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From: dab@opus.cray.com (Dave Borman)
Newsgroups: comp.protocols.tcp-ip
Subject: Re:  TELNET Buffering Woes
Message-ID: <8905051422.AA02452@oliver.cray.com>
Date: 5 May 89 14:22:00 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 53

> How is the local telnet supposed to know that ^C is the appropriate
> interrupt character for the remote machine?  Many systems permit the
> interrupt character to be set by the user or a program.  I don't think
> the TELNET protocol specifies a way to transmit this change to the
> client.

The whole idea of the IAC IP and the NVT is that you don't need to know
what the interrupt character on the remote side is.  You type the
interrupt character that is most natural to use on the client side of
the connection, it gets turned into IAC IP, and the server translates
the IAC IP into whatever is appropriate on the server side to cause an
interrupt.  Hence, if you are on your Unix machine, and you telnet to
some machine running foobarOS, you can type ^C and know that you will
interrupt the process on the remote side, regardless of what the
interrupt character on the remote side is.

> Also, the above assumes there's only one kind of interrupt.  On Unix,
> there's ^C (interrupt), ^Z (suspend), and ^\ (quit).  They all need
> output flushed, but they can't all send IAC IP.

Bingo.  You win the prize.  The 4.3BSD telnet uses BRK to send ^\, and
IP to send ^C.  It also has an option on the client side so that you
can tell the client what each character is.

There is a new option, LINEMODE, which add the capablity for both sides
to be in sync as to what character should be used for telnet code.  It
also adds three new new codes, ABORT, SUSP, and EOF, which on a Unix
machine would map to ^\ (quit), ^Z (suspend) and ^D (end-of-file).

You can probably expect to see the new RFC out sometime later this
month, a BSD implementation will also be made available (which I am
currently working on finishing up).  A draft version is available
for anonymous ftp from uc.msc.umn.edu, in pub/linemode.draft  (This
is NOT the final version.  The SLC area is changing).  Watch for
future announcements.

> What's really needed is a way to send out-of-band data to the telnet
> client, telling it to ignore output until it reads a mark.  Is it
> possible to use the URGent flag to implement this?
>
> Barry Margolin
> Thinking Machines Corp.
> 
> barmar@think.com
> {uunet,harvard}!think!barmar

It already exists.  It's called the "Synch" signal in RFC854, see
page 8.  A "Synch" consists of an IAC DM sent in urgent mode, and
causes the reader(client) to discard input (server output) until
it reads the IAC DM.


			-Dave Borman	dab@cray.com