tailieunhanh - Internetworking with TCP/IP- P52

Internetworking with TCP/IP- P52: 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 | Sec. Named Items And Syntax Of Names 469 The syntax of a name does not determine what type of object it names or the class of protocol suite. In particular the number of labels in a name does not determine whether the name refers to an individual object machine or a domain. Thus in our example it is possible to have a machine named gwen .purdue. edu even though cs .purdue. edu names a subdomain. We can summarize this important point One cannot distinguish the names of subdomains from the names of I individual objects or the type of an object using only the domain name I syntax. Mapping Domain Names To Addresses In addition to the rules for name syntax and delegation of authority the domain name scheme includes an efficient reliable general purpose distributed system for mapping names to addresses. The system is distributed in the technical sense meaning that a set of servers operating at multiple sites cooperatively solve the mapping problem. It is efficient in the sense that most names can be mapped locally only a few require internet traffic. It is general purpose because it is not restricted to machine names although we will use that example for now . Finally it is reliable in that no single machine failure will prevent the system from operating correctly. The domain mechanism for mapping names to addresses consists of independent cooperative systems called name servers. A name server is a server program that supplies name-to-address translation mapping from domain names to IP addresses. Often server software executes on a dedicated processor and the machine itself is called the name server. The client software called a name resolver uses one or more name servers when translating a name. The easiest way to understand how domain servers work is to imagine them arranged in a tree structure that corresponds to the naming hierarchy as Figure illustrates. The root of the tree is a server that recognizes the top-level domains and knows which server .