Each RPR ring supports as many as 255 nodes. You can have multiple ring setups by placing nodes on two or more rings and requiring those nodes to route packets from one ring to the next.
Data traffic is placed on the resilient packet ring in one of three classes of service: high, medium or low. High-priority reserves network bandwidth that can't be used by any other traffic, even if the bandwidth is idle. Low-priority traffic can't reserve any bandwidth; it uses only unused bandwidth. Medium-priority traffic can reserve bandwidth on the network, but any unused portion is made available to other medium- or low-priority traffic. The medium setting is most appropriate for time-sensitive and bursty traffic. Bandwidth would be provisioned for the high points in the burst; during the low points, the unused bandwidth would be available to other traffic.
Although low-priority traffic can be provisioned to 100 percent of the ring bandwidth, high priority can occupy only up to 50 percent. High-priority traffic is guaranteed to be delivered even during a fiber break because RPR can use both rings concurrently. In contrast, Sonet typically uses only one ring and keeps the other for redundancy. If a break occurs, the high-priority traffic is routed to the other ring. Of course, enough bandwidth must be available, taking into account high-priority traffic that may be on the second ring.
Because low-priority traffic can occupy any unused portion of the network bandwidth, a node on the network could grab all unused bandwidth and leave nothing for the other nodes. To prevent this from happening, RPR implements a fairness routine that shares unused bandwidth between nodes.
During a fiber break or during normal operations, if medium- and low-priority traffic exceed what the node is capable of transmitting, the node will invoke Weighted Fairness, determined by a "weight" that is placed on each node. This weight specifies the priority that a particular node has in placing traffic onto the rings. Any RPR node can send fairness messages, but when congestion occurs at a node with high access to the ring, it will send a message to the other nodes on the ring to throttle back their traffic so that its traffic gets first shot at the bandwidth. When a fairness request is received, nodes sending data through the requesting node will queue or drop packets marked fairness eligible (FE) in their header. High-priority traffic is never considered for drop, so its FE value is ignored. Once a node is no longer congested, other nodes are allowed their share of available bandwidth.
Broken Ring
If a fiber break occurs, RPR offers two methods of getting traffic around the ring: Wrap and Steer. Cisco Systems and Corrigent supported Wrap, while Nortel Networks was a proponent of Steer technology. As a compromise, RPR supports both, though Steer is the default (see chart "Getting Around a Fiber Break").
Wrap is synonymous to Sonet's BLSR (bidirectional line switched ring) technology. Traffic is sent to the nodes on either side of the break, then to the opposite ring and to its destination. Wrap functions without much intelligence; traffic is simply rerouted around a break without considering its destination. Steer, however, provides intelligence at the source node. During a fiber break, the source node determines the best path to the destination and places the packets on the appropriate rings, disregarding the ring the traffic was provisioned for.
Wrap places lower requirements on the nodes but causes more traffic delays. This can be a problem for time-sensitive traffic, such as voice and video packets. Steer helps minimize delays, but it places higher CPU requirements on source nodes. Each source node must determine on which side of the break a destination lies and place the packet on the best ring. The priority of each packet and the available bandwidth also must be considered. Wrap and Steer function simultaneously on each node and are part of the provisioning process. Each connection created is specified as Wrap or Steer.
Variants
Corrigent is adding value to its products by combining RPR with MPLS. By provisioning connections end to end, the company is simplifying node-to-node processing and manual provisioning. And by supporting MPLS, Corrigent provides bandwidth management and segregates customers on metropolitan area networks. The addition of MPLS is an excellent way to provide end-to-end TDM (time-division multiplexing) service over RPR.
Because RPR is a work in progress, variations are being used by members of the RPR Alliance. For example, Luminous Networks markets its RPR as RPT (Resilient Packet Transport), a superset of RPR that includes full Stratum-level clock synchronization for TDM services. By bringing clock sync to Ethernet, TDM services like voice and video can be delivered with the same quality they would have on a Sonet network, without jitter.
Cisco also has a variant, DPT (Dynamic Packet Transport), that is supported on most of its Gigabit Ethernet and Sonet products. DPT was built around SRP (Spatial Reuse Protocol), allowing effective use of idle bandwidth. SRP is part of the final RPR specification.
All RPR vendors say they will support the final RPR specification when it's finalized in March.
Darrin Woods is a Network Computing contributing editor. Previously he was a Network Computing senior technology editor and a WAN engineer for a telecom carrier. Write to him at dwoods@nwc.com.
RSS
