SLIP for 386/ix

Vernon Schryver vjs at calcite.UUCP
Wed Feb 14 18:13:21 AEST 1990


In article <511187 at nstar.UUCP>, larry at nstar.UUCP (Larry Snyder) writes:
> ISC is planning on releasing Slip for 386/ix next month.  I am under
> the idea that Slip with TCP/IP and RFS will allow me to run RFS (or NFS)
> over serial lines so that I may use FTP.  Is this true?  Can someone
> fill me in with the details?
>           Larry Snyder, Northern Star Communications, Notre Dame, IN USA 
>                 uucp: larry at nstar -or- ...!iuvax!ndmath!nstar!larry

Vanilla SLIP, as opposed to various "header compression" or "checksumming"
or "error correcting" varients or the new PPP (RFC-1xxx, I don't remember),
should not be used with NFS.  The reason is that classic SLIP uses a
simple 1-byte framing without error detection.  Most varieties of NFS
(anything based religously on the Sun 4.xBSD reference tapes as well as Sun
at least until recently) run with UDP checksumming turned off.  This means
that corrupted bits will not be noticed.  The result is file corruption for
either the client or on the server's disk.  NFS was originally implemented
on media with link-level error detection, and the designers felt that
saving the extra checksum computation was worthwhile.  Today checksumming
is the most expensive part of network transfers at high speed.  (That
statement comes from my day job at a workstation mfgr where we are sloshing
TCP at 6MByte/sec over some media.)

Application protocols such as FTP which run over TCP/IP do not suffer this
problem, because the TCP checksum cannot legally be turned off.

For interactive use, such as telnet or rsh, over vanilla SLIP you need a
9600 or better <<full-duplex>> modem.  Because of the 40 byte TCP/IP
header, 9600 or 19200 half-duplex made to look like full is not useable for
interactive applications.  With "TCP/IP header compression/prediction",
SLIP can be used with Telebit Trailblazer+ modems for interactive use.
Without header compression, the 19.2 half-duplex TB+ is not statisfactory
I do not know but doubt that the ISC SLIP does header compression yet.  One
might expect them to port Drew Perkin's PPP reference implementation soon.

Any V.32 modem is a good choice.  A Telebit T2500, switched between PEP for
file transfers and v.32 for interactive use can give >1KByte/sec FTP and
good interactive performance.  Notice I wrote 1KB, not the familiar 1.4KB
we all get from TB+'s and UUCP.

I know people doing NFS, YP, X, and rlogin with T2500's and even TB+'s 
with a SLIP with header compression as good as or better than VJ (if I
do say so).  I think they're crazy--NFS likes lots of network bandwidth.

The big win with SLIP is that you can do lots of stuff at once with the
same line.  If your SLIP driver does reasonable TOS queueing, you'll hardly
notice file transfers going on while you are doing interactive stuff.


Vernon Schryver
vjs at calcite.uucp


P.S.  I said nothing about RFS.



More information about the Comp.unix.i386 mailing list