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DESIGN AND ANALYSIS OF DISTRIBUTED ALGORITHMS phần 9

3. (ø = α = β = ø) tham nhũng: một tin nhắn được gửi bởi x để y tại thời điểm t, nhưng một trong những nội dung khác nhau nhận được y tại thời điểm t + 1. Trong khi bản chất của thiếu sót và tham nhũng là khá rõ ràng, đó là bổ sung có thể xuất hiện kỳ lạ và là nhân tạo lần đầu tiên. | 468 COMPUTING IN PRESENCE OF FAULTS 3. 0 a ft 0 corruption a message is sent by x to y at time t but one with different content is received by y at time t 1. While the nature of omissions and corruptions is quite obvious that of additions may appear strange and rather artificial at first. Instead it describes a variety of situations. The most obvious one is when sudden noise in the transmission channel is mistaken for a message. However the more important occurrence of additions in sytems is rather subtle When we say that the received message was not transmitted what we really mean is that it was not transmitted by any authorized user. Indeed additions can be seen as messages surreptitiously inserted in the system by some outside and possibly malicious entity. Spam being sent from an unsuspecting site clearly fits the description of an addition. Summarizing additions do occur and can be very dangerous. These three types of faults are quite incomparable with each other in terms of danger. The hierarchy of faults comes into place when two or all of these basic fault types can occur in the system see Figure 7.2 . The presence of all three types of faults creates what is called a Byzantine faulty behavior. Notice that most localized and permanent failures can be easily modeled by communication faults for instance omission of all messages sent by and to an entity can be used to describe the crash failure of that entity. Analogously with enough dynamic communication faults of the appropriate type it is easy to describe faults such as send and receive failures Byzantine link failures and so forth. In fact with at most 2 n 1 dynamic communication faults per time unit we can simulate the interaction of one faulty entity with its neighbors regardless of its fault type Exercise 7.10.39 . As in the previous section we will concentrate on the Agreement Problem Agree p . The goal will be to determine if and how a certain level of agreement i.e. value of p can be reached in spite