Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!magnus.acs.ohio-state.edu!tut.cis.ohio-state.edu!ucbvax!ASYLUM.SF.CA.US!romkey
From: romkey@ASYLUM.SF.CA.US (John Romkey)
Newsgroups: comp.protocols.tcp-ip
Subject: V3.2 vs V4 UDP and NFS
Message-ID: <9103230950.AA02367@asylum.sf.ca.us>
Date: 23 Mar 91 17:50:44 GMT
References: <17725@sdcc6.ucsd.edu>
Sender: daemon@ucbvax.BERKELEY.EDU
Reply-To: romkey@asylum.sf.ca.us
Organization: The Internet
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   Date: 23 Mar 91 01:46:58 GMT
   From: decwrl!ucsd.edu!sdcc6!jclark  (John Clark)
   Organization: University of California, San Diego
   References: <16241@uudell.dell.com>, <9103160909.AA01947@asylum.sf.ca.us>
   Sender: tcp-ip-relay@nic.ddn.mil

   With UDP and NFS based on it, is there a check sum on the
   reassembled packet. It seems that there is an assumption that the
   ethernet hardware CRC etc. is enough, but is that true.

IP carries a mandatory checksum over its headers. UDP carries an
optional checksum over its headers and data. The UDP checksum is
verified after the packet is reassembled by IP and passed on to UDP.
It can be set to 0 to indicate no sum was done.

Certain NFS implementations, like Sun's, do not generate UDP
checksums. Ethernet's CRC check is enough to validate the data *while
it's on the ethernet wire*. It doesn't protect the packets while
they're switched through routers, or copied into the network
interface, or transmitted over a different type of network link, for
instance a serial line.
		- john romkey			Epilogue Technology
USENET/UUCP/Internet:  romkey@asylum.sf.ca.us	voice/fax: 415 594-1141