MCast Component

Properties   Methods   Events   Config Settings   Errors  

The MCast component is used to multicast User Datagram Protocol (UDP) datagrams to multicast-capable hosts in multicast-capable subnets. It can act both as a client and a server and can communicate with any number of hosts simultaneously, as well as generate and receive broadcast packets.

Syntax

TipwMCast

Remarks

The MCast component is very similar to UDP, except for some additional properties that provide support for multicast datagrams. To send datagrams to a multicast group, simply assign the address of the group to RemoteHost. To receive datagrams from one or more groups, assign the group addresses to MulticastGroup one by one (after the component is activated). TimeToLive allows you to specify how many subnets a packet should be allowed to receive. ReceiveInterface and SendInterface allow you to select the network interface for multicast operations.

The component is activated/deactivated through the Active property. This property enables or disables sends or receives. The data can be sent in the same way as UDP, by calling the Send method with a Text parameter. The destination is specified using the RemoteHost and RemotePort properties. Inbound data are received through the DataIn event.

Property List

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The following is the full list of the properties of the component with short descriptions. Click on the links for further details.

Method List

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The following is the full list of the methods of the component with short descriptions. Click on the links for further details.

Event List

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The following is the full list of the events fired by the component with short descriptions. Click on the links for further details.

Config Settings

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The following is a list of config settings for the component with short descriptions. Click on the links for further details.

AcceptData Property (MCast Component)

This property indicates whether data reception is currently enabled.

Syntax

property AcceptData: Boolean read get_AcceptData;

Default Value

true

Remarks

This property indicates whether data reception is currently enabled. When False, data reception is disabled and the DataIn event will not fire. Use the PauseData and ProcessData methods to pause and resume data reception.

This property is read-only and not available at design time.

Active Property (MCast Component)

This property indicates whether the component is active.

Syntax

property Active: Boolean read get_Active;

Default Value

false

Remarks

This property indicates whether the component is currently active and can send or receive data. Use the Activate and Deactivate methods to control whether the component is active.

This property is read-only and not available at design time.

LocalHost Property (MCast Component)

The name of the local host or user-assigned IP interface through which connections are initiated or accepted.

Syntax

property LocalHost: String read get_LocalHost write set_LocalHost;

Default Value

''

Remarks

This property contains the name of the local host as obtained by the gethostname() system call, or if the user has assigned an IP address, the value of that address.

In multihomed hosts (machines with more than one IP interface) setting LocalHost to the IP address of an interface will make the component initiate connections (or accept in the case of server components) only through that interface. It is recommended to provide an IP address rather than a hostname when setting this property to ensure the desired interface is used.

If the component is connected, the LocalHost property shows the IP address of the interface through which the connection is made in internet dotted format (aaa.bbb.ccc.ddd). In most cases, this is the address of the local host, except for multihomed hosts (machines with more than one IP interface).

Note: LocalHost is not persistent. You must always set it in code, and never in the property window.

LocalPort Property (MCast Component)

This property includes the User Datagram Protocol (UDP) port in the local host where MCast binds.

Syntax

property LocalPort: Integer read get_LocalPort write set_LocalPort;

Default Value

0

Remarks

This property must be set before MCast is activated (Active is set to True). It instructs the component to bind to a specific port (or communication endpoint) on the local machine.

Setting it to 0 (default) enables the system to choose a port at random. The chosen port will be shown by this property after the connection is established.

This property cannot be changed once the component is Active. Any attempt to set this property when the component is Active will generate an error.

This property is useful when trying to send data to services that require a trusted port on the client side.

Loopback Property (MCast Component)

This property enables or disables loopback of data sent to the multicast group.

Syntax

property Loopback: Boolean read get_Loopback write set_Loopback;

Default Value

true

Remarks

This property enables or disables loopback of data sent to the multicast group. Setting this property to False disables loopback of data sent through the component (normally loopback is enabled). This property is directly related to the IP_MULTICAST_LOOP socket option.

This property is not available at design time.

MulticastGroup Property (MCast Component)

This property includes the multicast group to join or leave.

Syntax

property MulticastGroup: String read get_MulticastGroup write set_MulticastGroup;

Default Value

''

Remarks

This property contains the multicast group to join or leave. Multicast groups are joined by assigning values to this property after the component is activated. More than one group may be joined at the same time. To define which interface to use, the ReceiveInterface property is used.

Determining whether a group is left or joined is done by setting the first letter of MulticastGroup to '-' or '+'. The default operation is join.

To remove the component from a multicast group once it is joined, simply assign this groupname (address) to MulticastGroup prefixed by a minus sign '-'.

Example. Joining/Leaving Groups:

MCastControl.Active = true MCastControl.MulticastGroup = "+231.31.31.31" MCastControl.MulticastGroup = "+232.32.32.32" MCastControl.MulticastGroup = "-232.32.32.32" MCastControl.MulticastGroup = "-231.31.31.31"

ReceiveInterface Property (MCast Component)

This property includes the interface with which to receive multicast packets. If empty, the default is used.

Syntax

property ReceiveInterface: String read get_ReceiveInterface write set_ReceiveInterface;

Default Value

''

Remarks

This property is useful when there is more than one network interface, and the one used for receiving multicast datagrams is not the default. If you wish to set an interface other than the system default, provide the IP address of the interface to be set.

This property is used along with the MulticastGroup property to join one or more multicast groups. The joining is done after the component is activated by assigning proper values to MulticastGroup.

This property is not available at design time.

RemoteHost Property (MCast Component)

This property includes the address of the remote host (multicast group).

Syntax

property RemoteHost: String read get_RemoteHost write set_RemoteHost;

Default Value

''

Remarks

This property contains the address of the remote host (multicast group). To send a multicast datagram, specify a RemoteHost value in the range of 224.0.0.0 to 239.255.255.255. These addresses are reserved as multicast addresses by IP.

RemotePort Property (MCast Component)

This property includes the User Datagram Protocol (UDP) port on the remote host.

Syntax

property RemotePort: Integer read get_RemotePort write set_RemotePort;

Default Value

0

Remarks

This property contains the UDP port on the RemoteHost to send UDP datagrams to.

A valid port number (i.e., a value between 1 and 65535) is required.

SendInterface Property (MCast Component)

This property includes the interface from which multicast packets are sent. If empty, the default is used.

Syntax

property SendInterface: String read get_SendInterface write set_SendInterface;

Default Value

''

Remarks

This property is useful when there is more than one network interface, and the one used for sending multicast datagrams is not the default. If you wish to set an interface other than the system default, provide the IP address of the interface to be set.

This property is not available at design time.

TimeToLive Property (MCast Component)

This property includes the maximum number of subnets that a multicast datagram can traverse.

Syntax

property TimeToLive: Integer read get_TimeToLive write set_TimeToLive;

Default Value

7

Remarks

This property contains the maximum number of subnets that a multicast datagram can traverse. The default value for this property is 7. This is an arbitrary value. You need to specify a value suitable for your application.

Multicast datagrams with this property set to 0 are not transmitted on any subnetwork. Multicast datagrams with this property set to 1 are delivered only on the local network. Multicast datagrams with this property set to greater than 1 may be delivered to more than one subnetwork if one or more multicast routers are attached to the first-hop subnetwork.

A multicast router does not forward multicast datagrams with destination addresses between 224.0.0.0 and 224.0.0.255, inclusive, regardless of their TimeToLive. This particular range of addresses is reserved for the use of routing protocols and other low-level topology discovery or maintenance protocols, such as gateway discovery and group membership reporting.

Activate Method (MCast Component)

This method enables sending and receiving of data.

Syntax

procedure Activate();

Remarks

This method enables sending and receiving of data. When called, the component will create a communication endpoint (socket) that can be used for sending and receiving User Datagram Protocol (UDP) datagrams.

Config Method (MCast Component)

Sets or retrieves a configuration setting.

Syntax

function Config(ConfigurationString: String): String;

Remarks

Config is a generic method available in every component. It is used to set and retrieve configuration settings for the component.

These settings are similar in functionality to properties, but they are rarely used. In order to avoid "polluting" the property namespace of the component, access to these internal properties is provided through the Config method.

To set a configuration setting named PROPERTY, you must call Config("PROPERTY=VALUE"), where VALUE is the value of the setting expressed as a string. For boolean values, use the strings "True", "False", "0", "1", "Yes", or "No" (case does not matter).

To read (query) the value of a configuration setting, you must call Config("PROPERTY"). The value will be returned as a string.

Deactivate Method (MCast Component)

This method disables sending and receiving of data.

Syntax

procedure Deactivate();

Remarks

This method disables sending and receiving of data. When called, the component will destroy the existing socket and disable data communications.

DoEvents Method (MCast Component)

This method processes events from the internal message queue.

Syntax

procedure DoEvents();

Remarks

When DoEvents is called, the component processes any available events. If no events are available, it waits for a preset period of time, and then returns.

PauseData Method (MCast Component)

This method pauses data reception.

Syntax

procedure PauseData();

Remarks

This method pauses data reception when called. While data reception is paused, the DataIn event will not fire. Call ProcessData to reenable data reception.

ProcessData Method (MCast Component)

This method reenables data reception after a call to PauseData .

Syntax

procedure ProcessData();

Remarks

This method reenables data reception after a previous call to PauseData. When PauseData is called, the DataIn event will not fire. To reenable data reception and allow DataIn to fire, call this method.

Note: This method is used only after previously calling PauseData. It does not need to be called to process incoming data by default.

Reset Method (MCast Component)

This method resets the component.

Syntax

procedure Reset();

Remarks

This method will reset the component's properties to their default values.

ResolveRemoteHost Method (MCast Component)

This method resolves the hostname in RemoteHost to an IP address.

Syntax

procedure ResolveRemoteHost();

Remarks

This method resolves the hostname specified by RemoteHost to an IP address. The resolved value is available in the RemoteHost property after this method returns.

In most cases, calling this method is not necessary; the component will resolve the hostname automatically when needed. If DelayHostResolution is True, this method may be called to manually resolve RemoteHost, if desired.

Send Method (MCast Component)

This method sends data to the remote host.

Syntax

procedure Send(Text: TBytes);

Remarks

This method sends data to the remote host.

SendBytes Method (MCast Component)

This method sends data to the remote host.

Syntax

procedure SendBytes(Data: TBytes);

Remarks

This method sends data to the remote host. Calling this method is equivalent to calling the SendBytes/SendText method.

SendText Method (MCast Component)

This method sends data to the remote host.

Syntax

procedure SendText(Text: String);

Remarks

This method sends data to the remote host. Calling this method is equivalent to calling the SendBytes/SendText method.

DataIn Event (MCast Component)

This event is fired when data are received.

Syntax

type TDataInEvent = procedure (
  Sender: TObject;
  const Datagram: String;
  const DatagramB: TBytes;
  const SourceAddress: String;
  SourcePort: Integer
) of Object;

property OnDataIn: TDataInEvent read FOnDataIn write FOnDataIn;

Remarks

The DataIn event is fired every time a new datagram is received.

Datagram contains the packet as sent by the remote host.

SourceAddress contains the IP number (Internet address) of the remote host, and SourcePort contains the port from which the packet originated.

Note: Events are not re-entrant. Performing time-consuming operations within this event will prevent it from firing again in a timely manner and may affect overall performance.

Error Event (MCast Component)

Fired when information is available about errors during data delivery.

Syntax

type TErrorEvent = procedure (
  Sender: TObject;
  ErrorCode: Integer;
  const Description: String
) of Object;

property OnError: TErrorEvent read FOnError write FOnError;

Remarks

The Error event is fired in case of exceptional conditions during message processing. Normally the component raises an exception.

The ErrorCode parameter contains an error code, and the Description parameter contains a textual description of the error. For a list of valid error codes and their descriptions, please refer to the Error Codes section.

ReadyToSend Event (MCast Component)

This event is fired when the component is ready to send data.

Syntax

type TReadyToSendEvent = procedure (
  Sender: TObject
) of Object;

property OnReadyToSend: TReadyToSendEvent read FOnReadyToSend write FOnReadyToSend;

Remarks

The ReadyToSend event indicates that the underlying Transmission Control Protocol (TCP)/IP subsystem is ready to accept data after a failed Send.

Config Settings (MCast Component)

The component accepts one or more of the following configuration settings. Configuration settings are similar in functionality to properties, but they are rarely used. In order to avoid "polluting" the property namespace of the component, access to these internal properties is provided through the Config method.

UDP Config Settings

Socket Config Settings

Base Config Settings

Trappable Errors (MCast Component)

MCast Errors

SSL Errors

TCP/IP Errors