Chapter 7

s it always crucial to know that the message you have sent has arrived at its destination safely? If your answer is yes, explain why. If your answer is no, give appropriate examples. Answer:

No. Many status-gathering programs work from the assumption that packets may not be received by the destination system. These programs generally broadcast a packet and assume that at least some other systems on their network will receive the information. For instance, a daemon on each system might broadcast the system’s load average and number of users. This information might be used for process migration target selection. Another example is a program that determines if a remote site is both running and accessible over the network. If it sends a query and gets no reply it knows the system cannot currently be reached.

17.8 A distributed system has two sites, A and B. Consider whether site A can distinguish among the following:

a. b. c.

B goes down. The link between A and B goes down.

B is extremely overloaded, and its response time is 100 times longer than normal.

What implications does your answer have for recovery in distributed systems? Answer: One technique would be for B to periodically send a I-am-up message to A indicating it is still alive. If A does not receive an I-am-up message, it can assume either B—or the network link—is down. Note that an I-am-up message does not allow A to distinguish between each type of failure. One technique that allows A better to determine if the network is down is to send an Are-you-up message to B using an alternate route. If it receives a reply, it can determine that indeed the network link is down and that B is up.

If we assume that A knows B is up and is reachable (via the I-am-up mechanism) and that A has some value N that indicates a normal response time, A could monitor the response time from B and compare values to N, allowing A to determine if B is overloaded or not.

The implications of both of these techniques are that A...