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.
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
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
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
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
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
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
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
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
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
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.
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.
PostScript is a trademark of Adobe Systems, Inc.
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