Seamless handover is applied for user plane radio bearers mapped on RLC unacknowledged mode. These types are usually quite tolerant of losses, but less tolerant of delay (e.g. voice services). Therefore, seamless handover is designed to minimize complexity and delay, but can lead to loss of SDUs.
In teaching, for radio bearers to which seamless handover, PDCP entities including header compression contexts are reset and COUNT values are set to zero.
As a new key is generated from teaching anyway, there is no security reason to keep COUNT values . PDCP SDUs for transmission has not yet started will be sent after delivery to the target cell. At eNodeB , PDCP SDUs that have not yet been transmitted may be transmitted through X2 interface to the target eNodeB .