SLIP working group?


Barry Shein (bzs%bu-cs.bu.edu@buita.BU.EDU)
Wed, 30 Mar 88 11:49:13 EST


>The unfortunate proof is in UDP. Adding a CRC (not a checksum!) is a necessary
>result of some vendors (read SUN) reluctance to use checksums. The UDP spec
>unfortunately allows this. Although most people disagree with this decision
>(not to checksum UDP), we decided to add the CRC to SLIP so that you have a
>lower chance of getting burned if you are following the spec.

I've always thought that the optional checksum in UDP was a feature,
if I choose not to use the UDP checksum I may have some good reasons
(eg. I might be sending an ECC with each packet to avoid
retransmission on every error.) This defeats this and would force
someone to, well, even sending IP won't help, they'd have to re-write
the SLIP, bother. It's straightforward to turn on UDP checksumming
on a Sun, at worst you can fault them for not making it easier:

        % adb -w -k /vmunix /dev/mem
        udpcksum?W 1
        $q
        %

That could be put into a command script trivially enough. I think
putting a CRC into SLIP is a rather odd way to disagree with Sun's
design decision (which might still be wrong, that's irrelevant is my
point.) Add that to Rick's experiences that IP and TCP seem to do the
job well enough at that level, that UDP checksumming is trivial to
turn on in Unix and the fact that modems which will do end to end
checksumming are so common and inexpensive and we have an odd
situation, let's calculate a checksum 3 or 4 times as we route all
data?

Seems like over-engineering, at least until someone can come up with
better reasoning than doing it because the user may have turned it
(left it) off at a higher layer. The user may have *wanted* it off.

I'll anticipate the argument that the system admin who manages the
file server may have turned it off and is out of the user's control.
How is this putative user getting SLIP supported and NFS connections
allowed if they have no influence with the sysadmin? Is the protocol
spec the correct place to try to workaround bad personal relationships
between users' and their sysadmins? &c.

Of course, the real issue of simplicity of implementation has to be
factored in also, as Rick points out, in the end that's a critical
decision point.

        -Barry Shein, Boston University



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