Network Working Group D. Piscitello
Request for Comments: 1639 Core Competence, Inc.
Obsoletes: 1545 June 1994
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.
This paper describes a convention for specifying address families
other than the default Internet address family in FTP commands and
In the File Transfer Protocol (STD 9, RFC 959), the PORT command
argument <host-port> specifies the data port to be used to establish
a data connection for FTP (STD 9, RFC 959). This argument is also
used in the PASV reply to request the server-DTP to listen on a data
port other than its default data port. This RFC specifies a method
for assigning addresses other than 32-bit IPv4 addresses to data
ports through the specification of a "long Port (LPRT)" command and
"Long Passive (LPSV)" reply, each having as its argument a <long-
host-port>, which allows for additional address families, variable
length network addresses and variable length port numbers.
This is a general solution, applicable for all "next generation" IP
alternatives, as well as for other network protocols than IP. This
revision also extends FTP to allow for its operation over transport
interfaces other than TCP.
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, Steve Lunt, Jay Israel, Jon Postel,
Shawn Ostermann, and Tae Kyong Song, who contributed to this work.
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.
The <host-port> argument is also used by the PASV reply, and in
certain negative completion replies.
To accommodate larger network addresses anticipated for all IP "next
generation" alternatives, and to accommodate FTP operation over
network and transport protocols other than IP, new commands and reply
codes are needed for FTP.
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
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 initial values assigned to the <address-family> argument take the
value of the version number of IP (see Assigned Numbers, STD 2, RFC
1340); values in the range of 0-15 decimal are thus reserved for IP
and assigned by IANA. Values in the range 16-255 are available for
the IANA to assign to all other network layer protocols over which
FTP may be operated.
Relevant assigned <address-family> numbers for FOOBAR are:
4 Internet Protocol (IP)
5 ST Datagram Mode
16 Novell IPX
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
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 of the listener process at the server. 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 text message to accompany this reply code is:
228 Entering Long Passive Mode
(af, hal, h1, h2, h3,..., 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 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. (Note: it
has been suggested that the families could also be represented by
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" and other network layer protocol 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
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.
The mechanism defined here is simple, extensible, and meets both IPNG
and multi-protocol internet needs.
STD 9, RFC 959 Postel, J., and J. Reynolds, "File Transfer Protocol",
STD 9, RFC 959, USC/Information Sciences Institute,
STD 2, RFC 1340 Reynolds, J., and J. Postel, "Assigned Numbers",
STD 2, RFC 1340, USC/Information Sciences Institute,
July 1992. (Does not include recently assigned IPv7
STD 3, RFC 1123 Braden, R., Editor, "Requirements for Internet
Hosts - Application and Support", STD 3, RFC 1123,
USC/Information Sciences Institute, October 1989.
8. Security Considerations
Security issues are not discussed in this memo.
9. Author's Address
David M. Piscitello
Core Competence, Inc.
1620 Tuckerstown Road
Dresher, PA 19025