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+Network Working Group                                        E.I. Ancona
+Request for Comments: 42                       M.I.T. Lincoln Laboratory
+                                                           31 March 1970
+
+
+                            Message Data Types
+
+
+   Proposal:
+
+   We propose that the first eight bits of a normal message be reserved
+   for a message data type.  Adoption of this convention does not in any
+   way signify agreement as to the actual data types to be used.  It
+   merely establishes the convention that the first eight bits of every
+   normal message are not available for user data.
+
+   Discussion:
+
+                     Socket    Port
+                     |    |      |    ____________
+                     |    V      V   /            \
+                     V              /              \
+                         |=|    /==|                |
+             -------(+)->|Y|--><   |                |
+                         |=|    \==|                |
+                                   |    PROCESS     |
+                                   |                |
+                         |=|    /==|                |
+             -------(-)->|X|<--<   |                |
+                         |=|    \==|                |
+                                    \              /
+                                     \____________/
+
+   It is important that conventions regarding the contents of messages
+   be set up early so that there will not be a large proliferation of
+   such conventions between every pair of programs running on the
+   network.
+
+   As network usage grows, network languages may develop for specifying
+   both the syntax and semantics of messages.  However, even before such
+   conventions are developed, a simple way of describing such a
+   specification is by means of a message type which both sender and
+   receiver know how to interpret.
+
+   It is important that currently running programs still run with this
+   convention; thus, we propose that two system programs be written
+   which initially put in and test and remove the type information from
+   the message.  Let us call these two programs X and Y, for lack of
+
+
+
+Ancona                                                          [Page 1]
+
+RFC 42                     Message Data Types                 March 1970
+
+
+   better names.  In general, X and Y will perform transformations on
+   the data, e.g., change character sets or number formats.  As network
+   usage grows, X and Y might become table driven with the table
+   specified by the user.
+
+   Standard Types and Local Types:
+
+   We propose to distinguish between two kinds of message data types:
+   standard and local.
+
+   Since our two transformation programs cannot be expected to perform a
+   transformation between every possible data representation and the
+   data representation of the machine they are running on, and also
+   since the addition of a data representation should not necessarily
+   involve a change to X or Y, we propose that only a fixed number of
+   message types have meaning throughout the network.  These are
+   standard types.
+
+   There are two classes of local types: MYLOCAL and YOURLOCAL. A
+   message type MYLOCAL n implies: this is type n of the set of types of
+   the sending host.  YOURLOCAL n implies: this is type n of the set of
+   types of the receiving host.
+
+   Conventions:
+
+   A possible implementation of standard and local types is to define
+   standard type 0 to be YOURLOCAL and standard type 1 to be MYLOCAL. In
+   these cases, the second byte would be the local type number.
+
+   Local type 0 would mean user-specified, i.e., the message contents
+   are unchanged and unchecked.  Installations would define their own
+   local type numbers and these would normally be available from the
+   Network Information Center.
+
+   Thus initially, all messages sent to currently running programs will
+   be type 0, n and all messages received from currently running
+   programs will be type 1, n where n is the local type number of the
+   character set of the installation.
+
+   Examples of Possible Standard Types:
+
+        0.     YOURLOCAL
+        1.     MYLOCAL
+        2.     U.S. Ascii
+        3.     EBCDIC
+        4.     Mod 33 TTY Ascii
+
+
+
+
+
+Ancona                                                          [Page 2]
+
+RFC 42                     Message Data Types                 March 1970
+
+
+        5.     Load table driven translator table #n.  If, in the
+               future, the X and Y transformation boxes are table
+               driven, this gives the table.  The table number n is
+               stored in the second byte of the message.
+        6.     Use table driven translator table #n.
+        7.     Network standard graphics message.
+
+   Examples of Local Types:
+
+        1.     Local Character sets, e.g., Lincoln writer, DEC Ascii,
+               etc.
+        2.     Graphics local messages, e.g., TX-2 Apex display
+               executive calls, GSAM.
+
+
+
+         [ This RFC was put into machine readable form for entry ]
+         [ into the online RFC archives by Robbie Bennet 11/98   ]
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+Ancona                                                          [Page 3]
+