quality if the network can sustain a higher rate without encountering congestion. Logically, the basic protocol event flows are as follows: a. (5 points) What is the most critical advantage of this scheme compared with using TCP? This scheme reacts the congestion b. (5 points) What is the most critical advantage of this scheme compared with using UDP? This scheme reacts to packet losses c. (5 points) Briefly describe a method to compute the received rate. How frequently would you report the rate to the sender, and is there any reason for limiting the frequency such rate reports? Over a defined fixed time interval, compute the number of packets received and the number of payload bits in those packets. # bits / time = transmission rate in bps (1 point). Report the rate one in every few received packets (1 in 20 say) or once every few seconds (1 point). Reason is that we need several packets worth of Page 5
data to get a reasonable rate calculation and also to limit the return bandwidth demand. d. (5 points) Briefly describe a method to determine when to increase the sending rate. What concern should you have about the pace at which you increase this? Set a threshold, say 10%, of the current sending rate. If received rate as reported back is less than or equal to sending rate – threshold, then reduce sending rate. Only increase sending rate again if the network can successful transmit the current rate for several (say five) packets, as indicated by the returned rate reports (in other words, do not double the rate based only one packet getting through successfully!) Question 5 (20 points) a. (5 points) A large percentage of the DNS requests that arrive at the root DNS servers are requests for DNS domains that don’t exist (e.g. ). One way to reduce the load experienced by the root nameservers would be to use negative caching , that is the negative response returned by the nameserver would be cached at the local nameserver so subsequent requests would be answered locally. Explain the costs of this solutions and why it would not sufficiently reduce the load on the root nameservers due to non-existing domains. The hit rate would be very small. The cost is all the additional storage at the caching server required to cache all the bogus names. b. (5 points) What are the benefits of the hierarchical namespace in DNS? Scalability, matching organization structure and name conflict resolution. c. (5 points) A domain in DNS is served by multiple nameservers. Describe the two main benefits from having multiple nameservers. What factors determine whether these multiple nameservers will in practice deliver these benefits? Scalability and redundancy in the face of server and network failures. The multiple servers should fail independently and therefore should be located in different geographic locations and have separate connections to the Internet. d. (5 points) Denial of Service attacks against the root nameservers is a major problem these days. Given that no more than 13 IP addresses can be allocated for
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- Summer '19