this is an extension to the currently available security
configuration and activation methods provided by PDCP.
The new call allows to specify a Rx and Tx SN after which the
entire security config, i.e. integrity and ciphering, should
be enabled.
this is mainly required for the PDCP entity of the conformance
testing SS but could potentially also be used by the eNB
to enable security for PDCP after sending the SecModeCommand
for example.
the extra call was added to not break the existing API.
previously PDCP security (integrity and ciphering) could only
be enabled for both Rx and Tx at the same time.
this, however, caused an issue during the conformance testing in which,
in TC_8_2_1_1() for example, the eNB sends a SecModeCommand and a
RRC Reconfiguration in the same MAC TB. In this case, the eNB
needs to be able to enable DL security right after sending the SecModeCmd
in order to send the RRCReconfig encrypted. However, enabling UL security needs
to be postponed until after the SecModeComplete is received.
This patch allows to enable PDCP security for rx/tx independently if
that is needed. The default way is like before, enabling it for tx/rx at
the same time.