📄 rfc1434.txt
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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.
4. Protocol Specification
This section provides a description of the Switch-to-Switch
Protocols. Included is a set of high-level protocol flows and a
detail set of state transition tables. The states and the protocols
are described in terms that are intended to be generic to different
platforms. Emphasis of the technical details is to ensure
operability of the IBM 6611 with another vendor's implementation.
Notes are inserted at points where the IBM 6611 performs local
actions that are specific to the AIX platform upon which it operates.
4.1. Protocol Flow Diagrams
The switch-to-switch protocols are 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) protocols. In addition, the end systems can
establish an end-to-end connection for support of LLC Type 2
(connection oriented) protocols.
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 in defined by the concatenation of the destination MAC
address (DA), the source MAC address (SA), the destination link SAP
(DSAP) and source link SAP (SSAP).
4.1.1. Connect Protocols
The following figure depicts the protocol flows that are used for the
establishment of a circuit between a pair of Data Link Switches,
followed by the establishment of a connection between the pair of end
systems. The figure is drawn assuming that the two end systems are
SNA (the protocol flow for NetBIOS systems is described in a later
paragraph).
Dixon & Kushi [Page 11]
RFC 1434 DLS: Switch-to-Switch Protocol March 1993
Data Link Data Link Data Link Data Link
Control Switch Switch Control
-------------------- --------------------
+------------+ +------------+
|Disconnected| |Disconnected|
+------------+ +------------+
Test Command CANUREACH Test Comd.
----------> ---------------------------------------> ------->
(DSAP=Null) (DSAP=SSAP)
Test Response
ICANREACH <---------
Test Response <---------------------------------------
<---------- REACH ACK
--------------------------------------->
+------------+ +------------+
|Circuit Est.| |Circuit Est.|
+------------+ +------------+
SABME CONTACT
----------> ---------------------------------------> SABME
UA ------->
<----------
RNR UA
<---------- CONTACTED <-------
<---------------------------------------
+------------+ +------------+
| Connected | | Connected |
+------------+ +------------+
RR
<---------
Figure 4. DLS Connect Message Protocols
Upon receipt of a Test command from the origin station, the origin
DLS will send a CANUREACH (i.e., can you reach) message to the target
DLS. If the target DLS is not known to the origin DLS, the CANUREACH
message is sent to all remote Data Link Switches defined to the
origin DLS. The receipt of the CANUREACH message causes the target
DLS to send a Test command searching for the target station. The
target station will return a Test response, causing the target DLS to
return an ICANREACH (i.e., I can reach) message to the origin DLS.
If multiple Data Link Switches can reach the target station, the
origin DLS will receive multiple ICANREACH messages. The origin DLS
will select the first message and send a REACH_ACK (i.e., reach
acknowledgment) message to the selected Data Link Switch. During
this exchange of messages, both Data Link Switches change states from
the Disconected state to the Circuit Established state. Once the
circuit is established, Type-1 frames, such as XID, may be exchanged
between the origin and target stations.
Dixon & Kushi [Page 12]
RFC 1434 DLS: Switch-to-Switch Protocol March 1993
To establish a connection, the origin station sends a SABME command.
Upon receipt of this command, the origin DLS will send a CONTACT
message to the target DLS and return a UA response to the origin
station. To inhibit traffic flow until the connection is established
to the remote station, a RNR supervisory frame is sent to the origin
station. The CONTACT message will cause the target DLS to send a
SABME command to the target station, which in return will reply with
a UA response. Upon receipt of the UA response, the target DLS will
send a CONTACTED message to the origin DLS. The origin DLS will now
send an RR supervisory frame to the origin station. During this
exchange of messages, both Data Link Switches change states from the
Circuit Established state to the Connected state.
For NetBIOS end systems, the protocol flows are similar but employ
different frames and SSP messages. Instead of using a Test command
frame to initiate the circuit, a NetBIOS system will use a Name Query
frame. Receipt of a Name Query frame will cause the Data Link Switch
to issue a NETBIOS_NQ message instead of the CANUREACH message. In a
like fashion, the Test response is replaced with a Name Recognized
frame and the ICANREACH message is replaced with a NETBIOS_NR
message. As with the SNA protocol flows, the receipt of a NETBIOS_NR
message causes the origin Data Link Switch to respond with a
REACH_ACK message.
4.1.2. Link Restart Protocols
The following figure depicts the protocol flows that result from
restarting the end-to-end connection. This causes the Data Link
Switches to terminate the existing connection and to enter the
Circuit Established state awaiting the start of a new connection.
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
Data Link Data Link Data Link Data Link
Control Switch Switch Control
--------------------- ---------------------
+-----------+ +-----------+
| Connected | | Connected |
SABME +-----------+ +-----------+
-----------> RESTART_DL
DM -------------------------------------> DISC
<----------- -------->
UA
DL_RESTARTED (Case 1) <--------
<-------------------------------------
+-----------+ +-----------+
|Circuit Est| |Circuit Est|
+-----------+ +-----------+
........... or ...........
SABME
-----------> DL_RESTARTED (Case 2)
UA <-------------------------------------
<----------- +-----------+
|Circuit Est|
CONTACT +-----------+
RNR ------------------------------------>
<----------
Figure 5. DLS Link Restart Message Protocols
Upon receipt of a SABME command from the origin station, the origin
DLS will send a RESTART_DL message to the target DLS. A DM response
is also returned to the origin station and the data link is
restarted.
Upon receipt of the RESTART_DL message, the target DLS will issue a
DISC command to the target station. The target station is expected
to return a UA response. The target DLS will then restart its data
link and send an DL_RESTARTED message back to the origin DLS. During
this exchange of messages, both Data Link Switches change states from
Connected state to Circuit Established state.
If the origin station now resends the SABME command, the origin DLS
will send a CONTACT message to the target DLS. If the SABME command
is received prior to the receipt of the DL_RESTARTED message (case 2
in the figure), the CONNECT message is delayed until the DL_RESTARTED
message is received. The resulting protocol flows at this point
parallel those given above for the connect sequence.
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RFC 1434 DLS: Switch-to-Switch Protocol March 1993
4.1.3. Disconnect Protocols
The following figure depicts the protocol flows that result from the
end system terminating an existing connection. Not only is the
connection terminated, but the circuit between the Data Link Switches
is taken down.
Data Link Data Link Data Link Data Link
Control Switch Switch Control
-------------------- --------------------
+-----------+ +-----------+
| Connected | | Connected |
+-----------+ +-----------+
DISC
----------> HALT_DL
UA -------------------------------------> DISC
<---------- --------->
UA
DL_HALTED <--------
<-------------------------------------
+-----------+ +-----------+
|Disconnectd| |Disconnectd|
+-----------+ +-----------+
......... or ..........
+-----------+ +-----------+
| Connected | | Connected |
+-----------+ +-----------+
DISC TCP Connection Failure DISC
<-------- <------------------------------------> --------->
UA UA
--------> <--------
+-----------+ +-----------+
|Disconnectd| |Disconnectd|
+-----------+ +-----------+
Figure 6. DLS Disconnect Message Protocols
Upon receipt of a DISC command from the origin station, the origin
DLS will reply with a UA response and issue a HALT_DL message to the
target DLS. Upon receipt of the HALT_DL message, the target DLS will
send a DISC command to the target station. The target station will
then respond with a UA response, causing the target DLS to return a
DL_HALTED message to the origin DLS. During this exchange of
messages, both Data Link Switches change states from the Connected
state to the Disconnected state.
Dixon & Kushi [Page 15]
RFC 1434 DLS: Switch-to-Switch Protocol March 1993
If the TCP connection between two Data Link Switches fails, all
connections that are currently multiplexed on the failed TCP
connection will be taken down. This implies that both Data Link
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