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Network Working Group B. Cosell
Request for Comment: 435 BBN-NET
NIC: 13675 D. Walden
Category: TELNET, Protocols, Echoing BBN-NET
References: 318, 357 5 January 1973
TELNET Issues
This RFC discusses a number of TELNET related issues which have been
bothering us [1]. The basic, central issue we started from was that
of echoing. We worked downward from our difficulties to discover the
basic principles at the root of our unhappiness, and from there
worked back upwards to design a scheme which we believe to be better.
In this note we will discuss both the alternate scheme and its
underlying principles.
As something of a non sequitur, before discussing echoing we feel it
expedient to dismiss one possible stumbling block, outright. HIDE
YOUR INPUT may or may not be a good idea, this question not
concerning us at the moment. Whatever the case, the issue of hiding
input is certainly separable from that of echoing. We, therefore,
strongly recommend that a STOP HIDING YOUR INPUT command be
sanctioned to replace the multiplexing of this function on the NO
ECHO command. Once this has been done, the pair of commands HIDE
YOUR INPUT and STOP HIDING YOUR INPUT can be kept or discarded
together, and we can discuss the issue of echoing independently of
them.
Echoing
The basic observation that we made regarding echoing was that servers
seem to be optimized to best handle terminals which either do their
own echoing or do not, but not both. Therefore, the present TELNET
echoing conventions, which prohibit the server from initiating a
change in echo mode, seemed overly confining. The servers are
burdened with users who are in the 'wrong' mode, in which they might
not otherwise have to be, and users, both human and machine, are
burdened with remembering the proper echoing mode, and explicitly
setting it up, for all the different servers. It is our
understanding that this prohibition was imposed on the servers to
prevent loops from developing because of races which can arise when
the server and user both try to set up an echo mode simultaneously.
We will describe a method wherein both parties can initiate a change
of echo mode and show that the method does not loop.
Cosell & Walden [Page 1]
RFC 435 TELNET Issues 5 January 1973
This alternate specification relies on three primary assumptions.
First as above, the server, as well as the user, should be able to
suggest the echo mode. Second, all terminals must be able to provide
their own echoes, either internally or by means of the local Host.
Third, all servers must be able to operate in a mode which assumes
that a remote terminal is providing its own echoes. Both of these
last two result from the quest for a universal, minimal basis upon
which to build. It is fairly easy for a Host which normally supplies
echoes to disable the appropriate code, but it will difficult for a
Host which does not do echoing to integrate such routines into its
system similarly, it is easier for a local Host to supply echoes to a
terminal which cannot provides its own, but it borders on the
impossible to undo echoing in a terminal which has automatic echoing
built into it.
Our proposed specification would use the present ECHO and NO ECHO
commands as follows: ECHO, when sent by the server to the user, would
mean 'I'll echo to you' ECHO, when sent by the user to the server,
would mean 'You echo to me'. NO ECHO, when sent by the server to the
user, would mean 'I'll not echo to you'; NO ECHO, when sent by the
user to the server, would mean 'Don't you echo to me'. These are, of
course, nearly the same meanings that the commands currently have,
although most current implementations seem to invert the server-to-
user meanings.
In our specification, whenever a connection is opened both server and
user assume that the user is echoing locally. If the user would, in
fact, prefer the server to echo, the user could send off an ECHO
command. Similarly, if the server prefers to do the echoing (for
instance, because the server system is optimized for very interactive
echoing), the server could send off an ECHO command. Neither is
required to do anything, it is only a matter of preference. Upon
receipt of either command by either party, if that is an admissible
mode of operation the recipient should begin operating in that mode,
and if such operation reflects a change in mode, it should respond
with the same command to confirm that (and when) the changeover took
place. If the received command request an inadmissible mode of
operation, then the command's inverse should be sent as a refusal
(this must be NO ECHO, since neither party can refuse a change into
NO ECHO). To state these rules more formally:
1) Both server and user assume that a connection is initially in
NO ECHO mode.
2) Neither party can refuse a request to change into NO ECHO mode.
3) Either party may send an unsolicited command only to request a
change in mode.
Cosell & Walden [Page 2]
RFC 435 TELNET Issues 5 January 1973
4) A party only changes its echo mode when it receives an
admissible request.
5) When a command is received, the party replies with its echo
mode, unless it did not have to change mode to honor the
request.
Several properties of this scheme are worthy of note:
1) NO ECHO is retained as the nominal connection mode. A
connection will work in ECHO mode only when both parties agree
to operate that way.
2) The procedure cannot loop. Regardless of which party (or both)
initiates a change, or in what time order, there are at most
three commands sent between the parties [2].
3) Servers are free to specify their preferred mode of operation.
Thus, human, or machine, users do not have to learn the proper
mode for each server.
Three Principles
Let us mention the general principles we alluded to at the beginning
of this note. The principles are: default implementation, negotiated
options and symmetry. The principle of default implementation merely
states that for all options, defaults are declare which must be
implemented. It is this principle which leads us to seek out the
'minimum' for each option (to keep the required burden on everybody
as small as possible), and prevents loops in protocol. The principle
of negotiated options merely states that options must be agreed upon
by all (both) parties concerned. It is this principle which dictated
the positive/negative acknowledgement scheme. The principle of
symmetry merely states that neither party should have to 'know'
whether it is the server or the user. Our scheme, as described thus
far, is not totally symmetrical we will consider this matter in a
later section.
The ECHOING scheme we have described, together with the principles
stated above, form the heart of our comments on the TELNET protocol.
The remainder of this note consists of further ways in which the
protocol can be expanded on the whole, these suggestions are all
really only applications and development of the principles we have
already put forward. However, the fecundity of these expansions, and
the 'good feel' they have, make us yet more convinced of the '
rightness' of our original proposals.
Cosell & Walden [Page 3]
RFC 435 TELNET Issues 5 January 1973
Thus far, we have made a simple, concrete suggestion that we believe
should be immediately sanctioned. Looking beyond that proposal,
however, has suggestion a large number of further, more ambitious
changes. The remainder of this RFC describes ideas which we don't
feel have the immediacy of the proposal above, but should,
nonetheless, be kept in mind if the network community decides to
embark on revamping the protocol.
Synchronization
One complaint we have heard about the present convention for
establishing an echoing mode is about the lack of a provision to
synchronize a change of echoing mode with the user-to-server data
stream our scheme, too, is guilty on this count. John Davidson of
the University of Hawaii has documented, in RFC 357, a more elaborate
echoing scheme which doesn't have this problem. We, however, feel
that it is possible to eliminate most of the trouble involved with
normal changing of echo mode at a more modest cost than that required
by the highly interactive scheme described by Davidson. We can do
this by borrowing a small piece of that scheme. The rule we would
incorporate is that whenever a party initiates a request for a change
in echo mode, it then buffers, without transmitting or processing,
all data in the user-to-server data stream until it receives an
acknowledgement, positive or negative, at which time it deals with
the buffered data in the newly negotiated mode. Since with both our
proposed and the current schemes such a request is guaranteed to be
acknowledgement, the buffering time is bounded.
An important aspect of this technique of eliminating the
synchronization problem is that it need not ever become part of the
official protocol. Since its operation is entirely internal to the
server or user, each may independently weigh the value of elegance
against the cost of the required code and buffer space.
Other options
Abhay Bushan has suggested to us that whether the user and server
operate line-at-a-time or character-at-a-time mode (see RFC 318)
should also be a negotiated option. Further, he suggested that
whether the terminal follows the TELNET end-of-line convention or not
should also be negotiated. Thus, when a connection is opened, in
addition to being set to NO ECHO mode, the terminal would also be set
to LINE-AT-A-TIME and EOL modes. We could augment the command space
with the new commands LINE, NO LINE (=CHARACTER), EOL and NO EOL
(=separate CR and LF).
Cosell & Walden [Page 4]
RFC 435 TELNET Issues 5 January 1973
Once started in this direction, we found several further
applications. HIDE YOUR INPUT could be made an option, as could
Davidson's echoing scheme, and even the character set to be used!
Consider that an APL subsystem might well want to suggest to its user
that EBCDIC be used for the connection.
In mentionaing that the character set could be negotiated, it was
implicit that 7-bit USASCII was the default. The possibility of
having the default be straight binary suggests itself. If we
augmented the protocol with a QUOTE character, the byte after which
were to be always interpreted as data, then codes 128-255 could be
retained as the 'TELNET command space' independently of the data mode
in use by merely prefixing all data bytes in this region with a
QUOTE. If BINARY were a permissible data mode, then it is easy to
visualize many higher level protocols, e.g., perhaps, File Transfer
and Graphics, being built on top of, and into, the TELNET protocol.
What we would have accomplished is to promote TELNET from being a
constrained, terminal-oriented protocol to its being a flexible,
general protocol for any type of byte oriented communication. With
such a backbone, many of the higher level protocols could be designed
and implemented more quickly and less painfully -- conditions which
would undoubtedly hasten their universal acceptance and availability
[3].
Looking toward a better world of the future, we have come up with a
more compact and flexible command scheme. We'll describe it after
the next section.
Symmetry
Some of the TENEX group (in particular, Thomas, Burchfiel and
Tomlinson) have pointed out to us that although we have made the
rules for the protocol symmetrical, we have not made the meanings of
the commands symmetrical. For example, the interpretations of the
ECHO command -- 'I'll echo to you' and 'You echo to me' -- implicitly
assume that both the server and user know who is which. This is a
problem not only for server-server connections where it is not clear
which is the user, but also for user-user connections, e.g., in
linking Teletypes together, where it is not clear which is the
server.
Responding to this, we came to understand that there are only five
reasonable modes of operation for the echoing on a connection pair
[4]:
Cosell & Walden [Page 5]
RFC 435 TELNET Issues 5 January 1973
<------------------<
A Process 1 Process 2
>------------------>
neither end echoes
<------------------<
B Process 1 <--+ Process 2
^
>--^--------------->
one end echoes for itself
<------------------<
C Process 1 <--------------+ Process 2
^
>--------------^--->
one end echoes for the other
<--------------V---<
D Process 1 <--+ V Process 2
^ +--->
>--^--------------->
both ends echo for themselves
<-----V------------<
E Process 1 <--+ V Process 2
^ +------------>
>--^--------------->
one end echoes for both ends
The TENEX group suggested to us that four commands are sufficient to
deal with completely symmetric echoing. We have actually already
mentioned the four commands -- the two possible meanings for each of
ECHO and NO ECHO. Explicitly, the commands would be I'LL ECHO TO
YOU, YOU ECHO TO ME, DON'T ECHO TO ME and I'LL NOT ECHO TO YOU.
Echoing is now the negotiation of two options, and the initial,
default modes are DON'T ECHO TO ME and I'LL NOT ECHO TO YOU.
In the case where the server or user knows which he is, the
modification to the scheme is minimal since the commands never had
ambiguous meanings in these cases. When an 'end' truly doesn't know,
then things are a little more complicated -- for example, consider
both ends in I'LL ECHO TO YOU mode, but even then the problems are
not insurmountable.
Cosell & Walden [Page 6]
RFC 435 TELNET Issues 5 January 1973
Once the principle of symmetry is adopted, it is no longer possible
to use a function in two different ways. On pages 5 and 6 of RFC
318, Postel gives a description of INS and SYNC which indicates that
they are used to simulate a 'break' user-to-server, but flush the
output buffers server-to-user. Since we do believe in symmetry, we
suggest that the INS/DATA-MARK be treated the same in both directions
and that a new CLEAR YOUR BUFFER option be added.
Command Format
Extending full symmetry through the other options we have suggested,
we can now describes the compacted command format referred to
earlier.
Rather than having four commands for each option (I WILL, I WON'T,
YOU DO, YOU DON'T), there would be four 'prefixes' -- WILL, WON'T,
DO, DON'T -- which would be used before the single command devoted to
each option, WON'T and DON'T being the default modes. To give an
example, assume the codes for WILL and WON'T are 140 and 141, and the
codes for ECHO REMOTE and HIDE INPUT are 132 and 133. Then several
of the possible command combinations would be:
140 133 -- DO HIDE INPUT
140 132 -- DO ECHO REMOTE
141 132 -- WON'T ECHO REMOTE
141 133 -- WON'T HIDE INPUT
These are some of the commands that we believe should exist:
I WILL (140)
I WILL NOT (141)
YOU DO (142)
YOU DO NOT (143)
QUOTE (144)
SYNC (163)
SYNC REPLY (164)
ECHO REMOTE (132)
SEND A CHARACTER-AT-A-TIME (146)
SEND INDEPENDENT CR and LF (147)
SEND IN EBCDIC (162)
HIDE INPUT (133)
USE DAVIDSON'S ECHOING STRATEGY (145)
An important virtue of this command structure, and of our entire
viewpoint, is that Hosts need no longer even be aware of what all the
options are. If we call the mode of operation in which every
alternative is in its default state the 'NVT', then a site, of
Cosell & Walden [Page 7]
RFC 435 TELNET Issues 5 January 1973
course, must handle an NVT, but beyond that if it merely responds no
to any command it does not understand, then it can totally ignore
options it chooses not to implement. Thus, options would truly be
optional (for a change), not only to the user who may choose not to
invoke them, but also to the systems builders who may now choose not
to offer them!
We hereby volunteer to rigorously specify a version of TELNET which
embodies the principles we have described and to do so at any level
of complexity deemed sufficient by the network community.
Cosell & Walden [Page 8]
RFC 435 TELNET Issues 5 January 1973
Appendix: A Sample Implementation
The basis scheme we described represents most of what we have been
thinking about the further extensions are just that, extensions. We
fear, however, that some who are spiritually in league with us might
be frightened off by the magnitude of all the changes we suggest. To
combat this, we here provide an example of how simply and straight-
forwardly the basis scheme could be implemented for the TIP [5].
For each user terminal the TIP would keep three state bits: whether
the terminal echoes for itself (NO ECHO always) or not (ECHO mode
possible), whether the (human) user prefers to operate in ECHO or NO
ECHO mode, and whether the connection to this terminal is in ECHO or
NO ECHO mode. We call these three bits P(hysical), D(esired) and
A(ctual).
When a terminal dials up the TIP, the P-bit is set appropriately, the
D-bit is set equal to it, and the A-bit is set to NO ECHO. The P-
and A-bits may be manually reset by direct commands if the user so
desires for instance, a user in Hawaii on a 'full-duplex' terminal
might know that whatever the preference of a mainland server, because
of satellite delay his terminal had better operate in NO ECHO mode --
he would direct the TIP to change his D-bit from ECHO to NO ECHO.
When a connection is opened from the TIP terminal to a server, the
TIP would send the server an ECHO command if the MIN (with NO ECHO
less than ECHO) of the P- and D-bits is different from the A-bit. If
a NO ECHO or ECHO arrives from the server, the TIP will set the A-bit
to the MIN of the received request, the P-bit and D-bit. If this
changes the state of the A-bit, it will send off the appropriate
acknowledgement if it does not, then the TIP will send off the
appropriate refusal if not changing meant that it had to deny the
request (i.e., the MIN of the P- and D- bits was less than the
received A- request). If while a connection is open, the TIP
terminal user changes either the P- or D-bit, the TIP will repeat the
above tests and send off an ECHO or NO ECHO, if necessary. When the
connection is closed, the TIP would reset the A-bit to NO ECHO.
While the TIP's implementation would not involve ECHO or NO ECHO
commands being sent to the server except when the connection is
opened or the user explicitly changes his echoing mode, we would
suppose that bigger Hosts might send these commands quite frequently.
For instance, if a JOSS subsystem were running, the server might put
the user in NO ECHO mode, but while DDT was running, the server might
put the user in ECHO mode.
Cosell & Walden [Page 9]
RFC 435 TELNET Issues 5 January 1973
[1] We have assumed that TELNET is defined as suggested by Jon Postel
in RFC 318.
[2] Notice that a faulty implementation could achieve the effect of a
loop by repeatedly sending a command which has previously been
refused. We consider this a property of the implementation, not of
the scheme in general, a command which has be rejected should not be
repeated until something changes -- for instance, not until after a
different program has been started up.
[3] Will Crowther, with an eye towards building higher protocols upon
TELNET, has suggested that a SYNC command (not to be confused with
the existing SYNCH), and a SYNC REPLY be added to TELNET. For
example, a server might want to wait until the output buffer of a
user's terminal were empty before doing something like closing the
connection or passing the connection to another server. Although we
see no current use for the command pair, they seem to be a handy
enough building block that we recommend that they be included.
[4] It is perhaps appropriate to mention that most of the connections
in the network are TELNET connections, which are full duplex.
Wouldn't it be reasonable to make all Host/Host protocol connections
full duplex, rather than simplex? If, for some reason, one truly
needs a simplex connection, the reverse direction can always just be
ignored.
[5] Readers unfamiliar with the TIP may read the TIP Users Guide --
NIC 10916.
[This RFC was put into machine readable form for entry]
[into the online RFC archives by Helene Morin, Via Genie, 12/99]
Cosell & Walden [Page 10]