SAS enhances RPR routing

The Resilient Packet Ring standard, IEEE 802.17, combines the reliability, service guarantees and management capabilities of SONET with the statistical efficiency of packet networks. A key new optimization of the technology, the Spatially Aware Sublayer, 802.17b, provides spatial reuse of the ring bandwidth via Layer 2 bridging.

Spatial reuse refers to the ability to send packets on only a portion of the ring circumference rather than around the entire ring, thus conserving bandwidth. By substituting simple Ethernet switches for expensive Layer 3 routers as ring elements, this new technology makes RPR an attractive solution for large companies and service providers.

The IEEE's 802.17 Working Group began the standardization process for SAS following the ratification of the RPR standard in June 2004. Progress was swift; the current draft is stable and should be ratified in the second half of this year. With the technical details largely agreed on, products are shipping today that embody the standard and provide Layer 2 bridging in RPR.

Virtual bridge

An SAS-enabled RPR ring can be thought of as a virtual Ethernet bridge, with bridge ports represented by the nodes of the ring. The function of the technology is to allow the ring nodes to learn the Ethernet addresses of the endstations connected to them and then to make switching decisions based on this knowledge. When a destination address is encountered for the first time, the packet containing that address is sent to all stations on the ring. The graphic above illustrates this process.

As each station receives a packet, it learns the Ethernet address and corresponding RPR node for the endstation that originated the packet. Subsequently, when a packet arrives at the ring with this endstation address as its destination, the ingress RPR node searches its learning table and addresses the packet to the corresponding RPR node. RPR is specified with an extended header format that allows packets to be encapsulated with addresses in the RPR domain while retaining their original Ethernet addresses, which results in a transparent Ethernet transport.

Key benefit

The key benefit this process provides is spatial reuse - stations send traffic around the ring only to the node corresponding to the packet's destination. Without this knowledge, a Layer 2 RPR ring would be forced to broadcast all packets to all destinations on the ring.

The original RPR specification primarily imagined Layer 3 routers for ring nodes, which can achieve spatial reuse by directing packets only to the next hop on their routes. But requiring routing in ring nodes precludes RPR from many cost-sensitive applications and adds complexity. SAS-enabled RPR melds the economics of Ethernet with the reliability and operations support of SONET.

With the SAS enhancement, RPR network designers no longer face the no-win proposition of achieving spatial reuse via expensive Layer 3 routers or deploying an inefficient broadcast ring. Now Layer 2 Ethernet switches or transport equipment can be easily incorporated into RPR rings, extending the viability of RPR into service provider and large corporate networks.