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Network Working Group                                        E.I. AnconaRequest for Comments: 42                       M.I.T. Lincoln Laboratory                                                           31 March 1970Message 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 ofAncona                                                          [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 AsciiAncona                                                          [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   ]Ancona                                                          [Page 3]