OPCODE_MASK
#define OPCODE(v) ((v) & OPCODE_MASK)
#define OPCODE(v) ((v) & OPCODE_MASK)
#define OPCODE(packet) ((*buffer_at((packet), 2) & OPCODE_MASK) >> OPCODE_SHIFT)
(*buffer_at((packet), 2) = (*buffer_at((packet), 2) & ~OPCODE_MASK) | ((opcode) << OPCODE_SHIFT))