Computer Networks Summer 2013

Transcription

1 Computer Networks Summer 2013 Homework 3 Due Date: 6/23/2011 Problem 1 1. Suppose a TCP message that contains 2048 bytes of data and 20 bytes of TCP header is passed to IP for delivery across two networks of the Internet (i.e., from the source host to a router to the destination host). The first network uses 14-byte headers and has an MTU of 1024 bytes. The second uses 8-byte headers with an MTU of 512 bytes 1. Each network s MTU gives the size of the largest IP datagram that can be carried in a link-layer frame. Give the sizes and offsets of the sequence of fragments delivered to the network layer at the destination host. Assume all IP headers are 20 bytes. Consider the first network. Packets have room for = 990 bytes of IP-level data; because 990 is not a multiple of 8 each fragment can contain at most 8 990/8 = 984 bytes. We need to transfer = 2068 bytes of such data. This would be fragmented into fragments of size 984, 984, and 100. Over the second network, the 100-byte packet would be unfragmented but the 984-data-byte packet would be fragmented as follows. The network plus IP headers total 28 bytes, leaving = 484 bytes for IP-level data. Again rounding down to the nearest multiple of 8, each fragment could contain 480 bytes of IP-level data. 984 bytes of such data would become fragments with data sizes 480, 480, and Suppose hosts A and B have been assigned the same IP address on the same Ethernet, on which ARP is used. B starts up after A. What will happen to A s existing connections? After B broadcasts any ARP query, all stations that had been sending to A s physical address will switch to sending to B s. A will see a sudden halt to all arriving traffic. (To guard against this, A might monitor for ARP broadcasts supposedly coming from itself; station A might even immediately follow such 1 By the way, this is a hypothetical situation, since this MTU value is an illegally small MTU for IP.

2 broadcasts with its own ARP broadcast in order to return its traffic to itself. It is not clear, however, how often this is done.) 3. Assume that a station can send an ARP to query the network on startup for its own IP address. Explain how self-arp, might help with this problem. If B uses self-arp on startup, it will receive a reply indicating that its IP address is already in use, which is a clear indication that B should not continue on the network until the issue is resolved.

3 Problem 2 1. The IP header checksum only verifies the integrity of IP header. Discuss the pros and cons of doing the checksum on the header part versus on the entire packet. Error checking in the header is more important because the packet is routed according to the header information. In addition, the delivery of the data at the destination to the higher layers also requires the header information. Thus error checking of the header protects against miss-delivery of the information. Restricting the error checking to the header also simplifies the implementation in the nodes, requires less checksum bits, and prevents unnecessary packet discard. Some higher layers can tolerate some data errors, and higher layers also have the option of performing retransmission. 2. Identify the address class of the following IP addresses: ; ; ; ; An IP address has a fixed length of 32 bits, where the most significant bits identify the particular class. Therefore, to identify the address class we need to convert the dotted-decimal notation back into its binary counterpart, and compare the binary notation to the class prefixes. The first few bits (shown in red) of the address can be used to determine the class Class C Class B Class A Class B Class E 3. What are all the possible subnet masks for the Class C address space? List all the subnet masks in dotted-decimal notation, and determine the number of hosts per subnet supported for each subnet mask.

4 supports 126 hosts (not including the broadcast address) supports 62 hosts supports 30 hosts supports 14 hosts supports 7 hosts supports 3 hosts and are not practically usable.

5 Problem 3 A university has 150 LANs with 100 hosts in each LAN. Suppose the university has one Class B address. 1. Design an appropriate subnet addressing scheme that allows for flexibility. Show the subnet mask. A Class B address has 14 bits for the network ID and 16 bits for the host ID. To design an appropriate subnet addressing scheme we need to decide how many bits to allocate to the host ID versus the subnet ID. We can choose either 8 bits or 7 bits to identify the hosts. If we allocate 8 bits for to identify the host, as shown below, then there are sufficient subnet-id bits to cover up to 2 8 =256 LANs and enough host-id bits to cover up to 256 hosts for each LAN. The subnet mask in this case is Network ID Subnet ID Host ID If we allocate 7 bits for to identify the host, as shown below, then there are sufficient subnet-id bits to cover up to 2 9 =512 LANs and enough host-id bits to cover up to 128 hosts for each LAN. The choice between 7 or 8 bits to represent the hosts depends on which is likely to grow more, the number of subnets or the number of hosts in a LAN. Alternatively a variable-length prefix scheme using 7-bit host addresses, and grouping these form larger subnets provides greater flexibility in accommodating future changes. 2. Design an appropriate CIDR addressing scheme. CIDR addressing scheme involves devising a prefix length that indicates the length of the network mask. In this case, 8 bits are required to identify each LAN (since 127 < 150 < 255) and 7 bits are required to identify each host in each LAN (since 63 < 100 < 127). Therefore a CIDR address would use a 25-bit prefix, and thus have an address of the form address/ Perform CIDR aggregation on the following /24 IP addresses: /24; /24; /24; / / 22 = / 22 = / 22 = / 22 =

6 Mask = The resulting prefix is /22.

7 Problem 4 1. Assume that there are N routers in the network and that every router has m neighbors. Estimate the amount of memory required to store the information used by the distance-vector routing. Each node needs the distance to each neighbor and the distance from each neighbor to all destinations which is m(n 1) entries. Assuming E bytes for each entry the amount of memory is m(n 1)E. 2. Assume that there are N routers in the network and that every router has m neighbors. Estimate the amount of memory required to store the information by the link-state algorithm. Each node needs the information for all links across the network. Each node is connected to m links and there are N nodes in the network. Therefore the total number of links in the network is (1/2)Nm entries. Assuming E bytes per entry the amount of memory is (1/2)mNE. 3. Suppose a network uses distance-vector routing. Explain what happens if the router sends a distance vector with all zeros? A distance vector with all zeros means that the node has distance zero to all other nodes. This will prompt all neighbors to route all their packets through the given router. Eventually all packets in the network will be routed to this router, resulting in what can be characterized as a black hole. 4. Suppose a network uses link-state routing. Explain what happens if the router fails to claim a link that is attached to it. The link will be eventually omitted from the routing tables and as a result will not be utilized. This will result in some routing decisions that are not the shortest (optimal) paths. Loss of connectivity is also possible. Moreover, a routing loop may occur due to confusion among routers as to which links they are attached to. 5. Suppose a network uses link-state routing. Explain what happens if the router claims to have a link that does not exist. If a router claims to have a link that does not exist, it reports false topology information to other routers within the network. If this non-existing link belongs to a shortest path, all the packets that are sent via this path will be lost. This will affect the network performance severely.