Locked/Unlocked speed issues on T2500 (was Re: A/UX's buggy UUCP)

[Greg Andrews]

In article <D7Q_!R at smurf.sub.org> urlichs at smurf.sub.org (Matthias Urlichs) writes:
>In comp.unix.aux, article <1991Apr2.044623.299 at intacc.uucp>,
>  mann at intacc.uucp (Jeff Mann) writes:
>< 4. Therefore, if you are using a Telebit, you can't use auto baud rate
><    adjusting on incoming uucp calls. You must set S50=0 and use the normal
><    method of sending breaks to cycle getty until the proper speed is
><    attained.
>< 
>??? Why?
>
>UUCP defaults to using the g protocol. That protocol defaults to requiring
>less than 256 unacknowledged bytes; both the modem and the A/UX buffers are
>quite capable of storing that many unprocessed characters, therefore there
>can't be any soft overruns, therefore no flow control is necessary.
>

Why?  Because the modem can be used for interactive sessions - not
just uucp.

You're right that uucp doesn't need any flow control because the modem's
buffer is large (over 2.5K), and three packets worth of data can be
dumped into it without losing anything (assuming the standard packet
size of 64 data bytes).  Using the Mac's handshake signals for DTR and
DCD won't hurt uucp because a lack of flow control won't cause trouble.

However, interactive work is a completely different story.  If you lock
the interface at a high speed and don't have flow control, you run the
risk of losing data from buffer overruns.  Just dial in at a slower speed
like 2400 or 1200 bps, ls a large directory, and watch the bytes fall
on the floor.

XON/XOFF flow control in the modem can cause trouble for uucp, so
that's not a viable option.  You can disable XON/XOFF for uucp in
a chat script, but that doesn't help with incoming uucp calls.

Hardware handshaking would take away the ability for the Mac to detect
a disconnected session, causing security problems.

About the only thing left is to unlock the modem's speed so interactive
callers don't need to have overrun trouble and the system can detect
an unexpected hangup.  


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|  Greg Andrews   |       UUCP: {apple,amdahl,claris}!netcom!gandrews    |
|                 |   Internet: gandrews at netcom.COM                      |
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