Destination     5 bits
           Link            8 bits
           Trace           1 bit
           Spare           2 bits

   The destination is the numerical code for the HOST to which the
   message should be sent.  The trace bit signals the IMPs to record
   status information about the message and send the information back to
   the NMC (Network Measurement Center, i.e., UCLA).  The spare bits are
   unused.




 
RFC 1                        Host Software                 7 April 1969


Links

   The link field is a special device used by the IMPs to limit certain
   kinds of congestion.  They function as follows.  Between every pair of
   HOSTs there are 32 logical full-duplex connections over which messages
   may be passed in either direction.  The IMPs place the restriction on
   these links that no HOST can send two successive messages over the
   same link before the IMP at the destination has sent back a special
   message called an RFNM (Request for Next Message).  This arrangement
   limits the congestion one HOST can cause another if the sending HOST
   is attempting to send too much over one link.  We note, however, that
   since the IMP at the destination does not have enough capacity to
   handle all 32 links simultaneously, the links serve their purpose only
   if the overload is coming from one or two links.  It is necessary for
   the HOSTs to cooperate in this respect.

   The links have the following primitive characteristics.  They are
   always functioning and there are always 32 of them.

   By "always functioning," we mean that the IMPs are always prepared to
   transmit another message over them.  No notion of beginning or ending
   a conversation is contained in the IMP software.  It is thus not
   possible to query an IMP about the state of a link (although it might
   be possible to query an IMP about the recent history of a link --
   quite a different matter!).

   The other primitive characteristic of the links is that there are
   always 32 of them, whether they are in use or not.  This means that
   each IMP must maintain 18 tables, each with 32 entries, regardless of
   the actual traffic.

   The objections to the link structure notwithstanding, the links are
   easily programmed within the IMPs and are probably a better
   alternative to more complex arrangements just because of their
   simplicity.

IMP Transmission and Error Checking

   After receiving a message from a HOST, an IMP partitions the message
   into one or more packets.  Packets are not more than 1010 bits long
   and are the unit of data transmission from IMP to IMP.  A 24 bit
   cyclic checksum is computed by the transmission hardware and is
   appended to an outgoing packet.  The checksum is recomputed by the
   receiving hardware and is checked against the transmitted checksum.
   Packets are reassembled into messages at the destination IMP.

Open Questions on the IMP Software





 
RFC 1                        Host Software                 7 April 1969


   1.  An 8 bit field is provided for link specification, but only 32
   links are provided, why?

   2.  The HOST is supposed to be able to send messages to its IMP.  How
   does it do this?

   3.  Can a HOST, as opposed to its IMP, control RFNMs?

   4.  Will the IMPs perform code conversion?  How is it to be
   controlled?

II. Some Requirements Upon the Host-to-Host Software

Simple Use

   As with any new facility, there will be a period of very light usage
   until the community of users experiments with the network and begins
   to depend upon it.  One of our goals must be to stimulate the
   immediate and easy use by a wide class of users.  With this goal, it
   seems natural to provide the ability to use any remote HOST as if it
   had been dialed up from a TTY (teletype) terminal.  Additionally, we
   would like some ability to transmit a file in a somewhat different
   manner perhaps than simulating a teletype.

Deep Use