rfc1434.txt

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   In the figure, the transition labeled HALT_DL(ts) represents the
   HALT_PENDING state and the transition labeled RESTART_DL(ts)
   represents the RESTART_PENDING state.  Both these transitional states
   have only one input event and only one output action.  While in these
   states, the Data Link Switch is awaiting responses from the local
   station on the adjacent LAN (i.e., 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 the IBM 6611 implementation, 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.

4.2.2. State Transition Tables

   This section provides a detail representation of the Data Link
   Switch, as document by a set of state machines.  The first state
   machine documents the sending and receiving of SSP messages.  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.
























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RFC 1434             DLS: Switch-to-Switch Protocol           March 1993


     Event Name      Description
     ----------      -----------
     DLC_CONTACTED   Contact Indication:  DLC has received an SABME
                     command and will send a UA response, 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.
     DLC_XID         XID Indication:  DLC has received an XID command
                     or response to a non-null SAP.

                       Table 3.  Local DLC Events

     Action Name     Description
     -----------     -----------
     DLC_CONTACT     Contact Station Request:  DLC will send a 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_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.
     DLC_XID         XID Request:  DLC will send an XID command or an
                     XID response.

                      Table 4.  Local DLC Actions




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RFC 1434             DLS: Switch-to-Switch Protocol           March 1993


   The Data Link Switch may be described by a state transition table
   consisting of eleven states.  Each of these 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 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.

4.2.2.1   DISCONNECTED State

     Event                  Action(s)          Next State
     -----                  ---------          ----------
     Receive CANUREACH      DLC_START_DL       RESOLVE_PENDING
     Receive ICANREACH      Send REACH_ACK,    CIRCUIT_ESTABLISHED
                            DLC_RESOLVE_R
                            (See note 1)
     Receive DATAFRAME      DLC_DGRM
     Receive NETBIOS_NQ     DLC_DGRM           RESOLVE_PENDING
     Receive NETBIOS_NR     Send REACH_ACK,    CIRCUIT_ESTABLISHED
                            DLC_DGRM
                            (See note 2)
     DLC_RESOLVE_C          Send CANUREACH
     DLC_DGRM               If NB_Name_Query:
                            Send NETBIOS_NQ,
                            Else:
                            Send DATAFRAME

   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 based upon the origin address information received within the
   CANUREACH message or NETBIOS_NQ message and the target address
   information within the ICANREACH message or NETBIOS_NR message.  As a
   consequence, the amount of search traffic can be kept to a minimum.

   Upon receipt of a Test command or XID command to the null SAP (i.e.,
   DLC_RESOLVE_C signal from DLC), the Data Link Switch will check the
   topology table prior to sending the CANUREACH message.  If the target
   MAC address is in the table, the CANUREACH message will be sent to
   only those Data Link Switches that are known to be able to reach the
   given MAC address.  If the MAC address is not in the table, the
   CANUREACH message will be sent to all known Data Link Switches.
   Since the destination link SAP (DSAP) value is null, the Target Link
   SAP field in the CANUREACH message header is set equal to the Origin



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RFC 1434             DLS: Switch-to-Switch Protocol           March 1993


   Link SAP value.

   Upon receipt of a NB_Name_Query the Data Link Switch will check the
   topology table prior to sending the NETBIOS_NQ message.  If the
   target NetBIOS name is in the table, the NETBIOS_NQ message will be
   sent to only those Data Link Switches that are known to be able to
   reach the given NetBIOS name.  If the NetBIOS name is not in the
   table, the NETBIOS_NQ message will be sent to all known Data Link
   Switches.

   For SNA, the DISCONNECTED state is exited upon receipt of a CANUREACH
   message by a prospective target Data Link Switch, or upon receipt of
   an ICANREACH message by the origin Data Link Switch.  In the former
   case, the Data Link Switch will issue a Test command to the target
   station (i.e., DLC_START_DL signal is presented to DLC).  In the
   later case, a Test response is sent to the origin station (i.e.,
   DLC_RESOLVE_R will be issued) and a REACH_ACK message will be
   returned to the target Data Link Switch.

      Note 1-  The IBM 6611 will not send a Test response, but will send
      a Test command to the station that originated the resolve
      procedure (i.e., a DLC_START_DL will be issued) in order to start
      the data link.

   For NetBIOS, the DISCONNECTED state is exited upon the receipt of a
   NETBIOS_NQ message by the prospective target Data Link Switch, or
   upon the receipt of a NETBIOS_NR message by the origin Data Link
   Switch.  In the former case, the Data Link Switch will send a
   NB_Name_Query frame.  In the later case, the Data Link Switch will
   send a NB_Name_Recognized frame to the origin station and a REACH_ACK
   message will be returned to the target Data Link Switch.

      Note 2-  The IBM 6611 will also send a Test command (i.e., a
      DLC_START_DL will be issued) to the station that originated the
      name resolution in order to start the data link.

4.2.2.2   RESOLVE_PENDING State

     Event                  Action(s)              Next State
     -----                  ---------              ----------
     Receive DATAFRAME      DLC_DGRM
     DLC_DL_STARTED         Send ICANREACH         CIRCUIT_PENDING
     DLC_ERROR                                     DISCONNECTED
     DLC_DGRM               If NB_Name_Recognized: If
                            Send NETBIOS_NR        NB_Name_Recognized:
                            (See note),            CIRCUIT_PENDING
                            Else:
                            Send DATAFRAME



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RFC 1434             DLS: Switch-to-Switch Protocol           March 1993


   The RESOLVE_PENDING state is entered upon receipt of a CANUREACH
   message or a NETBIOS_NQ message by the target DLS.  In the former
   case, a data link is started, causing a Test command to be sent by
   the DLC.  In the later case, a NB_Name_Query frame is sent.  This
   frame is directed to a group MAC address.

   Any CANUREACH messages received in the RESOLVE_PENDING state will be
   responded to if a DLC_DL_STARTED signal is received.  The Data Link
   Switch may also update its topology information based upon the origin
   MAC address information in each CANUREACH 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 message should be
   returned to all Data Link Switches that had sent a CANUREACH message.
   In a similar fashion, the Data Link Switch may update its topology
   table upon the receipt of a NB_Name_Recognized frame and a NETBIOS_NR
   message will be returned to all Data Link Switches that have sent a
   NETBIOS_NQ message.

   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) or a NB_Name_Recognized frame is
   received (i.e., a DLC_DGRM signal is received).  The target Data Link
   Switch will then enter the CIRCUIT_PENDING state.

      Note:  The IBM 6611 will also send a Test command in order to
      start the data link to the station that responded to the Name
      Query frame (i.e., a DLC_START_DL will be issued).

4.2.2.3   CIRCUIT_PENDING State

     Event                  Action(s)         Next State
     -----                  ---------         ----------
     Receive CONTACT        DLC_CONTACT       CONTACT_PENDING
     Receive HALT_DL        DLC_HALT_DL       HALT_PENDING
     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          DLC_ENTER_BUSY
     DLC_ERROR              Send HALT_DL      DISCONNECT_PENDING
     DLC_XID                Send XIDFRAME
     DLC_DGRM               Send DGRMFRAME





Dixon & Kushi                                                  [Page 25]

RFC 1434             DLS: Switch-to-Switch Protocol           March 1993


   The CIRCUIT_PENDING state is entered by the target Data Link Switch
   following the sending of an ICANREACH message or the sending of a
   NETBIOS_NR message.  In this state it is awaiting the reception of a
   REACH_ACK message from the origin Data Link Switch.  If a connection
   does not exist with the target station (i.e., the normal case), the
   Data Link Switch will enter the CIRCUIT_ESTABLISHED state.

   If the target Data Link Switch happens to receive a SABME command
   from the target