RFC529 - Note on protocol synch sequences

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　　Network Working Group A. McKenzie
Request for Comments: 529 B. Thomas
NIC: 17165 R. Tomlinson
References: RFCs 454, 513, BBN-TENEX
NIC # 15372 K. Pogran
MIT-MULTICS
29 June 1973

A Note on Protocol Synch Sequences

This note is motivated by Wayne Hathaway's RFC513 which comments on
the interpretation of the TELNET SYNCH sequence (INS/Data Mark). We
agree with Wayne's observation that the phrase "interesting things",
as it appears and is explained in the TELNET Protocol Document (NIC#
15372), is much too imprecise to appear in a protocol specification.
However, we disagree with his proposal that the interpretation of the
TELNET SYNCH sequence should be redefined. Hathaway's comments led
us to examine the notion of "interesting things" with respect both to
TELNET protocol and to protocols built upon it.

We feel that the definition of the TELNET SYNCH sequence in the
TELNET Protocol Document is the proper one [1]. More important, we
feel that the (potential) difficulties with respect to the TELNET
SYNCH sequence noted in RFC513 are not the reflection of a TELNET
design flaw but rather reflect misuse of the TELNET SYNCH sequence by
"higher level" protocols (in particular FTP) that are based on
TELNET.

The remainder of this note examines the notion of a synch sequence
and suggests an approach to the design of protocols which are to use
the TELNET protocol as a basis.

The reason for defining a synch sequence for a protocol is to provide
a mechanism by which signals, represented as characters, that for one
reason or another are "stuck" in the pipeline between the sender and
the protocol interpreter, can promptly be brought to the attention of
the interpreter. Flow through the pipeline is, of course, controlled
by the receiver; the process operating the interpreter may be doing
something else at the moment, and may not be paying attention to the
incoming data stream. The sender would like to get the attention of
the receiving process, to have it read its incoming data stream and
take action as directed by the "interesting" characters in that
stream, which will, in general, be protocol commands. To accomplish
this, a "SYNCH sequence" is transmitted. A synch sequence consists
of:

1. An "out of band" signal which serves to get the attention of
the protocol interpreter; and

2. An "in band" marker which serves to mark how much of the data
stream is to be processed by the protocol interpreter in
response to the "out of band" signal.

For the TELNET protocol the "out of band" signal is the INS of Host-
Host Protocol and the "in band" marker is the TELNET Data Mark
character (DM). Ignoring for the moment the use of TELNET as a basis
for higher level protocols (such as FTP), the class of characters
"interesting" to a TELNET interpreter is the set of TELNET commands
(including the commands for option negotiation and sub-negotiation
[2]).

One might reasonably argue that this class could be enlarged by a
server Host to include the set of signals of interest to the terminal
support software of that particular Host. For example, in case of
TENEX such a set would include the "terminal interrupt" characters
enabled by the process reading from the TELNET connection (e.g., ^C,
^T, etc.). Other hosts, such as Multics, might look only for the
TELNET commands, such as Interrupt Process (IP), Abort Output (AO),
etc. Whether or not one chooses to consider additional signals as
interesting during the processing of a TELNET SYNCH sequence should
cause the implementer no problem:

He must treat all TELNET commands as interesting by interpreting
them. He may choose either to ignore such additional signals or
to pass them on to the process; in either case there is no
vagueness since the implementer knows which characters his
terminal support software considers interesting.

The difficulty noted in RFC513 concerning the vagueness of
"interesting things" occurs when a higher level protocol makes use of
the TELNET SYNCH sequence to force commands of interest to it through
to its interpreter. A higher level protocol designed in such a way
represents a violation of the protocol layering discipline:

The TELNET SYNCH mechanism is being misused by attempting to give
it meaning at two different levels of protocol.

The problem stems from the fact that, in general, a (increasing)
number of different higher level protocols can be designed with
TELNET as a base. A TELNET interpreter has no way of knowing the
higher level protocol interpreter (if any) to which it is passing
characters, and therefore, can not tell which things are
"interesting" to the higher level protocol interpreter. That is,
just as an NCP should not have to know whether the data it handles is

for a TELNET connection, an FTP data connection, etc., a TELNET
interpreter should not be required to know the kind of process for
which it is handling characters. This should, in fact, result in a
simplification of the design and implementation of TELNET protocol
interpreters.

This difficulty can be resolved by proper design of protocols that
make use of TELNET as a base. In particular, if in such a higher
level protocol it is important to be able to force commands through
to the protocol interpreter, the higher level protocol should include
its own synch sequence: i.e., an "out of band" signal used with an
"in band" data mark. The TELNET protocol provides the Interrupt
Process character (IP) for use as an "out of band" signal. A synch
sequence for a protocol built upon TELNET would be:

1. Insert the TELNET IP control character into the data stream;

2. Insert the higher level protocol data mark character (HDM) into
the data stream following whatever higher level protocol
commands are important at the time.

Receipt of the IP TELNET command causes the higher level protocol
interpreter to be interrupted, enabling it to scan the data stream
(up to and including the HDM) for commands it considers important.

As an example, consider the case of the File Transfer Protocol (RFC
454) and the problem of aborting a file transfer in progress. To
accomplish such an abort the FTP user (process) should:

1. Send the TELNET IP character;

2. Send the TELNET SYNC sequence, that is:

a. Send the TELNET Data Mark (DM);

b. Send the Host-Host Protocol INS;

3. Send the FTP ABOR command; and

4. Send the FTP data mark character [3].

The user (or process acting on his behalf) must transmit the TELNET
SYNCH sequence of step 2 above to ensure that the TELNET IP gets
through to the server's TELNET interpreter.

Endnotes

[1] I.e., any TELNET commands appearing before the Data Mark are to
be interpreted; the Data Mark is to be used to terminate the scan
initiated by the INS; characters that are not TELNET commands may be
discarded or passed to the user process as the implementer sees fit.

[2] We support Hathaway's proposal to fully parenthesize sub-
negotiations. Further, we believe that the "closing parenthesis"
should be a new command rather than a second SB command; this will
aid the receiver in recovering from errors, either in parsing at the
receiver or in generation at the transmitter. We disagree with his
proposal that sub-negotiations be discarded when encountered during
processing of a TELNET SYNCH.

[3] For FTP such a data mark character has not yet been defined and,
in fact, may not be necessary under the constraint that the FTP
command interpreter should look for exactly one command after being
interrupted; this is consistent with the general command-reply
orientation of FTP.

[This RFCwas put into machine readable form for entry]
[into the online RFCarchives by Helene Morin, Via Genie 12/1999]

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