Proposed Host-Front End Protocol
RFC 929

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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.

Lilienkamp & Mandell & Padlipsky                                [Page 1]



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