MSP2: Message Send Protocol 2

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,
   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
                  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
   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:

             +--------+---------+---------+---------+
           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
   "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
   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.

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