RCTE and stranger things

Rob Austein (SRA@XX.LCS.MIT.EDU)
Tue, 29 Sep 1987 19:49 EDT

I was going to say something about that, but decided to wait to see if
anybody had in fact read Dave's message. You did, so here goes.

Yes, SUPDUP as specified in the RFC is character at a time. However,
I believe that a very minor enhancement to the protocol would handle
that problem. The big advantages of SUPDUP (sales pitch) are that (1)
it works in a heterogenous environment (thus is better than rlogin),
and (2) has a wider view of the terminal than just the print head of a
hardcopy TTY (thus is better than TELNET). In particular, there are a
whole set of useful options under the heading of "The Intelligent
Terminal Protocol". Not all of these are documented in the SUPDUP
RFCs, for a full explanation of the ITS terminal system see the file
"MC: INFO; ITSTTY >" on MC.LCS.MIT.EDU. It's a bit long, so if you're
not up for a lot of reading, you want the parts on "Control of the
TTY" and "The Intelligent Terminal Protocol".

The model I'm using for the local/remote echo and wakup problem is the
TOPS-20 TEXTI% JSYS, which was mentioned obliquely a few messages ago
when somebody referred to TOPS-20 EMACS enhancements. For those who
aren't familiar with TOPS-20, one of the arguments to TEXTI% is a
break mask, a 128 bit vector indicating which characters should cause
the TEXTI% call to return. I believe that the EMACS extentions that
were mentioned were based on an extension to TEXTI% which would cause
any character with the meta bit (octal 200) turned on to act as a
wakeup. I may be wrong about this, I've never seen the code.

Presumably the entity that decides what the break mask should be is
the server (where applications programs are running) while the entity
that implements the break mask is the client (where the physical
display terminal is). So presumably the "change the break mask"
sequence would begin with a %TDxxx code. I can't think of any reason
why the client would want to tell the server about break masks, but if
so the process would be identical except for the escape character (a
30x code, presumably). Henceforth I'll refer to the entity sending
the break mask as the "sender" and the entity receiving the break mask
as the "recipient".

For the 12 bit character set SUPDUP permits, a complete break mask
would be rather cumbersome, but there's a natural way to compress
this. Make the first data byte a flag byte, with one flag per bucky
bit, one flag for characters with no bucky bits, and two unused bits.
The flag bits indicate which bucky bits the sender wants the client to
try to optimize; if a flag bit is set, a break mask is supplied, if a
flag is cleared, no break mask is supplied and the receiver should
fall back to the default behavior (wake on every character). The most
common message would presumably be one with the no-bucky-bit and
control-bucky-bit flags set and all others cleared, indicating that
any meta, super, or hyper characters are wakeups. In general, if a
program doesn't expect to see a class of characters, it should
probably wake up on them so that it can tell the user about typing
errors ASAP.

The flag byte is followed by a series of break masks (128 bits in 16
bytes, presumably). For completeness, this would have to be separate
break masks for each case that the sender has indicated in the flag
byte; ie, just because the sender wants to break on CONTROL-A and
META-A doesn't mean the server wants to break on CONTROL-META-A. This
is part of the reason for the flag byte, so that the sender needn't
send a lot of masks that are all ones.

All SUPDUP connections would still start out in character at a time
remote echo mode. Setting the break mask requests local echo of any
characters that are not breaks. Break characters are still handled
remotely. Setting the break mask with a zero flag byte (and thus no
following masks) would put the connection back in the default
character at a time mode.

One extension of this idea would be incremental changes to the break
mask; if anybody cares enough to do it, there's always the two unused
bits in the flag byte. But the above covers the basic scheme.

Yes, a similar mechanism could be used in TELNET, without having to
think about 12 bit characters and bucky bits, but TELNET is really not
a very good model for a display terminal. SUPDUP (and the abstract
model of terminals and capabilities that underlies it) is a much
better model. I think the existing software speaks for itself.


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