One of the inherent problems in the network is the fact that all responses
   from a remote HOST will require on the order of a half-second or so,
   no matter how simple.  For teletype use, we could shift to a
   half-duplex local-echo arrangement, but this would destroy some of the
   usefulness of the network.  The 940 Systems, for example, have a very
   specialized echo.

   When we consider using graphics stations or other sophisticated
   terminals under the control of a remote HOST, the problem becomes more
   severe. We must look for some method which allows us to use our most
   sophisticated equipment as much as possible as if we were connected
   directly to the remote computer.

Error Checking

   The point is made by Jeff Rulifson at SRI that error checking at major
   software interfaces is always a good thing. He points to some
   experience at SRI where it has saved much dispute and wasted effort.
   On these grounds, we would like to see some HOST to HOST checking.
   Besides checking the software interface, it would also check the
   HOST-IMP transmission hardware.  (BB&N claims the HOST-IMP hardware
   will be as reliable as the internal registers of the HOST.  We believe




 
RFC 1                        Host Software                 7 April 1969


   them, but we still want the error checking.)

III.  The Host Software

Establishment of a Connection

   The simplest connection we can imagine is where the local HOST acts as
   if it is a TTY and has dialed up the remote HOST.  After some
   consideration of the problems of initiating and terminating such a
   connection , it has been decided to reserve link 0 for communication
   between HOST operating systems.  The remaining 31 links are thus to be
   used as dial-up lines.

   Each HOST operating system must provide to its user level programs a
   primitive to establish a connection with a remote HOST and a primitive
   to break the connection.  When these primitives are invoked, the
   operating system must select a free link and send a message over link
   0 to the remote HOST requesting a connection on the selected link.
   The operating system in the remote HOST must agree and send back an
   accepting message over link 0.  In the event both HOSTs select the same
   link to initiate a connection and both send request messages at
   essentially the same time, a simple priority scheme will be invoked in
   which the HOST of lower priority gives way and selects another free
   link.  One usable priority scheme is simply the ranking of HOSTS
   by their identification numbers.  Note that both HOSTs are aware that
   simultaneous requests have been made, but they take complementary
   actions: The higher priority HOST disregards the request while the
   lower priority HOST sends both an acceptance and another request.

   The connection so established is a TTY-like connection in the
   pre-log-in state.  This means the remote HOST operating system will
   initially treat the link as if a TTY had just called up.  The remote
   HOST will generate the same echos, expect the same log-in sequence and
   look for the same interrupt characters.

High Volume Transmission

   Teletypes acting as terminals have two special drawbacks when we
   consider the transmission of a large file.  The first is that some
   characters are special interrupt characters.  The second is that
   special buffering techniques are often employed, and these are
   appropriate only for low-speed character at time transmission.

   We therefore define another class of connection to be used for the
   transmission of files or other large volumes of data.  To initiate
   this class of link, user level programs at both ends of an established
   TTY-like link must request the establishment of a file-like connection
   parallel to the TTY-like link.  Again the priority scheme comes into




 
RFC 1                        Host Software                 7 April 1969


   play, for the higher priority HOST sends a message over link 0 while
   the lower priority HOST waits for it.  The user level programs are, of
   course, not concerned with this.  Selection of the free link is done
   by the higher priority HOST.

   File-like links are distinguished by the fact that no searching for
   interrupt characters takes place and buffering techniques appropriate
   for the higher data rates takes place.

A Summary of Primitives

   Each HOST operating systems must provide at least the following
   primitives to its users.  This list knows not to be necessary but not
   sufficient.