Host Software

Document Type RFC - Unknown (April 1969; No errata)
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Network Working Group                                   Steve Crocker
Request for Comments: 1                                          UCLA
                                                         7 April 1969

                         Title:   Host Software
                        Author:   Steve Crocker
                          Installation:   UCLA
                          Date:   7 April 1969
             Network Working Group Request for Comment:   1



  I. A Summary of the IMP Software



     IMP Transmission and Error Checking

     Open Questions on the IMP Software

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

     Simple Use

     Deep Use

     Error Checking

III. The Host Software

     Establishment of a Connection

     High Volume Transmission

     A Summary of Primitives

     Error Checking

     Closer Interaction

     Open Questions

Crocker                                                        [Page 1]
RFC 1                        Host Software                 7 April 1969

 IV. Initial Experiments

     Experiment One

     Experiment Two


   The software for the ARPA Network exists partly in the IMPs and
   partly in the respective HOSTs.  BB&N has specified the software of
   the IMPs and it is the responsibility of the HOST groups to agree on
   HOST software.

   During the summer of 1968, representatives from the initial four
   sites met several times to discuss the HOST software and initial
   experiments on the network.  There emerged from these meetings a
   working group of three, Steve Carr from Utah, Jeff Rulifson from SRI,
   and Steve Crocker of UCLA, who met during the fall and winter.  The
   most recent meeting was in the last week of March in Utah.  Also
   present was Bill Duvall of SRI who has recently started working with
   Jeff Rulifson.

   Somewhat independently, Gerard DeLoche of UCLA has been working on
   the HOST-IMP interface.

   I present here some of the tentative agreements reached and some of
   the open questions encountered.  Very little of what is here is firm
   and reactions are expected.

I.   A Summary of the IMP Software


   Information is transmitted from HOST to HOST in bundles called
   messages.  A message is any stream of not more than 8080 bits,
   together with its header.  The header is 16 bits and contains the
   following information:

           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

Crocker                                                        [Page 2]
RFC 1                        Host Software                 7 April 1969


   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

IMP Transmission and Error Checking

   After receiving a message from a HOST, an IMP partitions the message
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