471
MIDTERM CMPT (50 minutes, 100 points)
1. How would you approach each of the following tasks, given you were working on a LINUX network? a. (3 points) Set the MTU (maximum transmission unit) for ethernet interface eth2 to 7200 bits. b. (4 points) Add a new entry to the routing table. The new entry you wish to add is for network 128.23.68.0/22. Packets destined for this network must leave the host through eth1 and travel to a router with address 162.12.17.41. 2. (10 points) For each of the following protocols draw an annotated diagram showing the encapsulation of a protocol message within an Ethernet frame. Indicate the protocol layer in which messages for each protocol are constructed. In your diagrams be sure to include all levels of encapsulation that apply (IP, TCP, UDP, etc.). Label each diagram to show the length of each header (see final page of exam for protocol message and header formats) and the type of header (UDP etc.). a. ARP b. ICMP c. DHCP d. BOOTP e. TCP 3. Consider how to capture packets passing a host on a connected network a. (2 points) Briefly explain the differences between a capture filter and a display filter in the ethereal packet capture application. b. (3 points) What capture filter would you use to capture ARP packets passing in both directions between your local host and the server seasons (172.16.1.1) c. (2 points) If your capture filter selects all UDP packets, how would you filter out the RIP and YPSERV packets (two protocols we have not yet studied) using a display filter. 4. (4 points) Briefly, explain the purpose of each of the following socket programming functions. a. select b. listen 5. Two networks belonging to company A have CIDR address blocks of 202.78.32.0/20 and 202.78.64.0/21. Company As router has IP address 202.79.64.23 and sends packets from the company site to the internet through interface eth2. Company As available block of addresses is 202.78.0.0/17, except for 202.78.48.0/22, which already belonged to company B when Company As network allocation was made. Company B has a single network with CIDR address block 202.78.48.0/22. Company Bs router has IP address 202.78.48.11 and sends packets from the company site to the internet through eth0. a. (2 points)How many IP addresses are allocated for each of company As networks? Briefly explain how you came to this conclusion. b. (3 points) Briefly explain the hierarchical address allocation scheme on which CIDR networks allocations are based. Your answer should not exceed six sentences. c. (4 points) A router at Company C, a company operating in the same city as Company A and Company B, has an entry or entries in its routing table to send information to Company A and Company B. Company Cs router uses interface eth1 to communicate with the internet. What would be the minimum number of entries in Company Cs routing table needed to guarantee
packets were correctly delivered to both company A an company B? What would those routing table entries be? Give the network id, gateway, netmask and interface for each routing table entry. d. (3 points) You work for Company A and are asked to allocate a new network. The network will need to service 512 hosts. What is an efficient CIDR address block allocation for this network. Your division of the company is allocated addresses in your companies CIDR block between 202.78.32.0 and 202.78.75.255, you must allocate your new network within this address range.
PART 2: PROBLEMS (60 of 100 marks) Choose
2 of the following 3 problems.
6. Consider the DHCP state machine l below a. (12 points) What path would be taken through the state machine to establish a new lease? Give a detailed step by step description of the interactions between DHCP servers and the client making the DHCP request. Relate the appropriate steps to the states and transitions illustrated for the state machine. Be sure to mention any DHCP messages (including their type, DHCPREQUEST ) sent and the source and recipient/s of each such message, in particular be sure to indicate if a particular message is broadcast or unicast.. b. (6 points) Explain why each message that is broadcast in your description in a) is broadcast rather than unicast. c. (3 points) Indicate, by drawing the relevant part of the state machine the diagram, path taken through the state machine when a lease is renewed. Assume all packets are delivered successfully, no packets are lost. d. (6 points) Indicate, by drawing the relevant part of the state machine diagram, the path taken through the state machine when a lease is renewed. Assume that there are multiple DHCP servers on the network segment in question and the DHCP server that granted the original server is not available. e. (3 points) Consider a network composed of multiple segments connected by routers. Can a client on a segment without a DHCP server reach a DHCP server on another network segment? If so what conditions must be satisfied, if not why?
INIT
DHCPREQUEST
7. (30 points) Explain what happens when you ping a host that does not have an entry in your local hosts ARP cache. You answer should be in the form of a chronological series of events that occur as the ping request and reply are assembled, sent and processed Include in your answer
What happens to the ARP cache on your host? What happens to the ARP cache on the destination host? What happens to all packets that will be sent to complete the ping (both requests and replies)?
8. A series of packets captured when a TCP connection was made to transfer a packet of data. The connection was made to a passive server socket. The packets, as produced by tcpdump, are shown below. Each packet is labeled with a capital letter to use to refer to that packet in your answers. Based on the packets below and the TCP state machine also shown below answer the following questions. a. (5 points) What is the MTU negotiated as the TCP connection is established? Explain how to determine the value of the MTU from the information supplied in the first two packets of the tcpdump below. b. (3 points) Could any of the packets in the TCP dump below be fragmented if they reached a network with a smaller MTU? Why or why not? (Be brief, 1-2 sentences) c. (14 points) Draw a partial TCP state machine showing only the states that are passed through when the captured packets below are transferred. Make sure to indicate (on your partial state machine diagram), using the labels added to the tcpdump output (A-H), which packet in the tcpdump corresponds to each packet that is received or sent to initiate a transition. d. (8 points) Discuss briefly which TCP flags are set in packets C, F and G and why those flags are set. Also explain briefly why the length of these packets is 52 octets.
A: 11:48:52.913278 IP (tos 0x0, ttl 64, id 32738, offset 0, flags [DF], proto 6, lengt...