Network Working Group Internet Engineering Task Force
Internet-Draft Telnet Working Group
D. Borman, Editor
Cray Research, Inc.
April 1990
Telnet Data Compression Option
Status of this Memo
This draft document will be submitted to the RFC editor as a protocol
specification. Distribution of this memo is unlimited. Please send
comments to Dave Borman.
1. Command Names and Codes
COMPRESS TBA
START 0
END 1
SUPPORT 2
REQUEST-START 3
REQUEST-END 4
ANY 0
LZ 1
2. Command Meanings
IAC WILL COMPRESS
The sender of this command is willing to send compressed data.
IAC WONT COMPRESS
The sender of this command refuses to send compressed data.
IAC DO COMPRESS
The sender of this command is willing to receive compressed data.
IAC DONT COMPRESS
The sender of this command refuses to accept compressed data.
IAC SB COMPRESS START compression-type IAC SE
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The sender of this command is stating that at this point in the
data stream, all following data will be compressed via the
"compression-type" method of data compression. A "compression-
type" value of ANY must not be used. The current types of
compression are listed in the current version of the Assigned
Numbers document[1].
IAC SB COMPRESS END IAC SE
The sender of this command is stating that at this point in the
data stream, all following data will no longer be compressed.
IAC SB COMPRESS SUPPORT compression-type-list IAC SE
The sender of this command is stating what types of compression it
will support.
IAC SB COMPRESS REQUEST-START compression-type IAC SE
The sender of this command requests that the remote side begin
compression of the telnet data stream. The "compression-type"
value is a request for a specific type of compression. If no
specific type is needed, a value of ANY may be used.
IAC SB COMPRESS REQUEST-END IAC SE
The sender of this command requests that the remote side stop
compression of the telnet data stream.
3. Default Specification
The default specification for this option is
WONT COMPRESS
DONT COMPRESS
meaning there will be no compression of the data stream.
4. Motivation
When large amounts of data are being generated over a Telnet connec-
tion, and the connection is over a slow link, it is useful to provide
some form of data compression to keep the number of bytes transmitted
down to a minimum to improve network throughput. This option pro-
vides a method for compression of part or all of the data stream.
5. Implementation Rules
WILL and DO are used only at the beginning of the connection to ob-
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tain and grant permission for future negotiations. If compression is
needed in both directions, then the COMPRESS option must be negotiat-
ed in both directions.
Once the two hosts have exchanged a WILL and a DO, the sender of the
DO COMPRESS must send a COMPRESS SUPPORT command to let the remote
side know what types of compression it is willing to accept. In the
request, a list of supported compression schemes is sent. Only the
sender of the DO may send a list of supported compression types ("IAC
SB COMPRESS SUPPORT compression-type-list IAC SE"). Only the sender
of the WILL may actually transmit compressed data. This is initiated
via the "IAC SB COMPRESS START compression-type IAC SE" command, and
terminated via the "IAC SB COMPRESS END IAC SE" command. If a START
is received, and then a second START is received before receiving an
END, the second START is assumed to terminate the first START com-
mand, and then begin a (possibly) new method of compression.
If the sender of the DO would like the remote side to begin sending
compressed data, it can send the "IAC SB COMPRESS REQUEST-START
compression-type IAC SE" command. The "compression-type" may contain
a specific type of compression that is requested, or it may contain
ANY if no specific type of compression is preferred. If the sender
of the DO would like the remote side to stop sending compressed data,
it can send the "IAC SB COMPRESS REQUEST-STOP IAC SE" command.
The current list of compression types are listed in the current As-
signed Numbers RFC [1].
If the receiver of the SUPPORT command does not support any of the
compression types listed in the SUPPORT command, it should send an
IAC WONT COMPRESS command to turn off the COMPRESS option.
If the receiver of a REQUEST-START receives a compression type that
is not supported, it may choose to use any other type of compression
that was received in the initial SUPPORT command.
The order of the compression types may be ordered to indicate a
preference for different compression types, the first type being the
most preferred, and the last type the least preferred.
Only the user data is compressed. Telnet options and commands are
inserted into the data stream after the user data has been
compressed, and removed from the data stream before the data is un-
compressed.
If the COMPRESS option has been enabled, and compressed data is being
received, the receipt of an "IAC WONT COMPRESS" implies the receipt
of an "IAC SB COMPRESS END IAC SE", e.g., compression of the data
stream is terminated.
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The following is an example of use of the option:
Host1 Host2
IAC DO COMPRESS
IAC WILL COMPRESS
IAC SB COMPRESS SUPPORT 1 2 3
IAC SE
[ Host2 is now free to send compressed data ]
IAC SB COMPRESS REQUEST-START 1
IAC SE
IAC SB COMPRESS START 2 IAC SE
[ Host2 did not support compression type 1, so it chose type 2.
All data from Host2 to Host1 is now compressed ]
IAC SB COMPRESS START 3 IAC SE
[ Host2 has now stopped using compression type 2, and is now us-
ing compression type 3. ]
IAC SB COMPRESS END IAC SE
[ All data from Host2 to Host1 is now uncompressed again ]
It is expected that any implementation that supports the Telnet
COMPRESS option will support all of this specification.
6. References
[1] Reynolds, Joyce, and Postel, Jon, "Assigned Numbers", RFC 993,
ISI, November 1986
Author's Address
David A. Borman, Editor
Cray Research, Inc.
1440 Northland Drive
Mendota Heights, MN 55120
Phone: (612) 681-3398
Mailing List: telnet-ietf@CRAY.COM
EMail: dab@CRAY.COM
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