RHEL6: Data successfully written to tcp socket never sent until socket closure.

Issue

• Our application does a successful write() call (ie. return value match the number of bytes to be sent), but the data never actually leave the network stack. The data is not seen in the network capture on the same server. We only see the data in the network capture, 3 minutes later when our application actually closes the socket.
• The application in question is an HTTP proxy, used by mobile network operator.
• This happens about once per day, at different time of day, on a random blade (customer has 37 blades or so).