Data Link Switching (DLSw)

Introduction

Data Link Switching (DLSw) is a means of transporting Systems Network Architecture (SNA) and NetBIOS traffic across a network which uses many different protocols. The original RFC 1434 described DLSw but this has been superceded by RFC 1795 which describes DLSw version 1. More recently scalability enhancements have been introduced in DLSw version 2. Cisco also has introduced some enhancements in it's DLSw+ which is backwardly compatible with both version 1 and version 2.

DLSw version 1

Benefits of DLSw over and above SRB are:

Gets around the SRB 7-hop limit.
Allows multiple connections across a network
Increases session response times.
Provides flow control.
Reroutes traffic around broken links.
Gets rid of the SRB heavy broadcast traffic.

In addition, implementations of DLSw can provide SDLC to LLC2 conversion, thereby doing away with the need for many Front End Processor (FEP) ports. Also, RFC 1490 can be supported, allowing LLC2 over Frame Relay and DLSw Prioritisation.

DLSw uses the Switch to Switch Protocol (SSP) in place of Source Route Bridging (SRB) between routers to create DLSw peer connections, locate resources, forward data, handle flow control and error recovery. TCP is used for encapsulation a standard, newer versions of DLSw are not restricted to TCP. The routers are called Data Link Switches. The Data Link Connections (DLCs) are terminated at the router so that the RIF ends at a virtual ring within the router. Because they are terminated locally the DLCs can be locally acknowledged so that there is no need for link layer acknowledgements or keep alive messages to run across the WAN, minimising session timeouts. Because the RIF ends at the peer router at each end you can have 6 more hops on each side of the virtual ring thereby extending the network. With RSRB the RIF is carried all the way through the virtual ring and therefore limits the number of hops. With DLSw the virtual ring can be different in each peer because of the RIF terminating.

DLSw routers are called Peers, and the connections between them are known as Peer Connections. Each Peer Connection can carry many circuits. DLSw carries the data-link control connection between the originating SNA (or NetBIOS) device and router, the data-link control connection between the destination SNA (or NetBIOS) device and router and the TCP connection between the routers. A router may have many peers (or partners).

DLSw operates this sequence of events:

Two TCP connections are established between the two participating routers called peers. If both peers are able to drop connections then the one with the highest IP address does the dropping. An Active Peer initiates connections with other known (configured) peers. A Passive Peer accepts connections with known peers and a Promiscuous Peer accepts connections from any peer whether configured or not.
The routers exchange their capabilities which include DLSw version number, NetBIOS support, TCP window sizes, known MAC addresses and known NetBIOS names, supported LSAPs and the number of TCP sessions.
The DLSw SSP process spoofs the data link connections between end devices by establishing these connections between the local SNA device and its local router. When an SNA device wants to connect to another device it sends an explorer frame (sometimes called a TEST frame or a XID frame). XID stands for Exchange Identification.
When the router receives the explorer frame it sends a CANUREACH EX explorer frame containing the destination MAC address (or NetBIOS name) to its peer router.
The remote peer that can see the particular MAC address replies with a ICANREACH frame.
There then follows a CANUREACH CS circuit setup frame that causes the peer routers to setup a circuit.
DLSw caches the circuit setup information so that there are no unnecessary explorer frames sent on subsequent connection requests. This Reachability cache lets the devices at each end know whether or not the router knows the whereabouts of the destination device.
SNA traffic can now flow while the routers handle the local ACK frames. The DLSw circuit is identified by the source and destination MAC addresses, the source and destination LSAPs and the dat-link control port ID.

When a DLSw peer comes across a destination device that it does not know about it sends a CANUREACH to all DLSw peers. These peers either send back a ICANNOT_REACH or ICANREACH. You can set up these ICANNOT_REACH or ICANREACH statically on the router and then this information is ent in the capabilities exchange.

The following schematic shows an example network where an AS400 needs to print on remote sites. The routers are set up as DLSw switches, each router is a remote peer to the other with itself being the local peer. In this example, the LANs are Ethernet and LLC2 needs to be set up on the LAN interface of the router in order to provide reliable transport at layer 2. Local acknowledgement of the sessions can be set up at the LAN interface of the router if the WAN is made up of slow links. If the WAN contains fast pipes between sites then local acknowledgement is not necessary since the host and clients can send acknowledgements across the WAN without the danger of sessions timing out due to congestion problems dropping frames. Transparent bridging is carried out to the router and then the RIF is added and the SSP takes the session frames to the remote peer where the RIF terminates. As far as the host and client are concerned, they sit astride the local bridge/router.

The routers multiplex the Data-Link Controls (DLC) across a TCP pipe and the IP backbone. Switch-to-Switch Protocol (SSP) is used to take the frames across a routed IP network using whatever IP routing protocol is in operation. At the remote peer router the IP header is stripped off and the SNA device sees the frame as if it was local.

Example

Consider the network below:

Host A wishes to talk SNA to host B across a Token Ring network. DLSw is used to get over the 7 hop limit on the Source Routed network. DLSw is configured on all the routers in between the end routers. All the routers are called Peers. Each router maintains a table detailing the known devices on the DLSw network and thus are able to provide a path through the network using Switch to Switch protocol (SSP).

The RIF (which enforces this 7 hop limit) terminates at the peer router where a 'virtual ring' is configured. Link layer acknowledgements terminate here and do not need to go across the WAN, the local router acknowledges the locally connected terminal instead.

TCP provides more reliable transport than Fast Switched Transport (FST). FST uses sequencing just like TCP but there is no facility for error correction, however the header is smaller, and provided that the WAN is reliable can provide a speedier link. The limitation with FST is that it can only be used for Token Ring, Ethernet LANs require TCP to be used.

DLSw version 2

DLSw 2 has been detailed in RFC 2066.

With DLSw v.1 two TCP sessions were required to be active between the peers all the time whether data was being sent or not. With DLSw v.2 only one TCP session is required and then ONLY when reliable LLC2 data is being transmitted AND just for the time period of the transmission.

If reliable transport is not needed then DLSw v.2 uses UDP instead of TCP, one example being NetBIOS broadcasts.

If packets need to be sent to multiple destinations then DLSw v.2 will use multicast services to reduce network overhead.

DLSw+

DLSw+ adds peer load sharing to DLSw's IP load sharing as well as queuing (custom, priority, weighted) and CoS.

DLSw+ makes use of backup peers for better availability, the use of border peers (that cache NetBIOS servers and SNA devices), peer groups and ring lists to minimise broadcasts. Peer biasing is supported along with SNA ToS to give more flexible ways of configuring the network.

You can create Ring Lists that define which ring's traffic can go to which peer. If you do not mind all peers and rings receiving all traffic then you do not need a ring list. You can also use port lists to control where broadcasts are sent.

If you want any to any connections then you have a alot of repeated broadcast traffic. This can be reduced by creating groups of peers and allocating a border peer to handle broadcast replication. In addition, you can configure On-demand Peers that establish and pull down peer connections as and when needed without having to specifically identify each peer.

DLSw+ can operate in a compatibility mode with RSRB on Cisco routers (these protocols are both proprietary to Cisco) or a standard mode to interoperate with DLSw (on other manufacturers routers). This decision is made at the capabilities exchange.

When DLSw+ initialises it exchanges extra information to standard DLSw including border peer and group information. Although two TCP connections are established to start with, the unused one is shut down. Also whereas standard DLSw uses TCP for the encapsulation, DLSw+ can use FST and direct encapsulation as well.

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