**********************************************************************
FTSC                             FIDONET TECHNICAL STANDARDS COMMITTEE
**********************************************************************

Publication:    FSP-1011
Revision:       3
Title:          Binkp - a protocol for transferring FidoNet mail over
                reliable connections
Authors:        Dima Maloff
                Nick Soveiko
                Maxim Masiutin
Revision Date:  31 July 2000
Expiry Date:    31 July 2002

----------------------------------------------------------------------

Abstract
--------

   This specification defines binkp - a protocol to handle a session
   between two Fidonet Technology systems over a reliable connection.
   Assumption that the connection is reliable makes possible to
   eliminate error-checking and unnecessary synchronization steps,
   achieving both ease of implementation and major performance
   improvement over connections with large unpredictable delays (e.g.
   Internet).

Status of this document
-----------------------

   This document is a Fidonet Standards Proposal (FSP).

   This document specifies an optional Fidonet standard protocol for
   the Fidonet community, and requests discussion and suggestions for
   improvements.

   This document is released to the public domain, and may be used,
   copied or modified for any purpose whatever.

Available formats
-----------------

   Binkp Specification is also available in HTML format at
   http://www.ritlabs.com/binkp/

Table of contents
-----------------

    1. Background
         1. Objectives
         2. Motivation for a New Protocol
    2. Definitions
    3. Protocol Overview
    4. Frame Format
         1. Notation
         2. Examples
    5. Protocol Commands and Their Arguments
         1. Classification
         2. File Name Issues
         3. Non-ASCII Characters in Command Argument symbol string
         4. Binkp Commands
         5. Example of Frame Exchange in a Simple binkp Session
    6. Protocol States
         1. Session Setup Stage
              1. Originating Side
              2. Answering Side
         2. File Transfer Stage
         3. Session Termination
    7. Recommended Protocol Extensions
         1. Non-Reliable Mode
         2. Multiple Batch Mode
         3. Multiple Passwords Mode
         4. Keyed Hashing Challenge-Response Authentication Mechanism
              1. Overview
              2. Sequence of Steps
              3. Generating and Transmitting Challenge Data
              4. Producing and Transmitting a Digest
              5. Indicating CRAM Capabilities
              6. Example of Frame Exchange During CRAM Authentication
              7. Notes on Hash Function Algorithms
    8. License
    9. Glossary
   10. References
   11. Acknowledgements
    A. Author Contact Data
    B. History

     ---------------------------------------------------------------

1. Background
-------------

  1.1 Objectives
  --------------

   It's been a long time since a new Fidonet protocol has been
   developed, [EMSI] definitions being published last time in 1991,
   not speaking about basic standards, [FTS-0001] and [FTS-0006].
   Fidonet is evolving everyday and new transport layers are being
   introduced into practice. This led to a situation when in certain
   Fidonet Regions a visible portion of traffic, especially long
   distance traffic generating high toll, is being carried by means of
   protocols that formally are not Fidonet standards. This creates an
   ambiguity for such systems in indicating their additional
   capabilities in Fidonet nodelist and in some instances, from being
   listed in the nodelist at all.

   This document attempts to document the current practice for
   communication between two Fidonet systems via a reliable channel,
   provide technical reference for Fidonet software developers and
   eventually improve Fidonet connectivity.

  1.2 Motivation for a new protocol
  ---------------------------------

   Existing Fidonet Technical Standards and Fidonet Reference Library
   documents [FTS-0001], [FTS-0006], [EMSI] specify both session
   handshake procedures and transmission capabilities that imply:
     * non-reliable communication channel between mailers
     * low round-trip times in the communication channel between
       mailers.

   This was commonplace a few years ago, when Fidonet systems were not
   using transport other than direct dial-up on a visible basis.
   Things have changed today, when other communication media becomes
   widely available on a day-to-day basis. This communication media
   typically provides implementation of Physical, Data Link, Network
   and Transport layers of the ISO/OSI Reference Model and facilitates
   relieving Session layer of inappropriate functions, such as error
   control, flow control, call management and data transparency
   [Halsall95]. Examples of such communication media are TCP/IP socket
   connection and HDLC family protocol connection.

   New communication media can be generally characterized by the
   reliable transmission service offered by it to the Session layer
   protocol. Reliable transmission implies that:
     * Data link and/or Transport layer protocols are responsible for
       error control and delivery of frames in correct sequence
     * Session layer and higher layer protocols are operating on top
       of connection-oriented mode
     * Quality of Service provisions (if any) result in unspecified
       delays between transmitter and receiver
     * connections are rarely aborted.

   Combination of these factors imposed the following requirements for
   the new Fidonet protocol:
     * error control can be eliminated throughout the session layer
       protocol for both handshake and default file transfer method
     * session setup procedure should minimize number of
       synchronization points for fast handshake
     * protocol should be insensitive to delays and robust with
       respect to timeouts
     * application flow control should be moved to file level;
       individual data frames do not need to be error checked nor
       acknowledged
     * protocol should be independent from both higher and lower layer
       protocols
     * protocol should be reasonably easy to implement and allow
       future extensions.

2. Definitions
--------------

   The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT" and "MAY"
   in this document are to be interpreted as specified in [FTA-0006].
   However, for readability, these words may sometimes not appear in
   all uppercase letters in this specification. Although it should not
   impact minimal realization of binkp protocol, it must be noted that
   Protocol Extensions may override, update or obsolete requirement
   levels indicated by the above keywords in chapters from 3 to 6
   inclusive.

   Calling party in this document is referred to as the Originating
   side and called party is referred to as the Answering side.
   Originating side here is the party that initiates the connection
   between two systems.

   Mailer in this document is a software that implements the protocol.

   Words "frame", "packet", and "block" when used in this document
   refer to binkp's Frames, unless explicitly stated otherwise.

   Other definitions that are not local to this document can be found
   in the Glossary.

   This document is organized as following:

   Frames section defines binkp's frames. Binkp/1.0 commands and their
   arguments section provides detailed description of all defined
   protocol commands together with recommendations for their usage.
   Actual binkp implementation may match it's own diagrams provided
   that such implementation remains fully compatible with current
   specification. Protocol states section gives rigorous state
   diagrams for the minimum realization of binkp. All mailers MUST
   support this minimum realization. Recommended Protocol Extensions
   section documents most important extensions to the basic protocol
   that are in use as of the time of this writing. The License,
   Glossary and References sections can be found at the end of this
   document.

3. Protocol Overview
--------------------

   Binkp is a Fidonet session layer protocol intended for use over
   data transparent bi-directional channels with reliable
   transmission. There are no other requirements for the service
   provided by the underlying protocol suite. Presentation and
   application layer protocols are not discussed here. Whenever TCP/IP
   socket is used, IANA registered port number for binkp 24554 SHOULD
   be used (as registered with the Internet Assigned Numbers
   Authority).

   Functionality of the minimum protocol realization makes provision
   for:
     * password protected sessions
     * 4D/5D addressing for Fidonet and technology compatible networks
     * exchange of Type 2 [FTS-0001], Type 2.2 [FSC-0045], Type 2+
       [FSC-0039] and [FSC-0048], Type 3 [FSC-0081] packets and
       [FTS-0006] arcmail in both directions, including poll and mail
       pickup, as well as transfer of any binary or ASCII files
     * handling WaZOO [FTS-0006] file requests
     * ensuring integrity of transmitted mail and files
     * simultaneous bi-directional transmission
     * maximizing performance over packet switched data networks

   Binkp uses only one synchronization point during session startup,
   that is password exchange. This feature facilitates fast session
   startup for high latency links. Sliding window flow control is
   incorporated on the file level. This ensures that a batch of small
   files is transmitted with the same efficiency as a one large file.

4. Frame Format
---------------

   Binkp is defined in terms of sending and receiving specifically
   formatted data blocks. We call them frames.

   Command frames carry protocol commands and may change protocol
   state. Data frames are usually appended to files being received by
   mailers or may be discarded, depending on the protocol state.

   The particular way of mapping an octet stream or a datagram stream
   of the transport layer into binkp frames may depend on the
   underlying protocol suite. At this time, we define such mapping for
   TCP/IP socket connection which can also be used for similar
   transports as well.

   The socket stream is being split into binkp frames in the following
   manner:

     7 6543210 76543210
    +-+-------+--------+--- ................ ---+
    |T|      SIZE      |          DATA          |
    +-+-------+--------+--- ................ ---+
    |<-   2 octets   ->|<- up to 32767 octets ->|
       (frame header)          (frame data)

   If T bit is 0, this is a data frame.

   If T bit is 1, this is a command frame.

   15 bits marked SIZE carry the size of the DATA part of the frame in
   octets (with the bit marked 0 being the least significant). That
   is, the actual length of a binkp frame is SIZE+2.

   The size of the DATA part may vary between 1 and 32767 octets. A
   correct realization should never set SIZE to 0. Upon receiving of a
   packet header with the SIZE field set to 0, the total length of the
   incoming packet must be treated as 2, this packet must be dropped,
   and the event should be logged.

   The first octet of a command frame data is the command ID. The ID
   must be between 0 and 127 inclusive.

   Other octets carry command arguments. Command arguments are an
   arbitrary symbol string that may be null-terminated. Treating of a
   null character in the middle of a command depends on realization
   (with the options being "treat as whitespace" or "treat as
   end-of-line"). The terminating null character (if any) is either
   stripped or used by mailers internally as an end-of-line marker.

  4.1 Notation
  ------------

   As stated before, command ID is a small number between 0 and 127.
   Every binkp command defined in this document has a symbolic name in
   the form M_XXX. Symbolic names are defined in binkp commands
   section. We will use symbolic names and not numeric command IDs to
   refer to commands everywhere in this document.

   The following notation is used to describe binkp's command frames:

         M_XXX "Data string"

   The actual numeric command ID for the command with the symbolic
   name of M_XXX should be written into the first octet of the DATA
   area of a binkp frame. "Data string" is a string to be copied into
   DATA area starting at second octet. SIZE should be set to the total
   length of "Data string" plus one for the octet to store the command
   number. T bit should be set to 1.

  4.2 Examples
  ------------

   M_OK "":

     7 6543210 76543210 76543210
    +-+-------+--------+--------+
    |1|      0        1|       4|
    +-+-------+--------+--------+
     |                |        +----- command ID (no arguments)
     |                +-------- frame length
     +- command frame flag

   M_NUL "TEST":

    +-+-------+--------+--------+-------+--------+--------+--------+
    |1|      0        5|       0|   T        E        S       T    |
    +-+-------+--------+--------+-------+--------+--------+--------+

5. Protocol Commands and Their Arguments
----------------------------------------

  5.1 Classification
  ------------------

   Protocol commands may be classified the following way:

     * By argument type:
        1. Mailer-parseable: M_ADR, M_PWD, M_FILE, M_GOT, M_GET,
            M_SKIP. Mailer MUST parse these commands and it is not
            recommended to log arguments of these commands as they
            are. Mailer-parseable commands can be further subdivided
            by containment of a file name in the argument.
             1. Contain a file name: M_FILE, M_GOT, M_GET, M_SKIP
                 commands contain a file name in their arguments.
             2. Do not contain a file name: M_ADR, M_PWD
        2. Human-readable: M_NUL, M_OK, M_EOB, M_ERR, M_BSY. Mailer
            MAY ignore and/or log arguments of these commands.
     * By protocol stage:
        1. Session setup stage: M_ADR (must be sent by both sides),
            M_PWD (must not be sent by the Answering side), M_OK (must
            not be sent by the Originating side). These commands MUST
            never be sent during the file transfer stage.
        2. File transfer stage: M_FILE, M_GOT, M_GET, M_SKIP, M_EOB.
            These commands MUST NOT be sent session setup stage.
        3. Any stage: M_NUL, M_ERR, M_BSY. These commands MAY be sent
            any time during the session.

  5.2 File Name Issues
  --------------------

   In Mailer-parseable commands that contain a file name, the file
   name MUST NOT include a whitespace (ASCII value 20 hex). The file
   name SHOULD NOT include symbols other than alphanumeric
   (A-Z,a-z,0-9) and safe characters as defined below in BNF. All
   other symbols are to be considered unsafe and SHOULD be escaped in
   the form of two hexadecimal digits preceded by a backslash (e.g. a
   whitespace must be transmitted as "\20").

   filename        = *pchar
   pchar           = unreserved | escape
   unreserved      = ALPHA | DIGIT | safe
   safe            = "@" | "&" | "=" | "+" | "%" | "$" | "-" | "_" |
                     "." | "!" | "(" | ")" | "#" | "|"
   escape          = "\" HEX HEX

   National characters should not be escaped, but rather transmitted
   using [UTF8] encoding (see section discussing non-ASCII characters
   below).

   The best current practice is that Mailer does not alter a file name
   without sysop's intention. If the mailer does provide such a
   mechanism, it MUST BE optional and it SHOULD BE off by default.

   The protocol does not impose limitations on the file name length
   other than those arising from the finite length of the binkp frame
   itself.

  5.3 Non-ASCII Characters in Command Argument Symbol String
  ----------------------------------------------------------

   Generally, mailer SHOULD use only characters from the ASCII range
   [32...126] in the symbol strings for command arguments. In case
   when there is a necessity to use non-ASCII characters, mailer
   SHOULD use the [UTF8] format of the multioctet Universal Character
   Set [ISO10646]. Mailer SHOULD use non-ASCII characters only if the
   other side have indicated it's support by transmitting M_NUL "OPT
   UTF8" frame during the session setup stage. Otherwise, mailer
   SHOULD assume that the remote does not support non-ASCII characters
   and SHOULD NOT use them in command arguments.

  5.4 Binkp Commands
  ------------------

   Format: symbolic_command_name command_ID

   M_NUL 0

           Command arguments contain human-readable information, such
           as nodelist info, sysop name, etc. This frame can also be
           used by some Mailers to exchange protocol options. Mailer
           MAY ignore and/or log arguments of M_NUL.

           e.g. "ZYZ Dima Maloff"

           The following format of M_NUL argument is recommended for
           compatibility purposes:

              * M_NUL "SYS system_name"
              * M_NUL "ZYZ sysop's_name"
              * M_NUL "LOC system_location"
              * M_NUL "NDL system_capabilities"
              * M_NUL "TIME remote_date_time"
                remote_date_time format is described in [RFC822].
                Example of valid remote_date_time is
                Sun, 06 Nov 1994 08:49:37 GMT
              * M_NUL "VER mailer_version protocol_version"
                note: binkp/1.0 mailers should send "binkp/1.0" string
                for protocol_version.
              * M_NUL "TRF netmail_bytes arcmail_bytes"
                traffic prognosis (in bytes) for the netmail
                (netmail_bytes) and arcmail and files (arcmail_bytes),
                both are decimal ASCII strings
              * M_NUL "OPT protocol options"
                here protocol options is a space separated list of
                binkp options and extensions supported by the mailer.
              * M_NUL "PHN string"
                phone number, ip address or other network layer
                addressing ID
              * M_NUL "OPM string"
                string is a message for the system operator that may
                require manual attention

   M_ADR 1

           List of 4D/5D addresses (space separated).

           e.g. "2:5047/13@fidonet 2:5047/0@fidonet"

   M_PWD 2

           Session password, case sensitive. After successful password
           authentication of the remote, originating side proceeds to
           the file transfer stage. This command MUST never be sent by
           the Answering side.

           e.g. "pAsSwOrD"

   M_OK 4

           Acknowledgement for a correct password. Upon receiving of
           this command, originating side goes to file transfer stage.
           This command MUST never be sent by the Originating side.
           Arguments may be ignored.

           e.g. ""

   M_FILE 3

           Space separated list of parameters for the next file to be
           transmitted: filename; size in bytes; unixtime; file
           transmission offset.

           In protocol extensions, negative values for the offset may
           have special meaning (see non-reliable mode for an example
           of such usage), basic implementation may treat negative
           values as an error.

           Size, time and offset parameters are decimal. Until the
           next M_FILE command is received, all data frames must carry
           data from this file in consecutive manner. There is no end
           of file identifier as the file size is known beforehand. If
           there are "extra" data frames, Mailer may append this data
           to the file. By default, transmission of each file should
           be started from offset 0. M_GET command sent by the remote
           MUST force the mailer to start transmission from the
           specified offset.

           e.g. "config.sys 125 2476327846 0"

           or, answering to M_GET with offset 100:

           "config.sys 125 2476327846 100"

   M_EOB 5

           End-of-Batch. M_EOB command must be transmitted after all
           the files have been sent.

           Arguments of the command may be ignored.

           e.g. ""

   M_GOT 6

           File acknowledgement, that must be transmitted upon
           receiving of the last data frame for current file.
           Arguments for this command shall be the same as for the
           M_FILE sent by remote, excluding the last argument, file
           offset, which is not transmitted back to the system which
           have sent M_FILE. M_GOT can also be transmitted while
           receiving a file, in which case transmitting party may
           interpret it as a destructive skip.

           e.g. "config.sys 125 2476327846"

   M_ERR 7

           This command indicates a fatal error. A party sending M_ERR
           should abort the session. Argument should contain an error
           explanation and may be logged. Mailer sends M_ERR in
           response for an incorrect password. Mailer NUST NOT abort a
           session without sending a M_ERR or a M_BSY frame (though
           state machine tables, for simplicity, may not include
           "transmit M_ERR" instructions).

           e.g. "Incorrect password"

   M_BSY 8

           M_BSY command is transmitted when the system encounters a
           non-fatal error typically due to temporary lack of
           resources to proceed with the session. The argument should
           contain an explanation of the situation and may be logged
           by remote. M_BSY may be sent at any time during the session
           (including session setup stage), not only the stages
           explicitly indicated in the finite state machine. The side,
           which have sent M_BSY, is in legal position to abort the
           session. Mailer MUST be able to accept M_BSY at any time.
           Though state machine tables, for simplicity, may not
           include handling of M_BSY command, Mailer MUST NOT be
           confused by reception of M_BSY command.

           e.g. "Too many servers are running already"

           If a mailer wishes to suggest the remote a time interval
           before the next session attempt, it may choose to transmit
           it in the following format:

           M_BSY "RETRY NNNN: explanation"

           where NNNN is interval in seconds (decimal string) and
           explanation is an arbitrary string containing explanation
           of the matter (optional).

   M_GET 9

           M_GET command is a request to (re)send files. Arguments of
           the command are the same as for the M_FILE command and
           refer to a file which we'd like to receive from the remote.

           Mailer may send M_GET when it doesn't like transmission
           file offset (e.g. file was partially received during one of
           the previous sessions).

           e.g. "config.sys 125 2476327846 100"

           Mailer reacts to this command as follows: according to the
           first three arguments (filename/size/unixtime), it
           determines whether the M_GET argument is the current file
           being transmitted to the remote (or a file that have been
           transmitted, but we are still waiting an M_GOT ack for it).
           If this is the case, it should

              * discard transmission in progress as soon as possible
              * perform seek() to the specified offset
              * proceed with transmission of the file requested
                starting with an appropriate M_FILE.

           For the example above, corresponding M_FILE will have the
           following arguments: "config.sys 125 2476327846 100"

           When the mailer is finished with transmitting data of the
           requested file it may proceed with transmission of other
           files it has for the remote.

   M_SKIP 10

           Non destructive skip. Parameter is a space separated list
           of filename, size and unixtime. This command indicates that
           the remote should postpone sending the file until next
           session.

           e.g. "config.sys 125 2476327846"

  5.5 Example of Frame Exchange in a Simple Binkp Session
  -------------------------------------------------------

   +-----------------------------------------------------------------+
   | Originating side                | Answering side                |
   |---------------------------------+-------------------------------|
   | M_NUL "SYS ..."                 | M_NUL "SYS ..."               |
   | M_NUL "ZYZ ..."                 | M_NUL "ZYZ ..."               |
   | M_NUL "LOC ..."                 | M_NUL "LOC ..."               |
   | M_NUL "VER ..."                 | M_NUL "VER ..."               |
   | M_ADR "2:2/2.2@fidonet"         | M_ADR "3:3/3.3@fidonet"       |
   | M_PWD "password"                | (waiting for a password from  |
   |                                 | remote)                       |
   |---------------------------------+-------------------------------|
   | (waiting for password           | M_OK "" (or M_ERR "Bad        |
   | acknowledgement)                | password")                    |
   |---------------------------------+-------------------------------|
   | (got M_OK)                      | M_FILE "file2 200 42342434 0" |
   |---------------------------------+-------------------------------|
   | M_FILE "file1 100 423424244 0"  | data                          |
   |---------------------------------+-------------------------------|
   | data                            | data                          |
   |---------------------------------+-------------------------------|
   | data                            | data                          |
   |---------------------------------+-------------------------------|
   | M_EOB                           | (got file1, acknowledging it) |
   |---------------------------------+-------------------------------|
   | (got file2, acknowledging it)   | M_GOT "file1 100 423424244"   |
   |---------------------------------+-------------------------------|
   | M_GOT "file2 200 42342434"      | data                          |
   |---------------------------------+-------------------------------|
   |                                 | M_EOB                         |
   +-----------------------------------------------------------------+

6. Protocol States
------------------

   The protocol has two major stages: session setup (different for
   originating side and answering side) and file transfer (where state
   machined for both sides are the same). Methods for initiating
   connection as well as numerical values for particular timeouts are
   dependent on the underlying layer's protocol suite and are not
   considered here. Mailer MAY allow configuration of timeouts in
   reasonably wide range to cover all supported transport protocols.

   The Finite State Machine notation is used throughout this section
   as defined by [FTS-0001].

  6.1 Session Setup Stage
  -----------------------

   Originating side should initiate a binkp session according to Table
   1. Answering side should be able to act according to Table 2. Any
   optional extensions of the handshake procedure MUST NOT confuse the
   other side, which may choose at it's discretion to follow this
   minimal implementation. Upon successful handshake, both sides
   follow Table 3 (file transfer stage). That's why terms Answering
   side and Originating side were chosen for this specification
   instead of Client and Server - both sides play the same roles, and
   their state machines differ in session setup stage only.

   Session setup stage has the following roles

     * Authentication (REQUIRED). Answering side, upon reception of a
       password (common secret word) from Originating side, decides
       whether the password really matches the list of presented
       addresses, and either acknowledges it by sending M_OK frame or
       rejects by sending M_ERR frame. This mechanism is called Basic
       Authentication Scheme and MUST be supported by all Mailers.
       Basic Authentication Scheme has the following limitations:
          * If Originating side presented multiple addresses, the
            password for all of the addresses must be the same (may be
            solved by Multiple passwords extension).
          * Cleartext reusable passwords are passed over a network
            (may be solved by CRAM extension).
          * Verification is made on Answering side only, thus
            Originating side has no way to verify Answering side (may
            be solved by dual CRAM or public-key cryptography, not
            discussed in this document).
     * Indicating protocol options (OPTIONAL). Sides may exchange
       specially formatted M_NUL messages to indicate supported
       extensions. Sides MAY use another technique to indicate
       extensions.

    6.1.1 Originating Side
    ----------------------

   Originating side sends M_ADR and M_PWD frames, waits for successful
   authentication acknowledgement from the Answering side (M_OK frame)
   and goes to file transfer stage. Originating side MUST NOT wait
   before sending M_ADR frame, i.e. this frame should be send just
   after setting up a connection on underlying layer. Originating side
   MUST NOT wait before sending M_PWD except after reception of M_ADR
   frame. The term wait in this paragraph means do not send anything
   while expecting data from remote.

                Table 1: Session setup, originating side
   +-----------------------------------------------------------------+
   | #  | Name       | Predicate(s)     | Action(s)           | Next |
   |----+------------+------------------+---------------------+------|
   | S0 | ConnInit   |                  | Attempt to          | S1   |
   |    |            |                  | establish           |      |
   |    |            |                  | connection          |      |
   |----+------------+------------------+---------------------+------|
   | S1 | WaitConn   | Connection       | Send M_NUL frames   | S2   |
   |    |            | established      | with system info    |      |
   |    |            |                  | (at least one M_NUL |      |
   |    |            |                  | "SYS ..." frame     |      |
   |    |            |                  | should be sent      |      |
   |    |            |                  | before M_ADR)       |      |
   |    |            |                  | Send M_ADR frame    |      |
   |    |            |                  | with system         |      |
   |    |            |                  | addresses           |      |
   |    |            |                  | Set Timer           |      |
   |    |            |                  | See if we have      |      |
   |    |            |                  | password for the    |      |
   |    |            |                  | remote              |      |
   |    |            |------------------+---------------------+------|
   |    |            | Connection       | Report no           | exit |
   |    |            | refused          | connection          |      |
   |----+------------+------------------+---------------------+------|
   | S2 | SendPasswd | Yes, we have a   | Send M_PWD          | S3   |
   |    |            | password         | "password" frame    |      |
   |    |            |                  | Reset Timer         |      |
   |    |            |------------------+---------------------+------|
   |    |            | No, there's no   | Send M_PWD "-"      | S3   |
   |    |            | password         | frame               |      |
   |----+------------+------------------+---------------------+------|
   | S3 | WaitAddr   | M_ADR frame      | See if answering    | S4   |
   |    |            | received         | side presented the  |      |
   |    |            |                  | address we've       |      |
   |    |            |                  | called              |      |
   |    |            |------------------+---------------------+------|
   |    |            | M_BSY frame      | Report remote is    | exit |
   |    |            | received         | busy                |      |
   |    |            |------------------+---------------------+------|
   |    |            | M_ERR frame      | Report error        | exit |
   |    |            | received         |                     |      |
   |    |            |------------------+---------------------+------|
   |    |            | M_NUL frame      | Ignore (optionally, | S3   |
   |    |            | received         | log frame argument) |      |
   |    |            |------------------+---------------------+------|
   |    |            | Other known      | Report unexpected   | exit |
   |    |            | frame received   | frame               |      |
   |    |            |------------------+---------------------+------|
   |    |            | Unknown frame    | Ignore              | S3   |
   |    |            | received         |                     |      |
   |    |            |------------------+---------------------+------|
   |    |            | Nothing happens  | Wait                | S3   |
   |    |            |------------------+---------------------+------|
   |    |            | Timer Expired    | Report timeout      | exit |
   |----+------------+------------------+---------------------+------|
   | S4 | AuthRemote | Yes, the address | See if we've sent a | S5   |
   |    |            | was presented    | password for this   |      |
   |    |            |                  | address             |      |
   |    |            |------------------+---------------------+------|
   |    |            | No, the address  | Report we called    | exit |
   |    |            | was not          | the wrong system    |      |
   |    |            | presented        |                     |      |
   |----+------------+------------------+---------------------+------|
   | S5 | IfSecure   | Yes, we've sent  | Wait for M_OK frame | S6   |
   |    |            | a password       |                     |      |
   |    |            |------------------+---------------------+------|
   |    |            | No, there was no | Report non-secure   | T0   |
   |    |            | password         | session             |      |
   |----+------------+------------------+---------------------+------|
   | S6 | WaitOk     | M_OK frame       | report secure       | T0   |
   |    |            | received         | session             |      |
   |    |            |------------------+---------------------+------|
   |    |            | M_BSY frame      | Report remote is    | exit |
   |    |            | received         | busy (Answering     |      |
   |    |            |                  | size MAY report     |      |
   |    |            |                  | busy after          |      |
   |    |            |                  | reception of        |      |
   |    |            |                  | caller's address)   |      |
   |    |            |------------------+---------------------+------|
   |    |            | M_ERR frame      | Report error        | exit |
   |    |            | received         |                     |      |
   |    |            |------------------+---------------------+------|
   |    |            | M_NUL frame      | Ignore (optionally, | S6   |
   |    |            | received         | log arguments)      |      |
   |    |            |------------------+---------------------+------|
   |    |            | Other known      | Report unexpected   | exit |
   |    |            | frame received   | frame               |      |
   |    |            |------------------+---------------------+------|
   |    |            | Unknown frame    | Ignore              | S6   |
   |    |            | received         |                     |      |
   |    |            |------------------+---------------------+------|
   |    |            | Nothing happens  | Wait                | S6   |
   |    |            |------------------+---------------------+------|
   |    |            | Timer Expired    | Report timeout      | exit |
   +-----------------------------------------------------------------+

    6.1.2 Answering Side
    --------------------

   Originating side sends M_ADR and waits for M_ADR and M_PWD frames
   from remote. Upon receptions of these frames, it decides whether
   the password really matches the list of presented addresses, and
   either acknowledges it by sending M_OK frame (and goes to file
   transfer stage) or rejects by sending M_ERR frame (and
   disconnects). The term wait in this paragraph means do not send
   anything while expecting data from remote.

                 Table 2: Session setup, answering side
   +-----------------------------------------------------------------+
   | #  | Name     | Predicate(s)        | Action(s)          | Next |
   |----+----------+---------------------+--------------------+------|
   | R0 | WaitConn | Incoming connection | Send M_NUL frames  | R1   |
   |    |          | established         | with system info   |      |
   |    |          |                     | (at least one      |      |
   |    |          |                     | M_NUL "SYS ..."    |      |
   |    |          |                     | frame should be    |      |
   |    |          |                     | sent before M_ADR) |      |
   |    |          |                     | Send M_ADR frame   |      |
   |    |          |                     | with system        |      |
   |    |          |                     | addresses          |      |
   |    |          |                     | Set Timer          |      |
   |    |          |---------------------+--------------------+------|
   |    |          | Nothing happens     | Wait               | R0   |
   |----+----------+---------------------+--------------------+------|
   | R1 | WaitAddr | M_ADR frame         | See if we have a   | R2   |
   |    |          | received            | password for any   |      |
   |    |          |                     | of the remote      |      |
   |    |          |                     | addresses          |      |
   |    |          |---------------------+--------------------+------|
   |    |          | M_ERR frame         | Report error       | exit |
   |    |          | received            |                    |      |
   |    |          |---------------------+--------------------+------|
   |    |          | M_NUL frame         | Log                | R1   |
   |    |          | received            |                    |      |
   |    |          |---------------------+--------------------+------|
   |    |          | Other known frame   | Report unexpected  | exit |
   |    |          | received            | frame              |      |
   |    |          |---------------------+--------------------+------|
   |    |          | Unknown frame       | Ignore             | R1   |
   |    |          | received            |                    |      |
   |    |          |---------------------+--------------------+------|
   |    |          | Nothing happens     | Wait               | R1   |
   |    |          |---------------------+--------------------+------|
   |    |          | Timer expired       | Report timeout     | exit |
   |----+----------+---------------------+--------------------+------|
   | R2 | IsPasswd | Yes, we have a      | Set Timer          | R3   |
   |    |          | password            |                    |      |
   |    |          |---------------------+--------------------+------|
   |    |          | Yes, but we have    | Send M_ERR frame   | exit |
   |    |          | several different   | Report             |      |
   |    |          | passwords for       | inconsistent       |      |
   |    |          | different addresses | password settings  |      |
   |    |          | of the remote       |                    |      |
   |    |          |---------------------+--------------------+------|
   |    |          | No, there's no      | Report non-secure  | T0   |
   |    |          | password            | session            |      |
   |----+----------+---------------------+--------------------+------|
   | R3 | WaitPwd  | M_PWD frame         | See if the         | R4   |
   |    |          | received            | password matches   |      |
   |    |          |---------------------+--------------------+------|
   |    |          | M_ERR frame         | Report error       | exit |
   |    |          | received            |                    |      |
   |    |          |---------------------+--------------------+------|
   |    |          | M_NUL frame         | Log                | R4   |
   |    |          | received            |                    |      |
   |    |          |---------------------+--------------------+------|
   |    |          | Other known frame   | Report unexpected  | exit |
   |    |          | received            | frame              |      |
   |    |          |---------------------+--------------------+------|
   |    |          | Unknown frame       | Ignore             | R4   |
   |    |          | received            |                    |      |
   |    |          |---------------------+--------------------+------|
   |    |          | Nothing happens     | Wait               | R3   |
   |    |          |---------------------+--------------------+------|
   |    |          | Timer Expired       | Report timeout     | exit |
   |----+----------+---------------------+--------------------+------|
   | R4 | PwdAck   | Yes, the password   | Send M_OK frame    | T0   |
   |    |          | matches             | Report secure      |      |
   |    |          |                     | session            |      |
   |    |          |---------------------+--------------------+------|
   |    |          | No, password does   | Report password    | exit |
   |    |          | not match           | error              |      |
   +-----------------------------------------------------------------+

  6.2 File Transfer Stage
  -----------------------

   File transfer stage is based on two major routines. We call them
   Receive Routine and Transmit Routine. These routines perform some
   actions depending on their state variables. State variables are
   RxState for Receive Routine and TxState for Transmit Routine.

   RxState := { RxWaitF | RxAccF | RxReceD | RxWriteD | RxEOB | RxDone
   }

   TxState := { TxGNF | TxTryR | TxReadS | TxWLA | TxDone }

                         Table 3: File Transfer
   +-----------------------------------------------------------------+
   | #  | Name         | Predicate(s)        | Action(s)      | Next |
   |----+--------------+---------------------+----------------+------|
   | T0 | InitTransfer | none                | Set Timer      | T1   |
   |    |              |                     | Set RxState to |      |
   |    |              |                     | RxWaitF        |      |
   |    |              |                     | Set TxState to |      |
   |    |              |                     | TxGNF          |      |
   |----+--------------+---------------------+----------------+------|
   | T1 | Switch       | RxState is RxDone   | Report session | exit |
   |    |              | and TxState is      | complete       |      |
   |    |              | TxDone              |                |      |
   |    |              |---------------------+----------------+------|
   |    |              | Data Available in   | call Receive   | T2   |
   |    |              | Input Buffer        | routine        |      |
   |    |              |---------------------+----------------+------|
   |    |              | Free space exists   | call Transmit  | T3   |
   |    |              | in output buffer    | routine        |      |
   |    |              |---------------------+----------------+------|
   |    |              | Nothing happens     | Wait           | T1   |
   |    |              |---------------------+----------------+------|
   |    |              | Timer Expired       | Report Timeout | exit |
   |----+--------------+---------------------+----------------+------|
   | T2 | Receive      | Receive routine     | Set Timer      | T1   |
   |    |              | returned OK         |                |      |
   |    |              |---------------------+----------------+------|
   |    |              | Receive routine     | Close all      | exit |
   |    |              | returned Failure    | opened files   |      |
   |    |              |---------------------+----------------+------|
   |    |              | Receive routine     | Call Receive   | T2   |
   |    |              | returned Continue   | routine again  |      |
   |----+--------------+---------------------+----------------+------|
   | T3 | Transmit     | Transmit routine    | Set Timer      | T1   |
   |    |              | returned OK         |                |      |
   |    |              |---------------------+----------------+------|
   |    |              | Transmit routine    | Close all      | exit |
   |    |              | returned Failure    | opened files   |      |
   |    |              |---------------------+----------------+------|
   |    |              | Transmit routine    | Call Transmit  | T3   |
   |    |              | returned Continue   | routine again  |      |
   +-----------------------------------------------------------------+

   Tables 4-6 are not actually state machines, but routines called
   during file transfer stage

   We define here a FIFO queue called "TheQueue", which is used to
   pass incoming M_GET / M_GOT / M_SKIP frames from Receive Routine to
   Transmit Routine. Receive routine itself does not react to these
   frames.

                        Table 4: Receive Routine
   +-----------------------------------------------------------------+
   |RxState |Predicate(s) |Condition(s) |Actions(s)|Next    |Return  |
   |--------+-------------+-------------+----------+--------+--------|
   |RxWaitF |Get a frame  |Haven't got a|none      |RxWaitF |OK      |
   |        |from Input   |complete     |          |        |        |
   |        |Buffer       |frame yet    |          |        |        |
   |        |             |-------------+----------+--------+--------|
   |        |             |Got Data     |ignore    |RxWaitF |OK      |
   |        |             |frame        |          |        |        |
   |        |             |-------------+----------+--------+--------|
   |        |             |Got M_ERR    |Report    |RxDone  |Failure |
   |        |             |             |Error     |        |        |
   |        |             |-------------+----------+--------+--------|
   |        |             |Got M_GET /  |Add frame |RxWaitF |OK      |
   |        |             |M_GOT /      |to The    |        |        |
   |        |             |M_SKIP       |Queue     |        |        |
   |        |             |-------------+----------+--------+--------|
   |        |             |Got M_NUL    |Log       |RxWaitF |OK      |
   |        |             |-------------+----------+--------+--------|
   |        |             |Got M_EOB    |Report End|RxEOB   |OK      |
   |        |             |             |of Batch  |        |        |
   |        |             |-------------+----------+--------+--------|
   |        |             |Got M_FILE   |none      |RxAccF  |continue|
   |        |             |-------------+----------+--------+--------|
   |        |             |Got other    |Report    |RxDone  |Failure |
   |        |             |known frame  |unexpected|        |        |
   |        |             |             |frame     |        |        |
   |        |             |-------------+----------+--------+--------|
   |        |             |Got unknown  |ignore    |RxWaitF |OK      |
   |        |             |frame        |          |        |        |
   |--------+-------------+-------------+----------+--------+--------|
   |RxAccF  |Decide how to|Accept from  |Report    |RxReceD |OK      |
   |        |accept       |beginning    |receiving |        |        |
   |        |Incoming File|             |file      |        |        |
   |        |             |-------------+----------+--------+--------|
   |        |             |Accept from  |Send M_GET|RxReceD |OK      |
   |        |             |offset (we do|Report    |        |        |
   |        |             |already have |receiving |        |        |
   |        |             |a part of    |file,     |        |        |
   |        |             |file)        |requested |        |        |
   |        |             |             |offest    |        |        |
   |        |             |-------------+----------+--------+--------|
   |        |             |Accept later |Send      |RxWaitF |OK      |
   |        |             |(or failed to|M_SKIP    |        |        |
   |        |             |create file) |Report we |        |        |
   |        |             |             |will      |        |        |
   |        |             |             |accept    |        |        |
   |        |             |             |file      |        |        |
   |        |             |             |later, not|        |        |
   |        |             |             |in current|        |        |
   |        |             |             |session   |        |        |
   |        |             |-------------+----------+--------+--------|
   |        |             |Refuse       |Send M_GOT|RxWaitF |OK      |
   |        |             |(delete on   |Report we |        |        |
   |        |             |remote)      |do not    |        |        |
   |        |             |             |accept    |        |        |
   |        |             |             |file      |        |        |
   |--------+-------------+-------------+----------+--------+--------|
   |RxReceD |Get a frame  |Didn't got a |none      |RxReceD |OK      |
   |        |from Input   |complete     |          |        |        |
   |        |Buffer       |frame yet    |          |        |        |
   |        |             |-------------+----------+--------+--------|
   |        |             |Got Data     |none      |RxWriteD|continue|
   |        |             |frame        |          |        |        |
   |        |             |-------------+----------+--------+--------|
   |        |             |Got M_ERR    |Report    |RxDone  |Failure |
   |        |             |             |Error     |        |        |
   |        |             |-------------+----------+--------+--------|
   |        |             |Got M_GET /  |Add frame |RxReceD |OK      |
   |        |             |M_GOT /      |to The    |        |        |
   |        |             |M_SKIP       |Queue     |        |        |
   |        |             |-------------+----------+--------+--------|
   |        |             |Got M_NUL    |Log       |RxReceD |OK      |
   |        |             |-------------+----------+--------+--------|
   |        |             |Got M_FILE   |Report    |RxAccF  |Continue|
   |        |             |             |partially |        |        |
   |        |             |             |received  |        |        |
   |        |             |             |file      |        |        |
   |        |             |-------------+----------+--------+--------|
   |        |             |Got other    |Report    |RxDone  |Failure |
   |        |             |known frame  |unexpected|        |        |
   |        |             |             |frame     |        |        |
   |        |             |-------------+----------+--------+--------|
   |        |             |Got unknown  |ignore    |RxReceD |OK      |
   |        |             |frame        |          |        |        |
   |--------+-------------+-------------+----------+--------+--------|
   |RxWriteD|Write data to|Write Failed |Report    |RxDone  |Failure |
   |        |file         |             |error     |        |        |
   |        |             |-------------+----------+--------+--------|
   |        |             |File Pos >   |Report    |RxDone  |Failure |
   |        |             |Reported     |write     |        |        |
   |        |             |             |beyond EOF|        |        |
   |        |             |-------------+----------+--------+--------|
   |        |             |File Pos =   |Close File|RxWaitF |OK      |
   |        |             |Reported     |Send M_GOT|        |        |
   |        |             |             |Report    |        |        |
   |        |             |             |File      |        |        |
   |        |             |             |Received  |        |        |
   |        |             |-------------+----------+--------+--------|
   |        |             |File Pos <   |none      |RxReceD |OK      |
   |        |             |Reported     |          |        |        |
   |--------+-------------+-------------+----------+--------+--------|
   |RxEOB   |Get a frame  |Didn't get a |none      |RxEOB   |OK      |
   |        |from Input   |complete     |          |        |        |
   |        |Buffer       |frame yet or |          |        |        |
   |        |             |TxState is   |          |        |        |
   |        |             |not TxDone   |          |        |        |
   |        |             |-------------+----------+--------+--------|
   |        |             |Got M_ERR    |Report    |RxDone  |Failure |
   |        |             |             |Error     |        |        |
   |        |             |-------------+----------+--------+--------|
   |        |             |Got M_GET /  |Add frame |RxEOB   |OK      |
   |        |             |M_GOT /      |to The    |        |        |
   |        |             |M_SKIP       |Queue     |        |        |
   |        |             |-------------+----------+--------+--------|
   |        |             |Got M_NUL    |Log       |RxEOB   |OK      |
   |        |             |-------------+----------+--------+--------|
   |        |             |Got other    |Report    |RxDone  |Failure |
   |        |             |known frame  |unexpected|        |        |
   |        |             |or data frame|frame     |        |        |
   |        |             |-------------+----------+--------+--------|
   |        |             |Got unknown  |ignore    |RxEOB   |OK      |
   |        |             |frame        |          |        |        |
   |--------+-------------+-------------+----------+--------+--------|
   |RxDone  |none         |none         |none      |RxDone  |OK      |
   +-----------------------------------------------------------------+

   We define the list called "PendingFiles". After we put the last
   byte of file into output buffer, we cannot yet consider the file as
   being successfully transmitted, thus we have to add the file to
   this list and then look for corresponding incoming M_GET / M_GOT /
   M_SKIP frames to remove the file from the list and decide whether
   the file was indeed received by remote or remote will accept this
   file later, or something else. After we have sent M_EOB frame, we
   must wait until PendingFiles list gets empty before disconnecting.

   If the connection accidentally breaks, all the files left in
   PendingFiles are considered unsent and will be re-transmitted in
   the next session. If the connection breaks when the remote did
   actually receive the file (but the corresponded confirmation frame
   (M_GOT) didn't came back to us) and we are resending this file
   again in the next session, remote may get two copies of the same
   file (file dupe). Binkp allows to reduce or totally suppress such
   dupes (at a cost of performance, of course), see Non-Reliable mode
   and "No Dupes" protocol extension (to be found in a separate
   document at a later date).

                        Table 5: Transmit Routine
   +-----------------------------------------------------------------+
   |TxState|Predicate(s)|Condition(s)  |Actions(s)  |Next   |Return  |
   |-------+------------+--------------+------------+-------+--------|
   |TxGNF  |Open next   |File opened OK|Send M_FILE |TxTryR |continue|
   |       |file from   |              |Report      |       |        |
   |       |outgoing    |              |sending file|       |        |
   |       |queue       |--------------+------------+-------+--------|
   |       |            |Failed to open|Report      |TxDone |Failure |
   |       |            |file          |failure     |       |        |
   |       |            |--------------+------------+-------+--------|
   |       |            |No more files |Send M_EOB  |TxWLA  |continue|
   |       |            |              |Report end  |       |        |
   |       |            |              |of batch    |       |        |
   |-------+------------+--------------+------------+-------+--------|
   |TxTryR |Check       |TheQueue is   |none        |TxReadS|continue|
   |       |TheQueue    |empty         |            |       |        |
   |       |            |--------------+--------------------+--------|
   |       |            |TheQueue is   |call ProcessTheQueue|continue|
   |       |            |not empty     |                    |        |
   |-------+------------+--------------+--------------------+--------|
   |TxReadS|Read data   |Read failed   |Report Error|TxDone |Failure |
   |       |block from  |--------------+------------+-------+--------|
   |       |file        |Read OK,      |Send data   |TxGNF  |OK      |
   |       |            |Reached EOF   |block frame |       |        |
   |       |            |              |Close       |       |        |
   |       |            |              |current file|       |        |
   |       |            |              |Add current |       |        |
   |       |            |              |file to     |       |        |
   |       |            |              |PendingFiles|       |        |
   |       |            |--------------+------------+-------+--------|
   |       |            |Read OK, not  |Send data   |TxTryR |OK      |
   |       |            |reached EOF   |block frame |       |        |
   |-------+------------+--------------+------------+-------+--------|
   |TxWLA  |Check       |TheQueue is   |none        |TxDone |OK      |
   |       |TheQueue    |empty and     |            |       |        |
   |       |            |RxState >=    |            |       |        |
   |       |            |RxEOB         |            |       |        |
   |       |            |--------------+------------+-------+--------|
   |       |            |TheQueue is   |none        |TxWLA  |OK      |
   |       |            |empty and     |            |       |        |
   |       |            |RxState <     |            |       |        |
   |       |            |RxEOB         |            |       |        |
   |       |            |--------------+--------------------+--------|
   |       |            |TheQueue is   |call ProcessTheQueue|continue|
   |       |            |not empty     |                    |        |
   |-------+------------+--------------+--------------------+--------|
   |TxDone |none        |none          |none        |TxDone |OK      |
   +-----------------------------------------------------------------+

   We define a list called KnownFiles. This list contains files that
   can be requested by the remote using M_GET command. This list shall
   at least contain all the files that are part of the PendingFiles
   list.

                    Table 6: ProcessTheQueue routine
   +-----------------------------------------------------------------+
   | Predicate(s)       | Condition(s)       | Actions(s)            |
   |--------------------+--------------------+-----------------------|
   | M_GET received     | requested file is  | Report unknown file   |
   |                    | not in the         |                       |
   |                    | KnownFiles list    |                       |
   |--------------------+--------------------+-----------------------|
   | M_GET received for | Requested pos is   | Close and finalize    |
   | a known file       | FileSize           | file.                 |
   |                    |                    | Report that remote    |
   |                    |                    | refused file being    |
   |                    |                    | transmitted.          |
   |                    |                    | Set TxState to        |
   |                    |                    | TxGetNextFile.        |
   |                    |--------------------+-----------------------|
   |                    | Requested pos is   | Set file pointer to   |
   |                    | less than FileSize | requested pos.        |
   |                    |                    | Report that remote    |
   |                    |                    | requested offset.     |
   |                    |                    | Set TxState to        |
   |                    |                    | TxReadSend.           |
   |                    |--------------------+-----------------------|
   |                    | Requested pos is   | Ignore frame          |
   |                    | greater than       |                       |
   |                    | FileSize           |                       |
   |--------------------+--------------------+-----------------------|
   | M_GOT file that is | none               | Close and finalize    |
   | currently          |                    | file                  |
   | transmitting       |                    | Report Remote refused |
   |                    |                    | file being            |
   |                    |                    | transmitted           |
   |                    |                    | Set TxState to TxGNF  |
   |--------------------+--------------------+-----------------------|
   | M_GOT file that is | File is in         | Finalize file         |
   | not currently      | PendingFiles list  | Report file has been  |
   | transmitting       |                    | sent                  |
   |                    |                    | Remove file from the  |
   |                    |                    | PendingFiles list     |
   |                    |--------------------+-----------------------|
   |                    | File is not in     | Ignore frame          |
   |                    | PendingFiles       |                       |
   |--------------------+--------------------+-----------------------|
   | M_SKIP file that   | none               | Close file (do not    |
   | is currently       |                    | finalize, we will     |
   | transmitting       |                    | send it later, not in |
   |                    |                    | current session)      |
   |                    |                    | Report remote will    |
   |                    |                    | accept this file      |
   |                    |                    | later                 |
   |                    |                    | Set TxState to TxGNF  |
   |--------------------+--------------------+-----------------------|
   | M_SKIP file that   | none               | Report remote will    |
   | is not currently   |                    | accept this file      |
   | transmitting       |                    | later                 |
   |                    |                    | Remove file from      |
   |                    |                    | PendingPiles, if      |
   |                    |                    | exists there          |
   +-----------------------------------------------------------------+

  6.3 Session Termination
  -----------------------

   A session may be terminated in any of the following cases:

   1. If transmitted or received M_ERR. In this case, the session
       should be deemed aborted due to a fatal error.
   2. If transmitted or received M_BSY. In this case, the session
       should be deemed aborted due to non-fatal error typically
       because of temporary lack of resources to proceed with the
       session.
   3. If all of the following applies:
          * all the files have been sent
          * we have received M_EOB from the remote side (there are no
            more files for us),
          * we have received acknowledgements for all the files sent,
          * we have received all the files re-requested by M_GET,
       In this case, the session should be deemed successfully
       completed.

   A session termination itself is not a protocol stage. Mailer may
   terminate a session at any time simply by issuing disconnect
   (shutdown) command to the underlying transport layer, provided any
   of the three conditions above are met. Mailer MUST take all proper
   steps to provide a graceful shutdown of the transport layer, as it
   is the transport layer that is responsible for all the data
   transmitted by one side to be received by another before
   disconnection, provided that shutdown of the transport layer
   protocol was successful.

7. Recommended Protocol Extensions
----------------------------------

   This section documents already implemented and proposed extensions
   for the binkp/1.0. These extensions are purely optional and are
   included here for the sake of compatibility with future
   implementations.

   Sides indicate supported protocol extensions by sending M_NUL
   frame(s) with "OPT list_of_extensions" string, where
   list_of_extensions is a space separated list of supported protocol
   extensions. Whenever multiple M_NUL "OPT ..." frames are received
   during the session, they SHOULD augment the current list of
   extensions rather than replace it, unless specifically stated
   otherwise for a particular option.

   Mailer SHOULD NOT use any extension unless exactly sure that this
   extension is supported by the remote. Mailer SHOULD use M_NUL "OPT
   ..." to indicate supported options. Other methods for indicating
   supported extensions are allowed as long as the provide full
   backwards compatibility.

  7.1 Non-reliable Mode
  ---------------------

   Non-reliable mode solves the problem with frequently aborted
   connections when the sides can not successfully complete file
   transfer before connection is broken. In this case, if the
   transmitting side starts retransmission from offset 0, performance
   degrades as by the time it receives M_GET from the remote, network
   buffers are already full and by the time they are freed for
   retransmission from requested offset, the connection might go down
   again.

   In order to circumvent this problem, a mailer can request the
   remote to enter non-reliable mode by sending a M_NUL "OPT NR" frame
   at any time during the session. After the remote acknowledges it by
   sending an M_NUL "OPT NR" frame indicating that the option is
   supported, both sides can assume that they are in non-reliable
   mode.

   When session is in non-reliable mode, the transmitting side may
   send -1 for the offset value in M_FILE command. If it does so, it
   should wait for the M_GET frame from the receiving side that
   explicitly specifies file offset and start transmitting file data
   from this offset. If the receiving side has indicated that it
   supports non-reliable mode by sending M_NUL "OPT NR" frame, it must
   recognize -1 as the file offset in M_FILE command as an explicit
   request for the file offset and transmit an appropriate M_GET frame
   as soon as possible.

   It should be understood that this option degrades performance over
   regular quality connections and it should be used only if
   absolutely necessary.

  7.2 Multiple Batch Mode
  -----------------------

   The session is in MB mode if both sides set "MB" flag in any of
   M_NUL "OPT" packets exchanged before sending of M_OK/M_PWD packets.

   In MB mode both sides restart session from RxDone into InitTransfer
   state if there were any command packets sent or received by any
   side between starting at InitTransfer and exchanging of M_EOB by
   the sides (RxDone state). Otherwise, the session terminates as
   usual.

   Multiple batches mode is intended to handle WaZOO [FTS-0006] file
   requests. If there were any WaZOO request files transferred in a
   batch, sides MAY process them and send resulting files in the next
   batch. Mailers MAY also generate list of files to send in
   additional batches by other techniques -- including rescanning of
   their spools or processing of other magic files transferred before
   in the same session.

  7.3 Multiple Passwords Mode
  ---------------------------

   Multiple password mode allows to specify different passwords for
   the different addresses of the remote.

   Originating side identifies it's multipassword capabilities by
   sending M_NUL "OPT MPWD" during session setup stage before sending
   any M_ADR commands and waits for response from the answering side.

   If answering side responds with the M_NUL "OPT MPWD", then it
   supports multiply passwords too. Answering side also always
   responds with it's own address list: M_ADR "adr1 adr2 adr3 ...". If
   M_NUL "OPT MPWD" was not received prior to the first M_ADR command,
   originating side should assume that the remote does not support
   multiple password mode and send a single password (if any) for one
   of the addresses of the remote.

   If the MPWD option was indicated by the answering side, originating
   side now may send M_PWD "pwd1 pwd2 pwd3 ..." with the number of
   entries in space separated password list equivalent to the number
   of addresses presented by the answering side. If there is no
   password for a particular address, it must send '-' character as a
   placeholder.

   If the passwords presented are consistent, answering side must
   acknowledge successful authentication by sending M_OK command.

  7.4 Keyed Hashing Challenge-Response Authentication Mechanism
  -------------------------------------------------------------

    7.4.1 Overview
    --------------

   Challenge-Response Authentication Mechanism (CRAM) allows to avoid
   passing cleartext, reusable passwords across the network. Since it
   utilizes Keyed-Hashing digests [Keyed], it does not require
   password to be stored in the clear on the Mailer's media, allowing
   storage of the intermediate results which are known as "contexts".

   Providing binkp-mailer is capable of [Keyed] digest calculation and
   conversion of a byte array to a hexadecimal string and back,
   implementation of CRAM is easily achieved by slightly modifying the
   state machine.

    7.4.2 Sequence of Steps
    -----------------------

   CRAM adds an additional synchronization step to binkp protocol. The
   description of this step follows:

   1. Answering side sends a unique set of data (challenge data) to
       the Originating side, encoded to a hexadecimal string.
   2. Originating side uses challenge data, decoded from received
       hexadecimal string, and a password to produce a digest by
       applying the keyed Hashing algorithm from [Keyed] where the key
       is the password and the digested text is the challenge data.
   3. When the answering side receives this response, it verifies the
       digest provided. If the digest is correct, the answering side
       should consider the Originating side authenticated and responds
       appropriately.

   Similar technique is used in [IMAP-AUTH].

    7.4.3 Generating and Transmitting Challenge Data
    ------------------------------------------------

   Size and contents of challenge data are implementation-dependent,
   but it SHOULD be no smaller than 8 bytes and no bigger than 64
   bytes. Answering side SHOULD never generate the same challenge
   data.

   Instead of generating a long challenge data, answering side MAY use
   a hash function to shorten it. In calculation of a challenge data
   answering side MAY also use connection/line number, caller's IP
   address, current time, etc.

   Answering side transmits challenge data in the very first M_NUL
   message, in the following way:

   M_NUL "OPT [othropt] CRAM-lsthf-cde [othropt]"

   lsthf is a list of aliases of supported hash functions, delimited
   by slash characters. The list begins with alias of the most
   preferred and ends with alias of the least preferred hash function.

   Currently defined aliases are: MD5 for [MD5] and SHA1 for [SHA-1].

   cde is the challenge data encoded to hexadecimal string, Lower-case
   ASCII characters MUST be used for encoding, but Mailer SHOULD also
   accept upper-case characters. The length of the string MUST be
   even, and the leading zeros MUST NOT be trimmed.

    7.4.4 Producing and Transmitting a Digest
    -----------------------------------------

   Originating side responds with:

   M_PWD "CRAM-chosenhf-khde [othropt]"

   where chosenhf is the alias of the chosen hash function and khde is
   the keyed hashed digest, encoded to a hexadecimal string.

   According to [IMAP-AUTH], keyed hashed digest is produced by
   calculating

   HASH((secret XOR opad), HASH((secret XOR ipad), challengedata))

   where HASH is chosen hash function, ipad and opad are 36 hex and 5C
   hex (as defined in [Keyed]) and secret is a password null-padded to
   a length of 64 bytes. If the password is longer than 64 bytes, the
   hash-function digest of the password is used as an input (16-byte
   for [MD5] and 20-byte for [SHA-1]) to the keyed hashed calculation.

    7.4.6 Indicating CRAM Capabilities
    ----------------------------------

   Answering side MUST send

   M_NUL "OPT [othropt] CRAM-lsthf-cde [othropt]"

   as a very first M_NUL message if it supports CRAM.

   It MAY send other non-M_NUL messages before though. Current
   specification doesn't define any such non-M_NUL message, they are
   reserved for protocol extension.

   Originating side MUST be ready to receive non-M_NUL before M_NUL in
   a CRAM session. Binkp state machine MUST ignore any received
   message of unknown type in order to be compatible with future
   extensions.

   If an originating side receives a first message that is a M_ADR or
   a M_NUL message that is not

   M_NUL "OPT [othropt] CRAM-lsthf-cde [othropt]"

   it MUST decide that the answering side doesn't support CRAM and MAY
   either disconnect or use old password exchange. If the sides have
   no any compatible hash function, originator may also either
   disconnect or use old password exchange. If an originating side
   decides to disconnect, it SHOULD send M_ERR frame with a proper
   explanation before disconnecting.

   When parsing M_NUL "OPT ..." string (coming from the answering
   side), originating side first splits it by using space delimiter to
   get a list of options, and then if an option begins with
   "CRAM-lsthf-", takes the remaining substring as a
   hexadecimal-encoded challenge data.

    7.4.7 Example of Frame Exchange During CRAM Authentication
    ----------------------------------------------------------

   (Password here is tanstaaftanstaaf)

   Originating :
     send M_NUL messages
     and M_ADR
     wait for first M_NUL message

   Answering   :
     send M_NUL "OPT ND CRAM-SHA1/MD5-f0315b074d728d483d6887d0182fc328"
     and other messages
     wait for M_PWD

   Originating :
     M_PWD "CRAM-MD5-56be002162a4a15ba7a9064f0c93fd00"

   Answering   :
     M_OK and continue session

    7.4.8 Notes on Hash Function Algorithms
    ---------------------------------------

   [MD5] and [SHA-1] are the most widely used cryptographic hash
   functions. [MD5] has been shown to be vulnerable to collision
   search attacks [Dobb]. This attack and other currently known
   weaknesses of [MD5] do not compromise the use of [MD5] within CRAM
   as specified in this document (see [Dobb]); however, [SHA-1]
   appears to be a cryptographically stronger function. To this date,
   [MD5] can be considered for use in CRAM for applications where the
   superior performance of [MD5] is critical. In any case,
   implementors and users need to be aware of possible cryptanalytic
   developments regarding any of these cryptographic hash functions,
   and the eventual need to replace the underlying hash function.

8. License
----------

   You can implement binkp protocol in your software as long as you
   agree to the following conditions:

   1. The protocol shall be referenced to as binkp and not in any
       other way. You shall include the author(s) of the protocol in
       your copyright statement for the software.
   2. Binkp shall always be backwards compatible with it's previous
       versions. Binkp allows development of the new capabilities
       without compromising interoperability with previous versions.
       Therefore, it is important that future developments of the
       protocol are not pursued in different directions by different
       people. If you have any suggestions regarding future
       developments of the protocol, make a reasonable effort to
       contact the author(s), so that the development efforts can
       coordinated in a way advantageous for everybody.
   3. If your implementation is not compatible with past, present or
       future binkp specifications, you shall reference to it as a
       "binkp variation" or "binkp derived".

   Remember that you may use, implement or utilize binkp, it's
   description or any other associated texts or documentations at your
   own risk, without any warranty, without even the implied warranty
   of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE

   Binkp author: Dima Maloff.

9. Glossary
-----------

   Many entries in this glossary are provided courtesy of Butterfly
   Glossary of Internet and Data Communication terms and RFC-1983.

   connection-oriented
           Data communication method in which communication proceeds
           through three well-defined phases: connection
           establishment, data transfer, connection release. TCP is a
           connection-oriented protocol.

   data link layer
           The OSI layer that is responsible for data transfer across
           a single physical connection, or series of bridged
           connections, between two Network entities.

   flow control
           A technique for ensuring that a transmitting entity does
           not overwhelm a receiving entity.

   HDLC
           (High level Data Link Control). Popular ISO standard
           bit-oriented, data link layer protocol derived from SDLC.
           HDLC specifies an encapsulated method of data on
           synchronous serial data links.

   IP
           (Internet Protocol). The Internet Protocol, defined in STD
           5, RFC 791, is the network layer for the TCP/IP Protocol
           Suite. It is a connectionless, best-effort packet switching
           protocol.

   network layer
           Layer 3 of the OSI reference model. Layer 3 is the layer at
           which routing, addressing and connection management take
           place.

   OSI (Open Systems Interconnection) Reference Model
           A seven-layer structure designed to describe computer
           network architectures and the way that data passes through
           them. This model was developed by the ISO (International
           Organization for Standardization) in 1978 to clearly define
           the interfaces in multivendor networks, and to provide
           users of those networks with conceptual guidelines in the
           construction of such networks.

   port
           A port is a transport layer demultiplexing value. Each
           application has a unique port identifier associated with
           it.

   physical layer
           The OSI layer that provides the means to activate and use
           physical connections for bit transmission. In plain terms,
           the Physical Layer provides the procedures for transferring
           a single bit across a Physical Media.

   Quality of Service
           (Also QoS). A measure of performance for a transmission
           system that reflects its transmission quality and
           availability of service.

   reliable transmission
           a type of transport service that:
              * recovers from errors by retransmitting errored frames
              * delivers frames in correct sequence (also known as
                stream-oriented)
              * usually is used in connection-oriented mode

   session layer
           Layer 5 of the OSI reference model. Coordinates session
           activity between applications, including application-layer
           error control, dialog control, and remote procedure calls.

   sliding window flow control
           Method of flow control in which a receiver gives
           transmitter permission to transmit data until a window is
           full. When the window is full, the transmitter must stop
           transmitting until the receiver advertises a larger window.

   socket
           Software structure operating as a communications and point
           within a network device.

   TCP
           Transmission Control Protocol. An Internet Standard
           transport layer reliable protocol defined in STD 7, RFC
           793. It is connection-oriented and stream-oriented.

   TCP/IP protocol suite
           Transmission Control Protocol over Internet Protocol. This
           is a common shorthand which refers to the suite of
           transport and application protocols which runs over IP.

   transport layer
           Layer 4 of the OSI reference model. The transport layer is
           responsible for reliable network communication between end
           nodes. It implements flow and error control and often uses
           virtual circuits to ensure reliable data delivery.

   unixtime
           number of seconds elapsed since 00:00:00 UTC, Jan. 1, 1970.

10. References
--------------

   [FTS-0001]
           A Basic FidoNet(r) Technical Standard, Revision 16. Randy
           Bush, Pacific Systems Group, September 30, 1995. FTS-0001.

   [FTS-0006]
           YOOHOO and YOOHOO/2U2. The netmail handshake used by
           Opus-CBCS and other intelligent Fidonet mail handling
           packages. Version 002, Vince Perriello. 30-Nov-1991.
           FTS-0006.

   [FSC-0039]
           M.Howard, A type-2 packet extension proposal, FSC-0039
           Version 4, 29-Sep-1990. FSC-0039.

   [FSC-0045]
           T.Henderson, Proposed new packet header, Version 1,
           17-Apr-1990. FSC-0045.

   [FSC-0048]
           J.Vroonhof, Proposed type-2 packet extension, Version 2,
           21-Oct-1990. FSC-0048.

   [FSC-0081]
           M.Staldal, A type-3 packet proposal, Version 1,
           01-Mar-1995. FSC-0081.

   [EMSI]
           Joaquim H. Homrighausen, EMSI/IEMSI protocol definition.
           May 3, 1991. FSC-0056.

   [FTA-1006]
           Key words to indicate requirement levels, Fidonet Technical
           Standards Committee administrative. FTA-1006.

   [Halsall95]
           Data Communications, Computer Networks and Open Systems, F.
           Halsall, 4th ed., Addison-Wesley, 1995, ISBN 0-201-42293-X.

   [Dobb]
           H. Dobbertin, "The Status of MD5 After a Recent Attack",
           RSA Labs' CryptoBytes, Vol. 2 No. 2, Summer 1996.

   [MD5]
           Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
           April 1992.

   [SHA-1]
           NIST, FIPS PUB 180-1: Secure Hash Standard, April 1995.

   [Keyed]
           Krawczyk, Bellare, Canetti, "HMAC: Keyed-Hashing for
           Message Authentication", RFC 2104, February 1997.

   [IMAP-AUTH]
           Klensin, "IMAP/POP AUTHorize Extension for Simple
           Challenge/Response", RFC 2195, September, 1997

   [RFC822]
           Standard for the format of ARPA Internet text messages. D.
           Crocker. Aug-13-1982. RFC 822, STD0011.

   [UTF8]
           UTF-8, a transformation format of ISO 10646. F. Yergeau.
           January 1998, RFC 2279.

   [ISO10646]
           ISO/IEC 10646-1:1993. International Standard -- Information
           technology -- Universal Multiple-Octet Coded Character Set
           (UCS) -- Part 1: Architecture and Basic Multilingual Plane.
           Five amendments and a technical corrigendum have been
           published up to now. UTF-8 is described in Annex R,
           published as Amendment 2.

11. Acknowledgements
--------------------

   This document is partially based on extracts from RFCs and FTSC
   publications too numerous to be acknowledged individually.

   The authors would like to thank Joaquim Homrighausen, Kim 'B'
   Heino, Rune Johansen and many others for fruitful discussions and
   suggestions regarding protocol design and specifications.

A. Author Contact Data
-----------------------

   Dima Maloff
   Fidonet: 2:5020/128
   E-mail: maloff@corbina.net
   WWW: http://www.corbina.net/~maloff/

   Maxim Masiutin
   Fidonet: 2:469/84
   E-mail: max@ritlabs.com
   WWW: http://www.ritlabs.com/

   Nick Soveiko
   Fidonet: 2:5030/23.101
   E-mail: nsoveiko@doe.carleton.ca
   WWW: http://www.doe.carleton.ca/~nsoveiko/

B. History
----------

   Rev.1, 19990611:
           First release

   Rev.2, 19991008:
              * Added new topic: "Definitions";
              * clarified the following topics: "Frame Format",
                "Protocol Commands and Their Arguments", "Keyed
                Hashing Challenge-Response Authentication Mechanism";
              * added "unixtime" item to Glossary topic;
              * corrected links in References topic.

   Rev.3, 20000731:
              * Table 6 in section 6.2, File transfer stage has been
                rewritten: TheListOfSendFiles replaced by PendingFiles
                which was defined earlier. introduced definition of
                KnownFiles list. new ProcessTheQueue routine w/respect
                to handling M_GET command
              * Section 5.2, File Name Issues was rewritten to clearly
                define safe and unsafe characters in filenames.
              * Section 5.3, Non-ASCII Characters was rewritten to
                clarify Unicode usage.
              * Expanded descriptions for M_NUL "TIME ...", M_NUL "TRF
                ...", added description of M_NUL "PHN ..." and M_NUL
                "OPM ..." frames in section 5.4 Binkp Commands.
              * IANA port number added to section 3, Protocol
                Overview.
              * M_GET description in section 5.4, Binkp Commands was
                rewritten for clarity.
              * M_BSY "RETRY ..." option documented.
              * Minor edits throughout the document to improve
                readability.