Huawei Technologies Huawei Technologies China Telecom China Mobile Cisco System The Path Computation Element Communication Protocol (PCEP) provides mechanisms for Path Computation Elements (PCEs) to perform path computations in response to Path Computation Clients (PCCs) requests. The Stateful PCE extensions allow stateful control of Multiprotocol Label Switching (MPLS) Traffic Engineering (TE) Label Switched Paths (LSPs) using PCEP. Furthermore, PCEP can be used for computing paths in SR networks. Stateful PCE provide active control of MPLS-TE LSPs via PCEP, for a model where the PCC delegates control over one or more locally configured LSPs to the PCE. Further, stateful PCE could also create and delete PCE-initiated LSPs itself. A PCE-based central controller (PCECC) simplify the processing of a distributed control plane by integrating with elements of Software-Defined Networking (SDN). In some use cases, such as PCECC or Binding Segment Identifier (SID) for Segment Routing (SR), there are requirements for a stateful PCE to make allocation of labels, SIDs, etc. These use cases require PCE aware of various identifier spaces where to make allocations on behalf of PCC. This document describes a mechanism for PCC to inform the PCE of the identifier space under its control via PCEP. The identifier could be MPLS label, SID or any other to-be-defined identifier to be allocated by a PCE.