rfc1795.txt

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                         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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RFC 1795                  Data Link Switching                 April 1995


   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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RFC 1795                  Data Link Switching                 April 1995


   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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RFC 1795                  Data Link Switching                 April 1995


   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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RFC 1795                  Data Link Switching                 April 1995


   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.
















Wells & Bartky                                                 [Page 26]

RFC 1795                  Data Link Switching                 April 1995


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: