JBoss.org Community Documentation Why is FC needed on top of TCP ? TCP has its own flow control !

The reason is group communication, where we essentially have to send group messages at the highest speed the slowest receiver can keep up with. Let's say we have a cluster {A,B,C,D}. D is slow (maybe overloaded), the rest is fast. When A sends a group message, it establishes TCP connections A-A (conceptually), A-B, A-C and A-D (if they don't yet exist). So let's say A sends 100 million messages to the cluster. Because TCP's flow control only applies to A-B, A-C and A-D, but not to A-{B,C,D}, where {B,C,D} is the group, it is possible that A, B and C receive the 100M, but D only received 1M messages. (BTW: this is also the reason why we need NAKACK, although TCP does its own retransmission).

Now JGroups has to buffer all messages in memory for the case when the original sender S dies and a node asks for retransmission of a message of S. Because all members buffer all messages they received, they need to purge stable messages (= messages seen by everyone) every now and then. This is done by the STABLE protocol, which can be configured to run the stability protocol round time based (e.g. every 50s) or size based (whenever 400K data has been received).

In the above case, the slow node D will prevent the group from purging messages above 1M, so every member will buffer 99M messages ! This in most cases leads to OOM exceptions. Note that - although the sliding window protocol in TCP will cause writes to block if the window is full - we assume in the above case that this is still much faster for A-B and A-C than for A-D.

So, in summary, we need to send messages at a rate the slowest receiver (D) can handle.