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rfc:rfc1312

Network Working Group R. Nelson Request for Comments: 1312 Crynwr Software Obsoletes: RFC 1159 G. Arnold

                                                Sun Microsystems, Inc.
                                                            April 1992
                      Message Send Protocol 2

Status of this Memo

 This memo defines an Experimental Protocol for the Internet
 community.  Discussion and suggestions for improvement are requested.
 Please refer to the current edition of the "IAB Official Protocol
 Standards" for the standardization state and status of this protocol.
 Distribution of this memo is unlimited.

Discussion

 The Message Send Protocol is used to send a short message to a given
 user on a given terminal on a given host.  Unix's write command
 offers a limited form of this service through its host-local write
 command.  This service is also known on some hosts as "SEND".
 As the Internet grows, more and more people are using hosts that do
 not run Internet protocols at all times.  These hosts may be able to
 use a simple protocol that can be implemented using UDP and IP.  The
 Message Send Protocol is one such protocol.
 Note that a message sending protocol is already defined using TCP.
 The SMTP protocol includes a "SEND" command that will direct mail to
 a user's terminal.  SMTP's SEND is not useful in this instance
 because SMTP's SEND is not implemented by the majority of vendors at
 this time, and is difficult to use by unskilled users.  For the
 purposes of standardization, we will include a TCP based Message Send
 Service.

Message Syntax

 The message consists of several parts, all of which must be present
 The first part is a single octet indicating the protocol revision,
 currently decimal 66, 'B'. The remaining parts are null-terminated
 sequences of eight-bit characters in the ISO 8859/1 alphabet. Some
 parts may be empty. All comparisons of parts (e.g., recipient,

Nelson & Arnold [Page 1] RFC 1312 Message Send Protocol 2 April 1992

 cookie, etc.) are case-insensitive. The parts are as follows:
 RECIPIENT      The name of the user that the message is directed to.
                If this part is empty, the message may be delivered to
                any user of the destination system.
 RECIP-TERM     The name of the terminal to which the message is to be
                delivered. The syntax and semantics of terminal names
                are outside the scope of this specification. If this
                part is empty, the "right" terminal is chosen. This is
                a system-dependent function.  If this part consists of
                the string "*", all terminals on the destination
                system are implied.  If the RECIPIENT part is empty
                but the RECIP-TERM is not, the message is written on
                the specified terminal.  If both the RECIPIENT and
                RECIP-TERM parts are empty, the message should be
                written on the "console", which is defined as some
                place where the message is most likely to be seen by a
                human operator or administrator.
 MESSAGE        The actual message. The server need not preserve the
                formatting and white-space content of the message if
                this is necessary to display it.  New lines should be
                represented using the usual Netascii CR + LF.
                (Following the Internet tradition, a server should
                probably be prepared to accept a message in which some
                other end-of-line convention is followed, but a
                conforming client must use CR + LF.)
                The message text may only contain printable characters
                from the ISO 8859/1 set, which is upward compatible
                from USASCII, plus CR, LF and TAB. No other control
                codes or escape sequences may be included: the client
                should strip them from the message before it is
                transmitted, and the server must check each incoming
                message for illegal codes. (A server may choose to
                display the message after stripping out such codes, or
                may reject the entire message.) If the MESSAGE part is
                empty, the message may be discarded by the server.
 SENDER         The username of the sender. (This and subsequent parts
                were not present in version 1 of the Message Send
                Protocol.) This part should not be empty. A server may
                choose to accept, reject or ignore messages in which
                the SENDER part is empty.
 SENDER-TERM    The name of the sending user's terminal. This part may
                be empty. The intention is that a recipient may reply

Nelson & Arnold [Page 2] RFC 1312 Message Send Protocol 2 April 1992

                to a message by sending the reply to the user SENDER
                at terminal SENDER-TERM on the originating system.
                (The sender's hostname should be retrieved from the
                transport software.)
 COOKIE         A magic cookie. This part must be present in all
                messages, but is only of significance for the UDP
                service. The combination of the sender's UDP port
                number and this cookie should be unique. A client may
                elect to transmit a particular message several times
                to increase the chances of its reception; a server may
                use the cookie and port to identify duplicate messages
                and discard them. A reasonable cookie is the time of
                day represented in a readable format. The maximum
                length of a cookie is 32 octets, excluding the
                terminating null.
 SIGNATURE      A token which, if present, may be used by the server
                to verify the identity of the sender. The use of the
                SIGNATURE part is discussed further in the section on
                Security, below.
 The total length of the message shall be less than 512 octets.  This
 includes all eight parts, and any terminating nulls.  UDP packets are
 limited to 512 octets.
 If this protocol is changed, the revision number will be changed.
 TCP Based Message Send Service
 One Message Send Service is defined as a connection based application
 on TCP.  A server listens for TCP connections on TCP port 18.  Once a
 connection is established a message is sent by the client over the
 connection.
 The server replies with a single character indicating positive ("+")
 or negative ("-") acknowledgment, immediately followed by an optional
 message of explanation, terminated with a null.  The positive
 acknowledgement means that the message was successfully delivered to
 some user/terminal, and that the negative acknowledgement means that
 the message was NOT delivered to any terminal.
 The positive acknowledgement message can contain information about
 what user and terminal the message was delivered to in the case of
 incomplete user/terminal fields in the message.  The negative
 acknowledgement can contain information about WHY the message was not
 delivered (no such user/terminal, system failure, user doesn't accept

Nelson & Arnold [Page 3] RFC 1312 Message Send Protocol 2 April 1992

 messages, etc).
 Multiple messages can be sent over the same channel.  The client
 should close first (the server may/should not close directly after
 the acknowledgement is sent) and the server may close after some
 timeout on the order of minutes. If the sever is unable to decode a
 message, or no message is received within a suitable timeout, it may
 close the channel (on the assumption that the sender may have
 formatted the data incorrectly).
 UDP Based Message Send Service
 Another Message Send Service is defined as a datagram based
 application on UDP.  A server listens for UDP datagrams on UDP port
 18.  When a datagram is received by the server, an answering datagram
 may be sent back to the client.  If the message was addressed to a
 particular user (i.e., the RECIPIENT part was non-empty) and was
 successfully delivered to that user, a positive acknowledgement
 should be sent (as described above). If the message was directed at
 any user (i.e., the RECIPIENT part is empty), or if the message could
 not be delivered for some reason, no reply is sent.
 The reason for this policy is that the UDP service may be used to
 broadcast messages addressed to a particular user on an unknown
 system or all users on all systems. In either case, it is
 inappropriate for all servers to send replies. An alternative
 approach might have been to require that a server only send a reply
 if a message was addressed explicitly to that system and was not
 broadcast. Unfortunately, the most popular network programming API
 does not provide an easy way for an application to determine this;
 furthermore such a policy would provide no feedback to the sender of
 a broadcast message to a particular recipient. The approach adopted
 here provides a reasonable compromise.
 Example of Message Encoding
 Consider a situation in which the user "sandy" is logged into the
 console of system "alpha", and wishes to send a message to the user
 "chris". "chris" is known to be logged in on the system "beta" but
 the exact terminal is unknown. The message consists of two lines of
 text, "Hi" followed by "How about lunch?".
 The message would be encoded as follows:

Nelson & Arnold [Page 4] RFC 1312 Message Send Protocol 2 April 1992

           +--------+---------+---------+---------+
         0 |    B   |    c    |    h    |    r    |
           +--------+---------+---------+---------+
         4 |    i   |    s    |  <NULL> |  <NULL> |
           +--------+---------+---------+---------+
         8 |    H   |    i    |   <CR>  |   <LF>  |
           +--------+---------+---------+---------+
        12 |    H   |    o    |    w    |         |
           +--------+---------+---------+---------+
        16 |    a   |    b    |    o    |    u    |
           +--------+---------+---------+---------+
        20 |    t   |         |    l    |    u    |
           +--------+---------+---------+---------+
        24 |    n   |    c    |    h    |    ?    |
           +--------+---------+---------+---------+
        28 |  <NULL>|    s    |    a    |    n    |
           +--------+---------+---------+---------+
        32 |    d   |    y    |  <NULL> |    c    |
           +--------+---------+---------+---------+
        36 |    o   |    n    |    s    |    o    |
           +--------+---------+---------+---------+
        40 |    l   |    e    |  <NULL> |    9    |
           +--------+---------+---------+---------+
        44 |    1   |    0    |    8    |    0    |
           +--------+---------+---------+---------+
        48 |    6   |    1    |    2    |    1    |
           +--------+---------+---------+---------+
        52 |    3   |    2    |    5    |  <NULL> |
           +--------+---------+---------+---------+
        56 | <NULL> |
           +--------+
 Note that the RECIP-TERM  and SIGNATURE parts are empty. The COOKIE
 is the string "910806121325", which in this implementation indicates
 that the message was sent at 12:13:25 on the 6th of August, 1991.
 The identity if the sending and receiving systems is not included in
 the message; the server must obtain this information from the
 transport service.
 Advisories
 Client and server implementations must follow the character set
 restrictions noted in the MESSAGE part description. Failure to do so
 may have undesirable effects on the operation of the receiver's
 terminal; more seriously, it may open up a significant security

Nelson & Arnold [Page 5] RFC 1312 Message Send Protocol 2 April 1992

 "hole". The checks must be made on any part of the message which may
 be displayed, including the sender's name and terminal.  This is one
 case where the admonition to "be liberal in what you accept" is not
 applicable. A server may chose to apply additional checks to an
 incoming message, and to reject any message which may pose a security
 risk. For example, a system using a PostScript-based display may
 reject a message which might be interpreted as an executable
 PostScript program.
 The underlying transport, whether TCP or UDP, is expected to provide
 checksums for the message and any response.
 The semantics of the various RECIPIENT and RECIP-TERM combinations
 may be confusing. The introduction of the "*" wildcard designation in
 the RECIP-TERM part makes it possible to send a message to all
 terminals on the designated system (if RECIPIENT is empty), or to all
 terminals at which a particular recipient has logged in.
 A positive acknowledgement may indicate only that the Message Send
 server was able to successfully invoke a local message delivery
 service. It may not be possible for true end-to-end semantics to be
 inferred.
 For example, a Message Send server may employ a local delivery
 mechanism which calls upon the services of a window system to display
 the message in a pop-up window. This process may take some
 significant time to complete, and it is unclear whether it is useful
 for the server to wait for an indeterminate period before returning
 an acknowledgement.  Therefore, this specification does not prescribe
 whether the acknowledgement is associated with delivery of the
 message to the local service, the display of the message, or
 confirmation by the user that the message has been read by, e.g.,
 dismissing the pop-up window.

Security Considerations

 Those who plan to implement this service must ensure that the
 following issues are reflected in the documentation of their
 products, and that their implementations include sufficient
 configuration controls to allow systems and network administrators to
 achieve the appropriate levels of usability and security.
 First, this service may allow someone to write on a user's terminal
 without the user giving his or her permission.  Where possible, users
 should be provided with a mechanism for disabling this.
 Second, it is extremely important for implementors to observe the
 rules for filtering message text as discussed under Message Syntax

Nelson & Arnold [Page 6] RFC 1312 Message Send Protocol 2 April 1992

 above. Failure to do this may introduce major security holes.
 The third issue concerns the verification of the sender's identity.
 If the recipient is fooled into believing that a message is from a
 particular user, various security issues may arise. For example, the
 recipient may send a reply containing confidential material.
 This service is primarily intended for "open" environments:
 controlled local area networks used by reasonably trusted
 participants, in which security considerations may be relaxed in the
 interests of ease of use and administration. In such an environment
 it is appropriate to trust the user name and source IP address as
 identifying the actual sender of the message.
 Within more security-conscious environments, this assumption is
 probably unacceptable. As has been widely noted, there is no way
 within the current Internet architecture to ensure that the source
 address of an IP datagram is correct. Hence it is entirely possible
 for someone to spoof the IP address.
 The obvious, and simplest, answer is to disallow the use of this
 protocol in such situations.  However a more constructive approach is
 to incorporate within the protocol some mechanism by which a server
 can reliably identify the sender.
 In this version of the protocol specification, we define a SIGNATURE
 part within a message. If this part is empty, the identity of the
 sender cannot be verified, and the server implementation may elect to
 reject all such requests.  If the part is not empty, it is treated as
 a case-insensitive text encoding of some security token. This RFC
 does not define the encoding or interpretation of this token. We
 expect that such matters will form part of future RFCs on security
 and privacy issues; at an appropriate time, this RFC will be re-
 issued to include references to these RFCs.

Acknowledgements

 PostScript is a trademark of Adobe Systems, Inc.

Nelson & Arnold [Page 7] RFC 1312 Message Send Protocol 2 April 1992

Authors' Addresses

 Russell Nelson
 Crynwr Software
 11 Grant St.
 Potsdam, NY 13676
 Phone:  (315) 268-1925
 EMail:  nelson@crynwr.com
 Geoff Arnold
 Sun Microsystems, Inc.
 2 Federal Street
 Billerica, MA 01821
 Phone:  (508) 671-0317
 EMail:  geoff@east.sun.com

Nelson & Arnold [Page 8]

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