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EXPERIMENTAL

Network Working Group                                          T. Rinne
Request for Comments: 1756                                          HUT
Category: Experimental                                     January 1995


                  REMOTE WRITE PROTOCOL - VERSION 1.0

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.

1. Background

   It is often convenient to use electronic communication somewhat
   lighter than electronic mail.  Sometimes even the use of the talk(1)
   *) program seems like overkill.  We like to offer to user something
   like UNIX **) command write(1) ***) except that it can also pass
   messages through the network instead of the single host.

   There have been few programs offering this kind of service, but they
   have either based on SUN-RPC protocol or used a strictly undocumented
   protocol.

   This document describes a simple Remote Write Protocol (RWP) that
   should have been documented at least 10 years ago.  But late is
   better than never.  Version number of the RWP protocol in this
   document is 1.0.

2. Overview

   RWP is a simple protocol that can be used to relay short messages
   through the network to other users.  RWP looks pretty much like
   Simple Mail Transfer Protocol (SMTP) ****) though it is a bit more
   complicated due to the interactive nature of the RWP session.

   The idea behind the RWP session is that client program that is
   relaying message to the host in which the target user is logged in
   opens the tcp or udp connection to the server program running in the
   target machine  Then the client gives the sender's and recipient's
   identification (usually login ids), actual message body and tells the
   server to deliver a message to the user.  On tcp-connection server
   returns a status from each action taken.  On udp-connection no
   responses are sent.  RWP sessions through udp are implemented to
   support message broadcasting.




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RFC 1756                 Remote Write Protocol              January 1995


   Message delivering methods are not defined within this document, but
   the basic method could be a simple write to users terminal.  This is
   basically what UNIX command write(1) does.  Depending on server
   implementation, the delivery method could be configurable personally
   by each user.

3. Description

   Server program answers to each command submitted by a response.  All
   responses have two parts: three number unique response code and a
   short textual explanation of the response.  Also whenever the server
   is ready to accept new commands a notification is submitted to the
   client.

   There are three kinds of commands in RWP.  The first group is for
   querying a status of the server.  The second group is actual message
   handling commands and the last set of commands are for RWP session
   control.

   When the server is ready to receive a command from the client, it
   sends a message code 100 to the client.  This message is for example
   as follows:

                    100 Ready.

    Server commands are as follows:

    Status Query

        HELP    Gives a short help message that contains legal
                RWP commands.  Help lines have code 510.  Example RWP
                implementation *****) gives a following response to
                HELP command:

                    510 Valid commands are:
                    510     BYE,    DATA,   HELP,   HELO,
                    510     RSET,   SEND,   PROT,   QUIT,
                    510     VRFY,   VER
                    510     FROM senderlogin
                    510     FHST senderhost
                    510     TO   recipentlogin [tty]
                    510     FWDS current_hop_count

        HELO    Says hello to the server.  Server response to HELO
                command has code 500.  For example:

                    500 Hello remote.host.  This is local.host speaking.




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RFC 1756                 Remote Write Protocol              January 1995


        PROT    Asks the RWP protocol version from the server.
                Response code to PROT command is 502.  Protocol
                version described in this document is RWP 1.0 and the
                response is as follows:

                    502 RWP version 1.0.

        VRFY    After the recipient of the message is set by to command
                described later, the possibility of message delivery
                can be queried by VRFY command.  If message can be
                delivered the response code is 108.  If message is
                about to be forwarded the response code is 110 and
                message is either form:

                    110 Recipient ok to forward.

                or if the server can tell the destination of the
                forwarding:

                    110 Recipient ok to forward <user@host.domain>.

                Other possible response codes are 669, 670, 671, 674
                and 677 and they all indicate that message delivery is
                by one way or another currently impossible.
                Description of the codes is later in this document.

                After the SEND command the server may also give
                autoreply from the remote user before the actual
                response code.  Autoreply lines are ones of code 300.

        VER     Asks the version of the server program.  Response code
                to VER command is 501 and the textual part of the
                response is the name and the version number of the RWP
                server, for example:

                    501 Rwrited version 1.0.

    Message Handling:

        FROM senderlogin
                Tells the server the identification information of the
                sender of the message.  Usually this id information is
                user's login id.  Response code to successful FROM
                command is 105, for example:

                    105 Sender ok.





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RFC 1756                 Remote Write Protocol              January 1995


        TO recipentlogin [tty]
                Tells the server the identification information of the
                intended recipient of the message.  Usually this id
                information is user's login id.  If tty is submitted,
                the message is delivered to that tty.  If tty is
                submitted between brackets '[]' the tty given is
                treated as a hint only.  Response code to successful
                TO command is 106.

        FHST original.host [forwarder1.host forwarder2.host ...]
                Tells the server the host name that the message
                originates to and the path of the hosts that has
                forwarded the message.  The host name of the machine
                that is currently submitting the message to the server
                should not be in the path list.

                This information is relevant if message is forwarded
                and it is not originally coming from the host that is
                forwarding it.  Response code to successful FHST
                command is 111.

        DATA    Tells the server to start receive the body of the
                message.  Response code to DATA command is 200, for
                example:

                200 Enter message.  Single dot '.' on line terminates.

                After response 200 the message lines are submitted to
                the server one after another.  Message is terminated
                by the line that contains a single dot '.'.  The
                termination of the message is acknowledged by the
                server with the response code 107.  Server does not
                notify client about receiving the single message
                lines.  If empty message is submitted (i.e. single dot
                is on the first line) the response code is 672 and
                DATA command only cancels possible previous DATA
                command.  Because of this all dots or at least dots
                that are standing alone in the line have to be quoted.

        SEND    Sends the message.  If commands FROM, TO and DATA are
                successfully given before SEND command, the message is
                delivered to the target user.  If delivery is
                successful the response code is 103.  If message is
                not delivered directly to the target user but instead
                forwarded to another host the response code is 104.
                Response codes 669, 670 and 671, 677 indicate an error
                on message delivery and codes 673, 674, 675 indicate
                that either command FROM, TO or DATA has not been



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RFC 1756                 Remote Write Protocol              January 1995


                successfully given before SEND command.  After the
                SEND command the server may also give autoreply from
                the remote user before the actual response code.
                Autoreply lines are ones of code 300.

        FWDS n  Tells the server that message has been forwarded n
                times.  If the server forwards the message to the
                another server, it increments the counter and tells
                the remote server the current count of forwards.
                Response code to the FWDS command is 110 if n is less
                than the server specific forward limit.  If this limit
                is exceeded the response code is 676.  If the response
                code is 676 the client can either quit the session and
                fail the message or it can give the message to the
                server despite the fact that the forward limit is
                exceeded.  If the message is given when forward limit
                is exceeded, the server tries to deliver it, but does
                not forward it to another server.  If forward count is
                given as -1, the message is considered as a autoreply
                and never forwarded.

    Session Control:

        RSET    Resets the RWP session.  FROM, TO and DATA -commands
                that are given before are canceled and they have to
                be given again before SEND command can be used.  Also
                possible FWDS and FHST commands are canceled.

        BYE     Terminates the RWP session.  Server gives a response
                code 101 and closes the connection.

        QUIT    Is the synonym to bye, but it's a lot more impolite.
                Response code is however 101 as in bye.

    Server specific command:

        QUOTE command

                Relay a command to the server.  If the QUOTE command
                is successfully completed response code 112 is
                returned.  If QUOTE command is failed the response
                code is 678.  If RWP server doesn't recognize the
                given QUOTE command the response code is 679.

                Currently reserved QUOTE commands are AGENT, CHARSET,
                IDENT, KEY and KEYID.





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RFC 1756                 Remote Write Protocol              January 1995


4. Response Codes

   Here are all legal response codes of RWP server followed by short
   textual explanation.  Only the numeral codes are important and texts
   can contain practically anything, however in response code 110 there
   is possibly useful information between '<' and '>' characters.  No
   characters '<' or '>' should be present in other responses.  Also
   response 502 has possibly interesting information about the RWP
   protocol version the server supports.

    100 Ready.

        The RWP server is ready to accept next command.

    101 Goodbye.

        The RWP server is closing connection.

    103 Message delivered.

        The SEND command is successfully completed and the message is
        delivered directly to its destination.

    104 Message forwarded.

        The SEND command is completed and message is forwarded to the
        user.

    105 Sender ok.

        The FROM command successful.

    106 Recipient ok.

        The TO command successful.

    107 Message ok.

        The DATA command successful.

    108 Recipient ok to send.

        The VRFY command successful and direct message delivery is
        possible.

    109 RSET ok.

        The RWP server has received the RSET command and reset itself.



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RFC 1756                 Remote Write Protocol              January 1995


    110 Ok to forward.

        or

    110 Ok to forward <user@host.domain>.

        The VRFY command successful and direct message delivery by
        forwarding is possible.  If response has also forwarding
        address the client can either forward the message itself or
        give it to server for forwarding.

    111 Original sender host ok.

        The FHST command successful and original sender host is set as
        given by the client.

    200 Enter message.  Single dot '.' on line terminates.

        The RWP server is ready to receive the message. Single dot on
        message line terminates the message.


    300 |I'm not in right now but I'll be back tomorrow
    300 |at 8 o'clock a.m.

        Automatical response to the delivered message.  Every line of
        this user defined reply message is delivered in its own 300
        line.  Response code 300 lines may appear only after SEND
        command before response code 103 (message delivered).  Client
        receiving autoreply 300 should show the text of the autoreply
        to the user.  Actual autoreply line begins after the '|'
        -character in the line.

    500 Hello remote.host.  This is local.host speaking.

        Response to the HELO command.  This message can also occur in
        the beginning of the conversation without the VER command and
        it can be ignored.

    501 Rwrited version X.X.

        Response to the VER command.  This message can also occur in
        the beginning of the conversation without the VER command and
        it can be ignored.







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RFC 1756                 Remote Write Protocol              January 1995


    502 RWP version 1.0.

        Response to the VER command.  This message can also occur in
        the beginning of the conversation without the VER command and
        it can be ignored.

    510 Valid commands are:
    510     BYE,    DATA,   HELP,   HELO,
    510     RSET,   SEND,   PROT,   QUIT,
    510     VRFY,   VER
    510     FROM senderlogin
    510     FHST senderhost
    510     TO   recipentlogin
    510     FWDS current_hop_count

        Response to the HELP command.

    511 Information to the user.

        Server specific informational response.  These responses may
        occur anytime during the conversation.  The client can ignore
        them.

    512 Debug information to the user.

        Server specific informational response.  Reserved for server
        debugging.  These messages may occur anytime during the
        conversation.  The client can ignore them.

    666 FATAL ERROR!

        The RWP server got into the fatal error situation and is about
        to exit immediately.  Client programs are strongly encouraged
        to close the connection.

    668 Syntax error.

        The RWP server has received an invalid command.

    669 Permission denied.

        The RWP server is unable to deliver the message because the
        target user has denied the send permission.

    670 User not logged in.

        The RWP server is unable to deliver the message because the
        target user is not logged in.



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RFC 1756                 Remote Write Protocol              January 1995


    671 No such user.

        The RWP server is unable to deliver the message because the
        target user does not exist.  Error code 670 can be used to
        replace this message.

    672 No message.

        The DATA command is terminated with empty message body.  No
        SEND command can be executed before a new DATA command is
        given.

    673 FROM command required.

        Tried to give the SEND command before FROM.

    674 TO command required.

        Tried to give the SEND command before TO.

    675 DATA command required.

        Tried to give the SEND command before DATA.

    676 Forward limit exceeded.

        Response to the FWDS command that had an argument that
        exceeded the server specific limit of message forwarding
        steps.

    677 Unable to forward message.

        or

    677 Unable to forward message to <user@host.domain>.

        Response to the SEND or VRFY command if message forwarding is
        attempted and the server specific limit of message forwarding
        steps has been exceeded or if message forwarding has otherwise
        failed.  If message forwarding fails with message 669, 670 or
        671, server will not use response 667 but gives response but
        instead it gives the response analogous with the error
        occured.  If message 677 includes address the message was to
        be forwarded, the client may try to deliver it itself.

    698 Unknown error.

        RWP server has faced an internal error that is not fatal.



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RFC 1756                 Remote Write Protocol              January 1995


    699 Unknown error.

        RWP server has faced an unknown error that is not fatal.

5. RWP Compliant Software

   Simple RWP 1.0 compliant server and client software RWrite-1.1 will
   be available during the fall 1994.

6. Security of RWP

   RWP version 1.0 does not offer any mean to verify the identity of the
   user connecting the RWP server program.  It's possible to identify
   the sender using ident-service, but not all hosts currently support
   that.  This vulnerability is analogous with the weakness of the SMTP
   protocol.  Cryptographic user verification and message hiding method
   is under development and is to be defined in RWP version 2.0 during
   the year 1995.

   RWP server also may offer a way to the intruder to get to know user
   ids within the target host by trying the TO and VRFY commands.  This
   vulnerability is also present in SMTP.  It is however possible to
   build servers so that they never give message 671 (no such user) but
   use response 670 (user not logged in) instead.

   Another way to increase security even within RWP-1.0 described in the
   document is to design RWP servers so that they do not deliver
   messages directly to user but instead connect to some kind of RWP
   agent process that is executed by each user willing to receive RWP
   messages.  This user configurable message agent could then decide
   whether to deliver the message to the user and which way of delivery
   to use.  Message agent is the best way to prevent hostile user from
   sending uncontrolled message flood to the user's terminal.

   Sample implementation (RWrite-1.0) of the RWP server includes the
   support for user configuration files in which each user can either
   allow or deny messages from some user(s), host(s) or network
   domains(s).  Support for message agents is currently under
   development.

   The user that is receiving the message should be able to define
   characters to be stripped from the incoming messages to prevent
   terminal mess-up.








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RFC 1756                 Remote Write Protocol              January 1995


7. RWP Connection Type

   It is suggested that tcp (and udp) port 18 should be allocated for
   rwp in future versions of RFCs listing the reserved tcp/udp/rpc
   ports.  Currently port 18 is assigned to the service called Message
   Send Protocol (msp) that is not known to be implemented.  Actually
   port 18 is not currently defined at all in the /etc/services -file of
   the any common UNIX-like system.  Entry for /etc/services -file is as
   follows

       rwrite    18/udp       # RWP rwrite
       rwrite    18/tcp       # RWP rwrite

   Given that RWP compliant daemon program is /usr/sbin/rwrited the
   entry for /etc/inetd.conf -file would be:

       rwrite  stream  tcp  nowait  nobody  /usr/sbin/rwrited rwrited

8. Character quotation

   To offer a safe method to transfer various character sets RWP defines
   a method to quote characters in both message and autoreply.  RWP uses
   quotation similar to MIME `quoted-printable' encoding.  Quoted
   character is presented as a '=' -sign followed by a two character hex
   code.  This means also that all '='-signs have to be quoted.
   Quotation is also needed when message contains a line with only a
   single dot '.' in it.

    For example:
                    '.'  ->  =2E
                    '='  ->  =3D
                    '\a' ->  =07
                    '\t' ->  =09

9. Security Considerations

   Security issues are not discussed in this memo.

10. Author's Address

   Timo J. Rinne
   Helsinki University of Technology.
   Cirion oy
   PO-BOX 250
   FIN-00121
   Helsinki, Finland

   EMail: Timo.Rinne@hut.fi



Rinne                                                          [Page 11]


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