Efficient computation of packet CRC from partial CRCs with application to the Cells-In-Frames protocol

Abstract

A Cyclic Redundancy Check (CRC) code is used by many communications protocols for packet error detection. Computation of CRC for an entire packet is easily implemented in hardware if packets are transmitted and received in contiguous form. In an ATM network, packets are fragmented into cells and, in the case of multiple virtual circuits, their transmission is overlapped, resulting in non-contiguous packets. For non-contiguous packets with a 32-bit CRC (CRC-32), as in the case of ATM Adaptation Layer 5 (AAL5), an efficient algorithm for computing the CRC for an entire packet based on combining packet fragment CRCs (e.g., cell CRCs) is developed in this paper. In this algorithm, the network hardware generates cell or partial packet CRCs and the host system software combines these CRCs into a full packet CRC. Several important properties for CRCs are described and proved, and the correctness of the algorithm developed from these properties is then also formally proved. The algorithm has direct application to the proposed Cells-In-Frames (CIF) architecture for support of ATM AAL5 services on Ethernet. When implemented in software, the algorithm is shown to be significantly faster than a table-based software computation of packet CRC-32. The algorithm is also applicable to networking devices that need to change the contents of a packet and quickly recompute the packet CRC based only on the changed portions of the packet.