tailieunhanh - distributed operating system phần 8

Tham khảo tài liệu 'distributed operating system phần 8', công nghệ thông tin, đồ họa - thiết kế - flash phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 414 CASE STUDY 1 AMOEBA CHAP. 7 the protection offered by the one-way function used to derive put-ports from get-ports. In addition if a server crashes it will pick a new FLIP address when it reboots. Attempts to use the old FLIP address will time out allowing the RPC layer to indicate failure to the client. This mechanism is how at-most-once semantics are guaranteed. The client however can just try again with the same put-port if it wishes since that is not necessarily invalidated by server crashes. FLIP over Wide-Area Networks FLIP also works transparently over wide-area networks. In Fig. 7-19 we have three local-area networks connected by a wide-area network. Suppose that the client A wants to do an RPC with the server E. A s RPC layer first tries to locate the put-port using a maximum hop count of 1. When that fails it tries again with a maximum hop count of 2. This time c forwards the broadcast packet to all the gateways that are connected to the wide-area network namely D and G. Effectively c simulates broadcast over the wide-area network by sending individual messages to all the other gateways. When this broadcast fails to turn up the server a third broadcast is sent this time with a maximum hop count of 3. This one succeeds. The reply contains E s network address and FLIP address which are then entered into A s routing table. From this point on communication between A and E happens using normal point-to-point communication. No more broadcasts are needed. LAN Fig. 7-19. Three LANs connected by a WAN. Communication over the wide-area network is encapsulated in whatever protocol the wide-area network requires. For example on a TCP IP network c SEC. COMMUNICATION IN AMOEBA 415 might have open connections to D and G all the time. Alternatively the implementation might decide to close any connection not used for a certain length of time. Although this method does not scale well to thousands of LANs for modest numbers it works quite well. In practice few .