EE 122 Fall 2010 Discussion Section III 5 October 2010

Transcription

1 EE 122 Fall 2010 Discussion Section III 5 October Question 1: IP Header This is the IPv4 header structure we will need for the problems Kisco Inc. is a startup making Internet equipment. You are hired to design and implement an IPv4 router. a. Which header fields must be updated before the router sends out a packet? The TTL field and the checksum. b. Your router needs to add a 4-byte option to some IPv4 packets. Which header fields, including your answer to the previous question, need to be updated? The TTL field, the checksum, the header length, the total length, and the option fields. c. You find that the router prototype doesn't read or write the time-to-live field. What problems does this cause? What is the right way to deal with time-to-live field? The problem is that a packet may be forwarded forever (perhaps in a loop). The way a router should deal with the TTL is to decrement it by one, or drop the packet if the TTL is zero and return the ICMP message Time exceeded.

2 d. To lower the hardware cost, you decide to ignore the header checksum, so your router doesn't check or update it. You test three interconnected router prototypes in your lab without finding any problems. So is this a good idea? Why or why not? On one hand, this is not a good idea because the packet may be corrupted in the real Internet, and routers from other venders will drop the packet if the checksum is wrong. However, note that IPv6 does not include a checksum because of the end-to-end principle and because checksumming is an expensive operation unless supported in hardware, which is the norm, but not the ideal. e. To accelerate packet forwarding, you decide to parse the last four bytes in the header as the destination IP address. But it occasionally fails to get the right address. Why? How would you solve this problem? The reason it occasionally gets the wrong address is that packets might have options in the header. In this case, the last four bytes is an option, not the destination address. To solve this problem, you should use the absolute offset from the beginning of the packet (i.e., bytes 17-20). f. What is the minimum length of an IPv4 header? If you have 1MB data to send but each IPv4 packet can only be 40 bytes, how many bytes do you actually send at the IP layer? (Assume no transport layer header.) The minimum length of an IPv4 header is 20 bytes. In the situation given you would end up sending 2 MB. g. You find that the 10th byte of an IPv4 packet is 0x06. What does this tell you about the packet? This tells you that the packet contains a TCP segment. Question 2: Packet Fragmentation After you finish the router prototype, you need to test it with different packet switching technologies, which may have different MTU. In a test, your router has 3 links with MTU as below: Link MTU

3 a. The router receives a 600-byte IPv4 packet from link 3, and it needs to send it to link 2. What will happen? Which header fields need to be updated? The router needs to fragment the packet into smaller packets. The fields to be updated are the total length, the flag, the fragment offset, the TTL, and the checksum. b. If the 600 byte IPv4 packet from link 3 has a 20 byte header, at least how many packets need to be sent over link 2? Write down the values of the total length, flag and fragmentation offset fields for each outgoing packet. At least two packets need to be sent. Packet 1 will be of length 420, with a flag of 1 and an offset of 0. Packet 2 will be of length 200 with a flag of 0 and an offset of 50. c. The router receives 20 fragmented packets from link 1, and needs to send them to link 3. How many packets will be sent over link 3? 20 packets will be sent, because the router doesn t reassemble packets. Question 3: Addressing Your company grows very fast. It soon has 550 computers that need Internet access at the same time. You ask two ISPs about buying IP addresses. ISP BTT allocates IP addresses in Class A, B or C. ISP Horizon allocates IP adresses using CIDR. Since you are limited by your budget, you always want the smallest IP address range that can support all of your computers. a. If you buy one class of IP adressess from ISP BTT, which class will you buy? How many addresses do you get? B classs, 2 16 = 65,536 addresses b. If you buy from Horizon, how many bits in the address are the network prefix? How many addresses do you get? 22 bits, 1,024 addresses

4 c. Another choice is buying several class C IP address blocks from BTT. How many C classes do you need? How many addresses do you get? Three C classes, 768 addresses d. Not all IP addresses can be used for end hosts. How many concurrent hosts can a Class C address block accommodate? 254 hosts, because you can t use the address ending in 255 (because it is the broadcast address) nor the address ending in 0 (because that is the netmask). e. You get an selling /12 addresses, and the price per IP address is much cheaper than your ISP s. Will you consider the offer? Why? No. Because 10/8 is a private address block, everyone can use it internally for free. Question 4: NAT Your router sells well, but some customers want to have the NAT feature included. a. What are some reasons that some customers might need NAT? Some reasons include that the customer may not have enough public IP addresses, and to hide hosts in the local network, so that external hosts can t access them directly.

5 b. How does a NAT processes IPv4 headers differently from a router? It replaces the source IP address with its own address before sending packets to the Internet, and it replaces the destination IP address with the internal host s IP address before sending packets back into the internal network. c. Host A and B are behind different NAT boxes. A's IP address is , and B's IP is If A sends a packet to , will B get the packet? Why or why not? No, because A and B are in two different private networks, and a packet must have a public IP address to be forwarded in the Internet. d. A customer uses /24 for their private network, but 300 computers need Internet access, though not at the same time. Which protocol will you suggest the customer use to assign IP addresses? Why? The customer should use DHCP, because using this protocol, one IP address can be shared among different hosts dynamically. Question 5: Middleboxes A firewall is typically placed at the edge of a network to protect against attacks and intrusions. It inspects all the packets that enter and leave the network. Argue if firewalls violate or conform to the end-to-end principles and fate sharing. Firewalls violate both principles. It violates the end-to-end principle, because it provides filtering functionality that can arguably be done at the host. It violates fate sharing because it has state regarding which packets to let through, which if it fails or malfunctions, can cause packets not to get through between two hosts on opposite sides of the firewall even though both hosts are still alive and functioning. Credits: Questions 1 4 were adapted from the second homework of the fall 2009 offering of EE 122. Question 5 is from

6 Prayag Narula.