COMPUTER NETWORKING Questions and Answers
R1. Let's review some of the terminology used in this textbook. Recall that the name of a
transport-layer packet is segment and that the name of a link-layer packet is frame. What is the
name of a network-layer packet? Recall that both routers and link-layer switches are called
packet switches. What is the fundamental difference between a router and link-layer switch?
Recall that we use the term routers for both datagram networks and VC networks. - ANSWER -
A network-layer packet is a datagram. A router forwards a packet based on the packet's IP (layer
3) address. A link-layer switch forwards a packet based on the packet's MAC (layer 2) address.
R2. What are the two most important network-layer functions in a datagram network? What are
the three most important network-layer functions in a virtual-circuit network? - ANSWER -The
two functions of datagram-based network layer are path determination and switching. The
additional function of a vc-based network is call setup.
R3. What is the difference between routing and forwarding? - ANSWER -Forwarding is about
moving a packet from a router's input link to the appropriate output link. Routing is about
determining the end-to-routes between sources and destinations.
R4. Do the routers in both datagram networks and virtual-circuit network use forwarding tables?
If so, describe the forwarding tables for both classes of networks. - ANSWER -Yes, both use
forwarding tables. For descriptions of the tables, see Section 4.2.
R5. Describe some hypothetical services that the network layer can provide to a single packet.
Do the same for a flow of packets. Are any of can provide to a single packet. Do the same for a
flow of packets. Are any of your hypothetical service provided by the Internet's network layer?
Are any provided by ATM's CBR service model? Are any provided by ATM's ABR service
model? - ANSWER -...
R6. List some applications that would benefit from ATM's CBR service model. - ANSWER -
Single packet: guaranteed delivery; guaranteed delivery with guaranteed maximum delay. Flow
of packets: in-order packet delivery; guaranteed minimal bandwidth; guaranteed maximum jitter.
None of these services is provided by the Internet's network layer. ATM's CBR service provides
both guaranteed delivery and timing. ABR does not provide any of these services.
,R7. Discuss why each input port in a high-speed router stores shadow copy of the forwarding
table. - ANSWER -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.
R8. Three types of switching fabrics are discussed in Section 4.3. List and briefly describe each
type. Which, if any, can send multiple packets across the fabric in parallel. - ANSWER -
Switching via memory; switching via a bus; switching via an interconnection network
R9. Describe how packet loss can occur at input ports. Describe how packet loss at input ports
can be eliminated (without using infinite buffers). - ANSWER -Packet loss occurs if queue size
at the input port grows large because of slow switching fabric speed and thus exhausting router's
buffer space. It can be eliminated if the switching fabric speed is at least n times as fast as the
input line speed, where n is the number of input ports.
R10. Describe how packet loss can occur at input ports. Describe how packet loss at input ports
can be eliminated (without using infinite buffers). - ANSWER -Packet loss can occur if the
queue size at the output port grows large because of slow outgoing line-speed.
R11. What is HOL blocking? Does it occur in input ports or output port? - ANSWER -HOL
blocking - a queued packet in an input queue must wait for transfer through the fabric because it
is blocked by another packet at the head of the line. It occurs at the input port.
R12. Do router have IP addresses? If so, how many? - ANSWER -Yes. They have one address
for each interface.
R13. What is the 32-bit binary equivalent of the IP address 223.1.3.27? - ANSWER -11011111
00000001 00000011 00011100
R14. Visit a host that uses DHCP to obtain its IP address, network mask, default router, and IP
address of its local DNS server. List these values. - ANSWER -8 interfaces; 3 forwarding tables
,R15. Suppose there are three routers between a source host and a destination host. Ignoring
fragmentation, an IP datagram sent from the source host to the destination host will travel over
how many interfaces? How many forwarding tables will be indexed to move the datagram from
the source to the destination? - ANSWER -50% overhead
R16. Suppose an application generates chunks of 40 bytes of data every 20 msec, and each chunk
get encapsulated in a TCP segment and then an IP datagram. What percentage of each datagram
will be overhead, and what percentage will application data? - ANSWER -The 8-bit protocol
field in the IP datagram contains information about which transport layer protocol the destination
host should pass the segment to.
R17. Suppose Host A sends Host B a TCP segment encapsulated in an IP datagram. When Host
B receives the datagram, how does the network layer in Host B know it should pass the segment
(that is, the payload of the datagram) to TCP rather than a UDP or to something else? -
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.
R18. Suppose you purchase a wireless router and connect it to your cable modem. Also suppose
that your ISP dynamically assign 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 wireless connect to your
wireless router. How are IP addresses assigned the five PCs? Does the wireless router use NAT?
Why or why not? - ANSWER -See Section 4.4.4
R19. Compare and contrast the IPv4 and the IPv6 header fields. Do they have any fields in
common? - ANSWER -Yes, because the entire IPv6 datagram (including header fields) is
encapsulated in an an IPv4 datagram
R20. It has been said that when IPv6 tunnels through IPv4 router, IPv6 treats the IPv4 tunnels as
link layer protocols. Do you agree with this statement? Why or why not? - ANSWER -Link state
algorithms: Computes the least-cost path between source and destination using complete, global
knowledge about the network. Distance-vector routing: The calculation of the least-cost path is
carried out in an iterative, distributed manner. A node only knows the neighbor to which it
should forward a packet in order to reach given destination along the least-cost path, and the cost
of that path from itself to the destination
R21. Compare and contrast link state and distance-vector routing algorithms. - ANSWER -Key:
, 1.Link-state algorithms is a global routing algorithm, and it requires each node to know the cost
of each link in the network. Also, whenever a link cost changes, new link cost must be sent to all
nodes.
Distance-vector algorithm is a decentralized routing algorithm, and it requires message
exchanges between directly connected Neighbors at each iteration. When link costs change ,the
DV algorithm will propagate the results of the changed link only if the new link cost result in a
changed least-cost path for one of the nodes attached to the link.
2.LS is an O(n^2)algorithm requiring O(nE)messages ,and that it potentially suffers from
oscillations. The DV algorithm can converge slowly and have routing loops .DV also suffer from
the count-to-infinity problem.
3. When a router fails, under LS, a router could broadcast an incorrect cost for one of its attached
links; under DV, a node can advertise incorrect least-cost paths to any/all destinations.
R22. Discuss how a hierarchical organization of the Internet has made it possible to scale to
millions of users. - ANSWER -Routers are aggregated into autonomous systems (ASs). Within
an AS, all routers run the same intra-AS routing protocol. Special gateway routers in the various
ASs run the inter-autonomous system routing protocol that determines the routing paths among
the ASs. The problem of scale is solved since an intra-AS router need only know about routers
within its AS and the gateway router(s) in its AS.
R23. Is it necessary that every autonomous system use the same intra-AS routing algorithm?
Why or why not? - ANSWER -No. Each AS has administrative autonomy for routing within an
AS.
R24. Consider Figure 4.37. Starting with the original table in D, suppose that D receives from A
the following advertisement:
Destination Subnet Next Router Number of Hops to Destination
z c 10
w __ 1
x __ 1
.........
Will the table in A change? If so how? - ANSWER -No. The advertisement tells D that it can get
to z in 11 hops by way of A. However, D can already get to z by way of B in 7 hops. Therefore,
R1. Let's review some of the terminology used in this textbook. Recall that the name of a
transport-layer packet is segment and that the name of a link-layer packet is frame. What is the
name of a network-layer packet? Recall that both routers and link-layer switches are called
packet switches. What is the fundamental difference between a router and link-layer switch?
Recall that we use the term routers for both datagram networks and VC networks. - ANSWER -
A network-layer packet is a datagram. A router forwards a packet based on the packet's IP (layer
3) address. A link-layer switch forwards a packet based on the packet's MAC (layer 2) address.
R2. What are the two most important network-layer functions in a datagram network? What are
the three most important network-layer functions in a virtual-circuit network? - ANSWER -The
two functions of datagram-based network layer are path determination and switching. The
additional function of a vc-based network is call setup.
R3. What is the difference between routing and forwarding? - ANSWER -Forwarding is about
moving a packet from a router's input link to the appropriate output link. Routing is about
determining the end-to-routes between sources and destinations.
R4. Do the routers in both datagram networks and virtual-circuit network use forwarding tables?
If so, describe the forwarding tables for both classes of networks. - ANSWER -Yes, both use
forwarding tables. For descriptions of the tables, see Section 4.2.
R5. Describe some hypothetical services that the network layer can provide to a single packet.
Do the same for a flow of packets. Are any of can provide to a single packet. Do the same for a
flow of packets. Are any of your hypothetical service provided by the Internet's network layer?
Are any provided by ATM's CBR service model? Are any provided by ATM's ABR service
model? - ANSWER -...
R6. List some applications that would benefit from ATM's CBR service model. - ANSWER -
Single packet: guaranteed delivery; guaranteed delivery with guaranteed maximum delay. Flow
of packets: in-order packet delivery; guaranteed minimal bandwidth; guaranteed maximum jitter.
None of these services is provided by the Internet's network layer. ATM's CBR service provides
both guaranteed delivery and timing. ABR does not provide any of these services.
,R7. Discuss why each input port in a high-speed router stores shadow copy of the forwarding
table. - ANSWER -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.
R8. Three types of switching fabrics are discussed in Section 4.3. List and briefly describe each
type. Which, if any, can send multiple packets across the fabric in parallel. - ANSWER -
Switching via memory; switching via a bus; switching via an interconnection network
R9. Describe how packet loss can occur at input ports. Describe how packet loss at input ports
can be eliminated (without using infinite buffers). - ANSWER -Packet loss occurs if queue size
at the input port grows large because of slow switching fabric speed and thus exhausting router's
buffer space. It can be eliminated if the switching fabric speed is at least n times as fast as the
input line speed, where n is the number of input ports.
R10. Describe how packet loss can occur at input ports. Describe how packet loss at input ports
can be eliminated (without using infinite buffers). - ANSWER -Packet loss can occur if the
queue size at the output port grows large because of slow outgoing line-speed.
R11. What is HOL blocking? Does it occur in input ports or output port? - ANSWER -HOL
blocking - a queued packet in an input queue must wait for transfer through the fabric because it
is blocked by another packet at the head of the line. It occurs at the input port.
R12. Do router have IP addresses? If so, how many? - ANSWER -Yes. They have one address
for each interface.
R13. What is the 32-bit binary equivalent of the IP address 223.1.3.27? - ANSWER -11011111
00000001 00000011 00011100
R14. Visit a host that uses DHCP to obtain its IP address, network mask, default router, and IP
address of its local DNS server. List these values. - ANSWER -8 interfaces; 3 forwarding tables
,R15. Suppose there are three routers between a source host and a destination host. Ignoring
fragmentation, an IP datagram sent from the source host to the destination host will travel over
how many interfaces? How many forwarding tables will be indexed to move the datagram from
the source to the destination? - ANSWER -50% overhead
R16. Suppose an application generates chunks of 40 bytes of data every 20 msec, and each chunk
get encapsulated in a TCP segment and then an IP datagram. What percentage of each datagram
will be overhead, and what percentage will application data? - ANSWER -The 8-bit protocol
field in the IP datagram contains information about which transport layer protocol the destination
host should pass the segment to.
R17. Suppose Host A sends Host B a TCP segment encapsulated in an IP datagram. When Host
B receives the datagram, how does the network layer in Host B know it should pass the segment
(that is, the payload of the datagram) to TCP rather than a UDP or to something else? -
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.
R18. Suppose you purchase a wireless router and connect it to your cable modem. Also suppose
that your ISP dynamically assign 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 wireless connect to your
wireless router. How are IP addresses assigned the five PCs? Does the wireless router use NAT?
Why or why not? - ANSWER -See Section 4.4.4
R19. Compare and contrast the IPv4 and the IPv6 header fields. Do they have any fields in
common? - ANSWER -Yes, because the entire IPv6 datagram (including header fields) is
encapsulated in an an IPv4 datagram
R20. It has been said that when IPv6 tunnels through IPv4 router, IPv6 treats the IPv4 tunnels as
link layer protocols. Do you agree with this statement? Why or why not? - ANSWER -Link state
algorithms: Computes the least-cost path between source and destination using complete, global
knowledge about the network. Distance-vector routing: The calculation of the least-cost path is
carried out in an iterative, distributed manner. A node only knows the neighbor to which it
should forward a packet in order to reach given destination along the least-cost path, and the cost
of that path from itself to the destination
R21. Compare and contrast link state and distance-vector routing algorithms. - ANSWER -Key:
, 1.Link-state algorithms is a global routing algorithm, and it requires each node to know the cost
of each link in the network. Also, whenever a link cost changes, new link cost must be sent to all
nodes.
Distance-vector algorithm is a decentralized routing algorithm, and it requires message
exchanges between directly connected Neighbors at each iteration. When link costs change ,the
DV algorithm will propagate the results of the changed link only if the new link cost result in a
changed least-cost path for one of the nodes attached to the link.
2.LS is an O(n^2)algorithm requiring O(nE)messages ,and that it potentially suffers from
oscillations. The DV algorithm can converge slowly and have routing loops .DV also suffer from
the count-to-infinity problem.
3. When a router fails, under LS, a router could broadcast an incorrect cost for one of its attached
links; under DV, a node can advertise incorrect least-cost paths to any/all destinations.
R22. Discuss how a hierarchical organization of the Internet has made it possible to scale to
millions of users. - ANSWER -Routers are aggregated into autonomous systems (ASs). Within
an AS, all routers run the same intra-AS routing protocol. Special gateway routers in the various
ASs run the inter-autonomous system routing protocol that determines the routing paths among
the ASs. The problem of scale is solved since an intra-AS router need only know about routers
within its AS and the gateway router(s) in its AS.
R23. Is it necessary that every autonomous system use the same intra-AS routing algorithm?
Why or why not? - ANSWER -No. Each AS has administrative autonomy for routing within an
AS.
R24. Consider Figure 4.37. Starting with the original table in D, suppose that D receives from A
the following advertisement:
Destination Subnet Next Router Number of Hops to Destination
z c 10
w __ 1
x __ 1
.........
Will the table in A change? If so how? - ANSWER -No. The advertisement tells D that it can get
to z in 11 hops by way of A. However, D can already get to z by way of B in 7 hops. Therefore,
