NAPIER UNIVERSITY SCHOOL OF COMPUTING

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1 NAPIER UNIVERSITY SCHOOL OF COMPUTING Level III SESSION 2000/2001 Duration: 2 hours Computer Networks and Distributed Systems MODULE NO: CO32006 (MM32021) There are SIX questions in this paper Attempt THREE questions. There are 4 pages in this paper. Examiners: W.Buchanan G.Russell A.Scott

2 Student Workload Lectures/Tutorials Practicals/Project Work Examination Module Organiser 24 hours 12 hours 2 hours Assessment Examination 40% (Any 3 from 6) Project 60% Aims The main learning outcomes are: A1. To provide an understanding of importance of the OSI model, and how standard protocols and networking types fit into this model. A2. To describe the advantages and disadvantages of differing network topology types, and how topology affects the performance of a network. A3. To outline the operation of typical networking technologies, especially Ethernet and ATM. A4. To describe methods of routing used on the Internet and with ATM networks. A5. To understand the architecture of the Internet, and how data is routed from one node to another, over interconnected networks. A6. To understand how data travels from one application to another, over a network. Module content The areas covered are: C1. Network fundamentals. OSI model. Data encapsulation. Network Topologies. Network elements: hubs, bridges, routers and switches. Peer-to-peer and client/server networks. C2. LAN/WAN network types. Practical network types, especially Ethernet and ATM. C3. Network protocols. TCP/IP. IP: Functions, IP addressing, IP routing, IP header, IP addressing, subnetting. TCP: Functions, TCP header, UDP header, three-way handshake. C4. Network security. Security: IP spoofing, session hi-jacking, and so on. Firewalls/Proxy servers. C5. Higher-level protocols. WWW: HTTP, architecture. Electronic Mail: Components, SMTP and MIME. Exam coverage The following table outlines how the examination covers the aims and content of the module. Aims Content Q A1 A2 A3 A4 A5 A6 C1 C2 C3 C4 C

3 Timetable Week Lecture/Tutorial 1 Lecture/Tutorial 2 Practical/Project Number 1 Unit 1 (Introduction) AS Unit 1 (OSI Model) WB Worksheet 1 (Network 2 Unit 2 (Networking Fundamentals) AS Tutorial 1 (Introduction) AS Introduction and Introduction to the WWW) 3 Tutorial 2 (Networking Worksheet 2 (ping) 4 Unit 3 (Ethernet) AS Fundamentals) Tutorial 3 (Ethernet) AS Worksheet 3 (tracert) 5 Unit 4 (ATM) AS Tutorial 4 (ATM) AS Worksheet 4 (netstat/ipconfig) 6 Unit 5 (TCP/IP) WB 7 Unit 6 (TCP/UDP) Tutorial 5 (TCP/IP) WB 8 WB Tutorial 6 (TCP/UDP) WB 9 Unit 7 (WWW) GR Tutorial 7 (WWW) GR Project 10 Unit 8 (Security) GR Tutorial 8 (Security) GR 11 Unit 9 (Firewalls) GR Tutorial 9 (Firewalls) GR 12 Unit 10 (Electronic Mail) GR Tutorial 10 (Electronic Mail) GR

4 1. (a) Give examples of real-time and non-real-time traffic and explain how ATM copes with the transmission of both types of profile. Explain why ATM does not provide reactive end-to-end flow control? Answer (b) (b) Explain the structure of an ATM cell header and the importance of each part. Using an example show how ATM cells are routed and explain how the VCI virtual paths and circuits route cells. (6) (6) (13) Total Marks [25] Coverage of aims: Content coverage: A3 (To outline the operation of typical networking technologies, especially Ethernet and ATM) A4 (To describe methods of routing used on the Internet and with ATM networks). C2 (LAN/WAN network types. Practical network types, especially Ethernet and ATM). Sting-in-the-tail : Related documents: The coverage of the importance of each of the parts of the ATM header. Sample answer (a) The following are a number of examples for the definition of real-time and non-real-time traffic: Delay sensitive Transmission type Data type Real-time control system Yes Short bursts Telephone/hi-fi music Yes Continuous/repeat File transfer/application progs. No High transfer rate (bursts) Teletex information No Slow transfer rate (bursts) Marking: 1 per example, [2] ATM overcomes the problems of transporting both computer type data and sampled real-time data by:

5 Analysing the type of connection to be made. For example, computer data requires a reliable connection, whereas real-time sampled data requires a connection with a low propagation time. [1] Analysing the type of data to be transmitted and knowing its traffic profile. Computer data tends to create bursts of traffic whereas real-time data will be constant traffic. [1] Reserving a virtual path for the data to allow the data profile to be transmitted within the required quality of service. [1] Splitting the data into small packets which have the minimum overhead in the number of extra bits. [1] (b) ATM Cell Header ATM cells (small fast packets ) Cell header ) VCI Label VCI (24 bits) (24 Control Checksum Contro Checksu (8 (8bits) (8 (8bits) Optional Option (32 bits) (32 Data Data (44 or 48 bytes) (44 or 48 The The virtual identifier (VCI) identifies the the route the the data. data. by by a fixed network time time Simple checksum for for detection. It It is is the the complement of of the the sum sum of of bytes in in the the Adaptation Layer fragmentation & assembly of of cells cells large large packets at at & The The control field field also also contains bits bits specify whether this this is is a flow flow control an an ordinary data data cell, cell, a bit bit to to whether this this packet can can be be congested network, and and Also, Also, a bit bit sets sets if if the the data data is is bytes Marking schedule: Diagram [2] Explanation of each Term in Header 1mark each [4]

6 (c) Routing example/ Virtual Paths ATM cell routing VCI labels between nodes in Routing IN OUT VCI=06 Routing IN OUT VCI= VCI=22 User 1 11 VCI=10 55 VCI=15 Transmissio: node UNI & negotiates destination, traffic type, peak 77 traffic needs, delay & cell requirement. Networkcomputes a route to determine best link for stage (to support requested service quality). The sends a connection set request to all 66 User 3 44 User 2 Virtual Channels and Virtual Paths A virtual is set-up between two users on a Cellstravel over this fixed path & through the reserved Oftenseveral virtual circuits travel through the same VC2 VC5 VC1 VC3 VC4 VC6 Virtual path Virtual path Transmission channel VP VP VC1 VC3 VC2 VC8 VC7 VC9 Virtual path VP Severalvirtual channels together(same start start end end points) to to create virtual path path Virtual Networkuser groups or or interconnected can can be be mapped to to virtual virtual paths paths and and are are thus thus administered Simplernetwork architecture which which consists groups (virtual paths) paths) with with individual (virtual Lessnetwork administration and and shorter times times arise arise from from fewer fewer set-up set-up Marking schedule: Routing example [7] Virtual channels and paths [6]

7 2. (a) Describe the structure of an IEEE Ethernet data frame and identify the usage of each field. (12) (b) What advantages do switches have over conventional hubs? Discuss both cut-through and store-and-forward switches. Explain their differences and incorporate the advantages and disadvantages of each. (13) Total Marks [25] Answer Coverage of aims: A1 (To describe the advantages and disadvantages of differing network topology types, and how topology affects the performance of a network) A3 (To outline the operation of typical networking technologies, especially Ethernet and ATM) Content coverage: C2 (LAN/WAN network types. Practical network types, especially Ethernet and ATM). Sting-in-the-tail : Related documents: To be able to contrast the methods used with different types of switches. Sample answer (a) 7 bytes 1 byte 6 bytes 6 bytes 2 bytes 4 bytes 96 bits Preamble Start delimiter Destination address Source address Length FCS Delay to 1500 bytes Data field (Logical Link Control) The preamble and delay components define the start and end of the frame. Preamble (seven bytes) precedes the Ethernet frame. Each byte of the preamble has a fixed binary pattern of and each node on the network uses it to synchronise their clock and transmission timings. It also informs nodes that a frame is to be sent and for them to check the destination address in the frame.

8 Start delimiter field (SDF) is a single byte (or octet) of It follows the preamble and identifies that there is a valid frame being transmitted. Source/destination addresses (2 or 6 bytes, Most Ethernet systems use a 48-bit MAC address for the sending and receiving node. Each Ethernet node has a unique MAC address, which is normally defined as hexadecimal digits. CRC (32 bits) 4 bytes for error checking, correct sequencing of bytes and multiple frames. LLC length (16 bits) 2 bytes. The LLC part may be up to 1500 bytes long and minimum 46 bytes long. Delay. The end of the frame there is a 96-bit delay period, which provides the minimum delay between two frames. This slot time delay allows for the worst-case network propagation delay. Marking Diagram [6] Description (1) mark for each field [7] (b) A network can be set up using segment switching and network hubs. [1] The store-and-forward switching technique reads the entire Ethernet frame before forwarding it, with the required protocol and at the correct speed, to the destination port. [2] The advantages of this are: Improved error check. Bad frames are blocked from the entering a network segment.[1] Protocol filtering. Allows the switch to convert from one protocol to another. [1] Speed matching. Ethernet 10Mbps or 100Mbps, ATM 155Mbps, FDDI 100Mbps. [1] The main disadvantage of this is: System delay. As the frame must be totally read before it is transmitted thus is a delay in transmission. [2] In the cut-through switching technique the data frame is forwarded to the destination before it is fully received. [2] The advantages of this are: Less delay (latency) between the reception and transmission of a data packet [1] The disadvantage of this technique is: Poor error checking. It does not have a chance to detect any errors, before it has started to transmit the received data frame. [2]

9 3. (a) With reference to the functions of the transport layer of the OSI model, discuss why the transport layer can be described as implementing a reliable, connection-oriented protocol. (9) (b) Explain, with reference to TCP and UDP headers, why TCP fits the description of being a reliable, connection-oriented protocol, but UDP does not. (9) (c) How does a TCP connection know when a remote connection has crashed, and why is it not possible for a previous connection to be confused with a current connection? (7) Total Marks [25] Answer Coverage of aims: Content coverage: Sting-in-the-tail : Related documents: A1 (To provide an understanding of importance of the OSI model, and how standard protocols and networking types fit into this model) A6 (To understand how data travels from one application to another, over a network) C1 (Network fundamentals. OSI model. Data encapsulation. Network Topologies. Network elements: hubs, bridges, routers and switches. Peerto-peer and client/server networks) C3 (Network protocols. TCP/IP. IP: Functions, IP addressing, IP routing, IP header, IP addressing, subnetting. TCP: Functions, TCP header, UDP header, three-way handshake). To be able to explain how a connection cannot be confused with a previous connection, and when a connection has crashed. Sample answer (a) With reference to the functions of the transport layer of the OSI model, discuss why the transport layer can be described as implementing a reliable, connection-oriented protocol. The main functions of the transport layer are: Synchronization and acknowledgement. Initially, when the transmitter makes contact with the receiver it makes a unique connection. The transmitter thus knows that the receiver is on-line, and willing to receive data. [2]

10 Acknowledgements and retransmissions. This allows the receiver to send back acknowledgements which tell the transmitter that the data segments have been received correctly. If no acknowledgements have been received, the transmitter can either resend the data, or can assume that the receiver has crashed and that the connection is to be terminated. [2] Flow control. This allows the receiver to tell the transmitter that it cannot receive any more data at present. This typically happens when the receiver has filled-up its receiving buffer. [2] Windowing. This is where the transmitter and the receiver agree on a window size when the connection is initially made. The window then defines the number of data segments that can be sent before the transmitter must wait for an acknowledgement from the receiver. [1] Multiple connections onto a single data stream. The transport layer takes data from one or more applications; it then marks them with a unique connection number and segment number. At the receiver these can be demultiplexed to the correct application program. [1] Reordering of data segments. All the data segments that are transmitted are marked with a sequence number. Thus if any are delivered in the incorrect order, or if any of them are missing, the receiver can easily reorder them or discard segments if one or more are missing. [1] (b) Explain, with reference to TCP and UDP headers, why TCP fits the description of being a reliable, connection-oriented protocol, but UDP does not. The TCP header contains: Source and destination port. This allows the source and destination to create a unique connection. [2] Sequence number. The allows data segments to be given unique value, thus data segments that are lost can be traced. [2] Acknowledgement number. This allows for data segments to be acknowledged. [2] Window. Allows for the negotiation of the window size which defines the number of data segments that can be sent before an acknowledgment is required. [2] UDP only defines a source and destination port [1]. (c) How does a TCP connection know when a remote connection has crashed, and why is it not possible for a previous connection to be confused with a current connection? The initial segment number [ISN] is generated from a 32-bit clock [1] which is incremented many times a second [1]. The clock has a rollover measured in days [1], data segments will not have the same data segment numbers from previous connections, as the each data segment identifies each of the bytes which are sent [2]. A connection will time-out before the 32-bit clock will rollover [2].

11 4. (a) Outline the difference between a static IP address and a dynamically-allocated IP address. How might a network manager use dynamically-allocated IP addresses to access to a network and/or the Internet? (7) (b) For a Class B address, derive the maximum number of hosts and the maximum number of subnets for a subnet mask of State all the assumption made. (8) (c) When a computer is to be connected a network and to the Internet, what networking parameters need to be set for it to communicate using TCP/IP? Also, briefly outline how a node is able to determine the MAC addresses of the hosts on a network, and how data packets are able to get passed a router and onto the Internet? (10) Total Marks [25] Answer Coverage of aims: Content coverage: Sting-in-the-tail : Related documents: A1 (To provide an understanding of importance of the OSI model, and how standard protocols and networking types fit into this model) A4 (To describe methods of routing used on the Internet and with ATM networks) A5 (To understand the architecture of the Internet, and how data is routed from one node to another, over interconnected networks) C3 (Network protocols. TCP/IP. IP: Functions, IP addressing, IP routing, IP header, IP addressing, subnetting. TCP: Functions, TCP header, UDP header, three-way handshake) To be able to explain who a computer can connect to a LAN and determine all the networking parameters, after being told the basic parameters, such as its own IP address, the IP address of the gateway, and so on. Many Internet experts would struggle to properly explain this. Sample answer (a) Outline the difference between a static IP address and a dynamicallyallocated IP address. How might a network manager use dynamicallyallocated IP addresses to access to a network and/or the Internet?

12 A static IP address is permanently assigned to a host, and is unique to that host [2], a dynamically-allocated IP address is granted by a server, such as a DHCP server [2]. A network manager may set up a table which maps MAC addresses to IP addresses [2], and only allow certain hosts to be granted an IP address[1]. (b) For a Class B address, derive the maximum number of subnets and the maximum number of hosts of each subnet for a subnet mask of State all the assumption made. Subnet mask is which gives a binary mask of [3]. Thus, as it is a Class B address the last two fields identifies the subnet and host part [1], there will thus be six bits for the subnet (64), and 10 bits for the hosts (1024) [2]. There are two addresses which are used for broadcast (all 1 s), and for the network (all 0 s). Thus there will only be 62 subnets, and 1022 hosts [2]. (c) When a computer is to be connected a network and to the Internet, what basic networking parameters need to be manually set up for it to communicate using TCP/IP? Also, briefly outline how a node is able to determine the MAC addresses of the hosts on a network, and how data packets are able to get passed a router and onto the Internet? Typical manual settings: Host IP address or DHCP server address [2]. Gateway IP address [1]. Domain name server IP address [1]. Subnet mask [1]. If a host does not know the IP address of a host on the network it sends out an ARP request with the required IP address, and the host with the correct IP address replies back with an ARP reply [3]. It then updates its ARP table. For the gateway, the host determines the MAC address of the router port with an ARP request/reply [1], it can then use the destination IP address with the MAC address of the router to send data packets over the router [1].

13 5. (a) Outline a set of best practices for creating a high-security network. (b) Explain how a firewall could be used to create a secure network, and outline the parameters in TCP data segments and in IP data packets which the firewall would filter with. (c) Explain the operation of an encryption tunnel using public-key encryption to create a secure connection. (8) (10) (7) Answer Coverage of aims: Content coverage: Sting-in-the-tail : Related documents: Total Marks [25] A1 (To provide an understanding of importance of the OSI model, and how standard protocols and networking types fit into this model) A5 (To understand the architecture of the Internet, and how data is routed from one node to another, over interconnected networks) A6 (To understand how data travels from one application to another, over a network) C3 (Network protocols. TCP/IP. IP: Functions, IP addressing, IP routing, IP header, IP addressing, subnetting. TCP: Functions, TCP header, UDP header, three-way handshake) C4 (Network security. Security: IP spoofing, session hi-jacking, and so on. Firewalls/Proxy servers) To be able to explain how a public key is passed between two parties, and how they use their private key to decode the data. Sample answer (a) Outline a set of best practices for creating a high-security network. Examples include (student should give eight): 1. BAN EXTERNAL CONNECTIONS. In a highly secure network, all external traffic should go through a strong firewall. There should be no other external connections on the network. If possible, telephone lines should be monitored to stop data being transferred over without going through firewall. [1] 2. BAN FLOPPY DISKS AND DATA STORAGE DEVICES. Employees should not be able to enter or leave the organization with any data on disk. Some organizations remove floppy disk drives from their computers to try and limit the possibility of transferring data. [1] 3. NO USER CAN INSTALL SOFTWARE. Viruses can be easily spread if users are allowed to install their own software. [1] 4. SECURE ACCESS TO RESOURCES. Typically users must use swipe cards, or some biometric technique to gain access to a restricted domain. [1]

14 5. LIMIT INTERNET ACCESS. Only key personnel should be given rights to access the external Internet. If possible the computers which access the Internet should be well protected against malicious programs. [1] 6. FIREWALLS USED BETWEEN DOMAINS. Internal hackers can be as big a problem as external hackers. Thus firewalls should be used between domains to limit access. [1] 7. BASE AUTHENTICATION ON MAC ADDRESSES. Network addresses do not offer good authentication of a user, as they can be easily spoofed. An improved method is to check the MAC address of the computer (as no two computers have the same MAC address). [1] 8. EVERY FILE AND OBJECT SHOULD HAVE UNIQUELY DEFINED PRIVILEDGES. Every file and resource should be uniquely setup for user privileges which can limit access. [1] Others might include: 9. EMPLOY SECURITY MANAGER. The security manager will be responsible for the design of the initial security model, and any changes to it. [1] 10. LOG EVERY EVENT. All the important security related events should be monitored within each domain. If possible they should be recorded over a long period of time. Software should be used to try and determine incorrect usage. (b) Explain how a firewall could be used to create a secure network, and outline the parameters in TCP data segments and in IP data packets which the firewall would filter with. Which ports are typically barred in a secure network? A firewall can be used to create secure networks by examining the TCP and IP parts of the TCP segment, and the IP data packet [1]. Typical tests are: Protocol. Typically either TCP, UDP or ICMP. [2] Source IP address. The firewall will grant and bar various source IP addresses either entering or leaving the firewall [1]. Destination IP address. The firewall will grant and bar various destination IP addresses either entering or leaving the firewall [1]. Source port. This will allow or bar certain TCP ports from entering or leaving the network for the source port [1] Destination port. This will allow or bar certain TCP ports from entering or leaving the network for a destination port [1]. Rules. Using one or more of the above [1]. Ports barred a typically FTP (because of the risk of transfer of files), and TELNET (because of the risk of remote log-in). [2] (c) Explain the operation of an encryption tunnel using public-key encryption to create a secure connection. Initially the two hosts each create two keys: a private one and a public one [2]. Each host then sends their public key to the other [2]. The data at one end is encrypted with the public key of the other, and sent. Only the private key of the other host can then decrypt the encrypted message [2]. When the connection is complete the keys are erased [1].

15 6. (a) Illustrate the different layers of the OSI 7-layered model, and provide an example of each layer. Also, with reference to the OSI model, identify the function of the three lower layers and discuss the function of the repeaters, bridges and routers. (b) Outline the operation of the HTTP protocol and how WWW caches are used in the transfer of WWW pages. What advantages does a cache-based system have over one without caches? Also explain why a WWW client knows that the file has been received correctly, or not. (13) (12) Answer Coverage of aims: Content coverage: Sting-in-the-tail : Related documents: Total Marks [25] A1 (To provide an understanding of importance of the OSI model, and how standard protocols and networking types fit into this model) A6 (To understand how data travels from one application to another, over a network) C1 (Network fundamentals. OSI model. Data encapsulation. Network Topologies. Network elements: hubs, bridges, routers and switches. Peer-to-peer and client/server networks. C5 (Higher-level protocols. WWW: HTTP, architecture. Electronic Mail: Components, SMTP and MIME). To be able to identify that the reliability is built into the TCP level and not at the session level. Sample answer (a) Illustrate the different layers of the OSI 7-layered model, and provide an example of each layer. Also, with reference to the OSI model, identify the function of the three lower layers and discuss the function of the repeaters, bridges and routers. Marking: Draw OSI model [2], with examples [2].

16 Functions of Physical: Electrical and Mechanical [2], Data link: Error detection, Framing and Physical Addressing [2] and Network: Addressing and Routing [2]. Repeaters operate at the physical layer and boost signals and clean digital pulses [1]. Bridges operate at the data link layer and operate on MAC addresses [1] Routers operate at the network layer on IP addresses [1]. (b) Describe the operation of the HTTP protocol and how caches in the transfer of WWW pages. Also explain why a WWW client knows that the file has been received correctly, or not. Marking: Basic operation of HTTP: Uses a TCP/IP connection [1]. Client issues a GET with the require name, and content-type [2]. Server returns back an integer response code (such as Okay), and the file [2]. With cache: 1. Client requests page, with date last modified. [1] 2. Request made to server for page with date last modified. [1] 3. Client receives a message that page has not changed. [1] 4. WWW client uses local or networked cache version if not modified since the previously loaded version. [1] A WWW cache reduces the time to download a file, it the file is unchanged [1]. The reliability is built into the TCP level, and not the session level, thus once the page is fully received, it can be assumed free of error [2].