A Path Computation Element (PCE) is an entity (component, application, or network node) that is capable of computing a network path or route based on a network graph and applying computational constraints. A Path Computation Client (PCC) is any client application requesting a path computation to be performed by a PCE. The Path Computation Element Protocol (PCEP) enables communications between a PCC and a PCE, or between two PCEs (defined in RFC 5440).
PCEP is a TCP-based protocol defined by the IETF PCE Working Group, and defines a set of messages and objects used to manage PCEP sessions and to request and send paths for multidomain traffic engineered LSPs (TE LSPs). It provides a mechanism for a PCE to perform path computation for a PCC’s external LSPs. The PCEP interactions include LSP status reports sent by the PCC to the PCE, and PCE updates for the external LSPs.
Figure 1 illustrates the role of PCEP in the client-side implementation of a stateful PCE architecture in an MPLS RSVP-TE enabled network.
A TCP-based PCEP session connects a PCC to an external PCE. The PCC initiates the PCEP session and stays connected to the PCE for the duration of the PCEP session. During the PCEP session, the PCC requests LSP parameters from the stateful PCE. On receiving one or more LSP parameters from the PCE, the PCC re-signals the TE LSP. When the PCEP session is terminated, the underlying TCP connection is closed immediately, and the PCC attempts to re-establish the PCEP session.
Thus, the PCEP functions include:
LSP tunnel state synchronization between a PCC and a stateful PCE—When an active stateful PCE connection is detected, a PCC tries to delegate all LSPs to this PCE in a procedure called LSP state synchronization. PCEP enables synchronization of the PCC LSP state to the PCE.
Delegation of control over LSP tunnels to a stateful PCE—An active stateful PCE controls one or more LSP attributes for computing paths, such as bandwidth, path (ERO), and priority (setup and hold). PCEP enables such delegation of LSPs for path computation.
Stateful PCE control of timing and sequence of path computations within and across PCEP sessions–An active stateful PCE modifies one or more LSP attributes, such as bandwidth, path (ERO), and priority (setup and hold). PCEP communicates these new LSP attributes from the PCE to the PCC, after which the PCC re-signals the LSP in the specified path.