rfc1795.txt

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   between Data Link Switches.  All values not listed are reserved for
   potential use in follow-on releases.




Wells & Bartky                                                 [Page 16]

RFC 1795                  Data Link Switching                 April 1995


   Command          Description                       Type   flags/notes
   -------          --------                         ------  -----------
   CANUREACH_ex     Can U Reach Station-explorer      0x03   SSPex
   CANUREACH_cs     Can U Reach Station-circuit start 0x03
   ICANREACH_ex     I Can Reach Station-explorer      0x04   SSPex
   ICANREACH_cs     I Can Reach Station-circuit start 0x04
   REACH_ACK        Reach Acknowledgment              0x05
   DGRMFRAME        Datagram Frame                    0x06   (note 1)
   XIDFRAME         XID Frame                         0x07
   CONTACT          Contact Remote Station            0x08
   CONTACTED        Remote Station Contacted          0x09
   RESTART_DL       Restart Data Link                 0x10
   DL_RESTARTED     Data Link Restarted               0x11
   ENTER_BUSY       Enter Busy                        0x0C   (note 2)
   EXIT_BUSY        Exit Busy                         0x0D   (note 2)
   INFOFRAME        Information (I) Frame             0x0A
   HALT_DL          Halt Data Link                    0x0E
   DL_HALTED        Data Link Halted                  0x0F
   NETBIOS_NQ_ex    NETBIOS Name Query-explorer       0x12   SSPex
   NETBIOS_NQ_cs    NETBIOS Name Query-circuit setup  0x12   (note 3)
   NETBIOS_NR_ex    NETBIOS Name Recognized-explorer  0x13   SSPex
   NETBIOS_NR_cs    NETBIOS Name Recog-circuit setup  0x13   (note 3)
   DATAFRAME        Data Frame                        0x14   (note 1)
   HALT_DL_NOACK    Halt Data Link with no Ack        0x19
   NETBIOS_ANQ      NETBIOS Add Name Query            0x1A
   NETBIOS_ANR      NETBIOS Add Name Response         0x1B
   KEEPALIVE        Transport Keepalive Message       0x1D   (note 4)
   CAP_EXCHANGE     Capabilities Exchange             0x20
   IFCM             Independent Flow Control Message  0x21
   TEST_CIRCUIT_REQ Test Circuit Request              0x7A
   TEST_CIRCUIT_RSP Test Circuit Response             0x7B

   Note 1: Both the DGRMFRAME and DATAFRAME messages are used to carry
   information received by the DLC entity within UI frames.  The
   DGRMFRAME message is addressed according to a pair of Circuit IDs,
   while the DATAFRAME message is addressed according to a Data Link ID,
   being composed of a pair of MAC addresses and a pair of link SAP
   addresses. The latter is employed prior to the establishment of an
   end-to-end circuit when Circuit IDs have yet to be established or
   during circuit restart when Data Links are reset.

   Note 2: These messages are not used for the DLSw Standard but may be
   used by older DLSw implementations.  They are listed here for
   informational purposes.  These messages were added after publication
   of RFC 1434 and were deleted in this standard (adaptive pacing is now
   used instead).





Wells & Bartky                                                 [Page 17]

RFC 1795                  Data Link Switching                 April 1995


   Note 3: These messages are not normally issued by a Standard DLSw,
   which uses the NB_*_ex messages as shown in section 5.4.  However if
   a Standard DLSw attempts to interoperate with older DLSw
   implementations, these messages correspond to the NETBIOS_NQ and
   NETBIOS_NR messages used in RFC1434 both to locate the resource and
   to setup a circuit.  This document does not attempt to provide a
   complete specification of the use of these messages.

   Note 4:  A KEEPALIVE message may be sent by a DLSw to a partner DLSw
   in order to verify the TCP connection (or other future SSP carrying
   protocol) is still functioning.  If received by a DLSw, this message
   is discarded and ignored.  Use of this message is optional.

   For the exchange of NetBIOS control messages, the entire DLC header
   is carried as part of the message unit.  This includes the MAC
   header, with the routing information field padded to 18 bytes, and
   the LLC header. The following message types are affected:
   NETBIOS_NQ, NETBIOS_NR, NETBIOS_ANQ, NETBIOS_ANR, and DATAFRAME when
   being used by NetBIOS systems.  The routing information in the DLC
   header is not used by the remote Data Link Switch upon receiving the
   above five messages.

   Any SSP message types not defined above if received by a DLSw are to
   be ignored (i.e., no error action is to be performed).  A Data Link
   Switch should quietly drop any SSP message with a Message Type that
   is not recognized or not supported.  Receipt of such a message should
   not cause the termination of the transport connection to the message
   sender.

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.








Wells & Bartky                                                 [Page 18]

RFC 1795                  Data Link Switching                 April 1995


   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.
















Wells & Bartky                                                 [Page 19]

RFC 1795                  Data Link Switching                 April 1995


   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.





Wells & Bartky                                                 [Page 20]

RFC 1795                  Data Link Switching                 April 1995


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



Wells & Bartky                                                 [Page 21]

RFC 1795                  Data Link Switching                 April 1995


                         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