[jboss-jira] [JBoss JIRA] (JGRP-1548) UNICAST2: send STABLE message after 'last received' message

[Bela Ban (JIRA)]

Bela Ban created JGRP-1548:
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             Summary: UNICAST2: send STABLE message after 'last received' message
                 Key: JGRP-1548
                 URL: https://issues.jboss.org/browse/JGRP-1548
             Project: JGroups
          Issue Type: Feature Request
            Reporter: Bela Ban
            Assignee: Bela Ban
             Fix For: 3.3


Contrary to UNICAST, which acks every message, UNICAST2 never acks messages, but only asks the sender to retransmit a message when a gap has been detected.
However, the drawback of this (negative ack) mechanism is the so called last-message-dropped problem: when A sends messages [1..5] to B, but 5 is dropped by the transport, as A doesn't retransmit messages until it gets an ack, B only gets messages [1..4].
B will *not* ask A to retransmit message 5, as B doesn't know A *sent* message 5 in the first place.
If A doesn't send message 6 for B to detect 5 is missing and asking A for retransmission, B won't get that message.
The way this is currently handled is with stable messages. A STABLE message is sent from B to A every stable_interval ms or whenever M bytes from A have been received. In the worst case, B will have to wait stable_interval ms until it finally receives message 5 from A.

SOLUTION:
In addition to time and size based STABLE messages, we could send a STABLE message whenever the batch of messages removed from the receive window has completed and the receive window is empty. 

This would send a STABLE message immediately when a single message has been received (and no other messages from A are in the receive window), but it would only send another STABLE message only when all (e.g.) 200 messages from A have been processed and the receive window is empty.

With this new mechanism, we could even remove the time-based STABLE messages !

Example:
- At time T0, messages M1 and M2 are received. A STABLE message for M2 is sent.
- At T+500 (ms), messages M3-M100 are received. A STABLE message for M100 is sent
- At T+1500, M101 is received. A STABLE message for M101 is sent.
- At T+2000, M102 is received. A STABLE message for M102 is sent.
- At T+2010, M103-M500 are received. A STABLE message form M500 is sent

This is similar to the ACK based scheme in UNICAST where we only send an ack for the last message in a batch (or for a single message if not batch has been received).

This new mechanism needs to be configurable: if enabled, the time-based STABLE mechanism would be disabled.

If the 

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