It is a link layer protocol for ring-based packet internetworking that is commonly used in optical fiber ring networks.
Ideas from the protocol are reflected in parts of the IEEE 802.17 Resilient Packet Ring (RPR) standard.
SRP behaves quite like the Point-to-Point Protocol (PPP) does in a Packet Over SONET (POS) environment.
This means that routers F and E can be sending data at 2.5 Gbit/s between each other concurrently, resulting in the total throughput of the ring being 5 Gbit/s.
These fields are as follows: Time to Live (TTL), Ring Identifier (R), Priority (PRI), Mode, and Parity (P).