header fields for udp checksum

udp_checksum

header_type udp_t {
    fields {
        srcPort : 16;
        dstPort : 16;
        length_ : 16;
        checksum : 16;
    }
}

header udp_t udp;

field_list udp_checksum_list {
    ipv4.srcAddr;
    ipv4.dstAddr;
    8'0;
    ipv4.protocol;
    udp.length_;
    udp.srcPort;
    udp.dstPort;
    payload;
}

field_list_calculation udp_checksum {
    input {
        udp_checksum_list;
    }
    algorithm : csum16;
    output_width : 16;
}

calculated_field udp.checksum {
    update udp_checksum if (valid(udp));
}

header_type custom_head_t {
   fields {
      round : 2;
   }
}

header custom_head_t custom_header;

action mod_payload() {
    add_to_field(custom_header.round, 1);
}