Network Working Group                                          Vint Cerf
Request for Comments: 22                                            UCLA
                                                        October 17, 1969


                   Host-Host Control Message Formats
	
   NWG/RFC 11 has been modified at UCLA; and will be republished.  In
   the meantime, it seems important to report a new control message
   format which does not use 7-bit ASCII character mode of transmission.

   All Host-Host control messages consist of sequences of 8-bit bytes of
   the form:

   <control byte> <parameter byte l> ... <parameter byte n>

   It is reasonable to transmit more than one control message in any
   given packet, although this is not mandatory.

   Presently, 9 control messages have been defined by UCLA; these are
   given in the table below along with their parameters.  The
   interpretation is given from the point of view of the transmitting
   host. ("L" or "Li" mean Link#, and are binary values.)

   Control byte     Parameter      Interpretation

    <0>             <L>           Please establish primary connection;
                                  our output link # is L

    <1>             <L,> <L2>     Please establish auxiliary connection
                                  parallel to our primary output link L.
                                  The auxiliary output link is L2.

    <2>             <L1> <L2>     DK primary.  Your primary output link
                                  to us was L; our primary output link
                                  to you is L2.

    <3>             <L1> <L2>     OK auxiliary.  Your auxiliary output
                                  link is Li, our auxiliary output link
                                  is L2.

    <4>             <L>           Not OK primary.  We cannot establish a
                                  primary connection.  Your primary
                                  output link number was L.

    <5>             <Li> <L2>     Not OK auxiliary.  We cannot establish
                                  an auxiliary connection.  Your primary
                                  output link no was L2.

    <6>             <L>           Please stop transmitting over link
                                  number L.  This is called the CEASE
                                  directive.

    <7>             <L>           We are CLOSING our output link number
                                  L.  You may get this message before
                                  the last message arrives over this
                                  link since control messages are higher
                                  priority than regular data messages.

    <8>             <L>           UNCEASE: that is, you may resume
                                  transmitting over output link number
                                  L.

   Each control message is embedded in the appropriate message structure
   e.g.:

               <-------------32 bits --------------->
               |           HEADER                   |
               |____________________________________|
               |      |       |           |         |
               | mark |  l    |  <L1>     |  <L2>   |
               |______|_______|___________|_________|
               |                 |                  |
               |     checksum    |     Padding      |
               |_________________|__________________|
                 typical control message (please
                 establish auxiliary link #L2
                 parallel to our primary link #l)

   The header for all HOST-HOST control messages is given below:

   0     3  4   7  8  9   10   14   LINK#      24              31
   _______________________________________________________________
   |       |      |     |       |               |////////////////|
   | FLAGS | TYPE |  H  |  SITE | 00000001      |////////////////|
   |_______|______|_____|_______|_______________|________________|

   where  FLAGS - 0000
          TYPE  - 0000 (regular message)
          H     - host #(0-3) at SITE (usually 0 for single HOST sites)
          SITE  - Site #
          LINK# - 00000001 (HOST-HOST control link)


       [ This RFC was put into machine readable form for entry ]
      [ into the online RFC archives by Alison De La Cruz 12/00 ]