Network Working Group E.I. Ancona
Request for Comments: 42
M.I.T. Lincoln Laboratory
31 March 1970
Message Data Types
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.
| | | ____________
| 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
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
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
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.
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:
2. U.S. Ascii
4. Mod 33 TTY Ascii