Packet Scale Rate Guarantee (PSRG) is a generic node model which underlies the definition of Expedited Forwarding (EF) proposed in the context of Internet Differentiated Services. For the case of FIFO nodes, PSRG is equivalent to the well-understood concept of adaptive service curve. However, in practice, many devices do not necessarily preserve the FIFO property, and therefore known FIFO results do not hold. This paper analyzes the properties of PSRG in the absence of FIFO assumption. Our analysis is based on a novel characterization of PSRG which avoids the use of virtual finish times; it is obtained by min-max algebra. We use it to show that delay bounds previously obtained for the FIFO case are still valid; this is not true for the characterization of the concatenation of two nodes.