rfc1795.txt

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3.2  Address Parameters

   A data link is defined as a logical association between the two end
   stations using Data Link Switching.  It is identified by a Data Link
   ID (14 bytes) consisting of the pair of attachment addresses
   associated with each end system.  Each attachment address is
   represented by the concatenation of the MAC address (6 bytes) and the
   LLC address (1 byte).  Each attachment address is classified as
   either "Target" in the context of the Destination MAC/SAP addresses
   of an explorer frame sent in the first frame used to establish a



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   circuit, or "Origin" in the context of the Source MAC/SAP addresses.
   All MAC addresses are expressed in non-canonical (Token-Ring) format.

    DATA LINK ID  (14 Bytes @ Control message offset 24 decimal)
   +-----------------------------+-----------------------------+
   | Target MAC Address                                        |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
   |                                                           |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
   |                                                           |
   +-----------------------------+-----------------------------+
   | Origin MAC Address                                        |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
   |                                                           |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
   |                                                           |
   +-----------------------------+-----------------------------+
   | Origin Link SAP             | Target Link SAP             |
   +-----------------------------+-----------------------------+


   An end-to-end circuit is identified by a pair of Circuit ID's.  A
   Circuit ID is a 64 bit number that identifies the DLC circuit within
   a single DLSw.  It consists of a DLC Port ID (4 bytes), and a Data
   Link Correlator (4 bytes).  The Circuit ID must be unique in a single
   DLSw and is assigned locally.  The pair of Circuit ID's along with
   the Data Link IDs,  uniquely identify a single end-to-end circuit.
   Each DLSw must keep a table of these Circuit ID pairs, one for the
   local end of the circuit and the other for the remote end of the
   circuit.  In order to identify which Data Link Switch originated the
   establishment of a circuit, the terms, "Origin" DLSw and "Target"
   DLSw, will be employed in this document.

    CIRCUIT ID   (8 Bytes)
   +-----------------------------+-----------------------------+
   | DLC Port ID                                               |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
   |                                                           |
   +-----------------------------+-----------------------------+
   | Data Link Correlator                                      |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
   |                                                           |
   +-----------------------------+-----------------------------+

   The Origin Transport ID and the Target Transport ID fields in the
   message header are used to identify the individual TCP/IP port on a
   Data Link Switch.  The values have only local significance.  However,
   each Data Link Switch is required to reflect the values contained in



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   these two fields, along with the associated values for DLC Port ID
   and the Data Link Correlator, when returning a message to the other
   Data Link Switch.

   The following figure shows the use of the addressing parameters
   during the establishment of an end-to-end connection.  The CANUREACH,
   ICANREACH, and REACH_ACK message types all carry the Data Link ID,
   consisting of the MAC and Link SAP addresses associated with the two
   end stations.  The CANUREACH and ICANREACH messages are qualified by
   the SSPex flag into CANUREACH_ex, ICANREACH_ex (explorer messages)
   and CANUREACH_cs, ICANREACH_cs (circuit start).  The CANUREACH_ex is
   used to find a remote MAC and Link SAP address without establishing
   an SSP circuit.  Upon receipt of a CANUREACH_cs message, the target
   DLSw starts a data link for each port, thereby obtaining a Data Link
   Correlator.  If the target station can be reached, an ICANREACH_cs
   message is returned to the origin DLSw containing the Target Circuit
   ID parameter.  Upon receipt, the origin DLSw starts a data link and
   returns the Origin Circuit ID to the target DLSw within the REACH_ACK
   message.  (Note for a full list of message types, see section 3.5.)

   +------------+                                +------------+
   |Disconnected|                                |Disconnected|
   +------------+   CANUREACH_cs (Data Link ID)  +------------+
       ------------------------------------------------->
         ICANREACH_cs (Data Link ID, Target Circuit ID)
       <------------------------------------------------
     REACH_ACK (Data Link ID, Origin Cir ID, Target Cir ID)
       ------------------------------------------------->
   +------------+                                +------------+
   |Circuit Est.|                                |Circuit Est.|
   +------------+                                +------------+
     XIDFRAME (Data Link ID, Origin Cir ID, Target Cir ID)
       <------------------------------------------------>
      CONTACT (Data Link ID, Origin Cir ID, Target Cir ID)
       ------------------------------------------------->
     CONTACTED (Data Link ID, Origin Cir ID, Target Cir ID)
       <-------------------------------------------------
   +------------+                                +------------+
   | Connected  |                                | Connected  |
   +------------+                                +------------+
        INFOFRAME (Remote Circuit ID = Target Circuit ID)
       ------------------------------------------------->
        INFOFRAME (Remote Circuit ID = Origin Circuit ID)
       <-------------------------------------------------

   During the exchange of the XIDFRAME, CONTACT, and CONTACTED messages,
   the pair of Circuit ID parameters is included in the message format
   along with the DATA LINK ID parameter.  Once the connection has been



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   established, the INFOFRAME messages are exchanged with the shorter
   header.  This header contains only the Circuit ID associated with the
   remote DLSw.  The Remote Data Link Correlator and the Remote DLC Port
   ID are set equal to the Data Link Correlator and the DLC Port ID that
   are associated with the origin or target Data Link Switch, dependent
   upon the direction of the packet.

3.3  Correlators

   The local use, and contents of the Data Link Correlator, Port ID and
   Transport ID fields in SSP messages is an implementation choice.
   These fields have local significance only.  The values received from
   a partner DLSw must not be interpreted by the DLSw that receives them
   and should be echoed "as is" to a partner DLSw in subsequent
   messages.  All implementations must obey the following rules in this
   section (3.3) on the assignment and fixing of these correlator fields
   for each transport connection or circuit:

   The Transport ID fields are learned from the first SSP message
   exchanged with a DLSw partner (the Capabilities exchange).  This
   field should not be varied by a DLSw after the capabilities exchange
   and must be reflected to the partner DLSw in every SSP control
   message.

   The Target Data Link Correlator, Target Port ID and Target Transport
   ID must remain the same once the Target DLSw has sent the
   ICANREACH_cs for a given circuit.  The Origin DLSw must store the
   values specified in the ICANREACH_cs and use these on all subsequent
   SSP messages for this circuit.

   The Origin DLSw must allow these fields to vary until the
   ICANREACH_cs is received.  Each SSP message issued for a circuit must
   reflect the values specified by the Target DLSw in the last SSP
   message for this circuit received by the Origin DLSw.  Binary zero
   should be used if no such message has yet been received for a given
   circuit (apart from the Target Transport ID which will have been
   learnt as specified above).

   The Origin Data Link Correlator, Origin Port ID and Origin Transport
   ID must remain the same once the Origin DLSw has issued the REACH_ACK
   for a given circuit.  The Target DLSw must store the values specified
   in the REACH_ACK and use these on all subsequent SSP messages for
   this circuit.

   The Target DLSw must allow these fields to vary until the REACH_ACK
   is received.  Each SSP message issued for a circuit must reflect the
   values specified by the Origin DLSw in the last SSP message for this
   circuit received by the Target DLSw.  Binary zero should be used if



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   no such message has yet been received for a given circuit (apart from
   the Origin Transport ID which will have been learnt as specified
   above).

   For the purposes of correlator exchange, explorer messages form a
   separate circuit.  Both DLSw partners must reflect the last received
   correlator values as specified above.  However correlators learned on
   explorer messages need not be carried over to a subsequent circuit
   setup attempt.  In particular, the Origin DLSw may elect to use the
   same values for the Origin Data Link Correlator and Origin Port ID
   when it issues a CANUREACH_cs after receiving an ICANREACH_ex or
   NETBIOS_NR_ex. However the Target DLSw must not assume that the
   CANUREACH_cs will specify any of the Target Data Link Correlator or
   Target Port ID that were exchanged on the explorer messages.

   Received SSP messages that require a valid Remote Circuit ID but
   cannot be associated with an existing circuit should be rejected with
   a HALT_DL_NOACK message.  This is done to prevent a situation where
   one DLSw partner has a circuit defined while the other partner does
   not. The exception would be a HALT_DL_NOACK message with an invalid
   Remote Circuit ID.  The HALT_DL_NOACK message is typically used in
   error situations where a response is not appropriate.

   The SSP messages requiring a valid Remote Circuit ID are all messages
   except the following: CANUREACH_ex, CANUREACH_cs, ICANREACH_ex,
   ICANREACH_cs, NETBIOS_NQ_cs, NETBIOS_NR_cs, DATAFRAME, NETBIOS_ANQ,
   NETBIOS_ANR, KEEPALIVE and CAP_EXCHANGE.

3.4  Largest Frame Size Field

   The Largest Frame Size (LF Size) field in the SSP Control Header is
   used to carry the LF Size bits across the DLSw connection.  This
   should be used to ensure that the two end-stations always negotiate a
   frame size to be used on a circuit that does not require the Origin
   and Target DLSw partners to re-segment frames.

   This field is valid on CANUREACH_ex, CANUREACH_cs, ICANREACH_ex,
   ICANREACH_cs, NETBIOS_NQ_ex and NETBIOS_NR_ex messages only. The
   contents of this field should be ignored on all other frames.

   Every DLSw forwarding a SSP frame to its DLSw partner must ensure
   that the contents of this frame reflect the minimum capability of the
   route to its local end-station or any limit imposed by the DLSw
   itself.

   The bit-wise definition of this field is as follows (bit 7 is the
   most significant bit, bit 0 is the least significant bit):




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     7   6   5   4   3   2   1   0
   +-------------------------------+
   | c | r | b | b | b | e | e | e |
   +-------------------------------+

     c   .   .   .   .   .   .   .  LF Size Control flag
                                    (significant on messages
                                    from Origin to Target
                                    DLSw only)

                                    0=fail circuit if route
                                      obtained requires a
                                      smaller LF size
                                    1=don't fail the circuit
                                      but return the LF size
                                      obtained even if it is
                                      smaller

     .   r   .   .   .   .   .   .  Reserved
     .   .   b   .   .   .   .   .  Largest Frame Bit Base
     .   .   .   b   .   .   .   .  Largest Frame Bit Base
     .   .   .   .   b   .   .   .  Largest Frame Bit Base
     .   .   .   .   .   e   .   .  Largest Frame Bit Extended
     .   .   .   .   .   .   e   .  Largest Frame Bit Extended
     .   .   .   .   .   .   .   e  Largest Frame Bit Extended

             <----- LF Bits ----->

   Refer to IEEE 802.1D Standard, Annex C for encoding of Largest Frame
   base and extended bit values.

   The Origin DLSw "Size Control" flag informs a Target DLSw that
   chooses to reply to *_cs messages on the basis of cached information
   that it may safely return a smaller LF Size on the ICANREACH_cs frame
   if it has had to choose an alternative route on which to initialize
   the circuit.  If this bit is set to 1, the Origin DLSw takes
   responsibility for ensuring that the end-stations negotiate a
   suitable frame size for the circuit. If this bit is set to 0, the
   Target DLSw must not reply to the CANUREACH_cs if it cannot obtain a
   route to the Target end station that support an LF Size at least as
   large as that specified in the CANUREACH_cs frame.

3.5  Message Types

   The following table lists the protocol data units that are exchanged
   between Data Link Switches.  All values not listed are reserved for
   potential use in follow-on releases.




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   Command          Description                       Type   flags/notes
   -------          --------                         ------  -----------
   CANUREACH_ex     Can U Reach Station-explorer      0x03   SSPex
   CANUREACH_cs     Can U Reach Station-circuit start 0x03