CSCE 416 Final

Suppose Host A sends 3 TCP segments back to back to Host B over a TCP connection. The sequence numbers of the 1st, 2nd, and 3rd segments are 1000, 1500, and 2500 respectively. When the 1st segment arrives at B, in the ACK that B sends to A, what will be the ACK number?

(d) 1500

How many sockets does a UDP server supporting 9 different clients simultaneously need?

1

With single parity bit, we can detect

1 bit and 3 bit errors

With two dimensional parity scheme, we can correct

1 bit errors

With two dimensional parity scheme, we can detect

1 bit errors

Suppose a host on a 10 Mbps Ethernet just detected a third collision. Note that the slot size under Ethernet is 512 bits. What is the probability that the wait time is less than 200 microseconds?

1/2

Suppose a host on a 10 Mbps Ethernet just detected a third collision. What is the probability that it retransmits without waiting (backoff time is 0)?

1/8

How many sockets does a TCP server supporting 9 di↵erent clients simultaneously need?

10

Assuming that the generator is 101, what would be the CRC bits corresponding to data bits 1001?

11

For a browser with no parallel TCP connections, getting a web page with 6 images with nonpersistent HTTP takes

14 round trips

Suppose Host A sends 3 TCP segments back to back to Host B over a TCP connection. The sequence numbers of the 1st, 2nd, and 3rd segments are 1000, 1500, and 2500 respectively. Suppose the 1st segment arrives, 2nd segment is lost, and then the 3rd segment arrives at B. When the 3rd segment arrives at B, in the ACK that B sends to A, what will be the ACK number?

1500

Suppose two hosts, A and B, are separated by 5,000 kilometers and are connected by a direct link of R = 1 Mbps. Suppose the propagation speed over the link is 2.5*10^5 kilometers/sec. What is the maximum number of bits that will be in the link at any time?

20000

A router that is attached to 3 networks through its 3 interfaces has

3 IP addresses

Suppose a host on a 10 Mbps Ethernet just detected a third collision. Note that the slot size under Ethernet is 512 bits. What is the maximum b

358.4 microseconds

Consider sending a 2400-byte datagram into a link that has an MTU of 700 bytes. What is the value of the length field in the last fragment?

360

Suppose Host A sends 3 TCP segments back to back to Host B over a TCP connection. The sequence numbers of the 1st, 2nd, and 3rd segments are 1000, 1500, and 2500 respectively. How much data is in the 1st segment?

500 bytes

In Figure 1, suppose ARP tables at all machines and router are empty. Assuming A sends an IP datagram to B, then B sends an IP datagram to A, how many total packets you see on the whole network.

8

For a browser with no existing TCP connections, getting a web page with 6 images with persistent HTTP without pipelining takes

8 round trips

Assuming data bits D, CRC bits R, and generator bits G, a receiver will declare that there is an error if

<D,R> is not divisible by G (remainder not 0)

Which type of DNS resource record contains the IP address of a hostname?

A

TCP waits until it has received three duplicate ACKs before performing a fast retransmit. Why do you think the TCP designers chose not to perform a fast retransmit after the first duplicate ACK for a segment is received?

A duplicate ACK indicates that a packet is delivered out of order. It does not necessarily mean that a packet is lost, instead it might arrive later out of sequence. However, if a sender receives 3 duplicate ACKs, i.e., 3 packets arrived out of order at the receiver, it is more likely that a packet is lost. The designers of the triple duplicate ACK scheme chose this as the right tradeoff between triggering a quick retransmission when needed, but not retransmitting prematurely in the face of packet reordering

What is a hidden terminal problem?

A sender can not hear another sender

Why do HTTP, FTP, and SMTP run on top of TCP rather than on UDP?

All these applications need reliable delivery, without strict timing requirements, and are elastic, i.e., they can manage with the available bandwidth. Hence TCP is more suitable than UDP

Why is an ARP query sent within a broadcast frame? Why is an ARP response sent within a frame with a specific destination MAC address?

An ARP query is sent in a broadcast frame because the querying host does not which adapter address corresponds to the IP address in question. For the response, the sending node knows the adapter address to which the response should be sent, so there is no need to send a broadcast frame (which would have to be processed by all the other nodes on the LAN)

Five application layers of the inernet protocol stack and key responsibilities

Application layer- application specific actions Transport layer- process to process data transfer network layer- routing datagrams link layer- data transfer between neighbors physical layer- bits on the wire

Suppose all switch tables in the given network are empty. If Z sends a packet to Y, which switches learn how to forward to Z?

B3, B2, and B1

How does BGP use the NEXT-HOP attribute? How does it use the AS-PATH attribute?

Basically, Next-Hop forces the router to do a recursive lookup in order to determine which egress interface should be used to send the packets out.

Why do HTTP, FTP, SMTP, and POP3 run on top of TCP rather than on UDP?

Because TCP is more reliable, and HTTP, FTP, SMTP, and POP3cannot be affordable using UDP while UDP cannot transmit packet and guarantee a well order delivery. TCP is a connected-oriented network so packet will be delivered to the destination

Which of the following is NOT true of client server architecture?

Clients communicate directly with each other

When a large datagram is fragmented into multiple smaller datagrams, where are these smaller datagrams reassembled into a single larger datagram?

Datagrams are reassembled at the destination, before being passed to the transport layer.

UDP uses which of the following fields in the segment header to determine the socket to pass the received data to?

Destination IP address, destination port number

Which of the following is NOT a service provided by DNS?

Dynamic IP address allocation

Which of the following is NOT a reason to choose TCP for file transfer?

File transfer needs to be secure

What information is used by a process to identify a process running on another host?

Host IP Address and Socket Port Number

For inter-process communication in the Internet, what information is used by a process to identify a process running on another host?

IP address of the host and port number of the socket

Which of the following services are provided by both TCP and UDP?

Inter-process communication

Which of the following is NOT a reason for an application developer to choose UDP over TCP?

More secure

What is NAT? Describe a scenario where you would use NAT.

Network Address Translation (NAT) is the process where a network device, usually a firewall, assigns a public address to a computer (or group of computers) inside a private network. The main use of NAT is to limit the number of public IP addresses an organization or company must use, for both economy and security purposes.

Assuming that the generator is 101 and the data bits are 1001, could the receiver catch the error if the 1st and 3rd bits are in error?

No

Suppose a computer is moved from one network to another network. Does its Ethernet address need to change?

No

TCP interprets packet loss as congestion and decreases congestion window. Is this response of TCP to packet loss appropriate for wireless networks?

No

In TCP, will the loss of an ACK always result in a retransmission of that packet? Why or why not?

No. Under TCP, acknowledgements are cumulative. Therefore, even if an ACK Is lost, a subsequent ACK could acknowledge the data that is being acknowledged by the lost ACK

Which of the following is NOT a part of Uniform Resource Locator (URL)?

Object size

Suppose all switches in the given network know how to forward to Y. If Z sends a packet to Y, which switches learn how to forward to Z?

Only B3 and B2

Suppose all switches in the given network know how to forward to Y. If Z sends a packet to Y, which hosts' interfaces see this packet?

Only Y

which is preferred for data networks: circuit switching or packet switching? why?

Packet switching is easier and more affordable than circuit switching. Since all the bandwidth can be used at once, packet switching is more efficient because it doesn't have to deal with a limited number of connections that may not be using all that bandwidth. Packet switching also requires a less complicated infrastructure that can easily respond should parts of the network fail, making it quicker and less expensive to add new nodes whenever they're needed.

Which of the following is NOT needed for maintaining state using cookies

Persistent HTTP where server keeps connection open

Which layers in the Internet protocol stack does a router process?

Physical, link, network

Which of the following delays vary depending on the network congestion?

Queuing delay

Which of the following is NOT an end system?

Router

Which is NOT true about packet switching with store-and-forward transmission?

Router begins forwarding as soon as it gets packet's header

Which layers in the Internet protocol stack does a router process? Whichlayers does a link-layer switch process? Which layers does a host process?

Routers process network, link and physical layers. Link layer switches process link and physical layers Hosts process all five layers.

TCP uses which of the following fields in the segment header to determine the socket to pass the received data to?

Source IP address, source port number, destination IP address, destination port number

If a TCP server were to support 5 different clients simultaneously, how many sockets would the TCP server need? How many sockets would a UDP server need? Briefly justify your answer

TCP is connection oriented and so needs 6 sockets, 1 welcome socket to receive new connection requests and 1 socket for each client. UDP needs only 1 socket, as it is connectionless.

why must the server program be executed before the client program? For the client server application over UDP, why may the client program be executed before the server program?

TCP requires a connection so the server must be ready in order to communicate. UDP does not require a connection so it does not worry about the server.

What information is used by a process running on one host to identify a process running on another host?

The IP address of the destination host and the port number of the destination socket.

Consider sending a packet from a source host to a destination host over a fixed route. List the delay components in the end-to-end delay. Which of these delays are constant and which are variable? Briefly justify your answer

The delay components are processing delay, transmission delay, propagation delay, and queuing delay. All of these delays are fixed, except for the queuing delay, which is variable

For a communication session between a pair of processes, which process is the client and which is the server?

The process which initiates the communication is the client; the process that waits to be contacted is the server.

Suppose all switch tables in the given network are empty. If Z sends a packet to Y, which hosts' interfaces see this packet?

W, X, and Y

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?

Web caching can bring the desired content "closer" to the user, perhaps to the same LAN to which the user's host is connected. Web caching can reduce the delay for all objects, even objects that are not cached, since caching reduces the traffic on links.

What is the problem with 2-way handshake for establishing a connection and how does 3-way handshake employed by TCP addresses that problem?

With 2-way handshake, due to retransmissions, it is possible that a server receives a connection request from a non-existing client. Then, the server may set asside some resources unnecessarily for this non-existent connection. It may also incorrectly accept some retransmitted data frames corresponding to an old connection. On the other hand, with 3-way handshake, TCP server responds with its own connection request to the client. It will consider a connection as established only after receiving an ACK, confirming that the client is live

What is a recursive query in DNS? How is that different from an iterative query?

With a recursive query, a DNS client requests the server to query subsequent DNS servers, if it does not have the answer, until it finds a definitive answer and reply to the client. With an iterative query, a DNS server, if it does not have the answer, may return information about the subsequent DNS server to be contacted by the client to obtain the answer.

Discuss why each input port in a high-speed router stores a shadow copy of the forwarding table.

With the shadow copy, the forwarding decision is made locally, at each input port, without invoking the centralized routing processor. Such decentralized forwarding avoids creating a forwarding processing bottleneck at a single point within the router.

Assuming that the generator is 101 and the data bits are 1001, could the receiver catch the error if the 1st and 2nd bits are in error?

Yes

Suppose a computer is moved from one network to another network. Does its IP address need to change?

Yes

Is it possible that an organization's Web server and mail server have exactly the same alias for a hostname (for example cs.ust.hk.)? . What would be the type for the resource record (RR) that contains the hostname of the mail server?

Yes, an organization can have the same alias name for both its Web server and its mail server. An MX resource record type contains the host name of the mail server.

Using DHCP, your laptop can obtain the following.

all of them

How does a host find wireless access points (APs) to associate with?

both active and passive scanning

ARP query packet is sent using what type of address?

broadcast

As the maximum propagation delay between two hosts increases, the efficiency of Ethernet

decreases

As the signal-to-noise ratio (SNR) increases, the probability of bits getting flipped during transmission on a wireless link

decreases

As the time to transmit a maximum-size frame decreases, the efficiency of Ethernet

decreases

do routers have IP addresses? If so, how many??

every router has 2 IP addresses

Data transfer between neighboring network elements is the responsibility of

link layer

Random access MAC protocols are efficient at

low load

As the bit errors increase, rate adaptation schemes in wireless networks switch to a transmission rate that is

lower

Suppose the 802.11 RTS and CTS frames are as long as the standard DATA and ACK frames. Would there be any advantage to using the RTS and CTS frames?

no

Which of the following fields in an Ethernet frame is used to synchronize receiver and sender clock rates?

preamble

Which of the following is more efficient?

slotted aloha

In case of having two entries in the forwarding table match a given destination IP address, longest prefix matching rule favors more

specific route

Two computers on the same IP subnet have the same

subnet part in their IP address

Process to process data transfer is the responsibility of

transport layer

ARP response packet is sent using what type of address?

unicast