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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
 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

Crocker [Page 3] 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

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

 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

Crocker [Page 4] 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

Crocker [Page 5] 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
 a)  Initiate TTY-like connection with HOST x.
 b)  Terminate connection.
 c)  Send/Receive character(s) over TTY-like connection.
 d)  Initiate file-like connection parallel to TTY-like connection.
 e)  Terminate file-like connection.
 f)  Send/Receive over file-like connection.

Error Checking

 We propose that each message carry a message number, bit count, and a
 checksum in its body, that is transparent to the IMP.  For a checksum
 we suggest a 16-bit end-around-carry sum computed on 1152 bits and
 then circularly shifted right one bit.  The right circular shift every
 1152 bits is designed to catch errors in message reassembly by the IMPs.

Closer Interaction

 The above described primitives suggest how a user can make simple use
 of a remote facility.  They shed no light on how much more intricate
 use of the network is to be carried out.  Specifically, we are
 concerned with the fact that as some sites a great deal of work has
 gone into making the computer highly responsive to a sophisticated
 console.  Culler's consoles at UCSB and Englebart's at SRI are at
 least two examples.  It is clear that delays of a half-second or so
 for trivial echo-like responses degrade the interaction to the point
 of making the sophistication of the console irrelevant.
 We believe that most console interaction can be divided into two

Crocker [Page 6] RFC 1 Host Software 7 April 1969

 parts, an essentially local, immediate and trivial part and a remote,
 more lengthy and significant part.  As a simple example, consider a
 user at a console consisting of a keyboard and refreshing display
 screen.  The program the user is talking typing into accumulates a
 string of characters until a carriage return is encountered and then
 it processes the string.  While characters are being typed, it
 displays the characters on the screen.  When a rubout character is
 typed, it deletes the previous non-rubout character.  If the user
 types H E L L O <- <- P <CR> where <- is rubout and <CR> is
 carriage-return, he has made nine keystrokes.  If each of these
 keystrokes causes a message to be sent which in return invokes
 instructions to our display station we will quickly become bored.
 A better solution would be to have the front-end of the remote program
 -- that is the part scanning for <- and <CR> -- be resident in our
 computer.  In that case, only one five character message would be
 sent, i.e., H E L P <CR>, and the screen would be managed locally.
 We propose to implement this solution by creating a language for
 console control.  This language, current named DEL, would be used by
 subsystem designers to specify what components are needed in a
 terminal and how the terminal is to respond to inputs from its
 keyboard, Lincoln Wand, etc.  Then, as a part of the initial protocol,
 the remote HOST would send to the local HOST, the source language text
 of the program which controls the console.  This program would have
 been by the subsystem designer in DEL, but will be compiled locally.
 The specifications of DEL are under discussion.  The following
 diagrams show the sequence of actions.

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

A. Before Link Establishment

       /                                                      \
      |     +-----------+                    +-----------+    |
      |     |           |                    |           |    |
      |     |           |                    |           |    |
      |     | terminal  |                    | terminal  |    |
      |     |           |                    |           |    |
      |     |           |                    |           |    |
      |     +-----+-----+                    +-----+-----+    |
      |           |                                |          |
      |           |                                |          |
      |           |                                |          |
      |     +-----+-----+                    +-----------+    |
      |     |     |     | Request connection |     |     |    |
 UCLA {     |     |     | -> over link 25    |     |     |    } SRI
      |     |   +-+-+   |  +-+          +-+  |   +-+-+   |    |
      |     |   | OS|---+-=|I|----------|I|=-+---| OS|   |    |
      |     |   +-+-+   |  +-+          +-+  |   +---+   |    |
      |     |           |                    |           |    |
      |     |           |                    |           |    |
      |     +-----------+                    +-----------+    |
      |      HOST: UCLA                        HOST: SRI      |
       \                                                     /

Crocker [Page 8] RFC 1 Host Software 7 April 1969

b. After Link Establishment and Log-in

       /                                                      \
      |     +-----------+                    +-----------+    |
      |     |           |                    |           |    |
      |     |           |                    |           |    |
      |     | terminal  |                    | terminal  |    |
      |     |           |                    |           |    |
      |     |           |                    |           |    |
      |     +-----+-----+                    +-----+-----+    |
      |           |                                |          |
      |           |                                |          |
      |           |                                |          |
      |     +-----+-----+ "Please send front"+-----------+    |
      |     |     |     | end control"       |     |     |    |
 UCLA {     |     |     |        ->          |     |     |    } SRI ___
      |     |   +-+-+   |  +-+          +-+  |  +--+---+ |    |    /   |
      |     |   | OS|---+-=|I|----------|I|=-+--|OS|NLS| +----+---|    |
      |     |   +-+-+   |  +-+          +-+  |  +------+ |    |   |___/
      |     |           |       DEL prog.    |           |    |   |    |
      |     |           |        <-          |           |    |   |____|
      |     +-----------+                    +-----------+    |
      |      HOST: UCLA                        HOST:SRI       |
       \                                                     /

Crocker [Page 9] RFC 1 Host Software 7 April 1969

c. After Receipt and Compilation of the DEL program

       /                                                     \
      |     +-----------+                    +-----------+    |
      |     |           |                    |           |    |
      |     |           |                    |           |    |
      |     | terminal  |                    | terminal  |    |
      |     |           |                    |           |    |
      |     |           |                    |           |    |
      |     +-----+-----+                    +-----+-----+    |
      |           |Trivial                         |          |
      |           |Responses                       |          |
      |           |                                |          |
      |     +-----+------+                    +-----------+   |
      |     |     |      |                    |     |     |   |
 UCLA {     |     |      |  Major Responses   |     |     |   } SRI ___
      |     |  +--+--+   |  +-+          +-+  |  +--+---+ |   |    /   |
      |     |  |DEL  |---+-=|I|----------|I|=-+--|OS|NLS| +---+---|    |
      |     |  |front|   |  +-+          +-+  |  +------+ |   |   |___/
      |     |  | end |   |                    |           |   |   |    |
      |     |  |prog.|   |                    |           |   |   |____|
      |     |  +-----+   |                    |           |   |
      |     |  | OS  |   |                    |           |   |
      |     |  +-----+   |                    |           |   |
      |     |            |                    |           |   |
      |     +------------+                    +-----------+   |
      |      HOST: UCLA                         HOST: SRI     |
       \                                                     /

Open Questions

 1.  If the IMPs do code conversion, the checksum will not be correct.
 2.  The procedure for requesting the DEL front end is not yet

IV. Initial Experiments

Experiment One

 SRI is currently modifying their on-line retrieval system which will
 be the major software component on the Network Documentation Center so
 that it can be operated with model 35 teletypes.  The control of the
 teletypes will be written in DEL.  All sites will write DEL compilers
 and use NLS through the DEL program.

Experiment Two

Crocker [Page 10] RFC 1 Host Software 7 April 1969

 SRI will write a DEL front end for full NLS, graphics included.  UCLA
 and UTAH will use NLS with graphics.
       [ This RFC was put into machine readable form for entry ]
       [ into the online RFC archives by Celeste Anderson 3/97 ]

Crocker [Page 11]

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