tx_next was increased only when all segments of an SDU were sent to
lower layers. This may lead to a situation where the receive entity
transmits a control PDU that is rejected because it sets its ACK to
tx_next+1 (if all segments were not transmitted when the control PDU
is sent).

What is done now is to increase tx_next as soon as we send the first
segment of an SDU to lower layers. A control PDU received from the
receive entity is now accepted and processed.

Specifications (38.322 5.2.3.1.1) say to increase tx_next when the
RLC entity receives an SDU from upper layers. But for us, since the
buffering of SDUs is done in the RLC layer and since the buffer size
may be big, we may increase tx_next too much. The choice has been
made to increase it only when we transmit an SDU to lower layers.
We may change this logic if needed. (But it won't be simple.)