Network Working Group D. Borman
Request for Comments: 1571 Cray Research, Inc.
Updates: 1408 January 1994
Category: Informational

Telnet Environment Option Interoperability Issues

Status of this Memo

This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.

1. Overview

RFC1408 [1], the specification for the Telnet Environment Option,
specifies definitions for VAR and VALUE that are reversed from the
BSD implementation of the Telnet Environment option. Since the BSD
implementation was the reference implementation that the RFCwas
supposed to document, and is the base for many existing
implementations, there exists an interoperability problem between
implementations with conflicting definitions for VAR and VALUE.

This document describes a method for allowing implementors to ensure
that their implementation of the Environment option will be
interoperable with as many other implementations as possible, by
providing a set of heuristics that can be used to help identify which
definitions for VAR and VALUE are being used by the other side of the
connection.

2. Client Telnet: Parsing a SEND

The SEND suboption should only contain VAR and USERVAR commands. It
should never contain a VALUE. If while parsing a SEND suboption, a
VALUE is encountered, the client should assume that the server has
reversed values for VAR and VALUE, and from that point on the client
should reverse those values, both in parsing the rest of the SEND
suboption, and when generating an IS or INFO suboption. If both
VALUE and VAR commands are encountered, the SEND command is not well
formed, and it is implementation dependent as to what will happen.

If there are not VAR or VALUE commands in the SEND suboption, then
the client cannot know what values the server is eXPecting for VAR
and VALUE. In this case, it should just assume that the server has
the correct definitions, and use the correct values for VAR and
VALUE.

3. Server Telnet: Parsing an IS or INFO

The IS or INFO in a suboption can only be legally followed by either
a VAR or a USERVAR. If an IS or INFO is immediately followed by a
VAR, then it can be assumed that the client has the correct
definitions for VAR and VALUE. If the IS or INFO is immediately
followed by a VALUE, then it can be assumed that the client has
reversed definitions for VAR and VALUE, and from that point on the
server should assume that the VALUE and VAR definitions are reversed.

If the IS or INFO is immediately followed by a USERVAR, further
hueristics must be applied to determine what are the client
definitions for VAR and VALUE. This is because it is legal for a
USERVAR to be followed by either a VAR or a VALUE. A VALUE after a
USERVAR gives the value for the USERVER. A VAR after a USERVAR
implies that the USERVAR is undefined.

The next thing to do is to scan the entire suboption, looking for two
consecutive instances of VAR or VALUE, or for a VAR or VALUE that is
empty. It is not legal for a suboption to contain two VALUEs without
an intervening VAR or USERVAR, and it is also not legal for a
suboption to contain an empty VAR. Thus, if two consecutive VARs or
an empty VALUE can be found, it can be assumed that the client that
generated the suboption uses the correct definitions for VAR and
VALUE. If two consecutive VALUEs or an empty VAR can be found, then
it can be assumed that the client that generated the suboption has
reversed definitions for VAR and VALUE, and from that point on the
server should assume that the VAR and VALUE definitions are reversed.

If things are still in douBT, the next test that can be applied is to
count up how many VARs, USERVARs and VALUEs were received.
(Consecutive USERVARs without an intervening VALUE or VAR should only
be counted as one.) Because a VALUE can only follow a VAR or a
USERVAR, there can never be more VALUEs than the sum of VARs and
USERVARs, and if all VARs and USERVARs have values, then there will
be exactly as many VALUEs as there are VARs and USERVARs. If the
number of VARs and USERVARs is the same as the total number of
VALUEs, then the client has correct definitions for VAR and VALUE.
If the number of VALUEs and USERVARs is the same as the total number
of VARs, then the client has reversed definitions for VAR and VALUE.

If the number of VALUEs is more than the sum of VARs and USERVARs, it
can be assumed that the client has reversed definitions of VAR and
VALUE, and if there are more VARs than USERVARs and VALUEs, then it
can be assumed that the client has the correct definitions for VAR
and VALUE. However, in order to get to this step, it has already
been determined that there are no consecutive VARs and VALUEs. A
little math will show that this means that the number of VALUEs will

never exceed the sum of VARs and USERVARs, and the number of VARs
will never exceed the sum of VALUEs and USERVARs. Hence, this check
is redundant and can be skipped.

If things are still in doubt, the values of the VAR commands can be
checked to see if they do indeed specify well known variables. If
any of them do, then the client is probably using the correct
definitions for VAR and VALUE. Otherwise, if any of the VALUEs
contain well know variable names, then the client probably has
reversed definitions for VAR and VALUE.

If all the above heuristics fail, then the server has done all it can
to determine what type of client it is, and it should just be assumed
that the client is using the correct definitions for VAR and VALUe.

4. Client Summary

The SEND suboption contains only VAR and USERVAR commands.
The server is ok.
The SEND suboption contains VALUE commands.
The server is reversed.
No VAR or VALUE commands are found.
Assume the server is ok.

5. Server Summary

IS/INFO is followed by VAR.
The client is ok.
IS/INFO is followed by VALUE.
The client is reversed.
There are two consecutive VARs.
The client is ok.
There are consecutive VALUEs.
The client is reversed.
There is an empty VALUE.
The client is ok.
There is an empty VAR.
The client is reversed.
The number of USERVARs and VARs is equal to the number of VALUEs.
Assume the client is ok.
The number of USERVARs and VALUEs is equal to the number of VARs.
Assume the client is reversed.
There are VARs with names that are well known.
Assume the client is ok.
There are VALUEs with names that are well known.
Assume the client is reversed.
Anything else.
Assume the client is ok.

References

[1] Borman, D., Editor, "Telnet Environment Option", RFC1408, Cray
Research, Inc., January 1993.

Security Considerations

Security issues are not discussed in this memeo.

Author's Address

David A. Borman
Cray Research, Inc.
655F Lone Oak Drive
Eagan, MN 55123

Phone: (612) 452-6650
EMail: dab@CRAY.COM

Telnet Working Group Mailing List: telnet-ietf@CRAY.COM