This document is about relay's architecture and protocol. It first introduces the overall EPS network architecture and functions, including both the core network and the access network. The structure of LTE transmissions and the protocol architecture for the user plane as well as the control planes are discussed. Then the document gives a general overview of relays. Generally, relays can be classified into 2 types, type-1 relay and type-2 relay. And depending on the spectrum used for the access link and backhaul link, type-1 relay can be further distinguished into inband relay and outband relay. This document focuses on the inband relays that have been standardized, including subframe enhancements, the user plane protocol architecture and the control plane protocol architecture for relay. Lastly, some procedures related to relay, such as relay startup procedure, embedded UE attach procedure and handover procedure, are addressed.