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As an example of why layered protocols are important, consider communication between two companies, Zippy Airlines and its caterer, Mushy Meals, Inc. Every month, the head of passenger service at Zippy asks her secretary to contact the sales manager’s secretary at Mushy to order 100,000 boxes of rubber chicken. Traditionally, the orders have gone via the post office. However, as the postal service deteriorates, at some point the two secretaries decide to abandon it and communicate by FAX. They can do this without bothering their bosses, since their protocol deals with the physical transmission of the orders, not their contents.. The physical layer is concerned with transmitting the 0s and 1s. How many volts to use for 0 and 1, how many bits per second can be sent, and whether transmission can take place in both directions simultaneously are key issues in the physical layer. In addition, the size and shape of the network connector (plug), as well as the number of pins and meaning of each are of concern here.. 2. Variable bit rate traffic but with bounded delay.. 2.3.5. Buffered versus Unbuffered Primitives. 3. A single point of failure in the system should be avoided.. whereMaxLifetime is the maximum time a message can live andMaxClockSkew is how far from UTC the clock might be at worst. Any timestamp older thanG can safely be removed from the table because all messages that old have died out already. If an incoming message has an unknown connection identifier, it is accepted if its timestamp is more recent thanG and rejected if its timestamp is older thanG because anything that old surely is a duplicate. In effect,G is a summary of the message numbers of all old messages. Every?T, the current time is written to disk.. After sending out requests asking permission to enter a critical region, a process sits back and waits until everyone else has given permission. As soon as all the permissions are in, it may enter the critical region. When it exits the critical region, it sendsOK messages to all processes on its queue and deletes them all from the queue.. [Картинка: any2fbimgloader76]. Accessing the private global variables is a bit tricky, however, since most programming languages have a way of expressing local variables and global variables, but not intermediate forms. It is possible to allocate a chunk of memory for the globals and pass it to each procedure in the thread, as an extra parameter. While hardly an elegant solution, it works.. Before considering the case of faulty processors, let us look at the “easy” case of perfect processors but communication lines that can lose messages. There is a famous problem, known as thetwo-army problem,which illustrates the difficulty of getting even two perfect processors to reach agreement about 1 bit of information. The red army, with 5000 troops, is encamped in a valley. Two blue armies, each 3000 strong, are encamped on the surrounding hillsides overlooking the valley. If the two blue armies can coordinate their attacks on the red army, they will be victorious. However, if either one attacks by itself it will be slaughtered. The goal of the blue armies is to reach agreement about attacking. The catch is that they can only communicate using an unreliable channel: sending a messenger who is subject to capture by the red army.. Unlike the OSI and Internet protocol suites, the TTP protocol consists of a single layer that handles end-to-end data transport, clock synchronization, and membership management. A typical packet format is illustrated in Fig. 4-29. It consists of a start-of-packet field, a control field, a data field, and a CRC field.. PRAM consistency is interesting because it is easy to implement. In effect it says that there are no guarantees about the order in which different processes see writes, except that two or more writes from a single source must arrive in order, as though they were in a pipeline. Put in other terms, in this model all writes generated by different processes are concurrent.. Fig. 6-40. Partial broadcasting of tuples and templates.. 7.1.4. The Amoeba Microkernel. 10.6.1. Security Model.