📄 rfc1434.txt
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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.Dixon & Kushi [Page 13] 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.Dixon & Kushi [Page 14] RFC 1434 DLS: Switch-to-Switch Protocol March 19934.1.3. 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 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⌨️ 快捷键说明
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