This is a purely informative rendering of an RFC that includes verified errata. This rendering may not be used as a reference.

The following 'Verified' errata have been incorporated in this document: EID 1053, EID 1780
Network Working Group                                         S. Crocker
Request for Comments: 33                                            UCLA
                                                                 S. Carr
                                                      University of Utah
                                                                 V. Cerf
                                                        12 February 1970

                         New HOST-HOST Protocol

   Attached is a copy of the paper to be presented at the SJCC on the
   HOST-HOST Protocol.  It indicates many changes from the old protocol
   in NWG/RFC 11; these changes resulted from the network meeting on
   December 8, 1969.  The attached document does not contain enough
   information to write a NCP, and I will send out another memo or so
   shortly.  Responses to this memo are solicited, either as NWG/RFC's
   or personal notes to me.

                     HOST-HOST Communication Protocol
                           in the ARPA Network*

   by C. Stephen Carr
   University of Utah
   Salt Lake City, Utah


   by Stephen D. Crocker
   University of California
   Los Angeles, California


   by Vinton G. Cerf
   University of California
   Los Angeles, California

   *This research was sponsored by the Advanced Research Projects
   Agency, Department of Defense, under contracts AF30(602)-4277 and


   The Advanced Research Projects Agency (ARPA) Computer Network
   (hereafter referred to as the "ARPA network") is one of the most
   ambitious computer networks attempted to date.  [1]  The types of

   machines and operating systems involved in the network vary widely.
   For example, the computers at the first four sites are an XDS 940
   (Stanford Research Institute), an IBM 360/75 (University of
   California, Santa Barbara), an XDS SIGMA-7 (University of California,
   Los Angeles), and a DEC PDP-10 (University of Utah).  The only
   commonality among the network membership is the use of highly
   interactive time-sharing systems; but, of course, these are all
   different in external appearance and implementation.  Furthermore, no
   one node is in control of the network.  This has insured reliability
   but complicates the software.

   Of the networks which have reached the operational phase and been
   reported in the literature, none have involved the variety of
   computers and operating systems found in the ARPA network.  For
   example, the Carnegie-Mellon, Princeton, IBM network consists of
   360/67's with identical software. [2]  Load sharing among identical
   batch machines was commonplace at North American Rockwell Corporation
   in the early 1960's.  Therefore, the implementers of the present
   network have been only slightly influenced by earlier network

   However, early time-sharing studies at the University of California
   at Berkeley, MIT, Lincoln Laboratory, and System Development
   Corporation (all ARPAA sponsored) have had considerable influence on
   the design of the network.  In some sense, the ARPA network of time-
   shared computers is a natural extension of earlier time-sharing

   The network is seen as a set of data entry and exit points into which
   individual computers insert messages destined for another (or the
   same) computer, and from which such messages emerge.  The format of
   such messages and the operation of the network was specified by the
   network contractor (BB&N) and it became the responsibility of
   representatives of the various computer sites to impose such
   additional constraints and provide such protocol as necessary for
   users at one site to use resources at foreign sites.  This paper
   details the decisions that have been made and the considerations
   behind these decisions.

   Several people deserve acknowledgement in this effort.  J. Rulifson
   and W. Duvall of SRI participated in the early design effort of the
   protocol and in the discussions of NIL.  G. Deloche of Thompson-CSF
   participated in the design effort while he was at UCLA and provided
   considerable documentation.  J. Curry of Utah and P. Rovner of
   Lincoln Laboratory reviewed the early design and NIL.  W. Crowther of
   Bolt, Beranek and Newman, contributed the idea of a virtual net.  The
   BB&N staff provided substantial assistance and guidance while
   delivering the network.

   We have found that, in the process of connecting machines and
   operating systems together, a great deal of rapport has been
   established between personnel at the various network node sites.  The
   resulting mixture of ideas, discussions, disagreements, and
   resolutions has been highly refreshing and beneficial to all
   involved, and we regard the human interaction as a valuable by-
   product of the main effect.


   Before going on to discuss operating system communication protocol,
   some definitions are needed.

      A HOST is a computer system which is a part of the network,

      An IMP (Interface Message Processor) is a Honeywell DDP-516
      computer which interfaces with up to four HOSTs at a particular
      site, and allows HOSTs access into the network.  The configuration
      of the initial four-HOST network is given in figure 1.  The IMPs
      from a store-and-forward communications network.  A companion
      paper in these proceedings covers the IMPs in some detail. [3]

   A message is a bit stream less than 8096 bits long which is given to
   an IMP by a HOST for transmission to another HOST.  The first 32 bits
   of the message are the leader.  The leader contains the following

      (a) HOST
      (b) Message Type
      (c) Flags
      (d) Link Number

   When a message is transmitted from a HOST to its IMP, the HOST field
   of the leader names the receiving HOST.  When the message arrives at
   the receiving HOST, the HOST field names the sending HOST.

   Only two message types are of concern in this paper.  Regular
   messages are generated by a HOST and sent to its IMP for transmission
   to a foreign HOST.  The other message type of interest is a RFNM
   (Request-for-Next-Message).  RFNM's are explained in conjunction with

   The flag field of the leader controls special cases not of concern

   The link number identifies over which of 256 logical paths (links)
   between the sending HOST and the receiving HOST the message will be
   sent.  Each link is unidirectional and is controlled by the network
   so that no more than one message at a time may be sent over it.  This
   control is implemented using RFNM messages.  After a sending HOST has
   sent a message to a receiving HOST over a particular link, the
   sending HOST is prohibited from sending another message over that
   same link until the sending HOST receives a RFNM.  The RFNM is 
EID 1780 (Verified) is as follows:

Section: GLOBAL

Original Text:


Corrected Text:

This typo occurs twice on page 4.
generated by the IMP connected to the receiving HOST, and the RFNM is sent back to the sending HOST after the message has entered the receiving HOST. It is important to remember that there are 256 links in each direction and that no relationship among these is imposed by the network.
EID 1053 (Verified) is as follows:

Section: 99On page 3, it says:

Original Text:

   It is important to remember that there are 356 links
   in each direction and that no relationship among these is imposed by
   the network.

Corrected Text:

   It is important to remember that there are 256 links
   in each direction and that no relationship among these is imposed by
   the network.
The purpose of the link and RFMN mechanism is to prohibit individual users from overloading an IMP or a HOST. Implicit in this purpose is the assumption that a user does not use multiple links to achieve a wide band, and to a large extent the HOST-HOST protocol cooperates with this assumption. An even more basic assumption, of course, is that the network's load comes from some users transmitting sequences of messages rather than many users transmitting single messages coincidently. In order to delimit the length of the message, and to make it easier for HOSTs of differing word lengths to communicate, the following formatting procedure is used. When a HOST prepares a message for output, it creates a 32-bit leader. Following the leader is a binary string, called marking, consisting of an arbitrary number of zeros, followed by one. Marking makes is possible for the sending HOST to synchronize the beginning of the text message with its word boundaries. When the last bit of a message has entered an IMP, the hardware interface between the IMP and HOST appends a one followed by enough zeros to make the message length a multiple of 16 bits. These appended bits are called padding. Except for the marking and padding, no limitations are placed on the text of a message. Figure 2 shows a typical message sent by a 24-bit machine. DESIGN CONCEPTS The computers participating in the network are alike in two important respects: each supports research independent of the network, and each is under the discipline of a time-sharing system. These facts contributed to the following design philosophy. First, because the computers in the network have independent purposes it is necessary to preserve decentralized administrative control of the various computers. Since all of the time-sharing supervisors possess elaborate and definite accounting and resource allocation mechanisms, we arranged matters so that these mechanisms would control the load due to the network in the same way that they control locally generated load. Second, because the computers are all operated under time-sharing disciplines, it seemed desirable to facilitate basic interactive mechanisms. Third, because this network is used by experienced programmers it was imperative to provide the widest latitude in using the network. Restrictions concerning character sets, programming languages, etc. would not be tolerated and we avoided such restrictions. Fourth, again because the network is used by experienced programmers, it was felt necessary to leave the design open-ended. We expect that conventions will arise from time to time as experience is gained, but we felt constrained not to impose them arbitrarily. Fifth, in order to make network participation comfortable, or in some cases, feasible, the software interface to the network should require minimal surgery on the HOST operating system. Finally, we except the assumption stated above that network use consists of prolonged conversations instead of one-shot requests. These considerations led to the notions of connections, a Network Control Program, a control link, control commands, sockets, and virtual nets. A connection is an extension of a link. A connection connects two processes so that output from one process is input to the other. Connections are simplex, so two connections are needed if two processes are to converse in both directions. Processes within a HOST communicate with the network through a Network Control Program (NCP). In most HOSTs, the NCP will be a part of the executive, so that processes will use system calls to communicate with it. The primary function of the NCP is to establish connections, break connections, switch connections, and control flow. In order to accomplish its tasks, a NCP in one HOST must communicate with a NCP in another HOST. To this end, a particular link between each pair of HOSTs has been designated as the control link. Messages received over the control link are always interpreted by the NCP as a sequence of one or more control commands. As an example, one of the kinds of control commands is used to assign a link and initiate a connection, while another kind carries notification that a connection has been terminated. A partial sketch of the syntax and semantics of control commands is given in the next section. A major issue is how to refer to processes in a foreign HOST. Each HOST has some internal naming scheme, but these various schemes often are incompatible. Since it is not practical to impose a common internal process naming scheme, an intermediate name space was created with a separate portion of the name space given to each HOST. It is left to each HOST to map internal process identifiers into its name space. The elements of the name space are called sockets. A socket forms one end of a connection, and a connection is fully specified by a pair of sockets. A socket is specified by the concatenation of three numbers: (a) a user number (24 bits) (b) a HOST number (8 bits) (c) AEN (8 bits) A typical socket is illustrated in Figure 3. Each HOST is assigned all sockets in the name space which have field (b) equal to the HOST's own identification. A socket is either a receive socket or a send socket, and is so marked by the lower-order bit of the AEN (0 = receive, 1 = send). The other seven bits of the AEN simply provide a sizable population of sockets for each used number at each HOST. (AEN stands for "another eight-bit number") Each user is assigned a 24-bit user number which uniquely identifies him throughout the network. Generally this will be the 8-bit HOST number of his home HOST, followed by 16 bits which uniquely identify him at that HOST. Provision can also be made for a user to have a user number not keyed to a particular HOST, an arrangement desirable for mobile users who might have no home HOST or more than one home HOST. This 24-bit user number is then used in the following manner. When a user signs onto a HOST, his user number is looked up. Thereafter, each process the user creates is tagged with his user number. When the user signs onto a foreign HOST via the network, his same user number is used to tag processes he creates in that HOST. The foreign HOST obtains the user number either by consulting a table at login time, as the home HOST does, or by noticing the identification of the caller. The effect of propagating the user's number is that each user creates his own virtual net consisting of processes he has created. This virtual net may span an arbitrary number of HOSTs. It will thus be easy for a user to connect his processes in arbitrary ways, while still permitting him to connect his processes with those in other virtual nets. The relationship between sockets and processes is now describable (see Figure 4). For each user number at each HOST, there are 128 send sockets and 128 receive sockets. A process may request from the local NCP the use of any one of the sockets with the same user number; the request is granted if the socket is not otherwise in use. The key observation here is that a socket requested by a process cannot already be in use unless it is by some other process within the same virtual net, and such a process is controlled by the same user. An unusual aspect of the HOST-HOST protocol is that a process may switch its end of a connection from one socket to another. The new socket may be in any virtual net and at any HOST, and the process may initiate a switch either at the time the connection is being established, or later. The most general forms of switching entail quite complex implementation, and are not germane to the rest of this paper, so only a limited form will be explained. This limited form of switching provides only that a process may substitute one socket for another while establishing a connection. The new socket must have the same user number and HOST number, and the connection is still established to the same process. This form of switching is thus only a way of relabelling a socket, for no charge in the routing of messages takes place. In the next section we document the system calls and control commands; in the section after next, we consider how login might be implemented. SYSTEM CALLS AND CONTROL COMMANDS Here we sketch the mechanisms of establishing, switching and breaking a connection. As noted above, the NCP interacts with user processes via system calls and with other NCPs via control commands. We therefore begin with a partial description of system calls and control commands. System calls will vary from one operating system to another, so the following description is only suggestive. We assume here that a process has several input-output paths which we will call ports. Each port may be connected to a sequential I/O device, and while connected, transmits information in only one direction. We further assume that the process is blocked (dismissed, slept) while transmission proceeds. The following is the list of system calls: Init <port>, <AEN 1>, <AEN 2>, <foreign socket> where <port> is part of the process issuing the Init _ <AEN 1> | and +- are 8-bit AEN's (see Figure 2) <AEN 2> | _| The first AEN is used to initiate the connection; the second is used while the connection exists. <foreign socket> is the 40-bit socket name of the distant end of the connection. The lower-order bits of <AEN 1> and <AEN 2> must agree, and these must be the complement of the lower-order bit of <foreign socket>. The NCP concatenates <AEN 1> and <AEN 2> each with the user number of the process and the HOST number to form 40-bit sockets. It then sends a Request for Connection (RFC) control command to the distant NCP. When the distant NCP responds positively, the connection is established and the process is unblocked. If the distant NCP responds negatively, the local NCP unblocks the requesting process, but informs it that the system call has failed. Listen <port>, <AEN 1> where <port> and <AEN 1> are as above. The NCP retains the ports and <AEN 1> and blocks the process. When an RFC control command arrives naming the local socket, the process is unblocked and notified that a foreign process is calling. Accept <AEN 2> After a Listen has been satisfied, the process may either refuse the call or accept it and switch it to another socket. To accept the call, the process issues the Accept system call. The NCP then sends back an RFC control command. Close <port> After establishing a connection, a process issues a Close to break the connection. The Close is also issued after a Listen to refuse a call. Transmit <port>, <addr> If <port> is attached to a send socket, <addr> points to a message to be sent. This message is preceded by its length in bits. If <port> is attached to a receive socket, a message is stored at <addr>. The length of the message is stored first. Control Commands A vocabulary of control commands has been defined for communication between Network Control Programs. Each control command consists of an 8-bit operation code to indicate its function, followed by some parameters. The number and format of parameters is fixed for each operation code. A sequence of control commands destined for a particular HOST can be packed into a single control message. RFC <my socket 1>, <my socket 2>. <your socket>, (<link>) This command is sent because a process has executed either an Init system call or an Accept system call. A link is assigned by the prospective receiver, so it is omitted if <my socket 1> is a send socket. There is distinct advantage in using the same commands both to initiate a connection (Init) and to accept a call (Accept). If the responding command were different from the initiating command, then two processes could call each other and become blocked waiting for each other to respond. With this scheme, no deadlock occurs and it provides a more compact way to connect a set of processes. CLS <my socket>, <your socket> The specified connection is terminated CEASE <link> When the receiving process does not consume its input as fast as it arrives, the buffer space in the receiving HOST is used to queue the waiting messages. Since only limited space is generally available, the receiving HOST may need to inhibit the sending HOST from sending any more messages over the offending connection. When the sending HOST receives this command, it may block the process generating the messages. RESUME <link> This command is also sent from the receiving HOST to the sending HOST and negates a previous CEASE. LOGGING IN We assume that within each HOST there is always a process in execution which listens to login requests. We call this process the logger, and it is part of a special virtual net whose user number is zero. The logger is programmed to listen to calls on socket number 0. Upon receiving a call, the logger switches it to a higher (even) numbered sockets, and returns a call to the socket numbered one less than the send socket originally calling. In this fashion, the logger can initiate 127 conversations. To illustrate, assume a user whose identification is X'010005' (user number 5 at UCLA) signs into UCLA, starts up one of his programs, and this program wants to start a process at SRI. No process except the logger is currently willing to listen to our user, so he executes Init, <port> = 1, <AEN 1> = 7, <AEN 2> = 7, <foreign socket> = 0 His process is blocked, and the NCP at UCLA sends RFC <my socket 1> = X'0100050107', <my socket 2> = X'0100050107', <your socket> = X'000000200' The logger at SRI is notified when this message is received, because it has previously executed Listen <port> = 9, <AEN 1> = 0. The logger then executes Accept <AEN 2> = 88. In response to the Accept, the SRI NCP sends RFC <my socket 1> = X'0000000200' <my socket 2> = X'0000000258' <your socket> = X'0100050107' <link> = 37 where the link has been chosen from the set of available links. The SRI logger than executes Init <port> = 10 <AEN 1> = 89, <AEN 2> = 89, <foreign socket> = X'0100050106' which causes the NCP to send RFC <my socket 1> = X'0000000259' <my socket 2> = x'0000000259' <your socket> = X'0100050106' The process at UCLA is unblocked and notified of the successful Init. Because SRI logger always initiates a connection to the AEN one less than it has just been connected to, the UCLA process then executes Listen <port> = 11 <AEN 1> = 6 and when unblocked Accept <AEN 2> = 6 When these transactions are complete, the UCLA process is doubly connected to the logger at SRI. The logger will then interrogate the UCLA process, and if satisfied, create a new process at SRI. This new process will be tagged with user number X'010005', and both connections wil be switched to the new process. In this case, switching the connections to the new process corresponds to "passing the console down" in many time-sharing systems. USER LEVEL SOFTWARE At the user level, subroutines which manage data buffer and format input designed for other HOSTs are provided. It is not mandatory that the user use such subroutines, since the user has access to the network system calls in his monitor. In addition to user programming access, it is desirable to have a subsystem program at each HOST which makes the network immediately accessible from a teletype-like device without special programming. Subsystems are commonly used system components such as text editors, compilers and interpreters. An example of a network-related subsystem is TELNET, which will allow users at the University of Utah to connect to Stanford Research Institute and appear as regular terminal users. It is expected that more sophisticated subsystems will be developed in time, but this basic one will render the early network immediately useful. A user at the University of Utah (UTAH) is sitting at a teletype dialed into the University's PDP-10/50 time-sharing system. He wishes to operate the Conversational Algebraic Language (CAL) subsystem on the XDS-940 at Stanford Research Institute (SRI) in Menlo Park, California. A typical TELNET dialog is illustrated in Figure 5. The meaning of each line of dialogue is discussed here. (i) The user signs in at UTAH (ii) The PDP-10 run command starts up the TELNET subsystem at the user's HOST. (111) The user identifies a break character which causes any message following the break to be interpreted locally rather than being sent on the foreign HOST. (iv) The TELNET subsystem will make the appropriate system calls to establish a pair of connections to the SRI logger. The connections will be established only if SRI accepts another foreign user. The UTAH user is now in the pre-logged-in state at SRI. This is analogous to the standard teletype user's state after dialing into a computer and making a connection but before typing anything. (v) The user signs in to SRI with a standard login command. Characters typed on the user's teletype are transmitted unaltered through the PDP-10 (user HOST) and on to the 940 (serving HOST). The PDP-10 TELNET will have automatically switched to full-duplex, character-by- character transmission, since this is required by SRI's 940. Full duplex operation is allowed for by the PDP-10, though not used by most Digital Equipment Corporations subsystems. (vi) and (vii) The 940 subsystem, CAL, is started. At this point, the user wishes to load a local CAL file into the 940 CAL subsystem, from the file system on his local PDP-10. (viii) CAL is instructed to establish a connection to UTAH in order to receive this file. "NETWRK" is a predefined 940 name similar in nature to "PAPER TYPE" or "TELETYPE". (ix) Finally, the user types the break character (#) followed by a command to his PDP-10 TELNET program, which sends the desired file to SRI from Utah on the connection just established for this purpose. The user's next statement is in CAL again. The TELNET subsystem coding should be minimal for it is essentially a shell program built over the network system calls. It effectively established a shunt in the user HOST between the remote user and a distant serving HOST. Given the basic system primitives, the TELNET subsystem at the user HOST and a manual for the serving HOST, the network can be profitably employed by remote users today. HIGHER LEVEL PROTOCOL The network poses special problems where a high degree of interaction is required between the user and a particular subsystem in a foreign HOST. These problems arise due to heterogeneous consoles, local operating systems overhead, and network transmission delays. Unless we use special strategies it may be difficult or even impossible for a distant user to make use of the more sophisticated subsystems offered. While these difficulties are especially severe in the area of graphics, problems may arise even for teletype interaction. For example, suppose that a foreign subsystem is designed for teletype consoles connected by telephone, and then this subsystem becomes available to network users. This subsystem might have the following characteristics. 1. Except for echoing and correction of mistyping, no action is taken until a carriage return is typed. 2. All characters except "^", and "<-" and carriage returns are echoed as the character is typed. 3. <- causes deletion of the immediately preceding character, and is echoed as that character. 4. ^ causes all previously typed characters to be ignored. A carriage return and line feed are echoed. 5. A carriage return is echoed as a carriage return followed by a line feed. If each character typed is sent in its own message, then the characters H E L L O <- <- P c.r. cause nine messages in each direction. Furthermore, each character is handled by a user level program in the local HOST before being sent to the foreign HOST. Now it is clear that if this particular example were important, we would quickly implement rules 1 to 5 in a local HOST program and send only complete lines to the foreign HOST. If the foreign HOST program could not be modified so as to not generate echoes, then the local program could not only echo properly, it could also throw away the later echoes from the foreign HOST. However, the problem is not any particular interaction scheme; the problem is that we expect many of these kinds of schemes to occur. We have not found any general solutions to these problems, but some observations and conjectures may lead the way. With respect to heterogeneous consoles, we note that although consoles are rarely compatible, many are equivalent. It is probably reasonable to treat a model 37 teletype as the equivalent of an IBM 2741. Similarly, most storage scopes will form an equivalence class, and most refresh display scopes will form another. Furthermore, a hierarchy might emerge with members of one class usable in place of those in another, but not vice versa. We can imagine that any scope might be an adequate substitute for a teletype, but hardly the reverse. This observation leads us to wonder if a network-wide language for consoles might be possible. Such a language would provide for distinct treatment of different classes of consoles, with semantics appropriate to each class. Each site could then write interface programs for its consoles to make them look like network standard devices. Another observation is that a user evaluates an interactive system by comparing the speed of the system's responses with his own expectations. Sometimes a user feels that he has made only a minor request, so the response should be immediate; at other times he feels he has made a substantial request, and is therefore willing to wait for the response. Some interactive subsystems are especially pleasant to use because a great deal of work has gone into tailoring the responses to the user's expectations. In the network, however, a local user level process intervenes between a local console and a foreign subsystem, and we may expect the response time for minor requests to degrade. Now it may happen that all of this tailoring of the interaction is fairly independent of the portion of the subsystem which does the heavy computing or I/O. In such a case, it may be possible to separate a subsystem into two sections. One section would be a "front end" which formats output to the user, accepts his input, and controls computationally simple responses such as echoes. In the example above, the program to accumulate a line and generate echoes would be the front end of some subsystem. We now take notice of the fact that the local HOSTs have substantial computational power, but our current designs make use of the local HOST only as a data concentrator. This is somewhat ironic, for the local HOST is not only poorly utilized as a data concentrator, it also degrades performance because of the delays it introduces. These arguments have led us to consider the possibility of a Network Interface Language (NIL) which would be a network-wide language for writing the front end of interactive subsystems. This language would have the feature that subprograms communicate through network-like connections. The strategy is then to transport the source code for the front end of a subsystem to the local HOST, where it would be compiled and executed. During preliminary discussions we have agreed that NIL should have at least the following semantic properties not generally found in other languages. 1. Concurrency. Because messages arrive asynchronously on different connections, and because user input is not synchronized with subsystem output, NIL must include semantics to accurately model the possible concurrencies. 2. Program Concatenation. It is very useful to be able to insert a program in between two other programs. To achieve this, the interconnection of programs would be specified at run time and would not be implicit in the source code. 3. Device substitutability. It is usual to define languages so that one device may be substituted for another. The requirement here is that any device can be modeled by a NIL program. For example, if a network standard display controller manipulates tree-structures according to messages sent to it then these structures must be easily implementable in NIL. NIL has not been fully specified, and reservations have been expressed about its usefulness. These reservations hinge upon our conjecture that it is possible to divide an interactive system into a transportable front end which satisfies a user's expectations at low cost and a more substantial stay-at-home section. If our conjecture is false, then NIL will not be useful; otherwise it seems worth pursuing. Testing of this conjecture and further development of NIL will take priority after low level HOST-HOST protocol has stabilized. HOST/IMP INTERFACING The hardware and software interfaces between HOST and IMP is an area of particular concern for the HOST organizations. Considering the diversity of HOST computers to which a standard IMP must connect, the hardware interface was made bit serial and full-duplex. Each HOST organization implements its half of this very simple interface. The software interface is equally simple and consists of messages passed back and forth between the IMP and HOST programs. Special error and signal messages are defined as well as messages containing normal data. Messages waiting in queues in either machine are sent at the pleasure of the machine in which they reside with no concern for the needs of the other computer. The effect of the present software interface is the needless rebuffering of all messages in the HOST in addition to the buffering in the IMP. The messages have no particular order other than arrival times at the IMP. The Network Control Program at one HOST (e.g., UTAH) needs waiting RFNM's before all other messages. At another site (e.g., SRI), the NCP could benefit by receiving messages for the user who is next to be run. What is needed is coding representing the specific needs of the HOST on both sides of the interface to make intelligent decisions about what to transmit next over the channel. With the present software interface, the channel in one direction once committed to a particular message is then locked up for up to 80 milliseconds! This approaches one teletype character time and needlessly limits full- duplex, character by character, interactions over the net. At the very least, the IMP/HOST protocol should be expended to permit each side to assist the other in scheduling messages over the channels. CONCLUSIONS At this time (February 1970) the initial network of four sites is just beginning to be utilized. The communications system of four IMPs and wide band telephone lines have been operational for two months. Programmers at UCLA have signed in as users of the SRI 940. More significantly, one of the authors (S. Carr) living in Palo Alto uses the Salt Lake PDP-10 on a daily basis by first connecting to SRI. We thus have first hand experience that remote interaction is possible and is highly effective. Work on the ARPA network has generated new areas of interest. NIL is one example, and interprocess communication is another. Interprocess communication over the network is a subcase of general interprocess communication in a multiprogrammed environment. The mechanism of connections seems to be new, and we wonder whether this mechanism is useful even when the processes are within the same computer. REFERENCES 1 L. ROBERTS "The ARPA network" Invitational Workshop on Networks of Computers Proceedings National Security Agency 1968 p 115 ff 2. R M RUTLEDGE et al "An interactive network of time-sharing computers" Proceedings of the 24th National Conference Association for Computing Machinery 1969 p 431 ff 3. F E HEART R E KAHN S M ORNSTEIN W R CROWTHER D C WALDEN "The interface message processors for the ARPA network" These Proceedings LIST OF FIGURES Figure 1 Initial network configuration Figure 2 A typical message from a 24-bit machine Figure 3 A typical socket Figure 4 The relationship between sockets and processes Figure 5 A typical TELNET dialog. Underlined characters are those types by the user. SRI _____ / \ | XDS | | 940 | \_____/ | +----------+ | IMP | +----------+ / | \ / | \ / | \ +----+ _____ / | \ | I | / \ ______ +----+ / | \| M |--| DEC | / \ | I |/ | | P | | PDP-10| | IBM |---| M | | +----+ \_____/ | 360/75 | | P |\ | \______/ +----+ \ | UTAH \ | UCSB \ | +----------+ | IMP | +----------+ | ___|___ / \ | XDS | |(sigma)-7| \_______/ UCLA Figure 1 Initial network configuration |<------------ 24bits ----------->| | | +---------------------------------+ | | | Leader (32 bits) | | __________________| | | 100 --- ----0 |<----16 bits of marking +--------------+------------------+ | | | | | Text of messages (96 bits) | | | +------------------------+--------+ | 100----- ----0| +-------^----------------+ | |______16 bits of padding added by the interface Figure 2 A typical message from a 24-bit machine 24 8 8 +----------------------+-----------+----------+ | User Number | | | +----------------------+-----------+----------+ | |___AEN | |___HOST number Figure 3 A typical socket |<--- connection --->| +---------+ +---------+ | | link | | | process |--(|--------------|)--| process | | | ^ ^ | | +---------+ | | +---------+ | | send socket receive socket Figure 4 The relationship between sockets and processes [ This RFC was put into machine readable form for entry ] [ into the online RFC archives by Lorrie Shiota 08/00]