Socket Buffers

* Socket Buffers
 - Independent of a specific protocol
 - avoid memory copy between layers

* Consists of two parts
 - Packet data
 - Management data (struct sk_buff)

* Allocation :
 after memcopy from device to kernel( Receive)
 Before memcopy from user to kernel(send)

* Free :
after memcopy from kernel to user(Receive)
after transmitting the data on device(send)

* Ack가 나가면 skb는 write queue에서 빠진다.
 User space에 복사가 되면 read queue에서 빠진다.