NUSE 
This is a library operating system (LibOS) version of Linux kernel, which will benefit in the couple of situations like:
- operating system personalization
- full featured network stack for kernel-bypass technology (a.k.a. a high-speed packet I/O mechanism) like Intel DPDK, netmap, etc
- testing network stack in a complex scenario.
For more detail, please refer the document.
https://github.com/libos-nuse/net-next-nuse/tree/nuse/Documentation/virtual/libos-howto.txt
Right now, we have 2 sub-projects of this LibOS.
- Network Stack in Userspace (NUSE) NUSE allows us to use Linux network stack as a library which any applications can directory use by linking the library. Each application has its own network stack so, it provides an instant virtualized environment apart from a host operating system.
- Direct Code Execution (DCE) DCE provides network simulator integration with Linux kernel so that any Linux implemented network protocols are for a protocol under investigate. Check http://www.nsnam.org/overview/projects/direct-code-execution/ for more detail about DCE.