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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   ]

Ancona [Page 3]