<pre>Network Working Group
Request for Comments:  705
NIC# 33644





                         FRONT - END PROTOCOL

                            B6700 VERSION




                           2 September 1975


This is a working document which has been developed as the specification
and guideline for design of a Burroughs B6700 attachment to an ARPA-Style
network.

The approach is to utilize a front-end processor with a new protocol for
network operation.  That protocol, described herein, has been built upon
the concepts expressed by M.A. Padlipsky, et al, in NIC# 31117, RFC# 647.

This proposed, site-specific, FEP implementation is the work of Gerald
Bailey and Keith McCloghrie of NSA and of David Grothe of ACC.  It has
already sustained some corrections provided by MAP.  It will be helpful
if interested networkers will review and provide comments to us.

Comments to BRYAN@ISI.

Roland Bryan - ACC                                                      1</pre>
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Request for Comments:  705
Front-End Protocol: B6700 Version



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                          FRONT-END PROTOCOL


                               PREFACE

This document describes the protocol to be used for connecting a general-
purpose computer system (host) to an ARPANET-like network via a "front-end"
computer.  The main body of the document is aimed at a reader who is not
conversant with all the details of network protocols.  However, a paragraph
marked with [n], refers a reader familiar with network protocols to the
n-th item of <a href="#appendix-A">Appendix A</a> which will amplify that particular paragraph.
Further information on the network protocols referred to in this document
can be obtained from the Network Information Center.

Appendix B contains diagrams showing the transitions between the different
connection states.  Appendices C and D give the implementation details of
this protocol in the Front-End and the Hosts.

This protocol is predicated upon the assumption that for each host, a line
protocol, at a lower level, will be established between the device-driver
modules in the Host and the Front-End, and that this line protocol provides
Front-End Protocol with error-free transmissions.


                             INTRODUCTION                               2

A host computer may be connected to a network for a variety of reasons.
Network connection may be an attempt to expand the usefulness of the
Host to the community of users which it serves by making network resources
available to them.  Conversely, the services which the Host provides may
be made available to a larger community of users, with the network providing
the method of access to those services.

In order for members of a network community to communicate in an intelligent
way, there must exist a set of protocols.  The implementation of these
protocols in a host computer is typically called the Network Control Program
(NCP).  The size and complexity of the NCP is proportional to the number and
complexity of protocols which it implements.  For an ARPANET like network,
both the number and complexity are substantial.


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A host which directly connects into the network must assume the responsibility
for implementing this set of protocols.  That is the "price of admission"
to become a network host.  It is not necessary to implement every protocol
and every option in every host, but even in the simplest case -- implementation
of an NCP is not a small task.  The intrusion into the normal operating
environment of the host is also not small.

An alternative method for network connection is to connect the host to some
intermediate processor, and in turn, directly connect that processor to the
network.  This approach is called "Front-Ending."  There are many arguments
which may be posed to justify a host connection to a network through a front-
end processor.  The most obvious being that the responsibility for
implementation of the network protocols (the NCP) can be delegated to the
front-end (FE), thereby reducing the impact on the host.

The purpose of this document is not to justify Front-Ending as a philosophy,
but rather, to introduce a protocol for communications between a host and
a front-end processor which is providing it network access.  The Front End
Protocol (FEP) is intended to permit the host to make use of the network
through existing protocols, without requiring that it be cognizant of the
complexities and implementation detail inherent in their execution.

The FEP is sufficiently general to permit its implementation in the host
to be in terms of the function the host is performing, or the services
which it is providing.  Of primary consideration in specification of FEP
was that it must provide the host with a sufficiently robust command
repertoire to perform its network tasks, while buffering it from the
details of network protocols.


                               CONCEPTS                                 3

Introduction                                                            3a

Before a detailed description of the command structures is undertaken it
seems appropriate to introduce several of the concepts upon which the FEP
is predicated.

The following section serves to briefly describe the FEP commands, and to
elaborate on the concepts of addressing and types of connections provided.



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Commands  (General)                                                     3b

<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  BEGIN Command</span>

This command is sent from the host to the front-end processor.  Its function
is to direct the establishment of one or more network connections.  The type
and number of connections is specified in the BEGIN command string.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  LISTEN Command</span>

Through this command the host indicates its willingness to accept requests
for connection arriving from other hosts.  It directs the front-end processor
to LISTEN for any such connection requests.  The number and type of
connections are specified in the command string.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  RESPONSE Command</span>

The front-end processor uses the RESPONSE command to indicate to the host that
a particular path specified in a BEGIN or LISTEN command is now open or that
the open attempt failed.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  MESSAGE Command</span>

Message text passing between the host and its front-end processor is sent in
this command string.  The MESSAGE command is bi-directional, and is the same
for host or front-end.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  INTERRUPT Command</span>

The INTERRUPT command is sent by either the host of FE.  Its most common use is
to convey that the user wishes to terminate what he is doing - i.e., he has
depressed the Control-C, ATTN, or INT key.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  END Command</span>

One or more connections may be closed by either the FE or the host issuing
this command.  The connection(s) which are affected by the action of the END
are specified in the command string.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  REPLY Command</span>

This command is required to be sent by both the host and FE to acknowledge
receipt of all command types (except REPLY).  The success or failure of the
command being acknowledged is conveyed in the REPLY command string.



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Connections                                                               3c

In order to engage in a meaningful conversation, the parties involved must
be connected.  A network connection is defined by the ARPA Host-Host Protocol
document (Nic #8246) as follows : "A connection couples two processes so
that output from one process is input to the other.  Connections are defined
to be unidirectional, so two connections are necessary if a pair of processes
are to converse in both directions."  The components of a connection, the
sockets, are defined: "... a socket forms the  reference for one end of a
connection, and a connection is fully specified by a pair of sockets.  A
socket is identified by a Host number and a 32-bit socket number.  The same
number in different Hosts represents different sockets."

The existing network protocols incorporate prescribed strategies for
selecting socket assignments, pairing sockets to form connections, and in
the number of connections required to implement the protocol.

Conversations, in most cases, are bi-directional.  Thus to simplify the
Host's procedures in these cases, FEP permits duplex connections on which
the Host can both send and receive.  Send only and Receive only connections
are also available for those situations where communication is one-way.

Thus, FEP provides the flexibility to reduce complexity in the Host, in
addition to accommodating existing protocols and allowing for the
development of new protocols.

Addressing                                                              3d

Conversations in FEP are uniquely identified at initiation by some combination
of Host address, Index number, Path number and Socket assignment.  The Host
address and Socket assignment are required to form the connection(s); there-
after the Index and Path are sufficient to identify the conversation.

Host Address

If, through the BEGIN command, the local Host explicitly directs the creation
of network connection(s), it must specify the address of the foreign host to
which it desires communication.  If the local host indicates a willingness to
communicate, through the LISTEN command, the Front-End processor will supply
the address of the connecting foreign host(s) in its RESPONSE command(s).


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Socket

A socket is either a send socket or a receive socket.  This property is
called the socket's gender.  The sockets at either end of a network
connection must be of opposite gender.  As previously defined a socket
forms the reference for one end of a network connection.  To the extent
possible, the FEP shields the Host from the responsibility of assigning
sockets for individual conversations.  However, because the
socket is a fundamental part of the addressing mechanism of the network,
the Host may need to be aware of socket assignments when establishing
connections.

It is through a "well-advertised" socket that a host provides services
to other members of the network community.  The Initial Connection
Protocol (ICP) [<a href="#ref-1">1</a>] is used to first connect to the well-advertised socket
in order to exchange the number of a presently unused socket which is then
used for the connections required so that the well-advertised socket can
be freed for others attempting to connect.

When establishing a conversation (with a BEGIN or LISTEN command) the
Host indicates in the value of the CONN-TYPE field whether the socket
specified is to be employed directly, or to be used as an initial
connection socket.

Index/Path Addressing                                                   3e

Indexes are values assigned by the local Host to identify network con-
versations.  When conversations are established (with the BEGIN or LISTEN
commands) the Host must specify an index value.  This value will be
associated with the resultant conversations for their duration.

It is often necessary to affiliate conversations [<a href="#ref-2" title="is in the implementation of FTP">2</a>].  To accommodate this,
data paths are defined such that each index has one or more path(s)
associated with it (a path can not exist except as a subordinate to an
index) and all network communication is transmitted on some path.

The maximum number of indexes which may be in use at any one time, and the
maximum number of paths within one index are installation parameters.

Index 0 is reserved for controlling other indexes, and logically represent the
"pipe" through which all other indexes "flow."



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Addresses in FEP command strings are conveyed by the pair of fields "INDEX"
and "PATH."  In commands which cause new indexes to be opened, or new data
paths to be added to an existing index (BEGIN or LISTEN), the PATH field
indicates the first path to be acted upon by this command.  For those
commands which do not create new paths or indexes, if PATH is 0, then all
paths associated with this INDEX are addressed; if PATH is non-zero, only the
specific path within the specified INDEX is addressed.

Path Types                                                              3f

A path can be one of three types:

        a.  DUPLEX - both the Host and the FE can issue MESSAGE commands
            on the path.

        b.  SEND - only the Host can issue MESSAGE commands on the path.

        c.  RECEIVE - only the FE can issue MESSAGE commands on the path.

The paths within an index may be a mixture of path types but one BEGIN/
LISTEN must be used for each contiguous set of the same type.

An FEP path is analogous to a network connection with the following exception.
Network connections are always simplex.  This is true for paths of type SEND
or RECEIVE.  However, a DUPLEX path is formed by the FE connecting two local
sockets to two foreign sockets.  This is a "duplex connection" which is
composed of two network (simplex) connections.

Modes of Establishing a Path                                            3g

One or more paths are established by the action of a single BEGIN or LISTEN
command, with the mode specified in the CONN-TYPE field of the command.
Each of the path types is established in one of two modes - directly or via
ICP.  The gender of the path (its ability to receive or send or both) is not
affected by the mode.

When any of the path types is specified with the ICP mode, the socket value
in the SOCKET field is used as the "well-advertised" socket and an actual
working socket will be exchanged according to the Initial Connection Protocol.
When the direct mode is indicated, the specified socket is used as the working
socket.



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In either mode, when multiple paths are indicated, the next higher socket
number values of the appropriate gender are selected for each path. [<a href="#ref-3">3</a>]

Translation                                                             3h

When the Host sets up a path(s) (with a BEGIN or LISTEN command) it identifies
what type of translation or data-mapping it requires the FE to perform on all
data transmitted on this path(s).  This is specified by two values - one
giving the format of the data transmitted between the FE and the network,
the other giving the format of the data between the Host and the FE. [<a href="#ref-4">4</a>]

Flow Control                                                            3i

All commands (except REPLYs) must be REPLYED to by the receiver.  The sender
is blocked from sending more commands on the same path until a REPLY has been
received.  The REPLY command serves two functions:  it indicates the
success/failure of the last transmission on the path, and it also indicates
a willingness of the receiver to accept more data on that path.  Receipt of
any valid REPLY on an open path is sufficient to unblock it for END or
INTERRUPT commands.  Thus a receiver who will not (or can not) accept more
data (MESSAGE commands) on a given path need not block the sender from
ENDing the path if he desires.  An indication of "READY" in the reply serves
to unblock the path for MESSAGE commands also.

In the normal case, the REPLY performs both functions concurrently.  However,
when the receiver is not ready to accept more data, he can REPLY indicating
only success/failure of the last command which should be sufficient to
allow the sender to free the transmission buffer, requeue the command for
retransmission if necessary, etc. and wait for another REPLY command
announcing the receiver's ability to accept more data.

Exceptional Conditions                                                  3j

When a command is received and can not be executed, the REPLY command is used
to notify the sender of the command.  To do this, the bits of CODE field of
the REPLY are set to show the CATEGORY of the error and its TYPE within that
category (see <a href="#section-3">Section 3</a>h).



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                               COMMANDS                                 4


Introduction                                                            4a

All communications between the Host and the FE is performed by means of
commands.  The commands are given names for documentation purposes but are
distinguished by the binary value of the first field of the command string.
Command strings will be padded with zeros up to the next multiple of an
installation defined parameter.  (This value will be dependent on the
capabilities of the hardware interface between the Host and the FE.)

Field lengths within a command string are specified as some number of bits.
These information bits will be right-justified within the least number of
bytes needed to hold them.  The size of a byte will be an installation
parameter which will normally be 8 bits but other values will be accommodated
as necessary.

The values and meanings of the CODE field of the REPLY command are given for
each command within the following descriptions:

1:  BEGIN                                                               4b

Format

        BEGIN INDEX PATH HOST SOCKET TRANS-TYPE CONN-TYPE NPATHS

Use

This command is sent only from the Host to the FE.  Its function is to direct
the FE to establish one or more logical connections (paths) on the specified
index between the Host and the FE.

Its use has three different modes (depending on the value of the PATH field) :

        mode (a) - to set up a new index and to direct the FE to attempt
        to establish network connections for the one or more paths
        specified within this index.



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        mode (b) - to attempt to establish network connections for an
        existing (but at present closed) path within the already set-up
        index.

        Mode (c) - to attempt to establish network connections for
        one or more new paths within the already set-up index.

Parameters

        a)  BEGIN is an 8-bit field with the value 1.

        b)  INDEX is a 16-bit field, specifying the index.  Note that
            the value 0 is reserved for special use (see <a href="#section-4">Section 4</a>).

        c)  PATH is an 8-bit field, specifying the path(s) which are
            to be established.  Its value identifies the mode of the
            BEGIN (see above) :

                mode (a) - its value must be 1.

                mode (b) - its value must be that of the path to be
                "re-opened."

                mode (c) - its value must be exactly one greater than
                the current number of paths defined within this index.

        d)  HOST is a 32-bit field specifying the foreign host with
            which connections are to be established.

        e)  SOCKET is a 32-bit field, specifying the first or only
            socket at the foreign host to which connections are to
            be made.

        f)  TRANS-TYPE is a 16-bit field which directs the FE to
            perform this type of translation on all data (i.e. TEXT
            in the MESSAGE command string) sent on every path being
            established by this command.  The first 8 bits specify
            the format of the data on the network side; the second
            8 bits specify the format of the data on the Host side.
            The values assigned to the particular formats (eq. ASCII,
            EBCDIC etc.) are installation parameters; however, the


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            value 0 will always mean "bit string" and thus if either
            of the 8-bit sub-fields contains 0, then no mapping will
            be performed.

        g)  CONN-TYPE is an 16-bit field, specifying the type and mode
            of connection(s) to be established for the specified path(s).
            Its value informs the FE how to associate sockets with
            indexes/paths (see Sections <a href="#section-2">2</a>f and 2g).

                Value            Type            Mode

                  7             Duplex          via ICP
                  6             Duplex          direct
                  5             Receive         via ICP
                  4             Receive         direct
                  3             Send            via ICP
                  2             Send            direct

        h)  NPATHS is an 8-bit field, specifying the number of paths which
        this command directs the FE to attempt to establish connections
        for.  If the BEGIN is of mode (b) then its value must be 1.
        Otherwise the sum of its value and the value of the PATH field
        is the new current number of paths plus one.

Error CODES in REPLY

        Category   Type                 Meaning

           3         1          PATH invalid for new index
           3         2          PATH invalid for old index
           3         3          PATH already open
           3         4          HOST unknown
           3         5          TRANSLATION-TYPE invalid
           3         6          CONNECTION-TYPE invalid
           3         7          NPATHS invalid for old path on old index
           3         8          Specified socket inconsistent with CONN-TYPE
           3         9          INDEX invalid, not ready for business
           4         1          No new connections - FE full
           4         2          No new connections - closing down soon


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2:  LISTEN                                                              4c

Format

        LISTEN INDEX PATH HOST SOCKET TRANS-TYPE CONN-TYPE NPATHS

Use

This command is sent only from the Host to the FE.

Its function is to direct the FE to "listen," i.e., to hold the specified paths
pending until such time as a request for connection (RFC) is received from the
network to the specified local socket. then to set up connections and to
respond with a RESPONSE command for each path.

Its use has three different modes (depending on the value of the PATH field) :

        mode (a) - to set up a new index and to listen on the specified local
        socket in order to establish connections for the specified paths.

        mode (b) - to listen on the specified socket in order to establish
        connections for the specified, existing (but at present closed)
        path within the already set-up index.

        mode (c) - to listen on the specified socket in order to establish
        connections for the specified new path(s) within the already set-up
        index.

By use of the HOST parameter, the FE can be directed to accept RFCs from any
host or only from the specified host.

Parameters

        a)  LISTEN is an 8-bit field with value 2.

        b)  INDEX is a 16-bit field specifying the index.

        c)  PATH is an 8-bit field specifying the first of the one or more
            paths which are to be held pending receipt of a RFC.  Its
            value identifies the mode of the LISTEN (see above) :



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                mode (a) - its value must be 1.

                mode (b) - its value must be that of the existing path.

                mode (c) - its value must be exactly one greater than
                the current number of paths within this index.

        d)  HOST is a 32-bit field specifying the host from which RFCs
            are to be accepted; a value of 0 implies from any host.

        e)  SOCKET is a 32-bit field specifying the local socket on which
            the FE is to listen for RFCs.

        f)  TRANS-TYPE is a 16-bit field specifying the type of translation
            the FE is to perform on all data sent on every path established
            as a result of this command.  Its values are the same as in the
            BEGIN command.

        g)  CONN-TYPE is an 16-bit field specifying the type and mode of the
            connection(s) to be established for the specified path(s) when
            an RFC is received.  Its values are the same as in the BEGIN
            command.

        h)  NPATHS is an 8-bit field specifying the number of paths which
            this command associates with the specified index and which are
            to be established.  If the LISTEN is of mode (b) then its value
            must be 1.  Otherwise the sum of its value and the value of the
            PATH field is the new current number of paths plus one, within
            this index.  Thus its value is the number of extra RFCs for
            which the FE is listening on this socket.

Error CODEs in REPLY

        Category      Type                      Meaning

           3            1       PATH invalid for new index
           3            2       PATH invalid for old index
           3            3       PATH already open
           3            4       HOST unknown
           3            5       TRANSLATION-TYPE invalid


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           3            6       CONNECTION-TYPE INVALID
           3            7       NPATHS invalid for old path on old index
           3            8       Specified socket inconsistent with CONN-TYPE
           3            9       INDEX invalid, not ready for business
           3           10       Socket already in use.
           4            1       No new listens - FE full
           4            2       No new listens - closing down soon

3:  RESPONSE                                                            4d

Format

        RESPONSE INDEX PATH CODE HOST SOCKET

Use

This command is sent only from the FE to the Host - once per path specified in
a BEGIN or a LISTEN command.

For paths specified in a BEGIN, it is sent to indicate the success or failure
of the connection attempt.  For paths specified in a LISTEN, it is sent at
the time when the FE has received a matching RFC and has established the
connection.

The HOST and SOCKET parameters are purely informational which the Host can
ignore if it so desires.  Their contents are only guaranteed if the connection
attempt succeeded.

Parameters

        a)  RESPONSE is an 8-bit field with value 3.

        b)  INDEX is a 16-bit field specifying the index.

        c)  PATH is an 8-bit field specifying the particular path.

        d)  CODE is a 16-bit field indicating the outcome of the
            connection attempt:


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                Value                   Meaning

                  0             Path successfully established.
                  1             Local IMP dead.
                  2             Foreign IMP inaccessible.
                  3             Foreign Host dead.
                  4             Foreign Host not responding.
                  5             Connection refused.

        e)  HOST is a 32-bit field specifying the foreign host to which the
            connection has been made.

        f)  SOCKET is a 32-bit field specifying the socket at the foreign
            host.  If the connection type is simplex, then it is the only
            foreign socket for this path; if duplex, then it is the lower
            of the two foreign sockets.

Error CODES in REPLY

                Category        Type       Meaning

                   3             11     INDEX unknown
                   3             12     PATH unknown
                   3             13     CODE invalid

4:  MESSAGE                                                             4e

Format

        MESSAGE INDEX PATH COUNT PAD TEXT

Use

This command is sent by either the Host or the FE to transmit data on the
specified path and index.

Parameters

        a)  MESSAGE is an 8-bit field with value 4.

        b)  INDEX is a 16-bit field specifying the index.


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        c)  PATH is an 8-bit field specifying the path.  Note that the value
            0 is used in the broadcast option (see <a href="#section-3">Section 3</a>j).

        d)  COUNT is a 16-bit field specifying the number of bits of data
            in the TEXT field.

        e)  PAD is an n-bit field, where n is an installation parameter.
            It contains only padding (in the present protocol specification)
            and can be used to enable the host to have the TEXT field start
            on a convenient boundary.

        f)  TEXT is a field containing COUNT bits of data being transmitted
            on this path.

Error CODES in REPLY

        Category        Type            Meaning

           2              1     This option not implemented
           3             12     PATH unknown
           3             14     No connection opened in this direction
           3             15     PATH blocked at this time, resent later
           3             16     PATH suspended at this time, resent later
           3             17     PATH closed
           3             17     COUNT too large
           4              3     Error in transmitting data, resend command
           4              4     Data lost, resent command.

5:  INTERRUPT                                                           4f
Format

        INTERRUPT INDEX PATH CODE
Use

This command is sent by either the Host or the FE.

Its most common use is to pass the information that a terminal user has
pressed his INT (or ATTN or Control-C) key, thereby requesting his
applications program to quit what it is doing for him.[<a href="#ref-5">5</a>]


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Parameters

        a)  INTERRUPT is a 8-bit field with value 5.

        b)  INDEX is a 16-bit field specifying the index.

        c)  PATH is an 8-bit field specifying the path on which the
            INTERRUPT is transmitted.  Note that the value 0 is used in
            the broadcast option (see <a href="#section-3">Section 3</a>j).

        d)  CODE is a 16-bit field.  It has no defined meanings as yet
            and should contain 0.

Error CODES in REPLY

        Category        Type            Meaning

           2              1     This option not implemented
           3             11     INDEX unknown
           3             12     PATH unknown
           3             14     No connection opened in this direction
           3             15     PATH blocked at this time, resend later
           3             17     PATH closed.

6:  END                                                                   4g

Format

        END INDEX PATH CODE

Use

This command is sent by either the Host or the FE, to terminate a connection.
If PATH is 0, then the index and all its paths are terminated, otherwise just
the specified path of the index is terminated.

Parameters

        a)  END is an 8-bit field with value 6.

        b)  INDEX is a 16-bit field specifying the index.


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        c)  PATH is an 8-bit field containing the path to be closed, or 0 if
            the whole index is to be closed.

        d)  CODE is a 16-bit field indicating the reason for the closure:

                Value        Meaning

                  0     Normal close
                  1     Retries exhausted
                  2     Foreign Host failure
                  3     Foreign IMP failure
                  4     Network failure
                  5     Local IMP failure.

            The "Retries exhausted" code indicates that the FE has been
            retrying a transmission to the foreign host without success.

Error CODES in REPLY

        Category        Type            Meaning

           3             11     INDEX unknown
           3             12     PATH unknown
           3             13     CODE unknown
           3             15     PATH blocked at this time, resend later
           3             17     PATH closed.

7:  REPLY                                                                  4h

Format

        REPLY INDEX PATH CODE

Use

This command is sent by both the Host and the FE to acknowledge receipt of
every other type of command (including those on index 0, see <a href="#section-4">Section 4</a>) and/or
to unblock that particular direction of an opened path for the transmission
of another command.

Note that the INDEX and PATH fields contain exactly the same as those of the
command being replied to.


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Parameters

        a)  REPLY is an 8-bit field with value 7.

        b)  INDEX is a 16-bit field specifying the index.

        c)  PATH is a 8-bit field specifying the path.

        d)  CODE is a 16-bit field indicating the success/failure of the
            command being REPLYed to, and the sender's readiness for more
            commands on the same path.  It is divided into four subfields -
            STATUS, COMMAND, CATEGORY, and TYPE.

                1)  STATUS is 4 bits wide

                    bit 0 (right-most) - READY
                    bit 1              - NOT-READY
                    bit 2              - ACK
                    bit 3              - NAK

                ACK=1 indicates that the sender (of the REPLY) has accepted
                the command (being REPLYed to).  NAK=1 indicates that the
                sender has discarded the command (with the reason given by
                the settings of the other bits of the CODE field).

                NOT-READY=1 indicates that the sender (of the REPLY) is
                willing to receive an END or INTERRUPT on this path.
                READY=1 indicates that MESSAGE commands will also be received.

                Normally only one REPLY command will be sent for each
                other command.  However MESSAGE, INTERRUPT, RESPONSE and
                invalid END commands can be replied to by a REPLY with
                ACK (or NAK)=1 and NOT-READY=1 and another REPLY, some
                time later, with READY=1. [<a href="#ref-6" title=" not directly related to a network RFNM; unless it is heavily loaded">6</a>]

                The ACK and NAK bits are mutually exclusive and should
                never both be on simultaneously, and similarly the READY
                and NOT-READY bits.

                Note that the READY/NOT-READY bit settings are only
                relevant when a path is open.


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                2)  COMMAND is 4 bits wide.  It indicates the command for
                    which this is a REPLY :

                        Value        Meaning

                          0     any of the following
                          1     BEGIN
                          2     LISTEN
                          3     RESPONSE
                          4     MESSAGE
                          5     INTERRUPT
                          6     END

                    The value 0 is defined for cases where a Host does not
                    wish to incur any overhead required to fill in the
                    non-zero value.

                3)  CATEGORY is 3 bits wide.  It specifies the category of
                    the error indicated by the ACK bit being off :

                        Value        Meaning

                          1     Command not recognized
                          2     Option not implemented
                          3     Invalid
                          4     Action failed.

                    Its value is relevant only when NAK=1.

                4)  TYPE is 5 bits wide.  It specifies which error occurred.
                    Its value is relevant only when NAK=1.  The possible values
                    and meanings for the various errors and their corresponding
                    CATEGORY subfield values are given under the description
                    of each command.

Sequencing                                                                4i

Once communication between the Host and the FE has been established and each
side is "Ready for Business" (see <a href="#section-4">Section 4</a>b) the Host may at any time send
BEGIN or LISTEN commands for new indexes.  The FE will acknowledge a BEGIN or


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LISTEN with a REPLY and the index is then set-up providing that the REPLY
indicates no errors.  Other BEGIN or LISTEN commands for the new paths on the
same index may be sent at any time after the index is set-up.

The FE will send a RESPONSE command for each path on completion of its attempts
to fulfill the Host's instructions.  If an attempt failed (indicated by the
CODE field) then the path remains closed and another BEGIN or LISTEN for that
path can be sent.  If the attempt was successful, then MESSAGE or INTERRUPT
commands can be sent after the Host has REPLYED to the RESPONSE.

An INTERRUPT or END command may be sent on any opened path after receiving
a REPLY for the previous command on the same path in the same direction.  A
MESSAGE command may be sent if in addition the READY bit was on in the last
REPLY command.

New paths on the same index may be opened at any time after the index has
been set-up, or particular paths may be ENDed and then have new BEGINs or
LISTENs for them issued.  An index remains set-up, even if all its paths are
closed, until an END command with PATH=0 is issued for the index.

Communication between the Host and the FE is terminated by an END with INDEX=0
and this will abort any remaining open paths and indexes.

Broadcasting                                                              4j

Broadcasting is an optional feature of the protocol.  If it has been enabled
by the installation parameter, then the Host may send a MESSAGE or INTERRUPT
command on a particular index with PATH=0.  This instructs the FE to send the
data contained in the TEXT field of the MESSAGE command (or an interrupt) on
every network connection corresponding to an open path of the specified index.

This feature will only occur on MESSAGEs from the Host to the FE (since no
utilization of it in the other direction is envisaged).

A broadcast MESSAGE is replied to with one or two REPLYs in the same way
as any other MESSAGE command.  Flow control within the index is maintained
as if broadcast MESSAGEs were sent on a separate path, i.e., flow control
on other paths is not directly affected.


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Note that for a broadcast MESSAGE command the FE will perform translation
on the data for each path in accordance with the BEGIN or LISTEN which
initiated that path.  Thus, care must be taken when all paths of the
particular index do not have the same format on the Host side specified in
their TRANS-TYPE (see <a href="#section-6">Section 6</a>b).

Index 0                                                                   5

Introduction                                                              5a

Index 0 provides a control link between the Host and the FE, and thus has no
network connections directly associated with it.  The commands on this index
are used to establish and terminate the connection between Host and FE and to
control other indexes.

Path 0                                                                    5b

Path 0 of Index 0 is used to pass global commands - i.e., those which do not
refer to any particular index or path.  The currently defined commands are :

        MESSAGE INDEX=0 COUNT PAD TEXT

            where TEXT = COMMAND [PARM1] [PARM2]
            COMMAND is 8 bits long
            PARM1 and PARM2 are 16 bits long

        a)  COMMAND=1 , PARM1=Hostid

        This is the "Ready for Business" command which must be sent by both
        Host and FE to establish communication between them.  Count gives the
        length of the TEXT field as usual.  If COUNT=8, then just the COMMAND
        field is present.  If COUNT=24, then both the COMMAND and Hostid are
        present.

        The FE will never send a Hostid.  The Host may send its Hostid in the
        event that the FE is connected to more than one IMP or if alternate
        routes to the network exist (e.g., via patch panels).

        Until both sides have sent this command no other command is valid.

        b)  COMMAND=2 , PARM1=M , PARM2=N


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        This is the "CLOSING" command which is a purely information indication
        that the sender's FEP module has been told that communication will be
        terminated in M minutes for a period of N minutes (N=0 implies
        unknown).

        No action is required of the receiver, however he may be able to
        distribute the information to his users.

        c)  COMMAND=3

        This is the "CONTINUE" command which indicates that any previous
        CLOSING command is now no longer true.

                END INDEX=0 PATH+0 CODE

        This command terminates the connection between the HOST and FE.  All
        other paths/indexes are automatically aborted and the FE will close
        all network connections.  The values of the CODE field are the same
        as in the general END command.

Path 1                                                                    5c

Path 1 is reserved for commands specific to a particular path or index.  No
commands are presently defined; they will be at a later date when more
experience has been gained on the need for them.

Path 2                                                                    5d

Path 2 of Index 0 is used for Operator-to-Operator communication between the
Host and the FE.  It is an optional feature which is enabled by an installation
parameter.

MESSAGE commands are formatted in the normal manner with the sender requesting
that the TEXT field be displayed to the receiver's system operator.

Scenarios                                                                   6

The following scenarios are included to provide the reader with a "feeling" for
FEP in a varied set of applications.  The examples selected relate to existing
ARPANET protocols or other networking applications, and do not represent an
exhaustive list of capabilities.                                            6a


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Fields which are variable or not relevant are not shown (for purposes of
clarity) in the command strings in the following examples.                6b

Host Implementation of User TELNET                                        6c

        BEGIN ndxa,PATH=1,host,SKT=1,,CONN-TYPE=duplex+ICP,NPATHS=1

The User TELNET process in the Host causes the BEGIN command to be issued.
When a successful RESPONSE has been returned by the FE, a typical duplex
TELNET connection will have been made to the Host specified in the BEGIN.

Host is Providing Server TELNET                                           6d

        LISTEN ndxa,PATH-1,HOST=0,SKT=1,,CONN-TYPE=duplex+ICP,NPATHS=32


With this one command the Server TELNET process in the Host has conditioned
the FE to LISTEN on Socket 1 (the well-advertised TELNET socket) and to
establish as many as 32 duplex data paths.  The FE, through the RESPONSE
command, will report each connection as it occurs.  Path 1 will represent
the first such duplex connection, etc.  The Host may then manage the data
paths individually.  Individual paths may be ended and placed back into a
LISTENing state by the Host.  If at any time an END command specifying this
INDEX with a PATH of 0 were to be sent by the Host, all connections would
be dissolved, and for all practical purposes, the Host is no longer willing
to provide Server TELNET services.

Host is Providing Server FTP                                              6e

        LISTEN ndxa,PATH=1,HOST=0,SKT=3,,CONN-TYPE=duplex+ICP,NPATH=1

As soon as a RESPONSE for this LISTEN comes from the FE, the Host Server FTP
process should select a new INDEX and issue a new LISTEN for ndxb on socket 3.
The duplex connection which has been made is the control path for the file
transfer.  Based upon control information passed between server and user on
ndxa (path 1) the server FTP will either:

        BEGIN ndxa,PATH=2,(hostid etc. from response),NPATHS=1
    OR
        LISTEN ndxa,PATH=2,(hostid etc. from response),NPATHS=1


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When a RESPONSE command has been received to the previous command, the data
connection (PATH 2) will have been made and transfer of data may begin.  The
values for TRANS-TYPE and CONN-TYPE for the LISTEN or BEGIN will be derived
from the exchange of information on the control path.

Host Is User FTP                                                          6f

        BEGIN NDXA,PATH=1,HOSTID,SKT-3,,CONN-TYPE-duplex+ICP,NPATH=1

when a RESPONSE to this command has been returned by the FE the control path
will have been connected.  The Host, after exchanging information on the
control path, may then proceed by issuing a BEGIN or LISTEN as in the Server
FTP example.

Teleconferencing                                                          6g

An INDEX with n PATHs permits up to n otherwise disassociated conversations
to be affiliated.  Each path can be manipulated individually, or all paths as
a group.  With the broadcast option -- a MESSAGE command specifying INDEX but
not specifying PATH will be broadcast to all open paths on that index.  Thus
each host may direct its messages to any or all parties.

A "conference" may be initiated by any host who issues a LISTEN with multiple
duplex paths on an agreed upon (but not necessarily well advertised) socket.
When some foreign host connects, an ordinary TELNET connection exists.  If,
however, a third or forth or more parties connect, the hosts already engaged
in the conversation may elect to inform the late comers of the members already
involved.  Each host may then elect to connect to as many other hosts as he
desires.  (The parties could agree as to who would BEGIN and who would LISTEN).
Following this scheme [it is not a protocol] all parties participate equally,
there is no moderator.  Each host decides to whom he will speak.  Using the
initial LISTEN, a variation on this would permit the LISTENer to be moderator
and require that he relay messages to other parties, as desired.

In summary, the data path mechanism permits a group of users to select an
agreed upon socket, appoint a "moderator," and at a prescribed time engage in
a conference without benefit of special protocol implementations in the FE
or in any of the hosts (except possibly the moderator).


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Example of the Use of Simplex Connections                                   6h

The Simplex connection types permits a host to LISTEN on a group of simplex
sockets of a particular gender.  If the host supported a group of line
printers, for example, the Line Printer Applications Program could perform a
LISTEN on a socket advertised to be his "Printing Socket," specifying as many
receive paths as he had printers.  Foreign hosts could then connect (via ICP)
to his print socket.  They would be given an appropriate working socket value
and then connect to an available printer.  In this way up to n foreign hosts
may be connected to his n printers at all times.  All that any needs to know
to avail themselves of printing services is the server host's print socket.
[<a href="#ref-1">1</a>]

Host Implementation                                                       7

Concepts                                                                  7a

The Front-End Protocol permits a Host to make use of the network through
existing low-level protocols, without requiring that it be cognizant of the
implementation details of those protocols.

Implementation of FEP in the Host is in terms of the function it is performing
or the service it is providing.  Information regarding sockets is available
to the sophisticated user, but can be ignored if not relevant to the problem
at hand.

The Host should provide the equivalent of a BEGIN, LISTEN, MESSAGE, INTERRUPT,
and END command.  In other words, the human user or applications level process
has at its disposal the full power of FEP.

The FEP module in the Host serves as a control mechanism to multiplex/
demultiplex traffic between itself and the FE.  In appearance and function it
is much like any multi-line interface driver.  It handles REPLYS, reports
errors, etc.  The FEP module must also assume the responsibility for assignment
of indexes.  This could easily be implemented as a "GETINDEX" subroutine
which would allow a user to ask for an index to be assigned to him.  The
user could then proceed to do BEGINs, LISTENs, etc. on that index.

A server process makes itself available to the network at large by issuing an
appropriate LISTEN.  The Host FEP module would not have to be aware of what
servers were implemented or in operation.  The server process, when it was


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activated, could do a "GETINDEX," followed by a LISTEN on its well-advertised
socket, and then proceed from there.  The Host FEP module simply associates
indexes to processes and passes the incoming traffic to the appropriate process
for analysis and response.  It maintains flow between itself and the FE through
the generation and receipt of REPLYs.

The type of data structures, or format of information employed in the
implementation of the FEP commands in the Host is, of course, up to the
implementor.  BEGIN could be a macro call, with the various information
passed as parameters to the Host FEP module -- which would then arrange it
into a command for delivery to the Front-End processor.  The important
consideration is not how the commands are implemented, but simply that their
function is provided.  It might be desirable, for instance, to implement the
Host such that the front-end processor looks like a special I/O device.  In
this case, it may be appropriate to implement a form of OPEN [for BEGIN or
LISTEN], a GET or PUT [for MESSAGE], CLOSE [for END], etc...

Regardless of the implementation details, it appears that, while it is the
responsibility of one control module to assign and manage INDEXes, data paths
are entirely the responsibility of the process which "owns" the INDEX.

Installation Parameters                                                   7b

To package the software for the FE for a given Host, that Host supplies a
number of parameters defining what FE capabilities it requires.  These
parameters are input to a system-generation procedure to produce the particular
FE code.

The parameters are:

        Byte Size

        This gives the size in bits, into a multiple of which each and every
        field of a command string will be right-justified (i.e., the
        information bits come last, preceded by as many padding bits as are
        needed to complete the least integral number of bytes).

        Its value will normally be 8 but other values will be accommodated
        as necessary.


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        Command String Padding

        This gives the size in bits of the width of the hardware interface
        between the Host and the FE, such that every command string
        transmitted in either direction will have padding appended to
        complete the least multiple of this width.

        In the typical implementation, this parameter will be 0 and any
        padding required will be appended/discarded by the line protocol
        underlying FEP.

        Pad Field Length

        This gives the size in bits of the PAD field in the MESSAGE command.
        This enable a Host to have the TEXT field start on a convenient
        boundary.

        Its value can be anywhere in the range 0 to 64.

        Maximum of MESSAGE

        This gives the maximum length of a MESSAGE command string.

        Because buffer allocation in the FE is based on this parameter,
        its value should be chosen with care.

        Maximum number of Indexes

        This gives the maximum number of indexes which may be set-up at any one
        time.

        Maximum Number of Paths

        This gives the maximum number of paths within one index which may be
        open at any one time.

        Translation Types

        This gives the set of required values and meanings of the TRANS-TYPE
        field of the BEGIN/LISTEN commands.  The TRANS-TYPE field is divided
        into two 8-bit subfields; the first giving the format of data on the


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        network side; the second giving the format of data on the Host side.
        The FE is required to translate between these formats all data
        contained in the TEXT field of MESSAGE commands.

        This parameter specifies the required formats and their values in the
        8-bit subfields.  The value 0 is reserved to mean "bit-string" and
        when it appears as either (or both) of the subfields it implies no
        translation is to be done.

        Broadcast Option

        This specifies whether the Host wants to be able to use the Broadcast
        feature (see <a href="#section-3">Section 3</a>j).

        Operator-to-Operator Communication Option

        This specifies whether the Host wants the ability to send messages
        to the FE operator or to have the Host's operator receive messages
        from the FE.

Other options may be included in the protocol at some later date and these will
be available through installation parameters similar to the Broadcast option.

Note that all of these parameters affect the size and complexity of the FE
code.  Thus it is important that their values be chosen carefully so as to
maximize FE efficiency while minimizing Host implementation effort.

For descriptions of individual Host implementations and a list of the options
available so far, see <a href="#appendix-D">Appendix D</a>.

FE Implementation                                                         8

FEP is device independent.  For the present however, an initial implementation
will be accomplished using the DEC PDP/11 computer as the FE device and the
front-end software is to be based upon an extended version of the original ELF
system developed at SCRL.

For more detailed information, see <a href="#appendix-C">Appendix C</a>.

        by :                                                              9
                G. W. Bailey    (BAILEY@OFFICE-1)
                K. McCloghrie   (MCCLOGHRIE@OFFICE-1)


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                              APPENDIX A                                  10

                              References


[<a id="ref-1">1</a>]     ICP is used in this document in a less strict manner than specified
        in NIC 7101, in that it is not necessarily two simplex connections
        that are set up as the result of the exchange of the socket number
        on the initial connection.

[<a id="ref-2">2</a>]     An example of connections needing to be affiliated, is in the
        implementation of FTP, where the control connection and the data
        connection have a defined relationship in their socket assignments.

[<a id="ref-3">3</a>]     Note that a range of socket numbers is reserved for use by an index
        when it is set-up (cf. AEN).

        However, socket numbers for the paths of an index are not necessarily
        contiguous.  For instance, when the next path opened after a SEND
        path is another SEND path, or when a path other than the first of an
        index is opened with ICP specified.  Nevertheless, if a protocol
        requires contiguous sockets, then the opening of the paths in a logical
        manner will provide the contiguity.

[<a id="ref-4">4</a>]     One possible translation will be from a Network Virtual Terminal on
        the network side to a local terminal type on the Host side.

[<a id="ref-5">5</a>]     The FE will directly equate the INTERRUPT command with the Host-Host
        protocol INR/INS commands.

[<a id="ref-6">6</a>]     Note that the READY indication in a REPLY is, in the general case,
        not directly related to a network RFNM; unless it is heavily loaded,
        the FE will be buffering possibly more than one message (in either
        direction) until flow control mechanism allow the messages to be sent
        on.

        However, it is possible that a particular Host might wish to have
        knowledge of receipt of a previous message before transmitting the
        next.  In this case, the FEP implementation could be set up to only
        indicate READY after receiving the RFNM and possibly only send RFNMs
        after receiving a REPLY containing an ACK.


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                              APPENDIX B                                  11

                            State Diagrams



In the state diagrams below the following notation is used:

        REPLY(A)    - REPLY with ACK=1, READY/NOT-READY irrelevant
        REPLY(N)    - REPLY with NAK=1, READY/NOT-READY irrelevant
        REPLY(R)    - REPLY with ACK=0, NAK=0, READY=1
        REPLY(A+R)  - REPLY with ACK=1, READY=1
        REPLY(N+R)  - REPLY with NAK=1, READY=1
        REPLY(A+NR) - REPLY with ACK=1, NOT-READY=1
        REPLY(N+NR) - REPLY with NAK=1, NOT-READY=1



                       State Diagram for INDEX



        / ------\                   /-------\           /-----\
        !       !BEGIN(new index)   !       !           !     !
        !       !-&gt;---------------&gt;-!Index  !           !     !
        !Index  !LISTEN(new index)  !Open   !           !     !
        !Closed !                   !Pending!           !Index!
        !       !           REPLY(N)!       !REPLY(A)   !Open !
        !       !-&lt;---------------&lt;-!       !-&gt;-------&gt;-!     !
        !       !                   \-------/           !     !
        !       !                                       !     !
        !       !             /-------\      END(Path=0)!     !
        !       !             !       !-&lt;-------------&lt;-!     !
        !       !     REPLY(A)!Index  !                 !     !
        !       !-&lt;---------&lt;-!Close  !REPLY(N)         !     !
        !       !             !Pending!-&gt;-------------&gt;-!     !
        \-------/             \-------/                 \-----/



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                        APPENDIX B (continued)

                     State Diagram for Whole Path


        /------\BEGIN       /----------\
        !      !-&gt;--------&gt;-!          !
        !      !LISTEN      !Connection!
        !Path  !            !Pending   !REPLY(A)        /-------\
        !Closed!    REPLY(N)!          !-&gt;------------&gt;-!       !
        !      !-&lt;--------&lt;-!          !                !       !
        !      !            \----------/                !Path   !
        !      !                                        !Conn-  !
        !      !            /-----\     RESPONSE(CODE&gt;0)! ecting!
        !      !            !     !-&lt;-----------------&lt;-!       !
        !      !            !Path !                     !       !
        !      !    REPLY(A)!Abort!          END(PATH&gt;0)!       !
        !      !-&lt;--------&lt;-!Pend-!-&lt;-----------------&lt;-!       !
        !      !            ! ing !                     !       !
        !      !            !     !REPLY(N)             !       !
        \------/            !     !-&gt;-----------------&gt;-!       !
                            \-----/                     !       !
                                                        !       !
                             /-------\                  !       !
                             !       !  RESPONSE(CODE=0)!       !
           /----\            !Path   !-&lt;--------------&lt;-!       !
           !    !            !Open   !                  !       !
           !Path!            !Pending!REPLY(N)          !       !
           !Open!    REPLY(A)!       !-&gt;--------------&gt;-!       !
           !    !-&lt;--------&lt;-!       !                  \-------/
           \----/            \-------/









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                        APPENDIX B (continued)

               State Diagram for Each Direction of Path




        /----\MESSAGE             /-------\             /-------\
        !    !-&gt;----------------&gt;-!       !REPLY(A+NR)  !       !
        !Path!INTERRUPT           !Command!-&gt;---------&gt;-!Message!
        !Open!                    !Blocked!REPLY(N+NR)  !Blocked!
        !    !                    !       !             !       !
        !    !          REPLY(A+R)!       !    INTERRUPT!       !
        !    !-&lt;----------------&lt;-!       !-&lt;---------&lt;-!       !
        !    !          REPLY(N+R)\-------/             !       !
        !    !                                  REPLY(R)!       !
        !    !-&lt;----------------------&lt;---------------&lt;-!       !
        !    !                                          !       !
        !    !END(PATH&gt;0)         /-------\  END(PATH&gt;0)!       !
        !    !-&gt;----------------&gt;-!       !-&lt;---------&lt;-!       !
        !    !                    !       !             !       !
        !    !          REPLY(N+R)!Path   !REPLY(N)     !       !
        !    !-&lt;----------------&lt;-!Close  !-&gt;---------&gt;-!       !
        \----/                    !Pending!             \-------/
                                  !       !
        /------\          REPLY(A)!       !
        !Path  !-&lt;--------------&lt;-!       !
        !Closed!                  !       !
        !      !                  \-------/
        \------/










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                              APPENDIX C

                       Front-End Implementation


Introduction

A Network Access System (NAS), developed for a DEC PDP/11 computer, supports
the current Imp-Host, Host-Host and ICP protocols.  The implementation of
these protocols facilitate process-process communications across the network
and multi-user TELNET access to foreign hosts.  This NAS provides the FE
environment in which FEP is implemented.

The NAS system is comprised of a Kernel or executive section and a Network
Control Program (NCP) plus a collection of modules to support device
interfaces, handle terminals, and implement applications, as appropriate.  The
software is modular and extensible.

The KERNEL

The Kernel of the system consists of a set of functional modules which perform
the task of resource management in a multiprocessing environment.  This allows
processes to be created, vie for processor service according to priority,
intercommunicate, and be terminated.  System primitives exist for various
tasks such as process creation and synchronization, storage allocation, and
sharing of the interval timer.

The term process used here describes an autonomous sequence of states brought
about by the PDP-11 processor; a process' state is characterized by the set of
processor registers, a stock, and process-owned storage areas.  Process share
storage areas which are accessed only (eq. pure code).  Processes also share
storage areas which may be updated (eq. control tables).  In this case an
allocation mechanism is utilized to prevent simultaneous ownership of an
updatable storage area.  The storage area is thus viewed as a sequentially
sharable resource which is allocated by the process, modified, and then
released.

Processes are given control of the processor by a single procedure called the
Dispatcher.  Processes are said to be in a ready state or in a waiting state.
When a process blocks itself, control is given to the highest priority ready
process.


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Each process has an associated input message queue.  This queue is the vehicle
for interprocess communication.  A process is blocked (put into a wait state)
when its input message queue becomes empty (voluntary wait), or when an
interrupt occurs (involuntary wait) because a higher priority process is to
receive control of the processor.  A process may voluntarily block itself
waiting for any signal, or it may block itself for a specific event to be
posted to its input message queue.

The Network Control Program

The NCP provides "third level" protocol functions to local processes.  It
contains a process which decodes and passes messages which have been received
from the IMP and placed on the IMP-Host queue.  This process interacts with
other processes which call the NCP to establish connections or to transmit
data.  Thus the NCP is essentially divided into two parts:

    1)  a process which handles incoming messages from the network,
        interprets IMP-Host and Host-Host control messages, and forwards
        regular messages on established connections; and

    2)  a set of primitives which allow local processes to establish
        connections to other processes across the network, and to perform
        requests for data to be transferred on these connections.

There are two primary data structures used by the NCP to monitor the status
of network connections.  The first is called the Host Table, and describes
that which is peculiar to each given host; the second is referred to as a
Connection Table and contains all information on the state of a local NCP
socket (connection).  Connection Tables may be created either through
external requests (e.q., an RFC is received from a remote host) or through
internal requests (e.g., a local process performs a LISTEN).

Flow control is that portion of the NCP which governs the flow of data on
connections.  There are two procedures which perform this task; one which
handles receive connections and one which handles send connections.  These
procedures receive control when an event has occurred which may now make it
possible to transfer data on a connection.

Both send and receive flow control procedures have the responsibility of moving
data between local process buffers and messages being received or transmitted
over the network.  In addition, they handle the formatting and unpacking of


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messages received.  Local processes are unaware that data is being transmitted
as discrete messages.

The NCP watchdog process monitors the state of network connections, checking
for error conditions and performing garbage collection tasks.  It receives
control at periodic intervals and scans the list of known hosts, looking for
existing connections.  For each host to which an input or output connection
exists, the Watchdog causes a Host-Host NOP message to be sent.  Thus if a
remote Host crashes while data is being awaited, local processes are informed
of the error condition.  The NCP takes notice of the remote crash when it
receives a IMP--Host type 7 control message (Destination Host Dead).  It then
automatically closes all connections to that Host, and notifies using processes
of that fact.

A second function of the NCP Watchdog is to check for connections hung because
of an outstanding RFNM.  If a RFNM is not received for a specified interval,
the message is discarded, and the associated connection closed.

The FEP Handler

The Front-End Protocol is implemented as a collection of related, but
specialized processes which manage network connections on the one side, and
manage FEP paths and indexes on the other.  Some FEP processes are NCP users.
They cause network connections to be made, rule on incoming RFCs, and both
accept and generate network data.  Other FEP processes support the Host.
These processes parse incoming commands, create indexes and paths, control
the generation of replies and generally manage the paths.  Certain FEP
processes control specialized tasks such as translation of data, servicing of
LISTEN commands and generation of RESPONSE commands.

Two data structures provide control information for FEP activities.  An Index
Table exists for each active index.  Each Index Table associates one or more
Path Table entries.  Information in the Path Table reflects the state of the
path, the translation type specified for data on this path, and necessary
information to associate the path to any appropriate NCP Connection Tables.
The Path Table is the common interface for all of the FEP modules.  Most FEP
processes are activated to service some event which is usually associated to
a path.  The action of the process will likely be dictated by the state of the
path as indicated by the Path Table entry, and may result in altering the state
of the path or the activation of one or more other FEP processes.


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Two message queues provide Host input and output to the FEP modules.  A line
protocol mechanism services these queues.  Commands from the Host are placed
on the FEP Input queue by the line protocol process and the FEP Host Input
process is signaled. When an FEP Host Output module places a Command for the
Host on the host Output queue it signals the line protocol process.

The FEP implementation is basically Host independent down to the level of the
Host Input and Host Output queues.

The Line Protocol Mechanism

The device interface and the line protocol between the FE and the Host are
installation dependent.  Because of this dependency, only a general discussion
of the Line Protocol Mechanism is possible in this context.  Detailed
descriptions of the specific line protocols are included in the section for
each Host.

The communications discipline and physical device characteristics may vary
considerably from host to host.  All FEP line protocols, however, will show
certain common characteristics.  The interface between the FEP handler and the
Line Protocol Mechanism will always be Host Input and Host Output queues.  All
line protocol mechanisms will be expected to guarantee the integrity of the
data.  This implies some form of flow control, error detection/correction and
retransmission capability, as well as normal transmit/receive responsibilities.
The Line Protocol Mechanism will be expected to report failure after
unsuccessfully attempting to perform an I/O operation.  The number of retries
etc. before reporting failure is an installation parameter.  The FEP Handler
works only in terms of FEP commands.  The line protocol may provide for block
transfers where each physical block is comprised of one or more FEP commands.
If such is the case, it is encumbent upon the Line Protocol Mechanism to
deblock the incoming Host commands before placing them in the Host Input queue.

The Line Protocol Mechanism will, in the general case, not manage any buffers.
After successfully transmitting a command to the Host it is responsible for
reporting the I/O complete, but the buffer space is freed or reused only by
the FEP process which "owns" that space.  The FEP Handler might use buffer
assignment to control the rate of incoming traffic.  When the FEP Host Input
queue is ready to accept an additional command, it would acquire a buffer and
signal the Line Protocol Mechanism, passing it a pointer to a buffer.  This


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is effectively a "read" request.  When the line protocol handler has filled
the buffer, it adds it to the Host Input queue and signals I/O complete to
the appropriate FEP process.

If the nature of the physical connection is such that the FE must accept
unsolicited input, it may be necessary for the Line Protocol Mechanism to
have its own buffer pool, in addition.  If this is the case, it must be
entirely managed by the line handler and transparent to the FEP Handler.

Data Translations

The TRANS-TYPE provisions in FeP may be employed for at least two general
services.  First, it can be used for normal character set substitutions.  This
is where, in the general case, there is a one-to-one relationship between the
two character sets.

The second service addresses the problem of data transformation.  In this case,
there need not be a one-to-one relationship between incoming data and outgoing
data.

The translation mechanism uses a token (e.g., a character) from the incoming
data stream to index into a translation table.  The result may be one of the
following:

        a)  do nothing, drop the character
        b)  output the character unchanged
        c)  substitute input character by output character
        d)  substitute input character by output string
        e)  activate a procedure indicated by the table
        f)  change the translation
        g)  test the translation mode and do any of the above depending
            on the result.

For each translation/transformation required by the Host a translation table
must be defined.  For simplicity and clarity the TRANS-TYPE field in the FEP
commands allows the user to specify Host side and Network side as independent
entities.  In actual execution the Host/Network pair addresses a translation
table which must have been previously defined.  Note that for a duplex path
two translation tables are necessary (A-&gt;B is not the same as A&lt;-B).

A collection of "standard" character sets will be addressed initially (EBCDIC,
ASC117, ASCII8, BCD, etc.) and at least NVT.  As new requirements are defined
these will be added to a library which will then be available to subsequent
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                              APPENDIX D

                         Host Implementations



To be written at a later date.




































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