can handle.

Consider the figure below, adapted from Figure 1.17 in the text, which draws the analogy between store-and-forward link transmission and propagation of bits in packet along a link, and cars in a caravan being serviced at a toll booth and then driving along a road to the next tollbooth. Suppose the caravan has 20 cars, and that the tollbooth services (that is, transmits) a car at a rate of one car per 1 seconds. Once receiving serving a car proceeds to the next tool both, which is 200 kilometers away at a rate of 20 kilometers per second. Also assume that whenever the first car of the caravan arrives at a tollbooth, it must wait at the entrance to the tollbooth until all of the other cars in its caravan have arrived, and lined up behind it before being serviced at the toll booth. (That is, the entire caravan must be stored at the tollbooth before the first car in the caravan can pay its toll and begin driving towards the next tollbooth). Are there ever two cars in service at the same time, one at the first toll booth and one at the second toll booth? Answer Yes or No

No: No, because cars can't get service at the next tollbooth until all cars have arrived

This question requires a little bit of background in probability (but we'll try to help you though it in the solutions). Consider the two scenarios below: A circuit-switching scenario in which Ncs users, each requiring a bandwidth of 20 Mbps, must share a link of capacity 150 Mbps. A packet-switching scenario with Nps users sharing a 150 Mbps link, where each user again requires 20 Mbps when transmitting, but only needs to transmit 20 percent of the time. Suppose packet switching is used. If there are 13 packet-switching users, can this many users be supported under circuit-switching? Yes or No.

No: Under circuit switching, the 13 users would each need to be allocated 20 Mbps, for an aggregate of 260 Mbps - more than the 150 Mbps of link capacity available.

Consider the circuit-switched network shown in the figure below, with circuit switches A, B, C, and D. Suppose there are 14 circuits between A and B, 20 circuits between B and C, 16 circuits between C and D, and 15 circuits between D and A. Suppose that 19 connections are needed from A to C, and 17 connections are needed from B to D. Can we route these calls through the four links to accommodate all 36 connections? Answer Yes or No

No: Using our answer from question 4, the sum of our needed connections is 36, and we have 30 available connections, so it is NOT possible.

Which of the following characteristics apply to HTTP only (and do not apply to SMTP)? Note: check one or more of the characteristics below.

Operates mostly as a "client pull" protocol. Uses a blank line (CRLF) to indicate end of request header. Uses server port 80.

Which of the following characteristics apply to SMTP only (and do not apply to HTTP)? Note: check one or more of the characteristics below.

Operates mostly as a "client push" protocol. Uses server port 25. Uses CRLF.CRLF to indicate end of message.

What is meant by transport-layer demultiplexing?

Receiving a transport-layer segment from the network layer, extracting the payload (data) and delivering the data to the correct socket.

Consider the rdt 2.0 sender and receiver shown below, with FSM transitions at the sender labeled S1, S2, and S3; and receiver transitions labeled R1 and R2.Which of the following sequences of transitions could possibly occur as a result of an initial rdt_send() call at the sender, and possible later message corruption and subsequent error recovery.

S1, R1, S2 S1, R1, S2, R2,S3 S1, R2, S3

DNS lookups often involve a combination of recursive and iterative queries.

True

The request at t=12. The request at t=5.

What are some reasons for discarding received-but- out-of-sequence packets at the receiver in GBN? Indicate one or more of the following statements that are correct.

The sender will resend that packet in any case. The implementation at the receiver is simpler.

Suppose one IPv6 router wants to send a datagram to another IPv6 router, but are connected together by intervening IPv4 routers. If the two routers use tunneling, then

The sending IPv6 router creates an IPv6 datagram and puts it in the data filed of an IPv4 datagram.

Which of the characteristics below are associated with a client-server approach to structuring network applications (as opposed to a P2P approach)?

There is a server that is always on. There is a server with a well known server IP address. HTTP uses this application structure.

client/server network structure

There is a server that is always on. There is a server with a well known server IP address. HTTP uses this application structure.

What is the purpose of the conditional HTTP GET request message?

To allow a server to only send the requested object to the client if this object has changed since the server last sent this object to the client.

What is the purpose of the If-Modified-Since field in a HTTP GET request message

To indicate to the server that the client has cached this object from a previous GET, and the time it was cached.

Manifest file. What is the purpose of a manifest file in a streaming multimedia setting?

To let a client know where it can retrieve different video segments, encoded at different rates

A gateway router must run both an intra-AS routing algorithm and an inter-AS routing algorithm.

True

Consider a queue preceding a transmission link of rate R. Suppose a packet arrives to the queue periodically every 1/a seconds. Also suppose all packets are of length L. Then the queuing delay is small and bounded as long as aL < R.

True

Which of the following human scenarios involve a protocol?

Two people introducing themselves to each other. One person asking, and getting, the time to/from another person. A student raising her/his hand to ask a really insightful question, followed by the teaching acknowledging the student, listening carefully to the question, and responding with a clear, insightful answer. And then thanking the student for the question, since teachers love to get questions.

Again, suppose a client is sending an HTTP GET request message to a web server, gaia.cs.umass.edu. The client-to-server HTTP GET message is the following (same as in previous problem): GET /kurose_ross_sandbox/interactive/quotation2.htm HTTP/1.1Host: gaia.cs.umass.eduAccept: text/plain, text/html, text/xml, image/jpeg, image/gif, audio/mpeg, audio/mp4, video/wmv, video/mp4,Accept-Language: en-us, en-gb;q=0.1, en;q=0.7, fr, fr-ch, da, de, fiIf-Modified-Since: Wed, 09 Sep 2020 16:06:01 -0700User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/535.11 (KHTML, like Gecko) Chrome/17.0.963.56 Safari/535.11What is the language in which the client would least prefer to get a response? [You may have to search around the Web a bit to answer this.]

United Kingdom English

For the client-server application over TCP, does the server program have to be executed before the client program?

Yes

Suppose an HTTP server sends the following HTTP response message a client:HTTP/1.0 200 OKDate: Wed, 09 Sep 2020 23:46:21 +0000Server: Apache/2.2.3 (CentOS)Last-Modified: Wed, 09 Sep 2020 23:51:41 +0000ETag:17dc6-a5c-bf716880.Content-Length: 418Connection: CloseContent-type: image/htmlWill the web server close the TCP connection after sending this message?

Yes, because this is HTTP 1.0

Consider the queuing delay in a router buffer, where the packet experiences a delay as it waits to be transmitted onto the link. The length of the queuing delay of a specific packet will depend on the number of earlier-arriving packets that are queued and waiting for transmission onto the link. If the queue is empty and no other packet is currently being transmitted, then our packet's queuing delay will be zero. On the other hand, if the traffic is heavy and many other packets are also waiting to be transmitted, the queuing delay will be long. Assume a constant transmission rate of R = 1100000 bps, a constant packet-length L = 3700 bits, and a is the average rate of packets/second. Traffic intensity I = La/R, and the queuing delay is calculated as I(L/R)(1 - I) for I < 1. In practice, does the queuing delay tend to vary a lot? Answer with Yes or No

Yes: Yes, in practice, queuing delay can vary significantly. We use the above formulas as a way to give a rough estimate, but in a real-life scenario it is much more complicated.

Consider the figure below, adapted from Figure 1.17 in the text, which draws the analogy between store-and-forward link transmission and propagation of bits in packet along a link, and cars in a caravan being serviced at a toll booth and then driving along a road to the next tollbooth. Suppose the caravan has 20 cars, and that the tollbooth services (that is, transmits) a car at a rate of one car per 1 seconds. Once receiving serving a car proceeds to the next tool both, which is 200 kilometers away at a rate of 20 kilometers per second. Also assume that whenever the first car of the caravan arrives at a tollbooth, it must wait at the entrance to the tollbooth until all of the other cars in its caravan have arrived, and lined up behind it before being serviced at the toll booth. (That is, the entire caravan must be stored at the tollbooth before the first car in the caravan can pay its toll and begin driving towards the next tollbooth). Are there ever zero cars in service at the same time, i.e., the caravan of cars has finished at the first toll both but not yet arrived at the second tollbooth? Answer Yes or No

Yes: Yes, one notable example is when the last car in the caravan is serviced but is still travelling to the next toll booth; all other cars have to wait until it arrives, thus no cars are being serviced

When a TCP segment arrives to a host, the socket to which the segment is directed depends on

the destination port number the source port number the source IP address of the datagram that encapsulated the segment

Routing

the global action of determining the source-destination paths taken by packets

Suppose the hit rate is .5 (instead of .4) in the caching example in the text. Then the average delay is

between 1.0 and 1.1 seconds

A browser will generate header lines as a function of

browser type and version user configuration of browser whether the browser has a cached version of the requested object

UDP Socket

socket(AF_INET, SOCK_DGRAM) creates this type of socket data from different clients can be received on the same socket the application must explicitly specify the IP destination address and port number for each group of bytes written into a socket provides unreliable transfer of a groups of bytes ("a datagram"), from client to server

Which of the following characteristics below are associated with a UDP socket? Check one or more that apply.

data from different clients can be received on the same socket the application must explicitly specify the IP destination address and port number for each group of bytes written into a socket provides unreliable transfer of a groups of bytes ("a datagram"), from client to server socket(AF_INET, SOCK_DGRAM) creates this type of socket

The transfer of an html file from one host to another is

loss-intolerant and time insensitive

For the Web application, the user agent is

none of the above: the user's keyboard the user's monitor the user's mouse

UDP has which of the following characteristics:

none of the above: three-way hand shake for connection establishment connection state at the server regulated send rate

TCP applies fast retransmit to a segment when

when it receives three ACKs for an earlier segment

With SYN cookies, when a server receives a SYN segment, it returns a SYNACK with:

with an initial sequence number that is a hash of the IP addresses and port numbers in the SYN segment (as well as of other things).

CP372

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