Extensions to current PCEP model are needed to allow a PCE to compute and instantiate Traffic Engineering paths, as well as a PCC to request a path subject to certain constraint(s) and optimization criteria in SR networks.

[RFC5440] describes Path Computation Element Protocol (PCEP) for
communication between a Path Computation Client (PCC) and a Path
Control Element (PCE) or between one a pair of PCEs. A PCE computes
paths for MPLS Traffic Engineering LSPs (MPLS-TE LSPs) based on
various constraints and optimization criteria.
[I-D.ietf-pce-stateful-pce] specifies extensions to PCEP that allow a
stateful PCE to compute and recommend network paths in compliance
with [RFC4657] and defines objects and TLVs for MPLS-TE LSPs.
Stateful PCEP extensions provide synchronization of LSP state between
a PCC and a PCE or between a pair of PCEs, delegation of LSP control,
reporting of LSP state from a PCC to a PCE, controlling the setup and
path routing of an LSP from a PCE to a PCC. Stateful PCEP extensions
are intended for an operational model in which LSPs are configured on
the PCC, and control over them is delegated to the PCE.

A mechanism to dynamically instantiate LSPs on a PCC based on the
requests from a stateful PCE or a controller using stateful PCE is
specified in [I-D.crabbe-pce-pce-initiated-lsp]. Such mechanism is
useful in Software Driven Networks (SDN) applications, such as demand
engineering, or bandwidth calendaring.

It is possible to use a stateful PCE for computing one or more SR-TE
paths taking into account various constraints and objective
functions. Once a path is chosen, the stateful PCE can instantiate
an SR-TE path on a PCC using PCEP extensions specified in
[I-D.crabbe-pce-pce-initiated-lsp] using the SR specific PCEP
extensions described in I-D.sivabalan-pce-segment-routing.
Additionally, using procedures described in this document,
a PCC can request an SR path from either stateful or a stateless PCE.
This specification uses the
PATH-SETUP-TYPE TLV and procedures specified in
[I-D.sivabalan-pce-lsp-setup-type].