Toke H├╕iland-J├╕rgensen has posted an
overview of how zero-copy networking works in the Linux kernel.

Since the memory is being copied directly from userspace to the
	network device, the userspace application has to keep it around
	unmodified, until it has finished sending. The sendmsg()
	syscall itself is asynchronous, and will return without waiting for
	this. Instead, once the memory buffers are no longer needed by the
	stack, the kernel will return a notification to userspace that the
	buffers can be reused.

https://lwn.net/Articles/1060953/
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