1)RU Request / Response Unit,请求/应答单元2)request-response unit,请求应答单元3)last request/response unit of chain (last RU of chain),链末请求/应答单元4)first-in-chain (FIC),链中第一请求答应单元5)only RU chain,链中唯一请求应答单元6)middle request/response of chain (middle RU of chain),中间请求应答单元链
