User space NIC meaning

Can somebody tell me what does User Space NIC exactly means and when it is used. And what are the advantages of running a开发者_运维技巧pplication in this way? I would be happy if you can support your reply with an example.

The software layers between the NIC driver and the application increase latency. If latency is to be minimized, there are two alternatives: run the application in the kernel, or interface the NIC more directly to the application in user space.

User space NIC interfaces are used in latency-critical applications such as packet filtering. A dedicated piece of hardware which encrypts every packet arriving on one NIC and sends it out through another NIC is likely to run an application with a user-space NIC interface.