High-speed SLIP/PPP

[Brian Lloyd]

In article <7275 at spdcc.SPDCC.COM> rbraun at spdcc.COM (Rich Braun) writes:
>SCO's TCP product includes a caveat that SLIP won't work faster than
>2400 baud for Unix 3.2.0 or earlier; presumably they've made some fixes
>to the serial port driver so it will handle faster data rates without
>flow control.

SCO's standard tty drivers for the internal 16450 ports (COM1 and
COM2) are pretty poor performers.  If you plan to do SLIP I
-*<STRONGLY>*- recommend an intelligent serial I/O card or plan to go
REEAAALLL SSLLOOWW.

>The engineering veep here shudders at the thought of hardware flow
>control.  I'm claiming one should support CTS/RTS in the interest of
>providing better performance.  But he does have a point:  I've spent
>many an hour tearing out my hair making RS-232 interfaces work against
>odd flavors of equipment, and we don't want our customers to go bald
>too.

First, SLIP connections do not stream data that often.  If the host
gets busy it may not have time to send back the TCP ACKs (assuming you
are using TCP atop IP atop SLIP).  In that case the sender's window
will close and he will stop sending.  This gives the busy host time to
catch it breath.  Given this, it is a matter of buffering in the asy
driver on the receiving end and how rapidly the host can empty that
buffer.  That is the reason that using an intelligent asy board is
such a win.  The intelligent boards usually have scads of buffer space
and the host can spend all of its time processing  the packets instead
of responding to interrupts.
>
>So, I pose the question to the net.  When designing a new product which
>supports SLIP, what kind of performance can I expect without flow
>control against various flavors of Unix?

I would expect to be able to send and receive at 19,200 bps if the
remote host has an intelligent comm board.  I could sustain that on a
286 running Microport's
SysV/AT UNIX with a Digiboard 8-port intelligent comm board.  Our
Telebit NetBlazer dial-up IP router will sustain many ports running
SLIP or PPP at 57.6Kbps without flow control.  We do support RTS/CTS
flow control anyway.  You don't have to use it but when you need it,
it's nice to have it.

>Will I have to get into the business of writing device drivers and/or
>OEMing "smart" serial cards?

Yes, your UNIX hosts will want to have an intelligent serial I/O card.
No, you won't need to write the driver since most vendors of the
aforementioned cards provide adequate drivers.  I don't know about
your personal application tho'.

>Am I crazy to consider developing an interface without RTS/CTS?  

Not crazy but think of it as insurance.  You will need it some day.

> Is the half-duplex hardware flow control supported by SCO widespread
>enough that I need to worry about supporting it?

I have never seen anyone else use it in the SLIP world.

>Finally, PPP (Point to Point Protocol) seems to address many inadequacies
>in SLIP.  For example, it's possible to use XON/XOFF flow control with
>PPP, because it remaps control characters out of the range 01-1F.  

But most just turn off the character mapping and use hardware flow
control, especially when talking to a modem.

>Is PPP catching on quickly,

Yes, very.  Most of the router vendors have adopted PPP for
interoperability on their sync links.  Telebit, Interactive, FTP,
Intercon, and others support async PPP in their products.  PPP has
many more advantages over SLIP besides the ability to escape the
control characters.

>and is it generally expected that new products
>supporting serial-line IP should also support PPP?  

ABSOLUTELY!!!

>(And, can I get a public-domain C implementation?)

Yes.  PPP and SLIP are in the KA9Q TCP/IP package.  Do an anonymous
FTP to ucdavis.edu or possibly thumper.bellcore.com and poke around.
This software is not public domain but it is there for you to learn
from and you can cut a deal with Phil Karn (karn at thumper.bellcore.com)
if you want to do something commercially.

-- 
Brian Lloyd, WB6RQN                              Telebit Corporation
Network Systems Architect                        1315 Chesapeake Terrace 
brian at napa.telebit.com                           Sunnyvale, CA 94089-1100
voice (408) 745-3103                             FAX (408) 734-3333