Re: Checksums, CRC's, and NFS


Philip Prindeville [CC] (philipp@Larry.McRCIM.McGill.EDU)
Sat, 9 Apr 88 14:27:21 EDT


>>places (i.e. over noisy asynch lines)?? If important files get bit
>
>The point I was trying to make and a lot of people missed is:
>>An async line is not usually noisy. You do not have to
>>automatically put a CRC on it. You probably don't use a
>>CRC between your terminal and the TTY MUX on the host. That's
>>because it's not necessary in 99% of the cases.

Of course the DCE/DTE wire does not need line error detection. It
doesn't have 50 volt ring signals in adjacent copper pairs or have
to travel through many relays, frequency boosters, or CODECs. And
it is only a few meters long, not several kilometers.

>>The same holds true for async lines in general. They don't
>>normally have a noise problem. If you have a serious noise
>>problem, you shouldn't be using a simple async scheme. Buy a
>>paid of decent modems and let them do the work. (E.g.
>>today you can buy a pair of telebit trailblazers for $1345.)

When I was living in a dorm years ago, I had line noise every time
the compressor is someones refrigerator started. Dial-up lines do
indeed have noise. And I certainly didn't have $1300 to drop on
a modem either (the one I had was an AJ rep's demo model).

>If you need a special case for your special environment, fine.
>However, don't claim that a protocol that doesn't have it is
>broken.

What the hell is wrong with a little robustness? I think we can all
recount stories where a little builtin robustness has saved our
butts at unexpected times. It might seem excessive now, but prove
invaluable later.

>No one seems to feel the need to put a CRC around the TCP/IP packet
>that is handed to the ethernet. Yet, given a broken ethernet board, you
>could be just as likely to have a corrupt packet as with SLIP without a
>CRC
>
>--rick

A computer's buss is hardly as hostile as a telephone line. A mile of
copper pair is a great inductor in a thunderstorm...

-Philip



This archive was generated by hypermail 2.0b3 on Thu Mar 09 2000 - 14:41:55 GMT