    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

    Network Working Group
    Request for Comments:  705
    Front-End Protocol: B6700 Version

                            ***WORKING DOCUMENT***

                              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 Appendix 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

    1.  BEGIN Command

    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.

    2.  LISTEN Command

    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.

    3.  RESPONSE Command

    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.

    4.  MESSAGE Command

    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.

    5.  INTERRUPT Command

    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.

    6.  END Command

    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.

    7.  REPLY Command

    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) [1] 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 [2].  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. [3]

    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. [4]

    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 Section 3h).

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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 Section 4).

            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 2f 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 Section 3j).

            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.[5]

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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 Section 3j).

            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 Section 4) 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. [6]

                    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 Section 4b) 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 Section 6b).

    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.
    [1]

    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 Section 3j).

            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 Appendix D.

    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 Appendix C.

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

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                                  APPENDIX A                                  10

                                  References

    [1]     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.

    [2]     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.

    [3]     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.

    [4]     One possible translation will be from a Network Virtual Terminal on
            the network side to a local terminal type on the Host side.

    [5]     The FE will directly equate the INTERRUPT command with the Host-Host
            protocol INR/INS commands.

    [6]     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)   !       !           !     !
            !       !->--------------->-!Index  !           !     !
            !Index  !LISTEN(new index)  !Open   !           !     !
            !Closed !                   !Pending!           !Index!
            !       !           REPLY(N)!       !REPLY(A)   !Open !
            !       !-<---------------<-!       !->------->-!     !
            !       !                   \-------/           !     !
            !       !                                       !     !
            !       !             /-------\      END(Path=0)!     !
            !       !             !       !-<-------------<-!     !
            !       !     REPLY(A)!Index  !                 !     !
            !       !-<---------<-!Close  !REPLY(N)         !     !
            !       !             !Pending!->------------->-!     !
            \-------/             \-------/                 \-----/

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                            APPENDIX B (continued)

                         State Diagram for Whole Path

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

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                            APPENDIX B (continued)

                   State Diagram for Each Direction of Path

            /----\MESSAGE             /-------\             /-------\
            !    !->---------------->-!       !REPLY(A+NR)  !       !
            !Path!INTERRUPT           !Command!->--------->-!Message!
            !Open!                    !Blocked!REPLY(N+NR)  !Blocked!
            !    !                    !       !             !       !
            !    !          REPLY(A+R)!       !    INTERRUPT!       !
            !    !-<----------------<-!       !-<---------<-!       !
            !    !          REPLY(N+R)\-------/             !       !
            !    !                                  REPLY(R)!       !
            !    !-<----------------------<---------------<-!       !
            !    !                                          !       !
            !    !END(PATH>0)         /-------\  END(PATH>0)!       !
            !    !->---------------->-!       !-<---------<-!       !
            !    !                    !       !             !       !
            !    !          REPLY(N+R)!Path   !REPLY(N)     !       !
            !    !-<----------------<-!Close  !->--------->-!       !
            \----/                    !Pending!             \-------/
                                      !       !
            /------\          REPLY(A)!       !
            !Path  !-<--------------<-!       !
            !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->B is not the same as A<-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
    users.

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                                  APPENDIX D

                             Host Implementations

    To be written at a later date.

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