Network Working Group                                          J. Mindel
Request for Comments: 1415                                     R. Slaski
                                                     Open Networks, Inc.
                                                            January 1993


                     FTP-FTAM Gateway Specification

Status of the Memo

   This RFC specifies an IAB standards track protocol for the Internet
   community, and requests discussion and suggestions for improvements.
   Please refer to the current edition of the "IAB Official Protocol
   Standards" for the standardization state and status of this protocol.
   Distribution of this memo is unlimited.

Abstract

   This memo describes a dual protocol stack application layer gateway
   that performs protocol translation, in an interactive environment,
   between the FTP and FTAM file transfer protocols.

   Two key assumptions are made:  1) POSIX file naming conventions and
   hierarchical organization, rather than proprietary conventions are in
   use; and 2) X.500 Directory Services are available.

Acknowledgments

   The authors of this RFC would like to express their appreciation to
   the individuals and organizations that participated in the
   implementation of the FTP-FTAM Application Layer Gateway and its
   fielding on the MILNET.  Implementation credits go to Mr. John Scott,
   formerly of the MITRE Corporation, while fielding credits are
   extended to James Graham and R. Greg Lavender of Open Networks, Inc.
   (formerly NetWorks One) and Robert Cooney of the Naval Computer and
   Telecommunications Station (NCTS) Washington.  Dr. Marshall Rose is
   to be commended for recognizing the importance of the FTP-FTAM
   gateway and promulgating it as a part of the ISO Development
   Environment (ISODE).   The following individuals have provided
   valuable editorial comments:  Larry Friedman, Donna Vincent and
   Michael Resnick of Digital Equipment Corporation; Robert Cooney of
   NCTS; and S.E. Hardcastle-Kille of University College London. Funding
   of the FTP-FTAM Gateway Request for Comments effort was provided by
   Open Networks Inc. and the Defense Information Systems Agency (DISA),
   formerly the Defense Communications Agency.  DISA sponsors include
   Len Tabacchi, George Bradshaw, Tom Clarke, and Betsy Turner.





Mindel & Slaski                                                 [Page 1]

RFC 1415             FTP-FTAM Gateway Specification         January 1993


Table of Contents

   1.  Introduction..................................................2
   1.1.   Relationship to Other Work ................................3
   1.2.   Overview of Gateway Operation .............................4
   2.  Gateway Architecture..........................................6
   3.  Network Naming and Addressing.................................8
   4.  Use of the Gateway Services...................................9
   4.1.   FTP-Initiated Gateway Service .............................9
   4.2.   FTAM-Initiated Gateway Service ...........................11
   4.3.   Summary of Usage .........................................12
   5.  Gateway State Variables and Transitions......................13
   5.1.   FTP-Initiated Gateway Service ............................14
   5.2.   FTAM-Initiated Gateway Service ...........................16
   6.  Document Type Support........................................18
   6.1.   Notes on NBS-9 ...........................................18
   7.  Functional Comparison of FTP and FTAM........................19
   7.1.   Loss of Functionality ....................................20
   8. Mapping of Protocol Functions and Representations.............20
   8.1.  FTP-Initiated Gateway Service .............................22
   8.2.  FTAM-Initiated Gateway Service ............................38
   9. Mapping between FTP Reply Codes and FTAM Parameters...........47
   9.1.  FTP Reply Codes to FTAM Parameters ........................48
   9.2.  FTAM Parameters to FTP Reply Codes ........................50
   9.3.  Future Mapping Problem ....................................54
   9.4.  Error Handling ............................................54
   10. Implementation and Configuration Guidelines..................54
   10.1.  Robustness ...............................................54
   10.2.  Well-Known TCP/IP Port ...................................55
   10.3.  Gateway Listener Processes ...............................55
   10.4.  Implementation Testing ...................................55
   10.5.  POSIX File Naming and Organization .......................55
   11. Security Considerations......................................55
   12. References...................................................56
   13. Authors' Addresses...........................................58

1. Introduction

   The TCP/IP and OSI protocol suites will coexist in the Internet
   community for several years to come.  As more and more OSI hosts are
   fielded on the Internet, the requirement for gateways between the two
   protocol suites becomes more pressing.

   This specification describes an application layer gateway providing
   interoperability between the TCP/IP File Transfer Protocol (FTP) and
   the OSI File Transfer, Access, and Management (FTAM) protocol.  The
   proposed application layer gateway is based on a bi-directional set
   of mappings between the FTP and FTAM protocols.  Since the protocols



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RFC 1415             FTP-FTAM Gateway Specification         January 1993


   have quite different command structures, the mappings between them
   are not one-to-one.  This paper assumes knowledge of the File
   Transfer Protocol (FTP) [RFC959] and the File Transfer, Access, and
   Management Protocol (FTAM) [ISO8571-1,2,3,4,5].

   Two important goals of the mappings are to:

      Provide FTP users with as much emulated FTP capability on an
      FTAM Responder as possible, and

      Provide FTAM users with as much emulated FTAM capability on an
      FTP Server as possible.

   Though it is anticipated that the application layer gateway will be
   implemented on full protocol suites of both TCP/IP and OSI, at least
   one implementation of such a gateway (included in the ISO Development
   Environment) can be configured to operate FTAM over either OSI or
   TCP/IP lower-layer services.

1.1. Relationship to Other Work

   Ideas presented in this specification are based on lessons learned in
   fielding the gateway on the MILNET, operational at NCTS Washington
   D.C. since 1989, and on the efforts of M. A. Wallace et al. of the
   National Institute of Standards and Technology (NIST) [NIST86].  In
   1986, NIST published a design document for an FTP-FTAM gateway.
   Since that time, at least one implementation (for a subset of the FTP
   and FTAM protocols) of the gateway has been developed [MITRE87] and
   is included with the ISODE.  This implementation is based on the NIST
   protocol translator gateway design [NIST86].

   This document's contribution to the advancement of the FTP-FTAM
   gateway concept is to:

      *  Enhance the user interaction capability provided by the ISODE
         implementation of the FTP-FTAM application layer gateway.

      *  Clarify and enhance the mappings (FTP to FTAM, FTAM to FTP)
         documented by NIST.

      *  Provide guidelines for fielding the FTP-FTAM application layer
         gateway on the Internet so that it is useful as an Internet
         resource.

      *  Produce a formal specification for the FTP-FTAM gateway suitable
         for implementors to use in building additional FTP-FTAM
         gateways.




Mindel & Slaski                                                 [Page 3]

RFC 1415             FTP-FTAM Gateway Specification         January 1993


      *  Provide a formal specification for organizations wishing to
         procure FTP-FTAM gateways.

1.2. Overview of Gateway Operation

   The gateway provides a virtual end-to-end application file transfer
   service.  As data is sent via FTP, the gateway immediately maps the
   requested function to FTAM and passes it to the FTAM host.  In a
   similar fashion, but using a different set of mappings, an FTAM
   request is sent to the gateway, immediately mapped to an FTP
   function, and passed along to the FTP host.

   In FTP, the two parties involved in a file transfer are the Client
   and Server.  The Client is responsible for initiating a connection to
   the Server.  Once the connection is established, all service requests
   originate from the Client.  The FTP-FTAM gateway does not support the
   FTP three node model.

   In FTAM, the two parties involved in a file transfer are the
   Initiator and Responder.  The Initiator is responsible for initiating
   a connection to the Responder.  Once the connection is established,
   either the Initiator or Responder may issue service requests to the
   other.

   The FTP-FTAM gateway provides two sets of services:

        1. FTP-Initiated Gateway Services

           Utilized when an FTP Client contacts the FTP-FTAM gateway to
           instigate a file transfer with an FTAM Responder.

        2. FTAM-Initiated Gateway Services

           Utilized when an FTAM Initiator contacts the FTP-FTAM
           gateway to instigate a file transfer with an FTP Server.

   The gateway services' names were selected to identify the roles that
   the FTP-FTAM gateway plays when performing file transfers.  For
   example, when a file transfer is instigated by an FTP Client, it
   contacts the FTP Server portion of the gateway, which maps protocol
   information to the FTAM Initiator portion of the gateway, which in
   turn contacts the remote FTAM Responder.  This example scenario uses
   the FTP-Initiated Gateway Services.

   Figure 1 illustrates the perspective of the application process in
   the FTP-Initiated service.  Figure 2 illustrates that of the FTAM-
   Initiated service.




Mindel & Slaski                                                 [Page 4]

RFC 1415             FTP-FTAM Gateway Specification         January 1993


          TCP Host                                  OSI Host

      +--------------+                        +------------------+

      |  FTP Client  |                        |  FTAM Responder  |

      +--------------+                        +------------------+

             |                                          |

             |                                          |

             |                                          |

             |            FTP-FTAM Gateway              |

             |    +--------------------------------+    |

             +--  |  FTP Server    FTAM Initiator  |  --+

                  +--------------------------------+


             Figure 1  -  FTP-Initiated Gateway Service



























Mindel & Slaski                                                 [Page 5]

RFC 1415             FTP-FTAM Gateway Specification         January 1993


          TCP Host                                  OSI Host

      +--------------+                        +------------------+

      |  FTP Server  |                        |  FTAM Initiator  |

      +--------------+                        +------------------+

             |                                          |

             |                                          |

             |                                          |

             |                                          |

             |            FTP-FTAM Gateway              |

             |    +--------------------------------+    |

             +--  |  FTP Client    FTAM Responder  |  --+

                  +--------------------------------+

             Figure 2  -  FTAM-Initiated Gateway Service

2. Gateway Architecture

   The gateway architecture, termed a protocol translator [NIST86], is
   depicted in Figure 3.  It implements TCP/IP and OSI protocol stacks
   with an application level process providing the link between the two.
   The link between FTP and FTAM is defined by two sets of protocol
   mappings, one each for the FTP-Initiated and FTAM-Initiated service
   sets.