    Document: FTS-0007
    Version:  003
    Date:     15-Oct-1990
    Updates:  FTS-0001

                      An Enhanced FidoNet(r) Technical Standard
                    Extending FTS-0001 to include SEAlink protocol
                        (Including Overdrive and File Restart)

                                 October 15, 1990

    Status of this document:

        This document specifies an optional standard for the FidoNet community.
        Implementation of the protocols defined in this document is not mandatory,
        but all implementations of these protocols are expected to adhere to this
        standard.  Distribution of this document is subject to the limitations of
        the copyright notice displayed below.

        Copyright 1989-90 by Philip L. Becker.  Portions of this document are
        copyright 1986-90 by Randy Bush and are incorporated with his consent.
        All rights reserved.  A right to distribute only without modification and
        only at no charge is granted.  Under no circumstances is this document to
        be reproduced or distributed as part of or packaged with any product or
        other sales transaction for which any fee is charged.  Any and all other
        reproduction or excerpting requires the explicit written consent of the
        copyright holders.

        Introduction

        While the basic FTS-0001 protocol has become reasonably standardized, it
        is technically inferior when used with modem speeds of 2400bps or higher,
        and results in considerably slower file transfers than more sophisticated
        protocols under many line and modem configurations.

        Very sophisticated protocols exist to allow absolute maximum efficiency,
        but these protocols are much more difficult to implement than the basic
        XMODEM used by FTS-0001.  A need exists for a standardized, easy to
        implement extension to the FTS-0001 protocol which can gain much better
        performance.  SEAlink is such an extension.  It is nearly as easy to
        implement as the FTS-0001 protocol with which it is fully backward
        compatible.  Despite its ease of implementation, it provides several
        significant performance advantages. Among these advantages are:

             o  Transparently communicates with strict FTS-0001 implementations.

             o  Transparently communicates with FTS-0001 variants which omit
                either the MODEM7 file name or the TeLink header block.

             o  Transparently becomes a sliding window XMODEM protocol when
                communicating with a like implementation.  This sliding window
                protocol gives significantly improved throughput when there is
                an end-to-end delay.

             o  Offers a negotiated streaming mode for high speed asymmetrical
                modems to further enhance throughput for such links.

             o  Offers a negotiated file restart capability which allows an
                interrupted transfer to restart where it left off, reducing
                time spent to retransmit the file.

        This document defines the  data structures and communication protocols
        which a FidoNet SEAlink implementation must provide.  The implementor of
        FidoNet compatible systems is the intended audience of this document.

        This document has the same overall format and state table descriptions
        as FTS-0001.  SEAlink is implemented by modifying the following tables:

           Session Layer:   Sender - S1
           Network Layer:   Batch File Sender - BS0
           Network Layer:   Batch File Receiver - BR0
           Data Link Layer: XMODEM/TeLink Sender - XS0
           Data Link Layer: XMODEM/TeLink Receiver - XR0

                                                                                 1
                                  Table of Contents

                                                                              Page
        The purpose of the SEAlink protocol ...................................  3

        How SEAlink negotiates its enhancements ...............................  4

        Basic requirements for a FidoNet Implementation .......................  5

        Levels of Compliance ..................................................  5

        The ISO/OSI Reference Model ...........................................  5

        Data Description Language .............................................  6

        Finite State Machine Notation .........................................  7

        Glossary of variables and terms .......................................  7

        Application layer .....................................................  8

        Presentation layer ....................................................  8

        Session layer protocol ................................................  8
             Session State Table: Sender (S0) .................................  9
             Session State Table: Receiver (R0) ............................... 10

        Transport layer ....................................................... 10

        Network layer ......................................................... 11
             Data Definition: MODEM7 file name ................................ 11
             Network State Table: Batch File Sender (BS0) ..................... 12
             Network State Table: Batch File Receiver (BR0) ................... 13

        Data Link Layer ....................................................... 14
             Data Definition: XMODEM data block (CRC) ......................... 14
             Data Definition: XMODEM data block (Checksum) .................... 15
             Data Definition: TeLink header block ............................. 15
             Data Definition: SEAlink header block ............................ 16
             Data Definition: SEAlink RESYNC packet ........................... 16
             DDL Definition: XMODEMBlock, TeLink header ....................... 17
             DDL Definition: SEAlink header, ACK, NAK, RESYNC block ........... 18
             Checksum and CRC calculation algorithms .......................... 19
             Data Link Layer protocol ......................................... 20
             Data Link State Table: XMODEM/TeLink/SEAlink Sender (XS0) ........ 21
             Data Link State Table: Transmitter ACK/NAK check (AC0) ........... 22
             Data Link State Table: XMODEM/TeLink/SEAlink Receiver (XR0) ...... 24
             Data Link State Table: Send NAK (SN0) ............................ 26
             Data Link State Table: Send ACK (SA0) ............................ 26
             Data Link State Table: MODEM7 Filename Sender (MS0) .............. 27
             Data Link State Table: MODEM7 Filename Receiver (MR0) ............ 27

                                                                                2
        The purpose of the SEAlink protocol

        The purpose of the SEAlink protocol is to provide a much higher level
        of capability than the XMODEM protocol provides, while retaining the
        ease of implementation which has made the XMODEM protocol ubiquitous.

        In order for an extended protocol to function in FidoNet, it has to be
        fully upward compatible with FTS-0001 mailers, and also those slight
        variants of FTS-0001 which can communicate well with FTS-0001 mailers.
        To meet this requirement, any extension to the FTS-0001 protocol has
        to be capable of transparently negotiating away its extended capabilities
        and communicate with strict FTS-0001 mailers (and their approved variants)
        properly and reliably.

        This means that an extended protocol must miminally do the following:

            o  Detect that the other mailer can or cannot support its extensions
               automatically, and within the framework of a legitimate FTS-0001
               protocol encounter.

            o  Support mail sessions with or without MODEM7 file names and with
               or without TeLink headers in either combination.

        To be useful such an extended protocol should also be able to reliably
        detect when the other mailer can support its extensions so they can
        be used to maximum benefits.

        The major problems which exist with a standard FidoNet FTS-0001 session
        result from the use of the XMODEM protocol.  This is a half duplex protocol
        which forces a line turnaround on every transmitted block.  As a result,
        any end-to-end delay in the transmission link is directly added to each
        transmitted data block twice (once for each line turnaround).

        To dramatize how easily XMODEM is impacted by even small line delays, let's
        examine a 2400bps call on a line with 500ms (1/2 second) delay on each line
        turnaround.  This is not an uncommon delay time on long distance calls.  A
        single data block in the XMODEM CRC format contains 133 characters.  This
        means it will be transmitted in 554ms.  The ACK/NAK response is a single
        character and will take 4ms.  Thus with no delay (an ideal link) an XMODEM
        transfer would send 128 data characters in 558ms for an effective data
        throughput of about 230cps.  With a 500ms line turnaround delay, these same
        128 data bytes will take 1558ms resulting in a throughput of 82cps.  If
        faster modem speeds are used, the percentage impact is even greater.

        The solution to this problem is to enhance the XMODEM protocol by adding
        a "sliding window" capability which allows more than one block to be sent
        before an acknowledgment is received.  This converts the protocol to a
        full duplex protocol, and if the "window size" (the number of blocks which
        may be sent before the sender must wait for an acknowledgment) is larger
        that the line turnaround delay, then the ideal throughput can be restored
        even to lines with long turnaround delays.  SEAlink is such a protocol.

        The standard SEAlink window size is 6 blocks, but the state tables given
        below implement a receiver which will operate correctly with any window
        size up to 127 blocks.  Thus an implementation which uses a larger window
        size will be totally compatible with the standard 6 block window versions.

        A second problem with the XMODEM protocol arises when asymmetrical high
        speed modems are used.  These modems achieve much higher throughput when
        data is sent in only one direction.  Since they provide error free links,
        a protocol which does not send any positive acknowledgments, but only
        reports any bad blocks received will achieve a significantly higher

                                                                                 3
        throughput than a protocol which is either full duplex or which turns
        around between each block such as XMODEM or normal SEAlink.  It is for
        this purpose that SEAlink Overdrive is provided.  It is a streaming version
        of SEAlink designed to provide much higher throughput on asymmetrical
        high speed links which provide end-to-end data flow control.

        Finally, there is the annoying problem which occurs when a large data file
        transfer has nearly completed and a loss of connection occurs.  Normally
        the entire file must be retransmitted on a new call, resulting in lost
        time and money.  The SEAlink RESYNC enhancement allows an interrupted
        file transfer to be resumed at the point it was interrupted thus minimizing
        the impact of such an interruption.

        How SEAlink Negotiates its enhancements

        SEAlink makes some assumptions about how FTS-0001 mailer implementations
        react to various stimuli in order to negotiate its enhancements.  For the
        sender, the test consists of two parts:

        1.  Send a SEAlink header and see if the other end acknowledges it.  In
            general it will, because most XMODEM implementations will think that
            the SEAlink header is a "previous block" and ACK and discard it.  If
            the receiver refuses to accept a SEAlink header block in three tries,
            then it clearly cannot do SEAlink protocol and the negotiation is over.

        2.  Since the receiver's acknowledgment of the SEAlink header is not a
            sufficient criteria to determine if the receiver in fact supports
            SEAlink, the sender dynamically examines the acknowledgments the
            receiver provides to determine if their format indicates support of
            SEAlink or not and adjusts its sending techniques accordingly.  This
            is also the technique used to detect whether the receiver is in fact
            supporting any extensions (such as SEAlink Overdrive) which have been
            requested in the header.

        For the receiver, the negotiation occurs during the receipt of the first
        valid block.

        1.   If the first block received is a valid SEAlink header, then the
             transmitter supports SEAlink and the receiver can switch to it.  This
             same header also indicates if the transmitter wants or can support the
             SEAlink options such as Overdrive and File RESYNC.

         2.  If the first block received is a valid TeLink header, then the
             transmitter supports a variant of FTS-0001 and SEAlink support may
             be assumed to be absent.

         3.  If the first block received is an XMODEM data block then SEAlink
             support may also be assumed to be absent.

        If the receiver gets a SEAlink header, it can then arbitrarily decide
        which of any requested options it wishes to use.  It may not use an option
        for which support is not indicated in the sender's SEAlink header block.

        The remainder of this document provides the details for a full SEAlink
        implementation with Overdrive and RESYNC support.  A glossary of terms and
        indicators is provided along with a full state table description of the
        protocol implementation.

                                                                                 4
        This document follows the format of FTS-0001 to allow ease of
        comparison of the two protocols.  This document could not have been
        generated without the tremendous efforts of those whose work resulted in
        FTS-0001.  FidoNet owes a great debt to those efforts.  The following
        introduction is reprinted from FTS-0001.

        The layered metaphor of the ISO Open Systems Interface reference model
        has been used to view FidoNet from a standard perspective.  As with most
        prospective ISO/OSI descriptions, FidoNet does not always make this easy.

       1. Basic Requirements for a FidoNet Implementation

          Compatibility is a set of abilities which, when taken as a whole, make
          it safe to list a net or node in the IFNA nodelist.  In other words,
          if another node should attempt contact, does it have a reasonable
          chance of successful communication?  This is a social obligation, as
          the calling system pays money for the attempt.  Conversely, an
          implementation should be able to successfully contact other systems,
          as life is not a one-way street.

          A FidoNet implementation must be able to call other nodes and transfer
          messages and files in both directions.  This includes pickup and poll.

          A FidoNet implementation must be able to accept calls from other nodes
          and transfer messages and files in both directions.  This includes
          pickup.

          A FidoNet implementation must be able to receive and process the IFNA
          format nodelist, and transfer nodelists to other nodes.  A companion
          document, FTS-0005, defines the IFNA format nodelist and how to
          interpret and process it.

          A FidoNet implementation must route messages which do not have files
          attached through net hosts as shown in an IFNA format nodelist.

       2. Levels of Compliance

          This documents represents an extended FidoNet implementation.  It
          defines a well tested extension which is optional but provides
          sufficient additional function that implementors should seriously
          consider it.  SEAdog(tm), from System Enhancement Associates,
          is the inspiration for this extended FidoNet implementation.
          System Enhancement Associates is the creator of the SEAlink protocol.

       3. The ISO/OSI Reference Model (cribbed from "Protocol Verification via
          Executable Logic Specifications", D. P. Sidhu, in Rudin & West)

          In the ISO/OSI model, a distributed system consists of entities that
          communicate with each other according to a set of rules called a
          protocol.   The model is layered, and there are entities associated
          with each layer of the model which provide services to higher layers
          by exchanging information with their peer entities using the services
          of lower layers.  The only actual physical communication between two
          systems is at the lowest level.

                                                                                 5
          Several techniques have been used in the specification of such
          protocols.  A common ingredient in all techniques is the notion of the
          extended finite state automata or machine.  Extensions include the
          addition of state variables for the storing of state information about
          the protocol.  The state of an automaton can change as a result of
          one of the following events:

          o Request from an upper network layer for service

          o Response to the upper layer

          o Request to the lower network layer to perform a service

          o Response from the lower layer

          o Interaction with the system and environment in which the protocol is
            implemented (e.g. timeouts, host operating system aborts, ...)

          A protocol specification, in a large part, consists of specifying
          state changes in automata which model protocol entities and in
          describing the data which they exchange.

          For historical reasons, the term packet is used in FidoNet to
          represent a bundle of messages, as opposed to the more common use as a
          unit of communication, which is known as a block in FidoNet.

       4. Data Description

          A language specific notation was avoided.  Please help stamp out
          environmental dependencies.   Don't panic, there are rectangular record
          layouts too.  The following defines the data description language used.

          (* non-terminals *)
          UpperCaseName - to be defined further on

          (* literals *)
          "ABC"         - ASCII character string, no termination implied
          nnH           - byte in hexadecimal

          (* terminals *)
          someName      - 16-bit integer, low order byte first (8080 style)
          someName[n]   - field of n bytes
          someName[.n]  - field of n bits
          someName(n)   - Null terminated string allocated n chars (incl Null)
          someName{max} - Null terminated string of up to max chars (incl Null)
          someName - String of up to max chars, NOT null terminated

          (* punctuation *)
          a b           - one 'a' followed by one 'b'
          ( a | b )     - either 'a' or 'b', but not both
          { a }         - zero or more 'a's
          [ b ]         - zero or one 'b'
          (* comment *) - ignored

          (* predeclared constant *)
          Null          = 00H

                                                                                 6
     5. Finite State Machine Notation

        .-----+----------+-------------------------+-------------------------+-----.
        |State| State    | Predicate(s)            | Action(s)               | Next|
        |  #  | Name     |                         |                         | St  |
        +-----+----------+-------------------------+-------------------------+-----+
        | fnn*|          |                         |                         |     |
        `-----+----------+-------------------------+-------------------------+-----'

        State #      - Number of this state (e.g. R13).
                       f  - FSM initial (Window, Sender, Receiver, ...)
                       nn - state number
                       *  - state which represents a lower level protocol  which
                            is represented by yet another automation.

        State Name   - Descriptive name of this state.

        Predicate(s) - Conditions which terminate the state.  If predicates are
                       non-exclusive, consider them ordered.

        Action(s)    - Action(s) corresponding to predicate(s)

        Next State   - Subsequent state corresponding to predicate(s)

        Ideally, there should be a supporting section for each state which
        should give a prose description of the state, its predicates, actions,
        etc.  So much for ideals.  The following is a list of all of the terms
        and variables used in the following state machine descriptions:

                         Glossary of variables and terms

        SEAlink - Flag indicating SEAlink or XMODEM mode

        SLO - Flag indicating overdrive if in SEAlink mode

        RESYNC - Flag indicating whether transmitting end can honor RESYNC
                 file positioning requests or only NAKs

        MACFLOW - Flag indicating whether the sender supports the Macintosh flow
                  control option.  This is an optional feature used by TABBY
                  because the Macintosh serial port does not support RTS/CTS.

        CRC - Flag indicating whether block check is done using CRC or Checksum

        T1 and T2 - Timeout Timers which run asynchronously with the code

        WINDOW - Number of unacknowledged blocks which may be transmitted

        SendBLK - Next 128 byte block number in file to send.  May not occur in
                  sequential order, so file positioning may be necessary when
                  it is time to send this block

        ACKBLK - Highest block number in file which has been acknowledged by
                 the receiver as received without error

        Last Blk - Block number of last 128 byte block (or partial block) in the
                   file being sent.

        NumNAK - Number of NAKs received since last ACK

        ACKs Rcvd - Number of ACKs received since the start of this file send

                                                                                 7
                         Glossary of variables and terms (cont.)

        ACKST - State of ACK/NAK machine during auto-detect of SEAlink or XMODEM
                style ACK/NAK block receipt

        RESYNC BLK# - Block number in file requested by a received RESYNC packet

        ARBLK8 - Block # (0-255) received in a SEAlink style ACK/NAK packet

        ARBLK - Block # in file (calculated from ARBLK8) which is the actual
                block being referenced in the SEAlink ACK/NAK packet

        blocknum - Block # (0-255) sent in a SEAlink style ACK/NAK packet

        WriteBLK - Block # in file to write next correctly received data block.
                   Note: Block 1 is the first byte of the file.

        CHR - Temp holding variable for received character during send operation

     B. Application Layer : the System from the User's View

        This is unchanged from FTS-0001.

     C. Presentation Layer : the User from the System's View

        This is unchanged from FTS-0001.

     D. Session Layer Protocol : Connecting to Another FidoNet Machine

        A session is a connection between two FidoNet machines.  It is currently
        assumed to be over the DDD telephone network via modems.  The calling
        machine starts out as the sender and the called machine as the receiver.
        The pickup feature is described by the sender and receiver changing
        roles midway through the session, after the sender has transferred the
        message packet and any attached files.  Due to the lack of security in
        the pickup protocol (danger of pickup by a fake node), extensions to the
        basic Session protocol have been developed.  This document describes only
        the minimum Session Layer protocol (as in FTS-0001).

        Once a connection has been established, each system should ensure that
        the physical connection remains throughout the session.  For physical
        layers implemented through modems, this means monitoring the carrier
        detect signal, and terminating the session if it is lost.

        Error detection at the physical layer should be monitored for both sent
        and received characters.  Parity, framing, and other physical errors
        should be detected.

        The only change to the Session Layer state tables from FTS-0001 is in the
        Sender state "S1", Predicate "1" (S1.1) entry.

                                                                                 8
        Sender

        .-----+----------+-------------------------+-------------------------+-----.
        |State| State    | Predicate(s)            | Action(s)               | Next|
        |  #  | Name     |                         |                         | St  |
        +-----+----------+-------------------------+-------------------------+-----+
        | S0  | SendInit |                         | dial modem              | S1  |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | S1  | WaitCxD  |1| carrier detected      | delay 1-5 seconds       | S2  |
        |     |  (*1)    | |                       | Set SLO if > 2400bps,   |     |
        |     |          | |                       | Reset SLO if <= 2400bps |     |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| busy, etc.            | report no connection    | exit|
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |3| voice                 | report no carrier       | exit|
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |4| carrier not detected  | report no connection    | exit|
        |     |          | | within 60 seconds     |                         |     |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | S2  | WhackCRs |1| over 30 seconds       | report no response <cr> | exit|
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| ?? <cr>s received     | delay 1 sec             | S3  |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |3| <cr>s not received    | send <cr> <sp> <cr> <sp>| S2  |
        |     |          | |                       |   delay ??? secs        |     |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | S3  | WaitClear|1| no input for 0.5 secs | send TSYNCH = AEH       | S4  |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| over 60 seconds       | hang up, report garbage | exit|
        |     |          | | and line not clear    |                         |     |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | S4* | SendMail |                         | (XMODEM send packet XS0)| S5  |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | S5  | CheckMail|1| XMODEM successful     | (Fido registers success)| S6  |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| XMODEM fail or timeout| hang up, report mail bad| exit|
        +-----+----------+-+-----------------------+-------------------------+-----+
        | S6* | SendFiles|                         | (BATCH send files BS0)  | S7  |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | S7  | CheckFile|1| BATCH send successful |                         | S8  |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| BATCH send failed     | hang up, rept files fail| exit|
        +-----+----------+-+-----------------------+-------------------------+-----+
        | S8  | TryPickup|1| wish to pickup        | note send ok            | R2* |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| no desire to pickup   | delay 5 secs            | exit|
        |     |          | |                       |   hang up, rept send ok |     |
        `-----+----------+-+-----------------------+-------------------------+-----'
        Although the above shows the sender emitting only one TSYNCH, it is
        recommended that a timeout of 5-20 seconds should initiate another TSYNCH.
        The receiver should tolerate multiple TSYNCHs.

        *1 - The action for (S1.1) is the only change from the corresponding
             FTS-0001 state table.

                                                                                 9
        Receiver

        The receiving FSM is given an external timer, the expiration of which
        will cause termination with a result of 'no calls' (R0.2).

        .-----+----------+-------------------------+-------------------------+-----.
        |State| State    | Predicate(s)            | Action(s)               | Next|
        |  #  | Name     |                         |                         | St  |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | R0  | WaitCxD  |1| carrier detected      |                         | R1  |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| external timer expires| report no calls         | exit|
        +-----+----------+-+-----------------------+-------------------------+-----+
        | R1  | WaitBaud |1| baud rate detected    | send signon with <cr>s  | R2  |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| no detect in ?? secs  | hang up, report no baud | exit|
        +-----+----------+-+-----------------------+-------------------------+-----+
        | R2  | WaitTsync|1| TSYNCH received       | ignore input not TSYNCH | R3  |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| 60 seconds timeout    | hang up, report not Fido| exit|
        +-----+----------+-+-----------------------+-------------------------+-----+
        | R3* | RecMail  |                         | (XMODEM rec packet XR0) | R4  |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | R4  | XRecEnd  |1| XMODEM successful     | delay 1 second          | R5  |
        |     |          | |                       |   flush input           |     |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| XMODEM failed         | hang up, rept mail fail | exit|
        +-----+----------+-+-----------------------+-------------------------+-----+
        | R5* | RecFiles |                         | (BATCH rec files BR0)   | R6  |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | R6  | ChkFiles |1| BATCH recv successful | delay 2 secs            | R7  |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| BATCH recv failed     | hang up, report bad file| exit|
        +-----+----------+-+-----------------------+-------------------------+-----+
        | R7  | AllowPkup|1| have pickup for sender| receiver becomes sender | S3* |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| nothing to pickup     | hang up, rept recv ok   | exit|
        `-----+----------+-+-----------------------+-------------------------+-----'

        There is no change in the Session Layer Receiver state table from FTS-0001.

     E. Transport Layer : ?????

        This is unchanged from FTS-0001.

                                                                                10
     F. Network Layer : the Network's View of the System, Routing and Packets

       1. Network Layer Data Definition : the Packet Header

          This is unchanged from FTS-0001.

       2. Network Layer Data Description : a File with Attributes

          The BATCH protocol uses the MODEM7 filename and/or SEAlink/TeLink/XMODEM
          file transfer protocols to transfer the file with attributes.

          When a file is transferred via FidoNet, an attempt is made to also
          pass the operating system's attributes for the file such as length,
          modification date, etc.  FidoNet does this via a special prefix block
          to the XMODEM file transfer using a protocol known as TeLink.  As the
          TeLink protocol relies on a modification to the XMODEM file transfer
          protocol, it is documented at the data link layer level.  Optionally,
          if both sender and receiver implement the SEAlink extension, file
          information is passed using the SEAlink header block which also
          contains feature negotiation information.

          The MODEM7 file name is redundant if there is also a TeLink or SEAlink
          block, in which case the name may be taken from either or both.  In this
          extended implementation, the MODEM7 file name is never required.  It
          is sent, however, if it appears that the other node is using a strict
          FTS-0001 implementation.  This implementation will adapt to an FTS-0001
          variant which only sends the TeLink header without the MODEM7 filename
          also so that it is compatible with all know variants of FTS-0001 which
          are currently in the FidoNet network.

                              FileName as Sent by MODEM7
           Offset
          dec hex
                  .-----------------------------------------------.
            0   0 |                   fileName                    |
                  ~                   8  bytes                    ~
                  |           left adjusted blank filled          |
                  +-----------------------------------------------+
            8   8 |                    fileExt                    |
                  ~                    3  bytes                   ~
                  |           left adjusted blank filled          |
                  `-----------------------------------------------'

                                                                                11
     3. Network Layer Protocol : BATCH File Finite State Machines

        BATCH File Sender

        .-----+----------+-------------------------+-------------------------+-----.
        |State| State    | Predicate(s)            | Action(s)               | Next|
        |  #  | Name     |                         |                         | St  |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | BS0 | MoreFiles|1| more files to send    |                         | BS1 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| no more files to send |                         | BS4 |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | BS1 | WaitType |1| rec NAK               | (MODEM7 FName send MS0) | BS2 |
        |     |    (*1)  +-+-----------------------+-------------------------+-----+
        |     |          |2| rec 'C'               | (SEAlink send file XS0) | BS3 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |3| rec other char        | eat character           | BS1 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |4| > 20 sec in BS1       | report name send bad    | exit|
        +-----+----------+-+-----------------------+-------------------------+-----+
        | BS2 | CheckFNm |1| MODEM7 Filename ok    | (TeLink send file XS0T) | BS3 |
        |     |    (*2)  +-+-----------------------+-------------------------+-----+
        |     |          |2| MODEM7 Filename bad   | report name send bad    | exit|
        +-----+----------+-+-----------------------+-------------------------+-----+
        | BS3 | CheckFile|1| File send ok          |                         | BS0 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| File send bad         | report file send bad    | exit|
        +-----+----------+-+-----------------------+-------------------------+-----+
        | BS4 | EndSend  |1| rec NAK or 'C'        | send EOT, report send ok| exit|
        |     |    (*3)  +-+-----------------------+-------------------------+-----+
        |     |          |2| 10 secs no NAK or 'C' | send EOT, report no NAK | exit|
        `-----+----------+-+-----------------------+-------------------------+-----'

        *1 - Note: Filenames must be upper case ASCII.  The data link layer uses
             lower case "u" as a control character during MODEM7 name transmission.

        *2 - Note: SEAdog (through version 4.51b) does not possess a state "XS0T".
             It therefore calls XS0 from state BS2, resulting in a MODEM7 file name
             being sent with no TeLink header on batch file transmissions when it
             is not in SEAlink mode.

        *3 - When no files remain, the sender responds to the receiver's NAK with
             an EOT.  The EOT is not ACK/NAKed by the receiver.

                                                                                12
       BATCH File Receiver

        .-----+----------+-------------------------+-------------------------+-----.
        |State| State    | Predicate(s)            | Action(s)               | Next|
        |  #  | Name     |                         |                         | St  |
        +-----+----------+-------------------------+-------------------------+-----+
        | BR0 | TestSL   |                         | Send 'C',               | BR1 |
        |     |          |                         |   Set T1 to 10 sec      |     |
        |     |          |                         |   Set T2 to 120 sec     |     |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | BR1 | CheckSL  |1| > 2 sec with no data  | Send 'C'                | BR1 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| Timer T2 expired      | (MODEM7 FName recv MR0) | BR2 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |3| Character Waiting     | "Peek" char to CHR (*1) | BR4 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |4| Timer T1 expired      | (MODEM7 FName recv MR0) | BR2 |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | BR2 | CheckFNm |1| no more files         | report files recd ok    | exit|
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| Filename ok           | (Rcv file Telink XR0)   | BR3 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |3| Filename bad          | report name recv bad    | exit|
        +-----+----------+-+-----------------------+-------------------------+-----+
        | BR3 | CheckFile|1| File received ok      |                         | BR0 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| File received bad     | report file recv bad    | exit|
        +-----+----------+-+-----------------------+-------------------------+-----+
        | BR4 | FindType |1| CHR = NUL             | eat character,          | BR1 |
        |     |          | |                       |   Reset T1 to 20 secs   |     |
        |     |   (*2)   +-+-----------------------+-------------------------+-----+
        |     |          |2| CHR = SOH             | (Rcv File SEAlink XR0B) | BR3 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |3| CHR = SYN             | (Rcv File Telink XR0B)  | BR3 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |4| CHR = EOT or          | eat character,          | exit|
        |     |          | | CHR = SUB             |   report files recd ok  |     |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |5| CHR = Other char      | eat character           | BR1 |
        `-----+----------+-+-----------------------+-------------------------+-----'

        *1 - "Peek" a character means to place it in CHR but leave it in the input
             buffer so the next read operation will re-read it.

        *2 - "Eat" a character means to remove it from the input buffer.

                                                                                13
     G. Data Link Layer : Error-Free Data Transfer

       1. Data Link Layer Data Definition : XMODEM/TeLink/SEAlink Blocks

          XMODEM transfers are in blocks of 128 uninterpreted data bytes
          preceded by a three byte header and followed by either a one byte
          checksum or a two byte crc remainder.  XMODEM makes no provision  for
          data streams which are not an integral number of blocks long.
          Therefore, the sender pads streams whose length is not a multiple of
          128 bytes with the end-of-file character (^Z for MS-DOS), and uses some
          other means to convey the true data length to the receiver (e.g.
          SEAlink or TeLink file info header block).

          Data blocks contain sequence numbers so the receiver can ensure it has
          the correct block.  Data block numbers are sequential unsigned eight bit
          integers beginning with 01H and wrapping to 00H.  A TeLink or SEAlink
          header block is given sequence number 00H.

          For files which are attached to the mail packet (but not the mail packet
          itself), if the sending system is aware of the file attributes as they
          are known to the operating system, then the first block of the XMODEM
          transfer may be a special TeLink block to transfer that information.
          This block differs in that the first byte is a SYN character as
          opposed to an SOH, and it is always sent checksum as opposed to CRC.
          Should the receiver be unwilling to handle such information, after four
          NAKs (or "C"s), the sender skips this special block and goes on to the
          data itself.

          In this extended protocol the TeLink header block may be replaced by
          the SEAlink header block which conveys protocol negotiation information
          in addition to the file attributes if both nodes implement SEAlink.

                            XMODEM Data Block (CRC mode)
           Offset
          dec hex
                  .-----------------------------------------------.
            0   0 |        SOH  -  Start Of Header -  01H         |
                  +-----------------------------------------------+
            1   1 |                 BlockNumber                   |
                  +-----------------------------------------------+
            2   2 |               BlockComplement                 |
                  +-----------------------------------------------+
            3   3 |                128 bytes  of                  |
                  ~                uninterpreted                  ~
                  |                    data                       |
                  +-----------------------------------------------+
          131  83 |             CRC high order byte               |
                  +-----------------------------------------------+
          132  84 |             CRC  low order byte               |
                  `-----------------------------------------------'

                                                                                14
                          XMODEM Data Block (Checksum mode)
           Offset
          dec hex
                  .-----------------------------------------------.
            0   0 |        SOH  -  Start Of Header -  01H         |
                  +-----------------------------------------------+
            1   1 |                 BlockNumber                   |
                  +-----------------------------------------------+
            2   2 |               BlockComplement                 |
                  +-----------------------------------------------+
            3   3 |                128 bytes  of                  |
                  ~                uninterpreted                  ~
                  |                    data                       |
                  +-----------------------------------------------+
          131  83 |                Checksum byte                  |
                  `-----------------------------------------------'

                           TeLink File Descriptor Block
           Offset
          dec hex
                  .-----------------------------------------------.
            0   0 |       SYN  -  File Info Header -  16H         |
                  +-----------------------------------------------+
            1   1 |                     00H                       |
                  +-----------------------------------------------+
            2   2 |                     FFH                       |  dec  hex
                  +-----------------------------------------------+
            3   3 |     File Length, least significant byte       |  0    0
                  +-----------------------------------------------+
            4   4 | File Length, second to least significant byte |  1    1
                  +-----------------------------------------------+
            5   5 |  File Length, second to most significant byte |  2    2
                  +-----------------------------------------------+
            6   6 |      File Length, most significant byte       |  3    3
                  +-----------------------------------------------+
            7   7 |            Creation Time of File              |  4    4
                  |                "DOS Format"                   |
                  +-----------------------------------------------+
            9   9 |            Creation Date of File              |  6    6
                  |                "DOS Format"                   |
                  +-----------------------------------------------+
           11   B |                 File  Name                    |  8    8
                  ~                  16 chars                     ~
                  |        left justified  blank filled           |
                  +-----------------------------------------------+
           27  1B |                    00H                        | 24   18
                  +-----------------------------------------------+
           28  1C |            Sending Program Name               | 25   19
                  ~                  16 chars                     ~
                  |         left justified  Null filled           |
                  +-----------------------------------------------+
           44  2B |            01H (for CRC) or 00H               | 41   29
                  +-----------------------------------------------+
           45  2C |                    fill                       | 42   2A
                  ~                  86 bytes                     ~
                  |                  all zero                     |
                  +-----------------------------------------------+
          131  83 |                Checksum byte                  |
                  `-----------------------------------------------'

                                                                                15
          Offset           SEAink File Descriptor Block
          dec hex
                  .-----------------------------------------------.
            0   0 |       SOH  -  Start of Header -   01H         |
                  +-----------------------------------------------+
            1   1 |                     00H                       |
                  +-----------------------------------------------+
            2   2 |                     FFH                       |  dec  hex
                  +-----------------------------------------------+
            3   3 |         File Length, (4 bytes, LSB first)     |  0    0
                  +-----------------------------------------------+
            7   7 |            Creation Date/Time of File         |  4    4
                  |   (4 bytes, LSB first, seconds since 1979)    |
                  +-----------------------------------------------+
           11   B |                 File  Name                    |  8    8
                  ~                  17 chars                     ~
                  |        left justified   Null filled           |
                  +-----------------------------------------------+
           28  1C |            Sending Program Name               | 25   19
                  ~                  15 chars                     ~
                  |         left justified  Null filled           |
                  +-----------------------------------------------+
           43  2B |            > 0 if SLO Requested               | 40   28
                  +-----------------------------------------------+
           44  2C |           > 0 if RESYNC Supported             | 41   29
                  +-----------------------------------------------+
           45  2D |           > 0 if MACFLOW Supported            | 42   2A
                  +-----------------------------------------------+
           46  2E |                    fill                       | 43   2B
                  ~                  85 bytes                     ~
                  |                  all zero                     |
                  +-----------------------------------------------+
          131  83 |            CRC high order byte                |
                  +-----------------------------------------------+
          132  84 |            CRC  low order byte                |
                  `-----------------------------------------------'

          Offset                SEAlink RESYNC packet
          dec hex
                  .-----------------------------------------------.
            0   0 |       SYN - Start of RESYNC packet - 16H      |
                  +-----------------------------------------------+
            1   1 |     ASCII Decimal 128 byte block number in    |
                  ~     file to restart sending.  (No leading     ~
            n     |     or trailing blanks, MSD first).           |
                  +-----------------------------------------------+
            n+1   |       ETX -  End of RESYNC packet  - 03H      |
                  +-----------------------------------------------+
            n+2   |     (*1)     CRC  low order byte              |
                  +-----------------------------------------------+
            n+3   |     (*1)     CRC high order byte              |
                  `-----------------------------------------------'

                  *1 - CRC does not include the SYN or ETX and is
                       in the reverse byte order from the CRC in a
                       normal XMODEM data packet.  The following is
                       a sample RESYNC packet for file block 27 (1BH).

                         SYN   '2'   '7'   ETX  CRCLO CRCHI
                       .-----+-----+-----+-----+-----+-----.
                       | 16H | 32H | 37H | 03H | 43H | 25H |
                       `-----+-----+-----+-----+-----+-----'

                                                                                16
        Data Description language definitions of block types:

          XMODEMData   =   XMODEMBlock    (* block of data with hdr and trailer *)
                         | SEALinkBlock   (* SEALink File Descriptor Block *)
                         | TeLinkBlock    (* TeLink File Descriptor Block *)
                         | ReSyncBlock    (* SEAlink RESYNC request packet *)
                         | ACK            (* acknowledge data received ok *)
                         | NAK            (* negative ACK & poll 1st block *)
                         | SEAlinkACK     (* acknowledge data received ok *)
                         | SEAlinkNAK     (* negative ACK & poll 1st block *)
                         | EOT            (* end of xfer, after last block *)
                         | "C"            (* 43H *)

          XMODEMBlock  = SOH              (* Start of Header, XMODEM Block *)
                         blockNumber[1]   (* sequence, i'=mod( i+1, 256 ) *)
                         blockCompl[1]    (* one's complement of blockNumber *)
                         data[128]        (* uninterpreted user data block *)
                         (CRC | Checksum) (* error detect/correction code *)

          TeLinkBlock  = SYN              (* File Info Header *)
                         00H              (* block no, must be first block *)
                         FFH              (* one's complement of block no *)
                         fileLength[4]    (* length of data in bytes *)
                    (*2) CreationTime[2]  (* time file last modified or zero *)
                    (*2) CreationDate[2]  (* date file last modified or zero *)
                         fileName(16)     (* name of file, not vol or dir *)
                         00H              (* header version number *)
                         sendingProg(16)  (* name of program on send side *)
                    (*1) crcMode[1]       (* 01H for CRC 00H for Checksum *)
                         fill[87]         (* zeroed *)
                         Checksum         (* error detect/correction code *)

        *1 -  Note that the crcMode is always set to 01H in current implementations
              as all TeLink/XMODEM implementations use the CRC method.  Therefore,
              it is always set to 01H by the sender, and is ignored by the receiver.

        *2 -  CreationDate and CreationTime are MS-DOS format as follows:

          CreationDate = year[.7]         (* 7 bits, years since 1980, 0-127  *)
                         month[.4]        (* 4 bits, month of year, 1-12 *)
                         day[.5]          (* 5 bits, day of month, 1-31 *)

          CreationTime = hour[.5]         (* 5 bits, hour of day, 0-23 *)
                         minute[.6]       (* 6 bits, minute of hour, 0-60 *)
                         biSeconds[.2]    (* 6 bits, seconds/2, 0-29 *)

                                                                                17
        Data Description Language definition of the block types added by this
        extended protocol specification:

         SEALinkBlock  = SOH              (* File Info Header *)
                         00H              (* block no, must be first block *)
                         FFH              (* one's complement of block no *)
                         fileLength[4]    (* length of data in bytes *)
                  (*1)   Creation[4]      (* Seconds since 1979 file last chgd *)
                         fileName(17)     (* name of file, not vol or dir *)
                         sendingProg(15)  (* name of program on send side *)
                         SLO[1]           (* 01H for Overdrive supported *)
                         RESYNC[1]        (* 01H for file Restart supported *)
                         MACFLOW[1]       (* 01H for Macintosh flow supported *)
                         fill[85]         (* zeroed *)
                         CRC              (* error detect/correction code *)

        *1 - Creation is a long integer number of seconds since January 1, 1979.

         SEAlinkACK    = ACK              (* indicator data block received ok *)
                         blockNumber[1]   (* sequence, i'=mod( i+1, 256 ) *)
                         blockCompl[1]    (* one's complement of blockNumber *)

         SEAlinkNAK    = NAK              (* indicator block not received ok *)
                         blockNumber[1]   (* sequence, i'=mod( i+1, 256 ) *)
                         blockCompl[1]    (* one's complement of blockNumber *)

         ReSyncBlock   = SYN              (* File Restart Position *)
                    (*1) RestartBlock<20> (* ASCII decimal file block # *)
                         ETX              (* End of block number text *)
                         CRC(rev order)   (* error detection code *)

         *1 - RestartBlock is a text ASCII version of the decimal block number
              in the file desired.  Note:  The first block of the file is block 1.

        Definitions of Single byte Character values used in protocol:

          ACK          = 06H              (* acknowledge data received ok *)
          NAK          = 15H              (* negative ACK & poll 1st block *)
          SOH          = 01H              (* start of header, begins block *)
          SYN          = 16H              (* start of TeLink file info blk *)
          EOT          = 04H              (* end of xfer, after last block *)
          ETX          = 03H              (* end of RESYNC request data field*)

                                                                                18
        Block Verification calculated values used by this protocol:

          CRC          = crc[2]           (* CCITT Cyclic Redundancy Check *)

          Checksum     = checksum[1]      (* low 8 bits of sum of data bytes
                                             using unsigned 8 bit arithmetic *)

        Calculating Checksum
        --------------------

        For blocks which use a checksum to do error detection, the checksum is
        calculated by initializing an accumulator to zero and doing successive
        addition of each character in the data field.  Carry is discarded on
        each addition.  The resulting 8 bit value is the checksum.

        Calculating CRC
        ---------------

        For blocks which use CRC to do error detection, the CRC is calculated
        using the CCITT V.41 generator polynomial.  An accumulator is initialized
        to zero, and then each character of the data field is processed by the
        CRC generator polynomial.  This process can be quite complex to explain,
        but is not so complex in practice.  The following CRC routine is
        given here as an aid to understanding the CRC generation process.

        8086 assembler routine to implement CCITT V.41 CRC algorithm
        ;---------------------------------------------------------------+
        ;       CRCUPD - Update CRC value from character in AL          |
        ;                                                               |
        ;  CRC is calculated using the CCITT V.41 generator polynomial. |
        ;    That polynomial is: X^16 + X^12 + X^5 + 1 (X^0)            |
        ;                                                               |
        ;  As an aid to understanding, remember that XOR is bitwise     |
        ;    addition without carry.                                    |
        ;---------------------------------------------------------------+
        CRCVAL  DW      0                   ;16 bit CRC accumulator
        ;
        CRCUPD: PUSH    AX                  ;All registers preserved
                PUSH    CX
                PUSH    DX
                MOV     DX,[CRCVAL]
                XOR     DH,AL               ;init X^16 term
                XOR     DL,DL
                MOV     CX,8
        CRCUP1: SHL     DX,1
                JNC     CRCUP2
                XOR     DX,1021h            ;X^12 + X^5 + 1
        CRCUP2: LOOP    CRCUP1
                XOR     DH,BYTE PTR[CRCVAL] ;finish X^16 term
                MOV     [CRCVAL],DX         ;update CRC accumulator
                POP     DX
                POP     CX
                POP     AX
                RET

                                                                                19
     2. Data Link Layer Protocol : XMODEM/TeLink/SEAlink Finite State Machines

        The protocol is receiver driven, the receiver polling the sender for
        each block.  If the receiver polls for the first block using a "C"
        (43H) as  the poll character, it would prefer to have the CRC-CCITT V.41
        polynomial remainder error detection code at the end of each block as
        opposed to a one byte unsigned checksum.  The sender will respond to
        the "C" poll if it can  comply. If the sender chooses checksum as
        opposed to CRC, it waits for the receiver to poll with  NAK (15H).
        Should the checksum method be preferable to the receiver, it polls
        with NAK rather than "C".

        The sender returns an EOT instead of a data block when no data remain.

        With this extended implementation, the sender and the receiver may send
        blocks or ACK/NAK responses while there is data being received, once the
        SEAlink protocol has been negotiated.  This full duplex operation allows
        the throughput gains of a sliding window protocol.  When SEAlink is not
        set the window size of 1 prohibits data being sent at the same time and
        restores the attributes of a standard XMODEM protocol.

        ------------------
        In this extended protocol, the FTS-0001 single state table
        "XMODEM Sender" is replaced by two state tables.

        The top level table is equivalent to the FTS-0001 XMODEM Sender table.
        It in turn calls the new state table named "Transmitter ACK/NAK Check"
        which implements the full duplex adaptive SEAlink/XMODEM dynamic switching
        along with the Overdrive and file Restart sending features of the extended
        protocol.

        -----------------
        In this extended protocol, the FTS-0001 single state table
        "XMODEM Receiver" is replaced by three state tables.

        The top level table is equivalent to the FTS-0001 XMODEM Receiver table.
        It in turn calls the two new state tables named "Send NAK" and "Send ACK"
        which implement the full duplex adaptive SEAlink/TeLink/XMODEM dynamic
        switching along with the Overdrive and file Restart receiving features of
        the extended protocol.

        Caution!!!!
        -----------
        Many current implementations keep file block numbers as 16 bit numbers.
        This limits the max file size to either 4 megabytes (if number is signed)
        or 8 megabytes (if number is unsigned).  To handle files up to the maximum
        size DOS allows (4 gigabytes) at least 25 bits plus sign are required for
        these numbers.  Good practice is to make file block numbers 32 bit values.

                                                                                20
        XMODEM/TeLink/SEAlink - Sender

        .-----+----------+-------------------------+-------------------------+-----.
        |State| State    | Predicate(s)            | Action(s)               | Next|
        |  #  | Name     |                         |                         | St  |
        +-----+----------+-------------------------+-------------------------+-----+
        | XS0 | XmtStart |  Normal Entry Point     | reset SEAlink flag,     | XS1 |
        |     |          |  for file send          |   SendBLK=1, ACKST=0,   |     |
        |     |          |                         |   ACKBLK= -1, WINDOW=1, |     |
        |     |          |                         |   ACKs Rcvd=0,          |     |
        |     |          |                         |   NumNAK=0,             |     |
        |     |          |                         |   T1=30 seconds,        |     |
        |     |          |                    (*1) |   Build SEAlink hdr blk |     |
        +-----+----------+-------------------------+-------------------------+-----+
        | XS0T| XmTeStrt |  Alternate entry from   | reset SEAlink flag,     | XS1 |
        |     |          |  Batch Send if TeLink   |   SendBLK=1, ACKST=0,   |     |
        |     |          |  mode send required     |   ACKBLK= -1, WINDOW=1, |     |
        |     |          |                         |   ACKs Rcvd=0,          |     |
        |     |          |                         |   NumNAK=0,             |     |
        |     |          |                         |   T1=30 seconds,        |     |
        |     |          |                         |   Build TeLink hdr blk  |     |
        +-----+----------+-------------------------+-------------------------+-----+
        | XS1 | CheckACK |                         | (Check ACK/NAK AC0)     | XS2 |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | XS2 | SendBlk  |1| NumNAK > 4 &          | If header = SEAlink     | XS0T|
        |     |   (*2)   | |   SendBLK = 0         +-------------------------+-----+
        |     |          | |                       | If header = TeLink,     | XS2 |
        |     |          | |                       |   NumNAK = 0,           |     |
        |     |          | |                       |   Incr ACKBLK,          |     |
        |     |          | |                       |   Incr SendBLK          |     |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| NumNAK > 10           | report too many errors  | exit|
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |3| Timer T1 expired      | report fatal timeout    | exit|
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |4| SendBLK > Last Blk+1  |                         | XS3 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |5| SendBLK >ACKBLK+Window|                         | XS1 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |6| SendBLK = Last Blk+1  | Send EOT, Incr SendBLK, | XS1 |
        |     |          | |                       |   Set T1 to 30 seconds  |     |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |7| SLO set & SEAlink set | Send SendBLK, (*3)      | XS1 |
        |     |          | |                       |   ACKBLK = SendBLK,     |     |
        |     |          | |                       |   Incr SendBLK,         |     |
        |     |          | |                       |   Set T1 to 60 seconds  |     |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |8| SLO reset or          | Send SendBLK, (*3)      | XS1 |
        |     |          | |     SEAlink reset     |   Incr SendBLK,         |     |
        |     |          | |                       |   Set T1 to 30 seconds  |     |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | XS3 | WaitEnd  |1| ACKBLK < Last Blk+1   |                         | XS1 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| ACKBLK = Last Blk+1   | report send success     | exit|
        `-----+----------+-+-----------------------+-------------------------+-----'

        *1 - Build SEAlink Header block with RESYNC set.  Set SLO if line speed >
             2400 bps, reset SLO otherwise.

        *2 - State (XS2.1) allows the receiver to refuse one or both header blocks.

        *3 - If SendBLK = 0, then send the SEAlink (or TeLink) header block.
             If SendBLK > 0, send the corresponding 128 byte file data block where
                             block #1 begins with the first byte of the file.

                                                                                21
        XMODEM/TeLink/SEAlink - Transmitter ACK/NAK Check

        .-----+----------+-------------------------+-------------------------+-----.
        |State| State    | Predicate(s)            | Action(s)               | Next|
        |  #  | Name     |                         |                         | St  |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | AC0 | ChkRcvd  |1| No character waiting  |                         | exit|
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| Character waiting     | Read character to CHR   | AC1 |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | AC1 | SLCheck  |1| ACKST > 2             |                         | AC2 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| ACKST <=2             |                         | AC6 |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | AC2 | SLVerify |1| ARBLK8 = 1's comp(CHR)| ARBLK = SendBLK -       | AC3 |
        |     |          | |                       | ((SendBLK-ARBLK8)&0FFh) |     |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| ARBLK8 # 1's comp(CHR)| Reset SEAlink flag,     | AC6 |
        |     |          | |                       |   WINDOW=1,             |     |
        |     |          | |                       |   ACKST=0               |     |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | AC3 | SLACKNAK |1| ARBLK not valid (*1)  |                         | exit|
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| ACKST = 3             | Set SEALink flag,       | AC5 |
        |     |     (*2) | |                       |   WINDOW = 6,           |     |
        |     |          | |                       |   ACKBLK = ARBLK,       |     |
        |     |          | |                       |   Incr ACKs Rcvd,       |     |
        |     |          | |                       |   ACKST=0               |     |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |3| ACKST = 4             | SendBLK = ARBLK,        | AC4 |
        |     |          | |                       |   ACKST=0               |     |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | AC4 | XMCheck  |1| NumNAK < 4            | Set SEAlink Flag,       | exit|
        |     |          | |                       |   WINDOW = 6            |     |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| NumNAK >= 4           | Reset SEAlink flag,     | exit|
        |     |          | |                       |   WINDOW = 1            |     |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | AC5 | SLOCheck |1| SLO Reset             |                         | exit|
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| ACKs Rcvd < 10        |                         | exit|
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |3| ACKs Rcvd >= 10       | Reset SLO               | exit|
        +-----+----------+-+-----------------------+-------------------------+-----+
        | AC6 | SL1Check |1| ACKST = 1 or 2        | ARBLK8 = CHR,           | AC6 |
        |     |          | |                       |   ACKST = ACKST+2       |     |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| SEAlink reset         |                         | AC7 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |3| ACKST = 0             |                         | AC7 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |4| ACKST > 2             |                         | exit|
        `-----+----------+-+-----------------------+-------------------------+-----'

                                (Continued on next page)

        *1 - ARBLK is valid only if all of the following are true:

             a. ARBLK >= 0
             b. ARBLK <= SendBLK
             c. ARBLK >  SendBLK-128

        *2 - Software error if ACKST is not 3 or 4 in state AC3

                                                                                22
        XMODEM/TeLink/SEAlink - Transmitter ACK/NAK Check

        .-----+----------+-------------------------+-------------------------+-----.
        |State| State    | Predicate(s)            | Action(s)               | Next|
        |  #  | Name     |                         |                         | St  |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | AC7 | ACKNAK   |1| CHR = ACK             | ACKST = 1               | AC8 |
        |     |          | |                       |   NumNAK = 0            |     |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| CHR = NAK or 'C'      | Set CRC/Chksm if 1st,   | AC9 |
        |     |     (*1) | |                       |   ACKST = 2,            |     |
        |     |          | |                       |   Incr NumNAK,          |     |
        |     |          | |                       |   Delay 0.6 seconds     |     |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |     (*1) |3| CHR = SYN             | Receive RESYNC packet,  | AC10|
        |     |          | |                       |   ACKST = 0             |     |
        |     |     (*2) +-+-----------------------+-------------------------+-----+
        |     |          |4| CHR = other           |                         | exit|
        +-----+----------+-+-----------------------+-------------------------+-----+
        | AC8 | XMACK    |1| SEAlink set           |                         | exit|
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| SEAlink reset         | Incr ACKBLK             | exit|
        +-----+----------+-+-----------------------+-------------------------+-----+
        | AC9 | XMNAK    |1| SEAlink set           |                         | exit|
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| SEAlink reset         | SendBLK = ACKBLK+1      | exit|
        +-----+----------+-+-----------------------+-------------------------+-----+
        | AC10| RESYNC   |1| RESYNC pkt invalid    | Send NAK                | exit|
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| RESYNC pkt valid      | Set SEAlink flag,       | exit|
        |     |          | |                       |   WINDOW = 6,           |     |
        |     |          | |                       |   SendBLK = RESYNC Blk#,|     |
        |     |          | |                       |   ACKBLK = SendBLK-1,   |     |
        |     |          | |                       |   Send ACK              |     |
        `-----+----------+-+-----------------------+-------------------------+-----'

        *1 - If the output is buffered in local computer memory, any characters
             which remain in the local buffer should be purged before leaving
             states (AC7.2) or (AC7.3) to aid in resynchronizing the link.

        *2 - If the implementation is honoring MACFLOW protocol, set the flag in
             the SEAlink header block and add the following state to (AC7):

        .-----+--------+---+-----------------------+-------------------------+-----.
        | AC7 |        |3.5| CHR = ^S (13H) &      | Delay 10 seconds or     | exit|
        |     |        |   |   SEAlink set &       |   until ^Q (11H) rcvd   |     |
        |     |        |   |   ACKST = 0           |                         |     |
        `-----+--------+---+-----------------------+-------------------------+-----'

                                                                                23
        XMODEM/TeLink/SEAlink - Receiver

        .-----+----------+-------------------------+-------------------------+-----.
        |State| State    | Predicate(s)            | Action(s)               | Next|
        |  #  | Name     |                         |                         | St  |
        +-----+----------+-------------------------+-------------------------+-----+
        | XR0 | RecInit  |                         | reset SEAlink flag,     | XR1 |
        |     |          |                         |   reset SLO flag,       |     |
        |     |          |                         |   reset RESYNC flag,    |     |
        |     |          |                         |   set CRC flag,         |     |
        |     |          |                         |   set blocknum=0,       |     |
        |     |          |                         |   set WriteBLK=1,       |     |
        |     |          |                         |   reset retry cnt       |     |
        +-----+----------+-------------------------+-------------------------+-----+
        | XR0B| BrecInit |  Alternate Entry from   | reset SEAlink flag,     | XR2 |
        |     |          |  Batch Receive (BR4.2)  |   reset SLO flag,       |     |
        |     |          |  or (BR4.3)             |   reset RESYNC flag,    |     |
        |     |          |                         |   set CRC flag,         |     |
        |     |          |                         |   set blocknum=0,       |     |
        |     |          |                         |   set WriteBLK=1,       |     |
        |     |          |                         |   reset retry cnt       |     |
        +-----+----------+-------------------------+-------------------------+-----+
        | XR1 | RecStart |                         | (Send NAK SN0)          | XR2 |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | XR2 | WaitFirst|1| 10 retries or 1 minute| report receive failure  | exit|
        |     |          | |   w/o valid input     |                         |     |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| > 3 retries or 30 secs| reset CRC flag          | XR1 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |3| EOT received          | (Send ACK SA0),         | exit|
        |     |          | |                       |   report no file        |     |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |     (*1) |4| TeLink block recd     | (Send ACK SA0),         | XR3 |
        |     |          | |                       |   reset retry cnt       |     |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |     (*2) |5| SEAlink block recd    | set SEAlink,            | XR4 |
        |     |          | |                       |   set RESYNC as         |     |
        |     |          | |                       |   indicated by header   |     |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |     (*3) |6| XMODEM data block 1   | Write block to file,    | XR3 |
        |     |          | |   received            |   Incr WriteBLK,        |     |
        |     |          | |                       |   Incr blocknum,        |     |
        |     |          | |                       |   (Send ACK SA0),       |     |
        |     |          | |                       |   reset retry cnt       |     |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |7| bad block or 2-10 secs| incr retry count        | XR1 |
        |     |          | |   without input       |                         |     |
        `-----+----------+-+-----------------------+-------------------------+-----'

                                  (Continued on next page)

        *1 - A TeLink header packet has the standard XMODEM data block format except
             that it begins with an SYN instead of an SOH character and has a block
             number of 0.  Note: SEAdog (through version 4.51b) does not possess
             (XR2.4) and therefore will consider a TeLink header a bad block and
             process it according to (XR2.7) when communicating with mailers which
             do not do SEAlink protocol.

        *2 - A SEAlink header packet has the standard XMODEM data block format
        *3   except that is has a block number of 0. The first block is an XMODEM
             data block if its block number is not 0.

                                                                                24
        XMODEM/TeLink/SEAlink - Receiver (cont.)

        .-----+----------+-------------------------+-------------------------+-----.
        |State| State    | Predicate(s)            | Action(s)               | Next|
        |  #  | Name     |                         |                         | St  |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | XR3 | WaitBlock|1| 10 retries or 1 minute| report receive failure  | exit|
        |     |          | |   w/o expected block  |                         |     |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| EOT received          | reset SLO flag,         | exit|
        |     |          | |                       |   (Send ACK SA0)        |     |
        |     |          | |                       |   report recd ok        | exit|
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |3| expected data         | Decrement blocknum,     | XR3 |
        |     |          | |   block-1 received    |   (Send ACK SA0)        |     |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |4| expected data         | Write block to file,    | XR3 |
        |     |          | |   block received      |   Incr WriteBLK,        |     |
        |     |          | |                       |   (Send ACK SA0),       |     |
        |     |          | |                       |   reset retry cnt       |     |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |5| SEAlink set &         | Discard block - resync  | XR3 |
        |     |          | |   expected block+1 to |   in progress,          |     |
        |     |          | |   expected block+127  | Send Conditional NAK(*5)|     |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |6| bad block or 2-10 secs| (Send NAK SN0),         | XR3 |
        |     |          | |   without input       |   incr retry cnt        |     |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | XR4 | Restart  |1| Want entire file      | (Send ACK SA0),         | XR5 |
        |     |          | |   or RESYNC not set   | reset retry cnt         |     |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| Want to resume an     |  WriteBLK = file restart| XR5 |
        |     |          | |   interrupted xfer    |    block number,        |     |
        |     |          | |   and RESYNC is set   |  blocknum=WriteBLK&0FFh,|     |
        |     |          | |                       |    (Send NAK SN0)       |     |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | XR5 | SetOvrdr |                         | Set SLO as indicated    | XR3 |
        |     |          |                         |   by SEAlink header     |     |
        `-----+----------+-------------------------+-------------------------+-----'
        Note: The routine that receives a header/data block should do the following:

        1. Report a bad block if the checksum or CRC is not correct or if the
           physical layer encounters errors (e.g. parity, framing, etc.) regardless
           of checksum or CRC.  Report a bad block if the length of the block is
           less than expected (Use a 5 second timeout to detect short blocks).
        2. If the block's sequence number does not match the complement, then
           report a bad block if not in SEAlink. If in SEAlink mode don't report
           this as a bad block, just restart the block search looking for SOH.
           Check for SOH, BLK, ~BLK characters in a "sliding" fashion and keep
           cycling until a valid start of block (or timeout) is detected before
           assembling the remainder of the block.  This procedure makes a resync
           occur much more rapidly, and with far fewer errors reported.
        3. If the sequence number and block are good but not the expected number,
           report state (XR3.3) if previous block.  If not in SEAlink, abort on any
           other out of sequence block.  If in SEAlink, report back state (XR3.5)
           or (XR3.6) as indicated by the out of sequence received block number.
        4. If an EOT is received on a data block prior to the filesize specified
           in the SEAlink or TeLink header block, a NAK should be issued to ensure
           that spurious data did not cause a premature EOF.  A NAK should also
           be issued for the first EOT received in an Xmodem transfer when the
           final file size is not known.  A second EOT without intervening data
           would ensure that the file really has been sent, and should be ACK'd.
        5. If you last sent an ACK, then send a NAK here.  If you last sent a NAK
           then only NAK evry 32 blocks after the first NAK.  This allows buffer
           drain in overdrive.  Use (Send NAK SN0) to send the NAK.
                                                                                25
        XMODEM/TeLink/SEAlink - Send NAK

        .-----+----------+-------------------------+-------------------------+-----.
        |State| State    | Predicate(s)            | Action(s)               | Next|
        |  #  | Name     |                         |                         | St  |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | SN0 | ClearLine|1| RESYNC flag set       | Send RESYNC packet      | SN3 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| SEAlink flag set      |                         | SN1 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |3| > 30 sec contin data  | report failure          | exit|
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |4| character waiting     | eat character           | SN0 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |5| clear line            |                         | SN1 |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | SN1 | SendNAK  |1| CRC flag set          | Send "C"                | SN2 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| CRC flag reset        | send NAK                | SN2 |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | SN2 | SEAlink  |1| SEAlink flag reset    |                         | exit|
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| SEAlink flag set      | send blocknum, ~blocknum| exit|
        +-----+----------+-+-----------------------+-------------------------+-----+
        | SN3 | AckResync|1| Rcv ACK               |                         | exit|
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| Rcv NAK               | Resend RESYNC packet    | SN3 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |3| Rcv Other Char        | eat character           | SN3 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |4| No char for 10 secs   | Resend RESYNC packet    | SN3 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |5| 30 seconds in SN3     |                         | exit|
        `-----+----------+-+-----------------------+-------------------------+-----'
        Note: RESYNC packet should send WriteBLK as its desired block number.

        XMODEM/TeLink/SEAlink - Send ACK

        .-----+----------+-------------------------+-------------------------+-----.
        |State| State    | Predicate(s)            | Action(s)               | Next|
        |  #  | Name     |                         |                         | St  |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | SA0 | ClearLine|1| SLO flag set          |                         | SA3 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| SEAlink flag set      |                         | SA1 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |3| > 30 sec contin data  | report failure          | exit|
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |4| character waiting     | eat character           | SA0 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |5| No char waiting       |                         | SA1 |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | SA1 | SendACK  |                         | Send ACK                | SA2 |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | SA2 | SEAlink  |1| SEAlink flag reset    |                         | SA3 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| SEAlink flag set      | send blocknum, ~blocknum| SA3 |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | SA3 | IncBlk   |                         | Incr blocknum           | exit|
        `-----+----------+-------------------------+-------------------------+-----'

                                                                                26
     3. Data Link Layer Protocol : MODEM7 Filename Finite State Machines
        (There is no change to the MODEM7 state tables from FTS-0001).

        MODEM7 Filename Sender

        .-----+----------+-------------------------+-------------------------+-----.
        |State| State    | Predicate(s)            | Action(s)               | Next|
        |  #  | Name     |                         |                         | St  |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | MS0 | WaitNak  |1| 20 retries or 1 minute| filename send failed    | exit|
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| NAK received          | send ACK & 1st ch of fn | MS1 |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | MS1 | WaitChAck|1| ACK rcd, fname done   | send SUB = 1AH          | MS2 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| ACK rcd, fname ~done  | send next ch of fname   | MS1 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |3| other char or 1 sec   | send "u", incr retry cnt| MS0 |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | MS2 | WaitCksm |1| cksum recd and ok     | send ACK, report fn ok  | exit|
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| cksum recd but bad    | send "u", incr retry cnt| MS0 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |3| no cksum in 1 sec     | send "u", incr retry cnt| MS0 |
        `-----+----------+-+-----------------------+-------------------------+-----'

        MODEM7 Filename Receiver

        .-----+----------+-------------------------+-------------------------+-----.
        |State| State    | Predicate(s)            | Action(s)               | Next|
        |  #  | Name     |                         |                         | St  |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | MR0 | SendNak  |1| 20 tries or 1 minute  | report filename failure | exit|
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2|                       | send NAK, incr try cnt  | MR1 |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | MR1 | WaitAck  |1| rcd ACK               |                         | MR2 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| rcd EOT               | report no files remain  | exit|
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |3| 5 secs & no ACK/EOT   |                         | MR0 |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | MR2 | WaitChar |1| recd EOT (can happen?)| report no files remain  | exit|
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| recd SUB              | send checksum byte      | MR3 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |3| recd "u"              |                         | MR0 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |4| recd char of name     | send ACK                | MR2 |
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |5| no char in 1 second   |                         | MR0 |
        +-----+----------+-+-----------------------+-------------------------+-----+
        | MR3 | WaitOkCk |1| recd ACK within 1 sec | report recd filename ok | exit|
        |     |          +-+-----------------------+-------------------------+-----+
        |     |          |2| recd "u" or other char|                         | MR0 |
        `-----+----------+-+-----------------------+-------------------------+-----'

        SUB  is the ASCII character ^Z or 1AH.  The checksum is the unsigned low
        order eight bits of the sum of the characters in the transferred filename
        including the SUB.

        Although 1 second timeouts are used successfully by Fido and SEAdog, some
        fear that this is too small a value for some satellite and packet networks.

                                                                                27

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