tailieunhanh - Internetworking with TCP/IP- P24

Internetworking with TCP/IP- P24: TCP/IP has accommodated change well. The basic technology has survived nearly two decades of exponential growth and the associated increases in traffic. The protocols have worked over new high-speed network technologies, and the design has handled applications that could not be imagined in the original design. Of course, the entire protocol suite has not remained static. New protocols have been deployed, and new techniques have been developed to adapt existing protocols to new network technologies | 198 User Datagram Protocol UDP Chap. 12 informing all senders . rebooting a machine can change all the processes but senders should not be required to know about the new processes . Third we need to identify destinations from the functions they implement without knowing the process that implements the function . to allow a sender to contact a file server without knowing which process on the destination machine implements the file server function . More important in systems that allow a single process to handle two or more functions it is essential that we arrange a way for a process to decide exactly which function the sender desires. Instead of thinking of a process as the ultimate destination we will imagine that each machine contains a set of abstract destination points called protocol ports. Each protocol port is identified by a positive integer. The local operating system provides an interface mechanism that processes use to specify a port or access it. Most operating systems provide synchronous access to ports. From a particular process s point of view synchronous access means the computation stops during a port access operation. For example if a process attempts to extract data from a port before any data arrives the operating system temporarily stops blocks the process until data arrives. Once the data arrives the operating system passes the data to the process and restarts it. In general ports are buffered so data that arrives before a process is ready to accept it will not be lost. To achieve buffering the protocol software located inside the operating system places packets that arrive for a particular protocol port in a finite queue until a process extracts them. To communicate with a foreign port a sender needs to know both the IP address of the destination machine and the protocol port number of the destination within that machine. Each message must carry the number of the destination port on the machine to which the message is sent as well as .