CCIE must know DLSW Technical Brief
DLSw (Data Link Switching) is developed by APPN (Advanced Peer-to-Peer Networking), Implementers Workshop (AIW, Implementation Working Group), and is used to carry SNA (System Network Architecture, a method of system network structure system. SNA is a network architecture corresponding to the OSI reference model launched by IBM in the 1970s. To realize the SNA protocol across the WAN transmission, one of the solutions is DLSw technology.
Using DLSw technology, it is also possible to implement SDLC (Synchronous Data Link Control) link protocol across TCP/IP transmission. First convert the SDLC format message to LLC2 format message, and then interconnect with the remote end through DLSw. In this way, DLSw also supports the interconnection of different media between LAN and SDLC.
DLSw currently has two versions: DLSw1.0 and DLSw2.0. The DLSw implemented based on RFC 1795 is version DLSw1.0; in order to improve product maintainability and reduce network overhead, the system implements DLSw2.0 version based on RFC 2166. In DLSw2.0, the function of supporting sending UDP inquiry messages in multicast and unicast mode is added. When the communication peer is also DLSw2.0, the two can use UDP packets to inquire about reachability information, and only establish a TCP connection when there is a data transmission demand.
There were many problems in version 1.0, so DLSW2.0 version came later:
Existing problems:
- The problem of TCP connection: All messages (including inquiry messages, circuit establishment request messages, and data messages) are transmitted using TCP connections. First establish two TCP connections. After the performance exchange is completed, disconnect one TCP connection. This caused a waste of network resources to a certain extent.
- Flooding of broadcast messages: When there is no reachable path information in the reachable information list of DLSw or there is too little reachable path information, the inquiry messages will flood the WAN through the established TCP connections.
- Poor maintainability: When the link is interrupted, DLSw1.0 uses two types of messages to notify the opposite end, but it cannot tell the opposite end what caused the link interruption. It is difficult to determine the problem.
DLSw2.0 improvements: - Use UDP packets to query peer addresses: In order to avoid establishing unnecessary TCP connections, DLSw2.0 generally does not use TCP connections to send inquiry packets, but uses UDP packets instead.
- Establish a single TCP channel: When there is a need to establish a link, a TCP connection is established between the source DLSw2.0 router and the target DLSw2.0 router.
- Enhanced maintainability: Five reasons for circuit interruption are defined: unknown error detected, DISC frame received by DLSw from the terminal, DLC error detected by the terminal, circuit standard protocol error and system initialization.
DLSW+:
Data link switching Plus (DLSw+) is a method of transmitting SNA and NetBIOS data in a wide area network or campus network. The terminal system can pass through the Token Ring network (Token Ring), Ethernet ( Ethernet), synchronous data link control (SDLC) protocol or optical fiber interface (FDDI) to connect to the Internet.
DLSw+ can convert data between different media, terminate the data link locally, keep responding, keepalive and close the polling information of the WAN. The data link layer terminates locally and also eliminates control timeouts caused by network congestion or re-routing. Finally, DLSw+ also provides a dynamic search for SNA or NetBIOS resource mechanism and high-number algorithms to minimize broadcast transmission.
In the document, DLSw+ routers can be regarded as peer routers, peers and partners. The connection between two DLSw+ routers is called a peer connection. A DLSw circuit includes the data link control connection between the initial terminal system and the initial router, the connection between the two routers (usually a TCP connection), and the data link control connection between the destination terminal system and the destination router. A single peer connection can support multiple circuits.
DLSW+ comparison DLSW standard adds four new points:
① Scalability-building an IBM network to reduce the amount of broadcast transmission and enhance network scalability.
② Practicality——Quickly and dynamically find related paths and optionally perform load balancing through multiple active peers and ports.
③ Transmission flexibility-high-performance transmission avoids network interruption caused by timeout.
④ Operation Mode-Dynamically detect the performance of peer routers and detect them according to their performance.
DLSW+ link establishment:
The establishment of a link for a group of end systems includes searching for target resources and setting up the data link connection of the end system. In the local area network, the SNA device sends a detection frame with the destination MAC address to look for other SNA devices. When a DLSw router receives the detection frame, it sends a canureach frame to every partner router it can reach. If one of the DLSw partners can reach the specified MAC address, it responds with an icanreach frame.
Each router and the local SNA designate the data link connection between the system and the TCP connection between the DLSw partner. This link is uniquely identified by the source and destination link numbers. Each link number is in turn composed of source and destination MAC addresses, source and destination The link service access point and a data link control number are defined. Once the link is established, the information frame can be transmitted.
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