COMPUTER NETWORKS FINAL EXAM
QUESTIONS AND ANSWERS
In our rdt protocol, why did we introduce sequence numbers?
in our rdt protocol, why did we introduce timers - Answer-Sequence numbers are
required for a receiver to find out whether an arriving packet contains new data or is a
retransmission. To handle losses in the channel. If the ACK for a transmitted packet is
not received within the duration of the timer for the packet, the packet (or its ACK or
NACK) is assumed to have been lost. Hence, the packet is retransmitted.
suppose Host A sends two TCP segments back to back to host B over a TCP
connection. The first segment have a sequence number of 90. The second has a
sequence number of 110.
How much data is in the first segment?
Suppose that the first segment is lost but the second segment arrives at B. In the
acknowledgement that Host B sends to Host A, what will be the acknowledgement
number? - Answer-a) 20 bytes b) ack number = 90
Given an IP address and mask of 180.3.70.5/16, design an IP addressing scheme that
satisfies the following requirements: Serve 50 subnets For first and second subnets,
what is the network address, broadcast address, first usable address, and last usable
address? - Answer-First subnet
180.3.68.0 --- NA
180.3.68.1 ---- First usable
180.3.71.254 ---- Last usable
180.3.71.255 --- BA
Second subnet
180.3.72.0 --- NA
180.3.72.1 ---- First usable
180.3.75.254 ---- Last usable
180.3.75.255 --- BA
Suppose Host A sends Host B a TCP segment encapsulated in an IP datagram. When
HostB receives the datagram, how does the network layer know it should pass the
segment(that is, the payload of the datagram) to TCP rather than to UDP or to
something else? - Answer-The 8-bit protocol field (upper layer) in the IP datagram
contains information about which transport layer protocol the destination host should
pass the segment to.
What field in the IP header can be used to ensure that a packet is forwarded through no
more than N routers? - Answer-Time-To-Live
Do routers have IP addresses? If so, how many? - Answer-Yes. They have one address
for each interface.
, Suppose you purchase a wireless router and connect it to your cable modem. Also
suppose that your ISP dynamically assigns your connected device (that is, your wireless
router) one IP address. Also suppose that you have five PCs at home that use 802.11 to
wirelessly connect to your wireless router. How are IP addresses assigned to the five
PCs? Does the wireless router use NAT? Why or why not? - Answer-Typically the
wireless router includes a DHCP server. DHCP is used to assign IP addresses to the 5
PCs and to the router interface. Yes, the wireless router also uses NAT as it obtains
only one IP address from the ISP.
It has been said that when IPv6 tunnels through IPv4 routers, IPv6 treats the IPv4
tunnels as link-layer protocols. Do you agree with this statement? Why or why not? -
Answer-Yes, because the entire IPv6 datagram (including header fields) is
encapsulated in an IPv4 datagram
What is the "count to infinity" problem in distance vector routing? - Answer-The count-
to-infinity problem refers to a problem of distance vector routing. The problem means
that it takes a long time for a distance vector routing algorithm to converge when there
is a link cost increase.
How does BGP use the NEXT-HOP attribute? How does it use the AS-PATH attribute?
- Answer-Routers use the AS-PATH attribute to detect and prevent looping
advertisements; they also use it in choosing among multiple paths to the same prefix.
The NEXT-HOP attribute indicates the IP address of the first router along an advertised
path to a given prefix.
List the four broad classes of services that a transport protocol can provide. For each
service class, indicate if either UDP or TCP (or both) provides such a service. - Answer-
security, guaranteed reliability of data transfer, guaranteed throughput, and guaranteed
timing. TCP can only provide reliable data transfer and UDP can't do any
Consider an e-commerce site that wants to keep a purchase record for each of its
customers. Describe how this can be done with cookies. - Answer-When the user visits
a website, the server will create a unique ID, creates an entry in the back-end database,
and returns the ID as the cookie number. The cookie is stored on the user's host and
managed by the browser. During each subsequent visit (and purchase), the browser
sends the cookie number back to the site. Thus the site knows when this user (more
precisely, this browser) is visiting the site.
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? - Answer-Web caching can bring the desired content "closer" to the
user, possibly 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.
QUESTIONS AND ANSWERS
In our rdt protocol, why did we introduce sequence numbers?
in our rdt protocol, why did we introduce timers - Answer-Sequence numbers are
required for a receiver to find out whether an arriving packet contains new data or is a
retransmission. To handle losses in the channel. If the ACK for a transmitted packet is
not received within the duration of the timer for the packet, the packet (or its ACK or
NACK) is assumed to have been lost. Hence, the packet is retransmitted.
suppose Host A sends two TCP segments back to back to host B over a TCP
connection. The first segment have a sequence number of 90. The second has a
sequence number of 110.
How much data is in the first segment?
Suppose that the first segment is lost but the second segment arrives at B. In the
acknowledgement that Host B sends to Host A, what will be the acknowledgement
number? - Answer-a) 20 bytes b) ack number = 90
Given an IP address and mask of 180.3.70.5/16, design an IP addressing scheme that
satisfies the following requirements: Serve 50 subnets For first and second subnets,
what is the network address, broadcast address, first usable address, and last usable
address? - Answer-First subnet
180.3.68.0 --- NA
180.3.68.1 ---- First usable
180.3.71.254 ---- Last usable
180.3.71.255 --- BA
Second subnet
180.3.72.0 --- NA
180.3.72.1 ---- First usable
180.3.75.254 ---- Last usable
180.3.75.255 --- BA
Suppose Host A sends Host B a TCP segment encapsulated in an IP datagram. When
HostB receives the datagram, how does the network layer know it should pass the
segment(that is, the payload of the datagram) to TCP rather than to UDP or to
something else? - Answer-The 8-bit protocol field (upper layer) in the IP datagram
contains information about which transport layer protocol the destination host should
pass the segment to.
What field in the IP header can be used to ensure that a packet is forwarded through no
more than N routers? - Answer-Time-To-Live
Do routers have IP addresses? If so, how many? - Answer-Yes. They have one address
for each interface.
, Suppose you purchase a wireless router and connect it to your cable modem. Also
suppose that your ISP dynamically assigns your connected device (that is, your wireless
router) one IP address. Also suppose that you have five PCs at home that use 802.11 to
wirelessly connect to your wireless router. How are IP addresses assigned to the five
PCs? Does the wireless router use NAT? Why or why not? - Answer-Typically the
wireless router includes a DHCP server. DHCP is used to assign IP addresses to the 5
PCs and to the router interface. Yes, the wireless router also uses NAT as it obtains
only one IP address from the ISP.
It has been said that when IPv6 tunnels through IPv4 routers, IPv6 treats the IPv4
tunnels as link-layer protocols. Do you agree with this statement? Why or why not? -
Answer-Yes, because the entire IPv6 datagram (including header fields) is
encapsulated in an IPv4 datagram
What is the "count to infinity" problem in distance vector routing? - Answer-The count-
to-infinity problem refers to a problem of distance vector routing. The problem means
that it takes a long time for a distance vector routing algorithm to converge when there
is a link cost increase.
How does BGP use the NEXT-HOP attribute? How does it use the AS-PATH attribute?
- Answer-Routers use the AS-PATH attribute to detect and prevent looping
advertisements; they also use it in choosing among multiple paths to the same prefix.
The NEXT-HOP attribute indicates the IP address of the first router along an advertised
path to a given prefix.
List the four broad classes of services that a transport protocol can provide. For each
service class, indicate if either UDP or TCP (or both) provides such a service. - Answer-
security, guaranteed reliability of data transfer, guaranteed throughput, and guaranteed
timing. TCP can only provide reliable data transfer and UDP can't do any
Consider an e-commerce site that wants to keep a purchase record for each of its
customers. Describe how this can be done with cookies. - Answer-When the user visits
a website, the server will create a unique ID, creates an entry in the back-end database,
and returns the ID as the cookie number. The cookie is stored on the user's host and
managed by the browser. During each subsequent visit (and purchase), the browser
sends the cookie number back to the site. Thus the site knows when this user (more
precisely, this browser) is visiting the site.
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? - Answer-Web caching can bring the desired content "closer" to the
user, possibly 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.
