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_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.
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.
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: | |
| | Send DATAFRAME | |
+----------------------+---------------------+----------------------+
| DLC_CONTACTED | Send CANUREACH_cs | CIRCUIT_START |
+----------------------+---------------------+----------------------+
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 using the explorer frames, as per the Explorer FSM in section
5.4. As a consequence, the amount of search traffic can be kept to a
minimum.
Upon receipt of a TEST command, broadcast XID or NetBIOS NAME_QUERY,
the Data Link Switch checks the topology table for the target MAC/SAP
or NetBIOS name. If there is no matching entry in the table, the
Data Link Switch uses the explorer FSMs in section 5.4 to locate the
target MAC/SAP or NetBIOS name.
When the first non-broadcast XID or SABME flows, the Data Link
Switch issues a CANUREACH_cs to attempt to start a circuit. The
CANUREACH_cs message is sent to only those Data Link Switches that
are known to be able to reach the given MAC address. The mechanism
by which a topology table entry is determined to be out-of-date and
is deleted from the table is implementation specific.
The DISCONNECTED state is exited upon the sending of a CANUREACH_cs
by the origin DLSw or the receipt of a CANUREACH_cs message by a
prospective target Data Link Switch. In the latter case, the Data
Link Switch will issue a Test command to the target station (i.e.,
DLC_START_DL signal is presented to DLC).
5.2.2 RESOLVE_PENDING State
+-------------------+-----------------------+-----------------------+
| Event | Action(s) | Next State |
+-------------------+-----------------------+-----------------------+
| Receive DATAFRAME | DLC_DGRM | |
+-------------------+-----------------------+-----------------------+
| DLC_DL_STARTED | If LF value of | If LF value of |
| | DLC_DL_STARTED | DLC_DL_STARTED |
| | is greater than or | is greater than or |
| | equal to LF Size of | equal to LF Size of |
| | CANUREACH_cs or LF | CANUREACH_cs or LF |
| | Size Control bit set: | Size Control bit set: |
| | Send ICANREACH_cs | CIRCUIT_PENDING |
| | else: | else: |
| | Send DLC_HALT_DL | HALT_PENDING_NOACK |
+-------------------+-----------------------+-----------------------+
| DLC_ERROR | | DISCONNECTED |
+-------------------+-----------------------+-----------------------+
| DLC_DGRM | Send DATAFRAME | |
+-------------------+-----------------------+-----------------------+
The RESOLVE_PENDING state is entered upon receipt of a CANUREACH_cs
message by the target DLSw. A data link is started, causing a Test
command to be sent by the DLC.
Several CANUREACH_cs messages can be received in the RESOLVE_PENDING
state. The Data Link Switch may update its topology information
based upon the origin MAC address information in each CANUREACH_cs
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_cs message must be
returned to the first partner DLSw from which a CANUREACH_cs was
received for this circuit, or an implementation may optionally reply
to all partners from which the CANUREACH_cs was received.
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). The target Data Link Switch then
enters the CIRCUIT_PENDING state.
5.2.3 CIRCUIT_START State
+----------------------+---------------------+----------------------+
| Event | Action(s) | Next State |
+----------------------+---------------------+----------------------+
| Receive CANUREACH_cs | If origin MAC addr | If DLC_START_DL |
| for circuit in | in CANUREACH_cs is | issued: |
| opposite direction | greater than origin | RESOLVE_PENDING |
| | MAC addr of circuit:| |
| | DLC_START_DL | |
| | else: | |
| | no action taken | |
+----------------------+---------------------+----------------------+
| Receive ICANREACH_cs | If LF Size Control | If LF Size Control |
| | bit set and LF Size | bit set and LF Size |
| | is not negotiable: | is not negotiable: |
| | Send HALT_DL_NOACK| DISCONNECTED |
| | else: | else if Connected: |
| | Send REACH_ACK, | CONNECT_PENDING |
| | Send appropriate | else: |
| | SSP message based | CIRCUIT_ESTABLISHED|
| | on the event | |
| | that generated | |
| | CANUREACH_cs | |
| | (see Note) | |
+----------------------+---------------------+----------------------+
| DLC_DGRM | Send DATAFRAME | |
+----------------------+---------------------+----------------------+
| DLC_ERROR | | DISCONNECTED |
+----------------------+---------------------+----------------------+
| CS_TIMER_EXP | | DISCONNECTED |
+----------------------+---------------------+----------------------+
| XPORT_FAILURE | | DISCONNECTED |
+----------------------+---------------------+----------------------+
The CIRCUIT_START state is entered by the origin Data Link Switch
when a DLC_XID or DLC_CONTACTED signal has been received from the
DLC.
The CIRCUIT_START state is exited upon receipt of an ICANREACH_cs
message. A REACH_ACK message is returned to the target Data Link
Switch. If the CIRCUIT_START state was entered due to a DLC_XID
signal, an XIDFRAME message containing the XID is sent to the target
Data Link Switch. If the CIRCUIT_START state was entered due to a
DLC_CONTACTED signal, a CONTACT message is sent to the target Data
Link Switch.
5.2.4 CIRCUIT_PENDING State
+----------------------+---------------------+----------------------+
| Event | Action(s) | Next State |
+----------------------+---------------------+----------------------+
| Receive CONTACT | DLC_CONTACT | CONTACT_PENDING |
+----------------------+---------------------+----------------------+
| Receive HALT_DL | DLC_HALT_DL | HALT_PENDING |
+----------------------+---------------------+----------------------+
| Receive HALT_DL_NOACK| DLC_HALT_DL | HALT_PENDING_NOACK |
+----------------------+---------------------+----------------------+
| 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 | If UA is sent in | |
| | response to SABME: | |
| | DLC_ENTER_BUSY | |
| | else: | |
| | no action taken | |
+----------------------+---------------------+----------------------+
| DLC_ERROR | | DISCONNECTED |
+----------------------+---------------------+----------------------+
| DLC_XID | Drop or hold until | |
| | REACH_ACK received | |
+----------------------+---------------------+----------------------+
| DLC_DGRM | Send DATAFRAME | |
+----------------------+---------------------+----------------------+
| XPORT_FAILURE | DLC_HALT_DL | HALT_PENDING_NOACK |
+----------------------+---------------------+----------------------+
The CIRCUIT_PENDING state is entered by the target Data Link Switch
following the sending of an ICANREACH_cs message. In this state it
is awaiting the reception of a REACH_ACK message from the origin Data
Link Switch.
If the target Data Link Switch happens to receive a SABME command
from the target station while in the CIRCUIT_PENDING state (i.e., a
DLC_CONTACTED signal received from the DLC), the reception of the
REACH_ACK message causes the target Data Link Switch to enter the
CONNECT_PENDING state and to send a CONTACT message to the origin
Data Link Switch.
If no such SABME is received, the receipt of the REACH_ACK causes the
Data Link Switch to enter CIRCUIT_ESTABLISHED state.
5.2.5 CONNECT_PENDING State
+----------------------+---------------------+----------------------+
| Event | Action(s) | Next State |
+----------------------+---------------------+----------------------+
| Receive CONTACTED | If UA was sent in | CONNECTED |
| | response to SABME: | |
| | DLC_EXIT_BUSY | |
| | else: | |
| | DLC_CONTACT | |
+----------------------+---------------------+----------------------+
| Receive HALT_DL | DLC_HALT_DL | HALT_PENDING |
+----------------------+---------------------+----------------------+
| Receive HALT_DL_NOACK| DLC_HALT_DL | HALT_PENDING_NOACK |
+----------------------+---------------------+----------------------+
| Receive DGRMFRAME | DLC_DGRM | |
+----------------------+---------------------+----------------------+
| Receive DATAFRAME | DLC_DGRM | |
+----------------------+---------------------+----------------------+
| Receive ICANREACH_cs | Send HALT_DL_NOACK | |
+----------------------+---------------------+----------------------+
| DLC_RESET | Send RESTART_DL | CIRCUIT_RESTART |
+----------------------+---------------------+----------------------+
| DLC_ERROR | Send HALT_DL | DISCONNECT_PENDING |
+----------------------+---------------------+----------------------+
| DLC_DGRM | Send DGRMFRAME | |
+----------------------+---------------------+----------------------+
| XPORT_FAILURE | DLC_HALT_DL | HALT_PENDING_NOACK |
+----------------------+---------------------+----------------------+
The CONNECT_PENDING state is entered when a DLC_CONTACTED signal has
been received from the DLC (i.e., a SABME command has been received).
A CONTACT message it then issued. The state is exited upon the
receipt of a CONTACTED message. If a DLC_RESET signal is received,
the local data link is restarted and a RESTART_DL message is sent to
the remote DLSw.
An ICANREACH_cs received after the transition to CONNECT_PENDING
state indicates that more than one CANUREACH_cs was sent at circuit
establishment time and the target station was found by more than one
Data Link Switch partner. A HALT_DL_NOACK is sent to halt the
circuit started by the Data Link Switch partner that originated each
such ICANREACH_cs.
Note: Some implementations will also send a Test command in order to
restart the data link to the station that sent the SABME command
(i.e., a DLC_START_DL will be issued).
5.2.6 CIRCUIT_ESTABLISHED State
+----------------------+---------------------+----------------------+
| Event | Action(s) | Next State |
+----------------------+---------------------+----------------------+
| Receive CONTACT | DLC_CONTACT | CONTACT_PENDING |
+----------------------+---------------------+----------------------+
| Receive HALT_DL | DLC_HALT_DL | HALT_PENDING |
+----------------------+---------------------+----------------------+
| Receive HALT_DL_NOACK| DLC_HALT_DL | HALT_PENDING_NOACK |
+----------------------+---------------------+----------------------+
| Receive XIDFRAME | DLC_XID | |
+----------------------+---------------------+----------------------+
| Receive DGRMFRAME | DLC_DGRM | |
+----------------------+---------------------+----------------------+
| Receive DATAFRAME | DLC_DGRM | |
+----------------------+---------------------+----------------------+
| Receive ICANREACH_cs | Send HALT_DL_NOACK | |
+----------------------+---------------------+----------------------+
| DLC_CONTACTED | Send CONTACT | CONNECT_PENDING |
| | If UA is sent in | |
| | response to SABME: | |
| | DLC_ENTER_BUSY | |
| | else: | |
| | no action taken | |
+----------------------+---------------------+----------------------+
| DLC_ERROR | Send HALT_DL | DISCONNECT_PENDING |
+----------------------+---------------------+----------------------+
| DLC_DGRM | Send DGRMFRAME | |
+----------------------+---------------------+----------------------+
| DLC_XID | Send XIDFRAME | |
+----------------------+---------------------+----------------------+
| XPORT_FAILURE | DLC_HALT_DL | HALT_PENDING_NOACK |
+----------------------+---------------------+----------------------+
The CIRCUIT_ESTABLISHED state is entered by the origin Data Link
Switch from the CIRCUIT_START state, and by the target Data Link
Switch from the CIRCUIT_PENDING state. The state is exited when a
connection is started (i.e., DLC receives a SABME command) or CONTACT
is received. The next state is CONTACT_PENDING or CONNECT_PENDING.
An ICANREACH_cs received after the transition to CIRCUIT_ESTABLISHED
state indicates that more than one CANUREACH_cs was sent at circuit
establishment time and the target station was found by more than one
Data Link Switch partner. A HALT_DL_NOACK is sent to halt the
circuit started by the Data Link Switch partner that originated each
such ICANREACH_cs.
5.2.7 CONTACT_PENDING State
+----------------------+---------------------+----------------------+
| Event | Action(s) | Next State |
+----------------------+---------------------+----------------------+
| Receive HALT_DL | DLC_HALT_DL | HALT_PENDING |
+----------------------+---------------------+----------------------+
| Receive HALT_DL_NOACK| DLC_HALT_DL | HALT_PENDING_NOACK |
+----------------------+---------------------+----------------------+
| Receive RESTART_DL | DLC_HALT_DL | RESTART_PENDING |
+----------------------+---------------------+----------------------+
| Receive DGRMFRAME | DLC_DGRM | |
+----------------------+---------------------+----------------------+
| Receive DATAFRAME | DLC_DGRM | |
+----------------------+---------------------+----------------------+
| DLC_CONTACTED | Send CONTACTED | CONNECTED |
+----------------------+---------------------+----------------------+
| DLC_ERROR | Send HALT_DL | DISCONNECT_PENDING |
+----------------------+---------------------+----------------------+
| DLC_DGRM | Send DGRMFRAME | |
+----------------------+---------------------+----------------------+
| XPORT_FAILURE | DLC_HALT_DL | HALT_PENDING_NOACK |
+----------------------+---------------------+----------------------+
The CONTACT_PENDING state is entered upon the receipt of a CONTACT
message, which causes the Data Link Switch to issue a DLC_CONTACT
signal to the DLC (i.e., DLC sends a SABME command). This state is
then exited upon the receipt of a DLC_CONTACTED signal from the DLC
(i.e., a UA response received).
If a RESTART_DL message is received, indicating that the remote Data
Link Switch has received a DLC_RESET signal, the local Data Link
Switch sends a DISC command frame on the adjacent LAN (i.e.,
DLC_HALT_DL signal) and enter the RESTART_PENDING state.
An ICANREACH_cs received after the transition to CONTACT_PENDING
state indicates that more than one CANUREACH_cs was sent at circuit
establishment time and the target station was found by more than one
Data Link Switch partner. A HALT_DL_NOACK is sent to halt the data
link started by the Data Link Switch partner that originated this
ICANREACH_cs.
5.2.8 CONNECTED State
+----------------------+---------------------+----------------------+
| Event | Action(s) | Next State |
+----------------------+---------------------+----------------------+
| Receive HALT_DL | DLC_HALT_DL | HALT_PENDING |
+----------------------+---------------------+----------------------+
| Receive HALT_DL_NOACK| DLC_HALT_DL | HALT_PENDING_NOACK |
+----------------------+---------------------+----------------------+
| Receive RESTART_DL | DLC_HALT_DL | RESTART_PENDING |
+----------------------+---------------------+----------------------+
| Receive DGRMFRAME | DLC_DGRM | |
+----------------------+---------------------+----------------------+
| Receive INFOFRAME | DLC_INFO | |
+----------------------+---------------------+----------------------+
| Receive DATAFRAME | DLC_DGRM | |
+----------------------+---------------------+----------------------+
| Receive XIDFRAME | If non-activation | |
| | XID3: | |
| | DLC_XID | |
+----------------------+---------------------+----------------------+
| Receive ICANREACH_cs | Send HALT_DL_NOACK | |
+----------------------+---------------------+----------------------+
| Receive ENTER_BUSY | DLC_ENTER_BUSY | |
+----------------------+---------------------+----------------------+
| Receive EXIT_BUSY | DLC_EXIT_BUSY | |
+----------------------+---------------------+----------------------+
| Rec TEST_CIRCUIT_REQ | Snd TEST_CIRCUIT_RSP| |
+----------------------+---------------------+----------------------+
| DLC_RESET | Send RESTART_DL | CIRCUIT_RESTART |
+----------------------+---------------------+----------------------+
| DLC_ERROR | Send HALT_DL | DISCONNECT_PENDING |
+----------------------+---------------------+----------------------+
| DLC_DGRM | Send DGRMFRAME | |
+----------------------+---------------------+----------------------+
| DLC_INFO | Send INFOFRAME | |
+----------------------+---------------------+----------------------+
| DLC_XID | If non-activation | |
| | XID3: | |
| | Send XIDFRAME | |
+----------------------+---------------------+----------------------+
| XPORT_FAILURE | DLC_HALT_DL | HALT_PENDING_NOACK |
+----------------------+---------------------+----------------------+
The CONNECTED state is entered from the CONNECT_PENDING state upon
the receipt of a CONTACTED message or from the CONTACT_PENDING state
upon the receipt of a DLC_CONTACTED signal.
The CONNECTED state is exited usually under one of two conditions: a
DLC_ERROR signal received from the DLC (e.g., a DISC command received
by the local DLC), or a HALT_DL message received from the other Data
Link Switch (e.g., a DISC command received by the remote DLC).
A SABME command (i.e., a DLC_RESET signal) received by either Data
Link Switch will also cause the two Data Link Switches to leave the
CONNECTED state and attempt to restart the circuit. Following the
receipt of a SABME, the local Data Link Switch sends a RESTART_DL
message to the other Data Link Switch and enters the CIRCUIT_RESTART
state. Upon the receipt of the RESTART_DL message, the remote Data
Link Switch sends a DISC command (i.e., DLC_HALT_DL signal) and
enters the RESTART_PENDING state.
An ICANREACH_cs received after the transition to CONNECTED state
indicates that more than one CANUREACH_cs was sent at circuit
establishment time and the target station was found by more than one
Data Link Switch partner. A HALT_DL_NOACK is sent to halt the
circuit started by the Data Link Switch partner that originated each
such ICANREACH_cs.
Note: Some implementations will also send a Test command in order to
restart the data link to the station that sent the SABME command
(i.e., a DLC_START_DL will be issued).
5.2.9 CIRCUIT_RESTART State
+----------------------+---------------------+----------------------+
| Event | Action(s) | Next State |
+----------------------+---------------------+----------------------+
| Receive DL_RESTARTED | If Connected: | If Connected: |
| | Send CONTACT | CONNECT_PENDING, |
| | | else: |
| | | CIRCUIT_ESTABLISHED |
+----------------------+---------------------+----------------------+
| Receive HALT_DL_NOACK| DLC_HALT_DL | HALT_PENDING_NOACK |
+----------------------+---------------------+----------------------+
| Receive DGRMFRAME | DLC_DGRM | |
+----------------------+---------------------+----------------------+
| DLC_ERROR | Send HALT_DL | DISCONNECT_PENDING |
+----------------------+---------------------+----------------------+
| DLC_DGRM | Send DGRMFRAME | |
+----------------------+---------------------+----------------------+
| XPORT_FAILURE | DLC_HALT_DL | HALT_PENDING_NOACK |
+----------------------+---------------------+----------------------+
The CIRCUIT_RESTART state is entered if a DLC_RESET signal is
received from the local DLC. This was caused by the receipt of a
SABME command while a connection was currently active. A DM response
will be issued to the SABME command and the Data Link Switch will
attempt to restart the end-to-end circuit.
The CIRCUIT_RESTART state is exited through one of two transitions.
The next state depends upon the time the local DLC has reached the
contacted state (i.e., a DLC_CONTACTED signal is presented) relative
to the receipt of the DL_RESTARTED message. This signal is caused by
the origin station resending the SABME command that initially caused
the Data Link Switch to enter the CIRCUIT_RESTART state. The two
cases are as follows:
1) DL_RESTARTED message received before the DLC_CONTACTED signal-
In this case, the CIRCUIT_ESTABLISHED state is entered.
2) DL_RESTARTED message received after the DLC_CONTACTED signal-
In this case, the CONNECT_PENDING state is entered.
5.2.10 DISCONNECT_PENDING State
+----------------------+---------------------+----------------------+
| Event | Action(s) | Next State |
+----------------------+---------------------+----------------------+
| Receive DL_HALTED | | DISCONNECTED |
+----------------------+---------------------+----------------------+
| Receive HALT_DL | Send DL_HALTED | |
+----------------------+---------------------+----------------------+
| Receive HALT_DL_NOACK| | DISCONNECTED |
+----------------------+---------------------+----------------------+
| Receive DATAFRAME | DLC_DGRM | |
+----------------------+---------------------+----------------------+
| DLC_DGRM | Send DATAFRAME | |
+----------------------+---------------------+----------------------+
| XPORT_FAILURE | | DISCONNECTED |
+----------------------+---------------------+----------------------+
The DISCONNECT_PENDING state is entered when a DLC_ERROR signal is
received from the local DLC. Upon receipt of this signal, a HALT_DL
message is sent. Once an DL_HALTED message is received, the state is
exited, and the Data Link Switch enters the DISCONNECTED state.
5.2.11 RESTART_PENDING State
+----------------------+---------------------+----------------------+
| Event | Action(s) | Next State |
+----------------------+---------------------+----------------------+
| Receive HALT_DL_NOACK| | HALT_PENDING_NOACK |
+----------------------+---------------------+----------------------+
| Receive DGRMFRAME | DLC_DGRM | |
+----------------------+---------------------+----------------------+
| DLC_DL_HALTED | Send DL_RESTARTED | CIRCUIT_ESTABLISHED |
+----------------------+---------------------+----------------------+
| DLC_ERROR | Send HALT_DL | DISCONNECT_PENDING |
+----------------------+---------------------+----------------------+
| DLC_DGRM | Send DGRMFRAME | |
+----------------------+---------------------+----------------------+
| XPORT_FAILURE | DLC_HALT_DL | HALT_PENDING_NOACK |
+----------------------+---------------------+----------------------+
The RESTART_PENDING state is entered upon the receipt of a RESTART_DL
message from the remote DLSw while the local Data Link Switch is in
either the CONTACT_PENDING state or the CONNECTED state, which causes
the local DLSw to issue a DISC command to the DLC. Upon the receipt
of the UA response (DLC_DL_HALTED), the data link is restarted, a
DL_RESTARTED message is returned to the remote DLSw, and the
CIRCUIT_ESTABLISHED state is entered.
Note: Some implementations will send a Test command in order to
restart the data link to the target station (i.e., a DLC_START_DL
will be issued) prior to sending the DL_RESTARTED message.
5.2.12 HALT_PENDING State
+----------------------+---------------------+----------------------+
| Event | Action(s) | Next State |
+----------------------+---------------------+----------------------+
| Receive HALT_DL_NOACK| | HALT_PENDING_NOACK |
+----------------------+---------------------+----------------------+
| Receive DATAFRAME | DLC_DGRM | |
+----------------------+---------------------+----------------------+
| DLC_DL_HALTED | Send DL_HALTED | DISCONNECTED |
+----------------------+---------------------+----------------------+
| DLC_ERROR | Send DL_HALTED | DISCONNECTED |
+----------------------+---------------------+----------------------+
| DLC_DGRM | Send DATAFRAME | |
+----------------------+---------------------+----------------------+
| XPORT_FAILURE | | HALT_PENDING_NOACK |
+----------------------+---------------------+----------------------+
The HALT_PENDING state is entered upon the receipt of a HALT_DL
message. This causes the local DLC to issue a DISC command. Upon the
receipt of the UA response (DLC_DL_HALTED), a DL_HALTED message is
returned to the remote DLSw and the DISCONNECTED state is entered.
5.2.13 HALT_PENDING_NOACK State
+----------------------+---------------------+----------------------+
| Event | Action(s) | Next State |
+----------------------+---------------------+----------------------+
| Receive DATAFRAME | DLC_DGRM | |
+----------------------+---------------------+----------------------+
| DLC_DL_HALTED | | DISCONNECTED |
+----------------------+---------------------+----------------------+
| DLC_ERROR | | DISCONNECTED |
+----------------------+---------------------+----------------------+
| DLC_DGRM | Send DATAFRAME | |
+----------------------+---------------------+----------------------+
The HALT_PENDING_NOACK state is entered upon the receipt of a
HALT_DL_NOACK message. This causes the local DLC to issue a DISC
command. Upon the receipt of the UA response (DLC_DL_HALTED), the
DISCONNECTED state is entered.
5.3 NetBIOS Datagrams
The NetBIOS protocols use a number of UI frames for directory
services and the transmission of datagrams. Most of these frames are
directed to a group MAC address (GA) with the routing information
field indicating spanning tree explorer (STE) (a.k.a. Single Route
Broadcast). The NB_Add_Name_Response and NB_Name_Recognized frames
are directed to a specific MAC address with the routing information
field indicating an all routes explorer frame (ARE) (a.k.a. All
Routes Broadcast) The NB_Status_Response frame, is directed to a
specific MAC address with the routing information field indicating a
specifically routed frame (SRF). The handling of these frames is
summarized in the following table.
+---------------------------+------------------+--------------------+
| Event | Action(s) | Comment |
+---------------------------+------------------+--------------------+
| DLC_DGRM for NETBIOS | Send NETBIOS_ANQ | Transmitted to all |
| group address: | | remote DLSw |
| NB_Add_Name_Query | | |
+---------------------------+------------------+--------------------+
| DLC_DGRM for a specific | Send NETBIOS_ANR | Transmitted to |
| address: | | specific DLSw |
| NB_Add_Name_Response | | |
+---------------------------+------------------+--------------------+
| DLC_DGRM for a specific | Send DATAFRAME | Transmitted to all |
| address: | | remote DLSw |
| NB_Status_Response | | |
+---------------------------+------------------+--------------------+
| DLC_DGRM for NETBIOS | Send DATAFRAME | Transmitted to all |
| group address: | | remote DLSw |
| NB_Name_in_Conflict | | |
| NB_Add_Group_Name_Query | | |
| NB_Datagram, | | |
| NB_Datagram_Broadcast | | |
| NB_Status_Query | | |
| NB_Terminate_Trace | | |
+---------------------------+------------------+--------------------+
The above actions do not apply in the following states:
CIRCUIT_ESTABLISHED, CONTACT_PENDING, CONNECT_PENDING, CONNECTED, and
CIRCUIT_PENDING. The handling of the remaining two UI frames used by
NetBIOS systems, NB_Name_Query and NB_Name_Recognized, are documented
as part of the DLSw state machine in the previous section (i.e.,
DISCONNECTED and RESOLVE_PENDING states). Furthermore, the handling
of NetBIOS datagrams (i.e., NB_Datagram) sent to a specific MAC
address is also governed by the DLSw state machine.
Note: Some implementations also issue Test frames during the
exchange of the NetBIOS, NB_Name_Query and NB_Name_Recognized. This
exchange of protocol data units occurs during the start of a data
link and is used to determine the routing information. Most other
implementations of NetBIOS will use the
NB_Name_Query/NB_Name_Recognized exchange to determine routes in
conjunction with resolving the NetBIOS names. These differences are
not reflected in the SSP protocols.
The handling of the NetBIOS specific SSP messages is given in the
following table.
+---------------+-------------------------+-------------------------+
| Event | Action(s) | Comment |
+---------------+-------------------------+-------------------------+
| NETBIOS_ANQ | DLC_DGRM: | Routed STE |
| | NB_Add_Name_Query | (NETBIOS Group Address) |
+---------------+-------------------------+-------------------------+
| NETBIOS_ANR | DLC_DGRM: | Routed ARE |
| | NB_Add_Name_Response | (Specific MAC Address) |
+---------------+-------------------------+-------------------------+
| NETBIOS_NQ_ex | DLC_DGRM: | Routed STE |
| | NB_Name_Query | (NETBIOS Group Address) |
+---------------+-------------------------+-------------------------+
| NETBIOS_NQ_cs | DLC_DGRM: | Routed STE |
| | NB_Name_Query | (NETBIOS Group Address) |
+---------------+-------------------------+-------------------------+
| NETBIOS_NR_ex | DLC_DGRM: | Routed ARE |
| | NB_Name_Recognized | (Specific MAC Address) |
+---------------+-------------------------+-------------------------+
| NETBIOS_NR_cs | DLC_DGRM: | Routed ARE |
| | NB_Name_Recognized | (Specific MAC Address) |
+---------------+-------------------------+-------------------------+
| DATAFRAME | DLC_DGRM | If NB_Status_Response: |
| | | Routed ARE |
| | | (Specific MAC Address) |
| | | Else: |
| | | Routed STE |
| | | (NETBIOS Group Address)|
+---------------+-------------------------+-------------------------+
The above actions apply to all DLSw states. The handling of NetBIOS
datagrams sent within DGRMFRAME messages is governed by the DLSw
state machine. The DGRMFRAME message type is employed instead of the
DATAFRAME message type once the end-to-end circuit has been
established. At that time, the message is addressed according to the
pair of Circuit IDs in the message header instead of relying upon the
MAC address information in the token ring header.
5.4 Explorer Traffic
The CANUREACH_ex, ICANREACH_ex, NETBIOS_NQ_ex, and NETBIOS_NR_ex SSP
messages explore the topology of the DLSw cloud and the networks
attached to it. These explorer frames are used to determine the DLSw
partners through which a MAC or NetBIOS name can be accessed. This
information may optionally be cached to reduce explorer traffic in
the DLSw cloud.
If a DLSw is aware from cached information that a given MAC address
or NetBIOS name is accessible through a given partner DLSw, it should
direct all circuit setup attempts to that partner. If the circuit
setup fails, or no such data is available in the MAC or name cache
database, the DLSw may fallback to issuing the setup attempt to all
DLSw partners on the assumption that the cached data is now out of
date. The mechanism for determining when to use such a fallback is
implementation defined.
DLSw implementations may also use a local MAC cache to enable
responses to CANUREACH_ex requests to be issued without the need for
TEST frame exchange (or equivalent) until the CANUREACH_cs is
received. Again, the fallback mechanism for determining when such
local cache data is out-of-date is implementation defined.
The use of either cache is an optional function in DLSw. An
implementation may choose to always issue explorer frames or to use
either or both types of cache.
The following sections describe the FSMs used for explorer frames.
The DLC events and actions are a subset of those described in section
5.2 for the main circuit FSM.
5.4.1 CANUREACH/ICANREACH Explorer FSM
The FSM described below is used to handle explorer frames routed by
MAC address. There is one instance of this FSM for each Data Link ID
(Target and Origin MAC/SAP pair) for which explorer traffic is
flowing. The states in this FSM are as follows.
State Name Description
---------- -----------
RESET The initial state.
SENT_EX Local DLSw has issued an explorer message
RECEIVED_EX Local DLSw has received an explorer message
5.4.1.1 RESET State
+----------------------+---------------------+----------------------+
| Event | Action(s) | Next State |
+----------------------+---------------------+----------------------+
| Receive CANUREACH_ex | If replying from | If DLC_RESOLVE sent, |
| | cache, send | RECEIVED_EX |
| | ICANREACH_ex | |
| | else if allowed to | |
| | test availability, | |
| | issue DLC_RESOLVE. | |
| | Optionally update | |
| | cache. | |
+----------------------+---------------------+----------------------+
| Receive ICANREACH_ex | Optionally update | RESET |
| | cache | |
+----------------------+---------------------+----------------------+
| DLC_RESOLVE_C | Send CANUREACH_ex | SENT_EX |
+----------------------+---------------------+----------------------+
RESET is the initial state for the CANUREACH/ICANREACH explorer FSM.
This state is exited when a DLC_RESOLVE_C request is received from
the DLC or a CANUREACH_ex is received from a remote DLSw.
A DLSw implementation may optionally reply from to CANUREACH_ex
messages on the basis of cached topology information, in which case
the DLC_RESOLVE exchange (i.e., TEST) is not required. If cache is
not used, or no match is found, and the DLC permits the use of TEST,
DLC_RESOLVE is issued to locate the target MAC and the state changes
to RECEIVED_EX. If no cache entry is available and TEST is not
allowed by the DLC, a received CANUREACH_ex frame is ignored.
5.4.1.2 SENT_EX State
+----------------------+---------------------+----------------------+
| Event | Action(s) | Next State |
+----------------------+---------------------+----------------------+
| Receive ICANREACH_ex | DLC_RESOLVE_R | RESET |
| | Optionally update | |
| | cache | |
+----------------------+---------------------+----------------------+
| DLC_RESOLVE_C | | SENT_EX |
+----------------------+---------------------+----------------------+
SENT_EX is entered when the DLSw has issued a CANUREACH_ex message to
locate a MAC address. This state is exited when a remote DLSw
returns a matching ICANREACH_ex, or after an implementation defined
timeout. DLC_RESOLVE events received in this state correspond to TEST
retries by the origin DLC station and are absorbed.
An implementation may choose whether to handle explorer frame
crossover either by using entirely separate FSM instances and simply
allowing both ends to issue TEST frames, or by detecting a reverse
CANUREACH_ex frame here and issuing an ICANREACH_ex message and
DLC_RESOLVE_R action.
5.4.1.3 RECEIVED_EX State
+----------------------+---------------------+----------------------+
| Event | Action(s) | Next State |
+----------------------+---------------------+----------------------+
| Receive CANUREACH_ex | Optionally update | RECEIVED_EX |
| | cache | |
+----------------------+---------------------+----------------------+
| Receive ICANREACH_ex | | RECEIVED_EX |
+----------------------+---------------------+----------------------+
| DLC_RESOLVED | Send ICANREACH_ex | RESET |
| | Optionally update | |
| | cache | |
+----------------------+---------------------+----------------------+
RECEIVED_EX is entered when the DLSw has received a CANUREACH_ex from
a remote DLSw and has issued a DLC_RESOLVE to locate the MAC address.
This state is exited when the DLC_RESOLVED response is received, or
after an implementation defined timeout.
If the target MAC is located, the DLSw must reply to the first
received CANUREACH_ex that caused the move to this state. If
additional CANUREACH_ex messages are received in this state from
other remote DLSw partners, the DLSw may optionally reply to these
messages too but it is not required to do so.
An implementation may choose whether to handle explorer frame
crossover either by using entirely separate FSM instances and simply
allowing both ends to issue TEST frames, or by detecting such a
reverse DLC_RESOLVE_C event here and issuing an ICANREACH_ex message
and DLC_RESOLVE_R action.
5.4.2 NETBIOS_NQ/NR Explorer FSM
The FSM described below is used to handle explorer frames routed by
NetBIOS names There is one instance of this FSM for each unique
combination of Source Name, Destination Name, Data 2 field and
Response Correlator.
State Name Description
---------- -----------
RESET The initial state.
SENT_EX Local DLSw has issued an explorer
message
RECEIVED_EX Local DLSw has received an explorer
message
SENT_REC_EX An explorer frame has been both sent
and received for the same (potential)
NetBIOS circuit.
5.4.2.1 RESET State
+----------------------+---------------------+----------------------+
| Event | Action(s) | Next State |
+----------------------+---------------------+----------------------+
| Receive NETBIOS_NQ_ex| DLC_DGRM(NAME_QUERY)| RECEIVED_EX |
| | Optionally update | |
| | cache. | |
+----------------------+---------------------+----------------------+
| Receive NETBIOS_NR_ex| Optionally update | RESET |
| | cache | |
+----------------------+---------------------+----------------------+
| DLC_DGRM (NAME_QUERY)| Send NETBIOS_NQ_ex | SENT_EX |
+----------------------+---------------------+----------------------+
The RESET state is the initial state for the NETBIOS_NQ/NR explorer
FSM. It is exited when the DLC receives either a NETBIOS_NQ_ex or a
DLC_DGRM containing a NetBIOS NAME_QUERY frame. If a NETBIOS_NQ_ex
message is received, the NAME_QUERY is propagated to the DLC and this
FSM moves to state RECEIVED_EX. If a NetBIOS NAME_QUERY frame is
received, the NETBIOS_NQ_ex is propagated either to the appropriate
DLSw partners (see below), and this FSM moves to state SENT_EX.
Unlike SNA traffic where the CANUREACH_ex/ICANREACH_ex exchange can
be omitted if the MAC location is already cached,
NETBIOS_NQ_ex/NETBIOS_NR_ex frames must always be issued during
NetBIOS session setup in order that the NetBIOS session numbers are
exchanged correctly between the DLC end stations. If the location of
a NetBIOS name is known from cached data, the NETBIOS_NQ_ex need only
be issued to the cached DLSw partners. Otherwise the NETBIOS_NQ_ex
should be issued to all partners that support NetBIOS.
5.4.2.2 SENT_EX State
+----------------------+---------------------+----------------------+
| Event | Action(s) | Next State |
+----------------------+---------------------+----------------------+
| Receive NETBIOS_NQ_ex| DLC_DGRM(NAME_QUERY)| SENT_REC_EX |
| | Optionally update | |
| | cache | |
+----------------------+---------------------+----------------------+
| Receive NETBIOS_NR_ex| DLC_DGRM(NAME_RECOG)| RESET |
| | Optionally update | |
| | cache | |
+----------------------+---------------------+----------------------+
| DLC_DGRM (NAME_QUERY)| Send NETBIOS_NQ_ex | SENT_EX |
| (different local | Optionally update | |
| session number than | cache | |
| existing searches) | | |
+----------------------+---------------------+----------------------+
SENT_EX is entered when the local DLSw issues a NETBIOS_NQ_ex to its
remote DLSw partners. This state is exited when a NETBIOS_NR_ex is
received from a remote DLSw, or if a matching NETBIOS_NQ_ex is
received from a remote DLSw (i.e., a NETBIOS_NQ_ex crossover case).
If the local NetBIOS end station issues a NAME_QUERY with a different
session number from any previous NAME_QUERY for this search, the
NAME_QUERY is propagated to the DLSw partners to ensure that the
exchange of NetBIOS session numbers is handled correctly.
5.4.2.3 RECEIVED_EX State
+----------------------+---------------------+----------------------+
| Event | Action(s) | Next State |
+----------------------+---------------------+----------------------+
| Receive NETBIOS_NQ_ex| DLC_DGRM(NAME_QUERY)| RECEIVED_EX |
| | Optionally update | |
| | cache | |
+----------------------+---------------------+----------------------+
| Receive NETBIOS_NR_ex| | RECEIVED_EX |
+----------------------+---------------------+----------------------+
| DLC_DGRM (NAME_QUERY)| Send NETBIOS_NQ_ex | SENT_REC_EX |
| | Optionally update | |
| | cache | |
+----------------------+---------------------+----------------------+
| DLC_DGRM (NAME_RECOG)| Send NETBIOS_NR_ex | RESET |
| | Optionally update | |
| | cache | |
+----------------------+---------------------+----------------------+
RECEIVED_EX is entered when the local DLSw receives a NETBIOS_NQ_ex
message from a remote DLSw. This state is exited when a
NAME_RECOGNIZED NetBIOS frame is received from the DLC, completing
the query, or when a matching NAME_QUERY is received from DLC (i.e.,
NAME_QUERY crossover).
5.4.2.4 SENT_REC_EX State
+----------------------+---------------------+----------------------+
| Event | Action(s) | Next State |
+----------------------+---------------------+----------------------+
| Receive NETBIOS_NQ_ex| DLC_DGRM(NAME_QUERY)| SENT_REC_EX |
| | Optionally update | |
| | cache | |
+----------------------+---------------------+----------------------+
| Receive NETBIOS_NR_ex| DLC_DGRM(NAME_RECOG)| RECEIVED_EX |
| | Optionally update | |
| | cache | |
+----------------------+---------------------+----------------------+
| DLC_DGRM (NAME_QUERY)| Send NETBIOS_NQ_ex | SENT_REC_EX |
| (different local | Optionally update | |
| session number than | cache | |
| existing searches) | | |
+----------------------+---------------------+----------------------+
| DLC_DGRM (NAME_RECOG)| Send NETBIOS_NR_ex | SENT_EX |
| | Optionally update | |
| | cache | |
+----------------------+---------------------+----------------------+
This state is required if an implementation wishes to manage NQ/NR
crossover cases from a single FSM instance by detecting 'opposite'
NAME_QUERY attempts between the same two NetBIOS names. If separate
FSM instances are used instead, this state is not required and the
transitions to it from other states can be removed.
SENT_RCV_EX is exited when the NAME_QUERY search in either direction
is resolved. If the local NetBIOS end station issues a NAME_QUERY
with a different session number from any previous NAME_QUERY it has
issued for this search, the NAME_QUERY is propagated to the DLSw
partners to ensure that the exchange of NetBIOS session numbers is
correctly handled.
5.4.2.5 NetBIOS Session Numbers
NetBIOS NAME_QUERY and NAME_RECOGNIZED frames exchange NetBIOS session
numbers between the end stations. For correct NetBIOS operation over
DLSw, it is important that all SSP NETBIOS_NQ_ex frames received by a
DLSw cause NetBIOS NAME_QUERY frames to flow on the LAN with the new
session number from the NETBIOS_NQ_ex. These frames cannot be replied
to from a cache of locally available NetBIOS names in the same way that
MAC addresses and CANUREACH_ex messages can be handled.
Also, NAME_QUERY messages are normally retried several times on the LAN.
The generation and absorption of such frames is outside the scope of the
FSM defined above.
6. Protocol Flow Diagrams
The Switch-to-Switch Protocol is used to setup and take down circuits
between a pair of Data Link Switches. Once a circuit is established,
the end stations on the local networks can employ LLC Type 1
(connectionless UI frames) protocols end-to-end. In addition, the end
systems can establish an end-to-end connection for support of LLC Type 2
(connection oriented I frames) protocols (Type 2 I frames go end-to-end,
supervisory frames are handled locally).
The term, Data Link, is used in this document to refer to both a
"logical data link" when supporting Type 1 LLC services, and a "data
link connection" when supporting Type 2 LLC services. In both cases,
the Data Link is identified by the Data Link ID defined in section 3.2.