Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download 1. What is flooding? Explain the need for sequence number and age
Transcript
CSE 4153 / 6153 Assignment 3 Fall 2008 1. What is flooding? Explain the need for sequence number and age in packets that are flooded. (5 points) Flooding: A packet arriving through one network interface of a router is forwarded to routers connected to all other interfaces. Sequence number keeps the flooding in check. Every router will flood a packet indicating the same source and sequence number only once. Age is required to prevent problems arising from wrapping around of sequence numbers. For example, assume that 4 bit sequence numbers are used to indicate values 0 to 15. Assume a scenario where a router receives a packet from a source S indicating a sequence number 5. Also assume that it had previously flooded a packet from S with sequence number 14. Now the router has to be able to distinguish unambiguously if the packet with sequence number 5 is fresher than the previously forwarded packet with sequence number 14. 2. Highlight the fundamental differences between distance vector and linkstate routing approaches. (6 points) In LS small link-state packets containing information about “a router and all its immediate neighbors” are flooded throughout the network. When LS is used a router A can determine the complete list of nodes in the path to every other node in the network. More specifically, all routers can compute the minimum spanning sink tree for all routers. In DV routers exchange large routing tables (consisting of information about every router in the network) with their neighbors. When DV is used the routers learn only about the next hop in the shortest path to reach any router. 3. Discuss the advantages and disadvantages of the three approaches for congestion control (warningbit, choke packets and hopbyhop choke packets) (6 points) Warning bit: No bandwidth overheads for control packets. One bit is set in the packet forwarded by a router (when it experiences congestion). The receiving host sets a bit in a packet sent in the reverse direction (for example, in the acknowledgment). The sender reduces the rate at which packets are sent. The disadvantage is the long delay before which the sender can start reducing traffic and thus relieve the congested router. Choke Packets: The router detecting congestion sends a warning packet directly to the sender. Provides quicker warning to the sender to reduce traffic. Hopbyhop Choke Packets: Router sends a choke packet to previous router which reduces rate in the line (network interface) through which the warning is received. Only if necessary the previous router forwards the waning to its previous hop. Provides very quick relief. 4. An IP packet received by a router has a 20 byte header and a payload of size 1000. The layer2 protocol used by the router does not support payload sizes greater than 400. Indicate the contents of the IP packets before and after fragmentation. Make some reasonable assumptions, and specify the values in the IP headers of every packet (the old packet and the fragments). (10 points) Each IP fragment can have at most 380 bytes of payload (380+20 bytes for header). Fragment offsets need to be a multiple of 8 – as 380 is not the maximum fragment size is 376 = 47*8. Need three fragments – first and second with 376 byte payload and the third with 248 byte payload. The fragment offsets are 0, 47, and 47*2 for the three fragments. The Identification number in the original un-fragmented packet and all fragments will be the same – assume 12345 Field Original Total Length 1020 Fragment 1 Fragment 2 Fragment 3 1020 1020 1020 Identification 12345 12345 12345 12345 DF 0 0 0 0 MF 0 1 1 0 Fragment Offset 0 0 47 94 5. A NAT with a public IP address 130.207.16.32 receives the following packets from hosts with private IP addresses behind the NAT Private IP Address PROTOCOL (in IP Source port (Source IP address Header) number (in in IP header) transport header) 10.2.5.1 TCP 2345 10.2.5.1 TCP 4456 10.2.5.1 UDP 2345 10.2.5.2 TCP 2345 10.2.5.3 UDP 2345 a) Are any changes made by the NAT to the destination IP address and the destination port number? (1 point) Only source IP (a private IP) and source port are modified by the NAT when a packet exits the network and into the public Internet. For packets that are received in response (all of which have the destination IP address of the NAT box) the NAT box will change the destination IP address (to the private IP address) and the destination port number. b) Depict the contents of the NAT table after the packets have relayed by the NAT (5 points) c) Show the contents of the three header fields (source and protocol in IP header and source port in transport header) to the three fields made by the NAT for all 5 cases. (5 points) Private IP PROTOCOL Address (in IP (Source Header) IP address in IP header) Source port Private IP number (in transport header) 10.2.5.1 TCP 2345 10.2.5.1 TCP 10.2.5.1 NAT Table Contents Protocol Port Assigned Port Time Stamp 10.2.5.1 TCP 2345 10000 T1 4456 10.2.5.2 TCP 4456 10001 T2 UDP 2345 10.2.5.1 UDP 2345 10000 T3 10.2.5.2 TCP 2345 10.2.5.2 TCP 2345 10002 T4 10.2.5.3 UDP 2345 10.2.5.3 UDP 2345 10001 T5 Private IP PROTOCOL Address (in IP (Source Header) IP address in IP header) Source port Source IP number (in transport header) Packets Exiting NAT Protocol Source Port 10.2.5.1 TCP 2345 10.2.5.1 TCP 10000 10.2.5.1 TCP 4456 10.2.5.2 TCP 10001 10.2.5.1 UDP 2345 10.2.5.1 UDP 10000 10.2.5.2 TCP 2345 10.2.5.2 TCP 10002 10.2.5.3 UDP 2345 10.2.5.3 UDP 10001
