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19.3.1. UDP configuration

UDP is the preferred protocol for JGroups. UDP uses multicast or multiple unicasts to send and receive messages. If you choose UDP as the transport protocol for your cluster service, you need to configure it in the UDP sub-element in the JGroups Config element. Here is an example.

<UDP mcast_addr="${jboss.partition.udpGroup:}" 
 down_thread="false" up_thread="false"/>

The available attributes in the above JGroups configuration are listed below.

  • ip_mcast specifies whether or not to use IP multicasting. The default is true. If set to false, it will send n unicast packets rather than 1 multicast packet. Either way, packets are UDP datagrams.

  • mcast_addr specifies the multicast address (class D) for joining a group (i.e., the cluster). If omitted, the default is .

  • mcast_port specifies the multicast port number. If omitted, the default is 45566.

  • bind_addr specifies the interface on which to receive and send multicasts (uses the -Djgroups.bind_address system property, if present). If you have a multihomed machine, set the bind_addr attribute or system property to the appropriate NIC IP address. By default, system property setting takes priority over XML attribute unless -Djgroups.ignore.bind_addr system property is set.

  • receive_on_all_interfaces specifies whether this node should listen on all interfaces for multicasts. The default is false. It overrides the bind_addr property for receiving multicasts. However, bind_addr (if set) is still used to send multicasts.

  • send_on_all_interfaces specifies whether this node send UDP packets via all the NICs if you have a multi NIC machine. This means that the same multicast message is sent N times, so use with care.

  • receive_interfaces specifies a list of of interfaces to receive multicasts on. The multicast receive socket will listen on all of these interfaces. This is a comma-separated list of IP addresses or interface names. E.g. ",eth1,".

  • ip_ttl specifies time-to-live for IP Multicast packets. TTL is the commonly used term in multicast networking, but is actually something of a misnomer, since the value here refers to how many network hops a packet will be allowed to travel before networking equipment will drop it.

  • use_incoming_packet_handler specifies whether to use a separate thread to process incoming messages. Sometimes receivers are overloaded (they have to handle de-serialization etc). Packet handler is a separate thread taking care of de-serialization, receiver thread(s) simply put packet in queue and return immediately. Setting this to true adds one more thread. The default is true.

  • use_outgoing_packet_handler specifies whether to use a separate thread to process outgoing messages. The default is false.

  • enable_bundling specifies whether to enable message bundling. If it is true, the node would queue outgoing messages until max_bundle_size bytes have accumulated, or max_bundle_time milliseconds have elapsed, whichever occurs first. Then bundle queued messages into a large message and send it. The messages are unbundled at the receiver. The default is false.

  • loopback specifies whether to loop outgoing message back up the stack. In unicast mode, the messages are sent to self. In mcast mode, a copy of the mcast message is sent. The default is false

  • discard_incompatibe_packets specifies whether to discard packets from different JGroups versions. Each message in the cluster is tagged with a JGroups version. When a message from a different version of JGroups is received, it will be discarded if set to true, otherwise a warning will be logged. The default is false

  • mcast_send_buf_size, mcast_recv_buf_size, ucast_send_buf_size, ucast_recv_buf_size define receive and send buffer sizes. It is good to have a large receiver buffer size, so packets are less likely to get dropped due to buffer overflow.

  • tos specifies traffic class for sending unicast and multicast datagrams.


On Windows 2000 machines, because of the media sense feature being broken with multicast (even after disabling media sense), you need to set the UDP protocol's loopback attribute to true.