[jboss-jira] [JBoss JIRA] (JGRP-1539) NAKACK2: xmit_interval sometimes triggers unneeded retransmissions

[Bela Ban (JIRA)]

    [ https://issues.jboss.org/browse/JGRP-1539?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12734936#comment-12734936 ] 

Bela Ban commented on JGRP-1539:
--------------------------------

OK, so anectotal evidence (small MPerf run with 2 nodes and 1 million 1K messages each) suggests:
- before: 263 / 164 retransmission requests (udp.xml)
- after: 4 / 3 retransmission requests (udp.xml)

- before: 118 / 0 retransmission requests (fast.xml)
- after: 0 /0 retransmission requests (fast.xml)
                
> NAKACK2: xmit_interval sometimes triggers unneeded retransmissions
> ------------------------------------------------------------------
>
>                 Key: JGRP-1539
>                 URL: https://issues.jboss.org/browse/JGRP-1539
>             Project: JGroups
>          Issue Type: Enhancement
>            Reporter: Bela Ban
>            Assignee: Bela Ban
>             Fix For: 3.3
>
>
> In NAKACK2 (and UNICAST2 as well), a timer task kicks in every xmit_interval milliseconds. If we just added a message out of order, then the retransmit task might send a spurious retransmission message. 
> Example:
> - Messages 1,2,3,4,5 are received at time T0 (in ms)
> - At time T1000, message 20 is received
> - At time T1001, the retransmit task kicks in and asks the sender for retransmission of messages [6-19]
> - At time T1009, messages 6-19 are received
> Problem: the out-of-order message 20 was received just before the retransmit task kicked in. Has the task waited for another 9 ms (until time T1009), the retransmission would not have been necessary.
> The underlying cause is that the retransmit tasks handles recently added gaps the same as gaps added before xmit_interval, as we don't maintain a timestamp for added messages.
> Another reason is also that the message bundler on the sender side might send messages in random order (see link below).
> SOLUTION:
> Investigate how to change the retransmission task such that it excludes gaps created within the last xmit_interval ms.

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