rfc1434.txt

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   Switches will send DISC commands to all the local systems that are
   associated with the failed connections.  Upon sending the DISC
   command, the Data Link Switch will enter the DISCONNECT state for
   each circuit.

4.2. DLS State Machine

   The following state tables describe the states for a single
   connection through the Data Link Switch.  State information is kept
   for each connection.  The initial state for a connection is
   DISCONNECT.  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, circuit 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 x'04', x'08', or x'0C'.  Typically the
   SAP would be used to determine if the Test frames should be sent to
   the DLS code in the router.  If both bridging and DLS are enabled,
   this allows the product to ensure that SNA frames are not both
   bridged and switched.

   For NetBIOS, circuit establishment is via the Name Query and Name
   Recognized frames.  These frames are used for both address resolution
   and source route determination.  Normally NetBIOS devices use SAP
   x'F0'.

4.2.1. Data Link Switch States

   The Switch-to-Switch Protocols will be formally defined through a
   single state machine.  The following table lists the eleven possible
   states.  A separate state machine is employed for each end-to-end
   circuit that is maintained by the Data Link Switch.  The three steady
   states are DISCONNECTED, CIRCUIT_ESTABLISHED, and CONNECTED.










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


     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 DLS is awaiting a REACH_ACK
                           response to an ICANREACH message.
     CIRCUIT_RESTART       The DLS that originated the reset is
                           awaiting the restart of the data link
                           and the DL_RESTARTED response to a
                           RESTART_DL 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 DLS is awaiting the CONTACTED
                           response to a CONTACT message.
     CONTACT_PENDING       The target DLS 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 DLS is awaiting
                           either a CANUREACH, an ICANREACH, a
                           NETBIOS_NQ, or a NETBIOS_NR message.
     DISCONNECT_PENDING    The DLS that originated the disconnect
                           is awaiting the DL_HALTED response to a
                           HALT_DL message.
     HALT_PENDING          The remote DLS 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).
     RESTART_PENDING       The remote DLS 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 DLS is awaiting either the
                           DLC_DL_STARTED indication following the
                           DLC_START_DL reqest (i.e., DLC is waiting
                           for a Test response as a result of sending a
                           Test command), or a NB_Name_Recognized
                           frame in response to a NB_Name_Query frame.

                   Table 2.  Data Link Switch States

   The following figure depicts the events that cause a transition to a
   new state.  The resulting action(s) are not explicitly shown.  The



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


   DISCONNECT_PENDING state will be entered whenever a DLC error
   condition occurs in any of the other states (except RESOLVE_PENDING),
   or when a DISC command is received by the DLC.
















































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


               DLC_RESOLVE_C+----+        +----DLC_ERROR---+
                            |    |       \|/               |
  +------------+            +>+--+---------+  CANUREACH  +-+----------+
  | Disconnect +--DL_HALTED-->|Disconnected|------------>|  Resolve   |
  |   Pending  |<+   +------->|            |<-------+    |  Pending   |
  +---------+--+ |   |        +-----+------+        |    +-----+------+
   /|\      | HALT_DL|         /|\  |/|\/|\         |          |
DLC_ERROR   +----+   |     +----+   | |  +------+   |    DLC_DL_STARTED
    |                |     |        |HALT_DL(ts)|   +------+   |
                     |     | ICANREACH|         |   HALT_DL(ts)|
       +--HALT_DL(ts)+     |        | |         |          |   |
       |               HALT_DL(ts)  | |     HALT_DL(ts)    |   |
       |                   |        | |         |          |   |
       |   +-RESTART_DL(ts)+----+   | |         |          |   |
       |   |               |   \|/ \|/|         |          |  \|/
  +----+---+---+           |  +-------+----+    |        +-+----------+
  |  Contact   |<-CONTACT--+--+   Circuit  |<-REACH_ACK--+   Circuit  |
  |  Pending   |           |  | Established| (DLC not    |   Pending  |
  +-----+------+           |  +----------+-+   contacted)+-+------+---+
        |   /|\            |   /|\ /|\   |      |          |      |
        |    |          +--+----+   |    |      |          |      |
        |    +-CONTACT--+--+--------+----+------+----------+  REACH_ACK
        |               |  |RESTART_DL(ts)      |       (DLC contacted)
    DLC_CONTACTED       |  +------+ |    |      +--------------+  |
        |               |         | |    | DLC_CONTACTED       |  |
        +---------------+-------+ | |    +-----------------+   |  |
        +-----DL_RESTARTED      | | |                      |   |  |
        |  (DLC not contacted) \|/| |                     \|/  | \|/
  +-----+------+              +---+-+------+  CONTACTED  +-----+------+
  |   Circuit  |<-DLC_RESET---+  Connected |<------------+   Connect  |
  |   Restart  |              |            |             |   Pending  |
  +----------+-+              +------------+             +-----+------+
       /|\   |                  DL_RESTARTED              /|\  |
        |    +----------------(DLC contacted)--------------+   |
        |                                                      |
        +-------------------------DLC_RESET--------------------+

      Figure 7.  DLS State Transitions  (ts = transitional state)

   The DISCONNECT state is the initial state for a new circuit.  One end
   station starts the connection via a TEST or XID command (i.e.,
   DLC_RESOLVE_C) or a NetBIOS Name Query command (not explicitly
   shown).  Upon receipt, the Data Link Switches exchange a set of
   CANUREACH, ICANREACH and REACH_ACK messages.  Upon completion of this
   three-legged exchange, both Data Link Switches will be in the
   CIRCUIT_ESTABLISHED state.  Two pending states also exist during this
   exchange.  The RESOLVE_PENDING state is entered by the target Data
   Link Switch awaiting a Test response to a Test Command and the



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


   CIRCUIT_PENDING state is entered by the target DLS awaiting the
   REACH_ACK reply to an ICANREACH 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
   the Data Link Switches.

   In the CIRCUIT_ESTABLISHED state, the receipt of a SABME command
   (i.e., DLC_CONTACTED) causes the origin DLS 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 DLS, 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 DLS sends a CONTACTED message and enters
   the CONNECTED state.  When received, the origin DLS 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 DLS will halt the local data
   link, send a HALT_DL message to the remote DLS, and enter the
   DISCONNECT_PENDING State.  When the HALT_DL frame is received by the
   other DLS, 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 DLS 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) 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 DLS will then send a DL_RESTARTED message back to the
   first DLS.  The receipt of the DL_RESTARTED message causes the first
   DLS 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.



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


   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).