main.p4

header_type my_header_t {
    fields {
        a: 8;
        b: 8;
    }
}

header my_header_t header_a;
header my_header_t header_b;

parser start {
    return header_b;
}

parser header_b {
    return header_a;
}

parser header_a {
    extract(header_a);
    return ingress;
}

table table1 {
    actions {
        action_add_header_and_forward;
    }
}

action action_add_header_and_forward() {
    add_header(header_b);
    modify_field(header_b.a, 0x10);
    modify_field(header_b.b, 0x20);
    modify_field(standard_metadata.egress_spec, 1);
}

control ingress {
    apply(table1);
}