Init KCP layer.

output: a callback for KCP layer to invoke when send data to transport layer
[
    output prototype:
        function output(info, buf)
            ...
        end
    info is brought when invoke lkcp_create
    buf is to be sent
]

ret: always 0

Create kcp object.

session: number mark session 
info: extra info, when KCP layer invoke callback to send data, KCP layer would brings that to output
[
    info prototype:
        info = {1, "who", ...}
    notice:
        the form of info must be sequential, and elements must be string or integer
]

kcp: kcp object

Set maximum window size: sndwnd=32, rcvwnd=32 by default

sndwnd: send window size
rcvwnd: recive window size

None

Config re-transmission and flow control

nodelay: 0:disable(default), 1:enable
interval: internal update timer interval in millisec, default is 100ms 
resend: 0:disable fast resend(default), 1:enable fast resend
nc: 0:normal congestion control(default), 1:disable congestion control

ret: always 0

Get when to invoke lkcp_update

current: current timestamp in millisec

when: timestamp in millisec when to invoke lkcp_update 

Update state (call it repeatedly, every 10ms-100ms), or you can ask 

current: current timestamp in millisec

None

User/upper level send

data: data to be sent

sent_len: below zero for error, otherwise succeed

Flush pending data

None

None

When you received a low level packet (eg. UDP packet), call it

data: data received from transport layer

ret: below zero for error, otherwise succeed 

User/upper level recv 

None

rcv_len: Less than or equal to 0 for EAGAIN, otherwise for rcv_len 
rcv_buf: if rcv_len greater than 0, rcv_buf is data to recv