Network Working Group B. Thomas
Request for Comments: 438 B. Clements
NIC: 13770 BBN-TENEX
References: 354,385,414,418 15 January 1973 FTP Server-Server Interaction
The current ARPANET File Transfer Protocol as specified by RFC 354
and updated by RFC's 385, 414 and 418 allows for "third host"
participation but does not specify a mechanism by which the process
at the third site may be the FTP server at that site. This RFC
suggests a simple extension to FTP which would allow an FTP user
process at one site to arrange for FTP server processes at other
sites to act cooperatively on its behalf.
Such server-server cooperation may appear to be of limited utility.
Consider, however, the requirements placed on FTP by a Resource
Sharing Executive (RSEXEC) program - a command language interpreter
which extends the range of a user's commands beyond the boundaries of
the user's local system. Among its services such as RSEXEC could
provide its users with a network-wide file system, perhaps allowing,
in certain contexts, the use of partially qualified pathnames which
omit site specification. Consider, for example the response of the
RSEXEC to the user command:
APPEND (FILE) PROG1.PL1 (TO FILE) PROG2.PL1
for the case in which the two files are at different sites (PROG1.PL1
at SITE1, PROG2.PL1 at SITE2) neither of which is the user's site. A
straightforward way for the RSEXEC to "perform" the APPEND would be
to establish FTP control connections to the FTP servers at SITE1 and
at SITE2, instruct the server at SITE1 to
using data connection C and instruct the server at SITE2 to
using the same data connection C.
Unfortunately, at present there is no way within FTP to arrange for
such server-server cooperation. This is due primarily to the lack of
symmetry in the way that FTP treats the ends of data connections
during connection establishment. It specifies one end to be the
"server" end, the other to be the "user" end and specifies different
means for establishing the connections from the two ends.
FTP could be modified to support server-server interaction by:
1. making the establishment of data connections symmetric, or;
2. providing a mechanism for instructing a server to establish its
end of a data connection as if it were a user.
The second alternative probably requires fewer changes to the
The following proposed extension to FTP uses the second method. It
involves the addition of a single new command (LSTN) and minor
modifications to several existing commands (SOCK, APPE, RETR, STOR):
a. The LSTN (Listen) command requests the FTP server to allocate a
socket for use as a data connection. To establish the
corresponding data connection the server is to "listen" on the
socket allocated when an appropriate transfer command is given.
syntax: LSTN <direction> CRLF
where <direction> is either "S" for send or "R" for receive.
The server responds to LSTN by:
1. refusing to allocate such a socket, or:
2. sending the user the number of the socket allocated (the 255
FTP server data socket reply could be used for this
b. Receipt of an appropriate STOR, RETR or APPE command following a
successful LSTN command causes the server to "listen" for an RFC
for the socket allocated. Data transfer may proceed after the
server receives an RFC for the socket and responds with a matching
RFC. Once established, a data connection corresponding to a
successful LSTN command has the same duration as one established
in the usual way.
c. The user may insure the security of his data transfer by using the
SOCK command to instruct the server to accept an RFC for the
listening socket only if it is from a specified host and socket.
d. The SOCK command is modified in two ways:
1. On success the reply must be the 255 FTP server data socket
reply; that is, the 255 reply can not be deferred until receipt
of a data transfer command. (This is to allow the user to
transmit the server's response to the program at the third
site; see the example below.)
2. After a LSTN command the SOCK command is to be interpreted by
the server as specification of the acceptable RFC for
subsequent data transfer command that use the allocated socket.
With this extension to FTP, the RSEXEC program could accomplish the
APPEND in the example above as follows:
to SITE1: to SITE2:
1. LSTN R CRLF
(let X = socket
2. SOCK SITE2,X CRLF
(let Y = socket in 255
reply from SITE1)
3. SOCK SITE1,Y CRLF
4. RETR PROG1.PL1 APPE PROG2.Pl1 CRLF
In closing it is appropriate to note that an experimental RSEXEC
program of the sort suggested above has been operational on TENEXs
for about 8 months. It currently uses a private, resource sharing
protocol (RSP) that includes file transfer operations. RSP supports
server-server cooperation; in RSP data connections are established in
a symmetric way (alternative 1 above).
[ This RFC was put into machine readable form for entry ]
[ into the online RFC archives by Mirsad Todorovac 5/98 ]