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Network Working Group B. Bressler Request for Comments: 478 B. Thomas NIC: 14947 BBN

                                                         26 March 1973

                  FTP Server-Server Interaction-II

 At the recent FTP meeting at BBN in Cambridge, one of the topics
 discussed was that of server-server interaction.  In a typical
 situation a user (A) conversing with two servers (B,C) is interested
 in retrieving a file from one site (B) and sending it to the other
 (C).

                        +------+
                        | USER |
                        |   A  |
                       /+------+\
                 control        control
                    /              \
              +------+            +------+
              |SERVER|    DATA    |SERVER|
              |   B  |----------->|  C   |
              +------+            +------+

 The consensus of the meeting was that mechanisms were necessary to
 make B and C aware of each other and to allow a data connection to be
 established without forcing each other to queue RFCs for local
 sockets before they exist.

 The proposed solution to this problem was a command called PASSIVE
 (PASV?).  The following is our conclusion as to the meaning of the
 command and how it would be used.

 Third party connections would be established using the SOCK command,
 which says "Be prepared to use socket S at Host H to establish your
 data connection", and the PASV command which says "open your data
 socket for listening, and upon receipt of a transfer command wait for
 an RFC rather than initiating one."

 A positive acknowledgement to the PASV command indicates that the
 data socket has been opened for listening.  When an RFC for its data
 socket arrives after it has positively acknowledged a PASV command,
 the server should respond with a matching RFC to open the data
 connection (assuming, of course, that the incoming RFC is consistent
 with the previous SOCK commands, if any).

Bressler [Page 1]  RFC 478 FTP Server-Server Interaction-II 26 March 1973

                          +---------------+
                          |               |
               +----------| USER PROCESS  |----------+
               |          |       A       |          |
             telnet       +---------------+        telnet
               |                                     |
               |                                     |
        +-----------+                         +-------------+
        |           |-------->      --------->|             |
        |  SERVER   |data sockets  data socket|   SERVER    |
        |     B     |    Sb           Sc      |     C       |
        |           |<--------      <---------|             |
        +-----------+                         +-------------+

 USER A TO SERVER B                     USER A TO SERVER C
 __________________                     __________________

 A->B   SOCK  HOST-C  SKT- Sc           A->C   SOCK  HOST-B  SKT-  Sb

 B->A       ACK                         C->A      ACK

 A->B   PASV

 B->A       ACK

 A->B   STOR                            A->C   RTRV

    1. After the PASV command has been acknowledged, the two data
       transfer commands can be sent in either order, since the
       LISTENING action takes place with the PASV command

    2. The user knows the socket numbers Sc and Sb to be the data
       sockets as specified by the protocol.

    3. Note that it is not essential for a SOCK command to be sent to
       the same Host to whom a PASV will be sent.  Sending one to him
       provides security in that the incoming RFC can be checked.

 RB/nlg

        [This RFC was put into machine readable form for entry]
   [into the online RFC archives by Helene Morin, Via Genie 12/1999]

Bressler [Page 2]