RPR technology uses a dual counter rotating fiber ring topology. Both rings (inner and outer) are used to transport working traffic between nodes. By utilizing both fibers, instead of keeping a spare fiber for protection, RPR utilizes the total available ring bandwidth. These fibers or ringlets are also used to carry control (topology updates, protection, and bandwidth control) messages. Control messages flow in the opposite direction of the traffic that they represent. For instance, outer-ring traffic-control information is carried on the inner ring to upstream nodes.

Figure 1. RPR Terminology

By using bandwidth-control messages, a RPR node can dynamically negotiate for bandwidth with the other nodes on the ring. RPR has the ability to differentiate between low- and high-priority packets. Just like other quality of service (QoS)–aware systems, nodes have the ability to transmit high-priority packets before those of low priority. In addition, RPR nodes also have a transit path, through which packets destined to downstream nodes on the ring flow. With a transit buffer capable of holding multiple packets, RPR nodes have the ability to transmit higher-priority packets while temporarily holding other lower-priority packets in the transit buffer. Nodes with smaller transit buffers can use bandwidth-control messages to ensure that bandwidth reserved for high-priority services stays available.

The RPR MAC

As a Layer-2 network protocol, the MAC layer contains much of the functionality for the RPR network. The RPR MAC is responsible for providing access to the fiber media. The RPR MAC can receive, transit, and transmit packets.

Figure 2. MAC Block Diagram

Receive Decision – Every station has a 48-bit MAC address. The MAC will receive any packets with a matching destination address. The MAC can receive both unicast and multicast packets. Multicast packets are copied to the host and allowed to continue through the transit path. Matching unicast packets are stripped from the ring and do not consume bandwidth on downstream spans. There are also control packets that are meant for the neighboring node; these packets do not need a destination or source address.

Transit Path – Nodes with a non-matching destination address are allowed to continue circulating around the ring. Unlike point-to-point protocols such as Ethernet, RPR packets undergo minimal processing per hop on a ring. RPR packets are only inspected for a matching address and header errors (TTL=0, Parity, CRC).

Transmit and Bandwidth Control – The RPR MAC can transmit both high- and low-priority packets. The bandwidth algorithm controls whether a node is within its negotiated bandwidth allotment for low-priority packets. High-priority packets are not subject to the bandwidth-control algorithm.

The bandwidth-control algorithm dynamically negotiates available bandwidth between the nodes on the ring. This applies only to the low-priority service. It ensures that nodes will not be disadvantaged because of ring location or changing traffic patterns. The algorithm only manages congestion, enabling nodes to maximize the use of any spare capacity. Nodes can be inserted or removed from the ring without any bandwidth provisioning by the host.

Topology Discovery

RPR has a topology discovery mechanism that allows nodes on the ring to be inserted/removed without manual management intervention. After a node joins a ring, it will circulate a topology discovery message to learn the MAC addresses of the other stations. Nodes also send these messages periodically (1 to 10 seconds). Each node that receives a topology message appends its MAC address and passes it to its neighbor. Eventually, the packet returns to its source with a topology map (list of addresses) of the ring.

Routers are able to use the address resolution protocol (ARP) mechanism to determine which RPR MAC address belongs to the destination address of an IP packet. RPR switches and bridges will have a list of stations that they can reach through a RPR MAC address. The topology map will be used to determine which direction on the ring will provide the best path to the destination.

Protection

RPR has the ability to protect the network from single span (node or fiber) failures. When a failure occurs, protection messages are quickly dispatched. RPR has two protection mechanisms:

Wrapping – Nodes neighboring the failed span will direct packets away from the failure by wrapping traffic around to the other fiber (ringlet). This mechanism requires that only two nodes participate in the protection event. Other nodes on the ring can send traffic as normal.

Figure 3. Wrapped Traffic Flow

Steering – The protection mechanism notifies all nodes on the ring of the failed span. Every node on the ring will adjust their topology maps to avoid this span.

Regardless of the protection mechanism used, the ring will be protected within 50 ms.

Physical Layer

RPR packets can be transported over both SONET and Ethernet physical layers. The SONET/SDH physical layer offers robust error and performance monitoring. RPR packets can be encapsulated within the synchronous payload envelope (SPE) using a high-level data-link control (HDLC)–like or generic framing protocol (GFP) encapsulation. A robust Layer-1 protocol, SONET/SDH provides information such as loss of signal and signal degrade for use by the RPR protection mechanism. When using a SONET/SDH physical layer, RPR can be carried over SONET/SDH TDM transport or dark fiber.

Ethernet provides an economical physical layer for RPR networks. RPR packets are transmitted with the required inter-packet gap (IPG).

RPR Systems using the SONET physical layer will not interoperate with Ethernet physical-layer-based systems on the same ring.