Fuzz testing tools generate malformed packets of protocols. Do they use their own TCP/IP stack to generate these malformed packets? Does the operating system's TCP/IP stack play any role in Fuzz testing? I am working on a MPLS Fuzz testing tool running on Windows 7 OS. The tool is testing a MPLS stack on a Linux machine. Does my test tool use the windows stack? I was told that windows does not support MPLS but the test tool works just fine.
Any tool that doesn't rely on kernel modifications will have to go through the OS's networking stack. This doesn't mean that they necessarily have to use the networking stack's TCP/IP support: many OSes support APIs like SOCK_RAW+IP_HDRINCL (Windows, BSD, OS X)/PF_PACKET (Linux) which lets you build your own packets (which do not have to be TCP, or even IP).
This means that userspace tools are free to bypass the TCP/IP handling in the OS and roll their own packets so long as the OS provides the necessary support (and most major ones do).