Spatial Reuse Protocol
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Overview
Spatial Reuse Protocol is a Cisco-developed MAC-layer protocol for ring-based packet internetworking, submitted to the IEEE 802.17 Resilient Packet Ring (RPR) Working Group for consideration as a standard.
Introduction
SRP was first developed as a layer 2 (data-link layer) protocol to link Cisco's Dynamic Packet Transport (DPT) protocol (a method of delivering packet-based traffic over a SONET/SDH infrastructure) to the physical SONET/SDH layer. DPT is a layer 3 (network layer) protocol, therefore it was necessary to develop an intermediate layer between DPT and SONET/SDH, SRP filled this role.
Analogy to POS
SRP behaves quite like the Point to Point Protocol (PPP) does in a Packet Over SONET (POS) environment. PPP acts an an abstraction layer between a layer 3 technology such as POS and a layer 1 technology such as SONET/SDH. Layer 1 and layer 3 protocols cannot interact directly without having an intermediate layer 2 protocol, in the case of DPT the layer 2 protocol is SRP.
Spatial Reuse Capability
DPT environments contain dual, counter-rotating rings, somewhat like FDDI. However, no tokens are used in DPT, it is a shared-medium technology. SRP has a unique bandwidth effeciency mechanism which allows multiple nodes on the ring to utilize the entirity of its bandwidth, this mechanism is called the Spatial Reuse Capability. Nodes in an SRP environment can send data directly from source to destination. Consider the following ring environment (for example, running at OC-48c [2.5 Gbps]) with 6 routers (A through F sequentially) on it. Routers A and D are sending data back and forth at 1.5 Gbps while routers B and C are sending data at 1 Gbps, this utilizes the entire 2.5 Gbps across routers A through D but still leaves routers F and E untouched. This means that routers F and E can be sending data at 2.5 Gbps between each other concurrently, resulting in the total throughput of the ring being 5 Gbps.
SRP Header
The SRP header is 16 bits (2 bytes) total. It contains 5 fields. These fields are as follows: Time to Live (TTL), Ring Identifier (R), Priority (PRI), Mode, and Parity (P). The TTL field is 8 bits, its only metric is hop count. The R field is 1 bit (either 0 or 1 designating the inner or outer ring). The PRI field is 3 bits designating the packet priority. The Mode field is 3 bits designating what type of data is contained in the payload. The P field is 1 bit.