tailieunhanh - Internetworking with TCP/IP- P55

Internetworking with TCP/IP- P55: 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. Sharing By File Transfer 499 Sharing By File Transfer The alternative to integrated transparent on-line access is file transfer. Accessing remote data with a transfer mechanism is a two-step process the user first obtains a local copy of a file and then operates on the copy. Most transfer mechanisms operate outside the local file system . they are not integrated . A user must invoke a specialpurpose client program to transfer files. When invoking the client the user specifies a remote computer on which the desired file resides and possibly an authorization needed to obtain access . an account or password . The client contacts a server on the remote machine and requests a copy of the file. Once the transfer is complete the user terminates the client and uses application programs on the local system to read or modify the local copy. One advantage of whole-file copying lies in the efficiency of operations once a program has obtained a copy of a remote file it can manipulate the copy efficiently. Thus many computations run faster with whole-file copying than with remote file access. As with on-line sharing whole-file transfer between heterogeneous machines can be difficult. The client and server must agree on authorization notions of file ownership and access protections and data formats. The latter is especially important because it may make inverse translations impossible. To see why consider copying between two machines A and B that use different representations for floating point numbers as well as different representations for text files. As most programmers realize it may be impossible to convert from one machine s floating point format to another s without losing precision. The same can happen with text files. Suppose system A stores text files as variable-length lines and system B pads text lines to a fixed length. Transferring a file from A to B and back can add padding to every line making the final copy different from the original. .