The trailer has four fields. The first two are each 1 byte long and are not used. Then comes a 2-byte field giving the packet length, and a 4-byte checksum over the packet, padding, and trailer.. The third approach is to take advantage of the fact that client-server communication is structured as a request from the client to the server followed by a reply from the server to the client. In this method, the client is blocked after sending a message. The server’s kernel does not send back an acknowledgement. Instead, the reply itself acts as the acknowledgement. Thus the sender remains blocked until the reply comes in. If it takes too long, the sending kernel can resend the request to guard against the possibility of a lost message. This approach is shown in Fig. 2-13(b).. [Картинка: any2fbimgloader69]. 7. Ricart and Agrawala’s algorithm has the problem that if a process has crashed and does not reply to a request from another process to enter a critical region, the lack of response will be interpreted as denial of permission. We suggested that all requests be answered immediately, to make it easy to detect crashed processes. Are there any circumstances where even this method is insufficient? Discuss.. In many distributed systems, it is possible to have multiple threads of control within a process. This ability provides some important advantages, but also introduces various problems. We will study these issues first. Then we come to the subject of how the processors and processes are organized and see that several different models are possible. Then we will look at processor allocation and scheduling in distributed systems. Finally, we consider two special kinds of distributed systems, fault-tolerant systems and real-time systems.. [Картинка: any2fbimgloader91]. For the rest of this section, we will discuss processor faults or crashes, but this should be understood to mean equally well process faults or crashes (e.g., due to software bugs). Two types of processor faults can be distinguished:. After the link fromA toB is removed,B ‘s reference count is reduced from 2 to 1, which on paper is fine. However,B is now Unreachable from the root of the file system(A). The three directories,B, D, andE, and all their files are effectively orphans.. Fig. 6-4. An example of how a cache ownership protocol works.. If the tuple is not present, or if the broadcast is not received at the machine holding the tuple, the requesting machine retransmits the broadcast request ad infinitem, increasing the interval between broadcasts until a suitable tuple materializes and the request can be satisfied. If two or more tuples are sent, they are treated like localouts and the tuples are effectively moved from the machines that had them to the one doing the request. In fact, the runtime system can even move tuples around on its own to balance the load. Carriero et al. (1986) used this method for implementing Linda on a LAN.. The third job of the kernel is to handle interprocess communication. Two forms of communication are provided: point-to-point communication and group communication. These are closely integrated to make them similar.. The final call,ResetGroup, is used to recover from crashes. It specifies how many members the new group must have as a minimum. If the kernel is able to establish contact with the requisite number of processes and rebuild the group, it returns the size of the new group. Otherwise, it fails. In this case, recovery is up to the user program.The Amoeba Reliable Broadcast Protocol. Although this strategy wastes space due to external fragmentation, both in memory and on disk, it has the advantage of extreme simplicity and high performance. A file on disk can be read into memory in a single operation, at the maximum speed of the disk, and it can be transmitted over the network at the maximum speed of the network. As memories and disks get larger and cheaper, it is likely that the cost of the wasted memory will be acceptable in return for the speed provided.. Some of the directories in each user’s root are similar to those in UNIX, such asbin, dev, andetc. However, others are fundamentally different. One of these ishome, which is the user’s home directory.. 15. Why does the bullet server have uncommitted and committed files?.