TCP IP Illustrated

First accessing an nfs tile must be transparent to

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Unformatted text preview: ilesystems on a server. This differs from FTP (Chapter 27), which provides file transfer. With FTP a complete copy of the file is made. NFS accesses only the portions of a file that a process references, and a goal of NFS is to make this access transparent. This means that any client application that works with a local tile should work with an NFS file, without any program changes whatsoever. NFS is a client-server application built using Sun RPC. NFS clients access tiles on an NFS server by sending RPC requests to the server. While this could be done using normal user processes - that is, the NFS client could be a user process that makes explicit RPC calls to the server, and the server could also be a user process-NFS is normally not implemented this way for two reasons. First, accessing an NFS tile must be transparent to the client. Therefore the NFS client calls are performed by the client operating system, on behalf of client user processes. Second, NFS servers are implemented within the operating system on the server for efficiency. If the NFS server were a user process, every client request and server reply (including the data being read or written) would have to cross the boundary between the kernel and the user process, which is expensive. In this section we look at version 2 of NFS, as documented in RFC 1094 [Sun Microsystems 1988b]. A better description of Sun RPC, XDR, and NFS is given in [X/Open 1991]. Details on using and administering NFS are in [Stern 1991]. The specifications for version 3 of the NFS protocol were released in 1993, which we cover in Section 29.7. Figure 29.3 shows the typical arrangement of an NFS client and an NFS server. There are many subtle points in this figure. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/nfs_netw.htm (7 of 23) [12/09/2001 14.47.56] Chapter 29. NFS: Network File System Figure 29.3 Typical arrangement of NFS client and NFS server. 1. It is transparent to the client whether it's accessing a local file or an NFS...
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