RFC1545 - FTP Operation Over Big Address Records (FOOBAR)

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　　Network Working Group D. Piscitello
Request for Comments: 1545 Bellcore
Category: Experimental November 1993

FTP Operation Over Big Address Records (FOOBAR)

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 paper describes a convention for specifying longer addresses in
the PORT command.

Introduction

This RFCspecifies a method for assigning long addresses in the
HOST-PORT specification for the data port to be used in establishing
a data connection for File Transfer Protocol, FTP (STD 9, RFC959).
This is a general solution, applicable for all "next generation" IP
alternatives, and can also be extended to allow FTP operation over
transport interfaces other than TCP.

Acknowledgments

Many thanks to all the folks in the IETF who casually mentioned how
to do this, but who left it to me to write this RFC. Special thanks
to Rich Colella, Bob Ullmann, Shawn Ostermann, Steve Lunt, and Brian
Carpenter who had the time and decency to comment on the initial
draft. :-)

1. Background

The PORT command of File Transfer Protocol allows users to specify an
address other than the default data port for the transport connection
over which data are transferred. The PORT command syntax is:

PORT <SP> <host-port> <CRLF>

The <host-port> argument is the concatenation of a 32-bit internet
<host-address> and a 16-bit TCP <port-address>. This address
information is broken into 8-bit fields and the value of each field
is transmitted as a decimal number (in character string

representation). The fields are separated by commas. A port command
is thus of the general form "PORT h1,h2,h3,h4,p1,p2", where h1 is the
high order 8 bits of the internet host address.

To accommodate larger network addresses anticipated for all IP "next
generation" alternatives, new commands and reply codes are needed for
FTP. This memo addresses these needs.

2. The LPRT Command

The LPRT command allows users to specify a "long" address for the
transport connection over which data are transferred. The LPRT
command syntax is:

LPRT <SP> <long-host-port> <CRLF>

The <long-host-port> argument is the concatenation of the following
fields;

o an 8-bit <address-family> argument (af)

o an 8-bit <host-address-length> argument (hal)

o a <host-address> of <host-address-length> (h1, h2, ...)

o an 8-bit <port-address-length> (pal)

o a <port-address> of <port-address-length> (p1, p2, ...)

The <address-family> argument takes the value of the version number
of IP (see Assigned Numbers, STD 2, RFC1340), or generally speaking,
an Internet layer protocol. Relevant assigned IPng version numbers
are:

Decimal Keyword
------ -------
0 reserved
1-3 unassigned
4 Internet Protocol (IP)
5 ST Datagram Mode
6 SIP
7 TP/IX
8 PIP
9 TUBA
10-14 unassigned
15 reserved

The value of each field is broken into 8-bit fields and the value of
each field is transmitted as an unsigned decimal number (in character
string representation, note that negative numbers are explicitly not
permitted). The fields are separated by commas.

A LPRT command is thus of the general form

LPRT af,hal,h1,h2,h3,h4...,pal,p1,p2...

where h1 is the high order 8 bits of the internet host address, and
p1 is the high order 8 bits of the port number (transport address).

3. The LPSV Command

The L(ONG) PASSIVE command requests the server-DTP to listen on a
data port other than its default data port and to wait for a
connection rather than initiate one upon receipt of a transfer
command. The response to this command includes the address family,
host address length indicator, host address, port address length, and
port address this server is listening on. The reply code and text
for entering the passive mode using a long address is 228
(Interpretation according to FTP is: positive completion reply 2yz,
connections x2z, passive mode entered using long address xy8). The
suggested textual message to accompany this reply code is:

228 Entering Long Passive Mode (af,hal,h1,h2,h3,h4...,pal,p1,p2...)

4. Permanent Negative Completion Reply Codes

The negative completion reply codes that are associated with syntax
errors in the PORT and PASV commands are appropriate for the LPRT and
LPSV commands (500, 501). An additional negative completion reply
code is needed to distinguish the case where a host supports the LPRT
or LPSV command, but does not support the address family specified.
Of the FTP function groupings currently defined for reply codes
(syntax, information, connections, authentication and accounting, and
file system), "connections" seems the most logical choice; thus, an
additional negative command completion reply code, 521 is added, with
the following suggested textual message:

521 Supported address families are (af1, af2, ..., afn)

Where (af1, af2, ..., afn) are the values of the version numbers of
the "next generation" or other protocol families supported. IP
address noted earlier.

5. Rationale

An explicit address family argument in the LPRT command and LPSV
reply allows the Internet community to experiment with a variety of
"next generation IP" alternatives within a common FTP implementation
framework. (It also allows the use of a different address family on
the command and data connections.) An explicit length indicator for
the host address is necessary because some of the IPNG alternatives
make use of variable length addresses. An explicit host address is
necessary because FTP says it's necessary.

The decision to provide a length indicator for the port number is not
as obvious, and certainly goes beyond the necessary condition of
having to support TCP port numbers. Currently, at least one IPng
alternative (TP/IX) supports longer port addresses. And given the
increasingly "multi-protocol" nature of the Internet, it seems
reasonable that someone, somewhere, might wish to operate FTP operate
over Appletalk, IPX, and OSI networks as well as TCP/IP networks.
(In theory, FTP should operate over *any* transport protocol that
offers the same service as TCP.) Since some of these transport
protocols may offer transport selectors or port numbers that exceed
16 bits, a length indicator may be desirable. If FTP must indeed be
changed to accommodate larger network addresses, it may be prudent to
determine at this time whether the same flexibility is useful or
necessary with respect to transport addresses.

6. Conclusions

The mechanism defined here is simple, extensible, and meets both IPNG
and possibly multi-protocol internet needs.

7. References

STD 9, RFC959 Postel, J., and J. Reynolds, "File Transfer Protocol",
STD 9, RFC959, USC/Information Sciences Institute,
October 1985.

STD 2, RFC1340 Reynolds, J., and J. Postel, "Assigned Numbers",
STD 2, RFC1340, USC/Information Sciences Institute,
July 1992. (Does not include recently assigned IPv7
numbers).

STD 3, RFC1123 Braden, R., Editor, "Requirements for Internet
Hosts - Application and Support", STD 3, RFC1123,
USC/Information Sciences Institute, October 1989.

8. Security Considerations

Security issues are not discussed in this memo.

9. Author's Address

David M. Piscitello
Bell Communications Research
NVC 1C322
331 Newman Springs Road
Red Bank, NJ 07701

EMail: dave@mail.bellcore.com

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