[jboss-jira] [JBoss JIRA] Commented: (JGRP-952) MERGE: UNICAST can lose messages on merging
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https://jira.jboss.org/jira/browse/JGRP-952?page=com.atlassian.jira.plugi...
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Bela Ban commented on JGRP-952:
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Addition to last comment: if we lose an ack, we have to make sure to ack subsequent
messages, e.g. as follows:
- A send #4 and #5 to B
- #4 is dropped
- B receives #5, adds it to its receiver window and doesn't ack it
- B receives #4, adds it to the receiver window for A
- B removes #4 acks it to A
- This ack is lost !
- B receives #4 again as a result of A retransmitting it
--> Because #4 has already been removed from A's receiver window at B, we'd
never ack it and A would continue resending #4 !
==> SOLUTION: when a message is received, ack it only if its seqno is lower than the
next seqno to be removed from B's receiver window at A. Otherwise, ack only on
remove() !
So in the example above, the spurious retransmission of #4 would get acked because
AckReceiverWindow.remove() set the next seqno to be removed to #5
MERGE: UNICAST can lose messages on merging
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Key: JGRP-952
URL: https://jira.jboss.org/jira/browse/JGRP-952
Project: JGroups
Issue Type: Bug
Reporter: Bela Ban
Assignee: Bela Ban
Fix For: 2.6.10, 2.8
The following use case loses messages:
- A sends #5 to B
- B expects #4 from A, but adds #5 and acks it
- A receives the ack(#5) and removes #5 from its sender window
- Now there is a partition such that B trashes its connection window form A, but A keeps
its window for B (A: {A,B}, B: {B})
- The partition heals and A sends #6 to B
- B asks A for its lowest seqno, A resends #4 (with a conn_id)
- B creates a receiver window for A with seqno=#4
- A resends #6
- B adds #6 to its window, but doesn't deliver it because it is missing #5
--> However, A will NEVER resend #5 because the ack(#5) from B removed #5 from A's
sender window !
==> Possible SOLUTION: when A gets the SEND_FIRST_SEQNO and there are (unacked)
messages in A's sender window, and they are not in order, then A will trash its
connection window and copy the pending messages into the new window (with new seqnos !)
before sending #1 (with conn_id)
==> This solution might lose the original message #5, but that's better than B
never being able to deliver any messages anymore !
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