Portable Bit Fields in packetC
Ralph Duncan, Peder Jungck, Dwight Mulcahy · Apress eBooks · 2011
Network packets place some protocol data in bit fields that are smaller than typical processor operand sizes. C language structures can represent such protocols but the uncertain layout and endian-specific nature of C’s bit fields cause problems. Research has ranged from alternative bit field constructs, through specialized bit registers to using analytic techniques to identify programs’ implicit subword usage. This paper describes the packetC language’s two-fold approach to handling protocols in a portable way. The language addresses bit field layout and operation uncertainties as language design matters that can be overcome with a container -oriented approach and unambiguous layout rules. It tackles the problems of endianness and of packet bit field processing by two means. On the language design level packetC imposes big endian byte allocation order for structure and packet array storage. Second, the language is built around a packet processing model that involves triggering a parallel copy of a program after the host system assembles the entire packet in a byte array, locates standard protocols within that packet and saves protocol location information. By providing both portable protocol representation and protocol layer offset calculation, packetC frees engineering resources to pursue other packet processing tasks.