Computer network questions for final exam preparation 21/08/2017

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1 Computer network questions for final exam preparation 21/08/2017 Chapter 3 1. What does UDP provide in addition to IP? Answer: provides two services not provided by the IP layer. - It provides port numbers to help distinguish different user requests and, optionally, and a checksum capability to verify that the data arrived intact. 2. What is protocol data unit (PDU) and what do we call it in (physical layer, data link layer, Network layer, and transport layer) Answer: look to ch3 slide 7 3. What is Multiplexing and demultiplexing Answer: look to ch 3 slide Suppose a process in Host C has a UDP socket with port number Suppose both Host A and Host B each send a UDP segment to Host C with destination port number Will both of these segments be directed to the same socket at Host C? If so, how will the process at Host C know that these two segments originated from two different hosts? Answer: Yes, both segments will be directed to the same socket. For each received segment, at the socket interface, the operating system will provide the process with the IP addresses to determine the origins of the individual segments. 5. Suppose that a Web server runs in Host C on port 80. Suppose this Web server uses persistent connections, and is currently receiving requests from two different Hosts, A and B. Are all of the requests being sent through the same socket at Host C? If they are being passed through different sockets, do both of the sockets have port 80? Discuss and explain. Answer: For each persistent connection, the Web server creates a separate connection socket. Each connection socket is identified with a four-tuple: (source IP address, source port number, destination IP address, destination port number). When host C receives and IP datagram, it examines these four fields in the datagram/segment to determine to which socket it should pass the payload of the TCP segment. Thus, the requests from A and B pass through different sockets. The identifier for both of these sockets has 80 for the destination port; however, the identifiers for these sockets have different values for source IP addresses. Unlike UDP, when the transport layer passes a TCP segment s payload to the application process, it does not specify the source IP address, as this is implicitly specified by the socket identifier. 6. Describe why an application developer might choose to run an application over UDP rather than TCP.

2 Answer: An application developer may not want its application to use TCP s congestion control, which can throttle the application s sending rate at times of congestion. Often, designers of IP telephony and IP videoconference applications choose to run their applications over UDP because they want to avoid TCP s congestion control. Also, some applications do not need the reliable data transfer provided by TCP. 7. Consider a TCP connection between Host A and Host B. Suppose that the TCP segments traveling from Host A to Host B have source port number x and destination port number y. What are the source and destination port numbers for the segments traveling from Host B to Host A? Answer: Source port number y and destination port number x. Chapter 4 1. 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? Answer: A network-layer packet is a datagram Recall that both routers and link-layer switches are called packet switches. What is the fundamental difference between a router and link-layer switch? Answer: 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. 2. What is the difference between routing and forwarding? Answer: The key differences between routing and forwarding is that forwarding is a router s local action of transferring packets from its input interfaces to its output interfaces, and forwarding takes place at very short timescales (typically a few nanoseconds), and thus is typically implemented in hardware. Routing refers to the network-wide process that determines the end-to-end paths that packets take from sources to destinations. Routing takes place on much longer timescales (typically seconds), and is often implemented in software. 3. 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 main function of the data plane is packet forwarding, which is to forward datagrams from their input links to their output links. For example, the data plane s input ports perform physical layer function of terminating an incoming

3 physical link at a router, perform link-layer function to interoperate with the link layer at the other side of the incoming link, and perform lookup function at the input ports. 4. What does the Network layer consist of? Look to chapter 4 slide 7 5. What does IP datagram format (IPv4) consist of? Look to chapter 4 slides Look to the example in slide Do routers have IP addresses? If so, how many Answer: Yes. They have one address for each interface 8. What is the 32-bit binary equivalent of the IP address ? Answer: What is the Class-based Addressing (A, B, C, D, and E) Answer: Chapter 4 slide How many subnets are there in the following figure? 11. There are three subnets

4 12. What are LAN Topologies(Physical)? advantages and disadvantage 1) Bus 2) Star 3) Ring 4) Switched 5) Daisy chains 6) Hierarchies Answers: slides What are WAN Topologies(Physical)? advantages and disadvantage Answer: chapter 4 slides Chapter 5 1. What does the Network layer consist of? Answer: chapter 5 slide 7 2. Review the examples of Dijsktra s algorithm (slides 14-19); you must understand, the process and steps of forward table generation 3. Review Distance vector algorithm (slides 22-28) Chapter 6 What is Address Resolution Protocol How to Detect and Correct Errors? What is Parity Checking of Error Detection What is Channel partitioning MAC protocols: TDMA

5 What is Channel partitioning MAC protocols: FDMA What is CSMA, CSMA/CD, CSMA/CA, Aloha, slotted and pure unslotted Aloha