AT&T Constraint-based path computation is a fundamental building block for traffic engineering systems such as multiprotocol label switching (MPLS) and generalized multiprotocol label switching (GMPLS) networks. Path computation in large, multi-domain or multi-layer networks is highly complex and may require special computational components and cooperation between the different network domains. There are multiple components in the Path Computation Element (PCE)- based path computation model, including PCE discovery and the PCE communication protocol. The PCE model is described in the "PCE Architecture" document and facilitates path computation requests from Path Computation Clients (PCCs) to PCEs. This document specifies generic requirements for a communication protocol between PCCs and PCEs, and between PCEs where cooperation between PCEs is desirable. Subsequent documents will specify application-specific requirements for the PCE communication protocol.