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Question 1(7 + 8 + 10 = 25 marks) a)In modern switched Ethernet networks, it is not unusual for a router to receive more trafficthan it is able to forward due to the relative speeds of the Ethernet network versus the links from the router to other networks or the Internet. In such circumstances, packets would be randomly dropped either by the router or by the interconnecting switch when buffers are filled. Briefly discuss how polling could be used to overcome this problem, and discuss two advantages and two disadvantages of this approach. b)Prior to the development of full-duplex Ethernet, Token Bus networks provided a means through which communication could be achieved on a shared medium without collisions. Briefly describe how devices transmitted on a network using Token Bus (ignore token management) and discuss the performance token bus as transmission speed increases. c)Collisions and broadcasts have a negative impact on the throughput of an Ethernet network. i)Describe how collisions and broadcasts influence the throughput on an Ethernet network using a bus topology. ii)Repeaters, hubs, bridges, and switches are all devices that can be used in the construction of Ethernet networks. Briefly discuss how each of these devices would influence the impact of collisions and broadcasts within a single Ethernet network. Hint: VLANs divide a network into two or more networks and is irrelevant to this question. Question 2(6 + 6 + 8 + 20 = 40 marks) a)IPv4 uses a checksum to determine if any errors have occurred in the transmission of a datagram, however this checksum only checks for errors in the IPv4 header. Discuss why only the header is checked by IPv4 for errors, and discuss whether it is necessary to check for errors at all given that Ethernet already checks the entire frame using CRC. b)Given a subnet mask of 255.255.192.0, demonstrate for an IP address of your choosing the calculation of the range of addresses that are valid from that network (host ID all 0s through to host ID of all 1s). Note: The IP address you choose must consist of four decimal numbers that are greater than or equal to 101, and must be odd numbers. c)The fragmentation mechanism provided by IPv4 can contribute to network congestion. Briefly describe how fragmentation works and, given that there is no change to higherlevel data, explain how this mechanism could contribute to congestion. d)Consider implementing a negative acknowledgement mechanism for IPv4 that would alert a source host to a datagram being discarded due to a corruption in the path between the communicating hosts at any router in the path or the destination host. i.Briefly describe how your protocol would work. ii.Illustrate the operation of your protocol using a time diagram/sequence diagram (see Session 5 Slide 39 for an example), including both successful and unsuccessful delivery of datagrams. iii.Discuss two advantages and two disadvantages of your protocol. Question 3(11 + 8 + 16 = 35 marks)
a)Both RIPv2 and OSPFv2 routing protocols receive updated information about the state of the network from their neighbours. However it can be said that the decisions made by vector-distance routing protocols (such as RIPV2) are based on “second hand” information, whilst the decisions made by link state routing protocols (OSPFv2) is based on “first hand” information. Briefly describe what information is contained in the updated information routers receive, where the information comes from, and why it is considered “second hand” or “first hand”. b)Given the following Link State Database and network topology: AB CD E F A - - 1 - 4 1 B - - - 3 1 6 C 1 - - - 2 - D - 3 - - - 2 E 4 1 2 - - - F 1 6 - 2 - - Demonstrate the working of the Shortest Path First algorithm and final routing table for one of the routers of your choosing. Your answer must indicate the paths and costs that will be taken by all datagrams through the network. c)For each of the fields listed below appearing in the IPv4 and IPv6 headers, briefly describe the purpose of those fields, explain how they are related, and explain why a change has been made: i.IPv4 defines both header length (HLEN) and total length fields, whilst IPv6 defines only a payload length. ii.IPv4 defines a protocol field, whilst IPv6 defines a next header field. iii.IPv4 defines a time to live (TTL) field, whilst IPv6 defines a hop limit field. Marking SchemeACEFBD
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