>> Date: Thu, 02 May 1996 12:33:21 -0700
>> From: Greg Wohletz <greg@duke.CS.UNLV.EDU>
>> 
>> >I've heard that suggested too (about udp_cksum in SunOS) but
>> >it never made any difference for me if it was on or off.
>> >I doubt that is the problem.
>> 
>> Well i  sure have.  I've experienced  both NFS file corruption and DNS
>> request corruption at  the hands  of  this.  BTW, the  person who said
>> that COMPAT_42 turns off udpcksum in netbsd 1.1  was correct.  In fact
>> this seems  to  be the only   thing that  COMPAT_42  does  (not  to be
>> confused  with TCP_COMPAT_42).  Unless there is  some  good reason for
>> this I'd suggest getting rid of that option, there just isn't any good
>> reason to ever turn off UDP checksums.
>
>Is there a good test to determine when this lossage exists?
>BTW, Why is the Ethernet CRC not a sufficient error check?

Who knows where your packet might travel.  The following is from:

	Requirements for Internet Hosts -- Communication Layers 

	Network Working Group, Internet Engineering Task Force 
	Request for Comments: 1122 
	R. Braden, Editor 
	October 1989 


4.1.3.4 UDP Checksums 

A   host MUST  implement  the  facility  to generate  and validate UDP
checksums. An application MAY optionally be able  to control whether a
UDP checksum will  be generated, but it  MUST default  to checksumming
on.

If a  UDP datagram is received  with a checksum  that is non- zero and
invalid, UDP MUST  silently discard the  datagram.  An application MAY
optionally be able to control whether  UDP datagrams without checksums
should be discarded or passed to the application.

DISCUSSION: 

Some applications that normally run   only across local area  networks
have chosen  to turn off  UDP checksums for  efficiency.  As a result,
numerous    cases   of  undetected  errors   have  been  reported. The
advisability   of   ever  turning  off     UDP  checksumming is   very
controversial.

IMPLEMENTATION: 

There is a common  implementation error in  UDP checksums. Unlike  the
TCP   checksum, the  UDP checksum  is   optional; the   value  zero is
transmitted  in the checksum  field of  a UDP  header to indicate  the
absence  of a  checksum. If  the transmitter  really calculates  a UDP
checksum of zero, it must transmit the checksum as all 1's (65535). No
special action  is required at the  receiver, since zero and 65535 are
equivalent in 1's complement arithmetic.


						--greg