Layered system

This constraint states that the REST system can have multiple layers and if a client is requesting for a response and getting a response, it can't be differentiated if it was returned from the server or another middle-ware server. So if one server layer is replaced by an other, it doesn't affect the client unless it provides what it is expected to provide. In short, one layer doesn't have knowledge beyond the next layer with which it is interacting directly.