Computer Network Exam Latest 2024 -
2025 | Computer Network Actual Exam Update 2024 Q uestions and Correct Answers Rated A+ 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 -Datagram -based network layer: forwarding; routing. Additional function of VC -base d network layer: call setup. 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-end routes between sources a nd destinations. Why each input port in a high -speed router stores a 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 d ecentralized forwarding avoids creating a forwarding processing bottleneck at a single point within the router. What are the three types of switching fabrics? -ANSWER -Switching via memory; switching via a bus; switching via an interconnection network. Why packet loss can occur at input ports and output ports? -
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 s peed is at least n times as fast as the input line speed, where n is the number of input ports. Packet loss can occur if the queue size at the output port grows large because of slow outgoing line -speed. What is HOL blocking? Does it occur in input ports or output ports? -
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. How a hierarchical organization of the Intern et 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 syste m 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. Is it necessary that every autonomous system use th e same intra -
autonomous system routing algorithm? Why or why not? -ANSWER -
No. Route selection within an AS is independent of how routes are chosen in another AS. So each AS can employ a different routing algorithm. What is NAT? Describe a scenario where you would use NAT. -
ANSWER -NAT stands for Network Address Translation, a technique to hide the private address spaces utilized by a private network. It is implemented at the edge router to map a globally unique IP add ress with a primate address inside the private network. Why does distance vector routing suffer from count -to-infinity problem? How come link state routing does not have this problem? -
ANSWER -When a link cost increases, a routing loop may be formed when d etermining the new least -cost path by the DV algorithm, causing the DV algorithm converge slowly, which results in the count -
to-infinity problem. In link state routing, routing paths are computed based on the global link state information maintained locall y by each node. When a link cost increases, this information will be propagated to the whole network. List three techniques that might contribute to slow down the adoption of IPv6. -ANSWER -NAT, CIDR, and DHCP Why must the virtual circuit number be change d from link to link in a virtual circuit network? -ANSWER -If otherwise, we need more number of bits for virtual circuit numbers, and all VC numbers must be unique among all routers in the path, which involves higher overhead to guarantee the uniqueness. What is the effect on routing due to count -to-infinity problem associated with distance vector protocol? How does IP deal with this problem? -ANSWER -Count -to-infinity problem may cause routing loops and therefore making the routing algorithm converge slowly . The count -to-infinity problem can be handled by Poisoned Reserve, which states that if z routes through y to get to the destination x, z will advertise to y that its distance to x is infinity. To avoid this problem, the RIP intra -AS routing protocol forc es the routing path to be at most 15 hops in length. When a packet is routed to the destination physical network via IP, how does the edge router at the destination network forward the packet to the destination host? -ANSWER -The edge router at the destina tion network employs the link layer protocol (e.g., Ethernet) of the destination network to forward the packet to the destination. Can IP packets circulate forever in an IP network, why or why not? -
ANSWER -No. IP datagram header contains a Time -to-live fi eld that is decreased by one each time the datagram is processed by a router. When this field reaches 0, the datagram must be dropped. How loops in paths can be detected in BGP? -ANSWER -BGP contains an attribute called AS -PATH, which lists the ASs through which the BGP advertisement for a prefix has passed. When does a router send an ICMP redirect message. Briefly explain the scenario. -ANSWER -Suppose a host H is connected to a network that has two routers attached to it, called R1 and R2. Suppose H uses R1 as the next hop to a destination and sends an IP packet to R1. Suppose R1 from its routing table determines that it has to route thru R2 to reach the destination. Since H1 can reach R2 directly, that would have been a better choice for the specific dest ination. In such a case, R1 sends an ICMP redirect message back to H asking it to use R2 instead of R1 as the next hop for packets addressed to that destination. If all the links in the Internet were to provide reliable delivery service, would the TCP rel iable delivery service be redundant? Why or why not? -ANSWER -Although each link guarantees that an IP datagram sent over the link will be received at the other end of the link without errors, it is not guaranteed that IP datagrams will arrive at the ultima te destination in the proper order. With IP, datagrams in the same TCP connection can take different routes in the network, and therefore arrive out of order. TCP is still needed to provide the receiving end of
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