RFC2066 - TELNET CHARSET Option

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　　Network Working Group R. Gellens
Request for Comments: 2066 Unisys
Category: Experimental January 1997

TELNET CHARSET Option

Status of this Memo

This memo defines an Experimental Protocol for the Internet

community. This memo does not specify an Internet standard of any
kind. Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.

Abstract

This document specifies a mechanism for passing character set and
translation information between a TELNET client and server. Use of
this mechanism enables an application used by a TELNET user to send
and receive data in the correct character set.

Either side can (subject to option negotiation) at any time request
that a (new) character set be used.

1. Command Names and Codes

CHARSET.......................42

REQUEST ....................01
ACCEPTED ...................02
REJECTED ...................03
TTABLE-IS ..................04
TTABLE-REJECTED ............05
TTABLE-ACK .................06
TTABLE-NAK .................07

As a convenience, standard TELNET text and codes for commands used in
this document are reproduced here (excerpted from [1]):

All TELNET commands consist of at least a two byte sequence: the
"Interpret as Command" (IAC) escape character followed by the code
for the command. The commands dealing with option negotiation are
three byte sequences, the third byte being the code for the option
referenced. ... [O]nly the IAC need be doubled to be sent as data,
and the other 255 codes may be passed transparently. The
following are [some of] the defined TELNET commands. Note that
these codes and code sequences have the indicated meaning only
when immediately preceded by an IAC.

NAME CODE MEANING

SE 240 End of subnegotiation parameters.

SB 250 Indicates that what follows is
subnegotiation of the indicated
option.

WILL 251 Indicates the desire to begin
performing, or confirmation that
you are now performing, the
indicated option.

WON'T 252 Indicates the refusal to perform,
or continue performing, the
indicated option.

DO 253 Indicates the request that the
other party perform, or
confirmation that you are expecting
the other party to perform, the
indicated option.

DON'T 254 Indicates the demand that the other
party stop performing, or
confirmation that you are no longer
expecting the other party to
perform, the indicated option.

IAC 255 Data Byte 255.

2. Command Meanings

A very simple meta-syntax is used, where most tokens represent
previously defined items (such as IAC); angle-brackets ("<>") are
used for items to be further defined; curly-braces ("{}") are used
around optional items; ellipses represent repeated sequences of
items; and quotes are used for literal strings.

IAC WILL CHARSET
The sender REQUESTS permission to, or AGREES to, use
CHARSET option subnegotiation to choose a character set.

IAC WON'T CHARSET
The sender REFUSES to use CHARSET option subnegotiation
to choose a character set.

IAC DO CHARSET
The sender REQUESTS that, or AGREES to have, the other
side use CHARSET option subnegotiation to choose a
character set.

IAC DON'T CHARSET
The sender DEMANDS that the other side not use the
CHARSET option subnegotiation.

IAC SB CHARSET REQUEST { "[TTABLE ]" <Version> } <char set
list> IAC SE

Char set list:

<sep> <character set> { ... <sep> <character set> }

This message initiates a new CHARSET subnegotiation. It can only be
sent by a side that has received a DO CHARSET message and sent a WILL
CHARSET message (in either order).

The sender requests that all text sent to and by it be encoded in one
of the specified character sets.

If the string [TTABLE] appears, the sender is willing to accept a
mapping (translation table) between any character set listed in <char
set list> and any character set desired by the receiver.

<Version> is an octet whose binary value is the highest version
level of the TTABLE-IS message which can be sent in response. This
field must not be zero. See the TTABLE-IS message for the permitted
version values.

<Char set list> is a sequence of 7-BIT ASCII printable characters.
The first octet defines the separator character (which must not
appear within any character set). It is terminated by the IAC SE
sequence. Case is not significant. It consists of one or more
character sets. The character sets should appear in order of
preference (most preferred first).

<Sep> is a separator octet, the value of which is chosen by the
sender. Examples include a space or a semicolon. Any value other
than IAC is allowed. The obvious choice is a space or any other
punctuation symbol which does not appear in any of the character set
names.

<Character set> is a sequence of 7-BIT ASCII printable characters.
Case is not significant.

If a requested character set name does not start with "X-" or "x-",
it MUST be registered with the Internet Assigned Number Authority
(IANA) [2].

The receiver responds in one of four ways:

If the receiver is already sending text to and expecting text from
the sender to be encoded in one of the specified character sets, it
sends a positive acknowledgment (CHARSET ACCEPTED); it MUST NOT
ignore the message. (Although ignoring the message is perhaps
suggested by some interpretations of the relevant RFCs ([1], [3]), in
the interests of determinacy it is not permitted. This ensures that
the issuer does not need to time out and infer a response, while
avoiding (because there is no response to a positive acknowledgment)
the non-terminating subnegotiation which is the rationale in the RFCs
for the non-response behavior.)

If the receiver is capable of handling at least one of the specified
character sets, it can respond with a positive acknowledgment for one
of the requested character sets. Normally, it should pick the first
set it is capable of handling but may choose one based on its own
preferences. After doing so, each side MUST encode subsequent text
in the specified character set.

If the string [TTABLE] is present, and the receiver prefers to use a
character set not included in <char set list>, and is capable of
doing so, it can send a translate table (TTABLE-IS) response.

If the receiver is not capable of handling any of the specified
character sets, it sends a negative acknowledgment (CHARSET
REJECTED).

Because it is not valid to reply to a CHARSET REQUEST message with
another CHARSET REQUEST message, if a CHARSET REQUEST message is
received after sending one, it means that both sides have sent them
simultaneously. In this case, the server side MUST issue a negative
acknowledgment. The client side MUST respond to the one from the
server.

IAC SB CHARSET ACCEPTED <Charset> IAC SE
This is a positive acknowledgment response to a CHARSET REQUEST
message; the receiver of the CHARSET REQUEST message acknowledges
its receipt and accepts the indicated character set.

<Charset> is a character sequence identical to one of the
character sets in the CHARSET REQUEST message. It is terminated
by the IAC SE sequence.

Text messages which follow this response must now be coded in the
indicated character set. This message terminates the current
CHARSET subnegotiation.

IAC SB CHARSET REJECTED IAC SE
This is a negative acknowledgment response to a CHARSET REQUEST
message; the receiver of the CHARSET REQUEST message acknowledges
its receipt but refuses to use any of the requested character
sets. Messages can not be sent in any of the indicated character
sets. This message can also be sent by the sender of a TTABLE-IS
message, if multiple TTABLE-NAK messages were sent in response.
This message terminates the current CHARSET subnegotiation.

IAC SB CHARSET TTABLE-IS <version> <syntax for version> IAC SE
In response to a CHARSET REQUEST message in which [TTABLE] was
specified, the receiver of the CHARSET REQUEST message
acknowledges its receipt and is transmitting a pair of tables
which define the mapping between specified character sets.

<Version> is an octet whose binary value is the version level of
this TTABLE-IS message. Different versions have different syntax.
The lowest version level is one (zero is not valid). The current
highest version level is also one. This field is provided so that
future versions of the TTABLE-SEND message can be specified, for
example, to handle character sets for which there is no simple
one-to-one character-for-character translation. This might
include some forms of multi-octet character sets for which
translation algorithms or subsets need to be sent.

Syntax for Version 1:

<sep> <char set name 1> <sep> < char size 1> < char count 1> <char
set name 2> <sep> <char size 2> <char count 2> <map 1> <map 2>

<Sep> is a separator octet, the value of which is chosen by the
sender. Examples include a space or a semicolon. Any value other
than IAC is allowed. The obvious choice is a space or any other
punctuation symbol which does not appear in either of the
character set names.

<Char set name 1> and <Char set name 2> are sequences of 7-BIT
ASCII printable characters which identify the two character sets
for which a mapping is being specified. Each is terminated by
<sep>. Case is not significant. If a character set name does not

start with "X-" or "x-", it MUST be registered with IANA. <Char
set name 1> MUST be chosen from the <char set list> in the CHARSET
REQUEST message. <Char set name 2> can be arbitrarily chosen.
Text on the wire MUST be encoded using <char set name 2>.

<Char size 1> and <char size 2> are single octets each. The
binary value of the octet is the number of bits nominally
required for each character in the corresponding table. It SHOULD
be a multiple of eight.

<Char count 1> and <char count 2> are each three-octet binary
fields in Network Byte Order [6]. Each specifies how many
characters (of the maximum 2**<char size>) are being transmitted
in the corresponding map.

<Map1> and <Map 2> each consist of the corresponding <char count>
number of characters. These characters form a mapping from all or
part of the characters in one of the specified character sets to
the correct characters in the other character set. If the
indicated <char count> is less than 2**<char size>, the first
<char count> characters are being mapped, and the remaining
characters are assumed to not be changed (and thus map to
themselves). That is, each map contains characters 0 through
<char count> -1. <Map 1> maps from <char set name 1> to <char set
name 2>. <Map 2> maps from <char set name 2> to <char set name
1>. Translation between the character sets is thus an obvious
process of using the binary value of a character as an index into
the appropriate map. The character at that index replaces the
original character. If the index exceeds the <char count> for the
map, no translation is performed for the character.

[Note to implementers: since TELNET works in octets, it is
possible for octets of value 255 to appear "spontaneously" when
using multi-octet or non-8-bit characters. All octets of value
255 (other than IAC) MUST be quoted to conform with TELNET
requirements. This applies even to octets within a table, or text
in a multi-octet character set.]

IAC SB CHARSET TTABLE-ACK IAC SE
The sender acknowledges the successful receipt of the translate
table. Text messages which follow this response must now be coded
in the character set specified as <char set name 2> of the
TTABLE-IS message. This message terminates the current CHARSET
subnegotiation.

IAC SB CHARSET TTABLE-NAK IAC SE
The sender reports the unsuccessful receipt of the translate table
and requests that it be resent. If subsequent transmission
attempts also fail, a TTABLE-REJECTED or CHARSET REJECTED message
(depending on which side sends it) should be sent instead of
additional futile TTABLE-IS and TTABLE-NAK messages.

IAC SB CHARSET TTABLE-REJECTED IAC SE
In response to a TTABLE-IS message, the receiver of the TTABLE-IS
message acknowledges its receipt and indicates it is unable to
handle it. This message terminates the current CHARSET
subnegotiation.

Any system which supports the CHARSET option MUST fully support
the CHARSET REQUEST, ACCEPTED, REJECTED, and TTABLE-REJECTED
subnegotiation messages. It MAY optionally fully support the
TTABLE-IS, TTABLE-ACK, and TTABLE-NAK messages. If it does fully
support the TTABLE-IS message, it MUST also fully support the
TTABLE-ACK and TTABLE-NAK messages.

3. Default

WON'T CHARSET

DON'T CHARSET

4. Motivation for the Option

Many TELNET sessions need to transmit data which is not in 7-bit
ASCII. This is usually done by negotiating BINARY, and using local
conventions (or terminal type kluges) to determine the character set
of the data. However, such methods tend not to interoperate well,
and have difficulties when multiple character sets need to be
supported by different sessions.

Many computer systems now utilize a variety of character sets.
Increasingly, a server computer needs to document character sets or
translate transmissions and receptions using different pairs of
character sets on a per-application or per-connection basis. This is
becoming more common as client and server computers become more
geographically disperse. (And as servers are consolidated into
ever-larger hubs, serving ever-wider areas.) In order for files,
databases, etc. to contain correct data, the server must determine
the character set in which the user is sending, and the character set
in which the application expects to receive.

In some cases, it is sufficient to determine the character set of the
end user (because every application on the server expects to use the
same character set, or because applications can handle the user's
character set), but in other cases different server applications
expect to use different character sets. In the former case, an
initial CHARSET subnegotiation suffices. In the latter case, the
server may need to initiate additional CHARSET subnegotiations as the
user switches between applications.

At a minimum, the option described in this memo allows both sides to
be clear as to which character set is being used. A minimal
implementation would have the server send DO CHARSET, and the client
send WILL CHARSET and CHARSET REQUEST. The server could then
communicate the client's character set to applications using whatever
means are appropriate. Such a server might refuse subsequent CHARSET
REQUEST messages from the client (if it lacked the ability to
communicate changed character set information to applications, for
example). Another system might have a method whereby various
applications could communicate to the TELNET server their character
set needs and abilities, which the server would handle by initiating
new CHARSET REQUEST negotiations as appropriate.

In some cases, servers may have a large set of clients which tend to
connect often (such as daily) and over a long period of time (such as
years). The server administrators may strongly prefer that the
servers not do character set translation (to save CPU cycles when
serving very large numbers of users). To avoid manually configuring
each copy of the user TELNET software, the administrators might
prefer that the software supports translate tables. (If the client
software received a translate table from the server and stored it,
the table would only need to be sent once.)

5. Description of the Option

When the client TELNET program is able to determine the user's
character set it should offer to specify the character set by sending
IAC WILL CHARSET.

If the server system is able to make use of this information, it
replies with IAC DO CHARSET. The client TELNET is then free to
request a character set in a subnegotiation at any time.

Likewise, when the server is able to determine the expected character
set(s) of the user's application(s), it should send IAC DO CHARSET
to request that the client system specify the character set it is
using. Or the server could send IAC WILL CHARSET to offer to specify
the character sets.

Once a character set has been determined, the server can either
perform the translation between the user and application character
sets itself, or request by additional CHARSET subnegotiations that
the client system do so.

Once it has been established that both sides are capable of character
set negotiation (that is, each side has received either a WILL
CHARSET or a DO CHARSET message, and has also sent either a DO
CHARSET or a WILL CHARSET message), subnegotiations can be requested
at any time by whichever side has sent a WILL CHARSET message and
also received a DO CHARSET message (this may be either or both
sides). Once a CHARSET subnegotiation has started, it must be
completed before additional CHARSET subnegotiations can be started
(there must never be more than one CHARSET subnegotiation active at
any given time). When a subnegotiation has completed, additional
subnegotiations can be started at any time.

If either side violates this rule and attempts to start a CHARSET
subnegotiation while one is already active, the other side MUST
reject the new subnegotiation by sending a CHARSET REJECTED message.

Receipt of a CHARSET REJECTED or TTABLE-REJECTED message terminates
the subnegotiation, leaving the character set unchanged. Receipt of
a CHARSET ACCEPTED or TTABLE-ACK message terminates the
subnegotiation, with the new character set in force.

In some cases, both the server and the client systems are able to
perform translations and to send and receive in the character set(s)
expected by the other side. In such cases, either side can request
that the other use the character set it prefers. When both sides
simultaneously make such a request (send CHARSET REQUEST messages),
the server MUST reject the client's request by sending a CHARSET
REJECTED message. The client system MUST respond to the server's
request. (See the CHARSET REQUEST description, above.)

When the client system makes the request first, and the server is
able to handle the requested character set(s), but prefers that the
client system instead use the server's (user application) character
set, it may reject the request, and issue a CHARSET REQUEST of its
own. If the client system is unable to comply with the server's
preference and issues a CHARSET REJECTED message, the server can
issue a new CHARSET REQUEST message for one of the previous character
sets (one of those which the client system originally requested).
The client system would obviously accept this character set.

While a CHARSET subnegotiation is in progress, data SHOULD be queued.
Once the CHARSET subnegotiation has terminated, the data can be sent
(in the correct character set).

Note that regardless of CHARSET negotiation, translation only applies
to text (not commands), and only occurs when in BINARY mode [4]. If
not in BINARY mode, all data is assumed to be in NVT ASCII [1].

Also note that the CHARSET option should be used with the END OF
RECORD option [5] for block-mode terminals in order to be clear on
what character represents the end of each record.

As an example of character set negotiation, consider a user on a
workstation using TELNET to communicate with a server. In this
example, the workstation normally uses the Cyrillic (ASCII) character
set [2] but is capable of using EBCDIC-Cyrillic [2], and the server
normally uses EBCDIC-Cyrillic. The server could handle the (ASCII)
Cyrillic character set, but prefers that instead the client system
uses the EBCDIC-Cyrillic character set. (This and the following
examples do not show the full syntax of the subnegotiation messages.)

CLIENT SERVER

WILL CHARSET WILL CHARSET

DO CHARSET DO CHARSET

CHARSET REQUEST Cyrillic
EBCDIC-Cyrillic

CHARSET ACCEPTED EBCDIC-
Cyrillic

Now consider a case where the workstation can't handle EBCDIC-
Cyrillic, but can accept a translate table:

CLIENT SERVER

WILL CHARSET WILL CHARSET

DO CHARSET DO CHARSET

CHARSET REQUEST [TTABLE] 1
Cyrillic

CHARSET TTABLE-IS 1 Cyrillic
EBCDIC-Cyrillic

CHARSET TTABLE-ACK

For another example, consider a case similar to the previous case,
but now the user switches server applications in the middle of the
session (denoted by ellipses), and the new application requires a
different character set:

CLIENT SERVER

WILL CHARSET WILL CHARSET

DO CHARSET DO CHARSET

CHARSET REQUEST [TTABLE] 1
Cyrillic EBCDIC-INT

CHARSET TTABLE-IS 1 Cyrillic
EBCDIC-Cyrillic

CHARSET TTABLE-ACK

. . . . . .

CHARSET REQUEST EBCDIC-INT

CHARSET ACCEPTED EBCDIC-INT

6. Security Considerations

Security issues are not discussed in this memo.

7. References

[1] Postel, J. and J. Reynolds, "Telnet Protocol
Specification", STD 8, RFC854, ISI, May 1983.

[2] Reynolds, J., and J. Postel, "Assigned Numbers",
STD 2, RFC1700, ISI, October 1994.

[3] Postel, J. and J. Reynolds, "Telnet Option
Specifications", STD 8, RFC855, ISI, May 1983.

[4] Postel, J. and J. Reynolds, "Telnet Binary
Transmission", STD 27, RFC856, ISI, May 1983.

[5] Postel, J., "Telnet End-Of-Record Option", RFC885,
ISI, December 1983.

[6] Postel, J., "Internet Official Protocol Standards",
STD 1, RFC1920, IAB, March 1996.

8. Author's Address

Randall Gellens
Unisys Corporation
25725 Jeronimo Road
Mail Stop 237
Mission Viejo, CA 92691
USA

Phone: +1.714.380.6350
Fax: +1.714.380.5912
EMail: Randy@MV.Unisys.Com

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