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RFC 1795

 
 
 

Data Link Switching: Switch-to-Switch Protocol AIW DLSw RIG: DLSw Closed Pages, DLSw Standard Version 1

Part 2 of 4, p. 18 to 43
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prevText      Top       Page 18 
4.  Circuit Priority

   At circuit start time, each circuit end point will provide priority
   information to its circuit partner.  The initiator of the circuit
   will choose which circuit priority will be effective for the life of
   the circuit.  If Priority is not implemented by the Data Link Switch,
   then "Unsupported" priority is used.

4.1  Frame format

   Circuit priority will be valid in the CANUREACH_cs, ICANREACH_cs, and
   REACH_ACK frames only. The relevant header field is shown below.  The
   Circuit Priority value is a byte value at offset 22 in an SSP Control
   Message.

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   The following describes the format of the Circuit Priority byte.

     7   6   5   4   3   2   1   0
   +-------------------+-----------+
   |   reserved        |    CP     |
   +-------------------+-----------+

   CP: Circuit Priority bits
           000 - Unsupported       (note 1)
           001 - Low Priority
           010 - Medium Priority
           011 - High Priority
           100 - Highest Priority
           101 to 111 are reserved for future use

   Note 1: Unsupported means that the Data Link Switch that originates
   the circuit does not implement priority.  Actions taken on
   Unsupported priority are vendor specific.

4.2  Circuit Startup

   The sender of a CANUREACH_cs is responsible for setting the CP bits
   to reflect the priority it would like to use for the circuit being
   requested.  The mechanism for choosing an appropriate value is
   implementation dependent.  The sender of an ICANREACH_cs frame will
   set the CP bits to reflect the priority it would like to use for the
   circuit being requested, with the mechanism for choosing the
   appropriate value being implementation dependent.  The receiver of
   the ICANREACH_cs will select from the priorities in the CANUREACH_cs
   and ICANREACH_cs frames, and will set the value in the CP field of
   the REACH_ACK frame that follows to the value to be used for this
   circuit.  This priority will be used for the life of the circuit.  A
   CANUREACH_cs or ICANREACH_cs with the circuit priority value set to
   Unsupported (CP=000) indicates that the sender does not support the
   circuit priority function.

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   Flow:

      DLSw A               DLSw B

   CANUREACH_cs (CP=011) ----->           Circuit initiator requests
                                          high Priority.

        <--------- ICANREACH_cs (CP=010)  Circuit target requests
                                          medium priority.

   REACH_ACK (CP=010) -------->           Circuit initiator sets
                                          the priority for this
                                          circuit to medium. The
                                          circuit initiator could
                                          choose either high or
                                          medium in this example.

5.  DLSw State Machine

   The following state tables describe the states for a single circuit
   through the Data Link Switch.  State information is kept for each
   connection.  The initial state for a connection is DISCONNECTED.  The
   steady state is either CIRCUIT_ESTABLISHED or CONNECTED.  In the former
   state, an end-to-end circuit has been established allowing the support
   of Type 1 LLC between the end systems.  The latter state exists when an
   end-to-end connection has been established for the support of Type 2 LLC
   services between the end systems.

   For SNA, LLC type 2 connection establishment is via the use of IEEE
   802.2 Test or XID  frames.  SNA devices send these frames to the null
   SAP in order to determine the source route information in support of
   bridging.  Normally SNA devices use SAP 0x04, 0x08, or 0x0C  (most SNA
   LLC2 devices that have a single PU per MAC address use a default of
   0x04).  Typically the SAP would be used to determine if the Test frames
   should be sent to the DLSw code in the router.  If both bridging and
   DLSw are enabled, this allows the product to ensure that SNA frames are
   not both bridged and switched.  Note that although typically SNA uses a
   DSAP and SSAP of 0x04, it allows for other SAPs to be configured and
   supports unequal SAPs.  This allows multiple PUs to share connections
   between two given MAC addresses (each PU to PU session uses one LLC2
   connection).

   For NetBIOS, LLC type 2 connection establishment is via the Name Query
   and Name Recognized frames.  These frames are used for both address
   resolution and source route determination.  NetBIOS devices use SAP
   0xF0.

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5.1  Data Link Switch States

   The Switch-to-Switch Protocol is formally defined through the state
   machines described in this chapter.  The following table lists the
   thirteen possible states for the main circuit FSM.  A separate state
   machine instance is employed for each end-to-end circuit that is
   maintained by the Data Link Switch.

   State Name            Description
   ----------            -----------
   CIRCUIT_ESTABLISHED   The end-to-end circuit has been
                         established.  At this time LLC Type 1
                         services are available from end-to-end.

   CIRCUIT_PENDING       The target DLSw is awaiting a REACH_ACK
                         response to an ICANREACH_cs message.

   CIRCUIT_RESTART       The DLSw that originated the reset is
                         awaiting the restart of the data link
                         and the DL_RESTARTED response to a
                         RESTART_DL message.

   CIRCUIT_START         The origin DLSw is awaiting a
                         ICANREACH_cs in response to a
                         CANUREACH_cs message.

   CONNECTED             The end-to-end connection has
                         been established thereby allowing
                         LLC Type 2 services from end-to-end
                         in addition to LLC Type 1 services.

   CONNECT_PENDING       The origin DLSw is awaiting the
                         CONTACTED response to a CONTACT
                         message.

   CONTACT_PENDING       The target DLSw is awaiting the
                         DLC_CONTACTED confirmation to a
                         DLC_CONTACT signal (i.e., DLC
                         is waiting for a UA response to
                         an SABME command).

   DISCONNECTED          The initial state with no circuit
                         or connection established, the
                         DLSw is awaiting either a
                         CANUREACH_cs, or an ICANREACH_cs.

   DISCONNECT_PENDING    The DLSw that originated the
                         disconnect is awaiting the DL_HALTED

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                         response to a HALT_DL message.

   HALT_PENDING          The remote DLSw is awaiting the
                         DLC_DL_HALTED indication following
                         the DLC_HALT_DL request (i.e., DLC
                         is waiting for a UA response to a
                         DISC command), due to receiving a
                         HALT_DL message.

   HALT_PENDING_NOACK    The remote DLSw is awaiting the
                         DLC_DL_HALTED indication following
                         the DLC_HALT_DL request (i.e., DLC
                         is waiting for a UA response to a
                         DISC command), due to receiving a
                         HALT_DL_NOACK message.

   RESTART_PENDING       The remote DLSw is awaiting the
                         DLC_DL_HALTED indication following
                         the DLC_HALT_DL request (i.e., DLC
                         is waiting for a UA response to a
                         DISC command), and the restart of
                         the data link.

   RESOLVE_PENDING       The target DLSw is awaiting
                         the DLC_DL_STARTED indication
                         following the DLC_START_DL request
                         (i.e., DLC is waiting for a Test
                         response as a result of sending a
                         Test command).

   The DISCONNECTED state is the initial state for a new circuit.  One
   end station starts the connection via an XID or SABME command (i.e.,
   DLC_XID or DLC_CONTACTED).  Upon receipt, the Data Link Switches
   exchange a set of CANUREACH_cs, ICANREACH_cs and REACH_ACK messages.
   Upon completion of this three-legged exchange both Data Link Switches
   will be in the CIRCUIT_ESTABLISHED state.  Three pending states also
   exist during this exchange.  The CIRCUIT_START state is entered by
   the origin Data Link Switch after it has sent the CANUREACH_cs
   message.  The RESOLVE_PENDING state is entered by the target Data
   Link Switch awaiting a Test response to a Test Command.  And lastly,
   the CIRCUIT_PENDING state is entered by the target DLSw awaiting the
   REACH_ACK reply to an ICANREACH_cs message.

   The CIRCUIT_ESTABLISHED state allows for the exchange of LLC Type 1
   frames such as the XID exchanges between SNA stations that occurs
   prior to the establishment of a connection.  Also, datagram traffic
   (i.e., UI frames)  may be sent and received between the end stations.
   These exchanges use the XIDFRAME and DGRMFRAME messages sent between

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   the Data Link Switches.

   In the CIRCUIT_ESTABLISHED state, the receipt of a SABME command
   (i.e., DLC_CONTACTED) causes the origin DLSw to issue a CONTACT
   message, to send an RNR supervisory frame (i.e., DLC_ENTER_BUSY) to
   the origin station, and to enter the CONNECT_PENDING state awaiting a
   CONTACTED message.  The target DLSw, upon the receipt of a CONTACT
   message, will issue a SABME command (i.e., DLC_CONTACT) and enter the
   Contact Pending state.  Once the UA response is received (i.e.,
   DLC_CONTACTED), the target DLSw sends a CONTACTED message and enters
   the CONNECTED state. When received, the origin DLSw enters the
   CONNECTED state and sends an RR supervisory frame (i.e.,
   DLC_EXIT_BUSY).

   The CONNECTED state is the steady state for normal data flow once a
   connection has been established.  Information frames (i.e., INFOFRAME
   messages) are simply sent back and forth between the end points of
   the connection.  This is the path that should be optimized for
   performance.

   The connection is terminated upon the receipt of a DISC frame or
   under some other error condition detected by DLC (i.e., DLC_ERROR).
   Upon receipt of this indication, the DLSw will halt the local data
   link, send a HALT_DL message to the remote DLSw, and enter the
   DISCONNECT_PENDING State.  When the HALT_DL frame is received by the
   other DLSw, the local DLC is halted for this data link, a DL_HALTED
   message is returned, and the DISCONNECTED state is entered.  Receipt
   of this DL_HALTED message causes the other DLSw to also enter the
   DISCONNECTED state.

   The CIRCUIT_RESTART state is entered if one of the Data Link Switches
   receives a SABME command  (i.e., DLC_RESET) after data transfer while
   in the CONNECTED state.  This causes a DM command to be returned to
   the origin station and a RESTART_DL message to be sent to the remote
   Data Link Switch. This causes the remote data link to be halted and
   then restarted.  The remote DLSw will then send a DL_RESTARTED
   message back to the first DLSw.  The receipt of the DL_RESTARTED
   message causes the first DLSw to issue a new CONTACT message,
   assuming that the local DLC has been contacted (i.e., the origin
   station has resent the SABME command).  This is eventually responded
   to by a CONTACTED message. Following this exchange, both Data Link
   Switches will return to the CONNECTED state.  If the local DLC has
   not been contacted, the receipt of a DL_RESTARTED command causes the
   Data Link Switch to enter the CIRCUIT_ESTABLISHED state awaiting the
   receipt of a SABME command (i.e., DLC_CONTACTED signal).

   The HALT_PENDING, HALT_PENDING_NOACK and RESTART_PENDING states
   correspond to the cases when the Data Link Switch is awaiting

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   responses from the local station on the adjacent LAN (e.g., a UA
   response to a DISC command). Also in the RESTART_PENDING state, the
   Data Link Switch will attempt to restart the data link prior to
   sending a DL_RESTARTED message.  For some implementations, the start
   of a data link involves the exchange of a Test command/response on
   the adjacent LAN (i.e., DLC_START_DL).  For other implementations,
   this additional exchange may not be required.

5.2  State Transition Tables

   This section provides a detailed representation of the Data Link
   Switch, as documented by a single state machine.  Many of the
   transitions are dependent upon local signals between the Data Link
   Switch entity and one of the DLC entities.  These signals and their
   definitions are given in the following tables.

   DLC Events:

   Event Name      Description
   ----------      -----------
   DLC_CONTACTED   Contact Indication:  DLC has received an SABME
                   command or DLC has received a UA response as a
                   result of sending an SABME command.

   DLC_DGRM        Datagram Indication:  DLC has received a UI frame.

   DLC_ERROR       Error condition indicated by DLC:  Such a
                   condition occurs when a DISC command is received
                   or when DLC experiences an unrecoverable error.

   DLC_INFO        Information Indication:  DLC has received an
                   Information (I) frame.

   DLC_DL_HALTED   Data Link Halted Indication:  DLC has
                   received a UA response to a DISC command.

   DLC_DL_STARTED  Data Link Started Indication:  DLC has
                   received a Test response from the null SAP.

   DLC_RESET       Reset Indication:  DLC has received an SABME
                   command during the time a connection is
                   currently active and has responded with DM.

   DLC_RESOLVE_C   Resolve Command Indication:  DLC has received
                   a Test command addressed to the null SAP, or an
                   XID command addressed to the null SAP.

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   DLC_RESOLVED    Resolve request:  DLC has received a TEST response
                   frame (or equivalent for non-LAN DLCs) but has not
                   reserved the resources required for a circuit yet.

   DLC_XID         XID Indication:  DLC has received an XID command
                   or response to a non-null SAP.

   Other Events:

   Event Name      Description
   ----------      -----------
   XPORT_FAILURE   Failure of the transport connection used by the
                   circuit.

   CS_TIMER_EXP    The CIRCUIT_START timer (started when the circuit
                   went into CIRCUIT_START state) has expired.


   DLC Actions:

   Action Name     Description
   -----------     -----------
   DLC_CONTACT     Contact Station Request:  DLC will send a SABME
                   command or a UA response to an outstanding SABME
                   command.

   DLC_DGRM        Datagram Request:  DLC will send a UI frame.

   DLC_ENTER_BUSY  Enter Link Station Busy:  DLC will send an
                   RNR supervisory frame.

   DLC_EXIT_BUSY   Exit Link Station Busy:  DLC will send an RR
                   supervisory frame.

   DLC_HALT_DL     Halt Data Link Request:  DLC will send a DISC
                   command.

   DLC_INFO        Information Request:  DLC will send an I frame.

   DLC_RESOLVE     Resolve request:  DLC should issue a TEST (or
                   appropriate equivalent for non-LAN DLCs) but need
                   not reserve the resources required for a circuit yet.

   DLC_RESOLVE_R   Resolve Response Request:  DLC will send a
                   Test response or XID response from the null SAP.

   DLC_START_DL    Start Data Link Request:  DLC will send a Test
                   command to the null SAP.

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   DLC_XID         XID Request:  DLC will send an XID command or an
                   XID response.


   Other Actions:

   Action Name     Description
   ----------      -----------
   START_CS_TIMER  Start the CIRCUIT_START timer.

   DLC_RESOLVE_R and DLC_START_DL actions require the DLC to reserve the
   resources necessary for a link station as they are used only when a
   circuit is about to be started.  The DLC_RESOLVE action is used for
   topology explorer traffic and does not require such resources to be
   reserved, though a DLC implementation may choose not to distinguish
   this from the DLC_START_DL action.  See section 5.4 for details of
   the actions and events for explorer frames.

   The Data Link Switch is described by a state transition table as
   documented in the following sections.  Each of the states is
   described below in terms of the events, actions, and next state for
   each transition. If a particular event is not listed for a given
   state, no action and no state transition should occur for that event.
   Any significant comments concerning the transitions within a given
   state are given immediately following the table representing the
   state.

   A separate state machine instance is maintained by the Data Link
   Switch for each end-to-end circuit.  The number of circuits that may
   be supported by each Data Link Switch is a local implementation
   option.

   The CANUREACH_ex, ICANREACH_ex, NETBIOS_NQ_ex, and NETBIOS_NR_ex are
   SSP messages that are not associated with a particular circuit.  The
   processing of these messages is covered in section 5.4.

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5.2.1  DISCONNECTED State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive CANUREACH_cs | DLC_START_DL        | RESOLVE_PENDING      |
   +----------------------+---------------------+----------------------+
   | Receive DATAFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | DLC_XID              | If source route     | If CANUREACH_cs was  |
   |                      | bridged frame with  | sent:                |
   |                      | broadcast indicated:|   CIRCUIT_START      |
   |                      |   Send CANUREACH_ex |                      |
   |                      | else:               |                      |
   |                      |   Send CANUREACH_cs |                      |
   |                      |   START_CS_TIMER    |                      |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM             | If NETBIOS          |                      |
   |                      | NAME_QUERY:         |                      |
   |                      |  Send NETBIOS_NQ_ex |                      |
   |                      | else:               |                      |
   |                      |  Send DATAFRAME     |                      |
   +----------------------+---------------------+----------------------+
   | DLC_CONTACTED        | Send CANUREACH_cs   | CIRCUIT_START        |
   +----------------------+---------------------+----------------------+

   It is assumed that each Data Link Switch will build a set of topology
   tables giving the identity of each Data Link Switch that can reach a
   specific MAC address or a specific NetBIOS name.  This table can be
   built  using the explorer frames, as per the Explorer FSM in section
   5.4.  As a consequence, the amount of search traffic can be kept to a
   minimum.

   Upon receipt of a TEST command, broadcast XID or NetBIOS NAME_QUERY,
   the Data Link Switch checks the topology table for the target MAC/SAP
   or NetBIOS name.  If there is no matching entry in the table, the
   Data Link Switch uses the explorer FSMs in section 5.4 to locate the
   target MAC/SAP or NetBIOS name.

   When the first non-broadcast XID or SABME flows,  the Data Link
   Switch issues a CANUREACH_cs to attempt to start a circuit.  The
   CANUREACH_cs message is sent to only those Data Link Switches that
   are known to be able to reach the given MAC address.  The mechanism
   by which a topology table entry is determined to be out-of-date and
   is deleted from the table is implementation specific.

   The DISCONNECTED state is exited upon the sending of a CANUREACH_cs
   by the origin DLSw or the receipt of a CANUREACH_cs message by a

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   prospective target Data Link Switch.  In the latter case, the Data
   Link Switch will issue a Test command to the target station (i.e.,
   DLC_START_DL signal is presented to DLC).

5.2.2  RESOLVE_PENDING State

   +-------------------+-----------------------+-----------------------+
   |        Event      |      Action(s)        |      Next State       |
   +-------------------+-----------------------+-----------------------+
   | Receive DATAFRAME | DLC_DGRM              |                       |
   +-------------------+-----------------------+-----------------------+
   | DLC_DL_STARTED    | If LF value of        | If LF value of        |
   |                   | DLC_DL_STARTED        | DLC_DL_STARTED        |
   |                   | is greater than or    | is greater than or    |
   |                   | equal to LF Size of   | equal to LF Size of   |
   |                   | CANUREACH_cs or LF    | CANUREACH_cs or LF    |
   |                   | Size Control bit set: | Size Control bit set: |
   |                   |   Send ICANREACH_cs   |   CIRCUIT_PENDING     |
   |                   | else:                 | else:                 |
   |                   |   Send DLC_HALT_DL    |   HALT_PENDING_NOACK  |
   +-------------------+-----------------------+-----------------------+
   | DLC_ERROR         |                       | DISCONNECTED          |
   +-------------------+-----------------------+-----------------------+
   | DLC_DGRM          | Send DATAFRAME        |                       |
   +-------------------+-----------------------+-----------------------+

   The RESOLVE_PENDING state is entered upon receipt of a CANUREACH_cs
   message by the target DLSw.  A data link is started, causing a Test
   command to be sent by the DLC.

   Several CANUREACH_cs messages can be received in the RESOLVE_PENDING
   state.  The Data Link Switch may update its topology information
   based upon the origin MAC address information in each CANUREACH_cs
   message.

   Upon the receipt of a DLC_DL_STARTED signal in the RESOLVE_PENDING
   state, the Data Link Switch may update its topology table base upon
   the remote MAC address information.  The ICANREACH_cs message must be
   returned to the first partner DLSw from which a CANUREACH_cs was
   received for this circuit, or an implementation may optionally reply
   to all partners from which the CANUREACH_cs was received.

   The RESOLVE_PENDING state is exited once the data link has been
   started (i.e., a DLC_DL_STARTED signal is received as a result of a
   Test response received by the DLC).  The target Data Link Switch then
   enters the CIRCUIT_PENDING state.

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5.2.3  CIRCUIT_START State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive CANUREACH_cs | If origin MAC addr  | If DLC_START_DL      |
   | for circuit in       | in CANUREACH_cs is  | issued:              |
   | opposite direction   | greater than origin |   RESOLVE_PENDING    |
   |                      | MAC addr of circuit:|                      |
   |                      |   DLC_START_DL      |                      |
   |                      | else:               |                      |
   |                      |   no action taken   |                      |
   +----------------------+---------------------+----------------------+
   | Receive ICANREACH_cs | If LF Size Control  | If LF Size Control   |
   |                      | bit set and LF Size | bit set and LF Size  |
   |                      | is not negotiable:  | is not negotiable:   |
   |                      |   Send HALT_DL_NOACK|   DISCONNECTED       |
   |                      | else:               | else if Connected:   |
   |                      |   Send REACH_ACK,   |   CONNECT_PENDING    |
   |                      |   Send appropriate  | else:                |
   |                      |   SSP message based |   CIRCUIT_ESTABLISHED|
   |                      |   on the event      |                      |
   |                      |   that generated    |                      |
   |                      |   CANUREACH_cs      |                      |
   |                      |   (see Note)        |                      |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM             | Send DATAFRAME      |                      |
   +----------------------+---------------------+----------------------+
   | DLC_ERROR            |                     | DISCONNECTED         |
   +----------------------+---------------------+----------------------+
   | CS_TIMER_EXP         |                     | DISCONNECTED         |
   +----------------------+---------------------+----------------------+
   | XPORT_FAILURE        |                     | DISCONNECTED         |
   +----------------------+---------------------+----------------------+

   The CIRCUIT_START state is entered by the origin Data Link Switch
   when a DLC_XID or DLC_CONTACTED signal has been received from the
   DLC.

   The CIRCUIT_START state is exited upon receipt of an ICANREACH_cs
   message.  A REACH_ACK message is returned to the target Data Link
   Switch.  If the CIRCUIT_START state was entered due to a DLC_XID
   signal, an XIDFRAME message containing the XID is sent to the target
   Data Link Switch.  If the CIRCUIT_START state was entered due to a
   DLC_CONTACTED signal, a CONTACT message is sent to the target Data
   Link Switch.

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5.2.4  CIRCUIT_PENDING State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive CONTACT      | DLC_CONTACT         | CONTACT_PENDING      |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL      | DLC_HALT_DL         | HALT_PENDING         |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL_NOACK| DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+
   | Receive REACH_ACK    | If Connected:       | If Connected:        |
   |                      |  Send CONTACT       |  CONNECT_PENDING,    |
   |                      |                     | else:                |
   |                      |                     |  CIRCUIT_ESTABLISHED |
   +----------------------+---------------------+----------------------+
   | Receive XIDFRAME     | DLC_XID             |                      |
   +----------------------+---------------------+----------------------+
   | Receive DGRMFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | Receive DATAFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | DLC_CONTACTED        | If UA is sent in    |                      |
   |                      | response to SABME:  |                      |
   |                      |   DLC_ENTER_BUSY    |                      |
   |                      | else:               |                      |
   |                      |   no action taken   |                      |
   +----------------------+---------------------+----------------------+
   | DLC_ERROR            |                     | DISCONNECTED         |
   +----------------------+---------------------+----------------------+
   | DLC_XID              | Drop or hold until  |                      |
   |                      | REACH_ACK received  |                      |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM             | Send DATAFRAME      |                      |
   +----------------------+---------------------+----------------------+
   | XPORT_FAILURE        | DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+

   The CIRCUIT_PENDING state is entered by the target Data Link Switch
   following the sending of an ICANREACH_cs message.  In this state it
   is awaiting the reception of a REACH_ACK message from the origin Data
   Link Switch.

   If the target Data Link Switch happens to receive a SABME command
   from the target station while in the CIRCUIT_PENDING state (i.e., a
   DLC_CONTACTED signal received from the DLC), the reception of the
   REACH_ACK message causes the target Data Link Switch to enter the
   CONNECT_PENDING state and to send a CONTACT message to the origin

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   Data Link Switch.

   If no such SABME is received, the receipt of the REACH_ACK causes the
   Data Link Switch to enter CIRCUIT_ESTABLISHED state.

5.2.5  CONNECT_PENDING State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive CONTACTED    | If UA was sent in   | CONNECTED            |
   |                      | response to SABME:  |                      |
   |                      |   DLC_EXIT_BUSY     |                      |
   |                      | else:               |                      |
   |                      |   DLC_CONTACT       |                      |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL      | DLC_HALT_DL         | HALT_PENDING         |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL_NOACK| DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+
   | Receive DGRMFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | Receive DATAFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | Receive ICANREACH_cs | Send HALT_DL_NOACK  |                      |
   +----------------------+---------------------+----------------------+
   | DLC_RESET            | Send RESTART_DL     | CIRCUIT_RESTART      |
   +----------------------+---------------------+----------------------+
   | DLC_ERROR            | Send HALT_DL        | DISCONNECT_PENDING   |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM             | Send DGRMFRAME      |                      |
   +----------------------+---------------------+----------------------+
   | XPORT_FAILURE        | DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+

   The CONNECT_PENDING state is entered when a DLC_CONTACTED signal has
   been received from the DLC (i.e., a SABME command has been received).
   A CONTACT message it then  issued.  The state is exited upon the
   receipt of a CONTACTED message.  If a DLC_RESET signal is received,
   the local data link is restarted and a RESTART_DL message is sent to
   the remote DLSw.

   An ICANREACH_cs received after the transition to CONNECT_PENDING
   state indicates that more than one CANUREACH_cs was sent at circuit
   establishment time and the target station was found by more than one
   Data Link Switch partner.  A HALT_DL_NOACK is sent to halt the
   circuit started by the Data Link Switch partner that originated each
   such ICANREACH_cs.

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   Note:  Some implementations will also send a Test command in order to
   restart the data link to the station that sent the SABME command
   (i.e., a DLC_START_DL will be issued).

5.2.6  CIRCUIT_ESTABLISHED State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive CONTACT      | DLC_CONTACT         | CONTACT_PENDING      |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL      | DLC_HALT_DL         | HALT_PENDING         |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL_NOACK| DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+
   | Receive XIDFRAME     | DLC_XID             |                      |
   +----------------------+---------------------+----------------------+
   | Receive DGRMFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | Receive DATAFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | Receive ICANREACH_cs | Send HALT_DL_NOACK  |                      |
   +----------------------+---------------------+----------------------+
   | DLC_CONTACTED        | Send CONTACT        | CONNECT_PENDING      |
   |                      | If UA is sent in    |                      |
   |                      | response to SABME:  |                      |
   |                      |   DLC_ENTER_BUSY    |                      |
   |                      | else:               |                      |
   |                      |   no action taken   |                      |
   +----------------------+---------------------+----------------------+
   | DLC_ERROR            | Send HALT_DL        | DISCONNECT_PENDING   |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM             | Send DGRMFRAME      |                      |
   +----------------------+---------------------+----------------------+
   | DLC_XID              | Send XIDFRAME       |                      |
   +----------------------+---------------------+----------------------+
   | XPORT_FAILURE        | DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+

   The CIRCUIT_ESTABLISHED state is entered by the origin Data Link
   Switch from the CIRCUIT_START state, and by the target Data Link
   Switch from the CIRCUIT_PENDING state.  The state is exited when a
   connection is started (i.e., DLC receives a SABME command) or CONTACT
   is received. The next state is CONTACT_PENDING or CONNECT_PENDING.

   An ICANREACH_cs received after the transition to CIRCUIT_ESTABLISHED
   state indicates that more than one CANUREACH_cs was sent at circuit
   establishment time and the target station was found by more than one

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   Data Link Switch partner.  A HALT_DL_NOACK is sent to halt the
   circuit started by the Data Link Switch partner that originated each
   such ICANREACH_cs.

5.2.7  CONTACT_PENDING State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL      | DLC_HALT_DL         | HALT_PENDING         |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL_NOACK| DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+
   | Receive RESTART_DL   | DLC_HALT_DL         | RESTART_PENDING      |
   +----------------------+---------------------+----------------------+
   | Receive DGRMFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | Receive DATAFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | DLC_CONTACTED        | Send CONTACTED      | CONNECTED            |
   +----------------------+---------------------+----------------------+
   | DLC_ERROR            | Send HALT_DL        | DISCONNECT_PENDING   |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM             | Send DGRMFRAME      |                      |
   +----------------------+---------------------+----------------------+
   | XPORT_FAILURE        | DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+

   The CONTACT_PENDING state is entered upon the receipt of a CONTACT
   message, which causes the Data Link Switch to issue a DLC_CONTACT
   signal to the DLC (i.e., DLC sends a SABME command).  This state is
   then exited upon the receipt of a DLC_CONTACTED signal from the DLC
   (i.e., a UA response received).

   If a RESTART_DL message is received, indicating that the remote Data
   Link Switch has received a DLC_RESET signal, the local Data Link
   Switch sends a DISC command frame on the adjacent LAN (i.e.,
   DLC_HALT_DL signal) and enter the RESTART_PENDING state.

   An ICANREACH_cs received after the transition to CONTACT_PENDING
   state indicates that more than one CANUREACH_cs was sent at circuit
   establishment time and the target station was found by more than one
   Data Link Switch partner.  A HALT_DL_NOACK is sent to halt the data
   link started by the Data Link Switch partner that originated this
   ICANREACH_cs.

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5.2.8  CONNECTED State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL      | DLC_HALT_DL         | HALT_PENDING         |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL_NOACK| DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+
   | Receive RESTART_DL   | DLC_HALT_DL         | RESTART_PENDING      |
   +----------------------+---------------------+----------------------+
   | Receive DGRMFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | Receive INFOFRAME    | DLC_INFO            |                      |
   +----------------------+---------------------+----------------------+
   | Receive DATAFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | Receive XIDFRAME     | If non-activation   |                      |
   |                      | XID3:               |                      |
   |                      |   DLC_XID           |                      |
   +----------------------+---------------------+----------------------+
   | Receive ICANREACH_cs | Send HALT_DL_NOACK  |                      |
   +----------------------+---------------------+----------------------+
   | Receive ENTER_BUSY   | DLC_ENTER_BUSY      |                      |
   +----------------------+---------------------+----------------------+
   | Receive EXIT_BUSY    | DLC_EXIT_BUSY       |                      |
   +----------------------+---------------------+----------------------+
   | Rec TEST_CIRCUIT_REQ | Snd TEST_CIRCUIT_RSP|                      |
   +----------------------+---------------------+----------------------+
   | DLC_RESET            | Send RESTART_DL     | CIRCUIT_RESTART      |
   +----------------------+---------------------+----------------------+
   | DLC_ERROR            | Send HALT_DL        | DISCONNECT_PENDING   |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM             | Send DGRMFRAME      |                      |
   +----------------------+---------------------+----------------------+
   | DLC_INFO             | Send INFOFRAME      |                      |
   +----------------------+---------------------+----------------------+
   | DLC_XID              | If non-activation   |                      |
   |                      | XID3:               |                      |
   |                      |   Send XIDFRAME     |                      |
   +----------------------+---------------------+----------------------+
   | XPORT_FAILURE        | DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+

   The CONNECTED state is entered from the CONNECT_PENDING state upon
   the receipt of a CONTACTED message or from the CONTACT_PENDING state
   upon the receipt of a DLC_CONTACTED signal.

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   The CONNECTED state is exited usually under one of two conditions: a
   DLC_ERROR signal received from the DLC (e.g., a DISC command received
   by the local DLC), or a HALT_DL message received from the other Data
   Link Switch (e.g., a DISC command received by the remote DLC).

   A SABME command (i.e., a DLC_RESET signal) received by either Data
   Link Switch will also cause the two Data Link Switches to leave the
   CONNECTED state and attempt to restart the circuit.  Following the
   receipt of a SABME, the local Data Link Switch sends a RESTART_DL
   message to the other Data Link Switch and enters the CIRCUIT_RESTART
   state.  Upon the receipt of the RESTART_DL message, the remote Data
   Link Switch sends a DISC command (i.e., DLC_HALT_DL signal) and
   enters the RESTART_PENDING state.

   An ICANREACH_cs received after the transition to CONNECTED state
   indicates that more than one CANUREACH_cs was sent at circuit
   establishment time and the target station was found by more than one
   Data Link Switch partner.  A HALT_DL_NOACK is sent to halt the
   circuit started by the Data Link Switch partner that originated each
   such ICANREACH_cs.

   Note:  Some implementations will also send a Test command in order to
   restart the data link to the station that sent the SABME command
   (i.e., a DLC_START_DL will be issued).

5.2.9  CIRCUIT_RESTART State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive DL_RESTARTED | If Connected:       | If Connected:        |
   |                      |  Send CONTACT       |  CONNECT_PENDING,    |
   |                      |                     | else:                |
   |                      |                     |  CIRCUIT_ESTABLISHED |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL_NOACK| DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+
   | Receive DGRMFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | DLC_ERROR            | Send HALT_DL        | DISCONNECT_PENDING   |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM             | Send DGRMFRAME      |                      |
   +----------------------+---------------------+----------------------+
   | XPORT_FAILURE        | DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+

Top       Page 36 
   The CIRCUIT_RESTART state is entered if a DLC_RESET signal is
   received from the local DLC.  This was caused by the receipt of a
   SABME command while a connection was currently active.  A DM response
   will be issued to the SABME command and the Data Link Switch will
   attempt to restart the end-to-end circuit.

   The CIRCUIT_RESTART state is exited through one of two transitions.
   The next state depends upon the time the local DLC has reached the
   contacted state (i.e., a DLC_CONTACTED signal is presented) relative
   to the receipt of the DL_RESTARTED message.  This signal is caused by
   the origin station resending the SABME command that initially caused
   the Data Link Switch to enter the CIRCUIT_RESTART state.  The two
   cases are as follows:

      1) DL_RESTARTED message received before the DLC_CONTACTED signal-
         In this case, the CIRCUIT_ESTABLISHED state is entered.

      2) DL_RESTARTED message received after the DLC_CONTACTED signal-
         In this case, the CONNECT_PENDING state is entered.

5.2.10  DISCONNECT_PENDING State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive DL_HALTED    |                     | DISCONNECTED         |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL      | Send DL_HALTED      |                      |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL_NOACK|                     | DISCONNECTED         |
   +----------------------+---------------------+----------------------+
   | Receive DATAFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM             | Send DATAFRAME      |                      |
   +----------------------+---------------------+----------------------+
   | XPORT_FAILURE        |                     | DISCONNECTED         |
   +----------------------+---------------------+----------------------+

   The DISCONNECT_PENDING state is entered when a DLC_ERROR signal is
   received from the local DLC.  Upon receipt of this signal, a HALT_DL
   message is sent.  Once an DL_HALTED message is received, the state is
   exited, and the Data Link Switch enters the DISCONNECTED state.

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5.2.11  RESTART_PENDING State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL_NOACK|                     | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+
   | Receive DGRMFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | DLC_DL_HALTED        | Send DL_RESTARTED   | CIRCUIT_ESTABLISHED  |
   +----------------------+---------------------+----------------------+
   | DLC_ERROR            | Send HALT_DL        | DISCONNECT_PENDING   |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM             | Send DGRMFRAME      |                      |
   +----------------------+---------------------+----------------------+
   | XPORT_FAILURE        | DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+

   The RESTART_PENDING state is entered upon the receipt of a RESTART_DL
   message from the remote DLSw while the local Data Link Switch is in
   either the CONTACT_PENDING state or the CONNECTED state, which causes
   the local DLSw to issue a DISC command to the DLC.  Upon the receipt
   of the UA response (DLC_DL_HALTED), the data link is restarted, a
   DL_RESTARTED message is returned to the remote DLSw, and the
   CIRCUIT_ESTABLISHED state is entered.

   Note:  Some implementations will send a Test command in order to
   restart the data link to the target station (i.e., a DLC_START_DL
   will be issued) prior to sending the DL_RESTARTED message.

5.2.12  HALT_PENDING State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL_NOACK|                     | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+
   | Receive DATAFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | DLC_DL_HALTED        | Send DL_HALTED      | DISCONNECTED         |
   +----------------------+---------------------+----------------------+
   | DLC_ERROR            | Send DL_HALTED      | DISCONNECTED         |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM             | Send DATAFRAME      |                      |
   +----------------------+---------------------+----------------------+
   | XPORT_FAILURE        |                     | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+

Top       Page 38 
   The HALT_PENDING state is entered upon the receipt of a HALT_DL
   message. This causes the local DLC to issue a DISC command.  Upon the
   receipt of the UA response (DLC_DL_HALTED), a DL_HALTED message is
   returned to the remote DLSw and the DISCONNECTED state is entered.

5.2.13  HALT_PENDING_NOACK State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive DATAFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | DLC_DL_HALTED        |                     | DISCONNECTED         |
   +----------------------+---------------------+----------------------+
   | DLC_ERROR            |                     | DISCONNECTED         |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM             | Send DATAFRAME      |                      |
   +----------------------+---------------------+----------------------+

   The HALT_PENDING_NOACK state is entered upon the receipt of a
   HALT_DL_NOACK message.  This causes the local DLC to issue a DISC
   command.  Upon the receipt of the UA response (DLC_DL_HALTED), the
   DISCONNECTED state is entered.

5.3  NetBIOS Datagrams

   The NetBIOS protocols use a number of UI frames for directory
   services and the transmission of datagrams.  Most of these frames are
   directed to a group MAC address (GA) with the routing information
   field indicating spanning tree explorer (STE) (a.k.a. Single Route
   Broadcast).  The NB_Add_Name_Response and NB_Name_Recognized frames
   are directed to a specific MAC address with the routing information
   field indicating an all routes explorer frame (ARE) (a.k.a. All
   Routes Broadcast)  The NB_Status_Response frame, is directed to a
   specific MAC address with the routing information field indicating a
   specifically routed frame (SRF). The handling of these frames is
   summarized in the following table.

Top       Page 39 
   +---------------------------+------------------+--------------------+
   |          Event            |     Action(s)    |      Comment       |
   +---------------------------+------------------+--------------------+
   | DLC_DGRM for NETBIOS      | Send NETBIOS_ANQ | Transmitted to all |
   |  group address:           |                  |   remote DLSw      |
   |   NB_Add_Name_Query       |                  |                    |
   +---------------------------+------------------+--------------------+
   | DLC_DGRM for a specific   | Send NETBIOS_ANR | Transmitted to     |
   |  address:                 |                  |   specific DLSw    |
   |   NB_Add_Name_Response    |                  |                    |
   +---------------------------+------------------+--------------------+
   | DLC_DGRM for a specific   | Send DATAFRAME   | Transmitted to all |
   |  address:                 |                  |   remote DLSw      |
   |   NB_Status_Response      |                  |                    |
   +---------------------------+------------------+--------------------+
   | DLC_DGRM for NETBIOS      | Send DATAFRAME   | Transmitted to all |
   |  group address:           |                  |   remote DLSw      |
   |   NB_Name_in_Conflict     |                  |                    |
   |   NB_Add_Group_Name_Query |                  |                    |
   |   NB_Datagram,            |                  |                    |
   |   NB_Datagram_Broadcast   |                  |                    |
   |   NB_Status_Query         |                  |                    |
   |   NB_Terminate_Trace      |                  |                    |
   +---------------------------+------------------+--------------------+

   The above actions do not apply in the following states:
   CIRCUIT_ESTABLISHED, CONTACT_PENDING, CONNECT_PENDING, CONNECTED, and
   CIRCUIT_PENDING.  The handling of the remaining two UI frames used by
   NetBIOS systems, NB_Name_Query and NB_Name_Recognized, are documented
   as part of the DLSw state machine in the previous section (i.e.,
   DISCONNECTED and RESOLVE_PENDING states).  Furthermore, the handling
   of NetBIOS datagrams (i.e., NB_Datagram) sent to a specific MAC
   address is also governed by the DLSw state machine.

   Note:  Some implementations also issue Test frames during the
   exchange of the NetBIOS, NB_Name_Query and NB_Name_Recognized.  This
   exchange of protocol data units occurs during the start of a data
   link and is used to determine the routing information.  Most other
   implementations of NetBIOS will use the
   NB_Name_Query/NB_Name_Recognized exchange to determine routes in
   conjunction with resolving the NetBIOS names. These differences are
   not reflected in the SSP protocols.

Top       Page 40 
   The handling of the NetBIOS specific SSP messages is given in the
   following table.

   +---------------+-------------------------+-------------------------+
   |     Event     |        Action(s)        |         Comment         |
   +---------------+-------------------------+-------------------------+
   | NETBIOS_ANQ   | DLC_DGRM:               | Routed STE              |
   |               |    NB_Add_Name_Query    | (NETBIOS Group Address) |
   +---------------+-------------------------+-------------------------+
   | NETBIOS_ANR   | DLC_DGRM:               | Routed ARE              |
   |               |    NB_Add_Name_Response | (Specific MAC Address)  |
   +---------------+-------------------------+-------------------------+
   | NETBIOS_NQ_ex | DLC_DGRM:               | Routed STE              |
   |               |    NB_Name_Query        | (NETBIOS Group Address) |
   +---------------+-------------------------+-------------------------+
   | NETBIOS_NQ_cs | DLC_DGRM:               | Routed STE              |
   |               |    NB_Name_Query        | (NETBIOS Group Address) |
   +---------------+-------------------------+-------------------------+
   | NETBIOS_NR_ex | DLC_DGRM:               | Routed ARE              |
   |               |    NB_Name_Recognized   | (Specific MAC Address)  |
   +---------------+-------------------------+-------------------------+
   | NETBIOS_NR_cs | DLC_DGRM:               | Routed ARE              |
   |               |    NB_Name_Recognized   | (Specific MAC Address)  |
   +---------------+-------------------------+-------------------------+
   | DATAFRAME     | DLC_DGRM                | If NB_Status_Response:  |
   |               |                         |  Routed ARE             |
   |               |                         |  (Specific MAC Address) |
   |               |                         | Else:                   |
   |               |                         |  Routed STE             |
   |               |                         |  (NETBIOS Group Address)|
   +---------------+-------------------------+-------------------------+

   The above actions apply to all DLSw states.  The handling of NetBIOS
   datagrams sent within DGRMFRAME messages is governed by the DLSw
   state machine.  The DGRMFRAME message type is employed instead of the
   DATAFRAME message type once the end-to-end circuit has been
   established. At that time, the message is addressed according to the
   pair of Circuit IDs in the message header instead of relying upon the
   MAC address information in the token ring header.

5.4  Explorer Traffic

   The CANUREACH_ex, ICANREACH_ex, NETBIOS_NQ_ex, and NETBIOS_NR_ex SSP
   messages explore the topology of the DLSw cloud and the networks
   attached to it.  These explorer frames are used to determine the DLSw
   partners through which a MAC or NetBIOS name can be accessed.  This
   information may optionally be cached to reduce explorer traffic in
   the DLSw cloud.

Top       Page 41 
   If a DLSw is aware from cached information that a given MAC address
   or NetBIOS name is accessible through a given partner DLSw, it should
   direct all circuit setup attempts to that partner.  If the circuit
   setup fails, or no such data is available in the MAC or name cache
   database, the DLSw may fallback to issuing the setup attempt to all
   DLSw partners on the assumption that the cached data is now out of
   date.  The mechanism for determining when to use such a fallback is
   implementation defined.

   DLSw implementations may also use a local MAC cache to enable
   responses to CANUREACH_ex requests to be issued without the need for
   TEST frame exchange (or equivalent) until the CANUREACH_cs is
   received.  Again, the fallback mechanism for determining when such
   local cache data is out-of-date is implementation defined.

   The use of either cache is an optional function in DLSw.  An
   implementation may choose to always issue explorer frames or to use
   either or both types of cache.

   The following sections describe the FSMs used for explorer frames.
   The DLC events and actions are a subset of those described in section
   5.2 for the main circuit FSM.

5.4.1  CANUREACH/ICANREACH Explorer FSM

   The FSM described below is used to handle explorer frames routed by
   MAC address.  There is one instance of this FSM for each Data Link ID
   (Target and Origin MAC/SAP pair) for which explorer traffic is
   flowing. The states in this FSM are as follows.

   State Name            Description
   ----------            -----------
   RESET                 The initial state.

   SENT_EX               Local DLSw has issued an explorer message

   RECEIVED_EX           Local DLSw has received an explorer message

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5.4.1.1  RESET State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive CANUREACH_ex | If replying from    | If DLC_RESOLVE sent, |
   |                      | cache, send         |   RECEIVED_EX        |
   |                      | ICANREACH_ex        |                      |
   |                      | else if allowed to  |                      |
   |                      | test availability,  |                      |
   |                      | issue DLC_RESOLVE.  |                      |
   |                      | Optionally update   |                      |
   |                      | cache.              |                      |
   +----------------------+---------------------+----------------------+
   | Receive ICANREACH_ex | Optionally update   | RESET                |
   |                      | cache               |                      |
   +----------------------+---------------------+----------------------+
   | DLC_RESOLVE_C        | Send CANUREACH_ex   | SENT_EX              |
   +----------------------+---------------------+----------------------+

   RESET is the initial state for the CANUREACH/ICANREACH explorer FSM.
   This state is exited when a DLC_RESOLVE_C request is received from
   the DLC or a CANUREACH_ex is received from a remote DLSw.

   A DLSw implementation may optionally reply from to CANUREACH_ex
   messages on the basis of cached topology information, in which case
   the DLC_RESOLVE exchange (i.e., TEST) is not required.  If cache is
   not used, or no match is found, and the DLC permits the use of TEST,
   DLC_RESOLVE is issued to locate the target MAC and the state changes
   to RECEIVED_EX. If no cache entry is available and TEST is not
   allowed by the DLC, a received CANUREACH_ex frame is ignored.

5.4.1.2  SENT_EX State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive ICANREACH_ex | DLC_RESOLVE_R       | RESET                |
   |                      | Optionally update   |                      |
   |                      | cache               |                      |
   +----------------------+---------------------+----------------------+
   | DLC_RESOLVE_C        |                     | SENT_EX              |
   +----------------------+---------------------+----------------------+

   SENT_EX is entered when the DLSw has issued a CANUREACH_ex message to
   locate a MAC address.  This state is exited when a remote DLSw
   returns a matching ICANREACH_ex, or after an implementation defined
   timeout. DLC_RESOLVE events received in this state correspond to TEST

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   retries by the origin DLC station and are absorbed.

   An implementation may choose whether to handle explorer frame
   crossover either by using entirely separate FSM instances and simply
   allowing both ends to issue TEST frames, or by detecting a reverse
   CANUREACH_ex frame here and issuing an ICANREACH_ex message and
   DLC_RESOLVE_R action.

5.4.1.3  RECEIVED_EX State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive CANUREACH_ex | Optionally update   | RECEIVED_EX          |
   |                      | cache               |                      |
   +----------------------+---------------------+----------------------+
   | Receive ICANREACH_ex |                     | RECEIVED_EX          |
   +----------------------+---------------------+----------------------+
   | DLC_RESOLVED         | Send ICANREACH_ex   | RESET                |
   |                      | Optionally update   |                      |
   |                      | cache               |                      |
   +----------------------+---------------------+----------------------+

   RECEIVED_EX is entered when the DLSw has received a CANUREACH_ex from
   a remote DLSw and has issued a DLC_RESOLVE to locate the MAC address.
   This state is exited when the DLC_RESOLVED response is received, or
   after an implementation defined timeout.

   If the target MAC is located, the DLSw must reply to the first
   received CANUREACH_ex that caused the move to this state.  If
   additional CANUREACH_ex messages are received in this state from
   other remote DLSw partners, the DLSw may optionally reply to these
   messages too but it is not required to do so.

   An implementation may choose whether to handle explorer frame
   crossover either by using entirely separate FSM instances and simply
   allowing both ends to issue TEST frames, or by detecting such a
   reverse DLC_RESOLVE_C event here and issuing an ICANREACH_ex message
   and DLC_RESOLVE_R action.


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