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Versions: 00 01 02 RFC 2840

Internet Engineering Task Force                        Jeffrey E. Altman
INTERNET-DRAFT                                             Frank da Cruz
<draft-altman-telnet-kermit-server-02.txt>           Columbia University
                                                         25 January 2000
                                                   Expires: 25 July 2000


                          TELNET KERMIT OPTION

                                DRAFT 02

STATUS OF THIS MEMO

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference mate-
   rial or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119.


ABSTRACT

This Internet-Draft describes an extension to the Telnet protocol to allow
the negotiation, coordination, and use of the Kermit file transfer and
management protocol over an existing Telnet protocol connection.


CONTENTS

  1. MOTIVATION
  2. DEFINITIONS
  3. COMMANDS AND CODES
  4. COMMAND MEANINGS
  5. KERMIT PROTOCOL IMPLICATIONS
  6. EXAMPLES
  6.1. EXAMPLE 1
  6.2. EXAMPLE 2
  6.3. EXAMPLE 3
  6.4. EXAMPLE 4
  6.5. EXAMPLE 5
  7. SECURITY CONSIDERATIONS
  8. REFERENCES
  9. AUTHORS' ADDRESS


1. MOTIVATION

The Kermit protocol [KER] performs error-corrected file transfer and
management over many types of connections, including terminal
connections, among diverse hardware and software platforms.  It is
supported by a large number of Telnet clients and is also widely
available on the Internet hosts to which Telnet connections are made.

Traditionally, the Kermit protocol connection is started manually by a
user, or perhaps by an automated script.  It is the user's
responsibility to start the Kermit server on one end of the connection
and the Kermit client on the other, or to start a Kermit "send"
operation on one end and a Kermit "receive" on the other.

This procedure grew out of necessity on ordinary direct-dial
connections, and serves its purpose within the limitations of that
context.  But it introduces timing and dexterity problems, and lacks an
effective way for each Kermit program to determine the "mode" of the
other, or even its very presence, and therefore to know with certainty
which operations and procedures are legal on the connection at any given
time.

When Kermit services are offered on the Internet, however, a strong
coupling can be established between the two end applications by having
the Telnet protocol [TEL] serve as a supervisor for Kermit sessions,
ensuring that a valid and known relationship is always obtained.  Kermit
sessions are, in effect, embedded within Telnet sessions, with Telnet
providing the mechanism for starting and stopping them and defining
which end is the Kermit client and which is the Kermit server, possibly
changing the relationship in response to user actions.


2. DEFINITIONS

Kermit server
  A software program that is ready to accept and act upon commands in
  the form of well-defined Kermit packets [KER].

Kermit client
  A software program that receives requests through its user interface
  from a human agent (or a script or other source) and translates them to
  command packets, which it sends to a Kermit server, thus initiating a
  Kermit protocol transaction such as the transfer of one or more files.

Availability of Kermit server
  For the purposes of this document, a Kermit server is said to be
  available if, through the negotiations described herein, its Telnet
  partner knows that it is a Kermit server.


3. COMMANDS AND CODES

Support for a Kermit server is negotiated separately in each direction,
allowing Kermit service to be embedded in the Telnet client, the Telnet
server, or in both.  The proposed Telnet extensions are, therefore,
symmetrical.

When the connection is first opened, Kermit service is unavailable in
both directions.

The availability of Kermit service is negotiated using the following
Telnet option:

  KERMIT           47 (assigned by IANA)

The state of the connection is controlled by the following Telnet
subnegotiation function codes:

  START-SERVER      0
  STOP-SERVER       1
  REQ-START-SERVER  2
  REQ-STOP-SERVER   3
  SOP               4
  RESP-START-SERVER 8
  RESP-STOP-SERVER  9


4. COMMAND MEANINGS

The KERMIT OPTION is negotiated using the standard Telnet mechanisms:

IAC WILL KERMIT
  The sender of this command incorporates a Kermit server and is willing
  to negotiate its use.

IAC WONT KERMIT
  The sender of this command does not incorporate a Kermit server or
  refuses to negotiate its use.

IAC DO KERMIT
  The sender of this command requests that the receiver negotiate use of
  a Kermit server.

IAC DONT KERMIT
  The sender of this command refuses to negotiate the use of a Kermit
  server.

Once WILL KERMIT is negotiated in a particular direction, subnegotiations
are used to indicate or request a change in state of the connection, or to
convey other information.  Subnegotiations may be sent at any time.

IAC SB KERMIT START-SERVER
  This command is sent by the WILL side to indicate that the Kermit
  server is now active; that is, that client-initiated Kermit packets
  will be accepted.

IAC SB KERMIT STOP-SERVER
  This command is sent by the WILL side to indicate that the Kermit
  server is no longer active, and therefore that it is not ready to
  accept Kermit packets.

IAC SB KERMIT REQ-START-SERVER
  This command is sent by the DO side to request that the Kermit server
  be started.  It must be responded to with either RESP-START-SERVER
  or RESP-STOP-SERVER depending upon whether or not the request was
  accepted.

IAC SB KERMIT REQ-STOP-SERVER
  This command is sent by the DO side to request that the Kermit server
  be stopped.  It must be responded to with either RESP-START-SERVER
  or RESP-STOP-SERVER depending upon whether or not the request was
  accepted.

IAC SB KERMIT RESP-START-SERVER
  This command is sent by the WILL side in response to REQ-START-SERVER
  or REQ-STOP-SERVER to indicate that the Kermit server is active
  after the request was accepted or denied.

IAC SB KERMIT RESP-STOP-SERVER
  This command is sent by the WILL side in response to REQ-START-SERVER
  or REQ-STOP-SERVER to indicate that the Kermit server is not active
  after the request was accepted or denied.

IAC SB KERMIT SOP <octet>
  Kermit Start Of Packet.  The sender of this command specifies the
  octet it will use to mark the beginning of the Kermit packets it
  sends.  This command must be sent by each connection partner upon the
  first WILL/DO pair to allow unambigious identification of Kermit
  packets in the data stream.  This subnegotiation must be sent whenever
  the Start of Packet character changes.  The values are restricted to
  ASCII C0 control characters other than Carriage Return and NUL.  The
  normal value is 1 (ASCII SOH).  The two Kermit partners normally use
  the same SOP, but may use distinct ones if desired.

IAC SB KERMIT SOP is necessary to allow each Telnet partner to recognize
subsequent incoming Kermit packets.  Data following the SOP is processed
by the Kermit packet analyzer.  All other Kermit protocol parameters are
automatically negotiated within the Kermit protocol upon the initial
exchange of Kermit packets [KER].

START-SERVER and STOP-SERVER commands must be sent by the WILL side
whenever the state of the Kermit server changes.  When WILL is
successfully negotiated the state of the WILL side is assumed to be
STOP-SERVER.  If the server is active, the WILL side must send a
START-SERVER to indicate the change in state.

The receiver of a REQ-START-SERVER or REQ-STOP-SERVER is not required to
agree to the request to change state.  The receiver must respond with
either RESP-START-SERVER or RESP-STOP-SERVER to indicate the state of the
Kermit Server subsequent to the request.  RESP-xxx-SERVER is sent instead
of xxx-SERVER to enable the sender of REQ-xxx-SERVER to distinguish
between the WILL side's spontaneous change in state and the response
to the DO side's request.

If the Kermit server receives a Kermit packet commanding it to cease
Kermit service (such as a FINISH, REMOTE EXIT or BYE packet [KER]), it
must send IAC SB KERMIT STOP-SERVER if the command is accepted.

These rules ensure that the Telnet client's user interface always knows
whether (and on which end) a Kermit server is available, and can
therefore present the user only with valid choices, and that changes in
state of one Telnet partner automatically switch the other to a
complementary and valid state.

While it is possible for a traditional telnet service (port 23) to
implement this option while at the same time supporting the existing
remote shell access functionality, it is not expected that this option
will be used in that manner.  Instead, this option is primarily meant
for use with dedicated Kermit services (port 1649) such as the
Internet Kermit Service [IKS].


5. KERMIT PROTOCOL IMPLICATIONS

The Kermit protocol is described elsewhere [KER].  It is an extensible
and self-configuring protocol, like Telnet, and thus any two proper
Kermit implementations should interoperate automatically.

In Kermit, as in Telnet, one particular octet is distinguished.  In
Telnet's case, it is IAC (decimal 255); in Kermit's it is the character
specified by the IAC SB KERMIT SOP negotiation, normally SOH (decimal 1,
Ctrl-A).  All Kermit packets must begin with the SOP and should not
contain the SOP character in an unquoted form.

Telnet protocol takes precedence over Kermit protocol; whenever an IAC
is detected, it is processed as the beginning of a Telnet command unless
quoted by another IAC.  Telnet commands can contain any characters at
all, including the SOP octet, transparently to the Kermit protocol, and
in fact Telnet commands are not seen by the Kermit protocol at all.

Kermit protocol must follow Telnet NVT rules in each direction when
Telnet binary mode is not negotiated for that direction.

If 8-bit transparency is desired, Telnet binary mode may be negotiated
upon entry to Kermit protocol in the appropriate direction, and the
previous mode (NVT or binary) restored upon exit from Kermit protocol.
Telnet binary mode can result in more efficient transfers, but is not
required for data transfer, since Kermit protocol does not require a
transparent path.


6. EXAMPLES

6.1. EXAMPLE 1

The Telnet server contains a Kermit server.  The Telnet client includes
Kermit protocol but does not implement the Telnet KERMIT Option.

  Telnet Server                   Telnet Client
  -----------------------------   -----------------------------
  <starts negotiations>
  WILL KERMIT
  DO KERMIT
                                  <responds to negotiations>
                                  DONT KERMIT
                                  WONT KERMIT

>From this point, no subnegotiations take place, and the Kermit
client/server relationship is under manual control of the user of the
Telnet client.  This will be the case (for example) with existing
Kermit-capable Telnet clients.

6.2. EXAMPLE 2

The Telnet server contains a Kermit server and starts a Kermit server
immediately after a connection is made.  The Telnet client does not
offer a Kermit server.

  Telnet Server                   Telnet Client
  -----------------------------   -----------------------------
  <starts negotiations>
  WILL KERMIT
  DO KERMIT
                                  <responds to negotiations>
                                  DO KERMIT
                                  SB KERMIT SOP <0x01>
                                  WONT KERMIT
  SB KERMIT SOP <0x01>

  <starts Kermit Server>
  SB KERMIT START-SERVER

At this point the Telnet client knows that a Kermit server is on the
other end of the connection, and so may customize its command set or
menus to allow only those commands that are valid as a client of a
Kermit server.

6.3. EXAMPLE 3

Telnet server and Telnet client both contain a Kermit server.  Telnet
client Kermit server is active whenever its terminal emulator is active,
and not active at other times.  The Telnet server is used for shell
access and does not start a Kermit Server unless requested.

  Telnet Server                 Telnet Client
  ---------------------------   -----------------------------
  <starts negotiations>
  WILL KERMIT
  DO KERMIT
                                <responds to negotiations>
                                DO KERMIT
                                SB KERMIT SOP <0x01>
                                WILL KERMIT
  SB KERMIT SOP <0x01>
                                <telnet client enters terminal emulator>
                                SB KERMIT START-SERVER

                                <client leaves terminal emulator>
                                SB KERMIT STOP-SERVER

                                <client requests Kermit service>
                                SB KERMIT REQ-START-SERVER
  <starts Kermit server>
  SB KERMIT RESP-START-SERVER
                                <client sends Kermit FINISH packet>
  <stops Kermit server>
  SB KERMIT STOP-SERVER
                                <client returns to terminal emulator>
                                SB KERMIT START-SERVER


6.4. EXAMPLE 4

Telnet server and Telnet client both contain a Kermit server.  Telnet
client's Kermit server is active whenever the terminal emulator is
active.  Telnet server is used solely for Kermit protocol and
automatically starts a Kermit Server upon accepting the connection.

  Telnet Server                 Telnet Client
  ---------------------------   -----------------------------
  <starts negotiations>
  WILL KERMIT
  DO KERMIT
                                <responds to negotiations>
                                DO KERMIT
                                SB KERMIT SOP <0x01>
                                WILL KERMIT

  SB KERMIT SOP <0x01>
                                <client enters terminal emulator>
                                SB KERMIT START-SERVER

  <in response to DO>
  SB KERMIT SOP <0x01>
  SB KERMIT START-SERVER
                                <client restricts command set to
                                 Kermit protocol commands>
                                SB KERMIT STOP-SERVER

                                <client performs Kermit protocol
                                 operations>

                                <client want to enter terminal mode>
                                SB KERMIT REQ-STOP-SERVER

  <Kermit Server refuses>
  SB KERMIT RESP-START-SERVER


6.5. EXAMPLE 5

This is an example of something that should not be allowed to happen.
Some Telnet clients that implement file transfer capabilities are
designed to accept incoming connections.  In this situation the Telnet
Client acts as a pseudo Telnet Server but without the ability to provide
shell access or many of the other functions associated with Telnet.  If
both Telnet clients support this option and contain a Kermit server that
is active during terminal emulation there is the potential for a
deadlock situation if scripting is also supported.  This is because
Telnet clients that support a script language do not process input while
waiting for the next command to be issued.

  Telnet Client One             Telnet Client Two
  ---------------------------   -----------------------------
  <starts negotiations>
  WILL KERMIT
  DO KERMIT
                                <responds to WILL>
                                DO KERMIT
                                SB KERMIT SOP <0x01>

  <in response to DO>
  SB KERMIT SOP <0x01>
  SB KERMIT START-SERVER
                                <responds to DO>
                                WILL KERMIT
                                SB KERMIT START-SERVER

  <client one restricts command
   set to Kermit protocol and
   disables Kermit Server>
  SB KERMIT STOP-SERVER
                                <client two restricts command
                                 set to Kermit protocol and
                                 disables Kermit Server>
                                SB KERMIT STOP-SERVER

At this point both clients have restricted their command set to Kermit
Protocol commands.  However, in both cases neither side is processing
input.  Therefore the following restriction MUST be enforced: A Telnet
partner may not restrict the command set if it accepted the incoming
connection.


7. SECURITY

Implementors of this Telnet Option must enforce appropriate user
authentication and file system access restrictions in conjunction with
their implementation of the Kermit file transfer protocol.  These issues
are beyond the scope of this Internet-Draft.


8. REFERENCES

[BCP] Bradner, Scott, RFC 2119, "Best Current Practice", March 1997.

[KER] da Cruz, Frank, "Kermit, A File Transfer Protocol", Digital Press/
      Butterworth Heinemann, Newton, MA, ISBN 0-932376-88-6 (1987).

[IKS] da Cruz, Frank, and Jeffrey E. Altman, "Internet Kermit Service",
      Internet Draft <draft-columbia-kermit-service-02.txt>,
      August 1998.

[TEL] Postel, J., and J. Reynolds, "Telnet Protocol Specification",
      RFC854, May 1983, et seq.; "Telnet Option Specification",
      RFC855, May 1983, et seq.


9. AUTHORS' ADDRESS

Jeffrey E. Altman
jaltman@columbia.edu

Frank da Cruz
fdc@columbia.edu

The Kermit Project
Columbia University
612 West 115th Street
New York NY 10025-7799
USA
http://www.columbia.edu/kermit/
http://www.kermit-project.org/

(End)


    Jeffrey Altman * Sr.Software Designer * Kermit-95 for Win32 and OS/2
                 The Kermit Project * Columbia University
              612 West 115th St #716 * New York, NY * 10025
  http://www.kermit-project.org/k95.html * kermit-support@kermit-project.org


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