faqs.org - Internet FAQ Archives

RFC 929 - Proposed Host-Front End Protocol


Or Display the document by number




Network Working Group                              Joel Lilienkamp (SDC)
Request for Comments: 929                          Richard Mandell (SDC)
                                         Michael Padlipsky (Mitre Corp.)
                                                           December 1984

                    PROPOSED HOST-FRONT END PROTOCOL

Status Of This Memo

   The reader should be aware of several things in regard to what the
   present document is up to.  First and foremost, IT IS A PROPOSAL FOR
   A STANDARD, NOT A STANDARD ITSELF.  Next, it assumes that the
   separate document, RFC 928, which is an introduction to the present
   document, has been read before it is. Next, it should be understood
   that "final cut" over this version of the document has been exercised
   by the author of RFC 928, not by the primary author of the present
   document, so any readers bothered by style considerations should feel
   free to blame the former, who's used to it, rather than the latter,
   who may well be guiltless.  (Editing at a distance finally become too
   hard to manage, so if I'm typing it myself I'm going to fiddle with
   it myself too, including, but not limited to, sticking my own section
   on the Conceptual Model in before Joel's words start, rather than
   leaving it in the Introduction.  MAP)

   Finally, it should be noted that this is not a finished document.
   That is, the intent is eventually to supply appendices for all of the
   protocol offloadings, describing their uses of protocol idiosyncratic
   parameters and even their interpretations of the standard per-command
   parameters, but in order to get what we've got into circulation we
   haven't waited until all such appendices have been written up.  (We
   do have notes on how to handle FTP, e.g., and UDP will be pretty
   straightforward, but getting them ready would have delayed things
   into still another calendar year, which would have been very annoying
   ... not to say embarrassing.) For that matter, it's not even a
   finished document with respect to what is here. Not only is it our
   stated intention to revise the protocol based upon implementation
   experience gained from volunteer test implementations, but it's also
   the case that it hasn't proven feasible to iron out all known
   wrinkles in what is being presented.  For example, the response codes
   almost certainly need clarification and expansion, and at least one
   of us doesn't think mandatory initial parameters need control flags.
   However, to try too hard for polish would be to stay in subcommittee
   for the better part of forever, so what you see is what we've got,
   but certainly isn't meant to be what you or we are stuck with.

   This RFC suggests a proposed protocol for the ARPA-Internet
   community, and requests discussion and suggestions for improvements.
   Distribution of this memo is unlimited.

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

Conceptual Model

   There are two fundamental motivations for doing outboard processing.
   One is to conserve the Hosts' resources (CPU cycles and memory) in a
   resource sharing intercomputer network, by offloading as much of the
   required networking software from the Hosts to Outboard Processing
   Environments (or "Network Front-Ends") as possible. The other is to
   facilitate procurement of implementations of the various
   intercomputer networking protocols for the several types of Host in
   play in a typical heterogeneous intercomputer network, by employing
   common implementations in the OPE.  A third motivation, of basing a
   network security approach on trusted mandatory OPEs, will not be
   dealt with here, but is at least worthy of mention.

   Neither motivation should be allowed to detract from the underlying,
   assumed desire to perform true intercomputer networking, however.
   Therefore, it is further assumed that OPEs will be attached to Hosts
   via a flexible attachment strategy, as described in [1]. That is, at
   the software level an explicit Host-Front End Protocol (H-FP) will be
   employed between Hosts and OPEs, rather than having OPEs emulate
   devices or device controllers already "known" to Host operating
   systems (in order to avoid introducing new code into the Host).

   For reasons discussed in the Introduction, an H-FP resolves into
   three layers.  The Link layer enables the exchange of bits between
   Host and OPE.  The Channel layer enables the bit streams to be
   demultiplexed and flow controlled  (both the Channel and Link layers
   may use preexisting per-Host mechanizations, it should be recalled).
   The Command (or "Service Access") layer is our primary concern at
   present. It serves as the distributed processing mechanism which
   allows processes on Hosts to manipulate protocol interpreters (PIs)
   in OPEs on their behalf; for convenience, it will be referred to as
   "the H-FP" here.  (It should be noted that the Link and Channel
   layers may be viewed as roughly equivalent to the inboard processing
   investment for a Host-comm subnet processor PI and device driver, so
   in practical terms the savings of resources achieved by outboard
   processing come from making the H-FP "smaller" than the inboard
   implementations of the protocols it allows to be offloaded.)

   The crucial property of the H-FP conceptually is that it stands as
   the interface between a (Host) process and a PI (which is actually
   outboard).  Usually, the model is that of a closed subroutine
   interface, although in some cases an interprocess communication
   mechanism model must be appealed to.  That is, the interactions
   between cooperating H-FP PIs in some sense mimic subroutine or IPC
   calls, from the perspective of Host processes calling upon their own
   H-FP PIs, which in turn are of course interfacing via just such

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

   mechanisms themselves. Another way of putting it is that "if the
   protocols were inboard," the processes invoking H-FP wouldn't know
   the difference.  H-FP, then, may be viewed as a roundabout way of
   letting Host processes "get at" various PIs.

   Naturally, the mechanization of the desired concept cannot be
   particularly literal.  After all, the Hosts and the OPEs are
   different processors, so we're not envisioning a passing through of
   parameters in an exact fashion.  However, in broad terms the model is
   just that of a somewhat funny interface between a process and a PI.
   (This should not be construed as ruling out the occurrence of events
   which prompt the OPE to initiate an exchange of commands with the
   Host, though; see the Introduction for more on the topic of
   "Symmetric Begins.")

Interaction Discipline

   The interaction between the Host and the OPE must be capable of
   providing a suitable interface between processes (or protocol
   interpreters) in the Host and the off-loaded protocol interpreters in
   the OPE.  This interaction must not, however, burden the Host more
   heavily than would have resulted from supporting the protocols
   inboard, lest the advantage of using an OPE be overridden.

   Channel Level Interaction

   As stated elsewhere, the Channel level protocol (implicitly in
   conjunction with the Link level) provides two major functions. These
   are demultiplexing the traffic from the Link level into distinct data
   streams, and providing flow control between the Host and the OPE on a
   per stream basis.  These hold even if the Host-OPE attachment is DMA.

   The data streams between the Host and the OPE are bidirectional. In
   this document, the basic unit of data transferred by the Channel
   level is referred to as a "chunk".  The primary motivation for this
   terminology is that the H-FP permits the Channel level to be one of
   several possible protocols, each with its own terminology.  For
   example, a chunk on an X.25 Channel would be a packet, while a chunk
   on the DTI H-FP channel would be a message.  While the Command level
   is, in a sense, "more efficient" when the chunk size is permitted to
   be large, the flexibility permitted in the choice of protocols at the
   Channel level precludes any assumptions about the chunk size.

   Each data stream is fully asynchronous.  A Channel protocol user can
   send data at any time, once the channel has been properly opened.
   (The Command level's logic may render some actions meaningless,
   however.) The data transfer service provided by the Channel protocol

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

   is reliable;  this entails delivery in the correct order, without
   duplication, and checked for bit errors.  All retransmission, error
   checking, and duplicate detection is provided by this protocol in a
   way that is transparent to the user.  (If the attachment is DMA,
   stream identification and chunk length must still be provided for.)

   The flow control at the Channel level is provided to prevent the OPE
   and the Host from overloading each other's resources by excessive
   transmissions.  In general, this flow control should not directly
   affect the outboard protocol interpreters' operation.  On the other
   had, this flow control has the same effect as explicit interface
   events that provide flow control between the user and the protocol
   interpreter (e.g., the Allocate event of the interface specification
   for TCP found in MIL-STD 1778).  Hence, such events do not need to be
   communicated explicitly at the Command level.  (If the attachment is
   DMA, flow control must still be provided for.)

   Should Hosts require an OPE to be attached via a Link Level that
   furnishes physical demultiplexing (e.g., a group of RS232 ports), any
   attempt to avoid furnishing reliability and explicit flow control, is
   done at their peril;  we have not chosen to assist such an
   enterprise, but neither have we precluded it.  (It would certainly
   violate the spirit of the thing, however.)

   Command Level Interaction

   The approach chosen for this H-FP is to base the interaction on a
   small set of commands, separately applicable to a given Channel Level
   channel. The commands are simple, but sufficiently flexible to permit
   the off-loading of the interpreters of the large number of protocols
   at various levels in the hierarchy.  This flexibility is made
   possible in part by the similar nature of the interfaces to most
   protocols, combined with the provision of "protocol idiosyncratic
   parameters". These parameters are defined for each offloaded protocol
   interpreter in the OPE.  The use of such parameters does not
   complicate the basic design of the OPE, since it must be customized
   for each off-loaded protocol anyway, and all that is required of the
   OPE for those parameters is to pass them to the off-loaded protocol
   interpreter.  Hence, an interface tailored to a particular protocol
   can be created in a straightforward and cost-effective way.

   The command dialog is more or less asynchronous.  Commands can be
   issued at any particular time (except when there is a pending
   command, which will be discussed below), and there is no need for
   dummy traffic on a channel when no commands are issued.

   Associated with each command is a response.  The purpose of this

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

   response is to indicate, at some level that depends in part on the
   particular protocol interpreter that is offloaded to the OPE, whether
   the command was successfully executed, and if unsuccessful, the
   reason.  Often, generating the response involves interaction with the
   protocol interpreter before a response can be generated.

   When a command is issued, the issuer must wait for a response before
   another command is issued.  The nature of the communication between
   the Host and the OPE is thus a lock step command/response dialog.
   There are two major exceptions to this principle, however. One
   exception is the abrupt form of the End command, which can be issued
   at any time to cancel any previously issued commands, and indicate
   that services are no longer desired.  The other exception is the
   Signal command.  Since a Signal is out-of-band and usually of high
   importance, forcing it to wait on a response would be undesirable.
   Hence, a Signal command can be issued while commands (other than
   Signal) are pending.  However, a Signal command should not be issued
   before a successful response to the Begin command has been received.
   Since it is possible for more than one command of different types to
   be pending at one time, a mechanism to distinguish responses is
   needed.  Since there are never two commands of the same type pending,
   including the command name in the response is sufficient to make this
   distinction.

   A special case command is the Transmit command.  Details of the
   Transmit command are provided in the next section. Essentially, the
   Transmit command is used to invoke the data transfer services of the
   off-loaded protocol (when issued by the Host) or to indicate the
   arrival of new data from the network (when issued by the OPE).  The
   nature of specific protocol interfaces for these events varies widely
   between protocols.  Some may block until the data is accepted by the
   remote counterpart (or "peer") protocol interpreter, while others may
   not.  Hence, there is a special parameter which indicates the nature
   of the Transmit command interface.  It can either require that the
   response should be generated immediately after determining the
   Transmit command is complete and formed properly, or can indicate
   that the response should not be generated until the appropriate
   interface event is given by the remote protocol interpreter.  The
   default action for all Transmit commands can be initialized using the
   Begin command and changed using the Condition command.  Also, the
   default action can be temporarily overridden by specifying a
   parameter with the Transmit command. The net result of this mechanism
   is to allow the Host to determine within reason just how lock-stepped
   transmissions are to be.  (It is assumed that the usual case will be
   to transfer the burden of buffering to the OPE by taking immediate
   responses, provided that doing so "makes sense" with the particular
   offloaded protocol in play.)

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

   Some protocols provide a block-oriented data transfer service rather
   than a stream-oriented one.  With such a service, the data associated
   with a transfer request is viewed as an integral unit.  For actual
   network transmission, the protocol may permit these units to be
   grouped or fragmented. However, the receiving end must deliver the
   data in the original, integral units. Protocols that conform to this
   model include some datagram protocols such as IP and UDP, and also
   some connection protocols such as NBS TP.

   To cater to these types of protocols, it is a convention that
   commands, their parameters, and any associated data be transferred
   between the Host and the OPE in a single chunk. Any data associated
   with an H-FP command is viewed as an integral unit which is used in
   the corresponding service request given to the outboard protocol
   interpreter or delivered as a complete unit to the process in the
   Host. Operation of stream-oriented protocols such as TCP will not be
   adversely affected by this convention.

   To accommodate Channel protocols that do not provide for arbitrarily
   large chunks, a mechanism at the Command level is required to permit
   the linking of multiple chunks into a single command, in order to
   transfer the burden of buffering as much as possible from the Host to
   the OPE.  The facility proposed here would consist of an indication
   at the beginning of each chunk which would distinguish integral
   commands, fragments of a command for which more fragments are yet to
   arrive, and the final fragment of a command.  The details of this
   mechanism are discussed in the section on the syntax of commands and
   responses.

   It is a convention for this H-FP that any data associated with a
   command must start on a word boundary (as defined by the local
   system).  Consequently, there is a need to provide padding within the
   commands.  Such padding is used only to fill to the next appropriate
   boundary, and has no semantic significance to the command interpreter
   (i.e., two commands that are identical except for the amount of
   padding should behave identically).  The details of this padding are
   discussed in the section on the syntax of commands and responses.

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

Syntax Rules

   At the Command Level, communication between the Host and the OPE
   takes the form of commands and responses.  A command is a request for
   some particular action, and the response indicates the success or
   failure of performing the requested action.

   All commands and responses are coded in ASCII characters. (Nothing
   precludes OPEs from accepting EBCDIC from Hosts that use it in native
   mode, but that is not required.) These characters are sent in some
   way convenient for the Host, and the OPE is sufficiently flexible to
   interpret them.  (i.e., OPEs are expected to accommodate Host
   idiosyncracies in regard to such things as use of 7-bit ASCII in a
   9-bit field.) This approach offers several advantages:

   Adaptabilities in most Hosts:  Most Hosts have the ability to
   generate and interpret ASCII character streams.  Hence, integrating
   H-FP into a Host will not require difficult software.

   Script generation:  Generation of test and operational command
   scripts will be simplified, since they will not need to contain
   special characters.

   Terminal Operation:  Using simple command streams simplifies the
   conversion of an OPE to a generic virtual terminal support machine.
   This is particularly useful during development and testing.

   Testing:  Testing will not require special hardware to interpret
   commands and responses.  A terminal or data line analyzer would be
   adequate.

   The specific format for the commands and responses will be discussed
   in the sections that follow. In those sections, the quote character
   is used to indicate strings.  The symbols "<" and ">" (referred to as
   angle brackets) are used as meta-characters.

   Syntax of Commands

   As alluded to in the section discussing the interaction discipline
   between the Host and the OPE, a function is provided by which a chunk
   can be used to carry either a complete command or a fragment of a
   command.  The mechanism chosen to provide this function entails use
   of the first character position in the chunk as a chunk usage
   identifier.  The character "C" in the first position indicates a
   chunk containing a single, complete command.  "F" in the first
   position indicates a chunk which is the first part of a multichunk
   command. "M" in the first position indicates the chunk is a middle

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

   part (neither the first nor the last chunk) of a command.  Finally,
   "L" indicates the chunk is the last chunk of a multi-chunk command.
   Hence, the following sequences of chunks (the letter corresponds to
   the chunk usage identifier in each chunk, and the angle brackets
   enclose a chunk) are legal:

      <C>
      <F><L>
      <F><M><M><L>

   while the following are not legal:

      <L>
      <M><L>
      <F><C>

   Tactics for handling multiple chunks with regard to OPE buffering
   limits are left to the ingenuity of OPE builders. The spirit is to
   take as much as you can, in order to relieve the Host of the
   necessity of buffering itself.

   A command always begins immediately following the indicator
   character, with possible intervening spaces.  This implies a chunk
   can contain at most one complete command.  The end of the command
   (not including the data) is signified by a newline (denoted as <nl>
   in this document) that does not appear inside a quoted string (see
   below).  The end of the data is designated by the end of the last
   chunk.

   Commands take the form of an ASCII string.  The command identifier is
   the first word of the chunk.  It consists of at least the first two
   letters of the command, in either upper or lower case (e.g., the
   sequences "BE", "Be", "bE", and "be" all identify the Begin command).
   Additional letters of the command name can be included if desired to
   aid readability of the command stream.

   Following the command identifier is a list of parameters. These
   parameters are also represented as ASCII strings, although the
   specific format will depend on the particular parameter.  The data to
   be transmitted is not considered a control parameter, however, and
   need not be ASCII data.

   Parameters are separated by one or more spaces.  Tabs, newlines, and
   other white space are not legal parameter separators.

   Parameter strings may be quoted, using the character <">. Any

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

   characters between the <"> characters are a part of the parameter,
   including spaces and newlines.  The character <"> that is part of the
   parameter is represented inside a quoted string as <"">.

   The order in which the parameters appear within the command is
   significant to their interpretation by the Host and by the OPE.
   Optional parameters may be skipped by using the characters ",," to
   indicate a NULL parameter.  Such a NULL parameter takes its default
   value.  Alternatively, each parameter has a MULTICS/UNIX style
   Control Argument/Flag associated with it that can be used to identify
   the parameter, without placing NULL parameters for each parameter
   skipped.  This flag consists of one or two ASCII characters, and
   either upper or lower case may be used.  For example, if the fourth
   parameter of a command had a flag of "-p" and the user wished the
   first three parameters to be null, he could use:

      command -p value

   or

      command -P value

   instead of

      command ,, ,, ,, value

   if it were more convenient for the Host to do so.  Flagged parameters
   must still appear in the correct sequence within the command,
   however.

   There may be data associated with some of the commands.  Any such
   data is placed into the chunk following all the parameters and the
   unquoted newline. Padding can be provided by placing spaces between
   the end of the final parameter string and the newline, so that data
   begins on a word boundary. The OPE will always pad to a host word
   boundary.  Padding by hosts is optional.

   Syntax of Responses

   Responses are actually just a special form of a command.  It is
   anticipated that all responses would fit into a single channel chunk,
   although the mechanisms described for multichunk commands can
   certainly be used in responses.  The ASCII string used to uniquely
   identify the response command is "RE" ("Re", "rE", and "re" are also
   permitted).

   After the response command identifier is the original command

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

   identifier, so the response can be associated with the proper
   command.  Following this identifier is a three ASCII digit response
   code, a set of protocol idiosyncratic parameters, and a textual
   message.  The protocol idiosyncratic parameters are used to transfer
   interface information between the Host and the OPE, and may not be
   needed when off-loading some protocol interpreters.  The textual
   message is intended for human interpretation of the response codes,
   and is not required by the protocol.  The three digits uniquely
   identify the semantics of the response, at least within the context
   of a particular command and particular outboarded protocol
   interpreter.

   Responses are numerically grouped by the type of information they
   convey.  The first digit identifies this group, and the last two
   digits further qualify the reply.  The following list illustrates
   this grouping.

      0XX Successful:  The command was executed successfully. The
          response code may contain further information.

      1XX Conditional Success:  The command was executed successfully,
          but not exactly according to the service and flow control
          suggestions.  If those suggestions were particularly important
          to the requester, he may wish to issue an End command.  The
          response code contains information on what suggestion or
          suggestions could not be followed.

      2XX Command Level Error:  An error at the command level has
          occurred.  This could include requesting services of a
          protocol not supported, or a problem in the way those services
          were requested.  This level does not include problems with the
          syntax of the command or its parameters.

      3XX Syntax and Parameter Errors:  An error in the syntax of the
          command or a problem with one of its parameters has occurred.
          A problem with a parameter may be other than syntactical, such
          as illegal address.

      4XX Off-loaded Protocol Interpreter Problems:  Some problem with
          the particular off-loaded protocol has occurred.

      5XX Local OPE Internal Problems:  Problems, such as insufficient
          OPE resources, or problems with OPE to subnet interface.

      6XX Security Problem:  Some problem with Host, network, or OPE
          security has occurred.  The response code indicates the
          problem.

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

      7XX Reserved for Future Expansion

      8XX Reserved for Future Expansion

      9XX Protocol Idiosyncratic Errors:  Some error occurred that is
          idiosyncratic to the particular off-loaded protocol being
          used.  The response code indicates the error.

Description of the Commands

   As stated above, communication between the Host and the OPE at the
   Command Level is accomplished using commands and responses.  Commands
   may be issued by either the Host or the OPE, and are used to
   stimulate activity in the other entity. Some commands may only have a
   meaningful interpretation in one direction, however.  A response
   indicates that the activity started by the command was completed, and
   a code indicates success or failure of the command, and perhaps other
   information related to the command as well.

   Associated with each command is a set of parameters.  The order in
   which the parameters appear is significant to the correct operation
   of the protocols.  More information on the syntax of command
   parameters can be found in the syntax descriptions.

   The commands are:

      - Begin: initiate communication between a process in the Host and
      an off-loaded protocol interpreter in the OPE.  (A Channel level
      stream/connection will typically have been opened as a prior step.
      All other commands, except No-op, apply to a stream on which a
      successful Begin has been done.)

      - Transmit: transmit data between a process in the Host and an
      off-loaded protocol interpreter in the OPE.

      - Signal:  cause an out-of-band signal to be sent by the
      off-loaded protocol interpreter to its peer, or indicate the
      arrival of such a signal from the remote side.

      - Condition: alter the off-loaded protocol interpreter's
      operational characteristics.

      - Status: transfer status requests or information between a
      process in the Host and an off-loaded protocol interpreter in the
      OPE.

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

      - End: indicate that services from the off-loaded protocol
      interpreter are no longer required, or will no longer be provided.

      - No-op:  performs no operation, but facilitates testing.

   These commands will be discussed in the following sections. Each of
   these sections includes a discussion of the purpose of the command, a
   description of each of the parameters used with the command, a list
   of responses for the command, an example of the command, and a set of
   notes for the implementor.  (An appendix will eventually be furnished
   for each protocol offloading, showing the use of its protocol
   idiosyncratic parameters as well as of the general parameters on a
   per-command basis.  Initially, only representative offloadings will
   be treated in appendices, with others to be added after the protocol
   gains acceptance.)

   Begin

      Purpose of the Begin Command

         The purpose of a Begin command is to initiate communication
         between the Host and the OPE on a particular stream or channel
         (the channel is opened as a separate step, of course). The
         interpretation of the command is somewhat dependent upon
         whether it was issued by the Host of the OPE.

         - If the command was issued by the Host, it means some process
         in the Host is requesting services of a protocol that was
         off-loaded to the OPE.  The user request results in the
         establishment of a channel connection between the Host and the
         OPE, and a Begin command to the Command interpreter in the OPE.

         - If the command was issued by the OPE, it means some protocol
         interpreter in the OPE has data for some process in the Host
         which is not currently known by the OPE.  An example would be
         an incoming UDP datagram on a new port, or if no Begin for UDP
         had been issued at all by the Host.  (An incoming TCP
         connection request could be handled by a response to the user's
         Passive Open request, which had previously caused a Begin
         request from the Host; an incoming TCP connection request to a
         port on which no Listen had been issued would cause an OPE
         generated Begin, however.)

         As indicated earlier, any particular Host is not required to
         support two-way Begins.

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

      Parameters of the Begin Command

         The Begin command has several parameters associated with it.
         These parameters contain information needed by the offloaded
         protocol to provide an adequate level of network service.  This
         information includes protocol, source and destination
         addresses, and also type of service and flow control advice.
         These parameters are discussed in detail below.

      Protocol

         The protocol parameter identifies that off-loaded protocol in
         the OPE to which Begin is directed, or which issued the Begin
         to the Host.  For example, if the user wished to utilize TCP
         services, and the TCP software was off-loaded into the OPE,
         then the Protocol parameter for the Begin command would be TCP.

         There are two categories of protocol parameters -- generic and
         specific.  A generic parameter identifies a type of protocol
         service required, but does not identify the actual protocol.
         Use of generic protocols allows a Host process to obtain
         network services without specific knowledge of what protocol is
         being used; this could be appropriate for use in situations
         where no specific aspect(s) of a specific protocol is/are
         required.  For example, the user may select a generic
         Host-to-Host connection protocol, and (at some point in the
         future) may actually receive services from either TCP or the
         NBS Transport Protocol, depending on the network (or even the
         foreign Host) in question.  A specific protocol parameter
         identifies some particular protocol, e.g., TCP, whose use is
         required for the given channel.

         The valid entries for the protocol field include:

            Generic   Specific  Comment

            GIP       IP        Datagram Internetwork Protocol
            HHP       TCP       Connection Transport/Host-Host Protocol
            GDP       UDP       Datagram Transport/Host-Host Protocol
            VTP       TEL       Virtual Terminal (Telnet) Protocol
            GFP       FTP       File Transfer Protocol
            MAIL      SMTP      Mail Transfer Protocol
            PROX      PROX      Proximate Net Interface Protocol

         (Note that the final line is meant to allow for a process in an
         OPE'd Host's getting at the PI of the Network Interface
         Protocol for whatever the proximate network is.  Of course, so

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

         doing only makes sense in specialized contexts.  We conceive of
         the desirability of "pumping bits at a peripheral" on a LAN,
         though, and don't want to preclude it, even if it would be
         impossible on many LAN's to deal with the problem of
         distinguishing traffic coming back on the LAN in this "raw"
         mode from normal, IP traffic.  Indeed, in some contexts it is
         likely that administrative considerations would preclude
         avoidance of IP even if technical considerations allowed it,
         but it's still the case that "the protocol" should provide a
         hook for going directly to the L I protocol in play.)

         There is no default value for this parameter.  If it is not
         present, the Begin command is in error.  The control flag for
         this parameter is -pr.

      Active/Passive

         The Active/Passive parameter indicates whether the issuer of
         the Begin command desires to be the Active or Passive user of
         the protocol.  This parameter is particularly relevant to
         connection-oriented protocols such as TCP, where the user may
         actively pursue connection establishment, or else may passively
         wait for the remote entity to actively establish the
         connection; it also allows some process to establish itself as
         the Host "fielder" of incoming traffic for a connectionless
         protocol such as IP.

         Active is requested using the single character "A".  Passive is
         indicated using the character "P".  The default value of this
         parameter is "A". Also, when the OPE issues the Begin command,
         the value must be "A".  The control flag for this parameter is
         -ap.

      Foreign Address Primary Component

         The addressing structure supported by H-FP is two level. Each
         address has two components, the primary and the secondary.  The
         exact interpretation of these two components is protocol
         specific, but some generalities do apply.  The primary
         component of the address identifies where the protocol is to
         deliver the information. The secondary component identifies
         which recipient at that location is to receive the information.
         For example, the TCP primary address component is the Host's
         Internet Address, while the secondary address component is the
         TCP port.  Similarly, IP's primary address component is the
         Host's Internet Address, and the secondary address component is
         the IP ULP field.  Some protocols provide only a single level

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

         of addressing, or the secondary level can be deduced from some
         other information (e.g., Telnet).  In these cases, only the
         primary component is used.  To cater to such cases, the
         secondary component parameter comes later in the parameter
         list.

         The Foreign Address Primary Component parameter contains the
         primary component of the destination address.  It may be in
         either a numeric or symbolic form.  (Note that this allows for
         the OPE to exercise a Name Server type of protocol if
         appropriate, as well as freeing the Host from the necessity of
         maintaining an in-board name to address table.) The default
         value for this parameter, although it only makes sense for
         Passive Begins, is "Any Host".  The control flag for this
         parameter is -fp.

      Mediation Level

         The mediation level parameter is an indication of the role the
         Host wishes the OPE to play in the operation of the protocol.
         The extreme ranges of this mediation would be the case where
         the Host wished to remain completely uninvolved, and the case
         where the Host wished to make every possible decision.  The
         specific interpretation of this parameter is dependent upon the
         particular off-loaded protocol.

         The concept of mediation level can best be clarified by means
         of example.  A full inboard implementation of the Telnet
         protocol places several responsibilities on the Host. These
         responsibilities include negotiation and provision of protocol
         options, translation between local and network character codes
         and formats, and monitoring the well-known socket for incoming
         connection requests.  The mediation level indicates whether
         these responsibilities are assigned to the Host or to the OPE
         when the Telnet implementation is outboard.  If no OPE
         mediation is selected, the Host is involved with all
         negotiation of the Telnet options, and all format conversions.
         With full OPE mediation, all option negotiation and all format
         conversions are performed by the OPE.  An intermediate level of
         mediation might have ordinary option negotiation, format
         conversion, and socket monitoring done in the OPE, while
         options not known to the OPE are handled by the Host.

         The parameter is represented with a single ASCII digit.  The
         value 9 represents full OPE mediation, and the value 0
         represents no OPE mediation.  Other values may be defined for

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

         some protocols (e.g., the intermediate mediation level
         discussed above for Telnet).  The default value for this
         parameter is 9.  The control flag for this parameter is -m.

      Transmit Response Discipline

         The Transmit Response Discipline parameter is used to set the
         desired action on the OPE's part for generating responses to
         Transmit commands.  Essentially the parameter determines when
         the OPE's response to the transmit command occurs (i.e.,
         immediately or delayed).

         The Transmit Response Discipline value is represented by a
         single ASCII character.  The character "N" is used for
         nonblocking Transmit commands, which implies that responses for
         Transmit commands should be generated as soon as the command
         has been examined for correctness (i.e., that the syntax is
         good and the parameters appear reasonable).  In other words,
         the outboard protocol interpreter has the data in its queue,
         but hasn't necessarily transmitted it to the net.  The
         character "B" is used for blocking Transmit commands, which
         requests that the response not be generated until the protocol
         interpreter has successfully transmitted the data (unless, of
         course, the Transmit command was badly formed). The default
         value for this parameter is "N", or a nonblocking Transmit
         command.  The control flag for this parameter is -tr.
         (Depending on the protocol in play, "successfully transmitted"
         might well imply that an acknowledgment of some sort has been
         received from the foreign Host, but for other protocols it
         might only mean that the given collection of bits has been
         passed from the OPE to the proximate net.)

      Foreign Address Secondary Component

         The addressing mechanisms supported by this level of H-FP are
         discussed above.  The Foreign Address Secondary Component
         parameter contains the value of the destination address's
         secondary component.  Some protocols do not require this
         parameter, or can obtain it from other information.  Therefore,
         the default value for this parameter is NULL.  A NULL secondary
         component might be an error for some protocols, however.  The
         secondary component can be expressed either numerically or
         symbolically.  The control flag for this parameter is -fs.
         (Note that it is intended to be "legal" to specify a Secondary
         Component other than the Well-Known Socket for the protocol in
         play; in such cases, the result should be that the virtualizing
         of the given protocol be applied to the stream, in the

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

         expectation that that's what the other side is expecting.  This
         is to cater to, for example, a Terminal-Terminal protocol that
         merely "does Telnet" to a socket other than the usual Logger.)

      Local Address Secondary Component

         The Local Address Secondary Component parameter contains the
         value of the local address's secondary component.  (The primary
         component is assumed to be the default for the Host, but can be
         altered as well; see below.) Some protocols do not require this
         parameter, or can obtain it from other information.  In some
         cases, the OPE may already know the value for this parameter
         and therefore not require it. The default value of this
         parameter is NULL.  The local address secondary component can
         be expressed either numerically or symbolically.  The control
         flag for this parameter is -ls.

      Begin Timeout Interval

         After a Begin command is issued, a timer can be started.  If
         the activity requested cannot be performed within some timed
         interval, then the Begin command may expire.  An expired Begin
         command returns a response code indicating a Begin timeout
         occurred.  The Begin Timeout Interval parameter contains the
         length of time the timer will run before the Begin timeout
         occurs.

         The parameter is represented as a string of ASCII digits
         indicating the time interval in seconds.  The default value of
         this parameter is infinity (i.e., the Begin command will never
         timeout).  The control flag for this parameter is -bt.

      Type of Service Advice

         The Type of Service Advice parameter contains information on
         the service characteristics the user desires from the offloaded
         protocol.  Included in this parameter is the precedence of the
         data transfer, and also indication of whether high throughput,
         fast response time, or low error rate is the primary goal.

         The format of this parameter is a letter immediately (i.e. no
         intervening spaces) followed by a digit.  The letter "T"
         indicates that high throughput is desired.  The letter "R"
         indicates minimal response time is the goal.  The letter "E"
         indicates that low error rates are the goal.  The letter "N"
         indicates there are no special service requirements to be
         conveyed.  The digit immediately following the character

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

         indicates the desired precedence level, with zero being the
         lowest, and nine being the highest.  The specific
         interpretation of this parameter is dependent on what service
         options are provided by the protocol.  The default value of
         this parameter is the lowest precedence (ROUTINE), and no
         special service requests.  The control flag for this parameter
         is -ts.

      Flow Control Advice

         The Flow Control Advice parameter contains information on the
         flow characteristics desired by the user.  Some applications
         such as file transfer operate more efficiently if the data is
         transferred in large pieces, while other, more interactive
         applications are more efficiently served if smaller pieces are
         used.  This parameter then indicates whether large or small
         data blocks should be used.  It is only relevant in stream or
         connection-oriented protocols, where the user sends more than a
         single piece of data.

         This parameter is represented by a single ASCII digit. A value
         0 means the data should be sent in relatively small blocks
         (e.g., character or line oriented applications), while a value
         9 means the data should be sent in relatively large blocks
         (e.g., block or file oriented applications). Other values
         represent sizes between those extremes.  The character "N"
         indicates that no special flow control advice is provided.  The
         actual interpretation of this parameter is dependent on the
         particular protocol in the OPE.  The default value of this
         parameter is no flow control advice. In this case, the protocol
         in the OPE will operate based only on information available in
         the OPE.  The control flag for this parameter is -fc.

      Local Address Primary Component

         This parameter contains the local address primary component. It
         is anticipated that under most circumstances, this component is
         known to both the Host and the OPE.  Consequently, this
         parameter is seldom required.  It would be useful if the Host
         desired to select one of several valid addresses, however.  The
         control flag for this parameter is -lp.

      Security

         The security parameters contain a set of security level,
         compartment, community of interest, and handling restriction
         information.  Currently, security is provided by performing all

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

         processing at system high level or at a single level.
         Consequently, these parameters are probably redundant, since
         the security information is known.  In the future, however,
         these parameters may be required.  Therefore a field is
         provided. The control flag for this parameter is -s.

      Protcol Idiosyncratic Parameters

         The remaining parameters are protocol idiosyncratic.  That is,
         each protocol that is off-loaded may have a set of these
         parameters, which are documented with a description of the
         off-loaded protocol.  The default value for these parameters is
         NULL, unless otherwise specified by a particular offloaded
         protocol.  The control flag for this set of parameters is -pi,
         which identifies the first protocol idiosyncratic parameters.
         Control flags for other protocol idiosyncratic parameters must
         be defined for each off-loaded protocol.

      Data

         After the Protocol Idiosyncratic Parameters, if any, and the
         required <nl>, if the protocol in play allows for it at this
         juncture the rest of the chunk will be interpreted as data to
         be transmitted.  That is, in connection oriented protocols data
         may or may not be permitted at connection initiation time, but
         in connectionless protocols it certainly makes sense to allow
         the H-FP Begin command to convey data. (This will also be
         useful when we get to the Condition command.)

      Responses

         The following responses have been identified for the Begin
         command:

            000    Command completed successfully
            101    Throughput not available; using maximum
            102    Reliability not available; using maximum
            103    Delay not available; using minimum
            110    Flow Control advice not followed; smaller blocks used
            111    Flow Control advice not followed; larger blocks used
            201    Failed; Begin not implemented in this direction
            202    Failed; timeout
            203    Failed; Begin command on already active channel
            300    Problem with multiple chunks
            301    Syntax problem with Begin command
            302    Protocol not supported in OPE/Host
            303    Active service not available

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

            304    Passive service not available
            305    Invalid Foreign Address Primary Component
            306    Invalid Transmit Discipline
            307    Invalid Foreign Address Secondary Component
            308    Invalid Local Address Secondary Component
            309    Invalid Timeout Interval
            310    Invalid Type of Service Advice
            311    Invalid Flow control Advice
            312    Invalid Local Address Primary Component
            401    Protocol Interpreter in OPE not responding
            402    Remote Protocol Interpreter not available
            403    Failed; insufficient protocol interpreter resources
            501    Failed; insufficient OPE resources
            601    Request violates security policy
            602    Security parameter problem

         Additionally, protocol idiosyncratic responses will be defined
         for each off-loaded protocol.

      Example of Begin Command

         The Begin command is the most complex of the H-FP Command
         Level. When the off-loaded protocol is TCP, the Begin command
         is used to open TCP connections.  One possible example of a
         Begin command issued by an inboard Telnet interpreter to open a
         TCP connection to ISIA, with no begin timeout interval, is:

            C BE TCP A ISIA 9 N 23 ,, ,, N0 S <nl>

         Where:

            TCP    The code for the protocol TCP
            A      Indicates Active Begin
            ISIA   The name of a Host at USC-ISI
            9      Mediation Level 9:  Full OPE mediation
            N      Non-blocking transmit
            23     Destination Telnet Port
            ,,     skip  over parameters  (Local Address Secondary,
                   Begin Timeout Interval)
            N0     Type of Service Advice:  No special Advice,
                   Normal Precedence
            S      Flow Control Advice: use small blocks

         This command will cause the OPE to invoke the TCP interpreter
         to generate the initial SYN packet to the well-known Telnet
         socket on Host ISIA.  It also informs the OPE to do all TCP
         related processing via the Mediation Level, accepts default

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

         Local Address parameters, and sets the Begin Timeout Interval
         to infinity.  The precedence of the TCP connection is Normal,
         and the TCP interpreter is informed that the data stream will
         consist of primarily small blocks.

      Notes to the Implementor

         Response 203 might seem silly to some readers, but it's there
         in case somebody goofed in using the Channel Layer.

   Transmit

      Purpose of the Transmit Command

         The purpose of the Transmit command is to permit the process in
         the Host to send data using an off-loaded protocol interpreter
         in the OPE, and also to permit the OPE to deliver data received
         from the network destined for the process in the Host.  The
         Transmit command is particularly relevant to connection and
         stream type protocols, although it has applications for
         connectionless protocols as well.  After the Begin command is
         issued successfully and the proper Response received, Transmit
         commands can be issued on the given channel.  The semantics of
         the Transmit command depend on whether it was issued by the
         Host or the OPE.

         - If the Host issues the Transmit command, a process in the
         Host wishes to send the data to the destination specified to
         the off-loaded protocol interpreter that was established
         (typically) by a previous Begin command on the given H-FP
         channel.

         - If the OPE issues the command, the OPE has received data
         destined for a process in the Host from a connection or stream
         supported by the off-loaded protocol that was established by a
         previous Begin command on the given H-FP channel.

      Parameters of the Transmit Command

         The Transmit command has one parameter associated with it. It
         is an optional parameter, to temporarily override the response
         discipline for this particular transmit command. Some protocols
         may have protocol-idiosyncratic parameters as well.  The
         transmit command also has data associated with it.  All
         parameters must precede the data to be transmitted.

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

      Response Discipline Override

         The Response Discipline Override parameter indicates the
         desired response discipline for that individual Transmit
         Command, overriding the default response discipline.  A single
         ASCII character is used to indicate the desired discipline.
         The character "N" indicates that this Transmit command should
         not block, and should return a response as soon as the data is
         given to the protocol interpreter in the OPE. The character "B"
         indicates that this Transmit command should block, meaning that
         a response should not be generated until the data has been sent
         to the destination.  The default value of this parameter is the
         currently defined Transmit Command response discipline.  The
         use of this parameter does not alter the currently defined
         Transmit command response discipline; the default is changed
         with the Condition command.  The control flag for this
         parameter is -rd.

      Protocol-Idiosyncratic Parameters

         Any other parameters to the Transmit command are
         protocol-idiosyncratic. That is, each protocol that is
         off-loaded has a set of these parameters, which are documented
         with a description of the off-loaded protocol.  The default
         value for these parameters is NULL, unless otherwise specified
         by a particular off-loaded protocol.  The control flag for this
         set of parameters is -pi, which identifies the first
         protocol-idiosyncratic parameters.  Control flags for other
         protocol-idiosyncratic parameters must be defined for each
         off-loaded protocol.

      Responses

         The following responses for the Transmit command have been
         identified:

            000    Transmit Command completed successfully
            201    Transmit Command not appropriate
            300    Problem with multiple chunks
            301    Syntax problem with Transmit Command
            302    Invalid Transmit Command Response Discipline
            401    Protocol Interpreter in OPE not responding
            402    Failure in remote protocol interpreter
            403    Failed; insufficient protocol interpreter resources
            501    Failed; insufficient OPE resources
            601    Request violates security policy

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

         Additionally, protocol-idiosyncratic responses will be defined
         for each off-loaded protocol.

      Example of Transmit Command

         The transmit command is used in TCP to provide the TCP write
         call.  An example of such a transmit command would be:

            C TR N <nl> <DATA>

         Where N indicates non-blocking transmission discipline, <nl> is
         the required command-ending newline, and <DATA> is presumed to
         be the user's data that is to be transmitted.

      Notes to the Implementor

         If you get a 403 or a 501 response and have sent a multiple
         chunk it probably makes sense to try a single chunk; if you've
         sent a single chunk, it makes sense to wait a while and try
         again a few times before giving up on the stream/channel.

   Condition

      Purpose of the Condition Command

         The primary purpose of the Condition command is to permit a
         process to alter the characteristics that were originally set
         up with the Begin command. (That is, "condition" is a verb.)
         These characteristics include the addresses, the mediation
         level, the type of service, and the flow control parameters
         from Begin. They may also include protocol-idiosyncratic
         characteristics. (Although Condition is usually thought of as a
         Host->OPE command, it may also be used OPE->Host in some
         contexts.)

         Condition is a generic command that may find little use in some
         off-loaded protocols.  In others, only some of the parameters
         identified may make sense.  For example, changing the
         destination address of a TCP connection involves closing one
         connection and opening another.  Consequently, in may make more
         sense to first issue an End command, and then a Begin with the
         new address.  In other protocols, such as IP or UDP, changing
         the address on each datagram would be a perfectly reasonable
         thing to do.

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

      Parameters of the Condition Command

         The Condition command has the same parameters as the Begin
         command.  Any parameters expressed in a Condition command
         indicate the new values of the characteristics to be altered;
         all parameters not expressed retain the current value.

         Although it is possible to express the change of any of the
         characteristics originally set up in the Begin command using
         the Condition command, there are some characteristics that do
         not make sense to alter, at least for some protocols. For
         example, once a connection is opened, it does not make much
         sense to change the Foreign Address Primary or Secondary
         Components.  Doing so is inconsistent with current versions of
         TCP, and would require the closing of the existing connection
         and opening a new one to another address.  Earlier versions of
         TCP did permit connections to be moved.  If a protocol that
         provided such a feature was implemented in the OPE, the
         changing the Secondary Address Components would be a reasonable
         thing to do.

      Responses

         The responses to the Condition command are the same as those to
         the Begin command.

      Example of Condition Command

         The Condition Command can be quite complex, and can be used for
         many purposes.  One conceived use of the condition command
         would be to change the type of service advice associated with
         the channel. An example of this (which also demonstrates the
         ability to skip parameters) is:

            C -ts T <nl>

         which causes the offloaded PI associated with the current
         channel to attempt to achieve high throughput (in its use of
         the comm subnet(s) in play).

      Notes to the Implementor

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

   Signal

      Purpose of Signal Command

         The purpose of the Signal Command (implicitly at least) is to
         permit the transfer of out-of-band signals or information
         between the Host and the OPE, in order to utilize (explicitly)
         out-of-band signaling services of the off-loaded protocol. The
         semantics of the Signal command depend upon whether it was
         issued by the Host or the OPE.

         - If the Signal command was issued by the Host, it means a
         process in the Host desires to send out-of-band data or an
         out-of-band signal.

         - If the Signal command was issued by the OPE, it means
         out-of-band data or an out-of-band signal arrived for the
         process associated with the channel in the Host.

      Parameters of the Signal Command

         The basic usage of the Signal command is with no parameters,
         which sends or reports the receipt of an out-of-band signal.
         Some protocols, such as the NBS Transport Protocol, permit the
         user to send data with the out-of-band signal.  Hence, data is
         permitted to accompany the Signal command.  There may also be
         protocol-idiosyncratic parameters for the Signal command.  If
         this is the case, these parameters would come before the data.

      Protocol-Idiosyncratic Parameters

         The parameters for the Signal command are protocol
         idiosyncratic.  That is, each protocol off-loaded has a set of
         these parameters.  The default value for these parameters is
         their previous values. Control flags for multiple
         protocol-idiosyncratic parameters must be defined for each
         off-loaded protocol.

      Responses

         The following responses have been identified for the Signal
         command:

            000    Command completed successfully
            201    Command not appropriate
            300    Problem with multiple chunks
            301    Syntax problem with Command

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

            401    Protocol Interpreter in OPE not responding
            402    Failure in remote protocol interpreter
            403    Failed; insufficient protocol interpreter resources
            501    Failed; insufficient OPE resources
            601    Request violates security policy

         Additionally, protocol-idiosyncratic responses will be defined
         for each off-loaded protocol.

      Example of Signal Command

         The major perceived use for the Signal command when offloading
         a connection protocol is sending an out-of-band signal with no
         data.  In such a case, the appropriate signal command would be:

            C SI <nl>

      Notes to the Implementor

         Some protocols may allow only only one outstanding signal at a
         time.  For these protocols, it is an implementation issue
         whether the OPE will buffer several signals, but a good case
         could be made for the position that a scrupulous OPE would
         reflect a 202 response back to the Host in such cases.

         There is some question as to the proper handling of the
         "expedited data" notion of some (particularly ISO) protocols.
         It might be more appropriate to deal with such a thing as a
         protocol idiosyncratic parameter on the Transmit command
         instead of using the Signal command (even if it's the closest
         approximation to an out-of-band signal in the given protocol).
         If it's provided using the Signal command, the expedited data
         should not be passed as ASCII, and should appear after the
         command-terminating newline character (and appropriate padding
         with space characters).

   Status

      Purpose of Status Command

         The purpose of the Status command is to permit the Host to
         request and obtain status information from the OPE, and vice
         versa. This includes status request of a conventional protocol
         interface (e.g., in TCP, there is a request to determine the
         state of a particular connection).

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

      Parameters of the Status Command

         The parameters for the Status command indicate whether it is a
         request or a response, and contain the status information.

         Request/Report

            This parameter indicates whether the command is a Status
            request or a Status report.  It consists of a single ASCII
            character.  Q indicates a request (query), and R indicates a
            report.  It should be noted that a report may be generated
            as the result of a query, or may be generated as the result
            of specific protocol mechanisms.

      Protocol-Idiosyncratic Parameters

         The parameters to the status command are
         protocol-idiosyncratic. That is, each protocol off-loaded has a
         set of these parameters.  The default value for these
         parameters is their previous values.  Among these parameters is
         an identifier of the type of status information contained or
         requested, and a value or set of values that contain the
         particular status information. The status information itself
         should be the last item in the command. The control flag for
         this set of parameters is -pi, which identifies the first
         protocol-idiosyncratic parameters.  Control flags for other
         protocol-idiosyncratic parameters must be defined for each
         off-loaded protocol.

      Responses

         The following responses have been identified for the Status
         command:

            000    Command completed successfully
            201    Command not appropriate
            300    Problem with multiple chunks
            301    Syntax problem with Command
            302    Inappropriate status request
            303    Inappropriate status response
            401    Protocol Interpreter in OPE not responding
            402    Failure in remote protocol interpreter
            403    Failed; insufficient protocol interpreter resources
            501    Failed; insufficient OPE resources
            601    Request violates security policy
            9xx    Protocol Idiosyncratic status responses

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

      Example of Status Command

         The status command can be particularly complex, depending on
         the protocol and particular type of status information.  One
         possible use of the status command when off-loading TCP is to
         communicate the status service request.  For performing this
         operation the status command would be:

            C ST Q <nl>

      Notes to the Implementor

   End

      Purpose of the End Command

         The purpose of the End command is to communicate that services
         of the off-loaded protocol are not required.  The semantics of
         the End command depends upon whether it was issued by the Host
         or the OPE.

         - If the Host issues the End command, it means the process in
         the Host no longer requires the services of the offloaded
         protocol.

         - If the OPE issues the End command, it means the remote entity
         has no more data to send (e.g., the off-loaded protocol is TCP
         and the remote user has issued a TCP close).

      Parameters of the End Command

         One parameter is associated with the End Command.  It indicates
         whether the termination should be "graceful" or "abrupt" (see
         below).

         Graceful/Abrupt

            The Graceful/Abrupt parameter indicates whether the End
            should be handled gracefully or abruptly.  If it is handled
            gracefully, then data in transit is allowed to reach its
            destination before service is actually terminated.  An
            abrupt End occurs immediately; all data transmitted from the
            Host but still pending in the OPE is discarded, and no new
            incoming data is sent to the Host from the OPE.

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

            The parameter is indicated by a single ASCII character.  The
            character "G" denotes graceful, and "A" denotes abrupt.  The
            default value for this parameter is graceful.

      Responses

         The following responses have been identified for the End
         command:

            000    Command completed successfully
            201    Command not appropriate
            300    Problem with multiple chunks
            301    Syntax problem with Command
            302    Illegal Type of End Command
            401    Protocol Interpreter in OPE not responding
            402    Failure in remote protocol interpreter
            403    Failed; insufficient protocol interpreter resources
            501    Failed; insufficient OPE resources
            601    Request violates security policy

         Additionally, protocol idiosyncratic responses will be defined
         for each off-loaded protocol.

      Example of End Command

         The syntax of the End command is relatively straightforward. It
         consists of a chunk that contains only a chunk usage
         identifier, the end command string, and the parameter
         indicating whether the end should be graceful or abrupt.  A
         possible valid (abrupt) End command would be:

            C EN A <nl>

      Notes to the Implementor

         Once an End has been issued in a given direction any other
         commands on the channel in the same direction are in error and
         should be responded to appropriately.

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

   No-op

      Purpose of the No-op Command

         The No-op command performs no operation.  Its purpose is to
         permit the Host and OPE to participate in a dialog which does
         not alter the state of communication activities, both for
         debugging purposes and to support features of certain protocols
         (e.g., Telnet's Are You There command).

      Parameters of the No-op Command

         There are no parameters associated with the No-op command.

      Responses

         There are only two possible legal responses to the No-op
         command.  They are:

            000    No-op Command Completed Correctly
            300    Problem with multiple chunks

      Example of No-op Command

         Syntactically the No-op command is quite simple.  It consists
         of a chunk that contains only the chunk usage identifier and
         the string for the command, and the newline.  One possible
         valid No-op command is:

            C NO <nl>

      Notes to the Implementor

         No-ops are included for use in testing and initial
         synchronization.  (The latter use is not mandatory, however.
         That is, no exchange of No-ops is required at start-up time,
         but it is conceivable that some implementations might want to
         do it just for exercise.) They are also traditional.

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

References

   (References [1]-[3] will be available in M. A. Padlipsky's "The
   Elements of Networking Style", Prentice Hall, 1985.)

   [1] Padlipsky, M. A., "The Host-Front End Protocol Approach",
   MTR-3996, Vol. III, MITRE Corp., 1980.

   [2] Padlipsky, M. A., "The Elements of Networking Style", M81-41,
   MITRE Corp., 1981.

   [3] Padlipsky, M. A., "A Perspective on the ARPANET Reference Model",
   M82-47, MITRE Corp., 1982.

   [4] Bailey, G., "Network Access Protocol", S-216,718, National
   Security Agency Central Security Service, 1982.

   [5] Day, J. D., G. R. Grossman, and R. H. Howe, "WWMCCS Host to Front
   End Protocol", 78012.C-INFE.14, Digital Technology Incorporated,
   1979.

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

APPENDIX

   Per-Protocol Offloading Descriptions

   1.  Command Level Interface to an Off-loaded TCP

      This appendix discusses the use of the commands described in the
      body of this document to provide an interface between a Host
      process and an off-loaded interpreter of the DoD's Transmission
      Control Protocol (TCP).  The interface described here is
      functionally equivalent to the interface found in the MIL-STD 1778
      specification of TCP.  It is not, however, identical, in that some
      features of the interface are particularly relevant only in an
      inboard implementation.

      The first section describes the mapping between the interface
      events of MIL-STD 1778 and the commands and responses of this
      H-FP, and highlights the unique features of the interface.  The
      next sections discuss the details of each command.  These details
      include the specialized usages of the command and the
      protocol-idiosyncratic parameters for that command.

      1.1.  Relation to MIL-STD 1778 Interface

         Most of the requests and responses of the TCP interface
         specified in MIL-STD 1778 are mapped directly to H-FP Commands
         and responses.  The exceptions are noted in the following
         descriptions.

         1.1.1. Requests

            Unspecified Passive Open, Fully Specified Passive Open,
            Active Open, and Active Open with Data requests are all
            implemented using variations of the Begin command.  The
            distinction between Passive and Active Open is made using
            the Active/Passive parameter of Begin.  The distinction
            between unspecified and fully specified lies in the presence
            or absence of the destination address fields.  An active
            open with data is identical to a normal active open, except
            for the presence of data following the command.

            The Send Service Request is implemented using the Transmit
            command.  Special protocol idiosyncratic parameters are
            provided for Urgent, Push, and changing the ULP timeout
            action and values.  The response to the Transmit command
            indicates that the appropriate Send call has been made.

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

            There is no corresponding response in the specified TCP
            interface; its only significance is that the Host can issue
            another Transmit command.

            The Allocate event is a specification feature of MIL-STD
            1778 to indicate the willingness of the user to accept
            incoming data across the interface.  However, because this
            is precisely the type of flow control provided by the
            Channel level, the Allocate event would be a superfluous
            mechanism.  Thus, there is no direct analogy to that event
            in the H-FP interface. A Host process indicates its
            willingness to accept new data by informing the channel via
            its flow control interface (if it has an explicit one).

            Close and Abort are provided by the End command.  Close uses
            the graceful version of the End command, while Abort uses
            the abrupt version.  The response indicates that the End
            command has been received and the corresponding Close or
            Abort was issued.  There is no corresponding response in the
            specified TCP interface.

            Status is provided by using the query form of the Status
            command.  The response to the Status command contains the
            information (see below).

         1.1.2. Responses

            The Open Id response is provided so that the user has a
            shorthand name by which to refer to the connection.  With an
            outboarded TCP interpreter, there is a one-to-one mapping
            between TCP connections and H-FP channels.  Hence, the Open
            Id event is not needed, since the channel ID is sufficient
            to indicate the desired connection.

            The Open Failure and Open Success responses are provided
            using OPE-generated responses to Begin commands (which
            provide the Active and Passive Service response primitives)
            issued by the Host.  The value of the response code
            indicates whether the Begin command succeeded or failed, and
            can be mapped to the appropriate Open Failure or Open
            Success indication by the Host.

            Deliver is provided by having the OPE issue a Transmit
            command.  As mentioned above, the "flow control" between the
            TCP interpreter and the Host is provided by the Channel
            layer, so no explicit interface events are needed.  The

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

            response to the Transmit command indicates the data was
            received by the Host process.  There is no corresponding
            response in the specified TCP interface.

            The Closing and Terminate service responses are provided
            using the End command. Closing is indicated using the
            graceful version of the command, while terminate is provided
            using the abrupt version.  The response indicates the End
            command was received by the Host process.  There is no
            corresponding response in the specified TCP interface.

            Status Response is provided by a response to the query
            version of the Status command.  The status information is
            communicated via protocol-idiosyncratic parameters following
            the Response code.

            Error messages are reported using the spontaneously
            generated version of the Status command issued by the OPE.
            The error message is provided in a parameter.  The response
            indicates the error message was received by the Host
            process.  There is no corresponding event in the specified
            TCP interface.

      1.2.  The Begin Command

         The Begin command is used in TCP in three major ways:

            1. To inform the OPE that a process in the Host wishes to
            open a connection to a particular port on a internet
            address.

            2. To inform the OPE that a process in the Host wishes to be
            informed when a connection attempt is made to any or to a
            specific port at this Host's internet address.

            3. To inform the Host that a connection attempt to the OPE
            has arrived, and there was no Begin of the second type
            (passive open) issued by the Host relevant to that
            particular port.

         1.2.1. Specialized Usage

            There are four major aspects to the specialized usage of the
            Begin command and its parameters.  These parameters are:

               1. The meaning of the Mediation Level parameter

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

               2. The selection of blocking treatment of Transmit
                  command

               3. The meaning of the address components

               4. The selection of the TCP Active Open with Data
                  primitive.

            The Mediation Level parameter has only two possible values
            when offloading TCP.  These are "9" and "0".  The normal
            usage of an off-loaded TCP uses the value "9", which means
            the Host is in no way involved in the operation of TCP.  The
            value "0" indicates the Host wishes to negotiate with the
            TCP options.

            The normal TCP Send event is non-blocking.  That is, when a
            user issues the send command, it counts on the reliability
            services of TCP, and is not explicitly notified when the
            data has reached the other end of the connection and been
            properly acknowledged. Hence, the default value for this
            parameter with TCP is "N".  There are some applications
            where the user may not wish to receive a response to a
            Transmit command until the data has been acknowledged by the
            other end of the connection.  In these cases, the value "B"
            should be used for this parameter.  If such a feature is not
            supported by the offloaded TCP interpreter, then it is
            acceptable to issue a 100 level Conditional acceptance
            indicating that blocking is not supported, but the Begin
            command will proceed using non-blocking Transmits.

            The primary address components of the local and remote
            addresses refer to the internet addresses of (or a symbolic
            Host name for) the respective Hosts.  The secondary
            components refer to the particular sockets at those internet
            addresses.  Normally, the secondary components (ports) are
            specified numerically. They may, however, be specified by
            name if the port is a well-known service port. In an Active
            Begin command, the remote addresses primary and secondary
            components must be specified.  The local address components
            need not be specified, unless the user wishes to indicate
            that the connection should be from a particular port or a
            particular internet address of a multi-homed Host.  In a
            Passive Begin command, the remote addresses are specified
            only if connection attempts from one particular Host are of
            interest.  The local address secondary component must be
            used to indicate on which port to perform the Listen.

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

            The way the TCP Active Open with data is provided is by
            including the data with the Begin Command.  This data is
            included in the same Channel level chunk, immediately
            following the newline.  If the data is more than a single
            chunk can hold, then the multi-chunk command feature of the
            H-FP must be used.

         1.2.2. Protocol-Idiosyncratic Parameters

            The protocol-idiosyncratic parameter identified for the TCP
            interface is the "ULP timeout" information.  This
            information includes whether the offloaded interpreter
            should abort the connection on a ULP timeout or report it to
            the inboard user, and also the numerical value of the
            timeout interval. The format chosen for this parameter is a
            single letter followed immediately (with no spaces) by an
            ASCII number. The letter can be either "R" or "A", and
            indicates that the ULP timeout should cause a report or an
            abort, respectively. The number is interpreted to be the
            timeout interval in seconds.

         1.2.3. Examples of the Command

            An example of an Active Begin command that might be issued
            by an inboard user Telnet is:

               C BE TCP A ISIA 9 N 23 ,, 60 R 0 -pi R120 <nl>

            ISIA is the destination Host, 23 is the well-known port
            number for Telnet connections, a Begin timeout of 60 seconds
            was chosen.  The desired type of service is to strive for
            good response time, the transmissions are expected to be in
            small units, and protocol-idiosyncratic parameter R120
            implies that a ULP timeout of 120 seconds should be
            reported.

            An example of a Passive Begin Command that might be issued
            by an inboard server Telnet is:

               C BE TCP P ,, 9 N ,, 23 ,, R 0 -pi R120 <nl>

            The major differences are that no remote address components
            are specified, and the local secondary address component is
            identified as the socket on which the Listen is being
            performed.  Also, the default ("infinite") timeout is taken.

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

      1.3.  The Transmit Command

         The Transmit command is used by the Host process to instruct
         the off-loaded TCP interpreter to send data to a remote site
         via the TCP connection associated with the command's channel.
         It is used by the OPE to deliver incoming data from the
         connection to the process in the Host.

         1.3.1. Specialized Usage

            The Transmit command must be capable of providing all the
            specialized features of the Send and Deliver Event.  These
            special features are Urgent, Push, and modification of the
            ULP Timeout action and/or interval.

            Urgent is a means to communicate that some point upcoming in
            the data stream has been marked as URGENT by the sender.
            While the actual Urgent bit travels through the connection
            out-of-band, it carries a pointer that is related to the
            sequence numbers of the in-band communication. Hence, the
            urgency must be indicated in the Transmit command rather
            than the Signal command.

            Push is a feature of the TCP Send Event that is used to
            indicate that the data in the Transmit command should be
            sent immediately (within the flow control constraints),
            rather than waiting for additional send commands or a
            timeout.  Push is indicated in the Transmit Command. The
            push feature has the same meaning when sent from the OPE to
            the Host.  If the Host implementation does no internal
            queuing, the flag has no meaning.

            The TCP Send event permits the user to modify the "ULP
            timeout action" and/or the "ULP timeout interval" associated
            with that connection.  When changed, the new values take
            effect for the remainder of the connection, unless changed
            later with another Send.  This feature is provided in this
            H-FP using the Transmit Command.

         1.3.2. Protocol-Idiosyncratic Parameters

            The three features identified above are provided using
            protocol-idiosyncratic parameters.

            The first such parameter is the Urgent parameter.  From the
            point of view of the interface, it is just a flag that
            indicates the data is urgent (the actual Urgent pointer is a

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

            concern of the off-loaded TCP interpreter, which is keeping
            track of the sequence numbers).  When issued by the Host
            process, the Urgent flag means the stream should be marked.
            When issued by the OPE, it means the receiver should go to
            (or remain in) the Urgent receive mode.  If the flag is not
            set in the Transmit issued by the OPE, then the receiver
            should remain in (or return to) the non-urgent receive mode.
            The value of this protocol-idiosyncratic parameter is "U" if
            the Urgent is set, or "N" if it is not set.  The default
            value for this parameter is "N".  Since this parameter is
            the first protocol-idiosyncratic parameter for the Transmit
            command, it requires no special flag, and can be indicated
            using the flag -pi.

            The second protocol-idiosyncratic parameter is the Push
            flag.  This parameter is only issued by the Host, since
            there is no Push in the TCP Deliver event.  Its value is "P"
            for push, or "N" for normal.  The default value of this
            parameter is "N".  Its control flag is -pu.

            The third protocol-idiosyncratic parameter is the ULP
            timeout action and value parameter.  The action part
            indicates whether the offloaded interpreter should abort the
            connection on a timeout or report it to the inboard user.
            The value part is the numerical value of the timeout
            interval.  The format used for this parameter is the same as
            in the Begin command, which is a single letter followed
            immediately (with no spaces) by an ASCII number.  The letter
            can be either "R" or "A", and indicates that the ULP timeout
            should cause a report or an abort, respectively.  The number
            is interpreted to be the timeout interval in seconds.  The
            default interpretation for this parameter is its previous
            value. The control flag for this parameter is -ul.

         1.3.3. Examples of the Command

            An example of a Transmit command issued by a Host process is

               C TR -pi N P R160 <nl> <DATA>

            where <DATA> is the data contained within the chunk.  This
            command is for a non-urgent but pushed TCP Send event, that
            also resets the timeout action and interval to Report with a
            value of 160 seconds. The response mode (i.e., nonblocking)
            is derived from the Begin command and not effected by
            transmit.

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

            An example of a Transmit command issued by the OPE is

               C TR -pi N <nl> <DATA>

            where <DATA> is the data contained within the chunk.  This
            command is for a non-urgent delivery (presumably, after a
            previous Urgent delivery).

      1.4.  The Condition Command

         The Condition command is used to modify the transmission
         characteristics of the connection.  The parameters that make
         sense to modify with TCP are the Transmit Response discipline,
         the Type of Service, and the Flow Control Advice.

         1.4.1. Specialized Usage

            There is no usage of the Condition command with an offloaded
            TCP interpreter that is particularly specialized.

         1.4.2. Protocol-Idiosyncratic Parameters

            There are no protocol-idiosyncratic parameters for the
            condition command for the off-loaded TCP. It would be
            possible for the ULP timeout action values to be changed
            with a condition command.  However, this is accomplished
            with the Transmit command, which more closely models the
            interface specified in MIL-STD 1778.  We propose that the
            condition command not provide this capability.

         1.4.3. Examples of the Command

            An example of the Condition command to change the flow
            control advice for a connection is

               C CO -fc 1 <nl>

            which indicates that relatively small transmission units are
            now expected.

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

      1.5.  The Signal Command

         As we currently understand it, TCP's URGENT feature provides an
         INband signal rather than a true out-of-band signal (and at
         least one of us deeply regrets this).  The actual URGENT bit is
         sent out-of-band, but it contains an URGENT pointer which
         relates the URGENT to its position in the data stream.  The
         actual semantics of the URGENT is left to the higher level
         protocol (e.g., Telnet says to discard all data up to the
         URGENT pointer).  Since the Signal command is allowed to cross
         a pending Transmit in the H-FP channel, it would be potentially
         dangerous to implement the interface to TCP URGENT using the
         Signal command since the wrong sequence number could be used as
         the urgent pointer.  Barring persuasive arguments to the
         contrary, it is proposed that Signal should not be used with
         TCP.

      1.6.  The Status Command

         The Status command maps directly into the TCP Status event when
         issued by a Host process. It is also used for the TCP error
         event when issued by the OPE.  There is currently some question
         as to how information from lower protocol levels (e.g., ICMP
         error messages) should be reported to TCP users. When these
         issues are resolved, there may be other uses for the Status
         command.  We solicit other ideas for the Status command with
         this report.

         1.6.1. Specialized Usage

            The major specialized usage of the Status command is to
            provide the error reporting service.  This usage is a form
            of the Status generated by the OPE.

         1.6.2. Protocol-Idiosyncratic Parameters

            When used as a TCP Status request (command issued by the
            Host process), there are no protocol-idiosyncratic
            parameters associated with the Status command.  The OPE
            response codes the TCP status.

            When used as a TCP error report (command issued by the OPE),
            there is one protocol-idiosyncratic parameter associated
            with the Status command.  It is an error description in the
            form of a text string. It requires no special control flag
            since the flag -pi is unambiguous and there are no other
            protocol-idiosyncratic parameters.

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

         1.6.3. Examples of the Command

            An example of the Status command issued by the Host process
            to request status information is

               C ST Q <nl>

            The status information is returned in the response to the
            status command.

            An example of the Status command issued by the OPE to report
            an error from the TCP interpreter is

               C ST R -pi "Connection already exists" <nl>

            which is issued when a TCP open (HFP Begin) is issued to an
            already opened (foreign) connection.

      1.7.  The End Command

         The End command is used to indicate that TCP services are no
         longer required.  Thus, it can be mapped into either the TCP
         Graceful Close or the TCP Abort events.  It is also used as the
         TCP Closing response (as contrasted with the response by the
         OPE to the close command), when issued by the OPE.

         1.7.1. Specialized Usage

            Because of the nature of the two-way close provided by TCP,
            there is a possibility that the Host and the OPE wish to
            gracefully terminate the connection at the same instant.  If
            this happens, then both the Host and the OPE would issue End
            commands at the same time.  To be prepared for this, it is
            necessary to make this the normal graceful closing sequence.
            In other words, both the Graceful Close request and the
            Closing response are mapped to End commands, and the
            response to one of those commands only indicates that the
            command has been received and executed, but not that the
            connection is actually fully closed.  The connection is
            gracefully closed when both End commands have been issued,
            and both successful responses have been received.

            With an abrupt end, a two-way exchange is not necessary.
            Only the Host or the OPE need issue it, for the connection
            to be aborted.

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

         1.7.2. Protocol-Idiosyncratic Parameters

            There are no protocol-idiosyncratic parameters for the End
            command used with TCP.

         1.7.3. Examples of the Command

            An example of the End command used to indicate either a TCP
            Close request (from the Host process) or TCP Closing
            response (from the OPE) is

               C EN G <nl>

            An example of the End command used as an Abort request (from
            the Host process) or as a Terminate response is

               C EN A <nl>

   2.  Command Level Interface to an Off-loaded Telnet

      This appendix is provided to discuss the use of the commands
      described in the body of this document to provide an interface
      between a Host process and an off-loaded interpreter of the Telnet
      protocol.

      The interface described here is not based on a formal interface.
      There are several reasons for this, including the lack of a widely
      accepted standard interface to Telnet, and its headerless nature.
      Consequently, the interface described here is very similar to the
      actual Telnet data stream.

      2.1.  The Begin Command

         The Begin command is used with Telnet to initiate Telnet
         connections.

         2.1.1. Specialized Usage

            There are three major specialized usages to the Begin
            command.  They are the meaning of the Mediation Level
            parameter, the way the number of incoming Telnet connections
            are supported, and the meaning of the secondary address
            components.

            The mediation level is used in Telnet to control which of
            the various Telnet activities are performed by the OPE, and
            which are controlled by the Host.  It has been determined

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

            that all monitoring of the Telnet Socket should be performed
            by the OPE.  Mediation level 9, which is the default,
            indicates the Host desires to play no role in Telnet
            operation. Level 5 means that protocol-idiosyncratic
            parameters to this Begin command indicate which incoming
            options the Host wishes to handle; all other options, and
            all NVT translations, are to be performed by the OPE. Level
            0 indicates that the Host will handle all options, while all
            NVT translations are to be performed in the OPE (see Section
            B.1.3).

            The Host can either accept the connections by fielding OPE
            generated Begins, or by issuing passive Begins to the OPE.
            The Host may wish to restrict the number of incoming Telnet
            connections that it will handle at any particular time.  It
            can do this by rejecting OPE-generated Begins above a
            certain number, or by limiting the number of Host-issued
            passive Begins.  However, precedence constraints dictate
            that the Host actually issue additional passive Begins or
            accept additional Begins from the OPE beyond the maximum
            number it is normally willing to support, so that
            high-priority service requests can be accommodated, possibly
            by preempting lower priority activities.

            The secondary address component is used to refer to specific
            ports. Normally, they are used only when the standard or
            default ports are not used, such as special purpose
            applications or testing.

         2.1.2. Protocol-Idiosyncratic Parameters

            The protocol-idiosyncratic parameters to the Telnet Begin
            command are the identifiers for the options which the host
            wishes to negotiate when using mediation level 5.  On other
            mediation levels, these parameters are not used.

         2.1.3. Examples of the Command

            An example of a passive Begin for an outboard Telnet
            protocol is:

               C BE TEL P ,, 5 N -fc 0 -pi 9 <nl>

            Where the parameters are:

               TEL   Code for the Telnet Protocol
               P     Passive Begin

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

               ,,    Skip the Foreign Address Primary Component
               5     Mediation Level is 5
               N     Non Blocking Transmits
               -fc   Skips over parameters up to Flow Control Advice
               S     Small Blocks are appropriate for Telnet
               -pi   Skips over parameters to the Protocol Idiosyncratic
                     List of Options to be Handled by the Host.
               9     Option Code for Line Length Option

            Here, no remote address component was specified, since the
            Host will accept connections from any Host.  Similarly, no
            local addresses are specified, since the default well-known
            socket for this Host is to be used.  In this example, the
            Host specifies it will handle the line length option (number
            9).  Other options are handled in the OPE.

            An example of an active Begin for an outboard Telnet
            protocol is:

               C BE TEL A ISIA 5 N -fc 0 -pi 9 <nl>

            This command is identical to the passive command, except
            that a remote primary address component is specified to
            identify the intended Host.  No remote secondary component
            is specified, since the well-known socket at that Host is to
            be used.  No local secondary address components are
            specified, since the connection can originate from any
            available socket of the appropriate type selected by the
            OPE.

      2.2.  The Transmit Command

         The Transmit Command is used to send data across a Telnet
         connection.

         2.2.1. Specialized Usage

            The Transmit command is used to transmit data over the
            Telnet connection.  There is one specialized aspect of the
            Transmit command used with an outboard Telnet interpreter.
            This is the provision of the Go Ahead feature of Telnet that
            supports half-duplex devices.

            Go Ahead is provided as a protocol idiosyncratic parameter
            to the Transmit.  It is only used if the Host will support
            it, however.  It is our opinion that Go Ahead is probably
            not a proper thing for the default case.

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

            Go Aheads are a matter between the Host and the terminal. It
            is difficult to offload the generation of Go Aheads to the
            OPE, since the OPE is not really cognizant of the semantics
            of the communication between the Host and the terminal.
            Hence, the OPE does not know when the Host is done
            transmitting and willing to pass "the turn" back to the
            terminal. Similarly when the remote site relinquishes
            control, the OPE includes Go Ahead in its TR.

            We don't believe this Go Ahead problem to be an indictment
            against outboard processing.  It merely illustrates that
            functionality not found in a Host cannot necessarily be
            provided by the OPE.  Hence, we provide this note to the
            implementor:  if the Host cannot generate the
            protocol-idiosyncratic Go Ahead parameter, then the DO
            Suppress Go Ahead must be issued immediately after the
            connection is established.

         2.2.2. Protocol Idiosyncratic Parameters

            The protocol idiosyncratic parameter is the Go Ahead
            indicator.  When present, the character "G" is used to mean
            the Go Ahead can be sent to the other end of the connection,
            but only after the data associated with that Transmit
            command is sent.  When the character is any other value, or
            is absent, the Go Ahead should not be sent.

         2.2.3. Examples of the Command

            An example of the Transmit command is:

               C TR -pi G <nl> <DATA>

            With this command, the Go Ahead is passed to the other side
            after the data is sent.

      2.3.  The Condition Command

         The Condition command is used with Telnet to modify the
         Transmission characteristics and to enable or disable Telnet
         options on a Telnet connection.

         2.3.1. Specialized Usage

            The Condition command takes on specialized usage with
            Telnet, in addition to its normal usage.  It is used to

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

            control the option selection and negotiation process, when
            such selection is performed by the Host (currently, this is
            done at mediation levels 5 and 1, but not at level 9).

            A set of protocol-idiosyncratic parameters has been defined
            for this purpose.  They are based heavily on the Telnet
            negotiation and subnegotiation mechanisms.  For simple
            negotiations there are two parameters, a negotiation type
            (from the set {DO, DONT, WILL, WONT}) followed by the code
            (numeric) or name (symbolic) for the desired option.  The
            codes for the options are identified below.  A basic
            difference between the H-FP interface to Telnet and the
            internal Telnet protocol is that additional parameters are
            included with the request (DO or WILL). The Telnet protocol
            subnegotiation is used internally to communicate that
            information in the Telnet data stream.  Option-specific,
            protocol-idiosyncratic parameters are used for these
            additional parameters.

            Both the Host and the OPE can issue these Condition
            commands. When issued by the Host, it means the user wishes
            to enable or disable a particular option. The OPE proceeds
            to issue the appropriate negotiation commands (i.e., IAC
            <DO> <code>) in the Telnet data stream.  When the results of
            the option negotiation are available, a response is
            generated by the OPE.  For the types DO and WILL, a 000
            Response indicates the appropriate acceptance (WILL or DO,
            respectively). A nonzero Response code may indicate
            negotiation failure or negotiation rejection (among other
            things).  For the types DONT and WONT, a 000 Response
            indicates the option will be disabled.  A negotiation
            rejection should not be expected in those cases.

            When the Condition command is issued by the OPE, it means
            the other end of the connection is negotiating a change.
            Here the response from the Host indicates the Host's desired
            action for the option negotiation.  Again, valid requests to
            disable options (DONT and WONT requests) should always get a
            000 Response.

         2.3.2. Protocol-Idiosyncratic Parameters

            There are two protocol-idiosyncratic parameters for primary
            negotiation using the Condition command.  These are the
            negotiation type and the option code.  The negotiation type
            is one of the set of {DO, DONT, WILL, WONT}.  The option
            code is a numeric value used to identify the particular

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

            option being negotiated.  The values for these codes are
            indicated here, but are identical to the codes used in the
            actual Telnet negotiation.  The codes are:

               Option Name     Option Code       Short Name

               Transmit Binary           0       Binary
               Echo                      1       Echo
               Suppress Go-Ahead         3       SuppressGA
               Approximate Message Size  4       NAMS
               Status                    5       Status
               Timing Mark               6       TimingMark
               RCTE                      7       RCTE
               Line Length               8       LineLength
               Page Size                 9       PageSize
               Carriage Return Disp     10       CRDisp
               Horizontal Tabstops      11       HTabStops
               Horizontal Tab Disp      12       HTabDisp
               Formfeed Disposition     13       FFDisp
               Vertical Tabstops        14       VTabStops
               Vertical Tab Disposition 15       VTabDisp
               Linefeed Disposition     16       LFDisp
               Extended ASCII           17       ExASCII
               Logout                   18       Logout
               Data Entry Terminal      20       DET
               Terminal Type            24       TermType
               Extended options list   255       ExOptions

            Options not listed here may of course be used. The code
            number should be the same as the option code used in Telnet
            negotiation.

            2.3.2.1.  Simple Options

               Options that do not require additional parameters use the
               simple negotiation mechanisms described briefly above and
               in greater detail in the Telnet documentation.  No
               additional parameters are required.  These options
               include the Transmit Binary, Echo, Suppress Go Ahead,
               Status, Timing Mark, and Logout options.

            2.3.2.2.  Approximate Message Size Option

               The Approximate Message Size option requires two
               parameters. The first indicates whether the approximate
               message size being negotiated applies to the local or the
               remote end of the connection.  DS means the size applies

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

               to the sender of the command (i.e., if the Host issues
               the command, DS means the local end of the connection;
               if issued by the OPE, DS means the remote end of the
               connection).  DR means the size applies to the receiver
               of the command (i.e., if the Host issues the command, DR
               means the remote end;  if issued by the OPE, DR means the
               local end of the connection).  This convention is
               consistent with the Telnet subnegotiation mechanisms.
               The second character is an ASCII encoded numeric value,
               which is a character count of the message size.

         2.3.3. Line Width and Page Size Options

            The Line Width and Page Size Options require two additional
            parameters.  The first indicates whether the line width or
            page size being negotiated applies to the local or the
            remote end of the connection, and uses the DS and DR
            convention described above.  The second parameter is an
            ASCII encoded numeric value, which is interpreted as follows
            (assuming the Condition command was issued by the Host):

               0         The Host requests that it handle length or size
                         considerations for the direction indicated by
                         the first parameter.

               1 to 253  The Host requests that the remote end handle
                         the size or length considerations for the
                         direction indicated by the first parameter, but
                         suggests that the value indicated be used as
                         the size or length.

               254       The Host requests that the remote end handle
                         the size or length considerations for the
                         direction indicated by the first parameter, but
                         suggests that the size or length be considered
                         to be infinity.

               255       The Host requests that the remote end handle
                         the tabstop considerations, and suggests
                         nothing about what the value should be.

            If the Condition command is issued by the OPE, then the
            roles of the Host and the remote end are reversed.

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

         2.3.4. Tabstop Options

            The Horizontal and Vertical Tabstops options require two
            option specific parameters.  The first is either DR or DS,
            as was described previously.  The second is a list of one or
            more ASCII encoded numeric values separated by spaces which,
            assuming the Condition command is issued by the Host, are
            individually interpreted as:

               0         The Host requests that it handle tabstops for
                         the direction indicated by the first parameter.

               1 to 250  The Host requests that the remote end handle
                         the tabstop considerations for the direction
                         indicated by the first parameter, but suggests
                         that the value(s) indicated should be used as
                         the tabstops.

               255       The Host requests that the remote end handle
                         the tabstop considerations for the direction
                         indicated by the first parameter, and suggests
                         nothing about what the value should be.

            If the Condition command is issued by the OPE, then the
            roles of the Host and the remote end are reversed.

         2.3.5. Character Disposition Options

            The Carriage Return Disposition option, the Horizontal Tab
            Disposition option, the  Formfeed Disposition option, the
            Vertical Tab Disposition option, and the Linefeed
            Disposition option are all considered character disposition
            options from the perspective of H-FP.  Two option-specific
            parameters are required for the character disposition
            options.  The first is the DR or DS code, which was
            described previously. The second is a single ASCII encoded
            numeric value, which is interpreted as (assuming that the
            Host issued the Condition command):

               0         The Host requests that it handle the character
                         disposition for this connection.

               1 to 250  The Host suggests that the remote end handle
                         the character disposition considerations, but
                         suggests that the value indicated should be
                         taken as the number of nulls which should be

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

                         inserted in the data stream following the
                         particular format character being
                         subnegotiated.

               251       The Host suggests that the remote end handle
                         the character disposition considerations, but
                         recommends that it replace the character with
                         some simplified character similar to but not
                         identical with it (e.g., replace a tab with a
                         space, or a formfeed with a newline).

               252       The Host suggests that the remote end handle
                         the character disposition considerations, but
                         recommends that it discard the character.

               253       The Host suggests that the remote end handle
                         the character disposition, but recommends that
                         the effect of the character be simulated using
                         other characters such as spaces or linefeeds.

               254       The Host suggests that the remote end handle
                         the character disposition considerations, but
                         recommends that it wait for additional data
                         before sending more data.

               255       The Host suggests that the remote end handle
                         the tabstop considerations, and suggests
                         nothing about what the value should be.

            Some of the codes between 251 and 254 are not used with some
            character disposition options. Refer to the ARPANET
            documentation for additional details.

            If the Condition command is issued by the OPE, then the
            roles of the Host and the remote end are reversed.

            2.3.5.1.  RCTE Option

               The Remote Controlled Transmission and Echoing option
               requires parameters to indicate the sets of break
               characters and transmit characters.  There are two
               option-idiosyncratic parameters for RCTE.  The first is a
               list of the character classes that make up the set of
               break characters, as defined in the RCTE documentation.
               The second is a list of character classes that make up
               the set of transmit characters, as defined in the RCTE
               documentation.  Since the two classes are optional and

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

               can be of arbitrary length, it is necessary to precede
               each list with a -bc (break characters) or -tc (transmit
               characters). The character classes are defined as

                  1 Upper Case Letters   A through Z
                  2 Lower Case Letters   a through z
                  3 Digits  0 through 9
                  4 Format effectors  <BS> <CR> <LF> <FF> <HT> <VT>
                  5 Non-format control codes, plus <ESC> and <DEL>
                  6 Punctuation  . , ; : ? !
                  7 Grouping    { [ ( < > ) ] }
                  8 Misc  ' ` " / \ % @ $ &   + - * = ^ _ | ~
                  9 <space>

            2.3.5.2.  Extended Option List

               The Extended Option List option requires a parameter to
               carry the number of the option on the extended list.
               There is thus one option specific parameter to the
               Condition command when used for this purpose, which is
               the number of the option on the extended option list.  It
               can be expressed in ASCII using an octal, decimal, or
               hexadecimal format.

            2.3.5.3.  Terminal Extension Options

               The Extended ASCII, SUPDUP, and Data Entry Terminal
               options of Telnet were all attempts to extend the basic
               capabilities of the Telnet data stream beyond the simple,
               scroll mode terminal model that was the basis of the
               original Telnet design.

               All of these options have limitations to their
               effectiveness.  The Extended ASCII option lacks a
               standardized interpretation of the bit patterns into
               extended ASCII characters.  The SUPDUP effort was
               actually an independent mode where a different virtual
               terminal protocol was used, and the option was there
               merely to switch to and from this protocol. The Data
               Entry Terminal option requires the excessive overhead of
               subnegotiation for each use of extended features.  All of
               these options lack the more valuable asset of widespread
               implementation and use.

               The way these options should be handled is not detailed
               in this appendix. It is clear that the Condition command
               could be used for initiating and terminating the use of

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

               these options.  The actual transmission of characters
               related to the extended terminal features should be
               provided by the Transmit command, either as part of the
               normal Host-to-OPE data stream or by using
               protocol-idiosyncratic parameters.

               A more recent option, the Terminal Type option, should be
               mentioned here.  It permits one end of a connection to
               request information about the terminal at the other end
               or send information about the terminal at the local end.
               This is convenient for systems that provide a wide
               variety of terminal support, but it clearly does not
               follow the model of reducing the MxN problem by use of a
               virtual terminal. Its use is very straightforward in the
               H-FP context.  It only requires sending the terminal type
               to the other end, and activating the Binary Transmission
               Option.

            2.3.5.4.  Status Option

               The Status option is enabled using the negotiation
               mechanism of Telnet.  However, the means to transfer
               status information between OPE and the Host is provided
               via the Status command.  Therefore, details of status
               negotiation are irrelevant to the interface to the
               outboard Telnet.

         2.3.6. Examples of the Command

            The following example shows the command issued by a Host to
            the OPE, requesting that the OPE negotiate with the other
            side so that remote echo is performed.

               C CO -pi DO 1 <nl>

            The numeral 1 is the option code for ECHO from the table
            above. All of the simple options listed above use this same
            basic format.

            The options with additional parameters use straightforward
            extensions of this syntax.  For example, a possible usage of
            Condition by the Host to set the approximate message size
            is:

               C CO -pi DO 4 DS 1024

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

            The 4 is the Option Code for the Approximate Message Size
            option, the DS indicates that Host's message size should be
            set, and 1024 is the desired size.

      2.4.  The Signal Command

         The Signal command is used with Telnet to provide the Telnet
         Interrupt Process and Abort Output services.

         2.4.1. Specialized Usage

            The Signal command is used with an outboard Telnet
            interpreter to interface to the Telnet synch mechanism.
            This mechanism is used with a protocol-idiosyncratic
            parameter, which indicates what particular command is being
            "synched." It is expected that normally, this Signal
            mechanism will only be used with the Interrupt Process and
            Abort Output Telnet signals.  When the Signal command is
            issued by the Host, it goes through the Channel
            (out-of-band) to the OPE, where the Telnet interpreter
            issues the corresponding Telnet signal and synch sequence.
            When such a sequence is received by the OPE, it immediately
            issues a Signal to the Host.  It is expected that a Host or
            OPE would not, in general, reject the Signal command unless
            it is badly formed.

         2.4.2. Protocol-Idiosyncratic Parameters

            The Telnet protocol-idiosyncratic parameter used with the
            Signal command identifies which Telnet signal is begin
            issued.  Normally, it would have the value of either "IP" or
            "AO", for Interrupt Process or Abort Output.  If absent, the
            default value is "IP".

         2.4.3. Examples of the Command

            An example of a Telnet Signal Command (in this case, to send
            an Interrupt Process signal) is:

               C SI IP <nl>

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

      2.5.  The Status Command

         The Status command is used with Telnet to obtain information
         about the Telnet connection and the options in effect.

         2.5.1. Specialized Usage

            The Status command has one specialized aspect when used to
            interface to an outboard Telnet interpreter.  That is to
            send and receive the Telnet Protocol status request
            subnegotiation message to and from the data stream.  In
            order to invoke the status command for this purpose,
            however, the user must have previously issued the Condition
            Status command, which causes the ability to request status
            to be negotiated.  The OPE, when it receives a valid Status
            request command, immediately responds to the user indicating
            the status.  The OPE can issue a status to request the
            Host's negotiated positions.

         2.5.2. Protocol-Idiosyncratic Parameters

            There are no protocol-idiosyncratic parameters to the Status
            query command. The Status Response command has a single
            protocol-idiosyncratic parameter.  It is an ASCII string
            containing the status of the various options (not at their
            default values).

         2.5.3. Examples of the Command

            An example of a Status Query command is:

               C ST Q

            An example of a Status Response command is:

               F ST R "WILL ECHO  DO SUPPRESS-GO-AHEAD
               L WILL STATUS  DO STATUS" <nl>

            In the previous example, note the opening quote is in the
            first chunk, and the closing quote is in the last chunk.
            This technique permits parameters to span chunk boundaries.

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

      2.6.  The End Command

         The End command is used to terminate the Telnet connection,
         either gracefully or abruptly.

         2.6.1. Specialized Usage

            The graceful termination of a Telnet requires End commands
            to be issued by both the Host and the OPE.  This specialized
            usage is identical to that of the outboard TCP interface,
            however.

         2.6.2. Examples of the Command

            An example of the graceful End command is:

               C EN G <nl>

            The abrupt End command is similar.

      2.7.  The No-op Command

         The No-op command is used with Telnet so the Host can determine
         if the OPE is active, and vice versa.

         2.7.1. Specialized Usage

            The No-op command has one specialized usage when offloading
            Telnet.  This is to provide the Telnet Are You There (AYT)
            feature.  When an (AYT) message is received by the OPE, it
            issues a No-op command to the Host. Upon receiving the
            response from the Host, the appropriate response is sent
            back in the data stream.

         2.7.2. Protocol Idiosyncratic Parameters

            There are no protocol-idiosyncratic parameters to the No-op
            command.

         2.7.3. Examples of the Command

            An example of the No-op command is:

               C NO <nl>

RFC 929                                                    December 1984
Proposed Host-Front End Protocol

   3. FTP Offloading

      TBS

   4. Mail Offloading

      TBS

   5. Whatever Offloading

      TBS

   Where TBS nominally = To Be Supplied, but really means: We'll argue
   through these once we get sufficiently positive feedback on the
   others (and on the H-FP as a whole).

 

User Contributions:

Comment about this RFC, ask questions, or add new information about this topic:

CAPTCHA