Comments on Host/Host Protocol document #1

Network Working Group                                        Dave Walden
Request for Comments: 65                       A/S Norsk Data-Elektonikk
                                                         August 29, 1970


  Comments on Host-Host Protocol Document No. 1 (S. Crocker - 8/3/70)

   Page 3.   Eliminate marking.  Instead, make all regular messages into
   two message: The first containing just the leader and indicating that
   the data follows in the second (next) message.  Do this both from the
   source Host to its IMP and from the destination IMP to its Host.
   Thus, no more hunting for the beginning of the data is necessary.
   Once this adjustment is made, an additional simplification is
   available.  If the maximum message length is a common multiple of the
   word sizes of all the computers in the network (perhaps 2880*2 bits),
   successive messages of long files can be dropped in place without
   shifting.

   Page 4.   Control messages should be sent to and from the _control
   socket_  -- not over the control link.  The concept of the control
   link causes a great big, unnecessary special case.

   Page 5.   Assigning sockets permanently to certain network resources
   should be encouraged and a directory of the socket/resource
   associations should be available somewhere in the network, perhaps in
   physical book form at each site.

   Page 6.  Links have no Host-Host purpose other than identifying a
   connection so that socket numbers don't have to be included in all
   messages and to simplify table look-ups in the NCPs.  However, since
   there are possibly 512 links* with the same number, links don't aid
   table look-ups very much.  Also finding the next available link to a
   particular destination is very ugly .  Therefore, I suggest limiting
   the number of links to a total of n (where n = 32, 64, or 256 or some
   other good number) for all destinations.  In other words, a
   particular link is only in use to one destination at a time(actually
   from one destination at a time since the receiver picks the link to
   be used for a connection).  This change makes picking the next
   available link very simple and,I feel,is a worthwhile change if only
   for this reason.  The question of simplifying table look-ups is a
   little more complex.  It is easy to use the link directly as an index
   into tables in the receive portion of the NCP since the receiver
   picks the link.  But a hash table or linear search or something is
   still necessary in the send portion of the NCP.  This too can be
   fixed with the following changes.  Add to STR a _pseudo link_  chosen
   by the sender. This link is sent in all non-control messages in the 8
   --------------------------------------
   *A destination number is 9 bits.

   bits to the right of the link in the leader.  The IMP must preserve
   these bits and return them with RFNMs and the receiver must use the
   pseudo link instead of the link in RET and INR.  The extra memory
   necessary to store the pseudo link in the NCP receive tables (which
   are indexed by link) and the link in the NCP send tables (which are
   indexed by pseudo link) is certainly less than the overhead necessary
   to maintain associative tables.

   Page 8.   The allocate mechanism seems very inconvenient for the
   receive portion of the NCP to use.  The receiver wants the allocation
   to be used up in units of the receiver's buffer size not in units of
   sender messages which may be variable length.  Otherwise the receiver
   has a memory compaction problem.

   Page 9.   The new irregular message to make the "cease" mechanism
   work are unnecessary, I think.  The sender can keep track (probably
   with a one bit counter) of ALLs and GVBs and ignore GVB 0s for which
   resume ALLs have already arrived.   This the receiver need not know
   whether the cease has been sent or not.

   Page 15.  If I implemented an NCP, all ERRs would be treated like
   NOP.  As an error control mechanism ERR is complicated and
   insufficient.  Who wants to debug a complicated mechanism which only
   catches bugs due to the primary mechanism being undebugged.  The one
   error control mechanism I would provide is a receive process to send
   process acknowledgment on every message.  If this is not received for
   too long, the send process can send the message again if it has been
   saving it.  This acknowledgment catches errors causing message loss
   at the process/NCP, NCP/NCP, Host/IMP, IMP/IMP, etc.  levels.
   Currently the Host/IMP interface is particularly lacking in useful
   error controls.  I wouldn't worry about kinds of errors check-sums
   are designed to pick up.  If dropped and picked up bits ever become a
   problem either add hardware to more interfaces or let the receive
   process not send the process to process acknowledgment if a software
   checksum does not check.

   The page 3 and page 6 comments involve a change to the IMP program.
   I feel a tiny bit guilty suggesting changes I don't have to implement
   any more.  However, I trust Crowther and Cosell will, as always,
   resist bad changes while making sensible ones.  The page 9 comment is
   aimed at avoiding a change in the IMP program.


         [ This RFC was put into machine readable form for entry ]
           [ into the online RFC archives by Luke Hollins 8/99]