    Document: FSC-0056
    Version:  001
    Date:     03-May-1991

                          EMSI/IEMSI Protocol Definitions
                              Joaquim H. Homrighausen
                                   May 3, 1991

        Status of this document:

         This FSC suggests a proposed protocol for the FidoNet(r) community,
         and requests discussion and suggestions for improvements.
         Distribution of this document is subject to the restrictions
         specified on the next page.

         Fido and FidoNet are registered marks of Tom Jennings and Fido
         Software.

          (Also known as EMSC-001; Electronic Mail Standards Document #001)
        ---------------------------------------------------------------------
          Copyright 1989-1991 Joaquim H. Homrighausen. All rights reserved.
        ---------------------------------------------------------------------

        Notice
        =====================================================================
        This document obsoletes EMSI_003 and any previous document describing
        the EMSI, UZAP, and/or IEMSI handshake protocol. I apologize for the
        lack of proper state charts. I am currently under a fairly heavy
        work-load and thought it would be better to release something half-
        decent than not to release anything at all.

        Restrictions
        =====================================================================
        EMSI/IEMSI may be used by any developer as long as these
        specifications are followed exactly. The IEMSI and EMSI specifications
        may be implemented independently of each other.

        EMSI/IEMSI may be used free-of-charge by any developer for any
        purpose, commercially or otherwise. In creating EMSI/IEMSI, we are
        taking the first step towards developing a clear protocol definition
        for state-of-the-art E-Mail systems to follow.

        This document and its NOTES file (EMSI.NOT) may be freely copied and
        distributed, but must NEVER be distributed in a modified form. If you
        have an enhancement request, please contact the author of this
        document; do not change it yourself.

        Permission is hereby granted to the FTSC (Fidonet Technical Standards
        Committee) and other technical organisations to republish this
        document in its entirety. Librarians may change the title page and
        page headers to match their library format as long as all copyrights
        and body text remain unaltered. The original document name and source
        (EMSC) must be mentioned in any republished versions of this
        document.

        No organization, company, person, or other being may impose any fees
        for any reason for providing this document. This document may not be
        sold or otherwise transferred for personal or company gain under any
        circumstances.

        Layout
        =====================================================================
        This document consists of four major parts; A short introduction and
        explanation of the EMSI/IEMSI handshake protocol, the EMSI
        definitions, the IEMSI definitions, and finally various notes and
        credits.

        PART I

        Introduction
        =====================================================================
        The EMSI/IEMSI handshake protocol allows for maximum flexibility in
        E-Mail session start-up and control. The YooHoo (FTS-6) standard,
        designed by Wynn Wagner III, was a good idea, but did not allow
        sufficient room for growth and cannot be used in 7-bit environments.
        EMSI/IEMSI should provide for virtually unlimited growth and
        expansion of its own scope. By providing variable-length packets,
        EMSI/IEMSI is capable of being as simple or as complex as necessary
        and entirely backwards compatible when new features and/or protocols
        are added.

        All EMSI/IEMSI packets and sequences consists of 7-bit printable
        ASCII characters. This format allows us to establish a universal
        handshake between "PCs" and "mainframes" alike. The more complicated
        the computer system, the more restrictions affect its I/O; there are
        many I/O channels that cannot transmit control characters such as XON
        and XOFF; for this, we have created a universal handshake protocol
        that uses all printable characters.

        EMSI/IEMSI does allow control and 8-bit ASCII characters to be
        transmitted. This is, however, accomplished by escaping the data
        and converting it to 7-bit printable ASCII characters.

        Data layer
        =====================================================================
        EMSI/IEMSI is a protocol based on multi-character sequences rather
        than single character flow control. There are several advantages of
        using several characters rather than just one, but there is also a
        drawback. On very poor-quality telephone lines, EMSI will most likely
        require several retransmissions of packets since line noise usually
        come in bursts. That aside, there is little advantage in using a
        protocol based on single characters.

        All EMSI/IEMSI sequences are terminated by a single <CR> unless
        otherwise specified. This is necessary to force some data collection
        equipment to flush their buffers. Appending <CR> to EMSI/IEMSI
        sequences in a FidoNet environment is a delicate matter and it is
        important that you follow the notes regarding this.

        Note regarding file requests
        ---------------------------------------------------------------------
        The file request concept mentioned in the EMSI document refers to
        WaZOO style file requests as specified in FTS-6. No other file
        request mechanism is supported in the EMSI specifications.

        Separator usage
        ---------------------------------------------------------------------
        To designate the fields within the EMSI/IEMSI packets and retain
        complete transparency, both start and stop characters are used.

        The ASCII1 type is used for all fields within the packet. This uses
        the brace characters to delimit the fields. The '{' (ASCII 123)
        character is the start byte and '}' (ASCII 125) is the stop byte.
        If a stop byte is used as literal data within a field, it must be
        transmitted twice. The end of a field is designated by a stop byte
        that is not followed by another identical stop byte.

        The ASCII2 fields are delimited in exactly the same way, but use the
        square brackets as delimiters. The '[' (ASCII 91) is the start byte
        and ']' (ASCII 93) is the stop byte. ASCII2 is used to delimit data
        within the ASCII1 extra_field information.

        7-bit data restriction
        ---------------------------------------------------------------------
        It is the developer's responsibility to ensure that the software
        generates EMSI/IEMSI packets and sequences containing only 7-bit
        (00H through 7eH) printable ASCII characters.

        It is recommended that all EMSI/IEMSI implementations strip the high-
        order bit of all received characters prior to processing the packet/
        sequence and prior to calculating CRC values.

        CRC values
        ---------------------------------------------------------------------
        The polynomial used to calculate a 16-bit CRC is the same polynomial
        used in the Xmodem file transfer protocol. The polynomial used to
        calculate a 32-bit CRC is the same polynomial used in the Zmodem file
        transfer protocol.

        Binary values
        ---------------------------------------------------------------------
        Since the EMSI environment specifies only 7-bit printable ASCII
        characters to be used, binary values, such as CRC and length
        descriptors are expressed as a four character hexadecimal string. The
        only exception to this is a 32-bit CRC value which is expressed as an
        eight character hexadecimal string.

        The application must treat them case insensitive, eg. ffaa should be
        treated identical to FFAA.

        Handling 8-bit data
        ---------------------------------------------------------------------
        Although EMSI only uses 7-bit printable ASCII characters, there is an
        escape mechanism that allows systems to transmit control and 8-bit
        ASCII characters without the requirement of an 8-bit data link. The
        escape character is a backslash character ('\') and is followed by two
        characters in hexadecimal notation. Eg. "\80" is the ASCII character
        128. To insert an actual backslash character, two backslashes are used
        ("\\"), or a backslash followed by the hexadecimal code for a
        backslash, eg. "\5c".

        The hexadecimal code following a backslash MUST always be two
        characters, ie. to insert ASCII 15 (hexadecimail 'f'), the result
        would be "0f". All hexadecimal sequences must be treated case
        insensitively.

        PART II - Electronic Mail Standard Idenfitication

        Connecting two EMSI capable systems
        =====================================================================
        This assumes that the two systems are connected and that no data
        has been transmitted by the Caller.

        It should be mentioned that sending/monitoring for the "YooHoo",
        "TSYNC", and other protocol start characters is optional and not
        required for a strict EMSI implementation.

        STEP 1, EMSI INIT

            Calling system                   Answering system
        +-+-------------------------------+----------------------------------+
        :1: Send <CR> until ANY character : Send EMSI_REQ and possible       :
        : : is received.                  : banner, etc.                     :
        +-+-------------------------------+----------------------------------+
        :2: Receive banner, etc. Monitor  : Monitor line for the EMSI_INQ    :
        : : line for the EMSI_REQ         : sequence and if received,        :
        : : sequence and if received,     : attempt to handshake immediately.:
        : : transmit EMSI_INQ and attempt :                                  :
        : : to handshake immediately.     :                                  :
        +-+-------------------------------+----------------------------------+
        :3: No EMSI_REQ sequence received,: Monitor line for EMSI_INQ and    :
        : : send EMSI_INQ twice followed  : possible other protocol start    :
        : : by possible other protocol    : characters and if received,      :
        : : start characters.             : attempt to handshake immediately.:
        : :                               :                                  :
        : : Transmit <CR>                 : Go to step 3.                    :
        +-+-------------------------------+----------------------------------+
        :4: If EMSI_REQ sequence received,:
        : : send EMSI_INQ and attempt to  :
        : : handshake immediately,        :
        : : otherwise repeat step 3.      :
        +-+-------------------------------+

        In steps 1 and 2, both the Calling and Answering system terminate all
        sequences with <CR>. In step 3, the Calling system does not terminate
        sequences with <CR> as it is explicitly transmitted after possible
        protocol start characters. In step 4, the Calling system once again
        terminate all sequences with a <CR>.

        STEP 2A, RECEIVE EMSI HANDSHAKE

        At this point, all sequences are terminated with a <CR>.

        +-+------------------------------------------------------------------+
        :1: Tries=0, T1=20 seconds, T2=60 seconds                            :
        +-+------------------------------------------------------------------+
        :2: Increment Tries                                                  :
        : :                                                                  :
        : : Tries>6?                      Terminate, and report failure.     :
        : +------------------------------------------------------------------+
        : : Are we answering system?      Transmit EMSI_REQ, go to step 3.   :
        : +------------------------------------------------------------------+
        : : Tries>1?                      Transmit EMSI_NAK, go to step 3.   :
        : +------------------------------------------------------------------+
        : : Go to step 4.                                                    :
        +-+------------------------------------------------------------------+
        :3: T1=20 seconds                                                    :
        +-+------------------------------------------------------------------+
        :4: Wait for EMSI sequence until EMSI_HBT or EMSI_DAT or any of the  :
        : : timers have expired.                                             :
        : :                                                                  :
        : : If T2 has expired, terminate call and report failure.            :
        : +------------------------------------------------------------------+
        : : If T1 has expired, go to step 2.                                 :
        : +------------------------------------------------------------------+
        : : If EMSI_HBT received, go to step 3.                              :
        : +------------------------------------------------------------------+
        : : If EMSI_DAT received, go to step 5.                              :
        : +------------------------------------------------------------------+
        : : Go to step 4.                                                    :
        +-+------------------------------------------------------------------+
        :5: Receive EMSI_DAT packet                                          :
        : +------------------------------------------------------------------+
        : : Packet received OK?                Transmit EMSI_ACK twice, and  :
        : :                                    go to step 6.                 :
        : +------------------------------------------------------------------+
        : : Go to step 2.                                                    :
        +-+------------------------------------------------------------------+
        :6: Received EMSI_DAT packet OK, exit.                               :
        +-+------------------------------------------------------------------+

        All processing of the information in the EMSI_DAT packet must be done
        after transmitting EMSI_ACK twice to the remote system. It is
        recommended that an EMSI_HBT sequence is issued once every seven
        seconds while such processing is taking place to avoid unnecessary
        handshake collissions. Emitting EMSI_HBT should only be done when
        it is obvious that the remote system is waiting for the second phase
        of the EMSI handshake to take place.

        STEP 2B, TRANSMIT EMSI HANDSHAKE

        At this point, all sequences are terminated with a <CR>.

        +-+------------------------------------------------------------------+
        :1: Tries=0, T1=60 seconds                                           :
        +-+------------------------------------------------------------------+
        :2: Transmit EMSI_DAT packet and increment Tries                     :
        : :                                                                  :
        : +------------------------------------------------------------------+
        : : Tries>6?                        Terminate, and report failure.   :
        : +------------------------------------------------------------------+
        : : Go to step 3.                                                    :
        +-+------------------------------------------------------------------+
        :3: T2=20 seconds                                                    :
        +-+------------------------------------------------------------------+
        :4: Wait for EMSI sequence until T1 has expired                      :
        : :                                                                  :
        : : If T1 has expired, terminate call and report failure.            :
        : +------------------------------------------------------------------+
        : : If T2 has expired, go to step 2.                                 :
        : +------------------------------------------------------------------+
        : : If EMSI_REQ received, go to step 4.                              :
        : +------------------------------------------------------------------+
        : : If EMSI_ACK received, go to step 5.                              :
        : +------------------------------------------------------------------+
        : : If any other sequence received, go to step 2.                    :             :
        +-+------------------------------------------------------------------+
        :5: Received EMSI_ACK, exit.                                         :
        +-+------------------------------------------------------------------+

        EMSI packet and sequence definitions
        =====================================================================

        =====================================================================
        EMSI Inquiry                                    **EMSI_INQ<crc16><CR>
        ---------------------------------------------------------------------
        EMSI Inquiry is transmitted by the calling system to identify it as
        EMSI capable. If an EMSI_REQ sequence is received in response, it is
        safe to assume the answering system to be EMSI capable.

        =====================================================================
        EMSI Request                                    **EMSI_REQ<crc16><CR>
        ---------------------------------------------------------------------
        EMSI Request is transmitted by the answering system in response to an
        EMSI Inquiry sequence. It should also be transmitted prior to or
        immediately following the answering system has identified itself by
        transmitting its program name and/or banner. If the calling system
        receives an EMSI Request sequence, it can safely assume that the
        answering system is EMSI capable.

        =====================================================================
        EMSI Client                                     **EMSI_CLI<crc16><CR>
        ---------------------------------------------------------------------
        EMSI Client is used by terminal emulation software to force a mailer
        front-end to bypass any unnecessary mail session negotiation and
        treat the call as an incoming human caller. The EMSI_CLI sequence may
        not be issued by any software attempting to establish a mail session
        between two systems and must only be acted upon by an answering
        system.

        =====================================================================
        EMSI Heartbeat                                  **EMSI_HBT<crc16><CR>
        ---------------------------------------------------------------------
        EMSI Heartbeat is used to prevent unnecessary timeouts from occurring
        while attempting to handshake. It is most commonly used when the
        answering system turns around to transmit its EMSI_DAT packet. It is
        quite normal that any of the timers of the calling system (which at
        this stage is waiting for an EMSI_DAT packet) expires while the
        answering system is processing the recently received EMSI_DAT packet.

        =====================================================================
        EMSI Data                      **EMSI_DAT<len16><data_pkt><crc16><CR>
        ---------------------------------------------------------------------
        EMSI Data is transmitted by both the calling and answering system at
        the appropriate time to exchange system information. Following the
        header is a four byte number representing the length of <data_pkt>
        excluding the CRC and terminating <CR>.

        The EMSI_DAT packet is a variable length packet. Since this is a
        synchronous protocol, the inbound data buffer should be purged
        between transmission of the <data_pkt> and <crc16> fields to prevent
        accidental EMSI_NAK sequences, etc.

        =====================================================================
        EMSI ACK                                        **EMSI_ACK<crc16><CR>
        ---------------------------------------------------------------------
        EMSI ACK is transmitted by either system as a positive
        acknowledgement of the valid receipt of a EMSI_DAT packet. This should
        only be used as a response to EMSI_DAT and not any other packet.
        Redundant EMSI_ACK sequences should be ignored.

        =====================================================================
        EMSI NAK                                        **EMSI_NAK<crc16><CR>
        ---------------------------------------------------------------------
        EMSI NAK is transmitted by either system as a negative
        acknowledgement of the valid receipt of a EMSI_DAT packet. This
        should only be used as a response to EMSI_DAT and not any other
        packet. Redundant EMSI_NAK packets should be ignored.

        The EMSI_DAT packet
        =====================================================================
        The EMSI_DAT packet is the core of an EMSI negotiated session. It
        contains information vital to the mail session. The following pseudo
        structure shows the layout of the EMSI_DAT packet.

        EMSI_DAT
            fingerprint,            "EMSI"
            system_address_list,
            password,
            link_codes,
            compatibility_codes,
            mailer_product_code,
            mailer_name,
            mailer_version,
            mailer_serial_number:    ASCII1;
            extra_field_1,
                ..
                ..
            extra_field_n:            EMSI_addon; (optional fields)
        end;

        The EMSI_addon structure is defined as follows:

        EMSI_addon
            product_ID,
            specific_data:            ASCII1;
        end;

        Following is an example of the actual data transmitted as an EMSI_DAT
        packet:

        {EMSI}{2:270/17}{}{8N1,PUA}{ZAP,ZMO,ARC,XMA}{44}{AirMail}{0.10}
        {Beta-2}{IDENT}{[Advanced Engineering S.A.R.L.][Luxembourg]
        [Joaquim Homrighausen][-Unpublished-][9600][MO,XA,HST,V32B,V42B]}

        EMSI_DAT field definitions
        ---------------------------------------------------------------------

        =====================================================================
        Fingerprint                                                      EMSI
        ---------------------------------------------------------------------
        The constant "EMSI". There is no need for a revision level since this
        basic format cannot change and remain backward compatible.

        =====================================================================
        System address list
        ---------------------------------------------------------------------
        The system address list is a list of system-specific identifiers for
        the E-Mail system separated by spaces.

        For FidoNet-technology based networks, it is required that
        Zone:Net/Node be presented, and .Point be omitted if zero. Zone and
        Net must not be zero.

        In other networks, an address such as "jhom@csource.oz.au" should be
        considered valid.

        =====================================================================
        Password
        ---------------------------------------------------------------------
        For systems using a session level password, it would be passed in
        this field. Note that the same password is used for all presented
        addresses and that it must be treated case insensitive.

        =====================================================================
        Link codes
        ---------------------------------------------------------------------
        Link codes is a string of flags that specify desired connect
        conditions. These codes are separated by commas. New codes may be
        added with prior approval from the author of this document.

        Calling system/answering system options:

            8N1,
            7E1,
            7O2,
            etc.       Communication parameters.

        Calling system options:

            PUA        Pickup mail for all presented addresses.
            PUP        Pickup mail for primary address only.
            NPU        No mail pickup desired.

        Answering system options:

            HAT        Hold ALL traffic.
            HXT        Hold compressed mail traffic.
            HRQ        Hold file requests (not processed at this time).

        =====================================================================
        Compatibility codes
        ---------------------------------------------------------------------
        Compatibility codes is a string of flags that specifies the
        capabilities and enabled features of the mailer. These codes are
        separated by commas. New codes may be added with prior approval from
        the author of this document.

        The calling system must list supported protocols first and descending
        order of preference (the most desirable protocol should be listed
        first). The answering system should only present one protocol and it
        should be the first item in the compatibility_codes field.

            Protocols
            -----------------------------------------------------------------
            DZA*    DirectZAP (Zmodem variant).
            ZAP     ZedZap (Zmodem variant).
            ZMO**   Zmodem w/1,024 byte data packets.
            JAN     Janus.
            KER     Kermit.

            Other codes
            -----------------------------------------------------------------
            NCP     No compatible protocols (failure).
            NRQ     No file requests accepted by this system.
            ARC     ARCmail 0.60-capable, as defined by the FTSC.
            XMA     Supports other forms of compressed mail.
            FNC     Filename conversion. This indicates that any transmitted
                    files must follow the MS-DOS restrictions of an eight
                    character file name followed by a three character
                    extension; eg. FILENAME.EXT

        (*) DirectZAP is a variant of ZedZap. The difference is that the
        transmitter only escapes CAN (18H). It is not recommended to use the
        DirectZAP protocol when two systems are connected via a packet
        switching network, or via another layer sensitive to control
        characters such as XON and XOFF.

        (**) The minimum protocol requirement for an EMSI implementation is
        to support plain Zmodem (16- or 32-bit CRC, 1,024 byte packets) which
        is represented by the ZMO flag in EMSI.

        =====================================================================
        Mailer product code
        ---------------------------------------------------------------------
        The hexadecimal representation of the EMSC product code assigned to
        the mailer. Currently, the EMSC codes are the same as the FTSC
        assigned codes.

        =====================================================================
        Mailer name
        ---------------------------------------------------------------------
        Specifies the name of the E-Mail system sending the EMSI packet.

        =====================================================================
        Mailer version
        ---------------------------------------------------------------------
        The version number of the mailer software, ie. "1.10", "2.00".

        =====================================================================
        Mailer serial number
        ---------------------------------------------------------------------
        The serial number, distribution source, version information, etc.
        This field is usually displayed like:

            Name<sp>Version/Serial_number

        eg.

            AirMail 0.10/Beta-2

        =====================================================================
        Extra fields
        ---------------------------------------------------------------------
        The extra fields make the EMSI handshake protocol extremely flexible.
        Any program or mailer may add fields to the end of the pre-defined
        structure so that program-specific data may be passed without the
        concern of interferring with other systems.

        There may be any number of extra fields added to the end of this
        structure. Each EXTRA_FIELD contains two ASCII1 strings:

        PRODUCT_IDENTIFIER      A unique "tag" that defines a specific
                                program (such as a mailer or a utility).

        SPECIFIC_DATA           ASCII text that is specific data to the
                                program defined in PRODUCT_IDENTIFIER. With
                                this structure, any program can add its own
                                data to the EMSI packet without affecting
                                other applications.

        It is recommended that you contact the author of this document should
        you have any EXTRA_FIELDS that you may think worthwhile for other
        developers to implement and support.

        Predefined extra fields
        ---------------------------------------------------------------------
        The following extra fields have been defined to date.

        PRODUCT_IDENTIFIER   :  IDENT

        Purpose              :  General identification of system that
                                includes all information to generate a St.
                                Louis-format nodelist entry.

        SPECIFIC_DATA        :  system_name,
                                city,
                                operator_name,
                                phone_number,
                                baud_rate,
                                flags:            ASCII2;

            SYSTEM_NAME         The name of the system given by the user.
                                This would normally be a company name, BBS
                                name or other identifying text.

            CITY                The geographical location of the system.

            OPERATOR_NAME       The name of the person primarily responsible
                                for the system.

            PHONE_NUMBER        The telephone number of the system, or
                                "-Unpublished-" if the telephone number is
                                unpublished. This MUST be in the standard
                                format COUNTRY-CITY-NUMBER. Leading zeros
                                should be stripped from the city code,
                                ie. Stockholm (Sweden) has a city code of 08,
                                included in an EMSI packet, it would read
                                46-8-<number>.

            BAUD_RATE           The maximum baud rate supported by the
                                system. This is NOT necessarily the same as
                                the highest DTE rate.

            FLAGS               The St. Louis (FTSC) nodelist flags
                                associated with the system.

        PART III - Interactive Electronic Mail Standard Idenfitication

        Connecting two IEMSI capable systems
        =====================================================================
        Two specific labels are used when discussing the IEMSI definitions.
        The Client, which in this case is the Terminal software, and the
        Server, which in this case is the interactive on-line software,
        such as a BBS package or database system. It is assumed that the
        Client and the Server have established a data link and that no
        data has been transmitted by the Server.

        STEP 1, IEMSI INIT

        There is no specific sequence of events in the IEMSI definition. The
        Client must monitor incoming data and if the EMSI_IRQ sequence is
        detected, it attempts to negotiate an IEMSI session with the Server.
        Under no circumstances is the Client allowed to transmit an EMSI_ICI
        packet prior to receiving the EMSI_IRQ sequence from the Server.
        All IEMSI sequences and EMSI sequences used during an IEMSI session
        are terminated by a single <CR>. There are no exceptions to this.

        STEP 2A, Server

        +-+------------------------------------------------------------------+
        :1: Tries=0, T2=60 seconds                                           :
        +-+------------------------------------------------------------------+
        :2: Transmit EMSI_IRQ sequence                                       :
        +-+------------------------------------------------------------------+
        :3: T1=20 seconds, increment Tries                                   :
        : :                                                                  :
        : : Tries>3?                        Discontinue IEMSI negotiation.   :
        +-+------------------------------------------------------------------+
        :4: Wait for EMSI_ICI packet until any of the timers have expired.   :
        : :                                                                  :
        : : If T2 has expired, discontinue IEMSI negotiation.                :
        : +------------------------------------------------------------------+
        : : If T1 has expired, go to step 2.                                 :
        : +------------------------------------------------------------------+
        : : If EMSI_ICI seen, go to step 5.                                  :
        : +------------------------------------------------------------------+
        : : Go to step 4.                                                    :
        +-+------------------------------------------------------------------+
        :5: Receive EMSI_ICI packet                                          :
        : +------------------------------------------------------------------+
        : : Packet received OK?             Transmit EMSI_ISI packet, and    :
        : :                                 go to step 6.                    :
        : +------------------------------------------------------------------+
        : : Packet not received OK?         Transmit EMSI_NAK and go to step :
        : :                                 3.                               :
        +-+------------------------------------------------------------------+
        :6: Tries=0                                                          :
        +-+------------------------------------------------------------------+
        :7: T1=20 seconds, increment Tries                                   :
        : :                                                                  :
        : : Tries>3?                        Discontinue IEMSI negotiation.   :
        +-+------------------------------------------------------------------+
        :8: Wait for EMSI_ACK/EMSI_NAK until any of the timers have expired. :
        : :                                                                  :
        : : If T2 has expired, discontinue IEMSI negotiation.                :
        : +------------------------------------------------------------------+
        : : If T1 has expired or EMSI_NAK received, transmit EMSI_ISI packet :
        : : again and go to step 7.                                          :
        : +------------------------------------------------------------------+
        : : If EMSI_ACK received, go to step 9.                              :
        : +------------------------------------------------------------------+
        : : Go to step 8.                                                    :
        +-+------------------------------------------------------------------+
        :9: IEMSI session successfully established, exit.                    :
        +-+------------------------------------------------------------------+

        The Server must monitor its incoming data channel for 'normal' data,
        ie. data not transmitted as IEMSI sequences, to detect if the user is
        attempting to log-in without the use of IEMSI. The only basic
        restriction this imposes on the Server is that user names and/or IDs
        may not start with the character '*' since all EMSI/IEMSI sequences
        start with this character.

        All processing of the information in the EMSI_ICI packet must be done
        after transmitting the EMSI_ISI packet and receiving two EMSI_ACK
        sequences in return.

        STEP 2B, Client

        Note that this assumes that the Client has seen the EMSI_IRQ sequence
        transmitted by the Server and that the negotiation is ready to take
        place.

        +-+------------------------------------------------------------------+
        :1: Tries=0, T2=60 seconds                                           :
        +-+------------------------------------------------------------------+
        :2: Transmit EMSI_ICI packet                                         :
        +-+------------------------------------------------------------------+
        :3: T1=20 seconds, increment Tries                                   :
        +-+------------------------------------------------------------------+
        :5: Tries>3 or T2 expired?            Discontinue IEMSI negotiation. :
        : +------------------------------------------------------------------+
        : : If T1 has expired, go to step 2.                                 :
        : +------------------------------------------------------------------+
        : : If EMSI_ISI seen, go to step 6.                                  :
        : +------------------------------------------------------------------+
        : : Go to step 5.                                                    :
        +-+------------------------------------------------------------------+
        :6: Receive EMSI_ISI packet                                          :
        : +------------------------------------------------------------------+
        : : Packet received OK?              Transmit EMSI_ACK packet twice, :
        : :                                  and go to step 7.               :
        : +------------------------------------------------------------------+
        : : Packet not received OK?          Transmit EMSI_NAK and go to step:
        : :                                  3.                              :
        +-+------------------------------------------------------------------+
        :7: IEMSI session successfully established, exit.                    :
        +-+------------------------------------------------------------------+

        All processing of the information in the EMSI_ISI packet must be done
        after transmitting two EMSI_ACK sequences in return.

        If either of the ICI or ISI packets are NAK'd three consecutive times,
        the session negotiation attempt is terminated and the Client proceeds
        as it would have done should the Server not have supported IEMSI.

        IEMSI packet and sequence definitions
        =====================================================================

        =====================================================================
        EMSI ACK                                        **EMSI_ACK<crc16><CR>
        ---------------------------------------------------------------------
        EMSI ACK is transmitted by either Client or Server as a positive
        acknowledgement of the valid receipt of an IEMSI packet such as
        EMSI_ISI and EMSI_ICI. During an IEMSI session, this sequence can,
        however, be used as a positive acknowledgement for other IEMSI
        packets. Redundant EMSI_ACK sequences should be ignored.

        =====================================================================
        EMSI NAK                                        **EMSI_NAK<crc16><CR>
        ---------------------------------------------------------------------
        EMSI NAK is transmitted by either Client or Server as a negative
        acknowledgement of the valid receipt of an IEMSI packet such as
        EMSI_ISI and EMSI_ICI. During an IEMSI session, this sequence can,
        however, be used as a negative acknowledgement for other IEMSI
        packets. Redundant EMSI_NAK sequences should be ignored.

        =====================================================================
        EMSI IRQ                                        **EMSI_IRQ<crc16><CR>
        ---------------------------------------------------------------------
        Similar to EMSI_REQ which is used by mailer software to negotiate a
        mail session. IRQ identifies the Server as being capable of
        negotiating an IEMSI session. When the Client detects an IRQ sequence
        in its inbound data stream, it attempts to negotiate an IEMSI
        session.

        =====================================================================
        EMSI IIR                                        **EMSI_IIR<crc16><CR>
        ---------------------------------------------------------------------
        The IIR (Interactive Interrupt Request) sequence is used by either
        Client or Server to abort the current negotiation. This could be
        during the initial IEMSI handshake or during other interactions
        between the Client and the Server.

        =====================================================================
        EMSI ICI                             **EMSI_ICI<len><crc32><CR>
        ---------------------------------------------------------------------
        The ICI packet is used by the Client to transmit its configuration
        and Server-related information to the Server. It contains Server
        parameters, Client options, and Client capabilities.

        =====================================================================
        EMSI ISI                             **EMSI_ISI<len><crc32><CR>
        ---------------------------------------------------------------------
        The ISI packet is used by the Server to transmit its configuration
        and Client-related information to the Client. It contains Server data
        and capabilities.

        =====================================================================
        EMSI ISM                             **EMSI_ISM<len><crc32><CR>
        ---------------------------------------------------------------------
        The ISM packet is used to transfer ASCII images from the Server to
        the Client. These images can then be recalled by the Client when
        the Server needs to display a previously displayed image. This will
        be further described in future revisions of this document.

        =====================================================================
        EMSI CHT                                        **EMSI_CHT<crc16><CR>
        ---------------------------------------------------------------------
        The CHT sequence is used by the Server to instruct the Client
        software to enter its full-screen conversation mode function (CHAT).
        Whether or not the Client software supports this is indicated in the
        ICI packet.

        If the Server transmits this sequence to the Client, it must wait for
        an EMSI_ACK prior to engaging its conversation mode. If no EMSI_ACK
        sequence is received with ten seconds, it is safe to assume that the
        Client does not support EMSI_CHT. If, however, an EMSI_NAK sequence
        is received from the Client, the Server must re-transmit the
        EMSI_CHT sequence. Once the on-line conversation function has been
        sucessfully activated, the Server must not echo any received
        characters back to the Client.

        =====================================================================
        EMSI TCH                                        **EMSI_TCH<crc16><CR>
        ---------------------------------------------------------------------
        The TCH sequence is used by the Server to instruct the Client
        software to terminate its full-screen conversation mode function
        (CHAT).

        If the Server transmits this sequence to the Client, it must wait for
        an EMSI_ACK prior to leaving its conversation mode. If no EMSI_ACK
        sequence is received with ten seconds, a second EMSI_TCH sequence
        should be issued before the Server resumes operation. If, however, an
        EMSI_NAK sequence is received from the Client, the Server must
        re-transmit the EMSI_TCH sequence.

        The EMSI_ICI packet
        =====================================================================
        The following pseudo structure shows the layout of the EMSI_ICI
        packet. Note that the information in the EMSI_ICI packet may not
        exceed 2,048 bytes.

        EMSI_ICI
            name,
            alias,
            location,
            data#,
            voice#,
            password,
            birthdate,
            crtdef,
            protocols,
            capabilities,
            requests,
            software,
            xlattabl:                ASCII1;
        end;

        EMSI_ICI field definitions
        ---------------------------------------------------------------------

        =====================================================================
        Name and Alias (or Handle)
        ---------------------------------------------------------------------
        The name and possible alias (AKA) of the user (Client). This must be
        treated case insensitively by the Server.

        =====================================================================
        Location
        ---------------------------------------------------------------------
        The geographical location of the user, ie. Stockholm, Sweden.

        =====================================================================
        data# and voice#
        ---------------------------------------------------------------------
        Unformatted data and voice telephone numbers of the user. Unformatted
        is defined as the full telephone number, including country and local
        area code. Eg. 46-8-90510 is a telephone number in Stockholm, Sweden.

        =====================================================================
        Password
        ---------------------------------------------------------------------
        The password for the user. This must be treated case insensitively by
        the Server.

        =====================================================================
        Birthdate
        ---------------------------------------------------------------------
        Hexadecimal string representing a long integer containing the birth-
        date of the user in UNIX notation (number of seconds since midnight,
        Jan 1 1970). This must be treated case insensitively by the Server.

        =====================================================================
        CrtDef
        ---------------------------------------------------------------------
        Consisting of four sub-fields separated by commas, this field
        contains from left to right: The requested terminal emulation
        protocol, the number of rows of the user's CRT, the number of columns
        of the user's CRT, and the number of ASCII NUL (00H) characters the
        user's software requires to be transmitted between each line of text.

        The following terminal emulation protocols are defined:

            AVT0    AVATAR/0+. Used in conjunction with ANSI. If AVT0 is
                    specified by the Client, support for ANSI X3.64 emulation
                    should be assumed to be present.
            ANSI    ANSI X3.64
            VT52    DEC VT52
            VT100   DEC VT100
            TTY     No terminal emulation, also referred to as RAW mode.

        =====================================================================
        Protocols
        ---------------------------------------------------------------------
        The file transfer protocol option specifies the preferred method of
        transferring files between the Client and the Server in either
        direction. The Client presents all transfer protocols it is capable
        of supporting and the Server chooses the most appropriate protocol.

            DZA*    DirectZAP (Zmodem variant)
            ZAP     ZedZap (Zmodem variant)
            ZMO     Zmodem w/1,024 byte data packets
            SLK     SEAlink
            KER     Kermit

        (*) DirectZAP is a variant of ZedZap. The difference is that the
        transmitter only escapes CAN (18H). It is not recommended to use the
        DirectZAP protocol when the Client and the Server are connected via a
        packet switching network, or via another layer sensitive to control
        characters such as XON and XOFF.

        =====================================================================
        Capabilities
        ---------------------------------------------------------------------
        The capabilities of the user's software. If more than one capability
        is listed, each capability is separated by a comma.

        The following capability codes are defined:

            CHT     Can do full-screen on-line conversation (CHAT).
            MNU     Can do ASCII image download (see ISM packet).
            TAB     Can handle TAB (ASCII 09H) characters.
            ASCII8  Can handle 8-bit IBM PC ASCII characters.

        =====================================================================
        Requests
        ---------------------------------------------------------------------
        The requests field specifies what the user wishes to do once the
        initial IEMSI negotiation has been successfully completed. If more
        than one capability is listed, each capability is separated by a
        comma.

        The following request codes are defined:

            NEWS    Show bulletins, announcements, etc.
            MAIL    Check for new mail.
            FILE    Check for new files.
            HOT     Hot-Keys.
            CLR     Screen clearing.
            HUSH    Do not disturb.
            MORE    Page pausing, often referred to as "More".
            FSED*   Full-screen editor.
            XPRS    <reserved>.

        (*) Note that this allows the Client to request use of a full-screen
        editor without requiring that it also supports a full-screen terminal
        emulation protocol.

        =====================================================================
        Software
        ---------------------------------------------------------------------
        The name, version number, and optionally the serial number of the
        user's software. Eg. {FrontDoor,2.00,AE000001}.

        =====================================================================
        XlatTabl
        ---------------------------------------------------------------------
        Used for character translation between the Server and the Client.
        This field has not been completely defined yet and should always be
        transmitted as {} (empty).

        The EMSI_ISI packet
        =====================================================================
        The following pseudo structure shows the layout of the EMSI_ISI
        packet. Note that the information in the EMSI_ISI packet may not
        exceed 2,048 bytes.

        EMSI_ISI
            id,
            name,
            location,
            operator,
            localtime,
            notice,
            wait,
            capabilities:            ASCII1;
        end;

        EMSI_ISI field definitions
        ---------------------------------------------------------------------

        =====================================================================
        ID
        ---------------------------------------------------------------------
        The name, version number, and optionally the serial number of the
        Server software. Eg. {RemoteAccess,1.10/b5,CS000001}.

        =====================================================================
        Name
        ---------------------------------------------------------------------
        The name of the Server system. Eg. {Advanced Engineering S.A.R.L.}.

        =====================================================================
        Location
        ---------------------------------------------------------------------
        The geographical location of the user, ie. Stockholm, Sweden.

        =====================================================================
        Operator
        ---------------------------------------------------------------------
        The name of the primary operator of the Server software. Eg. {Joaquim
        H. Homrighausen}.

        =====================================================================
        Localtime
        ---------------------------------------------------------------------
        Hexadecimal string representing a long integer containing the current
        time of the Server in UNIX notation (number of seconds since midnight,
        Jan 1 1970). This must be treated case insensitively by the Client.

        =====================================================================
        Notice
        ---------------------------------------------------------------------
        May contain copyright notices, system information, etc. This field
        may optionally be displayed by the Client.

        =====================================================================
        Wait
        ---------------------------------------------------------------------
        A single character used by the Server to indicate that the user
        has to press the <Enter> key to resume operation. This is used in
        conjunction with ASCII Image Downloads (see ISM packet).

        =====================================================================
        Capabilities
        ---------------------------------------------------------------------
        The capabilities of the Server software. No Server software
        capabilities have currently been defined.

        Credits and other notes
        =====================================================================
        The original EMSI specifications were designed by Chris Irwin and
        Joaquim H. Homrighausen. The original IEMSI specifications were
        designed by Joaquim H. Homrighausen and Andrew Milner.

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