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22.6.2. NAKACK

The NAKACK protocol is used for multicast messages. It uses NAK. Under this protocol, each message is tagged with a sequence number. The receiver keeps track of the sequence numbers and deliver the messages in order. When a gap in the sequence number is detected, the receiver asks the sender to retransmit the missing message. The NAKACK protocol is configured as the pbcast.NAKACK sub-element under the JGroups Config element. Here is an example configuration.

<pbcast.NAKACK max_xmit_size="60000" use_mcast_xmit="false" 
   retransmit_timeout="300,600,1200,2400,4800" gc_lag="0"
   down_thread="false" up_thread="false"/>

The configurable attributes in the pbcast.NAKACK element are as follows.

  • retransmit_timeout specifies the retransmission timeout (in milliseconds). It is the same as the timeout attribute in the UNICAST protocol.

  • use_mcast_xmit determines whether the sender should send the retransmission to the entire cluster rather than just the node requesting it. This is useful when the sender drops the pocket -- so we do not need to retransmit for each node.

  • max_xmit_size specifies maximum size for a bundled retransmission, if multiple packets are reported missing.

  • discard_delivered_msgs specifies whether to discard delivery messages on the receiver nodes. By default, we save all delivered messages. However, if we only ask the sender to resend their messages, we can enable this option and discard delivered messages.

  • gc_lag specifies the number of messages garbage collection lags behind.