When L2CAP channels are moved from one Controller to another data can be lost. Also, some applications require a residual （残留的;残余的）error rate much smaller than some Controllers can deliver. L2CAP provides error checks and retransmissions of L2CAP PDUs. The error checking in L2CAP protects against errors due to Controllers falsely（虚伪，不实地） accepting packets that contain errors but pass Controller-based integrity checks. L2CAP error checking and retransmission also protect against loss of packets due to flushing by the Controller. The error control works in conjunction with（与...协力 ）flow control in the sense that（也就是说） the flow control mechanism will throttle （调节）retransmissions as well as first transmissions
1.l2cap 提供了错误检查和重传l2cap pdus.
这种错误检查是为了对抗固件接收了那些通过了控制器的crc，hec检查的负负得正的错误包.
2.重传也用于保护由于控制机flush引起的丢包
（实际controller也有很多其他原因导致丢包，如检查到l2cap len 因为负负得正超过正常数据包大小，而遭到丢弃；如因为控制器自身buffer管理原因造成丢包；如没有及时收取hw的内存数据而hw ack对方后立即使用该buffer）
3.the error control 和flow control的协作也就是流程控制机制将会调节重传就好像是第一次传输一样.
