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LinuxDig.com Request For Comments

RFC Number : 929

Title : Proposed Host-Front End Protocol.


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.




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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


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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


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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


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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.)


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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.












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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


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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:





while the following are not legal:





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
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



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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


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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.


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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.




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- 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.




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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


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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


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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



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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


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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


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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


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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 , 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


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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

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


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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.




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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



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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

Where N indicates non-blocking transmission discipline, is
the required command-ending newline, and 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.





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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

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








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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


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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

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).




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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



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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

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.





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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

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.











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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

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.











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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.





























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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.



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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



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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



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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.



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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

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

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.




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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


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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

where 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.



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An example of a Transmit command issued by the OPE is

C TR -pi N

where 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

which indicates that relatively small transmission units are
now expected.










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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.


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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

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'

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.




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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

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

C EN A

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


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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

Where the parameters are:

TEL Code for the Telnet Protocol
P Passive Begin


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,, 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

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.


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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

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



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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
) 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


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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


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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.






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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



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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


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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
5 Non-format control codes, plus and
6 Punctuation . , ; : ? !
7 Grouping { [ ( < > ) ] }
8 Misc ' ` ' / % @ $ & + - * = ^ _ | ~
9

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


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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

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




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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









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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'

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.








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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

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





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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).


































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