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Network Working Group J. Postel Request for Comments: 48 S. Crocker

                                                        April 21, 1970
                    A Possible Protocol Plateau

I. Introduction

 We have been engaged in two activities since the network meeting of
 March 17, 1970 and, as promised, are reporting our results.
 First, we have considered the various modifications suggested from
 all quarters and have formed preferences about each of these.  In
 Section II we give our preferences on each issue, together with our
 Second, we have tried to formalize the protocol and algorithms for
 the NCP, we attempted to do this with very little specification of a
 particular implementation.  Our attempts to date have been seriously
 incomplete but have led to a better understanding.  We include here,
 only a brief sketch of the structure of the NCP.  Section III gives
 our assumptions about the environment of the NCP and in Section IV
 the components of the NCP are described.

II. Issues and Preferences

 In this section we try to present each of the several questions which
 have been raised in recent NWG/RFC's and in private conversations,
 and for each issue, we suggest an answer or policy.  In many cases,
 good ideas are rejected because in our estimation they should be
 incorporated at a different level.
    A. Double Padding
       As BBN report #1822 explains, the Imp side of the Host-to-Imp
       interface concatenates a 1 followed by zero or more 0's to fill
       out a message to an Imp word boundary and yet preserve the
       message length.  Furthermore, the Host side of the Imp-to-Host
       interface extends a message with 0's to fill out the message to
       a Host word boundary.
       BBN's mechanism works fine if the sending Host wants to send an
       integral number of words, or if the sending Host's hardware is
       capable of sending partial words.  However, in the event that

Postel & Crocker [Page 1] RFC 48 A Possible Protocol Plateau April 1970

       the sending Host wants to send an irregular length message and
       its hardware is only capable of sending word-multiple messages,
       some additional convention is needed.
       One of the simplest solutions is to modify the Imp side of the
       Host-to-Imp interface so that it appends only 0's.  This would
       mean that the Host software would have to supply the trailing
       1.  BBN rejected the change because of an understandably strong
       bias against hardware changes.  It was also suggested that a
       five instruction patch to the Imp program would remove the
       interface supplied 1, but this was also rejected on the new
       grounds that it seemed more secure to depend only upon the Host
       hardware to signal message end, and not to depend upon the Host
       software at all.
       Two other solutions are also available.  One is to have "double
       padding", whereby the sending Host supplies 10* and the network
       also supplies 10*.  Upon input, a receiving Host then strips
       the trailing 10* 10*.  The other solution is to make use of the
       marking.  Marking is a string of the form 0*1 inserted between
       the leader and the text of a message.  The original intent of
       marking was to extend the leader so that the sending Host could
       _begin_ its text on a word boundary.  It is also possible to
       use the marking to expand a message so that it _ends_ on a word
       Notice that double padding could replace marking altogether by
       abutting the text beginning against the leader.  For 32 bit
       machines, this is convenient and marking is not, while for
       other lengths, particularly 36 bit machines, marking is much
       more convenient than double padding.
       We have no strong preference, partially because we can send
       word fragments.  Shoshani, et al in NWG/RFC #44 claim that
       adjusting the marking does not cause them any problems, and
       they have a 32 bit machine.  Since the idea of marking has been
       accepted for some time, we suggest that double padding not be
       used and that marking be used to adjust the length of a
       message.  We note that if BBN ever does remove the 1 from the
       hardware padding, only minimal change to Host software is
       needed on the send side.
       A much prettier (and more expensive) arrangement was suggested
       by W. Sutherland.  He suggested that the Host/Imp interfaces be
       smart enough to strip padding or marking and might even parse
       the message upon input.

Postel & Crocker [Page 2] RFC 48 A Possible Protocol Plateau April 1970

    B. Reconnection
       A very large population of networkers has beat upon us for
       including dynamic reconnection in the protocol.  We felt it
       might be of interest to relate how it came to be included.
       After considering connections and their uses for a while, we
       wondered how the mechanism of connections compared to existing
       forms of intra-Host interprocess communication.  Two aspects
       are of interest, what formalisms have been presented in the
       literature, and what mechanisms are in use.  The formalisms are
       interesting because they lead to uniform implementations and
       parsimonious design.  The existing mechanisms are interesting
       because they point out which problems need solving and
       sometimes indicate what an appropriate formalism might be.  In
       particular, we have noticed that the mechanisms for connecting
       a console to the logger upon dial in, the mechanisms for
       creating a job, and the mechanisms for passing a console around
       to various processes within a job tend to be highly
       idiosyncratic and distinct from all other structures and
       mechanisms within an operating system.
       With respect to the literature, it appears there is only one
       idea with several variations, viz processes should share a
       portion of their address spaces and cooperatively wake up each
       other.  Semaphores and event channels are handy extensions of
       wake up signals, but the intent is basically the same.  (Event
       channels could probably function as connections, but it seems
       not to be within their intended use.  In small systems, the
       efficiency and capacity of event channels are inversely
       With respect to existing implementations, we note that several
       systems allow a process to appear to be a file to another
       process.  Some systems, e.g. the SDS-940 at SRI impose a
       master/slave relationship between two processes so connected,
       but other systems provide for a coequal relationship e.g. the
       AI group's PDP-6 system at MAC.  The PDP-6 system also has a
       feature whereby a superior process can "surround" an inferior
       process with a mapping from device and file names to other
       device and file names.  Consoles have nearly the same semantics
       as files, so it is quite reasonable for an inferior process to
       believe it is communicating with the console but in fact be
       communicating with another process.
       The similarity between network connections and existing
       sequential interprocess connections supports our belief that
       network connections are probably the correct structure for

Postel & Crocker [Page 3] RFC 48 A Possible Protocol Plateau April 1970

       using the network.  Moreover, the structure is clean enough and
       compatible with enough machines to pass as a formalism or
       theory, at least to the extent of the other forms of
       interprocess communication presented in the literature.
       Any new formalism, we believe, must meet at least the following
       two tests:
          1. What outstanding problems does it solve?
          2. Is it closed under all operations?
       In the case of network connections, the candidates for the
       first are the ones given above, i.e. all operations involving
       connecting a console to a job or a process.  Also of interest
       are the modelling of sequential devices such as tape drives,
       printers and card readers, and the modeling of their buffering
       (spooling, symbiont) systems.
       The second question mentions closure.  In applying the
       connection formalism to the dial-in and login procedures, we
       felt the need to include some sort of switching or
       reconnection, and an extremely mild form is presented in an
       SJCC paper, which is also NWG/RFC #33.  This mild form permits
       only the substitution of AEN's, and even then only at the time
       of connection establishment. However, it is a common experience
       that if an operation has a natural definition on an extended
       domain, it eventually becomes necessary or at least desirable
       to extend its definition.  Therefore, we considered the
       following extensions:
          1. Switching to any other socket, possibly in another Host.
          2. Switching even after data flow has started.
       There is even some precedent for feeling these extensions might
       be useful.  In one view of an operating system, we see all
       available phone lines as belonging to a live process known as
       the logger.  The logger answers calls, screens users, and
       creates jobs and processes.  One of the features of most
       telephone answering equipment is that many phone lines may
       serve the same phone number by using a block of sequential
       numbers and a rotary answering system.  In our quest for
       accurate models of practical systems, we wanted to be able to
       provide equivalent service to network users, i.e. they should
       be able to call a single advertised number and get connected to
       the logger.  Thus a prima facie case for switching is

Postel & Crocker [Page 4] RFC 48 A Possible Protocol Plateau April 1970

       Next we see that after the logger interrogates a prospective
       user, it must connect the user to a newly created job.  Data
       flow between the user and the logger has already commenced, so
       flow control has to be meshed with switching if it is desired
       not to lose or garble data in transit.
       With respect to inter-Host switching, we find it easy to
       imagine a utility service which is distributed throughout the
       network and which passes connections from one socket to another
       without the knowledge of the user.  Also, it is similar to the
       more sophisticated telephone systems, to standard facilities of
       telephone company operators, and to distributed private
       These considerations led us to investigate the possibility of
       finding one type of reconnection which provided a basis for all
       known models.  The algorithm did not come easily, probably
       because of inexperience with finite state automata theory, but
       eventually we produced the algorithm presented in NWG/RFC #36.
       A short time later, Bill Crowther produced an equivalent
       algorithm which takes an alternate approach to race conditions.
       Networkers seem to have one of two reactions.  Either it was
       pretty and (perhaps ipso facto) useful, or it was complex and
       (again perhaps ipso facto) unnecessary.  The latter group was
       far more evident to us, and we were put into the defensive
       position of admitting that dynamic reconnection was only
          1. pretty
          2. useful for login and console passing
       In response to persistent criticism, we have made the following
       change in the protocol.  Instead of calling socket <O,H,O> to
       login, sockets of the form <U,H,O> and <U,H,1> are the input
       and output sockets respectively of a copy of the logger or, if
       a job has been stared with user id U, these sockets are the
       console sockets.  The protocol for login is thus to initiate a
       connection to <U,H,O> and <U,H,1>.  If user U is not in use, a
       copy of the logger will respond and interrogate the caller.  If
       user id U is in use, the call will be refused.  This
       modification was suggested by Barry Wessler recently.  (Others
       also suggested this change much earlier; but we rejected it
       The logger may demand that the caller be from the same virtual
       net, i.e. the caller may have user id U in some other Host, or
       it may demand that the user supply a password matched to user

Postel & Crocker [Page 5] RFC 48 A Possible Protocol Plateau April 1970

       id U, or it may demand both.  Some systems may even choose to
       permit anybody to login to any user id.
       After login, AEN's 0 and 1 remain the console AEN's.  Each
       system presumably has mechanisms for passing the console, and
       these would be extended to know about AEN's 0 and 1 for network
       users.  Passing the console is thus a matter of reconnecting
       sockets to ports, and happens within the Host and without the
       In conversations with Meyer and Skinner after NWG/RFC #46 was
       received, they suggested a login scheme different from both
       Meyer's and ours in section above.  Their new scheme seemed a
       little better and we look forward to their next note.
       It is generally agreed that login should be "third-level", that
       is, above the NCP level.  We are beginning to be indifferent
       about particular logins schemes; all seem ok and none impress
       us greatly.  We suggest that several be tried.  It is some
       burden, of course, to modify the local login procedure, but we
       believe it imposes no extra hardship to deal with diverse login
       procedures.  This is because the text sequences and interrupt
       conventions are so heterogenous that the additional burden of
       following, say, our scheme on our system and Meyer's on Multics
       is minimal.
       We are agreed that reconnection should not be required in the
       initial protocol, and we will offer it later as an optional and
       experimental tool.  In addition, we would like to be on record
       as predicting that general reconnection facilities will become
       useful and will provide a unifying framework for currently ad
       hoc operating system structures.
    C. Decoupling Connections and Links
       Bill Crowther (BBN) and Steve Wolfe (UCLA) independently have
       suggested that links not be assigned to particular connections.
       Instead, they suggest, include the destination socket as part
       of the text of the message and then send messages over any
       unblocked link.
       We discussed this question a little in NWG/RFC #37, and feel
       there is yet an argument for either case.  With the current
       emphasis on simplicity, speed and small core requirements, it
       seems more efficient to leave links and connections coupled.
       We, therefore, recommend this.

Postel & Crocker [Page 6] RFC 48 A Possible Protocol Plateau April 1970

    D. Error Reporting
       As mentioned by J. Heafner and E. Harslem of RAND, it is
       important to treat errors which might occur.  A good philosophy
       is to guard against any input which destroys the consistency of
       the NCP's data base.
       The specific formulation of the error command given by Heafner
       and Harslem in NWG/RFC #40 and by Meyer in NWG/RFC #46 seems
       reasonable and we recommend its adoption.  Some comments are in
       order, however.
       A distinction should be made between resource errors and other
       types of errors.  Resource errors are just the detection of
       overload conditions.  Overload conditions are well-defined and
       valid, although perhaps undesirable.  Other types of errors
       reflect errant software or hardware.  We feel that resource
       errors should not be handled with error mechanisms, but with
       mechanisms specific to the problem.  Thus the <CLS> command may
       be issued when there is no more room to save waiting <RFC>'s.
       Flow control protocol is designed solely to handle buffering
       With respect to true errors, we are not certain what the value
       of the <ERR> command is to the recipient.  Presumably his NCP
       is broken, and it may only aggravate the problem to bombard it
       with error commands.  We therefore, recommend that error
       generation be optional, that all errors be logged locally in a
       chronological file and that <ERR> commands received likewise be
       logged in a chronological file.  No corrective action is
       specified at this time.
       In the short time the network has been up at UCLA, we have
       become convinced that the network itself will generate very few
       errors.  We have watched the BBN staff debug and test the IMP
       program, and it seemed that most of the errors affected timing
       and throughput rather than validity.  Hence most errors will
       probably arise from broken Hosts and/or buggy NCP's.
    E. Status Testing and Reporting
       A valuable debugging aid is to be able to get information about
       what a foreign NCP thinks is happening.  A convenient way to do
       this is to permit NCP's to send status whenever they wish, but
       to always have them do it whenever they receive a request.

Postel & Crocker [Page 7] RFC 48 A Possible Protocol Plateau April 1970

       Since we view this feature as primarily a debugging tool, we
       suggest that a distinct link, like 255, be used.  The intent is
       that processing of status requests and generating of status
       messages should use as little of the normal machinery as
       possible.  Thus we suggest that link 255 be used to send
       "request status" and "status is" commands.  The form follows
       the suggestion on page 2 of NWG/RFC #40.
       Meyer's <ECO> command is easily implemented and serves the more
       basic function of testing whether a foreign NCP is alive.  We
       suggest that the length of the <ECO> command be variable, as
       there seems to be no significance in this context to 48 bits.
       Also, the value of a (presumably) 8 bit binary switch is
       unclear, so we recommend a pair of commands:
                 <ECO>   <length>   <text>
                 <ERP>   <length>   <text>
                 <length> is 8 bits.
       Upon receipt of an <ECO> command the NCP would echo with the
       <ERP> command.
    F. Expansion and Experimentation
       As Meyer correctly points out in NWG/RFC #46, network protocol
       is a layered affair.  Three levels are apparent so far.
          1. IMP Network Protocol
          2. Network Control Program Protocol
          3. Special user level or Subsystem Level Protocol
       This last level should remain idiosyncratic to each Host (or
       even each user).  The first level is well-specified by BBN, and
       our focus here is on level 2.  We would like to keep level 2 as
       neutral and simple as possible, and in particular we agree that
       login protocol should be as much on level 3 as possible.
       Simplicity and foresight notwithstanding, there will arise
       occasions when the level 2 protocol should change or be
       experimented with.  In order to provide for experimentation and
       change, we recommend that only link numbers 2 through 31 be
       assigned to regular connections, with the remaining link
       numbers, 32 to 255, used experimentally.  We have already
       suggested that link 255 be used for status requests and
       replies, and this is in consonance with our view of the
       experimental aspects of that feature.

Postel & Crocker [Page 8] RFC 48 A Possible Protocol Plateau April 1970

       We also recommend that control command prefixes from 255
       downward be used for experimentation.
       These two conventions are sufficient, we feel to permit
       convenient experimentation with new protocol among any subset
       of the sites. We thus do not favor inclusion of Ancona's
       suggestion in NWG/RFC #42 for a message data type code as the
       first eight bits of the text of a message.
    G. Multiplexing Ports to Sockets
       Wolfe in NWG/RFC #38 and Shoshani et al in NWG/RFC #44 suggest
       that it should be possible to attach more than one port to a
       socket.  While all of our diagrams and prototypical system
       calls have shown a one-to-one correspondence between sockets
       and ports, it is strictly a matter of local implementation.  We
       note that sockets form a network-wide name space whose sole
       purpose is to interface between the idiosyncratic structures
       peculiar to each operating system.  Our references to ports are
       intended to be suggestive only, and should be ignored if no
       internal structures corresponds to them.  Most systems do have
       such structures, however, so we shall continue to use them for
    H. Echoing, Interrupts and Code Conversion
       1. Interrupts
          We had been under the impression that all operating systems
          scanned for a reserved character from the keyboard to
          interpret it as an interrupt signal.  Tom Skinner and Ed
          Meyer of MIT inform us that model 37 TTY's and IBM 2741
          generate a "long space" of 200-500 milliseconds which is
          detected by the I/O channel hardware and passed to the
          operating system as an interrupt.  The "long space" is not a
          character -- it has no ASCII code and cannot be program
          Well over a year ago, we considered the problem of
          simulating console interrupts and rejected the <INT> type
          command because it didn't correctly model any system we
          knew.  We now reverse our position and recommend the
          implementation of an INTERRUPT system call and an <INT>
          control command as suggested by Meyer in NWG/RFC #46.

Postel & Crocker [Page 9] RFC 48 A Possible Protocol Plateau April 1970

          Two restrictions of the interrupt facility should be
          observed.  First, when communicating with systems which scan
          for interrupt characters, this feature should not be used.
          Second, non-console-like connections probably should not
          have interrupts. We recommend that systems follow their own
          conventions, and if an <INT> arrives for a connection on
          which it shouldn't the <INT> should be discarded and
          optionally returned as an error.
       2. Echoing and Code Conversion
          We believe that each site should continue its current
          echoing policy and that code conversion should be done by
          the using process.  Standardization in this area should
          await further development.
          Ancona's suggestion of a table-driven front-end transducer
          seems like the right thing, but we believe that such
          techniques are part of a larger discussion involving
          higher-level languages for the network.
    I. Broadcast Facilities
       Heafner and Harslem suggest in NWG/RFC #39 a broadcast
       facility, i.e. <TER> and <BDC>.  We do not fully understand the
       value of this facility and are thus disposed against it.  We
       suspect that we would understand its value better if we had
       more experience with OS/360.  It is probably true in general
       that sites running OS/360 or similar systems will find less
       relevance in our suggestions for network protocol than sites
       running time-sharing systems.  We would appreciate any cogent
       statement on the relationship between OS/360 and the concepts
       and assumptions underlying the network protocol.
    J. Instance Numbers
       Meyer in NWG/RFC #46 suggests extending a socket to include an
       _instance_ code which identifies the process attached to the
       socket.  We carefully arranged matters so that processes would
       be indistinguishable.  We did this with the belief that both as
       a formal and as a practical matter it is of concern only within
       a Host whether a computation is performed by one or many
       processes.  Thus we believe that all processes within a job
       should cooperate in allocating AEN's.  If an operating system
       has facilities for passing a console from process to process
       within a job, these facilities mesh nicely with the current
       network protocol, even within reconnection protocol; but
       instance numbers interfere with such a procedure.

Postel & Crocker [Page 10] RFC 48 A Possible Protocol Plateau April 1970

       We suggest this matter be discussed fully because it relates to
       the basic philosophy of sockets and connections.  Presently we
       recommend 40 bit socket numbers without instance codes.
    K. AEN's
       Nobody, including us, is particularly happy with our name AEN
       for the low order 8 bits of the socket.  We rejected _socket_
       number_, and are similarly unhappy with Meyer's _socket_code_.
       The word socket should not be used as part of the field name,
       and we solicit suggestions.

III. Environment

 We assume that the typical host will have a time-sharing operating
 system in which the cpu is shared by processes.
 We envision that each process is tagged with a _user_number_. There
 may be more than one process with the same user number, and if so,
 they should all be cooperating with respect to using the network.
 We envision that each process contains a set of _ports_ which are
 unique to the process.  These ports are used for input to or output
 from the process, from or to files, devices or other processes.
 We also envision that each process has an event channel over which it
 can receive very short messages (several bits).  We will use this
 mechanism to notify a process that some action external to the
 process has occurred.
 To engage in network activity, a process _attaches_ a _local_socket_
 to one of its ports.  Sockets are identified by user number, host and
 AEN, and a socket is local to a process if their user numbers match
 and they are in the same host.  A process need only specify an AEN
 when it is referring to a local socket.
 Each port has a status which is modified by system calls and by
 concurrent events outside the process.  Whenever the status of a port
 is changed, the process is sent an event over its event channel which
 specifies which port's status has changed.  The process may then look
 at a port's status.
 These assumptions are used descriptive material which follows.
 However, these assumptions are not imposed by the network protocol
 and the implementation suggested by section IV is in no way binding.

Postel & Crocker [Page 11] RFC 48 A Possible Protocol Plateau April 1970

 We wish to make very clear that this material is offered only to
 provide clues as to what the implementation difficulties might be and
 not to impose any particular discipline.
 For example, we treat <RFC>'s which arrive for unattached local
 sockets as valid and queue them.  If desired, an NCP may reject them,
 as Meyer suggests, or it might hold them for awhile and reject them
 if they're not soon satisfied.  The offered protocol supports all
 these options.
 Another local option is the one mentioned before of attaching
 multiple ports to a socket.  We have shown one-one correspondence but
 this may be ignored.  Similarly, the system calls are merely
 System Calls
 These are typical system calls which a user process might execute.
 We show these only for completeness; each site will undoubtedly
 implement whatever equivalent set is convenient.
      We use the notation
      Syscall ( arg , arg ...; val ... )
                   1     2        1
      Syscall is the system call
      arg  etc. are the parameters supplied with the call, and
      val etc. are any values returned by the system call.
 Init (P,AEN,FS,Bsiz;C)
      P      Specifies a port of the process.
      AEN    Specifies a local socket.  The user number of this
             process and host number of this host are implicit.
      FS     Specifies a socket with any user number in any host,
             with any AEN.
      Bsiz   Specified the amount of storage in bits the user wants
             to devote to buffering messages.
      C      The condition code returned.
 Init attempts to attach the local socket specified by AEN to the port
 P and to initiate a connection with socket FS.  Possible returned
 values of C are

Postel & Crocker [Page 12] RFC 48 A Possible Protocol Plateau April 1970

      C = ok      The Init was legal and the socket FS is being
                  contacted.  When the connection is established or
                  when FS refuses, the process will receive an event.
      C = busy    The local socket was in use by a port on this or
                  some other process with the same user number.  No
                  action was taken.
      C = homosex The AEN and FS were either both send or both receive
      C = nohost  The host designated within FS isn't known.
      C = bufbig  Bsiz is too large.
 Listen (P,AEN,Bsize;C)
      P     Specifies a port of the process.
      AEN   Specifies a local socket.
      Bsiz  Specified a buffer size.
      C     The returned legality code.
 Codes for C are
      C = ok
      C = busy
      C = bufbig
 The local socket specifies by AEN is attached to P.  If there is a
 waiting call, it is processed; otherwise no action is taken.  When a
 call comes in, a connection will be established and the process
 notified via an event.
 Close (P)
      P Specifies a port of the process.
 Any activity is stopped, and the port becomes free for other use.
 Transmit (P,M,L1;L2,C)
      P     Specifies port with an open connection.
      M     The text to be transmitted.
      L1    Specifies the length of the text.
      L2    The length actually transmitted.
      C     The error code.

Postel & Crocker [Page 13] RFC 48 A Possible Protocol Plateau April 1970

 Transmission between the processes on either side of the port takes
 Codes for C are
      C = ok
      C = not open     if no connection is currently open and
                       otherwise uninhibited
 Status (P;C)
 The status of port P is returned as C.


 We view the NCP as having five component programs, three associative
 tables, some queues and buffers, and a link assignment table.  Each
 site will of course, vary this design to meet its needs, so our
 design is only illustrative.
 The Component Programs
    1. The Input Handler
       This is an interrupt driven input routine.  It initiates Imp-
       to-Host transmission into a resident buffer and wakes up the
       Input Interpreter when transmission is complete.
    2. The Output Handler
       This is an interrupt driven output routine.  It initiates
       Host-to-Imp transmission out of a resident buffer and wakes up
       the Output Scheduler when transmission is complete.
    3. The Input Interpreter
       This program decides whether the input is a regular message
       intended for a user, a control message, an Imp-to-Host message,
       or an error.  For each class of message, this program takes the
       appropriate action.
    4. The Output Scheduler
       Three classes of message are sent to the Imp
          (a) Host-to-Imp messages
          (b) Control messages
          (c) Regular messages

Postel & Crocker [Page 14] RFC 48 A Possible Protocol Plateau April 1970

       We believe that a priority should be imposed among these
       classes.  The priority we suggest is the ordering above. The
       Output Scheduler selects the highest priority message and
       gives it to the Output Handler.
    5. The System Call Interpreter
       This program interprets requests from the user.
 The two interesting components are the Input Interpreter and the
 System Call Interpreter.  These are similar in that the Input
 Interpreter services foreign requests and the System Call Interpreter
 services local requests.
 Associative Tables
 We envision that the bulk of the NCP's data base is in three
 associative tables.  By "associative", we mean that there is some
 lookup routine which is presented with a key and either returns
 successfully with a pointer to the corresponding entry, or fails if
 no entry corresponds to the key.
    1. The Rendezvous Table
       "Requests-for-connection" and other attributes of a
       connection are held in this table.  This table is accessed by
       local socket, but other tables have pointers to existing
          The components of an entry are:
          (a) local socket   (key)
          (b) foreign socket
          (c) link
          (d) queue of callers
          (e) text queue
          (f) connection state
          (g) flow state
          (h) pointer to attached port
          An entry is created when a user executes either an Init or a
          Listen system call or when a <RFC> is received.  Some fields
          are unused until the connection is established, e.g. the
          foreign socket is not known until a <RFC> arrives if the
          user did a Listen.

Postel & Crocker [Page 15] RFC 48 A Possible Protocol Plateau April 1970

    2. The Input Link Table
          The Input Interpreter uses the foreign host and link as a
          key to get a pointer to the entry in the rendezvous table
          for the connection using the incoming link.
    3. The Output Link Table
          In order to interpret RFNM's, the Input Interpreter needs a
          table in the same form as the Input Link Table but using
          outgoing links.
 Link Assignment Table
 This is a very simple structure which keeps track of which links are
 in use for each host.  One word per host probably suffices.
 The following diagram is our conception of the Network Control
 Program.  Boxes represent tables and Buffers, boxes with angled
 corners and a double bottom represent Queues, and jagged boxes
 represent component programs, the arrows represent data paths.
 The abbreviated names have the following meanings.
 ILT   - Input Link Table
 OLT   - Output Link Table
 LAT   - Link Assignment Table
 RT    - Rendezvous Table
 HIQ   - Host to Imp Queue
 OCCQ  - Output Control Command Queue
 ORMQ  - Output Regular Message Queue
 IHBuf - Buffer filled by the Input Handler from the IMP and
         emptied by the Input Interpreter
 OHBuf - Buffer of outgoing messages filled from the Queues
         by the Output Scheduler and emptied by the Output

Postel & Crocker [Page 16] RFC 48 A Possible Protocol Plateau April 1970

                            |  I M P  |
                              v     ^
                              |     |
  |                           |     |                              |
  |   /\/\/\/\/\/\/\          |     |     /\/\/\/\/\/\/\           |
  |   \            / <--------+     +---< \            /           |
  |   /  Input     \                      /  Output    \           |
  |   \   Handler  /                      \   Handler  / <----+    |
  |   /            \ >------+             /            \      |    |
  |   \/\/\/\/\/\/\/        |             \/\/\/\/\/\/\/      ^    |
  |                         v                              +-----+ |
  |                      +-----+                           | OH  | |
  |                      | IM  |                           | Buf | |
  |                      | Buf |                           +-----+ |
  |                      +-----+          /\/\/\/\/\/\/\/\    ^    |
  | /\/\/\/\/\/\/\/\        v      +----> \              /    |    |
  | \              /        |      |      /  Output      \ >--+    |
  | /              \ <------+      ^      \              /         |
  | \  Input       /           /-----\    /   Scheduler  \         |
  | /              \ >-------->| HIQ |    \              /         |
  | \  Interpreter /           |_____|    /              \         |
  | /              \ >----+    \_____/    \/\/\/\/\/\/\/\/         |
  | \/\/\/\/\/\/\/\/      |                ^     v    ^            |
  |   ^   ^    ^   \      |    /-----\     |     |    |    /-----\ |
  |   |    \    \   \     |    |  O  |     |     |    |    |  O  | |
  |   |     \    \   \    +--->|  C  |>----+     |    +---<|  R  | |
  |   v     v     v   \        |  C  |           |         |  M  | |
  | +---+ +---+ +---+  \       |  Q  |           v         |  Q  | |
  | |   | |   | |   |   \      |_____|      +---------+    |_____| |
  | |ILT| |LAT| |OLT|    \     \_____/      |         |    \_____/ |
  | |   | |   | |   |     \       ^         |   R T   |       ^    |
  | +---+ +---+ +---+      +------|-------->|         |       |    |
  |         v                     |         +---------+       |    |
  |         |                     ^              ^            |    |
  |         |            /\/\/\/\/\/\/\/\        |            |    |
  |         |            \              /        |            |    |
  |         +----------->/    System    \<-------+            |    |
  |                      \     Call     /                     |    |
  |                      /  Interpreter \>--------------------+    |
  |                      \              /                          |
  |                  +-->/              \>--+                      |
  |                  |   \/\/\/\/\/\/\/\/   |                      |
                     |                      |
                     +---< system calls <---+

Postel & Crocker [Page 17] RFC 48 A Possible Protocol Plateau April 1970

     [ This RFC was put into machine readable form for entry ]
 [ into the online RFC archives by Donald and Jill Eastlake 1999 ]

[Editor's note: The original hand-drawn diagram represented Queues by cylinders and component programs by "squishy ameoba like things".]

Postel & Crocker [Page 18]

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