sub new {
        my ( $caller, $arg_ref ) = @_;
        my $class = ref $caller || $caller;

        # make sure $arg_ref is a hash ref
        $arg_ref = {} unless ref($arg_ref) eq 'HASH';

        # make sure $arg_ref->{data_to_get} is an array ref
        $arg_ref->{data_to_get} = []
            unless ref( $arg_ref->{data_to_get} ) eq 'ARRAY';

        # make sure $arg_ref->{data_to_set} is a hash_ref
        $arg_ref->{data_to_set} = {}
            unless ref( $arg_ref->{data_to_set} ) eq 'HASH';

        # values from $arg_ref over-write the defaults
        my %arg = ( %fields_default, %{$arg_ref} );
        
        # A method must be specified, i.e. what type of packet is this?
        my $method = $arg{ucp_method};
        (          ( defined($method) )
                && ( exists $ucp_method_name->{$method} )
                && ( $ucp_method_name->{$method} ) )
            or do {
            croak('ucp_method invalid or not defined.');
            };

        # Set values and perform checks specific to each packet type
    SWITCH: {
            (          ( $method eq UCP_METHOD_DISCOVER )
                    or ( $method eq UCP_METHOD_ADV_DISCOVER )
                )
                && do {

                # Set values specific to discovery packets
                $arg{dst_broadcast} = BROADCAST_ON;
                $arg{dst_mac}       = MAC_ZERO;
                croak(    'Must specify IP address for '
                        . $ucp_method_name->{$method}
                        . ' msgs.' )
                    unless $arg{src_ip};
                last SWITCH;
                };

            # Mac address must be specified for all remaining method types
            if ( !defined $arg{dst_mac} ) {
                croak(    'Must specify dst_mac MAC address for '
                        . $ucp_method_name->{$method}
                        . ' msgs.' );
            }

            ( $method eq UCP_METHOD_GET_IP ) && do {

                # nothing further to do for get_ip
                last SWITCH;
            };

            ( $method eq UCP_METHOD_SET_IP ) && do {

                # The following data is required:
                #   UCP_CODE_SET_IP (0x03)
                #   IP address
                #   Netmask
                #   Gateway
                #   DHCP_ON / DHCP_OFF
                # Ought to validate the supplied data here
                # Otherwise, nothing further to do.
                last SWITCH;
            };

            ( $method eq UCP_METHOD_RESET ) && do {

                # Nothing more to do for reset method

                last SWITCH;
            };

            ( $method eq UCP_METHOD_GET_DATA ) && do {

                # Ought to validate the requested data here
                # Otherwise, nothing further to do
                last SWITCH;
            };

            ( $method eq UCP_METHOD_SET_DATA ) && do {

                # Should I validate any data here?
                last SWITCH;
            };

            # default action if ucp_method value recognised
            croak( 'Invalid ucp_method: ' . bytes_to_hex( $method, 4 ) );
        }

        my $self = bless {%arg}, $class;
        return $self;
    }

    sub packed {
        my $self = shift;

        # The first part of the msg is same for all msg types
        my $str .= $self->dst_broadcast;
        $str    .= $self->dst_type;
        $str    .= $self->dst_mac;         # mac stored packed
        $str    .= $self->src_broadcast;
        $str    .= $self->src_type;
        $str    .= $self->src_ip;
        $str    .= $self->src_port;
        $str    .= $self->seq;
        $str    .= $self->udap_type;
        $str    .= $self->ucp_flags;
        $str    .= $self->ucp_class;

        my $method = $self->ucp_method;
        $str .= $method;

    SWITCH: {
            (          ( $method eq UCP_METHOD_DISCOVER )
                    or ( $method eq UCP_METHOD_ADV_DISCOVER )
                    or ( $method eq UCP_METHOD_GET_IP )
                    or ( $method eq UCP_METHOD_RESET )
                )
                && do {
                last SWITCH;
                };
            ( $method eq UCP_METHOD_SET_IP ) && do {

                # IP Address, Netmask, Gateway
                my $dts = $self->data_to_set->{ip};
                $str .= exists $dts->{ip} ? inet_aton( $dts->{ip} ) : IP_ZERO;
                $str .=
                    exists $dts->{netmask}
                    ? inet_aton( $dts->{netmask} )
                    : IP_ZERO;
                $str .=
                    exists $dts->{gateway}
                    ? inet_aton( $dts->{gateway} )
                    : IP_ZERO;
                $str .= exists $dts->{ip} ? DHCP_OFF : DHCP_ON;
                last SWITCH;
            };
            ( $method eq UCP_METHOD_GET_DATA ) && do {

                $str .= $self->credentials;
                $str .= pack( 'n', scalar @{ $self->data_to_get } )
                    ;    # no. of data items
                foreach my $param_name ( @{ $self->data_to_get } ) {
                    if ( exists $field_offset_from_name->{$param_name} ) {
                        $str .= pack( 'n',
                            $field_offset_from_name->{$param_name} );
                        $str .= pack( 'n',
                            $field_size_from_name->{$param_name} );
                    }
                    else {
                        log( warn =>
                                "    Client param name [$param_name] not valid\n"
                        );
                    }
                }
                last SWITCH;
            };
            ( $method eq UCP_METHOD_SET_DATA ) && do {

                # set_data data is in the following format:
                #  - credentials
                #  - number of items
                #  - repeating group of:
                #    ( offset, data_length, data )
                $str .= $self->credentials;

             # no. of items is count of number of keys in data_to_set hash
                my $data = $self->data_to_set;
                $str .= pack( 'n', scalar( keys %{$data} ) );
                foreach my $pname ( keys %{$data} ) {
                    $str .= pack( 'n', $field_offset_from_name->{$pname} );
                    my $packed_data = $field_pack_from_name->{$pname}
                        ->( $data->{$pname} );
                    $str .= pack( 'n', length($packed_data) );
                    $str .= $packed_data;
                }
                last SWITCH;
            };
            ( $method eq UCP_METHOD_SET_IP ) && do {

                # set_ip data is in the following format:
                #  - ip address
                #  - subnet mask
                #  - gateway
                #  - ip mode (DHCP or static)
                my $data = $self->data_to_set;
                foreach my $fieldname (
                    qw(lan_network_address lan_subnet_mask lan_gateway lan_ip_mode)
                    )
                {
                    $str .= $field_pack_from_name->$data->{$fieldname}
                        ->( $self->data_to_set->{$fieldname} );
                }
                last SWITCH;
            };

            log(      error => '  msg method '
                    . $ucp_method_name->{$method}
                    . " not implemented\n" );
            return undef;
        }

        # print "packed msg in MessageOut.packed:\n" . HexDump( $str);
        return $str;
    }
}