TCP IP Illustrated

The reply of ok in line 8 doesnt mean that the

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Unformatted text preview: ip-illustrated/nfs_netw.htm (16 of 23) [12/09/2001 14.47.56] Chapter 29. NFS: Network File System 5 6 7 8 9 10 35.769875 (35.7590) 35.773432 0.0036) 35.775236 0.0018) 35.780914 0.0057) 35.782339 0.0014) 35.992354 0.2100) sun.7ad4 > bsdi.nfs: 104 getattr ( ( ( ( ( bsdi.nfs > sun.7ad4: reply ok 96 sun.7ad5 > bsdi.nfs: 112 lookup "Mail" bsdi.nfs > sun.7ad5: reply ok 28 sun.7ad6 > bsdi.nfs: 144 mkdir "Mail" bsdi.nfs > aun.7ad6: reply ok 128 Figure 29.8 NFS operations for cd to NFS directory, then mkdir. Changing our directory causes the client to call the GETATTR procedure twice (lines 14). When we create the new directory, the client calls the GETATTR procedure (lines 5 and 6), followed by a LOOKUP (lines 7 and 8, to verify that the directory doesn't already exist), followed by a MKDIR to create the directory (lines 9 and 10). The reply of OK in line 8 doesn't mean that the directory exists. It just means the procedure returned, tcpdump doesn't interpret the return values from the NFS procedures. It normally prints OK and the number of bytes of data in the reply. Statelessness One of the features of NFS (critics of NFS would call this a wart, not a feature) is that the NFS server is stateless. "The server does not keep track of which clients are accessing which files. Notice in the list of NFS procedures shown earlier that there is not an open procedure or a close procedure. The LOOKUP procedure is similar to an open, but the server never knows if the client is really going to reference the file after the client does a LOOKUP. The reason for a stateless design is to simplify the crash recovery of the server after it crashes and reboots. Example: Server Crash In the following example we are reading a file from an NFS server when the server crashes and reboots. This shows how the stateless server approach lets the client "not know" that the server crashes. Other than a time pause while the server crashes and reboots, the...
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