GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc2840

Network Working Group J. Altman Request for Comments: 2840 F. da Cruz Category: Informational Columbia University

                                                              May 2000
                        TELNET KERMIT OPTION

Status of this Memo

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

Copyright Notice

 Copyright (C) The Internet Society (2000).  All Rights Reserved.
 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 document 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
 2. DEFINITIONS. . . . . . . . . . . . . . . . . . . . . . . .  2
 3. COMMANDS AND CODES . . . . . . . . . . . . . . . . . . . .  3
 4. COMMAND MEANINGS . . . . . . . . . . . . . . . . . . . . .  3
 5. KERMIT PROTOCOL IMPLICATIONS . . . . . . . . . . . . . . .  5
 6. EXAMPLES . . . . . . . . . . . . . . . . . . . . . . . . .  6
 6.1. EXAMPLE 1. . . . . . . . . . . . . . . . . . . . . . . .  6
 6.2. EXAMPLE 2. . . . . . . . . . . . . . . . . . . . . . . .  7
 6.3. EXAMPLE 3. . . . . . . . . . . . . . . . . . . . . . . .  8
 6.4. EXAMPLE 4. . . . . . . . . . . . . . . . . . . . . . . .  9
 6.5. EXAMPLE 5. . . . . . . . . . . . . . . . . . . . . . . . 10
 7. SECURITY CONSIDERATIONS. . . . . . . . . . . . . . . . . . 11
 8. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . 11
 9. AUTHORS' ADDRESSES . . . . . . . . . . . . . . . . . . . . 11
 10. FULL COPYRIGHT STATEMENT. . . . . . . . . . . . . . . . . 12

Altman & da Cruz Informational [Page 1] RFC 2840 TELNET KERMIT OPTION May 2000

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.

Altman & da Cruz Informational [Page 2] RFC 2840 TELNET KERMIT OPTION May 2000

 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.

Altman & da Cruz Informational [Page 3] RFC 2840 TELNET KERMIT OPTION May 2000

 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 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 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 unambiguous identification of
    Kermit packets in the data stream.  This subnegotiation must be
    sent whenever the Start of Packet character changes.  The values

Altman & da Cruz Informational [Page 4] RFC 2840 TELNET KERMIT OPTION May 2000

    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 such as the
 Internet Kermit Service (port 1649) [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.

Altman & da Cruz Informational [Page 5] RFC 2840 TELNET KERMIT OPTION May 2000

 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.

Altman & da Cruz Informational [Page 6] RFC 2840 TELNET KERMIT OPTION May 2000

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.

Altman & da Cruz Informational [Page 7] RFC 2840 TELNET KERMIT OPTION May 2000

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

Altman & da Cruz Informational [Page 8] RFC 2840 TELNET KERMIT OPTION May 2000

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

Altman & da Cruz Informational [Page 9] RFC 2840 TELNET KERMIT OPTION May 2000

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.

Altman & da Cruz Informational [Page 10] RFC 2840 TELNET KERMIT OPTION May 2000

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 document.

8. REFERENCES

 [BCP] Bradner, S.,  "Key words for use in RFCs to Indicate
       Requirement Levels", BCP 14, RFC 2119, 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, F. and J. Altman, "Internet Kermit Service", RFC 2839,
       May 2000.
 [TEL] Postel, J. and J. Reynolds, "Telnet Protocol Specification",
       STD 8, RFC 854, May 1983.
 [TEL] Postel, J. and J. Reynolds, "Telnet Option Specification", STD
       8, RFC 855, May 1983.

9. AUTHORS' ADDRESSES

 Jeffrey E. Altman
 EMail:jaltman@columbia.edu
 Frank da Cruz
 EMail: 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/

Altman & da Cruz Informational [Page 11] RFC 2840 TELNET KERMIT OPTION May 2000

10. Full Copyright Statement

 Copyright (C) The Internet Society (2000).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Altman & da Cruz Informational [Page 12]

/data/webs/external/dokuwiki/data/pages/rfc/rfc2840.txt · Last modified: 2000/05/23 15:35 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki