const bit<16> ETHERTYPE_IPV4 = 0x0800;
const bit<16> ETHERTYPE_IPV6 = 0x86DD;

const bit<8>  IPPROTO_TCP   = 0x06;
const bit<8>  IPPROTO_UDP   = 0x11;

const bit<16> FIRST_CUSTOM_HEADER = 0x1234;
const bit<8> SECOND_CUSTOM_HEADER = 0x0;
const bit<8> THIRD_CUSTOM_HEADER = 0x0;
const bit<8> FORTH_CUSTOM_HEADER = 0x0;